Navigating the Materials World: A Guide to Understanding Materials Behavior

GUMPR 11/15/02 8:44 PM Page i Navigating the Materials World A Guide to Understanding Materials Behavior GUMPR 11/1...

Author: Caroline Baillie | Linda Vanasupa

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GUMPR 11/15/02 8:44 PM Page i

Navigating the Materials World

A Guide to Understanding Materials Behavior

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A Guide to Understanding Materials Behavior Edited by

Caroline Baillie Imperial College of Science

Linda Vanasupa California Polytechnic State University

Amsterdam Boston London New York Oxford San Francisco Singapore Sydney Tokyo

Paris

San Diego

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12/7/02

3:50 PM

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This book is printed on acid-free paper. Copyright 2003, Elsevier Science (USA). All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. Requests for permission to make copies of any part of the work should be mailed to the following address: Permissions Department, Harcourt, Inc., 6277 Sea Harbor Drive, Orlando, Florida, 32887-6777. Illustrations by Z*ghygoem [email protected] ACADEMIC PRESS An imprint of Elsevier Science 525 B Street, Suite 1900, San Diego, CA 92101-4495, USA http://www.academicpress.com Academic Press 84 Theobald’s Road, London WC1X 8RR, UK http://www.academicpress.com Library of Congress Control Number: 2002108381 International Standard Book Number: 0-12-073551-2 PRINTED IN CHINA 03 04 05 06 07 08

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CONTENTS

Acknowledgments

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Contributors xiii

1 Welcome to the Materials World

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The Clues: How to Discover the Facts 2 Effects 3 Concepts 3 Constructs 4 Your Mission 5 Clues About the Culture 7 Capabilities: Developing Your Potential for the Mission 9 Tools for the Mission 9 Planning Your Mission 10

2 Visiting the Travel Agency (or Selecting Your Material)

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Design 14 Materials 17 Relationships 18 Translations 19 Selection Criteria 20 Values and Experience 20 Engineering Properties 22 Natural Materials 23 Engineering Constructs in Use 24 v

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Contents Life Cycle Assessment Processing 26 Finishing 28 Summing Up 29

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3 The Tool Shop (or Characterizing Your Material)

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Overview 32 An Approach to Characterization 32 Step 1: The Question 32 Step 2: Techniques 34 Step 3: The Observations 38 Step 4: Interpretation 39 Step 5: Refine the Question 41 Some Important Concepts Associated with Characterizations 42 Phase 42 Crystal Structure 43 Scale and Magnification 44 Resolution 45 Pixels 47 Channels 48 Interaction Volume 48 Sensitivity 49 Accuracy 49 Your Mission 50 Acronyms Used in this Chapter 50

4 Entering the Metals Zone Introduction 54 Metals, Metals, Everywhere 54 Communication Is Needed for Any Relationship (Misconceptions) 55 Free Electrons (the Basic Chemistry of Metals) 58 In the Beginning There Was Adam . . . Well, Actually the Atom 58 Chemical Attraction 59 Electron Speed Limit: None? 59 Ticket to Anywhere 60 Bonding 63 Personality Traits of Metals 63 Help Me, I’m Melting . . . 63 Stretchy and Springy 64 Atomic Arrangements (Crystal Structures) 64 Strong, Yet Gentle 64 Travel Smartly and Pack Efficiently 65 Crystal Clear 67 And Now for Something Different 68

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Contents

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Defects (Imperfections in Crystals) 70 A Nonperfect, Yet Interesting, World 70 Small Things with Large Consequences 71 This One Is Too Small, This One Is Too Big, But This One Is Just Right (Microstructures) 72 Defect Interactions 73 Building Muscle (Strengthening Mechanisms) 73 Phase Stability and Transformations? 75 How Do We Get There From Here? (Processing) 75 It’s Just a Phase 76 Joy of Processing (Fabrication and Annealing) 78 Pygmalion . . . Microstructural Evolution 79 Conclusion 82 Putting It All Together 82

5 A Tour of Ceramic Land

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Common Misconceptions 86 Ceramics: What Are They? 87 Ceramics: How Are They Used? 88 Source of Ceramic Properties 91 Examples of Links Between Structure and Properties 92 Ceramics Melt at High Temperatures 92 Some Ceramics Can Emit Light 92 More on Crystal Structures 94 The Structure of Glass 95 Crystal Structures of Traditional Ceramics 95 Advanced Ceramics 98 How Do You Make a Ceramic Product? 99 Process Flow Diagram for a Traditional Ceramic 101 Ceramic Processes 103 Advanced Ceramic Processing 104 Ceramic Processes: Unifying Concepts 104 Performance 106 Parting Thoughts 108

6 Interfacing with Composites Structure 112 Interface 112 Bonds (Different Types of Relationships) Pull-Out 115 Debonding (Breaking Up) 116 Measuring the Bond 116 Laminate Design and Properties 119 Structure Property Relationships 120

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Contents Mechanics of Short Fiber: Shear Lag 123 External Environment 124 Design Tools 124 Failure Analysis 126 Failure Mechanisms 127 Processing 128 Open Mold Processing 129 Autoclave Molding 130 Compression Molding 130 Filament Winding 130 Pultrusion 130 Liquid Molding 130 Processing Methods for Thermoplastics 131

7 The Land of Polymers How Are Polymers Made? (How Do We Form the Team?) 137 What Stops the Team from Growing Too Big? 137 Termination 138 Inhibition 139 Autoacceleration 140 Chain Transfer 140 Nonlinear Step Polymerization 141 Copolymerization 141 Structure 142 Categorization 143 Crystallization 144 Crystal Defects 147 Processing Route 148 Making Things with Thermoplastics 148 Compression and Transfer Molding 148 Injection Molding 148 Extrusion 149 Blow Molding 149 Rotational Molding 149 Calendaring 149 Foaming 150 Vacuum Forming 150 Making Things with Thermosets 150 Interaction of Structure and Processing 150 Polymer Behavior 152 How Easily Will the Team Break Up and How Does It Behave? Static Behavior at Room Temperature 153 Five Regions of Deformation 154 Stress Relaxation 156

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Contents Recovery 156 Time Temperature Correspondence Models 157 Polymer Degradation 158

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8 Back to Nature

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You’ve Earned a Holiday! 162 Biological Natural Materials 163 Geological Natural Materials 163 Traditional Natural Materials 164 Protein-Based Traditional Natural Materials 164 Wool 164 Silk 165 Plant-Based Traditional Natural Materials 167 Natural Polymers 167 Natural Rubber 167 Starch 168 Lignin 169 Pectin 169 Cellulose 170 Hemicellulose 172 Cellulose Fibers 173 Wood 174 Leaf Fibers: Sisal 183 Bast Fibers: Jute, Hemp, and Flax 187

9 An Electronic Trip Through Semiconductors Welcome 196 The Concept Map 197 What Is a Semiconductor? 199 Electrons in Energy Space: Energy Bands 200 Generation of Charge Carriers 204 Thermal Generation of Charge Carriers 204 Photogeneration of Charge Carriers 207 Impurity Doping 208 Thermal Equilibrium in Semiconductors 211 Conductivity and Mobility 211 Carrier Mobility 212 Conductivity Dependence on Temperature 214 Optical Properties of Semiconductors 214 Light-Absorbing Semiconductors 214 Light-Emitting Semiconductors 215 Summing Up 216

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10 Accident and Emergency Introduction 220 Ductile or Brittle: A Competition 222 Ductile Failure: Fast Fracture 222 Ductile Failure: Creep 225 Brittle Failure: Fast Fracture 227 Brittle Failure: Fatigue 230

Index

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ACKNOWLEDGMENTS

The first thanks must go to Cordelia Sealy who enabled the whole thing and supported what seemed at times like a crazy plan—to bring together pairs of authors—Materials Scientists with Educational Specialists, to help students learn how to learn materials concepts. Caroline and Linda would like to thank all of the authors for involving themselves in what has become a hugely exciting project. We needed to meet from time to time, so thanks must go to the UK Centre for Materials Education for their support throughout and for enabling Caroline to meet with Linda and Emily at various times in the US, also to John for finding a reason to meet in Sweden. And finally, thanks to Caroline’s mum for believing.

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CONTRIBUTORS

Numbers in parentheses indicate the pages on which the authors’ contributions begin.

Emily L. Allen (195), San José State University, Department of Chemical and Materials Engineering, San José, California, USA Susan A. Ambrose (53, 85, 195), Carnegie Mellon University, Pittsburgh, Pennsylvania, USA Caroline Baillie (1, 13, 111, 135), Queens University, Faculty of Engineering, Kingston, Canada John Bowden (1), RMIT University, Bundoora Campus, Bundoora, Australia Andy Bushby (219), University of London, Queen Mary and Westfield College, Department of Materials, London, UK Katherine C. Chen (53), California Polytechnic State University, Materials Engineering Department, San Luis Obispo, California, USA Peter Goodhew (31), University of Liverpool, Department of Engineering, Liverpool, UK Adrian Lowe (161), Australian National University, Department of Engineering, Canberra, Australia Adam Mannis (31), University of Liverpool, UK Centre for Materials Education, Liverpool, UK Ton Peijs (111, 135), University of London, Queen Mary and Westfield College, Department of Materials, London, UK Chris Rose (13), University of Brighton, School of Architecture & Design, Three Dimensional Design & Materials Practice, Brighton, UK Linda Vanasupa (1, 85), California Polytechnic State University, Materials Engineering Department, San Luis Obispo, California, USA xiii

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CHAPTER 1

Welcome to the Materials World Caroline Baillie, John Bowde n, and Linda Vanasupa

0-12-073551-2 Copyright 2003, Elsevier Science (USA). All rights reserved.

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Navigating the Materials World You have just entered the Materials World. This guidebook is intended to help you as you navigate your way through the ideas and language of this new world. Imagine that you are thumbing a lift through this strange new land. What might you need to know? Some basic language How to interpret the signs you see around you What the laws are so that you don’t break them What the tools are that you can obtain to help you What transport is there—how to get around What forms of energy/light/heat are there, what is needed, and how do they affect the behavior of the inhabitants What currency they use Whether there is a social welfare state—who the donors and acceptors are What forms of war/disease/degradation might there be and what preventative measures to take How the inhabitants live—the sorts of structures they build How the inhabitants interact with one another As you journey through the materials world, you’ll want to discover these facts. Let’s consider how you might deal with the information.

The Clues: How to Discover the Facts In London, if you are a taxi driver, you get tested on your knowledge of the London streets. Not only which streets are where but also the quickest routes at different times of day.These are the “facts.”There may not be one right answer. Different drivers may know different tricks and ways of getting to the pick-up point.You then apply this knowledge to your driving capability, which includes your skills and attitude. If you are kind and helpful to your customers, you will get a tip. If you are racist, you might get the sack! If you are angry, you might find yourself taking a wrong turn. If you believe you can find the way and are resourceful, even if you have forgotten the street, you will get there.You will need to be creative and adaptive. This is true also of your journey through the Materials World. We know that some of you are in fact studying for a special qualification or degree to help you on your visit to the Materials World.The knowledge that you learn in your courses will be presented to you in lectures, tutorials, and laboratory classes, or you might take part in case studies or projects.You will end up with a whole collection of data, such as lecture notes, reports, assignments, etc.This guide is intended to help you make sense of all of the incoming data for yourself so that you can better achieve your Mission. In all cases there will be some facts that you have to understand and use. You will apply this knowledge with your capability developed by enhancing your skills and attitude. Your course may spell out which knowledge, skills, and attitudes you will develop in each area.

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Welcome to the Materials World

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What we find out about the world can be seen as fact. However, what we measure and observe can be interpreted in different ways. Imagine you are looking for hidden treasure.There are many clues about the location and the type of treasure you will find.There are many ways of finding it, and when you find it, it could be different from how you imagined it. Furthermore, the really clever explorer will be open-minded about the value of the treasure.You could have been expecting jewels and find only old books. To one explorer the old books would be rubbish; the other explorers might consider the possibility that they have found the Dead Seas Scrolls. In the Materials World, you never know quite what to expect. You might find some clues that mean very little to you, such as a strange line on an electron micrograph which turns out to be the most important dislocation in the planet causing the disruption of life as we know it! We never know what the truth is, although we continue to seek it. Considering the preceding, we can see that knowledge or facts can be divided up into three main types. EFFECTS Figure 1.1

Effects are the raw data. They are what you see, smell, and measure. They don’t necessarily mean anything to you at this point. As you move through the World you will find that you gain lots of data that will need to be interpreted, and you will also pick up the capability to do this. CONCEPTS Figure 1.2

Concepts are groups of ideas that link together so that we can start to try to understand the World. For example, “color” is a concept that is very difficult to actually define but we all know what it is and each of us has our own way of describing it. The Materials World has its fair share of these. Again you will develop capabilities that allow you to use these concepts and to understand them.

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CONSTRUCTS Figure 1.3

Constructs are ways in which we make sense of the observable facts. They are created by us and help us to interpret the data. We build laws and rules and equations and models that are based on the effects that we’ve observed.The real world does not behave exactly in this way and the data we obtain do not fit the models exactly. However, they give us a way of trying to predict what might happen so that we can build bridges with more reliability than we could 500 years ago. Even now, however, because of this uncertainty we “overdesign” and build in “error margins.” When you come across constructs in the Materials World, they are very valuable to you and you should collect as many as possible. But be aware that they might let you down at a critical moment if you rely on them too heavily, and always watch out for false constructs. In order to work out for yourself the best way in which you can use this guide, we need to explore more the different perspectives from which you can look at a concept within a typical course. Figure 1.4 relates different ways of experiencing materials. A degree course may be set up with this matrix in mind, enabling you to see things from different perspectives all the time, but it may be set up from one viewpoint, or completely randomly. Figure 1.4 Course matrix (examples only given) Composite structure

Processing

Structure

Properties Selection

Life Cycle

etc. Metals

Ceramics

Polymers

Composites

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A lecture may focus on one of these nodes (e.g., composite structure). This might be found in a whole course on composites or in a course on structures of all materials. The matrix helps you to see how this node fits with all other lectures. Now, back to your exploration of the Materials World. We are going to give you some basic clues about the materials culture and help you adapt to the way of life as easily as possible. But the first thing is for you to consider the goals of your Mission in this new land.

Your Mission This is not Mission Impossible, it is Mission Difficult —MISQUOTED FROM MISSION IMPOSSIBLE 2

The purpose of this guide is to help you experience materials in the Materials World as a full experience. In order to complete a Mission you really need to decide what it means for you and if you truly want to achieve it. The first step of solving any problem is to redefine what the problem is that you are actually solving. Imagine you are building a bridge over a river and you are brainstorming how you might do this. You should stop and think about why you want the bridge in the first place. Is it because you want to get over the river? In which case, why not use a boat? Is it because a lot of people need to get across? Why not dig a tunnel? Then your problem becomes how do I get across, rather than how do I build the bridge. So, what do you want to achieve in the Materials World? 1. Do you want to obtain lots of knowledge and gain capabilities so that you can explore even more of the world? Figure 1.5

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Navigating the Materials World It is important to explore with an open mind.You will need to question the constructs and see if they make sense to you, and see if they are in fact useful to your Mission. Don’t take them at face value.You will need to try to understand the concepts in your own way. Use the analogies and other tricks offered to you in this guide to help you do this. It is critical that you develop your own way of thinking about the knowledge. In this way, you will experience the concepts and get to live in the Materials World. 2. Do you want to find out a specific thing about the World or about yourself? You might, for example, be keen to find out how to make carbon fiber composite sports equipment.

Figure 1.6

You will need to be clear about what you want and then focus your plan on achieving this goal. As well as passing your exams, of course! Structure your daily tasks well.You will get to understand certain concepts but might not develop a full appreciation of the whole World. This is your choice. 3. Are you happy with having simply visited the World? Can you go home satisfied with your passport stamped saying that you’ve been there and successfully left? You are exactly the same as you were when you went in except for the knowledge that you have been there. You can probably survive with this Mission, although it will be tough going and you won’t enjoy it very much. (You might just pass a test about the Materials, but you will remember very little in later years because you have never fully come to understand them.) Only you can decide how far you can go with your Mission, but the results you aim at will determine how you approach your Mission, and of course it will determine what you achieve. This guide will help you

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Figure 1.7

to achieve the most from the Materials World that you can. It will help you to gain a full experience and achieve the most out of your Mission.

Clues About the Culture As you travel through the Materials World, you’ll get an impression of how the materials behave. This is almost like studying a foreign culture. As you study the culture, you may start to lump your observations into concepts and constructs. For example, you may decide on your travel to an unknown culture “These people are gregarious” as a general concept.Then, you may decide “These gregarious people always celebrate from Thursday evenings until dawn on Fridays,” as a construct, based on your observations. And, as you know, it’s possible to draw false conclusions based on your observations. We call this a false construct. How do these false constructs develop? Let’s consider the cultural concept of being a Goth. The Gothic movement may be viewed as an interplay between the music, the fashion, and the people who formed the subculture. If you are a Goth, you and your friends will dress in a similar style and enjoy the same music, you will go to the same clubs, and even sometimes read the same magazines and believe certain things about the world. However, you know there are millions of different ways of expressing your own individuality as a Goth. Some of you will see your influence as Joy Division, others as Siouxsie, and you all interpret the rules very differently. However, when your parents complain about Goths, they often treat you as if you must all be the same. Imagine that your parents begin to teach others about Goths, and they use the construct that they have developed by studying you and your

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Navigating the Materials World behavior. They assume certain things and summarize these in ways that simplify and miss the point. If you go to a club you will experience the Gothic culture fully. You will have your own way of experiencing this. This effect on you, and many others like you, will develop within you, an experience of the concept Gothic. The same effect on your friends will develop a different experience of this concept. Although you have lots of things in common with your friends, you will focus on different things in your set of experiences.You are all Gothic, but each one of you develops a different way of experiencing being Gothic. Your parents, on the other hand, probably assume that all Goths are the same, and if they think Goths are devil worshippers, then they assume you must be a devil worshipper. In the Materials World, you will experience many effects and if you are going to understand them in your own way, just as you would have your own way of understanding being Gothic, you need to experience the World with open eyes.You need to find ways of exploring the World in your own particular way, experiencing the effects and developing an understanding of the concept.You will experience effects of Materials in lab classes.You will be given constructs in lectures and you will be expected to use your understanding of these within tutorials, class work, case studies, and projects in order to develop an understanding of the concepts.What you need to do is ensure that you are aware of false constructs, as with the devil-worshipping constructs.You need to be able to question all constructs to make sure that you understand them, and why and how they are used. An example of a material construct might be that elastic materials behave like a spring.This is a model, which is used to help us visualize some aspects of elasticity. For example when elastic materials are stretched and released, they revert to their original shape. However, we can’t take this model too far because the material that real springs are made from is usually metal. Metals do not behave elastically, they behave in what we would describe as a plastic or ductile manner. If we stretch the spring beyond its elastic limit, it will deform and may never return to its original size and shape. This is a real construct but it must be used with caution. It is used to help us understand the real world but always assumes things that are not valid under certain conditions. The world is very complex, and sometimes we need simple models to help us understand things. However, if we use false constructs, or use constructs as if they are the same as reality, we will get into trouble. When we turn a partial concept of a full experience (the notion of being a Goth) into a construct, we call this a false construct. An example of a false construct (such as all Goths are devil worshippers) might be shown using Newton’s version of color. Color is experienced by all of us very differently and described by us in many ways. We know and feel what the redness of red is. However, we may learn that red is 632-nm wavelength of light. Newton used models and constructs to help us

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understand the world. He did so very effectively, but when we take his work and essentially turn color into a number, we are losing the experience of red. It is rather easy to do this in science.

Capabilities: Developing Your Potential for the Mission When you travel through the new World, you will need to gain various skills to meet your challenge. As you gain these skills, your task will become easier and you will be able to face even greater challenges. It is up to you to go as far as you possibly can and realize your full potential. Your potential as a learner is influenced by your background, your upbringing and school, your preferences and interests, and by the nature and timing of the challenges given to you. It is important for you to work within an optimal-tension zone in which you have energy for action driven by inner motivation or perhaps an externally imposed deadline. However, too much pressure will oppress you and you will not be creative or constructive. In the Materials World, you will be keen to take on some challenges and some will appear boring to you. However, if you don’t attempt them you will never achieve your ultimate Mission. TOOLS FOR THE MISSION You’ve decided on your Mission, you’re aware of the potential that you must constantly try to develop on your travels. In order to help you on your way, many of the Earth’s researchers have been developing tools to help you. Think of this as a large collection of characters (like the inven-

Figure 1.8

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Navigating the Materials World tor “Q” in James Bond films) creating clever devices for you to take on the challenge. You will need to know how they work but not necessarily how they were made. You will need to use them to find out about this guide we will refer to the tools the World. Throughout that will give you an idea of how with the explorer icon you might discover some evidence about the Materials World and see the immediate effects, begin to understand concepts, guard against false constructs, and enable yourself to differentiate between useful and false constructs.

PLANNING YOUR MISSION Think about the following questions: 1. 2. 3. 4. 5. 6. 7. 8.

What is my Mission? What knowledge will I need to achieve my Mission? What capabilities will I need? What do I need to do to develop my own potential? What tools do I need? What is my timing? Who can help me? What support/funding do I have?

Challenges

Throughout our travels you will come across different challenges. The more straightforward ones are level 1 challenges, and appear in a light green box. Those which require more consideration are level 2, and are in a dark green box. If you want to achieve Mission 1 listed previously, you will need to think about these.

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CHAPTER 2

Visiting the Travel Agency (or Selecting Your Material) Caroline Baillie and Chris Rose


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Navigating the Materials World You have been wandering around the Materials World for some months now and the time has come for you to make some decisions.Which material do you really want to get to know? What do you want to do with the knowledge that you gain? What would your world look like? What products would you design, how could they influence the way people live? This chapter will help you make your decisions so that whatever you want to do in the future, you can decide whether it will be in metals, ceramics, semiconductors, polymers, composites, or natural materials. You will be able to see how the potential of materials enables you to engineer the world a little more the way you would like it to be. The basic procedure of materials selection goes hand in hand with the component design. It is an iterative process, moving between stress analysis and identification of design related constraints, with materials properties including cost and life cycle assessment. Production methods and transport need to be considered right at the beginning.Various charts and computer programs have been created to short cut the procedures and to narrow the initial selection, but these should not be taken as the final proof of selection. All suggested solutions that are generated from such programs should be checked by detailed analysis once a small range of materials has been identified. However, the whole design process involves much more than simple mathematics and selection of materials to match the numbers. We try in this chapter to help you feel your way into the mind-set of design, both from a material engineer’s perspective as well as from a designer’s perspective. In order to do this we need to consider first of all what we mean by design and what we really mean by a material.

Design Design means mental plan, intention. One design may be inclusive, embracing many aspects. In discovering the materials world you can make choices, have visions and dreams, and realize intentions. However, this requires discovery, visiting, and revisiting, seeing things anew. Some things work in a sequence, some work in a recursive way. Improved perception often works by seeing the same thing from different viewpoints. Figure 2.1

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Visiting the Travel Agency (or Selecting Your Material)

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An interesting approach that many scientists and engineers take is to learn lessons from nature. The idea is that biology has solved many problems over time and if we can work out what the problem was that nature has solved, then we might be able to apply the same notion to our engineering challenges. An example of this is Velcro, inspired by the burrs of plants. The swimming motion of dolphins, the flight of birds, and the navigation systems of ants are issues being studied by scientists today. Different people think of learning from nature in different ways.

Identify at least nine examples of a problem solved by natural materials and structures. Then for each example, describe what characteristics you can understand that meet or solve the identified need.

If we choose to be inspired by nature through our interaction with it, even if not by direct emulation, then perhaps we can also learn how to connect with each other. It would mean learning how to live like nature in order to learn from nature. If we can learn how to be like nature in our manner of studying about nature, if we learn how to communicate with open eyes, and if we can open our eyes by connecting with other researchers, then maybe we will be in a position to have a meaningful respect for nature enough to learn its lessons. A common mistake we make is to copy blindly without remembering that plant and animal materials have a whole Ecosystem in which they must live.The reason for a particular choice of material or structure might relate to the way they need to feed,rather than the way they protect themselves from their enemies. In fact, if we truly learned how to mimic nature, it is probable that the Materials World wouldn’t have the problems it does with the environment.The aspects of a natural structure or system that we can appreciate are formed through the relational properties of the system and are an expression of

Think of natural forms, materials, or design solutions that are evidence of a relationship, and try to identify the connection or the manner in which the relationship may have a formative influence on the example.

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Navigating the Materials World its ecology. The Ecosystem approach could be mimicked in the Materials World. Following this perspective in attempting to understand the consequences of a design, design practice needs to be thought of as part of a story or a set of relationships. An ethically responsible approach to design will demonstrate a concern for the effects on all involved, both directly and indirectly. This involves an understanding of how knowledge and ideas can be communicated during the design process. However, an ethically responsible designer will also be aware of the potential transmission of knowledge and ideas through the design itself. Such objects (products) do exemplify a value system. We use constructs to represent our detailed knowledge of material properties. However, we refer to our experience in describing the effects or properties of materials. These experiences influence the descriptions we use in communicating with each other. Complete products or objects also have “properties” (or attributes) for the designer, and these properties are given visual and physical expression, which in turn is affected by selection of materials and the way materials are used in the final design. Theoretical issues in art and design practice also make use of constructs, but in this context such constructs tend to be of a social, cognitive, or philosophical nature. If we say “chair,” you can have a mental, imaginary response that is a generality of chair-type information and experience. Figure 2.2 shows Figure 2.2

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three specific chair design examples that embody contrasting design concepts and, therefore, contrasting relationships with client or audience communities, including those involved in manufacture and supply. Design is about attitude, backed by skill, knowledge, and experience. What affected the development from the general type of object (chair) into the particular and very different examples shown? • • • • • • • •

Other people’s input Choice and application of materials Matters of principle Different end uses or applications Transport, use, installation, method of purchase Research Ideas—visual, tactile Different audience or customer—different user criteria

Your challenge is to sketch out a chair form using specific materials (not a diagram) and make it “your” design. Is it similar to the examples given? Figure 2.3 How would your chair design go?

Materials Depending on who you are and what kind of knowledge and experience you have (e.g., artist, designer, biologist, chemist, physicist, engineer), your understanding of what a material is will be focused on different parts of this list.

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Navigating the Materials World Figure 2.4 From nano to micro to macro.

Materials can also be categorized in terms of their structures and properties. Figure 2.5 Structural Categories of Materials.

RELATIONSHIPS

Is the air in the chair a material? The air (gas) and the envelope (PVC) work together to make the chair. Each alone does not.

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The air has been given an application—it has relational property with the PVC because of the design intention. Figure 2.6

TRANSLATIONS Figure 2.7 Terminology of constructs vs. experience.

The scientist or engineer uses constructs (trial definitions of properties/data) and the artist, designer, or user uses experiential language. We all use both to the extent of our knowledge. The overlap gives common reference, and the differences must be acknowledged and explored to avoid problems in collaborative work.

List some of the words, descriptions, or expressions you may use either formally or informally to describe the materials or structures you encounter in daily life, whether they be irritating, enjoyable, interesting, unsatisfactory, impressive, etc. Then try to decide the status of these terms; are they colloquial, descriptive, precise, scientific, or what? What types of meaning do they have? Do they have an aesthetic or a technical validity?

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Selection Criteria The material we choose for our design will influence many aspects. It is not, as we have discovered, a simple case of choosing a material to fit certain criteria. If we select one material, the whole concept of the component may change; we may in fact choose to change the design to fit the material. It is as we have said, recursive. First of all we need to consider our own experience. This is something that cannot be taught but will develop overtime! VALUES AND EXPERIENCE We talk about the “feel” of a material. Qualitative terms in design may refer to qualities in a product, but they are really attributes of the materials used (e.g., comfortable, resilient, better quality, sound). Terms such as these refer to our experience. They are difficult to quantify, but others will know “exactly” what you mean if you are both looking at the same thing (i.e., if you are sharing an experience). The language refers to the art of design. Elegant, dramatic, challenging, awkward refer to sensibilities, which have complex references. Our senses and how they work mediate our knowledge. Aesthetic sensibilities are a form of connection with our knowledge and experience, much of which lies hidden and is highly complex in extent and subconscious. It is this realm of haptic or tacit knowledge that our aesthetic sensibilities can tap into. A skilled designer or maker eventually learns to see or feel appropriate properties in what is being produced. At larger scales of endeavor where this is not directly possible, the principle still holds true through the medium of representation or simulation. These are examples of different types of language being used to mediate ideas and knowledge.

Think of a conversation that may take place about an example of a design from home, workplace, public space, lab, studio etc. Then try to see to what exactly such a conversation is referring.

In the two challenge examples above it becomes possible to track the use of metaphors when human design is experienced or described. Try to contrast this with the functionality of natural materials and structures.

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Figure 2.8 (Dewulf and Baillie, CASE: Creativity in Art, Science, and Engineering, 1999)

A combination of the analytical and the poetic is a characteristic of successful design. This could be seen as left and right brain (yin and yang, etc.) both affecting the process; one aspect is concerned with “the sense of the whole” and the other aspects with a logical sequence of consequences (holistic-linear dynamic). When considering our values and experience, the criteria we use in our selection might include the following. How well is it fixed together? Badly fitting Cold/warm to touch, summer/winter Sticky Discarded Rough Better quality Slippery Comfortable Gooey

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Navigating the Materials World Figure 2.9 Because value-laden and experiential language is what your users will use in response to design, it is important to connect this vocabulary with qualities that your design will possess.

Touch, feel Chunky Resilient Gazelle-like “The look” “Good enough” Handling (processing) the material Cultural connotations Materials you are used to Unfamiliar materials—what are they? Material associations, e.g., glass in architecture = value free (transparent)? Decorative qualities Materials carry an investment of time/work Solid material or surface finish? (e.g., solid wood) Terms used in connection with perceived value (gold, tacky) ENGINEERING PROPERTIES We do of course need to consider all the engineering properties and make sure that the materials will stand up to the test. Figure 2.11 gives you an idea of the iterative procedure this entails.

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Figure 2.10 First choice of material

Materials data collection Including info on production, cost and LCA

First design

Stress analysis and needs analysis

First trial of choice of materials in new design

Specs and design with new material

Test and produce

Table 2.1 contrasts the properties of different materials. After working with these materials for some time, you will gain a general awareness of these properties. Artists and engineering designers will not use the same language to describe this experiential knowledge. In order to complete the technical specification and selection, tables and charts will need to be referred to for exact values.

Natural Materials Living natural structures exist in a state of maintenance, adaptation, and response. In other words they are not static. By analogy, effective design of components must acknowledge the dynamic processes at play in anything that is used or built. Natural materials have constraints that differ from engineered materials because they are restricted to fewer component molecules. These basic molecules have to provide for the compound roles and functions (e.g., plants use cells for transporting nutrients and for mechanical support). Microstructural design is more elaborate to compensate for this.

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24 Table 2.1

Metals

Ceramics

Good

Poor

Stiff Ductile Tough High-melting point Thermal shock resistance

Yield Hardness Fatigue strength Corrosion resistance

Good

Poor

Stiff Very low toughness High yield Thermal shock resistance Hardness Formability Corrosion resistance Moderate density

All natural materials are composites, mostly polymeric with the harder materials being made from forms of ceramic.

Engineering Constructs in Use A Performance Index is a group of materials properties that governs some aspect of the performance of a component. They are derived from simple models of the function of a component. If, for example, we need a light, stiff beam, we use equations in which we input stiffness of a beam for a certain mass and shape and we can create a simple expression that gives us the important factors in the materials selection. For example, a map or Material Property Chart shows the two most important properties plotted one against the other. Lines are drawn onto this plot to give the limits for optimum design according to these performance indices. There is of course software available to help you make full use of such plots. The factor to include for environmentally conscious materials selections is E1, the Eco-indicator, a measure of the total eco-burden associated with the production of a unit mass of material based on data from Life Cycle Assessment.

Life Cycle Assessment We need to consider the impact that our material choice and design will have on the society and the environment. Many companies insist on carrying out an LCA before their design can be realized. For example, the furniture company Wilkhan applies environmental LCA policies throughout the whole cycle of the companies operations. It covers the impact on employee health, of materials and processes used in the company, all aspects of the materials used in terms of safety, emissions, and recycling.

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Polymers

25

Composites (polymer composites)

Good

Poor

Good

Poor

Ductile and formable Corrosion resistant Low density

Low stiffness Yield Low Tg Toughness can be low

Stiff Strong Tough Fatigue resistant Corrosion resistant Low density

Formability Cost can be high Creep

An Environmental LCA helps us to quantify how much energy and material are used and how much waste is generated at each stage of a product’s life. The analysis takes places first, but after this the Life Cycle Assessment needs to take place, which is where the interpretation and value judgments come in. Consider the following. Is it worse to use up more energy in transport or to produce more factories? Is it worse to burn and create harmful gases or to create a landfill? Is it worse to dump or to use up energy in recycling? Is it worse to have the risk of food poisoning or waste food or increase packaging? An LCA contains an assessment of the impact on the environment at each stage: Resources Production Distribution Use Disposal or reuse

Choose an example of selection, use, or consumption in your own experience and try to analyze the variables in terms of environmental impact, and in turn what impact these variables have on the design challenge. Clues: availability, cost, energy, transportation, safety, process costs, waste, reuse/recycle, disposal, ownership.

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Processing Materials selection includes the choice of manufacturing method. This might seem a little back-to-front, but as you have discovered, everything you do affects something else. The choice of manufacturing method will affect cost and environmental impact, as well as final form and properties. Hence, it is an inherent part of any design. If your design is to be informed and developmental you cannot design something, choose a material, and then decide how to make it. Evolving a design requires consideration of an overall interactive set of variables. Furthermore there are many stages to the processing. We start with the raw material. In the case of wood, this might look a little like the end product, but rarely do we leave materials unprocessed or prepared (e.g., plywood, compressed wood). Processing techniques use up materials and energy and have side effects, such as dust, fumes, noise, and other potential health risks. Polymers start from petroleum, metals, and ceramics from natural rocks and sands, and composites of course from a mixture of processed materials. All of the above may be processed with gas or liquid, and possibly with solid additives as well. The first stages of the manufacturing process have been carried out by nature—turning atoms and molecules into gases, liquids, and solid matter (e.g., growth, fossilization). The next stage, of polymerization, steel processing, or glassmaking, are discussed elsewhere in this guide. Materials selection will be important at this stage because we might have to reject the material based on the cost of processing from the natural material (e.g., some expensive ceramics such as zirconia) or because of the impact on the environment (e.g., steel making, which is very energy consuming). The next stage involves taking the raw material and making it into the end product. This could involve shaping, joining, and bonding.

Forming/Processing

List as many ways you can think of that any kind of material including, natural and synthesized (manufactured) materials, can be processed into components. This is not an exhaustive list, and new processes are being discovered or developed continually. There are different varieties of the same generic process, such as molding (injection molding, rotational molding, die-casting).

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Your answers may have included the following. Heating/cooling Dissolving Catalyzing Alloying/mixing Bending Forming Pressing Punching Annealing Tempering Forging Mechanical connections Stitching Hot pressing Extruding Welding Bonding (adhesives, friction, etc)

Vaporizing Spraying Electroplating/electroforming Molding/casting Spark erosion Laminating Cutting processes (drilling, sawing, milling, routing, slicing, planing, etc.) Abrasion Finishing processes (grinding, sanding, polishing, coating, plating, painting, enameling, powder coating, glazing, dipping)

Mixing Impregnation Baking/curing/firing Spinning, turning Carving/sculpting Folding Foaming (bread making analogy) Deposition Sintering Growing Bio-assisted (living) processes involving metabolized materials or structures such as bone or skin

Finally we apply finishing processes to the product, which we will consider in the next section. Therefore, when deciding what method you want to use in order to make the product, you will need to consider many interrelated factors. Cost. How much does the equipment cost, the energy and the time, apart from the type of material that can be used with this method (e.g., some forms of polymer processing require thermoplastic polymers as opposed to thermosets—which process requires the polymer to melt)? Energy consumption.This will affect the cost and the impact on the environment. Safety. How safe is the process, what regulations do you need to adhere to, will this affect the cost in terms of liability, or the impact on the health or the environment in terms of harmful gases and other effluent? Quantity. Is it a batch process or continuous? Can we make 100 a minute or 1 every hour? This will affect the cost and energy consumption. Properties.What properties will be affected by the method (e.g., heat treatment, rate of cooling from the melt, orientation of chains for polymers, crystallization, grain size in metals, etc.)? Effect on mechanical and other properties?

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Navigating the Materials World Shape. Can it be made by molding or does it require power processing to make such a complex part? Can you use tubular techniques such as filament winding? What about directional properties you can get from composites? Sometimes forming has several stages to it, such as pressings from sheet material, which itself is a formed product. Fixings/joinings. Can you join another part to it easily with mechanical fixings or do you need to glue/weld? What is it like when complete? Combinations. Can you join other material parts to it as above? How is load transferred between parts? Are the fixings a weak point or an aspect leading to higher failure rates in assembly? Recycling. Can the product be recycled if you choose this manufacturing process or would it be better to choose a method that is essentially reversible, such as injection molding, where you can remelt the part?

Finishing Finally we need to think about the finished product and how it will look, handle, and perform, including shipping and handling. Corrosion prevention. What environmental factors will influence your material? How will this affect your choice? What coatings or treatments will you have to apply if it needs to be resistant to water or air or other specified materials? Think about environmental factors here as well because some treatments use harmful chemicals. Surface quality. What is the finish that you require and how can it be achieved and at what price, with what impact to the environment? Handling. Surface finishes often provide the primary feel of a product for the user. Aesthetic variation. Finishing processes are the primary means of exploring aesthetic variation. Imagery or text. Can be a part of finishing and surfacing operations: labels, silkscreen printing, laser engraving, etc. Packaging. Consumer choice, health and safety and marketing versus environmental factors. Examples of finishing include: Galvanizing Electroplating Powder coating

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Dry and wet chemical treatments Painting Polishing Grafting In order to finish the material, it must be gripped or handled in some way, so that must be considered when making selections.

Summing Up The impact of design on people and the environment embodies a range of values and experience.This is mediated by the combination of the ways of thinking and working outlined in this chapter. We cannot work effectively with only one or other type of approach. Knowledge, representations of that knowledge, and our own experiences combine to enable us to work effectively in design. Once you are sure of your goal, how you really want your ideas to be experienced in the world, once you have learned how to assess the potential of a material to fulfill your goal, you will be more likely to achieve your Mission in your exploration of the Materials World as well as in life.

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CHAPTER 3

The Tool Shop (or Characterizing Your Material) Pete r Goodhew and Adam Mannis


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Overview This chapter offers an approach to the characterization of all the materials mentioned in the other chapters of this book. We are giving you a sort of toolkit to help you explore and investigate many different materials. The chapter is presented in two parts. •

•

Part 1 outlines the thought processes that need to be running through your head while you think about characterizing any material. It tries to help you consider the important issues before you walk up to the microscope or spectrometer or thermal analysis equipment. Part 2 runs through some of the terminology that you will encounter as you read about various characterization techniques. Again, almost everything in this section is material-independent and applies equally to polymers, metals, ceramics semiconductors, or composites. The ideas are universal.

What this chapter does not do is to explain in detail how each technique works. There are literally hundreds of techniques used for characterization of materials. We are trying to give you a set of ideas that will enable you to understand what you read elsewhere, and most of our illustrations come from microscopy because it is one of the oldest and most widespread techniques. If we explained every technique we would end up writing a long textbook, which is not our intention.You will need to use other reference sources as you think your way into this topic. If you are unfamiliar with any or all of the acronyms used here (e.g., AFM), consult the list at the end of this chapter or the Dictionary of Materials Science & Engineering.

An Approach to Characterization For all materials characterization the essential cycle is as follows. Step Step Step Step Step

1: Ask a question 2: Choose a technique 3: Make an observation or measurement 4: Interpret your observation 5: Refine or redefine the question, return to step 2

STEP 1: THE QUESTION All the questions of interest in step 1 arise from the central task of materials science, which is to explain, as quantitatively and as predictively as

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possible, why each material exhibits a different combination of properties and is usually both heterogeneous and anisotropic. The conventional approach to this question is to attempt to relate properties to microstructure. It is then necessary to define and measure properties, and to identify, define, and measure the properties of the basic components of the microstructure. We call this characterization of the microstructure, although we rarely completely characterize the microstructure of any complex material (for instance a piece of wood, steel, or ceramic).

PROPERTIES Some properties in which you might be interested are: Strength, stiffness (modulus), toughness, hardness, conductivity (thermal or electrical), density, magnetic susceptibility, dielectric constant, refractive index, and so on. See if you can think of some more.

HETEROGENEOUS Heterogeneous means not having the same composition in all regions of the material. All two-phase materials are by definition heterogeneous.

ANISOTROPIC Anisotropic means having properties that are not the same in all directions of measurement. Wood is a good example; its properties along the grain are not the same as those across the grain. For any material, the questions relating to characterization are therefore likely to be along the following lines. •

•

•

What is it? Which microstructural phases or components does it contain? What are their identities, amounts, shapes, and distribution? Bulk composition? Composition gradients? How did it get into this state? Evidence for processing route, temperature, etc. Why could it get into this state? This is where the real materials science comes in, and the potential for predictive capability.

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Navigating the Materials World The nature of the question determines the nature of the technique that might provide the answer. The choice can be made only with an understanding of the existence, principles, and limitations of each technique. We will deal with some of the most useful techniques and their scope later in this chapter.

MICROSTRUCTURE The heterogeneous nature of most materials is visible only under a microscope and is therefore known as microstructure. We are usually interested in how many phases are present, what they are, and what shapes they adopt. All these features affect the measurable properties of a material. Sometimes (e.g., for many polymers) microstructure is hard to see, and more indirect techniques have to be used to detect the heterogeneity.

STEP 2: TECHNIQUES We must select a technique that is capable of answering our question (or at least part of the question). It must also be available or affordable. It is not the purpose of this book to present details of each and every characterization technique. There are books and software that cover all this ground very well. What we hope to do is to equip you with enough key concepts and arm you with a sufficient list of questions that you will be able to learn rapidly from such resources. If you adopt the approach presented here, there should be no technique you cannot understand in principle. You can then read up the details (or ask a practitioner) when you need to. Our aim, therefore, is to cover the key ideas that will enable you to assess (and eventually understand) confocal light microscopy, SEM, AFM, TEM (with EDX and EELS), X-ray diffraction, Raman, SIMS, XPS, thermal analysis techniques (such as DSC and TGA), and many others. Consider the following examples. •

•

•

In electron microscopy, beams of electrons are used to image the microstructure. For SEM, electrons are emitted from the surface of a material to form an image. For TEM, electrons pass through a material sample. In X-ray diffraction, a beam of x-rays impinges on a material and is scattered in various directions by the electron clouds of the atoms. In Raman spectroscopy, atoms of a material are excited to higher energy states by absorption of radiation of appropriate frequency from a monochromatic light source (usually generated by laser). The reemitted light has a different frequency.

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•

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Thermal analysis is used to study the effects of changes in temperature on materials.TGA records the mass of a sample as a function of temperature or time. DSC allows a quantitative measure of enthalpy changes. In SIMS, a beam of ions (i.e., charged particles) is used as the probe, which is scanned across the surface, emitting secondary ions to give an imaging technique.

Sketch a simple diagram that illustrates the principle of the method for SEM, EDX, XRD, and AFM. Examples of four methods (SEM, EDX, XRD, and AFM) are given for you in Figure 3.1. Figure 3.1 SEM: Electrons in Electrons out

EDX: Electrons in X -rays out

XRD: X -rays in X -rays out

AFM: Solid probe Deflection measured

Complete the set by drawing diagrams for TEM (with three variants: imaging, diffraction, and analytical), SIMS, STM, and XPS. Can you construct a similar style of diagram for DSC?

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Think about each technique in terms of the following. • • •

A probe A signal A variable

Ask yourself how big the probe is and how far it penetrates into the specimen. For example, a light beam cannot be focused too much finer than one wavelength (a bit less than half a micrometer), whereas an electron beam can be as fine as 0.2 nanometers. Light penetrates opaque materials very little (perhaps a nm or so) while x-rays may penetrate many materials up to tens or hundreds of micrometers (or, in the case of medical x-rays and soft tissue, many centimeters). For most purposes, you need think only in terms of rough approximations. Now ask yourself what the signal is, that is, what the instrument detects? A list of some of the possible signals is shown in the box on the next page (actually this is almost identical to a list of the possible probes!). Now Check Which Variable Is Used It is easy to assume that we just collect a signal at a point on the specimen, and if we collect this signal at many points in series we can form a “map” of the surface of specimen. (See examples in Table 3.1 in which the variables in this case are the Cartesian coordinates x and y). However, there are lots of other variables. We might collect our signal as a function of energy (as we do for an EDX or EELS spectrum), as a function of angle (as in a diffraction pattern), or as a function of specimen temperature.

ROUGH APPROXIMATIONS FOR THINKING ABOUT TECHNIQUES •

Wavelengths Light 0.5 mm (m = micro = 10-6) Electron 4 pm (p = pico = 10-12) X-ray 0.1 nm (n = nano = 10-9)

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Dimensions Interatomic spacings 0.1 nm

•

Density 1–10 Mgm-3

A LIST OF POSSIBLE SIGNALS FOR CHARACTERIZING A SPECIMEN • • • • • • • •

Low energy electrons (e.g., secondary) High energy electrons (e.g., transmitted, diffracted, or backscattered) Characteristic x-rays White x-rays Atoms or ions Light Gamma rays Heat

Finally, a very practical issue, consider how large the specimen can or must be.Then consider how much of the specimen can be examined. Refer to the example EDX map image in Figure 3.2. Figure 3.2

Continued

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Navigating the Materials World Ask yourself some questions about it. • • • •

Is it a map of the surface or of a layer of finite thickness? If the latter, how thick? What do the colors signify? How big are the various features?

I do not think that you can answer any of these questions, because there is not enough data provided with the map. Make sure that any image you reproduce can be interpreted.

Table 3.1

Some Specimen Sizes Light microscope SEM TEM AFM SIMS XRD XPS

Specimen

Examined (x, y)

Depth (z)

cm cm 3 mm cm cm cm cm

1 mm–1 mm 10 nm–100 mm 0.1 nm–10 mm 1 nm–100 mm 10 mm–100 mm mm mm

1 nm 1 nm–1 mm 10 nm–1 mm 1 nm 1 nm–1 mm 10 mm–1 mm 1 nm

STEP 3: THE OBSERVATIONS The observation you can make with an instrument is rarely in itself the answer to your question. An observation is likely to be an image (a micrograph of the microstructure, for instance), a spectrum (from an X-ray or electron spectrometer), or a diffraction pattern (electron, X-ray, neutron). The observation is useless unless you understand (and can extract) the information contained in it.

As a concrete example, consider the aerial photograph image, shown next. This is not usefully interpretable until you know which city, or unless there is a feature so distinctive that you can identify the city. Can you identify it without further help? With the photograph is a printout showing pollutant levels. Again this is totally useless unless you know where and when it was collected.

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Figure 3.3 is an aerial photograph of a major city in the United Kingdom. There is obviously some code involved because no city (that I know) is blue and red. (However, you might guess the city if I tell you that its football teams play in blue and red). Figure 3.3

Table 3.2 is a printout of pollution levels in a city location. There is no way of knowing where this data came from. Table 3.2 Pollution Levels Printout.

1996 1997 1998

CO, ppm

NO2, ppb

NOx, ppb

Ozone, ppb

PM10

5.26 4.83 4.05

393 302 299

962 830 780

205 203 206

152 173 145

An observation therefore needs to be tagged with further information, such as the following. •

• • •

Where it was made (on what sample of which material in which condition, using what instrument) When (date and time) Magnification (if it is an image) Other conditions that may help with interpretation

STEP 4: INTERPRETATION Proper interpretation of your observations will involve a combination of common sense and an understanding of the principles of the technique. For example, an SEM image of part of a coin is shown in Figure 3.4.

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Navigating the Materials World Common sense tells us that it is not a new coin because the image shows various dents and scratches. However, a further, incorrect, conclusion might be that it is a very clean and shiny coin. As it happens, this is not true because backscattered electrons were used to form the image, and these tend to pass straight through the layer of oxide and grease on the coin and show only the metal beneath. An alternative image using a light microscope or a SEM in secondary electron mode would have shown a quite different picture. The lesson here is that we should always ask ourselves not only what the image shows but also what the technique would not reveal. The former is often easy, whereas the latter requires understanding. A second example involves the TEM, with two images shown in Figure 3.5. These two TEM images show a nice set of dislocations. The TEM is particularly good at revealing dislocations, and in this image we can observe their configuration and preferred line directions. Or can we? For a start, we are only looking at a two-dimensional projection of a threedimensional arrangement. Second, the specimen has been set at such an orientation in the microscope that several dislocations are invisible (as the TEM image in Figure 3.5b shows).

Figure 3.4

Figure 3.5

A

B

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So, we were seeing a projected image of some of the dislocations, and only an understanding of diffraction contrast (about which whole books have been written) can help us to interpret the image properly. Following are some questions to ask while interpreting an observation. • What can be seen? • What might not be revealed by this technique in these conditions? • Where does this information come from? • What does this information appear to imply about the specimen? • Is this reasonable? • Is there any chance that I created this effect by what I did to prepare the specimen?

STEP 5: REFINE THE QUESTION Have you been able to answer the original question, or at least part of it? If not, ask yourself whether you need to use a different technique, or the same technique in a different way, and then start again.

Following are some examples of characterization questions. Can you suggest the technique(s) to use? • What is the average grain size of this alpha brass? • What are the small particles that are responsible for strengthening this aluminum alloy? • How well aligned are the fibers in this glass-fiber reinforced composite? • Is there a texture in this steel sheet intended for car body use? • How high and how far apart are the surface steps on this wafer of (single crystal) gallium arsenide? • What is the composition of the oxide that first forms on chromium in air? How thick is it? • Is this piece of rapidly cooled metal glassy or crystalline? • How does the doping level of boron change with depth in this wafer of silicon? • Is this sintered material, made from powder, fully dense? Continued

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Navigating the Materials World You might find the venn diagram of characterization techniques in Figure 3.6 helpful. It shows the type of information accessed by the various techniques mentioned in this chapter. Usually information is collected either from the surface (ask yourself how close) or from the inside or bulk of the specimen.The information itself usually relates to one or more of the factors along the top of the diagram. Figure 3.6 Shape

LM SEM

Structure

Composition

STM

Chemistry

XPS

AFM

STM Raman

XRD TEM

Surface

SIMS DSC EELS

Internal

EDX

Some Important Concepts Associated with Characterization It is important that you understand the special vocabulary used in microscopy, spectroscopy, and other characterization techniques. Terms in common use, with particular meanings, include the following. • • • • • • • • •

Phase Crystal structure Scale and magnification Resolution Pixel Channel Interaction volume Sensitivity and limits of detectability Accuracy

PHASE We need to interpret the word “phase” rather loosely.The important features for microstructural characterization are the size and shape of each region and (if it is crystalline) its structure and orientation. These are the questions we might want to ask.

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A phase is a region of defined structure, energetically distinct from other phases, which obeys specific rules that enable us to plot its conditions for existence on a phase diagram. However, for microstructural purposes it is often convenient to use the term to describe a region that has a distinct boundary with other regions, for example a glass fiber in an epoxy resin matrix. Neither of these can strictly be called a phase, but they are microstructurally distinct.

So, what are the techniques available to help us answer these questions about a phase? Always think first of the simplest (and usually cheapest) approach. Following is a list of possible suggestions. •

•

•

Light microscope (LM). For shape and size, if the region is large enough. Ask yourself how large a region would need to be for accurate measurement by LM, then ask yourself whether you see the true size and shape of a three-dimensional region in a two-dimensional image of a crosssection. Scanning electron microscope (SEM). For shape, size, and possibly orientation, if the region is large enough. Ask yourself how small (and also how large) a phase region should be to make the use of an SEM costeffective. Would you use an SEM to measure zinc grains on a piece of galvanized steel or the average aggregate size in a piece of concrete? Transmission electron microscope (TEM). For structure and orientation. Ask yourself over what (limited) range of sizes it is practicable to use a TEM to measure size and shape of phase regions.

CRYSTAL STRUCTURE You cannot determine the crystal structure of a specimen (e.g., of a grain of an alloy) simply from an image. Therefore LM, AFM, and conventional SEM are not useful here. Nor does the specimen composition directly tell us the structure, so analytical TEM, SIMS, and XPS are unlikely to be helpful. What is needed is a diffraction technique. Electron diffraction. This is easily carried out in almost any TEM. Remember that an electron diffraction pattern usually represents just a two-dimensional section through the crystal (strictly

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Navigating the Materials World through its reciprocal lattice) so that at least two diffraction patterns in different orientations are needed to identify a crystal. X-ray diffraction (XRD). This is a standard technique in many laboratories, but you need to have a significant amount of the phase you are trying to identify before you can identify it.

RECIPROCAL LATTICE Any crystal structure can also be represented by its reciprocal lattice. In this construction, dimensions are plotted as1/distance, giving the same scaling as found in a diffraction pattern.

Ask yourself how much material is being studied in a typical electron diffraction experiment compared with the amount involved in a typical XRD experiment.

SCALE AND MAGNIFICATION No micrograph is interpretable unless you have an idea of its scale. Your interpretation of the image in Figure 3.7 should depend on which scale marker you look at—if you see 10 mm, it is probably newsprint; if you see 1 nm (implying a magnification 10,000 times greater), it is probably a high-resolution TEM image of atomic columns. Try, therefore, to label every image with a scale marker (only one!). Scale markers are much better than magnifications written into figure

Figure 3.7

____ 10mm or 1nm?

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captions because they remain accurate even when the original image is magnified or reduced (for instance when it is displayed to a class or on a screen at a conference). When thinking about the performance of a microscope, magnification is not a particularly useful concept. We can increase the magnification of an image indefinitely simply by displaying it on a larger screen. If the image displayed on your computer screen were to be shown on the huge screen at your local football field, its magnification would be some thousand times greater, but no more detail would be visible. Resolution is more important.

EMPTY MAGNIFICATION The average human eye can detect detail as fine as 0.1 mm. If we magnify an image to such an extent that the finest detail it contains is larger than 0.1 mm, then the extra magnification is referred to as empty magnification.

RESOLUTION Spatial resolution (more accurately known as resolving power) is usually the most important indicator of performance for an imaging system such as a microscope. Angular resolution is important in diffraction patterns. Energy resolution is equally significant in spectrometers used for x-ray spectrometry (e.g., EDX), electron spectrometry (e.g., EELS and XPS), and mass spectrometry (e.g., SIMS, although here it is usually called mass resolution). In all cases, resolution refers to our ability to discriminate between close features. In a microscope, we are concerned about features in the image, such as how close can two points be before we can no longer tell them apart from one single slightly larger point? In a diffraction pattern, the question is how close together can two diffraction spots or lines be (in reciprocal space, see earlier) before we can no longer confidently describe them as separate? In a spectrometer, we ask ourselves how close can two peaks be before we can no longer tell there are two? This is often related to the width of each peak. We will have a more detailed look at some of these, but first it is important to emphasize that an instrument may have an intrinsic resolving power (i.e., it is never going to be able to do better than this), but with a particular specimen the resolution may be considerably worse than this instrumental limit.

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Navigating the Materials World INSTRUMENT VERSUS SPECIMEN RESOLUTION An SEM may be able to form an electron beam of diameter only 2 nm. The ultimate instrumental resolution could therefore be 2nm. However, if we use this instrument to examine a specimen of low density and low electron emission such as carbon, beam penetration and poor secondary electron collection statistics could reduce the attainable resolution to 5 or 10 nm.

Image Resolution In most instruments, the instrumental resolution is limited by the effect of diffraction at a circular aperture. This effect either controls the size of the probe (as in an SEM or STEM) or is responsible for smearing out sharp features in the imaging lenses (in the TEM).You can see these effects nicely simulated on the MATTER Web site at www.matter.org.uk (see Figure 3.8 for a sample). Diffraction effects lead to an equation for resolution of the form g/a so resolving power is usually improved by using a smaller wavelength (i.e., for electrons, a higher energy) and a larger aperture.

Figure 3.8

Spectral Resolution (Energy Resolution) Energy resolution in a spectrum is usually limited by the instrument or its detector. This is most clearly seen in the x-ray spectrum collected by an energy dispersive detector (EDX). The statistics of electron collection in the EDX detector cause the peak from even a sharp line to be broadened, as shown in Figure 3.9. This means that two peaks close together cannot be distinguished separately.This is much the same effect as for diffraction-limited spatial resolution (compare the two images from MATTER in Figure 3.8 and Figure 3.9).

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Figure 3.9

PIXELS Almost all images generated today are stored in digital form. In order to do this, the image is considered as a set of discrete points, each of which has a finite size and an intensity (for a black and white image), or color (for a color image). These are called pixels (short for “picture elements”). Clearly no detail finer than one pixel in size can be contained in an image.

Pixels in a Typical Image

A good digital camera might record images with 4 million pixels. This corresponds to a square image being broken down into 2000 elements across and 2000 elements high. If this image were displayed as a picture 10 cm by 10 cm, how big would the pixels be? Could your eye resolve these pixels?

Pixels and Magnification

Image magnification is simply the size of the image pixel divided by the size of the object pixel. If an image is to be displayed at a magnification of 10,000 times with no empty magnification, how big (or rather, how small) should the object pixels be?

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Navigating the Materials World Pixels and Resolution

The resolution available in an image cannot be better than the object pixel size. Does the image pixel size affect resolution?

CHANNELS In any spectral data (e.g., EDX spectrum, EELS spectrum, or mass spectrum), the intensity (i.e., number of counts) is collected over an energy range. For example, an EDX spectrum will often consist of 1000 intensity values, covering perhaps 10 kV of x-ray energies. Each channel is therefore set to count x-rays with a 10 electron-volt range of energies. We say that the channel width is 10 eV. The energy resolution available in a spectrum can clearly not be better than the channel width. INTERACTION VOLUME In any technique, using a probe to excite a secondary effect (that is virtually all characterization techniques) there is a finite volume within which the probe interacts with the specimen. This is “where it all happens” and is known as the interaction volume. The simplest case, of the SEM, is illustrated in Figure 3.11. We must be careful to distinguish between the interaction volume itself and the (often smaller) region from which the detected signal comes. Again the SEM provides a good example: the region from which sec-

Figure 3.10 Channels in an EDX spectrum. Each channel is 10 eV wide, so the second channel collects x-rays with energies between 10 eV and 20 eV.

0

50 eV

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Figure 3.11 Interaction volume in the SEM. Sec e-

X-

Interaction volume

ondary electrons emerge and can be detected is much smaller than the region (the interaction volume) within which they can be generated. However if we are using the SEM to excite x-rays, the volume from which they come is often the same size as the interaction volume because characteristic x-rays can penetrate further than 20 keV electrons in most solids. SENSITIVITY All analytical techniques have a limit of detection, often called (not quite correctly) their sensitivity. This is an important feature of any analysis because it is never true to say, “our analysis revealed that there is no iron present.” The most we can say is, “the level of iron in this specimen is below the level at which we could detect it.” Unfortunately, it is often not easy to determine what the sensitivity of each technique is, particularly because it will depend to some extent on the specimen, not just on the instrument itself. Estimates of instrumental sensitivities often rely on assessing the noise or background in the signal being studied. For example when detecting characteristic iron x-rays by EDX, there will be a signal (i.e., some x-ray counts at the energy appropriate to iron) even when there is no specimen present, or no iron in the region being analyzed. Estimating sensitivity or detection limit is usually a matter of asking, “at what level of signal could I detect this superimposed on the background?” The amount corresponding to this signal is the minimum detectable. Note that sensitivity is not the same as precision or accuracy— see next concept. ACCURACY Precision or accuracy is a measure of the repeatability of a measurement. Estimates of this quantity are usually made by looking at the scatter of results when an apparently identical measurement is repeated many times.

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Your Mission Now, armed with your tool kit, you can begin your Mission in the Materials World!

Acronyms Used in this Chapter AFM DSC EDX EELS LM Raman SEM SIMS STEM STM TEM TGA XPS XRD

Atomic Force Microscopy Differential scanning calorimetry Energy dispersive x-ray analysis Electron energy loss spectrometry Light microscopy Raman spectroscopy Scanning electron microscopy Secondary ion mass spectrometry Scanning transmission electron microscopy Scanning tunneling microscopy Transmission electron microscopy Thermo gravimetric analysis X-ray photoelectron spectroscopy X-ray diffraction

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CHAPTER 4

Entering the Metals Zone Kathe rine C. Che n and Susan A. Ambrose


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Introduction METALS, METALS, EVERYWHERE Metals are found everywhere in the Materials World, and a world without any metals would truly be unimaginable. Although some objects, such as forks, could be made of plastic rather than sterling silver, some applications like skyscrapers or jet planes could not even exist without metals. So, what makes this class of materials so special? Let’s immerse ourselves in the Metals World for a while in order to gain a better appreciation and understanding of these materials.

Take a mental inventory of all the items you encounter during the day that are metals. Choose a few of those objects and write them down. Then, answer the following. • •

• •

What are the requirements for the function of each item? What are the specific properties of the metals that fulfill those requirements? Why do metals have those properties in the first place? How can we alter those properties to optimize the performance?

Although most of us have some feel for what metals are, let’s delve more deeply into the subject and investigate what makes certain materials a metal and why metals behave the way that they do.We’ll also dispel some of the common misconceptions about metals along the way.We’ll explore metals across several length scales, from the atomic level up to the bulk, and establish how the properties are a direct outcome of the structure. Different schemes or frameworks of how to think about some of the concepts will be presented. Keep in mind that the key to designing or tailoring desired performances of metals (and our goal for this chapter) is a strong foundation in understanding the processing-structure-properties relationships of materials. Most likely, you’ve already had one or more courses in Materials. This chapter is intended to strengthen and deepen some of your existing knowledge. Many terms should already be familiar to you. Some parts of the chapter might be quite easy for you, whereas other parts may require (or inspire) you to look up additional information elsewhere. Because the Materials World can look slightly different from various angles, some concepts (perhaps familiar to you from different classes or textbooks) might be presented or connected in a different way (Figure 4.1). Certain phrases that are descriptive of “concepts” are used to help understand the “effects” or behavior of metals. Table 4.1 lists the effects, concepts, and constructs that are presented in this chapter.

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Entering the Metals Zone

55

Figure 4.1 Concept map for the Metals World. METALS are comprised of structure (atomic arrangement)

atoms of metallic elements

arranged into

form

determin

microstructures

ed by y d b equilibrium e that comprise of n held together by in mi ter manifested in de crystalline amorphous phases metallic phase aff as in as in bonding e meaning diagrams cts meaning is single multiphase nondirectional no Long Range such as phase such as repeating units can be results in such as Order such as solid precipitates demonstrate intermetallics can be eutectics & solution in a matrix can result unit cells “free & mobile 100% pure e.g. symmetry eutectoids alloys from electrons” e.g. metal Ni3Al determined represent e.g. represented e.g. e.g. Ni3Al in TiAl processing by Pearlite in determines Ni-base Aluminum crystal NiTi (a-Fe + Fe3C) XRD such as superalloys Nickel structures CuAl2 in Titanium properties Brass: Cu-Zn rapid e.g. such as age-hardened Iron can show sterling silver: Cu-Ag solidification such as FCC Al alloys high electrical HCP as measured by can result in corrosion conductivity BCC explained or by anisotropy and oxidation Tensile such nonequilibrium electronic Tests as high thermal band Defects bond conductivity structure can be can be strength give can be explained point 2D: interfaces by defects processing high strength shiny, opaque between e.g. ca appearance meaning and unique meaning n yet also grains phases i n vacancies properties cr high stiffness generally history of ea such as impurities fabrication called can be se or elastic modulus sample alloying ecements such as grain techniques influenced by precipitates ductile such as good boundaries which scales high ith strengthening e.g. corrosion tw with due to annealing or rac strength mechanisms resistance inte 1D: dislocations casting heat treatments can high melting e.g. plastic deformation good powder temperates dampening processing as seen orm solid solution of to f tions single xtal by c o n ra strengthening inte from move since to growth ment of strain hardening due forming TEM (cold working) precipitation hardening grain refinement

ea

m

as

pa ck

eff i ci

en

tly

as

su ed

affects

by

aff

ec

can be

ts

can be

s

de clu

leads to

ally

con

e no hav

usu

lack of

to reduce

due to

m ay

po ss es s

tain

COMMUNICATION IS NEEDED FOR ANY RELATIONSHIP (MISCONCEPTIONS) Before we start our journey into the Metals World, we had better dispose of any false, preconceived notions. Following is a list of some of the more common misconceptions about metals (made by other people, of course). Although there may be some truths contained within the statements, we have to be careful about wild generalizations.

Misconceptions • • • •

•

Phases are made up of one single element. Mixing of elements always results in a solid solution alloy. High strength in a metal causes high melting temperatures. The crystal structure of metals has little impact on the behavior of metals. Metals are always crystalline.

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Table 4.1. Effects, Concepts, and Constructs.

Effects Metals are found in ores in nature

Concepts

Constructs

Free electrons, “sea of electrons”: delocalized electrons shared

Electropositive elements

Nondirectional bonds

Corrosion potential

Metals are good electrical and thermal conductors

Mobile electrons: carriers of charge and thermal energy

Electrical, thermal conductivity Wiedmann –Franz Law: L = k/sT

Metals are shiny and opaque

Scattering of electron flow by obstacles

Metallic bonding

Metals are malleable Metals are prone to corrosion

Electron band structure

Vacant energy bands available, no energy gap

Resistivity effects: Material defects Elect. resistivity equations

Metals are usually strong

Bond strength

E—elastic modulus

Metals stay solid up to high temperatures

Elastic deformation—bond stretching

Hooke’s law elastic deformation: s = ee Tm—melting temperature Correlation between E and Tm

X-ray diffraction shows periodic nature of metallic structures Different property values along different directions

Atomic arrangements Metal atoms as efficiently packed hard spheres

Crystal structures: bcc, fcc, hcp directions and planes anisotropy

Crystals: periodicity, symmetry

Noncrystalline or amorphous metals, no long-range order

TEM, metallography

Imperfections in crystals

Measured strength < theoretical strength

Dislocations allow atomic rearrangements

Defects: Vacancies, interstitials, Alloying elements Dislocations Plastic deformation: s = ken Interfaces, grain boundaries

Microstructures: link between atomic and bulk scales

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Table 4.1. Continued

Effects

Concepts

Constructs

Strengthening by: alloying coldworking annealing/aging

Defect-dislocation interactions: obstacles to dislocation movement leads to strengthening

Strengthening mechanisms: solid solution strengthening strain hardening precipitation hardening grain refining: Hall–Petch

Solid solutions: alloys vs. intermetallics

Microstructural evolution due to:

Phase diagrams: phase stability phase transformations

Multiphase structures

Thermodynamics equilibrium reduction in energy Vs. Kinetics solidification rates diffusion

Fabrication: casting directional solidification rapid solidification powder processing

• • • • • •

•

Heat and mass transfer: Fick’s laws Processing-structureproperties relationships

All metals are strong and ductile. Vacancies are large voids in a material. Imperfections (or defects) are rare in a metal and are undesirable. Dislocations make a metal weak and are unwanted in materials. A single dislocation will cause macroscopic shape change. Any material with small grains will be stronger than another material with large grains. The same metal will always have the same properties.

Do any of the misconceptions sound familiar to you? Make sure that you revisit the list after reading the chapter to see if you can explain the errors.

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Free Electrons (the Basic Chemistry of Metals) IN THE BEGINNING, THERE WAS ADAM . . . WELL, ACTUALLY THE ATOM Quite simply, metals are primarily made up of metallic elements. A quick scan of the Periodic Table (Figure 4.2) reveals that most elements are metals. The metallic elements are electropositive and are quite willing to give up or share their valence electrons. The valence electrons are “free” to roam around within the material and are not bound to any particular atom. Collectively, the free electrons form a “cloud” or “sea of electrons” that surrounds the metallic ion cores (Figure 4.3) and forms the metallic bonds between the atoms in order to form a solid. The electrons in a metal can be thought of as the dough that surrounds and binds together the chips in a chocolate chip cookie. Furthermore, the fluidity of the sea of electrons and uniformity of the electron cloud aptly illustrate how the metallic bonds are nondirectional, meaning that the bonds are not fixed at any particular angle relative to one another (as in a ceramic structure). The directionality of the bonds can have certain consequences, such as the ability of atoms to move relative to one another, and thus metals tend to be malleable or ductile when compared with ceramics. The concept of free electrons is extremely helpful in explaining many of the effects that we see with metals. Using such a framework, we can easily understand and predict chemical, electrical, thermal, optical, and mechanical properties.

Figure 4.2 Periodic table with metallic elements highlighted. IA 1 H 3 Li 11 Na 19 K 37 Rb 55 Cs 87 Fr

O II A 4 Be 12 Mg 20 Ca 38 Sr 56 Ba 88 Ra

IIIA

IB

IIB

28 Ni 46 Pd 78 Pr

29 Cu 47 Ag 79 Au

30 Zn 48 Cd 80 Hr

5 B 13 Al 31 Ga 49 In 81 Tl

65 Tb 97 Bk

66 Dy 98 Cf

67 Ho 99 Es

68 Er 100 Fm

VIII IIIB

IVB

VB

VIB 24 Cr 42 Mo 74 W

VII B 25 Mn 43 Tc 75 Re

21 Sc 39 Y 57 La 89 Ac

22 Ti 40 Zr 72 Hr

23 V 41 Nb 73 Ta

26 Fe 44 Ru 76 Os

27 Co 45 Rh 77 Ir

58 Ce 90 Th

59 Pr 91 Pa

60 Nd 92 U

61 Pm 93 Np

62 Sm 94 Pu

63 Eu 95 Am

64 Gd 96 Cm

IV A 6 C 14 Si 32 Ge 50 Sn 82 Pb

VA 7 N 15 P 33 As 51 Sb 83 Bi

VI A 8 O 16 S 34 Se 52 Te 84 Po

69 Tm 101 Md

70 Yb 102 No

71 Lu 103 Lw

VII A 9 F 17 Cl 35 Br 53 I 85 At

2 He 10 Ni 18 Ar 36 Kr 54 Xe 86 Rn

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Figure 4.3 Sea of electrons represents metallic bonds in metals. e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e

CHEMICAL ATTRACTION Metals can be quite the friendly bunch because they are generous with their electrons. The free electrons are available for chemical interactions and can combine with other elements to form new bonds (i.e., new relationships). New compounds (e.g., intermetallics) or alloys can then result (depending on thermodynamics, of course). As a result, metals are usually found in nature as ores and the ores (usually oxides or sulfides) need to be refined or processed to recover the metal itself. However, there are some exceptions, and these “noble metals” (i.e., gold, silver, copper, and platinum) can sometimes be found naturally in the pure, elemental state. Unfortunately, the friendly nature of metals also leaves them vulnerable to chemical attack. As we all well know, leaving a steel (Fe) nail out in the rain (H2O) leads to corrosion and results in rust (Fe(OH)3). Perhaps you’ve even realized that most food storage containers are nonmetallic for good reason. Sometimes, those free electrons can cause trouble. Other times, the metal (such as Al or Cu) can form a thin, adherent oxide layer on the surface, which protects the interior of the material from further oxidation. ELECTRON SPEED LIMIT: NONE? Although the electrons in a metal may be free to interact with other elements, many times the electrons are content enough to just be within the metal itself. However, the valence electrons are not static; they are highly mobile and can travel large distances. A current results from the flow of electrons. The ability of the electrons to travel freely results in high electrical conductivity typical of metals. Although there is a range of electrical conductivities among metals, the values are much higher when compared with other material types, such as conventional polymers and ceramics. Thus, music aficionados use Cu wires to hook up their stereo speakers, and interconnections between microelectronic components are often Cu or Al metals.

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TICKET TO ANYWHERE Electrical properties are often understood in terms of the electronic band structure. Generally, metals always have adjacent, empty energy levels available for the electrons and there are no energy gaps, as in semiconductors or insulators (Figure 4.4). Thus, there’s not much stopping that electron flow, and electrical conductivities are high for metals. In fact, the characteristic appearance of metals can also be understood with the electronic band structure. The continuous energy levels that are available in metals allow almost all the visible wavelengths to be absorbed and reemitted. Thus, metals are opaque and give off that metallic luster (from the continuous range of wavelengths).

ENERGY

Figure 4.4 Electronic band structures at 0 K for A) metals and B) semiconductors and insulators. Metals have no energy gap and, thus, available adjacent energy levels for electrons. High electrical and thermal conductivity results in metals. empty band

empty conduction band band gap

filled band

(a)

filled valence band (b)

Cool to the Touch

Suppose you had a stainless steel plate, brick, and plastic sheet all at room temperature (or perhaps you are reading at a desk with a wood top and metal frame, and sitting on a chair made with plastic or cloth parts). How would these different material types feel? Would all the samples have the same sensation to your touch? All the items are at the same temperature, yet the metal feels “cold.”The sensation you feel is not the temperature, but heat being transported away from your fingers. Because there is a gradient in heat, the heat will be conducted via the material ( from you to the object).

In this example, we are experiencing thermal conductivity (i.e., flow of heat), and metals are often cold to the touch. Instead of thinking of electrons as carriers of electronic charge, think of them as carriers of thermal energy. In metals, the free and mobile electrons can rapidly carry away or

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dissipate heat, and thus leave your fingers cold. Thus, metals have high thermal conductivities. In other materials, phonon contributions (or lattice vibrations) may be the dominant mode of thermal conduction, rather than electrons.

Can you think of some applications where the conductivity of heat is important?

Because free electrons are the mechanism for both electrical conduction (s) and thermal conduction (k) in metals (Figure 4.5), the two values are related through a constant in the Wiedemann-Franz law. L=

k sT

where T is the absolute temperature, and L is 22.4 ¥ 10-8 W-W/(K)2.

Figure 4.5 Concept map showing the relationship between thermal and electrical conductivity in metals.

Free & mobile electrons results in

results in high thermal conductivity

high electrical conductivity due to no band gap results in metallic luster

correlates Wiedemann-Franz Law

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Navigating the Materials World The framework of free and mobile electrons also enables us to predict which material features will affect conductivity and what the effect will be. Just as roadblocks slow down traffic as you drive along the highway, anything that blocks the path of moving electrons creates resistance to electron flow and thus reduces the conductivity. Although electrons do not have to travel along specified paths (or roads), obstacles within materials can cause scattering (Figure 4.6) and thereby reduce the net motion of electrons. Figure 4.6 Schematic drawing of the decrease in net electron flow due to obstacles that cause scattering and result in resistance. (Source: Adapted from Callister, Jr., Materials Science and Engineering: An Introduction, 5th ed., New York: John Wiley & Sons, 2000.)

scattering events

NET ELECTRON MOTION

Resistance Is Futile . . .

What are some obstacles in a lattice that can increase the resistivity of metals? a) Increases in temperature cause greater vibrations of atoms about their position in the lattice. Essentially, these lattice vibrations create larger scattering centers or obstacles that block the path of electrons. Thus, the resistivity of a material can increase because of temperature, as the following equation demonstrates. rt ro a T

= = = =

rt = ro + aT resistivity due to temperature effects initial resistivity of material temperature coefficient of resistivity temperature Continued

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b) Foreign atoms in metals, such as impurity or alloying atoms, act much like hurdles for runners. The point defects can get in the way of traveling electrons and add resistance to the electron flow. c) Dislocations can also be barriers to electron flow. The conductivity of a heavily worked metal (which thus has a high dislocation density) is much lower than the conductivity of that same metal in the annealed state. The total resistivity (rtotal) of a metal may then be represented by summing the resistivity from each factor: rtotal = rt + ri + rd ri = resistivity due to impurities rd = resistivity due to dislocations

Bonding PERSONALITY TRAITS OF METALS So far, we’ve been treating metals collectively as a group. But, not all materials are created equally and each metal has its own identity and unique properties. What are some of the features you would use to differentiate among the different metals? From a structural engineer’s standpoint, materials are often first screened by their melting temperature and strength. Metals tend to have high melting temperatures and be quite strong, and thus metals are often used in applications for transportation and buildings. HELP ME, I’M MELTING . . . Different metals have different melting temperatures, but why? Can a basic trait such as the melting temperature provide clues as to how a material will behave? Let’s go back to the fundamentals and consider the different states of matter on the atomic level. The average distance between atoms in the solid or “condensed state” is smaller than that in a liquid and also much smaller than that in a gas (Figure 4.7). Melting is simply a phase transformation from solid to liquid, and thus atoms are required to be separated further apart. To achieve the phase transition, the temperature is increased and heat (i.e., energy) must be supplied to the system. Now for a moment, consider the difficulty in separating Linus (of the Peanuts gang) and his security blanket versus you and an old pair of smelly socks full of holes. The amount of attraction or bonding will obviously have a direct influence in the amount of work required to separate objects. Thus, the melting temperature is a measure of how much the atoms want to stay together, and accordingly, stronger bond strengths result in higher melting temperatures.

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Navigating the Materials World Figure 4.7 The average distance between atoms in a A) gas, B) liquid, and C) solid.

A

B

C

Look up the melting temperature (Tm) and elastic modulus (E) for several different metals. Does an overall trend develop between Tm and E? (You can also plot the two properties on different axes.) Why might a trend exist?

STRETCHY AND SPRINGY During elastic (i.e., nonpermanent) deformation, atoms are pulled in different directions through bond stretching but return to their original positions once the load or force is released. Thus the relationship between stress (s) and strain (e) is linear and follows Hooke’s Law. s = Ee The elastic modulus (E) or stiffness of materials is related to the strength of the bonds. Thus stronger bonds result in higher elastic moduli and, generally, higher yield strengths. By establishing structure-property relationships of materials, we can appreciate why the melting temperature scales with the elastic modulus for metals (Figure 4.8). Both Tm and E are correlated through the bonding strength. However, one does not cause the other. Does the same trend (or correlation) also exist for ceramics?

Atomic Arrangements (Crystal Structures) STRONG, YET GENTLE Although there are some general characteristics common to most metals, there are other properties that can vary greatly. For example, some metals

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Figure 4.8 Concept map that relates the elastic modulus and melting temperature of metals. bond strength determines

determines due to

elastic modulus

melting temperature

due to bond stretching

correlates with

solid to liquid phase transformation from greater interatomic separation

follows Hooke’s Law

(such as Cu and Au) are ductile, whereas other metals (such as Mg and Be) are brittle. Furthermore, some ductile metals become brittle at low temperatures (e.g., the tragic Titanic ship). Why? Again, we return to the atom for better understanding. This time we concentrate on the atomic arrangement within the solid, or in other words, the crystal structure. TRAVEL SMARTLY AND PACK EFFICIENTLY We already know that metals like to share electrons, but metal atoms are also friendly, gregarious entities that like to be close to one another. In fact in some systems, the atoms are arranged the closest possible! Since bonding in metals is nondirectional, the atoms may be modeled as hard spheres, and we can represent the atomic arrangements of metals with the packing of spheres (or BB’s or marbles or bowling balls . . .).

Convince yourself of the most efficient (or densest) packing in two dimensions (2-D). Place several of the same sized marbles (or coins) in a container. Sometimes gently shaking the container will send the marbles into the most packed arrangement (which also happens to be the lowest energy configuration). You can also use geometry to convince yourself that a hexagonal array of spheres is the most efficient packing arrangement.

Just as how stuffing large amounts of clothing into a tiny suitcase requires efficient packing, the atoms in a metal will also try to pack efficiently, and the atomic packing factor (APF) of metals tend to be high. APF =

total volume of atoms in unit cell volume of unit cell

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What are the three predominant crystal structures of metals? Calculate the APF for each structure. Figure 4.9 shows the metal crystal structures of body-centered cubic (BCC), face-centered cubic (FCC) and hexagonal close packed (HCP) with relationships between the lattice constant (a) and the atomic radius (r). The FCC and HCP structures are actually variations on the stacking of hexagonal arrays (Figure 4.10), and both structures have an APF of 0.74, whereas the BCC structure has an APF of 0.68. Figure 4.9 Metallic crystal structures: A) body centered cubic (BCC), B) face centered cubic (FCC), and C) hexagonal close packed (HCP). (a)

(c)

(b)

a0

a0

4r = a0 √ 2

a0

a0 = 2r

a0 √ 3 = 4r

Figure 4.10 Closed packed structures based the stacking of close-packed planes (or hexagonal arrays). A) The stacking sequence of ABCABC results in the C) FCC crystal structure with stacking of the {111} planes. B) The stacking sequence of ABAB forms the D) HCP crystal structure with stacking of {0001} planes. (Source: Adapted from Shackelford, Introduction to Materials Science for Engineers, 5th ed., Englewood Cliffs, NJ: Prentice-Hall, 2000.) A A A C B B A A A CB C A A

A

Close-packed planes C

A B

A B

A

A

(b) Stacking of close-packed planes

(a) Stacking of close-packed planes Normal to close-packed A planes

A B

B

Normal to close-packed planes

A

Close-packed planes B A A

(c) Face-centered cubic

(d) Hexagonal close-packed

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CRYSTAL CLEAR The orderly arrangement of the atoms in each metal crystal structure leads to particular periodicity and specific symmetry. In effect, repeating units (e.g., atoms or unit cells) that extend out in three dimensions can build up the entire material or a lattice. Most metals are crystalline because they exhibit long-range order. The field of crystallography is devoted to the precise description of atoms within structures, and X-ray diffraction is used to experimentally determine crystal structures. Most metals are polycrystalline, meaning that they consist of many different grains. The grains form a mosaic-like pattern in which the orientation of the atoms (or lattice) is consistent within each grain but different from neighboring grains (Figure 4.11). An analogy would be desks in classrooms of a building. The desks (atoms) might be systematically arranged facing a particular direction in a classroom (grain), yet in the next room, the desks are still nicely organized but face a different direction. A single crystal would thus have only one particular orientation of the lattice of atoms throughout its entirety. So why do we care so much about crystal structures? Not only are the atomic arrangements interesting from a geometric standpoint, but they also play a vital role in material properties.The atomic arrangements differ along specific crystallographic directions and planes, which leads to different property values along those different directions and planes. Or in fewer words, anisotropy results. Figure 4.11 Polycrystalline material is made up of many grains in different orientations. (Photo courtesy of A. Kelly, LANL.)

grain

A

grain boundaries

B

Sketch the atomic arrangement within the {110} plane in the BCC crystal structure. Identify the , , and directions. How are the atomic arrangements along these directions different? What kind of consequences would result? In the BCC structure, atoms can be considered to be touching one another or are “close packed” along the direction, but not or Continued

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Navigating the Materials World (Figure 4.12).Thus, the different atomic arrangements along those directions result in anisotropy. Similarly, you would get different results traveling along a street with bumper-to-bumper traffic versus an empty road. Figure 4.12 A) The BCC unit cell with a (110) plane highlighted. B) The atomic arrangements within the {110} plane differ along , , and , and can result in anisotropy.

(b)

(a)

Values of electrical conductivity, elastic modulus, and coefficients of thermal expansion (CTE) can vary along different directions within a crystalline material and may have an impact on engineering applications. Moreover, plastic deformation involving dislocation movement or slip occurs on specific (and usually the closest packed) crystallographic planes and directions (i.e., slip systems). Figure 4.13 shows how slip occurs more easily along the close-packed directions. Thus, the ease of deformation relates back to how the atoms are arranged, and the FCC metals, such as Cu and Au, are ductile because of the large number of slip systems available. On the other hand, HCP metals, such as Mg and Be, tend to be less deformable and brittle because the crystal structure does not allow easy passage of dislocations. BCC metals (such as the steel used on the Titanic) can change from behaving ductile to brittle if the temperature falls below the ductile-to-brittle transition temperature (DBTT). Because FCC metals tend to have high symmetry and lots of available slip systems, the deformation is not affected by temperature. AND NOW FOR SOMETHING DIFFERENT If solidification from the melt is quick enough such that the atoms do not have enough time to order onto a repeating lattice, the metal will be noncrystalline (Figure 4.14). Although metals are usually crystalline, special processing techniques (such as rapid solidification and lattice frustration) have been developed to achieve noncrystalline or amorphous metals. In essence, the kinetics can prevent equilibrium, and the product can be a metastable phase.

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These unique metals are also sometimes known as metglass (i.e., metallic glass). The noncrystallinity implies random arrangement of atoms and no long-range order (which is much more common in polymers). There is no longer any periodicity to the atom locations, but some short-range order exists (as found in liquids). Amorphous metals have some unique properties and will be discussed later.

Figure 4.13 Schematic of how slip can occur more easily along a close-packed direction (AA¢) in a metal. (Source: Adapted from K.L. Watson, Materials in Chemical Perspective, City: John Wiley & Sons, 1975.) A

X Y Z

A¢

B¢ A¢

A X

B

Y

Z

B¢ X

Y

Z

Figure 4.14 Atomic arrangements represented by bricks in A) crystalline and B) noncrystalline materials.

(a)

(b)

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Are polycrystalline materials isotropic (i.e., same in all directions) or anisotropic? What happens to properties when polycrystalline material becomes textured (i.e., preferred orientation of grains)? Would amorphous materials be isotropic or anisotropic? How could you use anisotropic material to your advantage? Single crystals (or grains) are inherently anisotropic, but polycrystalline materials usually have a random mix of different grains in different orientations (see Figure 4.11), and thus overall, the material will be isotropic. However, a textured polycrystalline material can show different behavior in certain directions because of the textured structure and, thus, can be anisotropic. An amorphous material has no crystal structure and the atoms are, more or less, randomly and uniformly arranged. Thus, amorphous materials tend to be isotropic.

Defects (Imperfections in Crystals) A NONPERFECT, YET INTERESTING, WORLD Ok, now we have a model for how atoms are arranged and bonded together in metals, but we don’t really live in a perfect world. Is there a more appropriate way to view metals? In addition, our theoretical predictions of property values (such as elastic modulus) based on a perfect lattice don’t match up to measured values. Why? Ever notice (or maybe you’ve only thought about) how much energy it takes to keep your room completely neat and orderly? Now think about the incredible amount of energy that would be needed to keep every single atom in its proper location within the lattice of a material. (Realize that there are roughly 1022 atoms in a tiny cubic centimeter!) Entropy works against us with our rooms and the atoms within a lattice! So realistically, a few things are going to be misplaced. In fact, thermodynamics tells us that there should actually be some imperfections (such as vacancies) in crystals. Imperfections can be treated as objects imposed on perfect crystals and also having additional energy terms associated with them. These imperfections or defects actually make things interesting. Likewise, people who are a little different from the norm (like geniuses, artists, class clowns?) make society and life more interesting. We will soon see that defects play a very large and important role in determining properties. Similar to “The Force” in Star Wars, defects can be good or bad. Sometimes, materials engineers purposely introduce defects into materials, whereas other times, great efforts are employed to rid them. Most

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importantly, though, is to understand the effects of these defects in order to deal with them. SMALL THINGS WITH LARGE CONSEQUENCES Metals will often contain foreign atoms in varying amounts and desirability. Impurities, especially interstitials (such as C, O, and N), are common in castings and can be difficult to remove completely. However sometimes, alloying elements are added on purpose. Much like modernday (and successful) alchemy, new materials or alloys can be formulated to have desired properties.You are probably already familiar with C in Fe that make up steels and how brass is a mixture or solid solution of Cu and Zn. Another example is iridium added to platinum to give the alloy greater hardness, machinability, and surface finish that are important in jewelry and other applications. We’ve already discussed the effects of dislocations on conductivity, but how about mechanical properties? Dislocations were actually first devised as a theoretical construct to explain why the strengths of materials were much lower than those calculated from bond energies. Only after the transmission electron microscope (TEM) was invented were dislocations actually seen and proven. Dislocations are crystalline imperfections that keep most materials from realizing their theoretical maximum strength, yet at the same time also enable metals to plastically deform. In the plastic deformation regime, atoms are rearranged to result in permanent changes (Figure 4.15). Stress and strain are no longer linear, and follow: s = Ken s = stress K = strength coefficient e = strain n = strain-hardening exponent

A metal with very few dislocations might be very strong, but the tradeoff would be that deformation would be very limited and the material would be brittle (as in the case of ceramics). The almost contradictory and complex nature of defects makes the Materials World so fascinating. Figure 4.15 Movement of dislocations enables atomic rearrangement or plastic deformation. (Source: Adapted from W. D. Callister, Jr., Materials Science and Engineering: An Introduction, 5th ed., New York: John Wiley & Sons, 2000.)

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Navigating the Materials World Interactions of dislocations will be discussed later in the section on strengthening mechanisms. Interfaces between different phases (e.g., precipitate and matrix) and grain boundaries are additional defects that affect material properties. These defects act like the border patrol that might restrict the flow of things (e.g., electrons or dislocations) across the boundaries. Thus, electrical conductivity can decrease in a metal if the grain size is reduced (because the grain boundary area increases). Although interfaces and grain boundaries may seem all-powerful by keeping things in or out, they are also usually more prone to chemical attack or corrosion (which allows etchants to reveal microstructures).

If defects are imperfections in the crystal structure, can there be defects in amorphous metals? How would properties be different if the same metal could be compared in the crystalline versus the amorphous form? Why might amorphous metals be used for razor blades or golf clubs? There are actually no defects in amorphous metals, at least not in the same sense as in crystalline materials. Without that orderly atomic arrangement, dislocations and grain boundaries just don’t exist anymore. Thus, golf clubs made out of amorphous metals have been claimed to have more of an elastic response to propel the golf ball further because no plastic deformation (i.e., dislocation movement) occurs. However, not all metals can be made to be amorphous and stable at operating temperatures. Special processing is needed, and often times, the actual alloy composition is critical. The demise of common disposable razor blades is usually corrosion. Because grain boundaries are often the first place for chemical attack, amorphous metals without boundaries offer better resistance and longer life.

THIS ONE IS TOO SMALL, THIS ONE IS TOO BIG, BUT THIS ONE IS JUST RIGHT (MICROSTRUCTURES) In the course of discussing crystalline imperfections, we’ve gone from the atomic level (~Å = 10-10 m) up to the micron level (~mm = 10-6 m).We’ve also discussed how bulk properties that we see and experience on large length scales are directly connected to structures on much smaller length scales. In between the atomic and bulk scales lay the microstructure, where materials scientists and engineers spend a lot of time. The

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microstructure is the vital link that bridges basic science (e.g., chemistry and physics) with the other engineering disciplines (e.g., civil and mechanical engineering). Examining the microstructure of metals can involve several different techniques, but metallography is probably the most common and standard method. Figure 4.16 shows some examples of microstructures revealed by metallography. The imperfections in a material are usually revealed through microscopy, and ironically, the perfect material (and perhaps perfect people?) would be featureless, and quite boring. Examination of the microstructure can reveal much about the metal’s history and potential. Indeed, a picture is worth a thousand words.

Figure 4.16 Microstructure of A) dendrites in a casting and B) twinned grains in deformed Zr revealed through metallography. (Photo courtesy of A. Kelly and R. Reiswig, LANL.)

A B

Defect Interactions BUILDING MUSCLE (STRENGTHENING MECHANISMS) Imagine yourself as a dislocation.Your mission is to travel through a crystal as part of a mass movement to effect some macroscopic shape change (i.e., plastic deformation). As you image yourself walking through your school building, you are a dislocation moving through a metal, and during your journey you meet up with different things in your path. In a crystal, if the movement of a dislocation is hindered or is stopped, deformation becomes more difficult and thus, in turn, the material gets stronger.

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What are the possible obstacles you (i.e., a dislocation) could encounter during your travels? How would the deformation and strength of the metal be affected?

Isolated objects, such as desks or chairs, would certainly get in your way, and likewise, point defects (such as vacancies and foreign atoms) interfere with the motion of dislocations. The localized strain fields associated with the point defects can pin or slow down dislocations. For instance, pure gold and silver are quite soft, and Cu is often added as alloying additions to solid solution strengthen the metals. Now suppose for the moment that the rest of your classmates are also dislocations. All of you are moving in the classroom at the same time, and not necessarily all going in the same direction. Traffic jams and dislocation tangling ensues! As metals deform, more and more dislocations are created, and continued deformation becomes harder and harder, which leads to strain hardening. You may have experienced strain hardening before with the repeated bending of a paper clip. The first bend may be easy, but following attempts become more difficult as dislocations build up and the metal becomes stronger. Finally, microcracks form and fracture occurs. Thus, cold working is a strengthening mechanism for metals. Annealing reverses the effects of cold working by reducing the dislocation density and returns the material back to its original properties. Large objects, such as a cluster of desks or bulky equipment, would certainly be obstacles in your path and slow you down. Likewise, different phases or precipitates within a metal alloy force the dislocations to cut through or go around the obstructions. Al–Cu alloys are annealed or age–hardened to produce a series of extra phases throughout the metal to increase the strength.Thus, precipitation hardening (or dispersion strengthening) is another means of strengthening metals. Grain boundaries are also barriers to dislocation motion. Imagine running into the wall of the classroom. (Ouch!) Movement ceases, and at the same time other classmates can run into you as dislocations pile up at the boundary. (Ouch again!) Only with enough force can you continue onto the next room. Therefore, reducing the grain size of a metal, or grain refining, can result in increases in strength. The Hall-Petch equation illustrates the relationship between average grain size and yield strength (s).

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s = so + kd-1/2 so = initial yield stress of particular material d = average diameter of grains k = constant for particular material

Nanocrystalline metals are intriguing new materials that have grain sizes down in the nm (10-9 m) range, rather than the typical mm range. Why would these materials possess unique properties?

Identify and explain the strengthening mechanisms in the age-old tradition of blacksmithing to harden and form steels. A number of different effects are present with blacksmithing, such as solid solution strengthening and precipitation hardening (from the diffusion of C, N, O from the atmosphere into Fe) and strain hardening (from the working of the metal). As you can see, defects in a material set up a wild obstacle course for dislocations. Much of the behavior of materials can only be understood with defects at the microscopic level. The interactions of defects with dislocations produce significant effects that help us understand the Materials World.

Phase Stability and Transformations? HOW DO WE GET THERE FROM HERE? (PROCESSING) We’ve spent a lot of time establishing structure–property relationships of metals, but how do we create the proper structures to get the desired properties (Figure 4.17)? What elements we use, how we fabricate the metal, and any subsequent work (e.g., annealing, working) done to the system all represent the processing (or history) of the material and ultimately determines the structure (both atomic and micron-level). Knowledge about phase stability and kinetics, as well as a bit of experience, will help guide us.

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Navigating the Materials World Figure 4.17 The Materials World follows the interrelationship among processing, structure, properties, and performance. performance

processing

properties

structure

Figure 4.18 The pearlite microstructure in steels consists of alternating layers of ferrite and cementite. (Photo courtesy of A. Kelly, LANL.)

IT’S JUST A PHASE When setting out on a long hike, a map of the terrain is quite helpful, and when combining different elements together, a map of what to expect is useful too. Luckily, we have phase diagrams that map out the equilibrium phases (as determined by thermodynamics) according to alloy composition and temperature. A phase is a distinct entity or component with a particular crystal structure and composition (or range of compositions). The mixing of metallic elements does not always result in a single-phase solid solution. If the ratio of certain elements is just right, a new compound phase (with a specific composition and ordered crystal structure) can form, and is called an intermetallic. Examples of intermetallics are NiTi, Ni3Al, NbCr2, and TiAl. Furthermore, sometimes two or more phases will coexist in the metal, and a multiphase system results. For instance, the pearlite microstructure in steels (Figure 4.18) is composed of the two equilibrium phases, ferrite (Fe with a small amount of C), and cementite (Fe3C).

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Figure 4.19 is the equilibrium phase diagram for the Ti-Cr system. 1. Identify the phase fields that contains: a. single-phase HCP solid solution b. single-phase BCC solid solution c. single-phase intermetallic, TiCr2 d. two phase, BCC + TiCr2 2. Suggest a process to produce 40% (by volume) of TiCr2 precipitates in a bcc matrix, a. given an overall alloy composition of Ti-50 at % Cr. b. given an annealing temperature of 1000°C 1. a. In the low temperature ranges,Ti has very little solubility for Cr, and the single-phase HCP region is marked (aTi). Ti goes through an Figure 4.19 The equilibrium Ti-Cr phase diagram. (Source: Reproduced with permission of Handbook of Phase Diagrams for Binary Alloys, Okamoto, Ed., Materials Park, Ohio: ASM International, 1989.) 2000

0

10

20

30

Weight Percent Chromlum 40 50 60 70

80

90

100 1863°C

1800 1670°C

L

Temperature °C

1600 1410°C

1400

1370°C

(b Ti,Cr) ~1270°C ~1220°C

1200

–gTiCr2

1000

b TiCr2 882°C ~800°C

800 667°C 600

a TiCr2

(aTi) 0 Ti

10

20

30

60 70 40 50 Atomic Percent Chromlum

80

90

100 Cr

Ti-Cr Crystal Structure Data

Phase

Compoaition, at.% Cr

(b Ti,Cr)……… 0 to 100 (aTi)…………… 0 to 0.2 aTiCr2………… 63 to 65 b TiCr2………… 64 to 66 gTiCr2………… 64 to 66 Metastable phase w……………… …

Pearson symbol cI 2 hP2 cF 24 hP 12 hP 24

Space group – Im 3m P 63/mmc – Fd 3m P 63/mmc P 63/mmc

hP 3

– P 3m1

Strukturbericht designation

Prototype

A2 A3 C 15 C 14 C 36

W Mg Cu2Mg MgZn2 MgNi2

…

w CrTi

Continued

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Navigating the Materials World allotropic (or crystal structure) transformation from HCP to BCC at 882°C. b. The high-temperature region marked (bTi,Cr) represents a complete solid solution of Ti and Cr. The same crystal structures (i.e., BCC), similar atomic sizes, and similar electronic structures of b-Ti and Cr satisfy the Hume-Rothery rules for substitutional solid solutions. c. The TiCr2 intermetallic occurs at roughly 63 to 66 at a percentage of Cr, and below temperatures of 1370°C. There are three different crystal structures (i.e., allotropes) of TiCr2. d. There are two distinct phase fields that contain two phases: (Ti-rich, b) + TiCr2 and (Cr-rich, b)+ TiCr2.These two-phase fields are bounded on each side by the single-phase fields of b and TiCr2. 2. The processing should include a high-temperature solutionizing or homogenization step in the single-phase b region, and then an annealing step to form the TiCr2 precipitates. (Later, see the similarities and differences with precipitation hardening of Al-Cu alloys.) a. Using tie lines and the lever rule on the phase diagram, an annealing temperature of ~1210°C can be used. b. Nominal alloy compositions of roughly Ti-41.5 at % of Cr or Ti82.2 at % Cr could be used.

JOY OF PROCESSING (FABRICATION AND ANNEALING) Several different processing techniques exist for metals, and many of the particulars will depend on the materials system and the intended use. Thus, only the basics will be covered here. Processing is invariably tied to the microstructural evolution of a material, and is the means in which we can attempt to manipulate properties. Casting of metals involves the solidification of a liquid melt in a mold to form an ingot. Depending on the rate of heat removal, microstructural inhomogeneities, such as dendrites (see Figure 4.16a) or columnar grains, might form (Figure 4.20). Segregation of different elements in an alloy may also result in chemical inhomogeneities within the metal. To some extent, annealing or homogenization treatments may be used to produce a more uniform structure through diffusion (i.e., atomic transport) mechanisms. However, long anneals can also produce grain growth and coarsening. Alternatively, rapid solidification techniques are employed to create chemical homogeneity, fine grain structures (i.e., microstructural homogeneity), and, even sometimes, metastable phases. Other times, very slow cooling rates are employed to carefully control the microstructure, as in growing single crystals for integrated circuits or turbine blades, or the directional solidification of eutectics. Forging

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Figure 4.20 Schematic drawing of the side view of a casting with nonuniform grain structures caused by nonuniform cooling rates.

Table 4.2. Annealing Processes that Are Driven by a Reduction in Energy and Occur by Diffusion.

Process Recovery Recrystallization Sintering Grain growth Coarsening

Driving Force Rearrangement of dislocations into lower energy configuration Creation of new, strain-free grains Reduction in total surface energy Reduction in total grain boundary energy Reduction in total interfacial energy

involves deformation during fabrication, and thus usually high temperatures are used and simultaneous annealing can occur. Annealing of heavily worked materials can result in recovery and recrystallization. Metal parts may also be fabricated by powder processing. Sintering of the powders requires diffusion and is achieved through high temperatures and pressures. As Table 4.2 shows, many of the processes that can occur during annealing are driven by the reduction of energy of some sort. The mechanism for these processes involves diffusion, and thus occurs at high homologous temperatures (T/Tm). In addition, if high operating temperatures are to be used for the material, caution must be taken to ensure that possible microstructural changes (e.g., grain growth), which might degrade properties, do not take place. PYGMALION . . . MICROSTRUCTURAL EVOLUTION In many storylines, there is a battle between two forces (e.g., Man versus Nature, the good versus the bad, etc.), and oftentimes in processing, thermodynamics is pitted against kinetics. Thermodynamics tells us what the destination is, whereas kinetics tells us the story of how and if we get there.

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Navigating the Materials World For example, the practical Fe-C phase diagram (Figure 4.21) indicates that for steel alloys ( q > 40° (Figure 6.12). The reason for this is that no interaction between the different failure modes is accounted for. To account for this, a number of interactive failure criteria have been developed, such as the Tsai-Hill and Tsai-Wu criterion. MECHANICS OF SHORT FIBER: SHEAR LAG In the case of discontinuous fiber composites, the properties of the composite may be described by an adapted rule of mixtures that includes knockdown factors for the reinforcement efficiency as a result of a reduced fiber length and fiber orientation. Ec = h1hoEf Vf + Em Vm Where h1 ho are efficiency parameters. The first parameter, hl, describes the length efficiency of the fiber and depends on fiber length, matrix, and fiber modulus and is in the order of 0.5 to 0.8. The second parameter, ho, addresses the effect of fiber orientation and is 3/8 in the case Figure 6.12 longitudinal tensile failure

shear failure transverse tensile failure

normalised tensile strength

20

40

q degrees

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Navigating the Materials World of random in-plane (2-D) fiber orientation and 1/5 in the case of 3-D random fiber orientation.

EXTERNAL ENVIRONMENT There are many external factors that influence the strength of the relationship and its behavior in a society or a material. For the couple, the climate, the amount of work and financial pressure, and the attitude of friends, will all influence the strength of the bond. For materials, the temperature, the presence of oxygen and moisture, and of course applied stress will all determine the survival of the bond. Can we actually predict any of this for a full material? Once we have more than one layer of composite what happens? Once we have more than one layer in this community, how complicated does it get and are there any rules or laws to predict how the community behaves? Economists, town planners, and market researchers try to predict the behavior of people and provide models so that we can build shops, plan the country’s policies, and provide for the community. In a similar way, we have studied the way in which composites behave.We have modeled these and approximate the behavior according to the simplest usable mathematical prediction. Just as predicting the behavior of people is never accurate, the same is true of materials, but we have a first attempt in which to measure up against when we do some experiments. DESIGN TOOLS The anisotropic nature of fiber-reinforced composites requires designers to adopt a different design approach from the way they normally design Figure 6.13 External environment.

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with conventional isotropic materials such as metals. Laminated composite materials are generally multilayer materials, each layer being a lamina consisting of an array of aligned fibers in a matrix. The lamina is strong and stiff in the fiber direction and weak and compliant in the transverse direction. The lamina can be regarded as a highly anisotropic material with one-dimensional properties. Hence, for use in a real threedimensional world we need to account for these highly anisotropic properties by combining layers of different orientations within a laminate. A laminate is composed of lamina at different angles to a reference direction, being generally the principal loading direction. As a result, the properties of the laminate can be adapted to the loading situation. In composite design, it is not only the part but also the material, i.e., laminate that is designed to the actual application. Because designers want to make full use of the potential of advanced composites, a number of design tools have been developed. Some of design tools available are network theory (carpet plots), laminate plate theory, or finite element method (FEM). Carpet Plots Carpet plots are simple ways of demonstrating the laminate behavior on the basis of network theory without having to go through the math each time! In network theory, the resin does not take up any forces. They are only approximate and can be used as a guide for further work. They can show the properties of laminates with any combination of 0, 90, or +/-45 plies. Figure 6.14 The carpet plot.

Laminate Plate Analysis The laminate can be designed to the actual application. Elastic properties of a laminate can be predicted from those of the lamina using laminate plate theory, which takes into account the complex interactions between the individual lamina. Although the procedure involves straightforward but

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Navigating the Materials World extensive matrix algebra manipulation, this is often done using commercially available software packages. Properties of a lamina are input Laminate configuration is input Elastic properties of the laminate are computed Strength of the laminate is also computed by use of one of several failure criteria Loadings may also be input and strains, deflections, and failure predicted Does not determine the laminate configurations – optimum configuration Does not choose the appropriate composite system Does not address matter of ply lay up Nowadays, the FEM is certainly the most widely used method for designing composite parts. Many software companies offer modulus specifically developed for composite parts and capable of taking characteristic features, such as anisotropy, into account.

FAILURE ANALYSIS In a laminated composite, because the plies are all bonded together, upon loading, the strain in all plies will be the same. On the other hand, the stresses will vary according to the modules of the individual plies. For example, the highest stress occurs in the ply with the highest modulus (e.g., 0-ply), whereas the ply with the lowest stiffness (90-ply) takes the lowest stress.The first damage to occur in the multiply laminate is often predicted by first-ply failure. Upon loading, the ply with the lowest failure strain, being often the 90-ply, will fail first. Note that, although the 90-ply has also the lowest strength, it is its low failure strain that makes it fail first. Generally, the design of composite parts is largely governed by maximum allowable deformations. To be on the safe side, composite parts should be designed in such a way that the specified maximum deformations and the acceptable or critical strain levels of the individual plies will not be exceeded. In other words, the part should be dimensioned so as to prevent any cracking. Initial microcracks in some layers tend to grow and allow aggressive media to penetrate into the laminate and damage it. This will reduce the service life of the part and is, therefore, undesirable. If the critical strain level is exceeded, this will not result in immediate failure of the part, but it will reduce the service life. In order to avoid any damage in the laminate, many laminates that contain 90° plies are designed on the basis of first-ply failure and with a design criteria of allowable strain. Alternatively, parts may be designed on the basis of desired high strength. Here the designer must make sure that the part will be able to sustain a certain maximum load. Failure of individual layers may be

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acceptable as long as this does not result in total failure of the part. Failure of individual layers within a laminate does not necessarily mean that the laminate is no longer able to sustain the load because other plies may be capable of withstanding greater loads before they fail. However, analysis of the behavior beyond the first ply failure is complicated because it is unclear to what extent the damaged ply continues to bear some load. FAILURE MECHANISMS Just as we have theories to help us predict the behavior of a society, we also try to understand what it is that fractures a society. What causes the breakdown, what is it that causes people to fight one another, to steal from one another, and to commit crimes? What are the main failure events in composites? There are a number of microscopic failure phenomena operative in composites.We have talked already about debonding as a microscopic failure phenomena, but there are a number of other microscopic failure mechanisms operative in composites that all lead to toughening of the composite. This is because the failure pathway of the crack is lengthened. More small cracks and debonding mean a tougher composite. The addition of fibers to brittle matrices leads to the introduction of a large number of microscopic failure mechanisms that will lead to an increase in toughness of the composite. Some toughening mechanisms, all based on creating new surfaces, that may be operative in composites are as follows. •

Crack deflection. The interaction between the reinforcing fiber and crack front can cause the crack to be deflected causing a reduction in stress intensity in the matrix and toughening of the composite material.

Figure 6.15

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Navigating the Materials World •

•

Debonding. The process of debonding creates new surfaces in the composite and, therefore, requires energy. Pull-out. This toughening mechanism occurs after debonding and in the case of continuous fiber composites, also fiber fracture. In pull-out, a force is required to overcome frictional forces that usually originate from residual stresses associated with resin shrinkage during curing and thermal contraction.

In a single-ply or unidirectional composite, if we consider a crack that is traveling through the matrix material, which is weaker then the fiber as the crack reaches the fiber, there will be three stresses to worry about. s1 will tend to cause fiber failure, s2 will cause tensile separation or debonding, and s3 will cause shear failure and debonding. Figure 6.16 s1 s3 s2

1. stresses at crack tip

2. crack tip at interface

3. interface splitting and crack opening

Take a balloon and make a fibrous network by sticking tape onto the balloon in the shape of a triangle. Prick inside the triangle with a pin. What happens and why?

Processing In the case of composite materials, knowledge of fabrication processes is essential because with composites we design and manufacture not just the part but the material as well. Therefore, in order to be able to design a composite part, one must know the method by which this part will be produced. Each manufacturing process has its special characteristics that the designer of composite parts must take into account. Another selection criterion is the size of the production run. We have a lot of choices

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Figure 6.17 Processing.

to make: material form, intermediate form (prepreg), processing method, processing parameters, microstructure (crystalline alignment, voids), and mechanical properties. Even within the materials form we have many choices. Reinforcement format and orientation Roving Web, mat, chopped strand, random, aligned short fiber/cloth weight, weave Nonwoven Bulk forms: perform, braided forms Processing methods for thermosets Hand lay-up and spray-up OPEN MOLD PROCESSING A mold having the desired shape is first coated with a release coat that prevents bonding of the part to the mold. If a smooth surface of the part is required, such as in the case of a boat hull, a gel coat is applied to the mold. In the case of hand lay-up, a woven fiber mat or chopped strand mat is then applied and impregnated with a thermosetting resin, such as unsaturated polyester. Rollers may be used for consolidation, followed by

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Navigating the Materials World curing of the resin at the required temperature. In the case of spray-up the chopped fibers and resin are applied using a spray gun.

AUTOCLAVE MOLDING Autoclave molding is the standard aerospace industry process for fabrication with prepregs. These prepregs have been an important innovation in composite manufacturing technology. Prepreg is preimpregnated tape consisting of fibers in unidirectional or fabric form with polymer resin. The advantages of using prepregs are that the resin and fibers are combined in a controlled way and at the right portions. In the case of thermosetting resins, the resin is partially cured and the tape, therefore, must be kept refrigerated to prevent full curing until final use. The prepregs are consolidated in an autoclave, which is simply a large pressure cooker into which the mold with lay-up is placed and subjected to the required pressure and temperature for curing.The lay-up is placed in the autoclave under a vacuum bag, which should remove volatile gasses during the cure process and results in very low void content (0.1%). COMPRESSION MOLDING Similar to unidirectional prepregs for autoclave molding, sheet molding compound (SMC) has been an important innovation in the manufacturing of high volume parts using compression molding technology, such as in the automotive industry. In contrast to prepreg tape, which consists of continuous fibers in a thin sheet, SMC is a relatively thick sheet and is based on chopped glass fibers in an unsaturated polyester resin. An alternative to SMC is bulk molding compound (BMC), which consists of capped fibers and resin in bulk form. Both SMC and BMC are molded using a matched die compression molding process. FILAMENT WINDING Filament winding is a process widely used for the manufacturing of hollow tubular products, such as pressure vessels, drive shafts, piping, and tubing. The process involves wet winding of resin-coated fiber rovings or fabric tapes onto a rotating mandrel. Alternatively, dry winding using prepreg tapes also can be used. PULTRUSION In pultrusion, fiber rovings and fiber preforms are impregnated in a resin bath and pulled through a heated die to form structural elements, such as beams and profiles. The process is relatively fast but restricted to structures whose shape does not change along their length. LIQUID MOLDING Liquid molding techniques include resin transfer molding (RTM), structural reaction injection molding (SRIM), vacuum-assisted resin transfer

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molding (VARTM), and vacuum infusion. In all processes, a preform consisting of fiber mats and a foam core is first produced to fit the shape of the mold. In the case of RTM and SRIM, the preform is then closed in metal mold, and liquid resin is injected under pressure to impregnate the fiber preform.The major differences between the two are that with RTM the resin and hardener are premixed before injection, whereas with SRIM the resin and hardener are mixed when they are injected into the mold. Because SRIM works with very low viscosity systems that are injected under high pressures, production cycles are faster than for RTM.VARTM or resin infusion also uses liquid molding technologies, but here the resin is injected in the cavity using a vacuum. VARTM and resin infusion have the advantages that only a single-sided mould can be used with a flexible vacuum bag to close the mold. The use of single-sided molds is much cheaper than an expensive metal-matched die mold for RTM or SRIM, and the process is, therefore, used often as an environmentally friendly alternative to open-mold processes for large product, such as boat hulls. PROCESSING METHODS FOR THERMOPLASTICS Processing methods for thermoplastics are similar to those given in Chapter 7 for thermoplastic polymers without fibers. Compression molding and thermoforming can also incorporate mats of random or oriented fibers, but injection molding and extrusion require short fibers that get broken down even more by the process. There are quite a few choices for thermoplastics processing at the intermediate form stage. Intermediate forms for reinforcement Film stacking Commingled fibers Pultruded band Powder impregnated bundles

What factors affect the interaction of processing and behavior of composites? Imagine the sorts of things that can affect the behavior of couples in society, such as education, socialization, and upbringing; the basic materials or the people as they are born; and the way they are treated before birth (pre-treatment), as young children (conditioning), and as young adults in education and socialization (treatment). This is the same as our composites: pretreatment, conditioning, and treatment/coatings. Continued

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Navigating the Materials World Matrix type. Thermoplastic/set/elastomers Matrix properties. Viscosity for flow and impregnation Fiber type or particle Fiber length and flaw distribution Fiber orientation and Vf, textile form, intermediate form Fiber treatment/sizing/matrix Permeability. Cloth and mat, open, rovings control micropermeability and are rate controlling Fiber surface. Wetting, surface energy Processing method Associated crystallinity and void content Temperature. Raising will reduce viscosity and gel time Pressure. Increase in pressure gradient increases flow and infiltration; may cause displacement Anisotropy. Resin flow more difficult in normal direction

So, what would affect your choice of composite for any application? What would make you choose a particular combination of fiber and matrix? How would life cycle assessment affect your choice? How does the processing route affect your choice? How do all of the above affect the properties of the component? What have you learned from your new relationships in the Composite Land? Did you achieve your Mission there?

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CHAPTER 7

The Land of Polymers Caroline Baillie and Ton Peijs


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Navigating the Materials World The land of polymers awaits you. You see before you many long chains, some of them short, some long, some lined up, some forming helices, and constantly moving in a sort of dance with each other.Where do you start to explore the culture and behavior of these strange beings? In order to understand this land we are going to use the analogy of forming a group of friends or a team of acrobats in which the team players are monomers, or basic units of the chain, and the team is the polymer. How do you choose the people in this group, what are their characteristics, how do they behave together, what external factors affect them, etc.? Polymers are a very important category of material in nature and in engineered structures. In order to understand fully and exploit the unique interaction between structure and properties in these materials, we need to consider the synthesis of the polymer from the raw material because different processing routes will affect the properties; there are many possible ways of varying the final polymer produced. We need to explore the structure of polymers using characterization methods from the tool shop. We also need to measure the properties of polymers and determine the behavior of polymers as engineering materials and why they behave the way they do under different conditions as a function of the structure. If we have a thorough understanding of all of the above, we are ready to be able to tailor a particular polymer to an application for optimum performance. This guidebook will lead you through the concepts behind the above considerations. What is the survival kit knowledge for the land of polymers? You can gain a lot of basic information by observation. More detailed knowledge requires characterization methods as shown in chapter 3.

When you see a plastic product, what does it tell you about the underlying polymer? •

• •

•

•

What does the product look like? Is there a join line on the bottle? What method and what polymer were used? What is the difference between thermosets and thermoplastics? What does it feel like? Hard, soft, flexible? Is it cold to the touch? What affects these properties? What is the life cycle assessment? Is it better to use biodegradation or recycling? Which has the least impact on the environment? What sorts of factors are involved in a life cycle assessment? How does a polymer differ from a metal and from a ceramic? Continued

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PET bottles can be blow-molded because their viscosity increases with stress. HDPE and PE are thermoformed, so they have a join line Transparent often can mean it is glassy or amorphous because the light is scattered. White is often semicrystalline. Thermoplastics have secondary bonds between chains, which can be broken with heat. Thermosets cross-link and form primary bonds between the chains. After cross-linking, they cannot be melted. The polymer may behave in a ductile or brittle manner depending on the temperature with respect to its Tg, as well as the rate of application of stress. Polymers have poor conductivity because their electrons are shared covalently within chains and are highly localized. The life cycle of a polymer will include a cradle-to-grave approach in which all aspects are considered from raw material availability to processing fumes, energy consumption, and waste. Polymers differ from metals because they have no sea of electrons to conduct electricity. They differ from both ceramics and metals because they have long chainlike molecules.

How Are Polymers Made? (How Do We Form the Team?) Polymers are made from petroleum products. There are two general ways in which polymers are made from the raw purified materials. The first is step-growth polymerization. This is the mechanism by which the polymer chain grows step-wise by reactions that occur between any two molecular species. Step-growth polymerizations (Figure 7.1a) occur when two monomers or polymers combine to form a longer chain. In order for them to keep reacting, they need to have more than one functional group, or they will just get together as a couple. Teams grow in this way by joining forces. Polymerizations in which a polymer chain grows only by reaction of a monomer with a reactive end-group on the growing chain are known as chain-growth polymerizations (initiation of free radicals on the monomer by initiators, heat or REDOX reaction, followed by propagation in which the free radical reacts with the next chain). In this case there is a team leader (the free radical) who selects more team members one at a time (Figure 7.1b). See Table 7.1.

What Stops the Team from Growing Too Big? With step polymerization, once there are no more reactive groups to encourage chains to join up, the chain stops growing. With chain poly-

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Figure 7.1

A) Step

B) Chain

Table 7.1 What’s the Big Difference?

Step

Chain

Any two molecular species can react (any old teams get together)

Repeating units added one at a time (Fussy team leader)

Monomer disperses early (singles get paired off quickly)

Monomer decreases steadily throughout (lots of bachelors and bachelorettes left at the end)

Molecular weight rises steadily throughout

Long reaction times have little affect on Mw

Long reaction times give high Mw

merization, there are two common ways to stop the chain from growing: combination and disproportionation. TERMINATION Combination occurs when two chains (teams) couple together to form a single polymer molecule (team). This is called head to head and it stops the growth because two team leaders have got together and decided they don’t want any more expansion (the free radicals have complimented each other and electrons balance) (Figure 7.2a). The other way that termination can occur is by disproportionation when a hydrogen atom can be abstracted from one growing chain to another (Figure 7.2b). This is as if the best team member has been taken away by another team, thereby effectively disabling the first.

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Figure 7.2 Termination. A) Combination

B) Disproportionation

INHIBITION Sometimes we can slow down growth. Some substances can react with the free radical centers to produce species that are incapable of reinitiating polymerization. If the procedure is not very efficient, the substance is described as a retarder, and if efficient, it is described as an inhibitor. Imagine the team leader drinks too much to care about new team members. Figure 7.3 Retarders and inhibitors.

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What other ways are there for influencing the length of the chain? Imagine the team is growing in size, what limits it or encourages this growth? An enthusiastic team leader? Friendly team leaders? Warm climate? Lots of funds? Good beer? Chain growth. Formation of free radicals (leaders), heat and light, termination, chain transfer, autoacceleration, inhibition or retardation, elimination of unreacted species, inhibitor concentration, ceiling temperature (temperature above which reaction will not occur) versus high temperatures for high rate of reaction. Step growth: functionality of more than one. But three forms a network. Stoichiometry, ring formation.

The following aspects are particularly important. AUTOACCELERATION When the monomer concentration is high, there is often a sharp increase in the rate of reaction because the conversion increases. It occurs as a consequence of increase in viscosity of the reaction medium, and chains have difficulty moving to positions whereby termination can occur. This is like a big party in which there are so many big teams that the leaders find it hard to locate each other and thus form alliances. Figure 7.4 Autoacceleration.

CHAIN TRANSFER Values of degree of polymerization can be much lower than expected, indicating the presence of additional reactions that terminate the growth

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of a chain radical. These reactions are known collectively as chain transfers. They effectively stop the growth of one chain and start another. The team leader takes their best players to another team. NONLINEAR STEP POLYMERIZATION If a monomer with a functionality greater than two is used in the early stages, the polymer has a branched structure and the reaction proceeds much faster than for a linear step polymerization. As the reaction continues, further branching leads to complex networks. Two teams get together and form friendships between team members. It’s hard to break the groups apart. Figure 7.5 Networking.

COPOLYMERIZATION Copolymers combine the advantages of two or more monomers. Just like two teams of different strengths combining to get the best of both. Degree of Polymerization How do we measure the length of a chain or the molecular weight that is related to this? We can use viscosity methods (viscometry), optical methods (light scattering), osmometry (osmosis), or gas phase chromatography (GPC).

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Navigating the Materials World Figure 7.6 Copolymer.

The molar mass of a polymer is the mass of one mole of the polymer. For network polymers, the only meaningful molar mass is that of the polymer chains existing between junction points. The molar mass of a homopolymer is related to the degree of polymerization, which is the number of repeat units (x) in the polymer chain. M = xMo where Mo is the molar mass of the repeat unit. For copolymers, the sum of the products xMo for each type of repeat unit is required to define molar mass. The Team consists of x numbers of members.

Structure We can ask four questions about the structure. 1. What is the structure? Does it have a category or classification system? In our case, what is the basic form that the acrobats take? 2. What do we focus on? The bonds, the atoms, or the whole; which scale is important and when? In our case, how do we concern ourselves with the individuals or with the team? 3. Micro/macro. What is the relationship between the molecules and the chains? How are they bonded?

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In our case, how do the acrobats hold each other and support others? 4. Why is it built that way? Structure/property relationships. Or, in our case, what is the team goal and how can they achieve this? CATEGORIZATION Two important terms in structural categorization are “configuration” and “conformation.” Configuration is usually used for a description of chains in which the geometric variations can be interchanged only by breaking a bond. Isotactic polymers. Identical configuration around each successive center. Syndiotactic polymers. Opposite configuration around each successive center Atactic polymers. Random stereochemistry Conformation is used when referring to the three-dimensional geometric arrangement of the polymer, which changes easily when bonds are rotated Figure 7.7 (A) Syndiotactic polymers; (B) Isotactic polymers; (C) Atactic polymers

A

B

C

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Navigating the Materials World Figure 7.8 Conformation.

Both configuration and conformation are determined by infrared spectroscopy or nuclear magnetic resonance (NMR). CRYSTALLIZATION During polymerization or processing of the final product, crystallization can occur. For the polymer chain, it is mostly heat treatment that will Figure 7.9 Crystal.

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produce crystallinity. Polymers are held at a certain temperature (Tc) to form the crystal structures. What Is a Crystal? The polymer chain folds in an ordered manner and forms a platelike lamella. Isolated chain-folded lamellae single crystals can be obtained only from solutions. As molecules become entangled more complex forms are obtained. If a melt-crystallized polymer is prepared in the form of a thin film and then viewed in an optical polarizing microscope, spherulites can be seen, which show a Maltese cross pattern under crossed polars. Molecules are aligned tangentially and extinction patterns are due to the orientation of the crystals. Spherulites stop growing when they impinge upon one another. Twisted lamellar crystals radiate from a central nucleus and terminate at spherulite boundaries. The spherulite consists of an aggregate of ribbon-like chain folded crystallites approximately 10 nm thick that radiate from the center outward. X-ray diffraction (and differential scanning calorimetry [DSC]) is used to discover crystal structure and degree of crystallinity. Polymers can never crystallize 100% because there will always be folds and defects. Electron microscopy can be used to study texture. TEM of thin sections allows us to see a replica of the fracture surface, or etched (preferentially attacks amorphous region) SEM of the fracture surface allows us to see the ribbon structure and twists. The degree of crystallization has an important effect on the physical properties of a polymer. The process is thought to be made up of two stages: 1) nucleation and 2) growth. In the primary nucleation step, a few molecules pack side by side to form a small cylindrical crystalline embryo. How Do Crystals Grow? Growth of the polymer crystals takes place by secondary nucleation on a preexisting surface. The first step involves lying down of a molecular strand on a smooth surface. This is followed by the addition of further segments through a chain-folding process. Chain folding occurs for flexible molecules, and for more rigid molecules extended chain crystals are obtained. Heating to temperatures just below the melting temperature increases the lamellar thickness. The driving force is the reduction in free energy gained by lowering the surface area of a lamellar crystal so it becomes thicker and less wide. This is called annealing, heating to high temperatures in which there is sufficient thermal energy to allow molecular motion to take place.

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Navigating the Materials World What Factors Affect Crystal Growth?

Factors, such as team fitness, their background, previous training, and the presence of difficult team members can influence the team structure. There are many factors that affect the rate and extent of crystallinity of polymers, such as processing variables, presence of orientation in the melt and melt temperature, tacticity, molar mass, chain branching, and the presence of additives.

What Form Do Crystals Take? The crystal structures of several hundred polymers have been determined by X-ray diffraction. The unit cell of any crystal structure can be assigned to one of the seven basic crystal systems (triclinic, monoclinic, etc.). The packing and symmetry determines the space group. Polymer molecules pack into crystals either in the form of zigzags or helices. They are also characterized by the form and shape and number of atoms and turns. Finally, we can state unit cell dimensions and angles between axes, chain repeats per unit cell, and densities. Why Does One Polymer Crystallize but not Another? There are certain structural requirements that are essential before a polymer molecule can crystallize. Polymer chains must be linear (a small number of branches or copolymerization limits but does not suppress crystallization completely). Tacticity affects crystallization.—generally, isotactic and syndiotactic polymers will crystallize, whereas atactic polymers will not. Measurement of crystallinity can be done by differential scanning calorimetry or by X-ray diffraction. Low density PE has about 50 to 60% crystallinity with short and long branches from the main chain. High density PE has about 85 to 90% crystallinity with no long chain branching. Why Are Some Crystals in the Shape of Zigzags and Others in the Form of Helices? Polymers with short repeating units assume a helical conformation in the crystal. Substitution of the H in PE for larger atoms gives interference, and there will be a higher potential energy in the transposition. Polyvinyl chloride can be planar zigzag when in syndiotactic form, but in isotactic form the Cl molecules would be too big and distort the structure.

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Figure 7.10 Helical conformation caused by large substitution atoms.

CRYSTAL DEFECTS What are the defects in any team structure? What causes the team to fall apart or the game to be lost? There are many reasons why teams fail Figure 7.11 Defects in the structure.

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Navigating the Materials World and often the defects are within the individuals, but sometimes it is the interface between the players, or the team leaders themselves. The following defects have been identified in polymers, and although the names are the same as those given in metals, they are alike only in the sense of their general shape or function and not how they are formed. Point defects. Chain ends, folds or copolymer units, molecular kinks Dislocations. Growth spirals (screw) and misalignments (edge)

Processing Route Starting from the polymer chain and finishing with a plastic component involves many stages. Even a small change in heat treatment caused by a different processing route can change the polymer from semi crystalline to amorphous. Figure 7.12

MAKING THINGS WITH THERMOPLASTICS Because thermoplastics can melt, there are several methods of processing that we can use. Polymer manufacturing methods normally involve elevated temperatures and the application of pressure to flow, fill, or conform the polymer to the shape of a mold. Amorphous polymers are processed (around 100°C) above their glass transition temperature, whereas semicrystalline polymers are processed above their melting temperature. Molding is the most common way to process polymers. COMPRESSION AND TRANSFER MOLDING In compression molding, the polymer is placed between a stationary and moving parts of a mold. The mold is closed and heat and pressure are applied so the material becomes plastic and flows to fill the mold. Transfer molding, a variation on compression molding, first melts the polymer in a separate chamber before it is transferred or injected into the heated mold. Transfer molding is often used for thermosetting polymers. INJECTION MOLDING Most thermoplastics products are made by injection molding. Here a polymer charge is preheated in a cylindrical chamber to a temperature at which it will flow. This charge is forced through a nozzle into a relatively

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cold closed-mold cavity by means of high hydraulic pressures where it will solidify. The pressure is maintained until the polymer is solidified. Finally, the mold is opened and the part is ejected. The mold then closes and the whole cycle is repeated. The most outstanding feature of injection molding is the fact that very complex products can be made at high speed (typically 10 to 30 seconds). Examples of products include housing for coffee machines, vacuum cleaners, computers, mobile phones, skishoes, motorcycle helmets, garden furniture, etc. EXTRUSION The extrusion process is a process in which a viscous thermoplastic is transported using a screw through a heated chamber, where it is first melted to form a continuous mass that is than forced through a die shaped to give the final product. Solidification takes place outside the die through air blowers or water baths. A wide variety of shapes can be made by extrusion, including profiles, channels, tubing, pipes, filaments, film, and sheeting. Examples of products include drainage and electrical pipes, water hoses, and window frames.The extrusion of film can be done either by direct-sheet extrusion or by the film-blowing process. In the latter process, a tubular die is used from which a hollow tube is extruded. The tube is extruded vertically towards a film tower. The tube is blown into a thin cylindrical film by air trapped in the film bubble.At the top of the tower the by-now cool bubble is collapsed and wound up to form either hollow tubing (e.g., plastic bags) or slit into flat film (e.g., black agricultural plastic film). BLOW MOLDING The blow-molding process for plastic bottles is similar to that for blowing glass bottles. First a piece of plastic tubing is extruded or injection molded and subsequently placed in a two-piece mold having the desired shape of the container. By means of compressed air, the plastic tube, which is still in the rubbery state, is then forced to conform to the shape of the mold.This technique is widely used for the manufacture of bottles and containers. ROTATIONAL MOLDING In this technique a powdered polymer is placed in a heated closed mold that is being rotated biaxially. As the polymer powder melts, it coats the inner walls of the mold to a uniform thickness. The method is used for producing large hollow parts, such as containers but also kayaks are made using this technique. CALENDARING Calendaring is a process used for the continuous manufacture of (mainly PVC) sheet or film. Granular resin is passed between heated rolls under high pressure, with a gradual reduction in roll separation as the material progresses through the unit. Calendaring is widely used for flooring products.

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FOAMING The production of plastic foams is accomplished by generating a gas in a liquid polymer at elevated temperature. VACUUM FORMING In vacuum forming, a sheet of plastic is heated to the rubbery phase and laid over a hollow cavity mold and a vacuum is drawn on the cavity. Atmospheric pressure forces the sheet to conform to the mold. After cooling of the sheet, the vacuum is released and the formed object removed. Examples of vacuum formed products include coffee cups and containers for dairy products such as butter, yogurt, or cream. MAKING THINGS WITH THERMOSETS Thermosets do not melt, but after mixing the two components harden usually in some sort of mold. They are formed by mixing two parts together to activate network formation (sometimes an accelerator is used also). As they gel or set and cannot be remelted, they are formed by different types of molding. The liquid is poured into the mold and then cured at a particular temperature profile. Often this requires a cure and a postcure to relieve any thermal stresses associated with the curing process.

What can affect the properties of the component? Imagine the sorts of things that can affect the behavior of the team, the way they were made, their individual personalities, prior knowledge, socialization, culture, etc. Water absorption Pressure Orientation Oxidization Specific heat and, therefore, heat needed for processing Melt viscosity: Newtonian or non-Newtonian Crystallization and shrinkage and the temperature profile: rate of application, cooling holding temperature, etc. Polymer relaxation times Sharp melting point

Interaction of Structure and Processing Processing, structure, and behavior are intimately related (e.g., how the team behaves as a result of their background as well as in their relationships). We cannot list all of these here, but we will mention a few so that

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you can start to develop the thinking patterns. It’s all based on common sense. If I change this, what must happen to that? Structural factors and processing variables will influence mechanical properties directly or indirectly via a change in Tm, Tg, or crystallinity.

Figure 7.13 Factors that affect the interaction of processing, stucture, and behavior of a polymer. (A) Material choice; (B) Condition; (C) Processing variables; (D) Orientation

A

B

C

D

What factors affect the interaction of structure/processing and behavior? Of course the original choice of polymer molecules is very important, but it can become much more complex as many factors are interrelated. What factors in the categories below will influence 1) the melting temperature of the polymer (Tm), 2) rate of crystallization, and 3) degree of crystallinity? 1. Processing variables • Temperature of crystallization a) Defects and faults in the structure lower Tm. This depends on the temperature of crystallization. The rate of heating also affects Tm due to annealing just before the Tm. b) Competing increase of diffusion rate with temperature and reducing free energy change will produce a peak between Tg and Tm at which the highest rate will occur. c) If the rate of crystallization is not too high, it is possible to quench to an amorphous state and then reheat to crystallize to any degree desired by changing Tc. Continued

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Navigating the Materials World Orientation of molecules in the melt a) If the polymer chains in the melt have preferential orientation (e.g., through a die in injection molding), this will assist crystallization and polymers will have Tm nearer to the true equilibrium melting point. b) The number of nucleation points increases with alignment of chains and the rate of crystallization increases. c) As above, the degree of crystallinity also increases •

2. Structural variables • Chain structure and tacticity a) If chains are held in a crystalline structure,Tm will be higher. As flexibility is increased Tm is lowered. b) The rate is influenced by chain flexibility, that is, melt viscosity and the degree of alignment needed in the crystal. The greater the ability to rotate chains, the greater the rate. c) Polymers never crystallize to 100% because it is impossible for all chains to get into position. Tacticity is very important. A small drop in tacticity results in a large drop in crystallinity, and the polymer rapidly turns into an amorphous structure. Molecular weight a) Introduction of chain ends into the polymer increases the number of defects, and chains are more able to move. Tm is therefore lowered. b) As the molecular weight is reduced, the melt viscosity reduces and hence the rate of crystallization increases. c) As molecular weight decreases, the degree of crystallinity gradually increases and chains move into position more easily. At a certain point, the molecular weight is so low that the material is no longer a true polymer (grease). •

Chain branching and cross linking a) Branching and cross-linking produce defects in the crystalline lattice, which will decrease Tm. b) Restraints on polymer chain movement affect the velocity of crystal nucleation and growth. c) Cross-linking rapidly lowers the crystallinity. •

Polymer Behavior HOW EASILY WILL THE TEAM BREAK UP AND HOW DOES IT BEHAVE? Polymers can behave as elastic solids, viscous fluids, or as elastomers, or anywhere in between. This is why they are such an important category of material.

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STATIC BEHAVIOR AT ROOM TEMPERATURE How many ways can a polymer fail? How many ways can a team break up? How do arguments start, how are these influenced by the stress of the team, and the resulting strain experienced. How tough is the team to withstand sudden unexpected impact? How resistant is the team to local environments? Above we saw how the structure and processing conditions affected the Tm and crystallinity. But how do the thermal properties and crystallinity in turn affect the mechanical properties? Measured by the use of a tensile testing machine, we see four possible types of failure as shown in Figure 7.14A: necking and failure (e.g., PVC, PC), uniform extension and no neck (e.g., rubber-modified polystyrene), and cold drawing after necking (e.g., PE, PP). Molecules begin to align themselves in the direction of the applied load (strain induced crystallization) and the material is much stiffer (strain hardening). Fibrillation is often seen at breaks—the forces between the molecules in the highly drawn state are insufficient to hold the material together in the transverse direction. PP and most semicrystalline polymers can cold draw. It is often found in polymers with slim molecules that can crystallize. The fourth failure type shows some brittle and some ductile behavior;—it can undergo very large extensions without forming a neck it will behave as an elastomer (e.g., cross-linked rubber) and will return to its original dimensions if the load is removed before reaching the failure stress. If a lightly cross-linked rubber can crystallize during deformation this results in stiffening.Very highly crystallized polymers will have poor mechanical

Figure 7.14 Types of ductile failure. (A) Necking and failure (e.g., PVC, PC); (B) Uniform extension and no neck (e.g., rubber-modified polystyrene); (C) Cold drawing after necking (e.g., PE, PP); (D) Some brittle and some ductile behavior

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Navigating the Materials World properties (toughness) because they have no intercrystalline links, and they will be very brittle and behave like elastic solids. Most polymers can show all types of failure depending on the strain rate and temperature. Therefore, it is critical that we know the Tg and Tm of a polymer in relation to service temperature. If the polymer selected is too close to Tg, disaster could result.Your skis could suddenly flop over while standing them on the sun terrace of your ski resort! The use of polymers is often limited by their low melting temperature or glass transition temperature. Both Tm and Tg are affected by the structure of the polymer chains.

What Structural Factors Might Affect Tm and Tg?

Mainly affected by intermolecular and intramolecular interactions (chain stiffness, side groups). The value of Tm depends on the molar mass and degree of chain branching. Chain ends and branches can be thought of as impurities that depress Tm. An increase in stiffness of the main chain, the presence of polar groups and the type and size of side groups present on the backbone will increase Tg and Tm. Plasticizers (and fillers) lower Tg. Random copolymers do not readily form crystalline polymers.The random nature of the chain is inconsistent with packing chains in a regular ordered lattice. The Tg does not involve fitting into a crystal lattice and, therefore, there is no reason for structural irregularity to interfere with the Tg. However, structural irregularity does interfere with crystallinity, which influences Tm. A good demonstration to try for yourselves is to consider spaghetti. If you cook traditional long spaghetti or short lengths you will find a great difference in their properties.

FIVE REGIONS OF DEFORMATION If we consider the effect of temperature on polymer properties, we can see that linear amorphous polymers (or lightly cross-linked) (e.g., polycarbonate, polystyrene) show five regimes of deformation, of which each modulus has certain characteristics. They are as follows. 1. The glassy regime, with a high modulus (around 3 Gpa) 2. The glass-transition regime (modulus drops steeply from 3 Gpa to a few Mpa)

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Figure 7.15 Glassy

Glass transition Modulus Rubbery viscous decomposition Normalised temperature T Tg

3. The rubbery regime (low modulus, few Mpa) 4. The viscous regime, when the polymer starts to flow 5. The regime of decomposition in which chemical breakdown starts This behavior is due to the fact that polymers possess the concept Viscoelasticity. A viscoelastic material has no unique modulus; it varies with time, temperature, and rate or frequency. Mathematical models may describe viscoelastic behavior, but they cannot necessarily be interpreted in terms of molecular structure. The characteristic features of a viscoelastic solid are as follows. •

•

•

They have a time-dependent strain response to a constant stress: creep They have a time-dependent stress response to a constant strain: relaxation Removal of applied stress leads to recovery over a period of time (i.e., strain goes down as time goes up after removal of load)

Real polymers are actually nonlinear viscoelastic materials, but we often assume linear viscoelasticity, which is not unreasonable at low strains. In a tensile test polymer, behavior is very sensitive to many parameters, including temperature and strain rate. The duration of the test is short, just few minutes. In service we have polymers under load for long times and creep tests are needed to determine long-term properties. A constant load or stress is applied and the strain measured as a function of time. Strain is time dependent from the instant the load is applied. This is called creep. Keep the load on your team and you will eventually see the strain.

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Navigating the Materials World Figure 7.16

The family of creep curves can be replotted as isochronous (constant t and T) or isometric (constant strain and T) curves. STRESS RELAXATION The counterpart of creep is stress relaxation in which the sample is subjected to constant strain and the decay of stress is observed with time. Keep your team constantly stretched and exercised and they will destress. We can define the stress relaxation modulus Er(t) as the value of stress at time t/applied strain. It is worth noting that Er(t) does not equal the inverse of the creep compliance modulus for viscoelastic materials, although it would for elastic materials. The characteristic time from this plot is tx or relaxation time. RECOVERY If we unload the material after a creep test, the polymer will recover but it will take some time. If we take away the stress the team will recover. TIME TEMPERATURE CORRESPONDENCE To obtain a complete log Er(t) of log time curve at one temperature (i.e., a master curve), we would have to run the relaxation test for a very long time. However, we can make use of time temperature equivalence, which is applicable to all linear viscoelastic behavior in polymers. Linear viscoelastic behavior of a polymer is defined if we have two out of three of the following: 1. Master curve at one temperature 2. Modulus temperature curve for one time 3. Shift factors relative to some reference temperature (not necessarily Tg)

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MODELS Viscoelasticity can be modeled by analogical constructs. We use the analogy of a spring to represent an elastic material and a dashpot to represent a viscous material. As with humans, polymers do not actually behave in the way the models predicts, but we get as close as we can so it is easier to determine what might happen under different circumstances. If we did not expect people to sit on chairs in restaurants, we would not provide seats. However, occasionally someone might sit on the table, and this would not go unnoticed. The Maxwell Model The Maxwell model is shown in Figure 7.17. It predicts the behavior of real polymers for relaxation but not creep and recovery. Kelvin, or Voigt, Model The Kelvin, or Voigt, model (Figure 7.18) gives an acceptable first approximation to creep and recovery behavior but does not account for relaxation. Some compromise may be achieved by combining the two models. Boltzmann Superposition Principle In service, materials are often subjected to complex stress histories, and obviously it is not practical to obtain experimental data that refers to all combinations of loading. We can predict the response of a linear viscoelastic material subjected to a particular loading schedule. The Boltzmann superposition principle proposes that for a linear viscoelastic material, the strain response to a complex loading history is simply the algebraic sum of the strains due to each step in load. Figure 7.17

spring

dashpot

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Navigating the Materials World Figure 7.18

spring

dashpot

Polymer Degradation How does your team degrade on exposure to local environments and how best to stop it? What are the external factors that break bonds between the players or eat away at an individual? These could be aging, drugs, alcohol, lack of good sleep or food, arguments at home, broken families, childhood traumas, etc. All of these will gradually disrupt the order and erode the effectiveness of the players and the team unless stabilized in some way. Figure 7.19 Molecular degradation.

Thermal degradation Oxidative degradation

Degradation by hydration

Swelling and dissolution

Chemical degradation

Biodegradation

Photodegradation

Mechanical degradation

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So what would affect your choice of polymer for any application? What would make you choose a polymer in the first place? How does the life cycle assessment affect your choice? How does the processing route affect your choice? How do all of the above affect the properties of the component? Does your team need reinforcement?

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CHAPTER 8

Back to Nature Adrian Lowe


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You’ve Earned a Holiday! You are now entering the Natural World. Perhaps the simplest definition of a natural material is any material occurring that is not artificially synthesized (i.e., it is created by nature). Natural materials in the form of polymers, fibers, or composites are by their very definition some of the oldest materials there are. In theory, any naturally occurring material could be dealt with in this chapter, but the chapter would have to be thousands of pages long. What you will read about in this chapter deals with materials that are studied in the context of modern technological materials advancement. A simplistic natural materials classification system illustrating this is shown in Figure 8.1. So, a natural material is something that nature uses to build things?

A prime example of this is wood. Nature uses this material as tree trunks to provide support for large plants (i.e., trees), whereas humans have modified these tree trunks into simple geometric shapes (e.g., planks, etc.) that are then used in the design and construction of boats and buildings and numerous other products. However, there is much more to wood than this, as you will discover. Another mechanism for simplifying the field of study would be on the basis of related scientific discipline. Polymers, fibers, and composites are traditionally studied using materials science, whereas rocks are the realm of geologists, and biological materials tend to involve biologists and medical fields. However, it is a reflection on current progress that these boundaries are becoming ever more vague, and from an engineering perFigure 8.1 Natural materials classification diagram highlighting key categories and selected examples. Biomaterials Cells

Proteins

Biological DNA Fossil fuels

NATURAL MATERIALS

Proteinbased

Lignin

Rocks

Soil

Silk

Wool

Natural Polymers Starch

Gasses

Geological

Hair

Traditional

Genes

Pectin Cellulosics Hemicellulose

Cellulose

Cotton

Banana

Cellulose fibers Sisal

Hemp

Coir Wood Flax Jute

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spective, virtually transparent. Even within the boundaries of polymers, fibers, and composites, there are far too many candidate materials to be introduced in this brief chapter. We will, therefore, concentrate on those materials that are widely accepted as being the most useful outside of their natural functions and that are currently of scientific interest.

What types of natural materials are there? Typically, naturally occurring materials can be grouped into three categories: biological, geological, and traditional.

Biological Natural Materials Although this class of materials is of great scientific interest, it is really outside the scope of general materials science.There are areas where some overlap does occur, most notably in the areas of biomaterials and biomimetics. Biomaterials essentially involve the study of materials that can be used within the human body to initiate improvements in some condition. These materials can be existing bodily materials or materials specially created. Examples include artificial skin and heart valves, replacement tendons, and hip replacements. The science of biomimetics involves creating artificial structures that mimic biological equivalents. For example, some nut shell materials are extremely tough and strong, as are certain bone structures, and one of the major biomimetic research thrusts at the present time involves creating artificial carbon fiber-based structures that possess their structural complexity and hopefully their structural response. Other biological natural materials of interest include deoxyribonucleic acid (DNA) and related chemicals, cells, and cell membranes and blood. These materials, although important, are well outside the scope of this chapter.

Geological Natural Materials This category is probably inappropriately named because it includes atmospheric gases and crude oil in addition to rocks, soils, and solid fossil fuels. Although the staple knowledge base for geologists and the occasional climatologist, these materials are not of much interest to materials

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Navigating the Materials World scientists and engineers. However, when crude oil is converted to various organic chemicals, the polymer scientists start to take over. This category can be loosely expanded to include traditional engineering materials, such as metals and ceramics, because these tend to be found within rocks.

Traditional Natural Materials Traditional natural materials is merely a convenient umbrella name to describe pretty much what is left. Such materials are those that exist naturally and usually in abundance that can be extracted from nature and processed for use in other environments. These materials are either naturally occurring polymers within the plant kingdom or protein-based materials that occur within the animal kingdom. It would be impossible to provide a concise description of all possible candidate materials, and so subsequent description will be restricted to those materials that are currently receiving the most scientific interest.

Are plant and animal materials really that different? Absolutely. Plant fibers tend to be based on cellulose and other polysaccharides, whereas animal fiber (typically hair-type materials) tends to be protein based (e.g., keratin, which is found in hair and bone).

Protein-Based Traditional Natural Materials In keeping with the general ideas of this chapter, this section will restrict itself to very well-known fibrous materials that have found a niche in society for countless years. WOOL Wool is the most common and best known of the animal fiber family. Wool may be differentiated from human and other animal hair mainly by the nature of the scales that cover the outer surface of each fiber. Wool scales are numerous, tiny, and pointed and are attached only at their bases, thus allowing the fibers to interlock under pressure, leading to the production of felt. The number of scales varies with the fineness and curliness of the fiber. Importantly, wool can absorb up to 30% of its weight in moisture without giving the feeling of dampness. This ability to absorb water means that wool fiber is very easy to dye effectively.

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Now I know why when I spill drinks on my woolen carpet, they disappear rather than float.

The physical and mechanical properties of wool are entirely dependent on their chemical composition, and this varies significantly from animal to animal. For instance, wool from the alpaca or llama is extremely soft, whereas wool from the Angora goat (mohair) is quite harsh. The flexibility exhibited by wool allows it to be successfully processed by knitting techniques and other processes that require a high degree of fiber bending without any loss of structural integrity. Of course, sheep are by far the biggest source of wool, and significant variations in quality and feel are possible from various breeds. That’s nice, but is wool used for anything technical?

Other than clothing, one highly technical use for wool and other animaltype fibers is in the production of boron carbide fibers. Mixing fibers with boric acid under controlled conditions yields this highly temperature resistant and inert ceramic fiber. It also is used occasionally as a fiber reinforcement in thermoplastic matrix composites, although there are bonding problems and little high temperature strength. Figure 8.2 Alpaca—a rather cute wool production unit.

SILK Silk is a natural protein fiber and is obtained from the cocoon of the silkworm. Silkworm larvae secrete a clear, viscous fluid through specially modified glands and through a mouth shaped like a spinneret. This fluid hardens on contact with air to form strands of silk. Perhaps the most important factor relating to silk is that it is just about the strongest natural fiber there is, and it is in fact stronger than steel. It is also highly hydrophilic. After harvest, the threads from several cocoons are subsequently unwound together to form a single strand of raw silk. This fine thread is the basic component of all silk yarn and fabric. Some of the gum, (sericin) which the silkworm uses to hold the cocoon

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Navigating the Materials World together, remains to assist the delicate fiber during processing. It is subsequently washed away, although some remains to form a natural fiber sizing. So if we could make structures out of silk, they would be stronger than existing metal ones.

Indeed! There is a large quantity of research being undertaken globally to see if this is possible, with work also looking at alternatives, such as nutshells and seashells. Such technology falls under the umbrella of Biomimetics, the science of mimicking nature’s structures for engineering applications. Even stronger is spider silk, although the strands are so thin, this strength is not very apparent to you or me as we walk through a spider’s web. However, the base protein is receiving a great deal of interest from engineers and genetic scientists who are splicing spider genes with animal eggs (typically goats) to produce animals whose milk contains the spider silk protein. Spider silk has been measured as having a potential strength in excess of 2 GPa and is widely regarded as having about 5 times the strength of steel while having 30 times more flexibility than nylon. It is also more waterproof than silk fiber. The main protein in silk is glycine, which is able to form flat extended chains that can pack together very tightly. It is glycine that is responsible for giving silk fabric its sheen, strength, and feel. So, we could all soon be driving in cars made from spider silk?

Possible but unlikely. Producing sheet material from thin fibers is still a very problematic idea.

Figure 8.3 Typical silkworm larvae and cocoon.

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Plant-Based Traditional Natural Materials NATURAL POLYMERS Unlike synthetic polymers, humans cannot take the credit for the creation of natural polymers, and unlike natural polymers, humans have not had countless millions of years in which to perfect their structures and properties. Our somewhat narrow outlook renders the concept of a naturally occurring polymer a difficult one.This is because we associate polymers with plastics and the numerous brightly colored products made from them. However, nature has a very different use for polymers, being the very stuff of which plants are predominantly made. Scientists have even given them a different name, and they are often referred to as macromolecules.

Can you think of any natural polymers? The best-known natural polymers are natural rubber (from trees) and starch (from many things, including potatoes), as well as lignin (plant fibers) and pectin (plant fibers and fruits). Proteins are natural polymers too.

NATURAL RUBBER Natural rubber is based around a simple repeat unit called polyisoprene —(C5H8)—n. Society has been exploiting this polymer (obtained initially from the sap of the hevea tree but now known to exist in other species) for nearly a thousand years. In its raw form, polyisoprene is a sticky liquid (called latex) with little structural integrity, and so it is cross-linked with sulfur (vulcanization) to give it structure and also maximize its highly elastomeric properties. One of its earliest uses was with the Aztecs who used rubber to make bouncing balls. Polyisoprene is one of the rare examples of a natural polymer that can be artificially synthesized as an almost perfect replicate. After the onset of vulcanization as a process, rubber became extremely useful as an engineering material.Vulcanized rubber is impermeable to gases and relatively inert to chemical attack, abrasion, electricity, and low-level heat, as well as having highly unique frictional characteristics. The processing of rubber involves the tapping of the latex by partially cutting through the tree and collecting the latex in a cup at the base of the tree. Using this method, about 50 g of latex is obtained and the run can be repeated every 2 days. The rubber particles are then

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Navigating the Materials World removed from the latex suspension using dilute formic acid, forming rubber crumbs on the surface of the mixture, and these crumbs are subsequently washed and dried and packaged.

Can you think of uses for rubber other than for erasers? By far and away the biggest use of natural rubber is in vehicle tires, although synthetic rubber is making inroads into this market. However, natural rubber has superior heat transfer properties. Natural rubber is also used for conveyor belts, hoses, condoms, rubber bands, and sporting equipment. However, one of the biggest drawbacks is the inability to break down used car tires into biodegradable or recyclable constituents.

STARCH Starch is a high molecular weight water-soluble polysaccharide and is best known as a major constituent of hydrocarbon foods, such as potatoes and bread. It is a byproduct of the agricultural industry and is produced in many millions of tons per year. It has certain chemical similarities with cellulose (they are both polysaccharides for instance), but it has a spherulitic crystalline form, rather than a fibrous crystalline structure, and hence occurs as particles and not as fibers. Starch occurs in plant cells as a fine granular material around 0.5 mm in size. It serves no structural function, and merely acts as a food reserve for the host plant. More than 50% of the starch extracted from plants is converted into dextrose and syrups. Starch is processed from starch-rich plant cells (corn, potato, tapioca, etc.) by grinding them such that the starch granules pass into an aqueous solution and eventually separate out.The mixture is then filtered, centrifuged, and dried to give a powder. Unlike many other commercial extractives, starch retains certain structural and compositional anomalies that can be used to identify the original plant source. Structurally, starch consists of two basic repeat units: amylopectin and amylose. Amylopectin is insoluble in water, but it will swell. It is also a very branched structure. Typically, amylopectin constitutes around 80% of the entire starch molecule. Amylose, by comparison, is very linear in structure although it was the first natural polymer discovered that composed of helical chains.When isolated, amylose is used to produce maltodextrose, a common food additive. Apart from food, starch is used as a fabric stiffener and also as biodegradable food packaging. Because it is soluble in water, starch is used as a thickener and binding agent in industries, such as papermaking and as a food additive. It is the amylose content in starch that has been suc-

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cessfully used in the United States to produce starch films and tubes, although synthetic plastics have now largely superseded this. In a gelatinized form (i.e., a very thick paste), starch can be modified with the addition of simple copolymers and ammonia to form a substance that will readily bond with simple thermoplastics to form useful injection moldable composites. LIGNIN Lignin is a complex amorphous hydrocarbon based around phenyl propane that contains both aliphatic and aromatic components. In its natural form, lignin is the matrix material that permeates the network of cellulose fibers and is also part of the sheathing material that holds the cells together, giving the cellular structure rigidity and compressive strength. It is also present to waterproof the cells and to protect against enzyme degradation. Lignin is often a waste product from fiberproduction techniques, and even in this waste form, several uses have been found for this material. It can be used as an additive to concrete, as a fuel oil, as a fertilizer, and as a reinforcing agent for rubber-based materials. It has even been used as a filler in polypropylene composites, although such technologies are still in their infancy. Despite a great deal of study, the structure of lignin is not yet fully understood. Because lignins break down under ultraviolet light, the weather resistance of lignin products (e.g., wood products) can be improved by chemically modifying the lignin with photostabilizers such as alkyl phenols. PECTIN As all jam makers know, pectin is a sugar-based polymer. In plants, it is used as a glue that works in tandem with the lignin to give the plant structure a degree of structural integrity. During most fiber-removal processes, lignin and pectin are the components that are removed to leave a predominantly cellulose-based fibrous structure. Fine, but do any of these materials exist as fibers, such as carbon or glass?

Any plant with any sort of structural integrity essentially comprises fibrous regions that have great strength in a specific direction. They allow the plant to stand tall and defy gravity instead of flopping down in a heap on the ground. Trees are very heavy objects, as witnessed by the damage they do to cars and roofs when they fall over in a storm, and it is only through fibers located in the tree trunk that they are able to remain rigid. Cellulosics is the generic name often given to plant fibers and specifically relates to the chemicals and polymers that make up the structure of the fiber and surrounding material. However, it is an overused term and often

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Navigating the Materials World comes to mean different things to different people. This section will confine itself to talking about the remaining major plant fiber constituents: cellulose and hemicellulose. These polymers are rarely seen isolated in their natural state and are invariably extracted from source materials and subsequently processed into other materials. Another term that is often used is “lignocellulosic materials,” and this specifically relates to cellulose microfibrils embedded in a matrix consisting of lignin and hemicellulose.

CELLULOSE Cellulose is in fact a member of the polysaccharide family like starch, but is far more abundant within the plant kingdom and due to its crystalline nature and strong hydrogen bonding, is insoluble in virtually everything. It is essentially a polymerized form of glucose, which makes its insolubility even more puzzling. In this form, it is also indigestible, so you can’t get your sugar fix by chewing on wood! A typical cellulose structure consists of 200 to 6000 glucose repeat units linked by acetal bridges (Figure 8.4). The glucose units are covalently bonded both within and between units, thus rendering cellulose strong and stiff and therefore a good candidate for a fibrous reinforcement material. Note that the –CH2OH side groups alternate between positions two and six on the ring structures. The isotactic variant of this (i.e., all occurring at position two) is the base structure of another important polysaccharide already discussed—starch. The crystalline nature of cellulose comes from the fact that linearly oriented clusters of molecules are extensively bonded together in ordered structures through hydrogen and van der Waals bonding. The chemical structure of cellulose shows a predomination of OH groups.These groups allow secondary bonding to occur, but they are also very hydrophilic and thus readily attract water. So that’s why wood swells when it gets wet!

Once it has been logged, wood is dried prior to use because wood is too soft to be used structurally in its natural form. However, the drying process results in shrinkage, so that in furniture and other manufactured prod-

Figure 8.4 The structure of the glucose-based cellulose repeat unit. OH CH2

H OH

H O

OH

H

H

O

OH

H

H OH OH O

H

OH

O

CH2 OH n

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ucts, if water contacts the bare wood surface, highly localized swelling, or rehydration, can occur, causing a blister or stain. That’s why it’s always a good idea to coat or chemically treat anything made from wood. Cellulose is extracted from wood and other plant fibers by means of the viscose process, which can produce regenerated cellulose fibers (e.g., rayon) or cellulose films (e.g., cellophane). Unfortunately, it is a very energy-intensive process and involves the mixing of cellulose wood/plant pulp with concentrated alkali (usually sodium hydroxide).This mixing will dissolve the cellulose by breaking it down into smaller molecular units. The mixture is then reacted with carbon disulphide to form cellulose xanthate, which can be siphoned off. This solution is then treated with sulfuric acid and sodium hydrogen sulfate to convert the xanthate into cellulose fibers. These fibers are then dried and spun to a greater length and are generally less crystalline than their corresponding raw material, and like naturally occurring cellulose fibers, they will also absorb significant amounts of water and other polar solvents causing significant swelling. Rayon was originally used as the precursor material for carbon fibers and with the escalating cost of synthetic polymers, production of carbon fiber from cellulose is receiving increased attention. Does cellulose have any uses other than in wood?

Well, carbon fiber production has been mentioned, but cellulose can be processed into several well-known chemical compounds. Cellulose Nitrate Originally formed by reacting cotton cellulose with nitric acid and sulfuric acid, cellulose nitrate is renowned for its explosive properties and hence was used as an alternative to gunpowder. It has also been used as an alternative to ivory in products such as snooker balls, but perhaps its most interesting use was in early cinema film and sound tapes. Over the years, these tapes have become increasingly unstable and as a result, started exploding while being stored in cans in archives around the world. There is now a race on to try and stabilize the remaining tapes before they too explode, and the result of this is that large quantities of film and sound footage from the 1940s and earlier have been lost. Interestingly, cellulose nitrate was the first commercial artificial fiber (Chardonnet silk).The prohibitive reactivity of cellulose nitrate can be somewhat reduced by reducing the amount of nitrogen present during the reaction. Low nitrogen formulations are often used as lacquers. Cellulose Acetate Formed by the reaction of cellulose with acetic acid, cellulose acetate is the thermoplastic medium used to supercede cellulose nitrate in the film and sound industries. Due to its possessing less crystallinity, cellulose

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Navigating the Materials World Figure 8.5 The structure of cellulose nitrate. O

NO2

CH2

OH

H O H

H H

O

OH

H

H

OH

H

H OH OH O

O

CH2 OH

Figure 8.6 The structure of cellulose acetate. O

CH3

O

CH2 O

H OH

H

H

O

O

H

O O H

H

H

H

O

CH3 O

OH OH O

O

CH2

O

O

CH3 O

O H3C

O

acetate is more soluble than cellulose and can exist in several states, depending on how long the material has been hydrolyzed. Although far more stable than its predecessor, it has now been largely replaced by polyester-based materials. Perhaps the best-known use of cellulose acetate is as a fibrous material for clothing (i.e., acetate fibers). Cellulose acetate fibers are produced by dry spinning. The polymer is dissolved in acetone, filtered, and then pumped through a spinneret.The solvent instantly evaporates, giving continuous lengths of fibers. HEMICELLULOSE Hemicellulose is also a polysaccharide, but unlike the highly linear cellulose molecule, hemicellulose is highly branched and thus has restricted crystallinity. In its natural environment, it is found with lignin as a matrix material and is often left attached to the cellulose fiber after the lignin has been removed during fiber processing. It differs from cellulose in that it is composed of numerous different sugar units, rather than just glucose, thus adding to the complexity of the molecule. In the papermaking industry, hemicelluloses are responsible for providing fiber-to-fiber bonding. So, wood is a simple composite material consisting of a matrix and a fibrous reinforcement.

Simplistic but essentially true. Early attempts at the structural modeling of wood used a simple fiber array model consisting of cellulose fibers

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embedded in a hemicellulose matrix. However, living materials are by their very nature extremely complex (other chemicals, interfacial bonding, etc.), so they are certainly not simple, but wood is definitely an example of a true composite material.

Cellulose Fibers It is only recently that the study of cellulosic fibers has been taken out of the sphere of nature and into the sphere of science and engineering. In addition to traditional uses for this family of materials (e.g., fabrics), advances in their use as composite reinforcements are now frequent. Although the most abundant cellulosic fiber is wood, there are many others such as sisal, hemp, jute, and flax. Table 8.1 shows that wood is clearly dominant, although the balance will somewhat change as wood as a resource becomes scarcer and environmental concerns come to the forefront. So, what are cellulose fibers made of?

All cellulose fibers are effectively composites in their own right because they consist of several natural polymers and other additives. Again, “cellulose fibers” is one of those generic terms that can be open to interpretation, but in this instance describes any naturally occurring fiber with a predominant component of cellulose, and it is cellulose that is certainly the secret behind the usefulness of these materials. Cellulosic fibers tend to comprise a degree of crystalline cellulose that has been stabilized by hydrogen bonding to yield a potential stiffness of 145 GPa or higher. However, stiffness values in excess of 80 GPa are not usually attained in practice. In addition to cellulose, plant fibers contain hemicellulose, lignin, pectin, waxes, and other chemicals.The relative composition of these poly-

Table 8.1. Selected Natural Fiber Sources and Their Annual World Production.

Source plant Wood Cotton Jute Flax Sisal Hemp Coir

World production (103 tons)

Origin

1,750,000 18,450 2,300 830 378 214 100

Stem Fruit Stem Stem Leaf Stem Fruit

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Navigating the Materials World mers and the nature of the other additions (typically the alcohol-based waxes) give cellulose-based plants the subtle differences that define one species from another. It should be pointed out that cellulose also can be fabricated artificially and, with the exception of some structural differences, is remarkably similar to the naturally occurring equivalent. Cellulosic fibers tend to be anisotropic because they are required only to give strength and support along the plant or leaf stem. Typically, cellulose fibers consist of hollow fibrils that run lengthways along the source plant, with each fibril consisting of several layers. Each layer has differing amounts of cellulose and associated polymer and is of varying thickness. It should not be overlooked that nature has had millions of years to perfect these fibers structurally to optimize their performance in their required environments. We have only been dabbling for a few decades, and so it is not unreasonable to assume that there is still a lot we don’t understand about these materials. One major problem that humans have yet to fully overcome revolves around the damage accrued by the fibers during removal and subsequent processing. The theoretical strength of many of these fibers (and other natural materials such as silk) is highly competitive, opening up numerous possibilities if these strengths can one day be attained. There is significant research effort around the world that is focused on trying to optimize processing techniques with this in mind. Can we make composites from natural fibers?

Certainly it is true to say that the majority of research into natural fibers is in the area of composite materials. In general, it has been found that the performance of natural fiber composites is improved if they are used as hybrid reinforcements with (most typically) glass fibers, and typically, glass additions of up to 25% have been found to be optimal in terms of properties and cost. More and more, cellulose-based fibers are being used in the composites industry as replacements for glass fibers because they are cheaper, sometimes stronger, and easier to use. In the era of environmental awareness, such materials are receiving enormous attention because of their biodegradability, nontoxicity, low cost, and combustibility. However, they do suffer from excessive moisture absorption, poor wetting, and poor interfacial bonds because of their hydrophilic nature and the presence of waxy surface layers.

WOOD Wood became useful to humans as soon as they discovered fire, and ever since, it has been the most abundant of all engineering materials. It

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is virtually impossible to spend a 24-hour period on this planet and not come into contact with a wood product. Its uses range from simple decorative ornaments up to large buildings and anything in between. Wood is, of course, also the main constituent of the paper you are reading this on. On a simple level, wood is often classed as a composite consisting of cellulose fibers embedded within a lignin matrix, and this is true to some extent for other plant fibers.The structure is, of course, much more complicated than that and really must be introduced on three levels. Essentially, there are only two types of wood: hardwood that comes from deciduous trees (those that shed their leaves annually) and softwood that comes from conifers (cone-bearing trees with needle-like leaves). Wood grows as the tree converts carbon dioxide and water into the molecules that constitute cellulose, hemicellulose, and lignin.This growth takes place typically in the cambium region, between the bark and the outermost sapwood. Because growth is radial, processed wood in the form of planks is highly anisotropic, and because wood shrinks when it dries (raw, new wet wood is structurally very weak), significant warping can occur.Therefore, wood is cut according to certain cutting patterns.

Wood Structure The structure of wood will be dealt with comprehensively here for two reasons: first, it is the most common natural engineering material and, therefore, deserves a bit of space. Second, many of the structures shown here are very similar to those seen in other cellulosic fibers, and so they can be treated as a reference model to some degree for fibrous materials introduced later. Structurally, is wood complex or quite simple?

Figure 8.7 Showing four types of cutting pattern used in the harvesting of wood.

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Navigating the Materials World Wood is extremely complex and structurally needs to be discussed on three levels in order to fully understand its behavior. Structure Level 1: Molecular Structure—Figure 8.8 shows that wood is very complex on a microlevel. These fibers are called tracheids, and it is these tracheids that give wood its engineering properties. However, the walls of these tracheids are themselves complex structures and consist of five aspects. Primary wall. This structure is relatively very thin and consists of cellulose microfibrils randomly arranged within a matrix material of lignin and some hemicellulose. Outer wall. The outer wall is made up of an essentially regular flat helical pattern of cellulose microfibrils embedded within a ligninbased matrix. Middle wall. This structure is the thickest layer in the cell and is mainly a lignin matrix with small amounts of hemicellulose. Here the microfibrils are arranged in a steep helix that almost renders them axially aligned. Inner wall. The inner wall is structurally the same as the outer wall. Lumen. The lumen is merely a conduit for water and other essentials to pass through. In addition, there is a cell coating known as the middle lamella. This region is predominantly lignin and pectin and assists in binding the cells together. But what is actually in wood?

Figure 8.8 Wood microstructure showing the different components in a cell wall.

microfibril outer wall primary wall

lumen inner wall middle wall middle lamella

9¥9 cell matrix

50 microns

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Compositionally, there is on average 45% cellulose fibers within a tracheid structure, and within the matrix regions there is around 40% lignin, 40% hemicellulose, 20% water, and 10% other chemicals. Surprisingly, the composition and structure of the cell walls does not vary significantly from tree species to tree species; the differences are due to the other chemicals that sit within the matrix regions. Structure Level 2: Microstructure—Typically, the tracheids are around 2 to 4 mm long and in the order of 40 mm wide (Figure 8.9). The cell cross-sectional shape ranges from square to hexagonal and various iterations in between. Axially, as the tracheids vary significantly in length, welldefined layers every few millimeters do not occur. Instead, the tapered ends of the tracheid allow them to slot in between other tracheids to create an almost continuous fiber structure all along the tree trunk. The tracheids are able to act as axial conduits because the fiber ends are partially open due to discontinuities in the secondary wall structure and are only bonded to fibers above or below them by their primary walls. On a microlevel, it is possible to see small numbers of transversely oriented ray cells whose primary function is to store food. These are clearly seen on the image overleaf (labeled “r”), which also illustrates the stark differences in cell structure that occurs during the growth season. Early on (i.e., spring/summer), the earlywood cells form (labeled “ew”) into a low density, large-celled structure, whereas the growth that occurs towards the end of the year is much slower and hence the cells are smaller and more densely packed. This is the latewood and is labeled “lw” in Figure 8.10.These regions are darker in color than the earlywood and are responsible for the characteristic growth ring pattern seen in wood structures. The region labeled “rc” is a resin canal and is used to transport fluids around the tree.

Figure 8.9 Showing the tracheid-based structure of wood as well as transversely oriented ray cells. Ray

Cell

2–4 mm

20–40 mm

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Navigating the Materials World Figure 8.10 Scanning electron micrograph revealing the microstructure of a typical softwood pine tree. The transition between earlywood (ew) and latewood (lw) is clearly seen, as are resin canals (rc) and transverse cells (r). The tracheids observed running vertically are around 2 mm long. Reproduced from Colling and Vasilos, Industrial Materials Part 2: Polymers, Ceramics and Composites, City: Prentice Hall, 1995.

lw rc ew

r

Structure Level 3: Macrostructure—Figure 8.11 shows a cross-section from the trunk of a softwood tree and details the macrofeatures present in wood structure. The outer bark (“ob”), which consists of dead cells, sits as a layer around the live bark (“ib”), which in turn surrounds the thin cambium (“cz”) region. This region is only one cell thick, and it is repeated subdivision of the cambium cells that forms the bark and the wood. Interior to this is the wood (“X”), which is alternatively known as xylem. This region is characterized by the growth rings (“gi”), which signify alternating layers of earlywood and latewood. The thickness of these rings and the relative size of the earlywood versus the latewood rings varies significantly with age. Additionally, wood may be categorized as either sapwood or heartwood in which sapwood exists at the outer regions of the trunk and is generally around 30% of the total trunk thickness. The sapwood is responsible for moisture conduction up from the roots and tends to be rich in food stores. The heartwood is composed of essentially dead cells and is harder and stronger than the sapwood and more resistant to attack. The region labeled “p” is the pith region, which is extremely soft and signifies the earliest growth from seed.

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Figure 8.11 Cross-section from the trunk of a softwood tree. Reproduced from Colling and Vasilos, Industrial Materials Part 2: Polymers, Ceramics and Composites, City: Prentice Hall, 1995.

ob

ib gi

cz

p

Differences Between Softwood and Hardwood Firstly, it is important to realize that the terms softwood and hardwood do not refer to the physical hardness of the wood. Softwoods are trees that have needle-like leaves and produce cones, such as Douglas Fir and Radiata Pine, whereas hardwoods have broad leaves, such as sycamore, oak, and ash.The cell structure of hardwoods is more complex that softwood structure, as in addition to tracheids and transversely oriented cells, they contain extra vessel cells as well as extra fibers that are still cellulose based but are longer and thinner than normal tracheids. Chemically, softwood generally has more lignin present in the matrix material, and it is chemically different to hardwood lignin and also considerably less hemicellulose. So, how strong is wood and does it matter what type of wood is used?

Due to the fibrillar nature of the tracheids, wood is very anisotropic. For instance, tensile strengths of more than 6 MPa can be obtained in the fiber direction, compared with 2.5 MPa transverse to this. Also, the cutting pattern plays a part in this. Prior to use, the timber must be seasoned to reduce its moisture content and harden the material. Indoor wood typically has a moisture content of 14%, whereas outdoor timber has around 22% moisture content. Also, chemicals, such as preservatives, must be used to protect the timber from rotting and other forms of degradation. Flame retarders can be added as well to give the wood some degree of fire resistance. As already mentioned, wood will shrink as it dries out, and this shrinkage is often twice as much in the tangential direction than in the radial direction, again highlighting the importance of getting a

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Navigating the Materials World correct cut. Another factor to consider is the occurrence and number of knots.

Do you know what knots are? How might they affect the properties of wood?

Knots occur in the structure of wood because of the presence of branches. Branches will start to form early in many trees’ lives, and as the trunk thickens with age, the early branch growth will get swallowed up accordingly.This results in a completely different structure in these regions because the branch cells will be aligned radially within the trunk, as opposed to axially like the original tracheids. These regions are known as intergrown knots and are different to encased knots. These are knots that initially form in the same manner, but here, the branch falls off after a period of time (common in many tree species because growth is concentrated on the upper canopy).The knot is then encased by normal axial cellular growth and becomes swallowed up by the trunk. Knots in general will adversely affect the properties of the surrounding wood because of local distortion of the tracheids, which have to pass around the knot, rather than through it. Also, because of the completely different orientation, knots often fall out of the wood, or timber, once it has been dried and shrunk, or conversely become larger with respect to the surrounding wood and induce large local residual stresses. The negative influence that knots can have on the strength of a timber beam can be demonstrated easily, as shown in Figure 8.12. The beam has been loaded in bending and has prematurely failed by cracking between the knots and the base structure. The crack path also roughly follows the distortion of the wood grain pattern caused by the presence of these knots. So when buying wood for structural building, try to avoid knots. These days, it is mostly softwood (and in particular, pine) that is used to provide wood for structural purposes. This is mainly because of its abundance, speed of growth, the straightness of the trunks, and the lack of branches on the lower regions of the tree. Because wood possesses different mechanical properties in the three major loading directions (axial, tangential, and radial), it is known as an orthotropic material, and the even-

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Figure 8.12 Effect of knots on the failure of a timber beam. Note the similarity between the crack path and the locally distorted grain structure. From Young, et al. The Science and Technology of Civil Engineering Materials, City: Prentice Hall, 1998.

tual performance of any structure made from wood will need to have been carefully designed, and the choice of bulk material (planks, dowels, etc.) will need to reflect this. For example, for a typical piece of ash (a common hardwood tree species), the Young’s modulus in these three directions are 15.8 GPa (axial), 1.51 GPa (radial), and 0.8 GPa (tangential). These differences are striking and are reflected in softwood species as well and also in other properties such as strength. Mechanically, there are strong differences between earlywood and latewood, and if for whatever reason the growth patterns have at some stage caused strong seasonal variations, it is likely that the tree cannot be used for structural wood products. This is another reason why certain pine species are favored as structural timber sources because the rejection rates due to problems such as this are minimal. Wood Composites An unusual title, you may think, because wood is clearly a composite in its own right. However, wood as a structural unit is used in several other structural products. Perhaps the best known is plywood, which consists of thin wood sheets called veneers, bonded to each other at 90 degrees and is thus analogous to a traditional cross-ply composite. These veneers are peeled from a green log then dried and coated according to their final use and bonded together using special adhesives. By their nature, plywood structures are relatively thin, because the thicker the structure, the less the obtained property benefits when compared with bulk wood, and thehigher the costs. Although the purists often frown on plywoods as not being real wood, there are several advantages to using plywoods as structural or cladding materials because the majority of the anisotropy present in bulk wood has been designed out to give a more predictable material. A second well-known material is particleboard. This material consists of small wood particles in the form of chips, flakes, or sawdust that are

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Navigating the Materials World compressed (under temperature and pressure) with a suitable resin into sheets ranging from 1 millimeter thick up to 50 millimeters thick and beyond. If there is sufficient consolidation, these boards are both strong and isotropic and are generally very easy to machine into other shapes. They are also quite popular because they use predominantly wood waste products from other processing types, and this keeps the material costs down. Unfortunately, the materials are not water resistant and will swell more severely than bulk wood (unless coated), and many of the resins used to bind the wood particles together are toxic. Typical examples of these materials include Formica and MDF (Medium Density Fiberboard).

Why shouldn’t you get your MDF or chipboard furniture wet? Swelling will occur within minutes and is irreversible as resin dissolution occurs. Also, the resin binders used in MDF are quite toxic, so don’t lick it afterwards

Apart from wood, how many other cellulose fiber types are there?

Too many to mention. It is probably the same number as there are species of plants around the globe. However, only a very small number are being used as engineering materials, although this will no doubt change in the years to come.

Figure 8.13 Schematics showing the structure of plywood (a) and particleboard (b).

(a)

(b)

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LEAF FIBERS: SISAL Sisal is an extremely abundant fiber and is obtained from the leaves of the Agave plant (Figure 8.14), which is a succulent that grows in many parts of the tropics and subtropics, especially in Brazil and Tanzania. Like all natural fibers, sisal fibers are strong, lightweight, safe, and nonabrasive. They are also inexpensive, which is imperative if they are to be viable in the areas where they are grown.The plant consists of radially spaced waxy leaves up to 2 meters long, and once the plant has reached a certain age, a central flowering stem appears, after which the plant dies. The leaves need to be harvested once the plant has reached maturity, but before it significantly ages. So the whole leaf is strong enough to be used mechanically!

No. Invariably with plant leaves, there are too many other fluids and chemicals to allow the leaves as such to be used. At best, the leaves need to be totally dehydrated, but more commonly, an extraction process is used to remove the fibrous strong material from the other chemicals and water-based matrix. With sisal, although there are three types of fibers that can be extracted from the leaves, only the fibers known as mechanical fibers are mechanically strong enough to survive the extraction process. These fibers occur close to the surface of the leaf and run the whole length and provide structural rigidity. Ribbon fibers are weaker versions and occur in the center of the leaf. They provide little mechanical strength and taper to a point at the extremity of the leaf to give a sharp, waxy tip. The xylem fibers are little more than a loose collection of thin-walled cells and are extremely fragile.

Figure 8.14 Typical Agave plant.

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Navigating the Materials World Figure 8.15 Schematic of an Agave plane and a sisal leaf, showing three types of fiber.

plant leaf cross-section ribbon fibres mechanical fibres xylem fibrous region

A typical plant can produce up to 450 useable leaves in its lifetime, with each leaf yielding around 1000 fibers. These fibers possess a horseshoe-type cross-section and are usually as long as the leaf, although the extremities tend to be mechanically inferior and break off during processing. How do you get the fibers out and what are they made of?

Extraction of the fibers from the Agave leaves can be either mechanical or chemical. The mechanical process is termed decortication and results in an average fiber yield between 3 and 7% by weight. The process involves scraping the leaves from top to tail and then pressing the resulting pulp and washing it to dissolve the majority of the leaf, leaving the fibers in an almost pure, white, form. These fibers are then dried and cleaned. Chemical extraction traditionally involves soaking the leaves in seawater for around 3 months, resulting in dissolution of the nonfibrous parts of the leaf. More recently, solvents such as sodium hydroxide have been used to shorten the soaking time, but these tend to be environmentally unfriendly. Sisal fibers typically consist of around 78% cellulose, 8% lignin, 10% hemicellulose, with small amounts of ash and wax products. However, high polysaccharide contents are also possible in plants from certain geographical regions. Physically, the fibers are conservatively around 1.5 meters long and 150–300 mm in diameter, and each fiber consists of a bundle of hollow subfibers that are reinforced spirally by a cellulose/lignin composite structure, similar to wood.The waxes that are present (they are there to induce

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intercellular bonding) mean that it is difficult to get the fibers to form strong interfacial bonds with synthetic polymers, such as polypropylene. Also, the fibers are hydrophilic (attract water), whereas synthetic polymers tend to be hydrophobic (repulse water) and this serves to compound the issue. The surface of sisal fibers is rather scaly, and that is typical of plant fibers, but otherwise, cellulosic fibers are rather featureless. So why is sisal so special?

Basically, it’s their length that makes sisal fibers unique. Practically every other natural fiber type has an average fiber length measured in the order of millimeters or centimeters. Leading synthetic engineering fibers, such as carbon or glass, tend to be continuous (i.e., very long). Long fibers tend to give stronger and stiffer composites than short fiber ones, and sisal is the only natural fiber type that can be reasonably equated to continuous. As a general rule, plant fibers possess a direct relationship between mechanical response (modulus, strength, etc.) and cellulose content, although geometric factors, such as spiral angle, cannot be overlooked. Additionally, for sisal fibers, toughness, strength, and modulus all increase with increasing leaf age, although this effect is less pronounced if the fibers are to be used at elevated temperatures. Typically, sisal fibers are still mechanically happy in temperatures in excess of 100°C. So just how strong are they?

Table 8.2 details various mechanical and physical properties of sisal and other natural fibers. As a comparison, glass fiber is included. Bast fibers are fibers obtained from plant stems, whereas leaf fibers come from plant leaves and seed fibers come from plant seeds. Ramie fibers are obtained from the stalks of a type of Asian grass.

Figure 8.16 Typical sisal fiber (diameter = c.150µm).

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Table 8.2. Typical Mechanical and Physical Properties of Selected Fiber Types.

E Glass

Flax

Bast Fibers Hemp Jute Ramie

Seed fibers Coir Cotton

Leaf Fibers Banana Sisal

Density (g/cm3)

2.55

1.4

1.48

1.46

1.5

1.25

1.51

1.5

1.33

Strength (MPa)

2400

c.1100

c.750

600

500

270

400

980

650

Modulus (GPa)

73

70

70

20

44

6

12

—

38

Specific modulus

29

36

47

14

29

5

8

—

29

Failure strain (%)

3

1.4

1.6

1.8

2

20

3–10

—

2–3

Moisture absorption (%)

—

7

8

12

12–17

10

16

—

11

Raw price ($/Kg)

1.3

0.5–1.5

0.6–1.8

0.35

1.5–2.5

0.25–0.5

1.5–2.2

1.5–2.5

0.6–0.7

Fabric price ($/Kg)

1.7–3.8

2–4

2–4

1.5

—

—

1.9–2.6

—

—

Determination of these properties is difficult because there are significant response variations according to where on the fiber the measurements were taken. For example, in sisal, the fiber root area (closest to the main plant) is weaker but more ductile than the fiber region closest to the leaf extremity. Clearly, this property variation along the fiber length complicates property prediction, especially if the fibers are used as a composite reinforcement phase. These days, the term “natural fiber composite” describes a material in which a synthetic polymer has been reinforced by a naturally occurring particulate or fiber, rather than one that is wholly naturally occurring. As with most natural fiber reinforcements, sisal fiber should be chemically treated to improve the resulting composite interfacial strength to a competitive level. Common treatments include titanates, silanes, zirconates, oxidatives, isocyanate, alkali, and amides, although there is some debate as to how these treatments adversely affect the structural integrity of the fibers. However, it has been shown that the properties of the resulting composites are not significantly influenced by treatment type. Sisal fibers have been alloyed with most common synthetic resin categories, with perhaps sisal/polypropylene being the most studied.

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Are these fibers used on their own?

Sisal fibers currently find extensive use in countries such as India in products such as rugs, floor mats, and wall hangings. They can also be pulped and used in sisal paper. Sisal paper products possess high tear resistance compared with normal paper, high absorption characteristics, and higher recyclability. Sisal is also used as a substitute to asbestos and fiberglass in buildings and as a general cordage material. Sisal fibers possess many other properties that are currently being exploited, including thermal insulation, sound insulation, and fire resistance. In addition it is, of course, biodegradable. Sisal fibers are initially coarse and inflexible, but if they are woven correctly, they can be turned into baskets, braided ropes, and even hats (Figure 8.17), such as those coming out of French fashion houses at the moment. Further processing allows the fibers to be woven into an almost silklike fabric that is being increasingly used in hot-weather clothing. Recent developments have involved car companies going into partnership with sisal producers in some of the poorest regions of Africa who supply them with sisal fibers for car seats. Indeed, automotive applications are probably the fastest growing markets for natural fibers in general.

Figure 8.17 Sisal hat. Designed by Nathalie Sergent.

Sisal comes from leaves, what about bast fibers?

BAST FIBERS: JUTE, HEMP, AND FLAX Excluding wood, jute fibers are probably the second most common natural fiber material (after cotton), and hence are just about the cheap-

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Navigating the Materials World est. It is mostly grown in central and eastern India, in addition to Bangladesh, China, Indonesia, and Brazil.The fibers are obtained from the bark of two cultivated species of cochorus (woody herbs), namely capsularis (white jute) and olitorius (tossa jute). Unlike sisal fiber, which is known as a hard leaf fiber, jute fibers are known as bast fibers and are relatively soft and flexible. The hemp plant is harvested for its fibers, and additionally its seed, seed meal, and seed oil. It is a distinct variety of the plant species cannabis sativa L. and due to a similar leaf shape, it is commonly confused with marijuana. Hemp fibers are another example of a soft bast vegetable fiber, which are characterized by long primary fibers on the outer portion of an exceptionally long, thin, strong stem. It will happily grow in Middle European-type climates, requiring similar soil and climate conditions to corn and on average will yield around 3.5 tons per hectare of fiber. It is also a highly resilient crop and doesn’t require pesticides or fungicides and will grow up to 4 meters in the space of a few months. Originally from the Nile Delta, most flax fibers of commercial interest are now grown in the Low Countries and Northern France, and flax has been cultivated for thousands of years and harvested primarily for its fibers, which are used in the production of linen. Flax (linum usitatissimum) is a bast fiber plant that closely resembles oil seed rape in size and physical appearance, although its flowers are either blue or white. Typically, the central stem is a woody hollow trunk, around 1 to 1.5 meters high, and the fibers wind around the core and are attached by pectintype substances. The seedpods that form at the top of the stem are of equal importance because they are the source of linseed oil, used as a lacquer and in paints and other coatings. How are bast fibers made?

Traditionally, jute fibers are manufactured by soaking the bark in water for about 2 weeks to loosen the fibers from the pith and this also removes the thin skin. The fibers are then peeled away, washed, and dried before bundling. This process is often known as retting. Unfortunately, this process is a very variable one (because of insufficient available clean water and inhomogeneities along the bark surface), and the fibers often have to undergo a secondary process to make them sufficiently smooth for use in fabrics by removing residual pectin. In order for the fibers to be used effectively in composites, they have to be degummed (retted), which involves the removal of the adhesive interfiber materials, such as lignin, pectin, and hemicellulose. This is performed either with enzymes or chemicals, and other methods such as steam explosion and decortication are also used. Although decortication results in coarse fibers, these subsequent methods result in fibers that are significantly smaller (5–10 mm) in

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diameter. Processed hemp is a predominantly cellulose-based fiber that degrades at temperatures in excess of 200°C, and typical fiber lengths range from 20 to 100 millimeters. Hemp fibers are often dew-retted in which the fibers are stripped from the stalk and are left out in the open for many weeks so that the fiber-binding pectins can be broken down by atmospheric moisture.The fibers themselves are on average 20 centimeters in length and are easily stored in bundles. Flax fibers are harvested by pulling the plants out of the ground and are also dew retted. Enzymes coupled with rain and sunlight dissolve the pectins, leaving the fibers, which can then be bundled and used. The fibers are then rippled (removed from the seedpods) scutched (removed from the remains of the woody stem), washed, and dried to give a healthy yield of high quality fiber. Flax and hemp are often indistinguishable, especially during the processing stages. However, they are easily distinguished during the retting process because flax fibers are arranged in left-handed spirals and hemp fibers are arranged in right-handed spirals. What about composition and behavior?

Compositionally, jute fibers consist of 60% cellulose, 12% hemicellulose, 18% lignin, and around 10% other chemicals (waxes, etc.), and the strength and stiffness of jute fibers can be improved by up to 15% by means of an alkali (usually NaOH) treatment, a factor commonly exploited in jute reinforced-composite materials. The main effect of this treatment is that it improves fiber crystallinity by reducing the amount of cellulosic material present. Currently in India, there is extensive interest in the softening of jute fibers by biological methods. Compositionally, hemp fibers are around 67% cellulose, 17% hemicellulose, 1% pectin, 3% lignin, 2% proteins, 1% fats and waxes, and 9% moisture. In terms of mechanical properties, hemp composites are very dependent on fiber length, which really suffers during many common processing techniques, such as extrusion and injection molding. The type of fiber treatment also plays a minor role, especially when alkali-based treatments are used. In hemp, property differences of upwards of 30% have been reported by merely changing from decorticated fibers to steamexploded fibers. In addition, varying the treatments can have significant effects on the fiber surface morphology and resulting interfacial characteristics. Typically, steam-exploded fibers are smoother and tend to bundle less than decorticated fibers. Structurally, flax fibers exhibit many structural similarities with other natural fiber materials, such as a central region (lumen) possibly surrounded by three secondary cellular walls and an external primary cell wall. Perhaps more specifically to flax, the primary wall consists of pectin, which can be further stiffened by the addition of lignin. Lignin addition

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Navigating the Materials World also causes dehydration and this assists in providing an adhesive bond that holds the fibers together. The secondary cell walls are merely cellulose fibrils networked with hemicellulose, although lignin can also provide a significant strengthening role here. Compositionally, flax differs from other plant fibers because it possesses very little lignin (2% by weight). Additional constituents include 67% cellulose, 11% hemicellulose, and around 20% other chemicals. Flax fibers are strong and have low density, although they do have low thermal resistance and are quite variable in quality. In addition, flax fibers are quite polar by nature. Although this is a common phenomenon in most natural fibrous materials, it seems to be more of an issue with flax. This polar bond results in very poor interfacial bonding characteristics when used as a composite reinforcement and is somewhat alleviated by chemically treating the matrix material to optimize the system polarity. Such a process is common with polypropylene in which maleic anhydride is often used to increase the polarity of the plastic and hence promote better bonding with natural fibers. Alternatively, simple boiling of the fibers will decrease their polarity by removing pectins, but other fiber damage may occur. What are they used for?

In fiber form, jute is used as a packaging material and especially as a food packaging material, as well as coarse fabric and cordage. Jute fibers are often used in the production of hessian cloth for clothes, bags, and other apparel. Flax fibers are made into linen, but increasingly are being used in automobile and construction materials as a replacement for glass. After having been stripped of fibers, hemp stalks are often cut down into smaller pieces, 5 to -10 centimeters, and in this form are known as ‘hurds.’ Because hurds have over more than twice the absorbency of wood, they make an excellent animal bedding and garden mulch. The high cellulose content of hemp means that it is also used in the production of cellulose-based plastic. The main uses of hemp fibers are in the areas of insulation materials, and as reinforcements for compression molded parts, such as those used in the automotive industry. Indeed, companies such as Mercedes-Benz use hemp-reinforced thermoplastics as the vast majority of their internal detail. Hemp fibers are easily spun and woven into a fine linen-like fabric and can be used for clothing, home furnishings, and carpeting. Hemp fabric is also used as sails, awnings, and tarpaulins because of its antimildew and antimicrobial resistance and is often mixed with wool or cotton to give superior clothing fabrics. Hemp has been used commercially for thousands of years for paper, lighting oil, paints, and even food. Historically, the hemp plant is highly significant for the United

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States because the first American flag was made from hemp, and hemp paper was used for the Declaration of Independence and the Constitution. Compared with normal wood-based paper, which can be recycled effectively only 3 times, hemp paper can be economically recycled 10 times or more. In the building industry, hemp fiber is used as a replacement to asbestos in fiber-reinforced cement. Some Difficulties to Overcome The use of hemp and flax as a composite constituent is a fairly recent development, although the interest generated by these materials, especially in reinforcing cheap and recyclable thermoplastics, is increasing rapidly. In addition to dew-retted fiber, flax can be Duralin® treated. These are fibers processed to give improved moisture properties and rot sensitivity and have been proven to absorb less moisture than traditional flax fibers. They are widely suspected to give enhanced interfacial properties, as reflected by composite strength and stiffness studies. Transcrystallinity is also a big issue in flax and jute composites. As well as being structurally anomalous to the rest of the matrix material, transcrystals induce a degree of anisotropy into the composite that may not have been expected. The amount of transcrystallinity that occurs is very dependent on the fiber or matrix treatment and will invariably lead to a strengthening of the interfacial region. In addition, the surface morphology of flax fibers is somewhat featureless, unlike many other cellulosic fibers. What about seed fibers?

Coir fibers, obtained from coconut, have a cellulose content lower than in sisal and also possesses a higher spiral angle, and this renders the fibers weaker and certainly less stiff. As with jute fibers, coir fibers can be improved by treating with sodium hydroxide, and although improvements

Figure 8.18 Showing (top) spherulites and (bottom) transcrystals around a typical fiber.

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Navigating the Materials World in strength are modest, increases in stiffness of up to 40% can be obtained. Typically, coir fibers have a modulus of around 3 GPa and a tensile strength of 100 MPa. Cotton is characterized by being almost pure cellulose with no lignin whatsoever. For this reason, cotton was used as the definitive source of cellulosic material long before wood became a candidate. Cotton is harvested as small hairs (up to 50 millimeters long) that grow from the seed pods. These hairs are not all the same length and it is only the longer ones that are harvested as a textile fiber.The remaining hairs (called linters, on the order of 2 to -3 millimeters long) are still sometimes used in the production of cellulose. Are there any other natural materials of note?

A common natural polysaccharide that is receiving a great deal of attention these days is chitin (Figure 8.19). Chitin is structurally very similar to cellulose, with one of the -OH links replaced by a NHCOCH3. Like cellulose, it is used primarily as structural support, but unlike cellulose, it is found predominantly in animals (such as arachnids and crustaceans) as an exoskeleton (e.g., seashells). Pure chitin can be extracted from seashells by acid dissolution and subsequent alkali boiling to remove the proteins. It is largely used in the biomimetics area to synthesize materials based on reconstituted shells and currently have enormous potential as engineering materials. Seashells are hard, strong, insoluble, yet extremely flexible and tough and one of the major goals of biomimetisists around the world is to be able to simulate and produce a synthetic equivalent of this material in a usable form (e.g., sheets and beams) and use the excellent property range in structural applications. So now I know everything about natural materials!

Hmm—no, but this has been a good start. You now know about the various classes of natural materials and how study of these classes often impinges on other branches of science. The differences between plant-

Figure 8.19 Chemical structure of chitin. H3C

OH CH2

H OH

H O

OH

H

H

NH H

OH

OH O

H H3C

O H

NH C

O C

O

CH2 OH

O

n

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based and animal-based materials has been illustrated, as has the incredible mechanical properties that can be achieved from some of these materials. However, we have only scratched the surface regarding current technologies, uses, and research areas relating to these materials. There is, though, a wealth of information stored in numerous information sources, including, of course, the Internet. Happy Traveling!

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CHAPTER 9

An Electronic Trip Through Semiconductors Emily Alle n and Susan A. Ambrose


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Welcome Welcome to the land of semiconductors. Until now, you’ve probably been traveling to places that were somewhat familiar to you, full of materials that you encounter in your everyday life. Metals and ceramics, for example, have been explored by humans for millennia. However, now you’re in a world of materials whose properties weren’t even known until late in the 20th century. To attain a deep understanding of semiconductor materials, you’ll probably need to visit the worlds of quantum mechanics and electrical engineering; those destinations might be later in your journey. On your visit here, though, we’ll show you the useful properties of semiconductor materials, and why they are so valuable to our modern world. Because you’re living in the 21st century, you know that semiconductor materials are used all around you in advanced technology. Computers and communication equipment, sensors, transportation, automation, and entertainment devices all have at their hearts one or more semiconductor chips to provide functional control and data storage.These chips are actually composed of small pieces of semiconductor material with electronic circuits embedded in them. The unique properties of semiconductors allow us to make versatile components of electronic circuits, such as resistors, capacitors, diodes, and transistors, all on the same small piece of material. We won’t discuss those circuits here, or even the component devices that make up the circuits. Instead, we’ll focus only on the properties and behaviors of the semiconductor materials. Table 9.1 shows some common applications of semiconductor materials. Like all materials, the crystallographic and defect structure of the semiconductor material determines its structural, mechanical, electrical, thermodynamic, and other properties. It is generally the electrical and optical properties of semiconductors that make them useful to us, so that is what we’ll look at in this chapter.We use analogies to help you build an understanding of abstract concepts.We’ll also teach you how to make a concept map, which you have seen used in other worlds but perhaps did not understand their origin. Throughout this chapter, we’ll ask you to stop reading, engage in a challenge, or develop your concept map. This will help you learn how to learn new concepts. The challenges include simple equations and calculations to help you understand relationships among concepts. To understand how and why semiconductor materials are used, we’ll have to travel inside the material and look at things from the point of view of the electrons. We’ll look at a few behaviors that are specific to semiconductors, such as variable conductivity, photoconductivity, and the special role played by impurities.We’ll walk through some concepts using analogies, such as generation and recombination, electrons and holes, thermal equilibrium, carrier

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An Electronic Trip Through Semiconductors

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Table 9.1. Some Applications of Semiconductor Materials.

Application

Purpose

Typical Semiconductor

Microprocessors

Complex logic circuits designed to perform various functions

Silicon

Memory chips

Store digital data

Silicon

Microwave communications

Logic circuits to control processing and conversion of microwave signals; used in cell phones and other systems

GaAs

LEDs

Devices that emit light of a single color; used for displays

InGaP alloys AlGaAsP alloys

Semiconductor lasers

Devices that emit highintensity coherent light of a single wavelength

Same as LEDs, with many layers of various alloys

Infrared detectors

Sensing living things

Cadmium telluride (CdTe)

Photovoltaic cells

Convert sunlight into electricity

Silicon (crystalline or amorphous); GaAs; other alloys

mobility, and intrinsic and extrinsic behavior. Along the way, we’ll introduce the very useful construct of electronic band structure.

The Concept Map Let’s start with examining a concept map, which we’ll use throughout this chapter to navigate the land of semiconductors. Figure 9.1 is a concept map showing some of the basic features of semiconductor materials. Each bubble refers to an important concept, and each concept is related to many other concepts. The relationships are shown by connecting lines, traced by one or two words describing the relationship between the two concepts. Once a basic understanding of this field has been achieved, you should be able to discuss each concept, and each relationship, in either qualitative or quantitative terms. In some cases a relationship can be easily summarized by an equation or group of equations. Developing your ability to navigate the concept map is equivalent to developing your understanding of this field. As your understanding grows

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Figure 9.1 Concept map showing some of the basic features of semiconductor materials.

pure elements

Semiconductors

can be or

are useful for their

covalent bonds:

have

Si C Ge

compounds atomic structure

electrical properties

ionic bonds:

have

are determined by

determines bonding

optical properties

crystal structure

energy band structure

determines

may allow

defines values of

GaAs SiC GaN, InP

Photon emission or

increases extrinsic

carrier concentration

carrier mobility

Impurity doping

Impurity doping

Photon absorption

Decreases mobility Temperature

results in

Increases intrinsic Temperature

n-type or p-type electrical conductivity

Photoconductivity

to encompass concepts not on this map, either because they are from a broader context or they are more detailed than what is shown here, you can create your own concept maps, which will help deepen your understanding. There are no wrong or right concept maps, but some maps are more useful in organizing your thoughts or demonstrating relationships to other people. You can use a concept map drawn by an expert to help you study new ideas. What do you know about the concepts in each bubble? Do you understand the connections between each concept? Can you make the calculations suggested by those connections? You can also use a concept map drawn by an expert to help you understand the way the expert sees the world of semiconductors, to help you situate new information within a larger context, and to help you make meaningful relationships between and among concepts. In this chapter, there are sections discussing each part of the concept map, with the same headings as the bubbles on the concept map. For example, in Figure 9.1 there is a connector between temperature and carrier concentration. To check that you understand how these two are related, find the sections in this chapter that discuss these concepts. Also, study the challenge calculations. If you can navigate the concept map in this chapter, then you have a good beginning understanding of semiconductor materials.

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Making Your Own Concept Map

Concept maps can be used by learners to organize the information they know about any subject. Try it yourself. Make a concept map for something you know a lot about, say, a hobby, sports, or the subject of an early physics class. First, write down the 10 most important concepts: these are the Key Concepts. Next to each Key Concept, write down two other key concepts from the list that are most closely associated with it. Then, lay out the 10 key concepts in bubbles such that you can easily draw connecting lines between the most connected ideas. Next, fill in a verb along each line, a verb that describes how the two concepts are related. Rearrange your concept map to display any underlying structure or hierarchy among the concepts. As your understanding of a discipline grows, your concept map will both expand and deepen.

What Is a Semiconductor? Simply put, a semiconductor is a material that sometimes conducts electrical current. Fortunately, engineers can control when and if the semiconductor will act as a conductor and when it will act as an insulator, by controlling the properties of the material through processing. Some semiconductor devices, such as diodes and transistors, function as amplifiers, switches, and memory devices by using the various electrical properties of semiconductors. Other devices, such as solar cells and photodiodes, work by either emitting or absorbing light, and use the optical properties of the material. This chapter does not discuss semiconductor devices, but rather provides some guidelines on how to think about semiconductor materials and their properties. Semiconductor materials are either covalently or ionically bonded, sometimes with a mixture of the two types of bonding. Elemental semiconductors, such as silicon and germanium, have very strong directional, tetrahedral covalent bonds resulting in a diamond cubic crystal structure. It is the sharing of electrons between neighboring Group IV atoms that gives rise to the configuration of the covalent bonds. Diamond is one of the best insulator materials, but if properly treated it can also act as a semiconductor and has potentially useful properties at high temperature. Compound semiconductors, such as GaAs, InP, and CdTe, are ionically bonded and can also be thought of as ceramic materials. These semiconductors have similar crystal structures (often the zincblende structure) and similar properties to the elemental semiconductors. The very strong ionic and covalent bonding found in semiconductors makes them very hard,

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Navigating the Materials World stiff, and brittle, as well as good conductors of heat. There are also advanced semiconductor materials made of organic polymers that behave similarly to traditional crystalline semiconductors. These materials offer the possibility of flexible, large devices that can be easily mass produced for applications such as large flat panel displays and traffic and automobile lights. The very useful electrical properties of semiconductors derive from the fact that the allowed energies of the valence electrons in a crystalline semiconductor are arranged in specific ranges of energy called energy bands, with a forbidden range of energies between the bands, called the energy bandgap. It is the presence of this bandgap that allows semiconductor materials to be engineered into useful and controllable devices. The conductivity of the semiconductor is an important property, just as it is in metals, where the conductivity is much higher than in semiconductors. However, metal conductivity does not vary as dramatically as does semiconductor conductivity. The optical properties of semiconductors also come from the existence of the bandgap. A gap in allowed energies of the electrons means that relatively large transitions between electronic states can be made, which in turn means there can be an interaction between photons (particles of light) and the material. Both absorption of light and emission of light can be made to happen in some semiconductors, and the useful optical range of these materials is generally in the infrared, visible, and ultraviolet range of the electromagnetic spectrum, making these materials highly valuable for communications and detection systems.

Electrons in Energy Space: Energy Bands The land of semiconductor materials requires you to open your mind to new dimensions. To understand and design semiconductor materials and devices, you must be comfortable with the world of electrons. Notice that the concept map in Figure 9.1 doesn’t list the word electrons anywhere. That is because every aspect on the concept map is related to electrons and their electronic energy state. An electronic state refers to the energy, momentum, and spin of the electron, which varies depending on whether the electron is in an isolated atom, molecule, or solid. Electrons live in a different dimension, they live in energy space. Even though our everyday mind tells us that an electron in an electrical current travels from point a to point b along a physical path, it is often useful to think about an electron moving in energy space instead of real space. When an electron is accelerated, like all particles, its velocity increases; and like all particles, its kinetic energy also increases. But because an electron also can be thought of as a wave, when it is accelerated its wavelength and frequency also change. Thus, if we visit the electron in its energy space instead of real physical space, it is easier to follow the electron’s movements.

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In a solid, just as in an isolated atom, an electron is restricted to having only specific energies, unlike a football or other large particle. However, in a solid there are many electrons close together (more than 1028 m-3) and to obey the Pauli Exclusion Principle, each of these electrons must be in a different electronic state. In the semiconductor land, we keep track of these many allowed states and their energies by using energy band diagrams.The allowed energy states of the electrons are arranged in energy bands in a solid. Figure 9.2 shows a partially filled band of allowed electron energy states in a material.The vertical dimension of the band is kinetic energy, whereas the horizontal dimension is spatial. Depending on how many electrons are in the solid, all of the allowed states, none of the allowed states, or some of the allowed states may be filled by electrons. Notice in Figure 9.2 that the band is flat in the x-direction. This is because if the material does not have an electric potential applied to it, its band structure is the same everywhere in the material. Figure 9.3 shows what happens when we apply a potential to the crystal, with the positive terminal on the left-hand side. The potential energy of the electrons is higher at the negative end of the material, so they will tend to move toward the positive terminal and then around the external circuit. As the electrons move from right to left in the band, they move into new electronic states that were previously empty. Thus we can think of the acceleration of electrons under an electric field as the movement of electrons into new states. Electrons, like all particles, have both kinetic and potential energy. An externally applied potential determines the potential energy, but the electron’s place in the band indicates its kinetic energy. Figure 9.2 Partially filled band of allowed electron energy states in a material. Top of Band KE

Bottom of Band X

Figure 9.3 Reaction to applying a potential to the crystal, with the positive terminal on the left-hand side. Top of Band VPE Bottom of Band

X +

–

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Navigating the Materials World When there is no external power source, the potential energy of electrons is the same throughout the crystal. However, the electron kinetic energies are determined by which energy levels they occupy. We might ask from where these electrons in the energy band come? When a solid forms, it is the valence electrons that form bonds between atoms. Electron sharing in covalent semiconductors, and ionic bonding in compound semiconductors, occurs such that the bond configurations are satisfied. It is these valence electrons that become most important electronically. To satisfy the Pauli Exclusion Principle, one of the basic rules of quantum mechanics, each valence electron must be in a different electronic state. So they spread out their energies to fill a specific range of energies called the valence band. In the valence band of a semiconductor (at absolute zero temperature), every state is filled by an electron. If a potential is applied to this band, there can be no acceleration of electrons, and hence no current, because there are no empty states for electrons to move into. Electrons in the Valence Band

How many electrons are there in the valence band in one cubic millimeter of silicon? If the width of the band in energy space is 2 eV, how close are the energy states to each other? The valence electrons in a silicon atom are the two 3p electrons and two 3s electrons; the atomic density of silicon is about 5 ¥ 1028 m-3. Thus, in one cubic millimeter of silicon there are 2 ¥ 1020 valence electrons. All of these electrons have to fit into the band in separate energy states; thus, the energy states are only about 10 -20 eV apart. Note: we disregarded the spin of the electrons, and also we assumed all the energy levels are equally spaced, which is not the case. But we can see that the electron states are spaced very closely together. In a semiconductor, there is another higher band of allowed energies called the conduction band. The conduction band is also a set of allowed energy states, and at absolute zero there are no electrons occupying any of these states, because they are all stuck in the valence band. It is the existence of these two bands that gives a semiconductor its properties.As you can see in Figure 9.4, there is an energy gap between the valence band and the conduction band.There are no allowed states in the energy gap, so an electron has to gain significant energy to get into the conduction band. With a full valence band and empty conduction band, a semiconductor material is no different than an insulator. On the other hand, metals have partly full conduction bands all the time, which is what makes them such good conductors.There are empty states in the conduction band for electrons to be accelerated into, resulting in current conduction, as shown in Figure 9.4. For a semiconductor to be conductive, electrons must somehow be placed into the conduction band.That’s what we’ll explore next.

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An Electronic Trip Through Semiconductors Figure 9.4 Energy gap between the valence band and the conduction band. Conduction Band

Energy Bandgap Valence Band

THE ELECTRON APARTMENT BUILDING Imagine that all the electrons in a piece of material are students living in apartment buildings, but only two can live in each room. The building has many floors and on each floor are many rooms. The two students in each room live either on the right-hand side (RH) or left-hand side (LH) of the room. Each student has a unique address, for example, Building 1, Floor 3, Room 6, RH side. In some materials, the rooms in the buildings may be completely full; in others they may be partly full or even empty. In this analogy, the buildings are what we call energy bands, the floors are energy levels, and the rooms are specific energy states. All the states at one level (i.e., all the rooms on one floor) have the same energy.The only difference between the two electrons in each room is their spin state.Thus the electrons in our buildings obey the Pauli exclusion principle—each electron is in a unique quantum state, that is, it has its own unique address. In a semiconductor, we are interested in only two of the buildings. One building is completely full (the valence band), and another building next door is completely empty (the conduction band). Figure 9.5

203

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Generation of Charge Carriers Electrons excited into the conduction band can carry charge in a current and are called mobile charge carriers, conduction electrons, or sometimes free electrons.We refer to the concentration of conduction electrons as the carrier concentration. In insulators, the bandgap energy is very large (>3 eV). In semiconductors it is small enough to be useful (

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