Chapter 5 - Chapter 5: Diffusion in Solids ISSUES TO...

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Chapter 5: Diffusion in Solids ISSUES TO ADDRESS. .. • How does diffusion occur? • Why is it an important part of processing? • How can the rate of diffusion be predicted for some simple cases? • How does diffusion depend on structure and temperature? Chapter 5 - 1
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Diffusion Diffusion - Mass transport by atomic motion Mechanisms Gases & Liquids – random (Brownian) motion Solids – vacancy diffusion or interstitial diffusion Chapter 5 - 2
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Diffusion Interdiffusion : In an alloy, atoms tend to migrate from regions of high concentration to regions of low concentration. Initially Adapted from Figs. 5.1 and 5.2, Callister 7e . After some time Chapter 5 - 3
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Diffusion Self-diffusion : In an elemental solid, atoms also migrate. Label some atoms After some time A B C D A B C D Chapter 5 - 4
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Diffusion Mechanisms Vacancy Diffusion: • atoms exchange with vacancies applies to substitutional impurities atoms rate depends on: -- number of vacancies -- activation energy to exchange. Chapter 5 - 5 increasing elapsed time
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Diffusion Mechanisms energy ( activation energy ) to make atom-vacancy exchange vacancy distortion of planes Chapter 5 - 6 increasing elapsed time
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Diffusion Simulation • Simulation of interdiffusion across an interface: • Rate of substitutional diffusion depends on: --vacancy concentration --frequency of jumping. (Courtesy P.M. Anderson) Chapter 5 - 7
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Diffusion Mechanisms • Interstitial diffusion – smaller atoms can diffuse between atoms. More rapid than vacancy diffusion Adapted from Fig. 5.3 (b), Callister 7e . Chapter 5 - 8
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Example - Interstitial diffusion of C in Fe Japanese swords Chapter 5 - 9
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Processing Using Diffusion Adapted from chapter -o pening photograph, Chapter 5, Callister 7e. (Courtesy of Surface Division, Midland -R oss.) Case Hardening : - Diffuse carbon atoms into the host iron atoms at the surface. - Case hardened gear is an e xample of interstitial diffusion • Result: The presence of C atoms makes iron (steel) harder. Chapter 5 - 10
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Processing Using Diffusion Doping silicon with phosphorus for n -type semiconductors: Chapter 5 - 11 magnified image of a computer chip 0.5mm light regions: Si atoms light regions: Al atoms Adapted from chapter- opening photograph, Chapter 18, Callister 7e.
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Chapter 5 - 12 The Periodic Table • Columns: Similar Valence Structure Electropositive elements: Readily give up electrons to become + ions. Electronegative elements: Readily acquire electrons to become - ions.
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Chapter 5 - Chapter 5: Diffusion in Solids ISSUES TO...

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