Mse05 - 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

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1 Chapter 5 - 1 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: Diffusion in Solids Chapter 5 - 2 Diffusion Diffusion - Mass transport by atomic motion Mechanisms • Solids – vacancy diffusion or interstitial diffusion
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2 Chapter 5 - 3 Interdiffusion : In an alloy, atoms tend to migrate from regions of high conc. to regions of low conc. Initially Adapted from Figs. 5.1 and 5.2, Callister 7e . Diffusion After some time Chapter 5 - 4 Self-diffusion : In an elemental solid, atoms also migrate. Label some atoms After some time Diffusion A B C D A B C D
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3 Chapter 5 - 5 Diffusion Mechanisms Vacancy Diffusion: • atoms exchange with vacancies • applies to substitutional impurities atoms • rate depends on: --number of vacancies --activation energy to exchange. increasing elapsed time Chapter 5 - 6 • Simulation of interdiffusion across an interface: • Rate of substitutional diffusion depends on: --vacancy concentration --frequency of jumping. (Courtesy P.M. Anderson) Diffusion Simulation
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4 Chapter 5 - 7 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 Adapted from chapter-opening photograph, Chapter 5, Callister 7e. (Courtesy of Surface Division, Midland-Ross.) Case Hardening : --Diffuse carbon atoms into the host iron atoms at the surface. --Example of interstitial diffusion is a case hardened gear. • Result: The presence of C atoms makes iron (steel) harder. Processing Using Diffusion
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5 Chapter 5 - 9 Doping silicon with phosphorus for n -type semiconductors: • Process: 3. Result: Doped semiconductor regions. silicon Processing Using Diffusion magnified image of a computer chip 0.5mm light regions: Si atoms light regions: Al atoms 2. Heat it. 1. Deposit
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This note was uploaded on 03/18/2010 for the course ME 2580 taught by Professor Vandenbrink during the Spring '08 term at Western Michigan.

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Mse05 - 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

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