ch06 - 1/19/2012 Chapter 6: Diffusion Diffusion Diffusion -...

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1/19/2012 1 Chapter 6 - 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 6: Diffusion Chapter 6 - 2 Diffusion Diffusion - Mass transport by atomic motion Mechanisms Gases & Liquids – random (Brownian) motion Solids – vacancy diffusion or interstitial diffusion Chapter 6 - 3 Interdiffusion : In an alloy, atoms tend to migrate from regions of high conc. to regions of low conc. Initially Adapted from Figs. 6.1 and 6.2, Cal ister & Rethwisch 3e . Diffusion After some time Chapter 6 - 4 Self-diffusion : In an elemental solid, atoms also migrate. Label some atoms Diffusion A B C D After some time A B C D Chapter 6 - 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 6 - 6 • Simulation of interdiffusion across an interface: Rate of substitutional diffusion depends on: -- vacancy concentration -- frequency of jumping. (Courtesy P.M. Anderson) Diffusion Simulation QuickTime™ and a Cinepak decompressor are needed to see this picture.
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1/19/2012 2 Chapter 6 - 7 Diffusion Mechanisms • Interstitial diffusion – smaller atoms can diffuse between atoms. More rapid than vacancy diffusion Adapted from Fig. 6.3 (b), . Chapter 6 - 8 Adapted from chapter-opening photograph, Chapter 6, Cal ister & Rethwisch 3e. (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 Chapter 6 - 9 Doping silicon with phosphorus for n -type semiconductors: • Process: 3. Result: Doped semiconductor regions. silicon
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This note was uploaded on 02/18/2012 for the course BME 340 taught by Professor Dr.garybledsoe during the Spring '12 term at MO St. Louis.

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ch06 - 1/19/2012 Chapter 6: Diffusion Diffusion Diffusion -...

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