201 Week 5 2008 Posted

201 Week 5 2008 Posted - Chapter 5: Diffusion in Solids...

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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
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Chapter 5 - 2 Diffusion Diffusion - Mass transport by atomic motion Mechanisms Gases & Liquids – random (Brownian) motion Solids – vacancy diffusion or interstitial diffusion – Diffusion similar to a musical chair process, facilitated by atoms jumping into adjacent vacancies or interstitial sites.
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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
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Chapter 5 - 4 Self-diffusion : In an elemental solid, atoms also migrate. Label some atoms After some time Diffusion A B C D B C D • How would you follow this migration?
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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
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Chapter 5 - 6 Diffusion Mechanisms • Interstitial diffusion – smaller atoms can diffuse between atoms. More rapid than vacancy diffusion Adapted from Fig. 5.3 (b), Callister 7e .
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Chapter 5 - 7 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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Chapter 5 - 8 Case Hardening cont’d How is carbon diffused in? • Gas carburizing or plasma carburizing using a carbon-containing gas • Typical carbon concentration in the % range • Surface concentration (C s) controlled by gas pressure
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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 0.5 mm light regions: Si atoms light regions: Al atoms 2. Heat it.
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201 Week 5 2008 Posted - Chapter 5: Diffusion in Solids...

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