Lecture09 - Diffusion Diffusion • Interdiffusion: In an...

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1 1 Diffusion Diffusion - Mass transport by atomic motion Mechanisms • Gases & Liquids – random (Brownian) motion • Solids – vacancy diffusion or interstitial diffusion 2 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, Cal ister 7e . Diffusion After some time 3 Self-diffusion : In an elemental solid, atoms also migrate. Label some atoms After some time Diffusion A B C D A B C D 4 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 5 • Simulation of interdiffusion across an interface: • Rate of substitutional diffusion depends on: --vacancy concentration --frequency of jumping. (Courtesy P.M. Anderson) Diffusion Simulation 6 Diffusion Mechanisms • Interstitial diffusion – smaller atoms can diffuse between atoms. More rapid than vacancy diffusion Adapted from Fig. 5.3 (b), Cal ister 7e .
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2 7 Adapted from chapter-opening photograph, Chapter 5, Cal ister 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 8 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.
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Lecture09 - Diffusion Diffusion • Interdiffusion: In an...

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