Ch 5 2009 - Diffusion in Solids ISSUES TO ADDRESS How does...

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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? Diffusion in Solids
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Diffusion Diffusion - Mass transport by atomic motion 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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Interdiffusion : In an alloy, atoms tend to migrate from regions of high conc. to regions of low conc. Initially Interdiffusion After some time
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Self-diffusion : In an elemental solid, atoms also migrate. Label some atoms After some time Self-Diffusion A B C D A B C D
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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 Increasing elapsed time
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Diffusion Mechanisms Interstitial diffusion – Smaller atoms can diffuse between atoms. More rapid than vacancy diffusion
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How do we quantify the amount or rate of diffusion? Measured empirically Make thin film (membrane) of known surface area Impose concentration gradient Measure how fast atoms or molecules diffuse through the membrane ( 29 ( 29 s m kg or s cm mol time area
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Ch 5 2009 - Diffusion in Solids ISSUES TO ADDRESS How does...

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