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Unformatted text preview: Chapter 5  Chapter 5: Diffusion Learning objectives: 1. Name and describe the 2 atomic mechanisms of diffusion. 2. Distinguish between steadystate and nonsteadystate diffusion. 3. Write Ficks first and second laws in equation forms, and define all parameters, and note the kind of diffusion for which each of these equations is normally applied. 4. Write the solution to Ficks second law for diffusion into a semi infinite solid when the concentration of diffusing species at the surface is held constant. Define all parameters in this equation. 5. Calculate the diffusion coefficient for some material at a specified temperature, given the appropriate diffusion constants. Chapter 5  2 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  3 Diffusion Diffusion Mass transport by atomic motion Mechanisms Gases & Liquids random (Brownian) motion Solids vacancy diffusion or interstitial diffusion Chapter 5  4 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  5 Selfdiffusion : In an elemental solid, atoms also migrate. Label some atoms After some time Diffusion A B C D A B C D Chapter 5  6 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  7 Simulation of interdiffusion across an interface: Rate of substitutional diffusion depends on: vacancy concentration frequency of jumping. (Courtesy P.M. Anderson) Diffusion Simulation Chapter 5  Substitutional Diffusion: applies to substitutional impurities atoms exchange with vacancies rate depends on: # of vacancies activation energy Mechanisms Diffusion Couple Chapter 5  9 Diffusion Mechanisms Interstitial diffusion smaller atoms can diffuse between atoms....
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 Summer '09
 Jone

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