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ch. 5 notes

ch. 5 notes - concentration gradient E Steady State...

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MSE 209 CH.5 Diffusion in Solids I. Diffusion- material transport by atomic transport A. Atom movement driven by thermal energy 1. Heat causes atoms to vibrate 2. Vibration amplitude increases with T B. Rate of diffusion 1. Uses Arrhenius law 2. Q and C are material properties 3. Q characterizes the energy barrier that atoms must have to overcome to move 4. Atom migration occurs by mechanics of vacancy migration\ - = RT Q C rate exp kT Q D D e N N - =
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5. Atoms jump randomly 6. Interdiffusion in an alloy: atoms tend to migrate from higher to lower concentration 7. Self diffusion: in an elemental solid atoms also migrate C. Another method of diffusion is movement via interstitial sites D. Fick’s Laws Quantify diffusion rates 1. Steady State- Fick’s 1 st law- net flux of diffusing species moves down the
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Unformatted text preview: concentration gradient E. Steady State diffusion occurs when the concentration gradient is stable dx dC D J x-= 1. Flux is defined as the number of atoms diffusing through unit area and per unit time 2. Concentration drives the diffusion rate it is the “driving force” F. Diffusion coefficient 1. D is the measure of mobility of diffusing species 2. D is a strong function of T G. Temperature dependence: follows the Arrhenius equation 1. Q is calculated by determining the slope of the line H. There is a wide range of diffusion coefficients for materials -= RT Q D D d exp ---= 2 1 2 1 1 1 log log 3 . 2 T T D D R Q d...
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