HW10 Solutions F10 - ENMA 300: Problem Set #10 1. Under...

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Page 1 of 3 ENMA 300: Problem Set #10 1. Under what conditions can Fick’s first law be used to solve diffusion problems? It can be used to solve diffusion problems provided the concentration gradient does not change over time. 2. Silica can be either a crystalline solid or a glass at room temperature. Is diffusion faster in glassy or crystalline silica? Why? The mechanism of diffusion can be vacancy-mediated or interstitial. Glassy SiO 2 : In the case of a glassy material there is no lattice, so the concept of vacancies is ill-defined. Diffusion is interstitial as the packing density of glass is lower, offering larger interstitial voids for diffusion. Crystalline SiO 2 : Vacancy-mediated diffusion is slow, because the Si-O bond is very strong therefore generating a very small concentration of vacancies in the lattice at room temperature. Interstitial diffusion is slower than in the glassy state, because the interstices are smaller in volume compared to glassy SiO 2 . 3. Ge is covalently bonded while Cu possess metallic bonds. Self-diffusion in each takes places via vacancy-exchange mechanism. For comparable temperatures, which would have a larger diffusion coefficient? Why? The diffusion process through the vacancy-exchange mechanism depends on creation of vacancies and vacancy/atom interchange. At comparable temperatures, for Ge and Cu, the diffusion coefficient for that material which has a higher vacancy concentration and a lower site-exchange activation energy would be higher. A covalent bond as opposed to a metallic bond is directional. It is also difficult to create
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HW10 Solutions F10 - ENMA 300: Problem Set #10 1. Under...

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