Structure.homework-#8 answer

Structure.homework-#8 answer - MTLE-2100 Structure of...

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MTLE-2100 Structure of Engineering Materials, Homework #8 --Answer 1. Diffusion coefficient of Na + in a NaCl single crystal is shown in the attached figure. This crystal has a small quantity of impurity CaCl 2 and Na + diffuses by the vacancy mechanism. Diffusion coefficient by the vacancy mechanism is given, in general, by D = D 0 exp (- E m /kT) exp(-E fs /2kT), where E m is the energy needed to migrate Na + and E fs is the energy needed to form a pair of Schottky defect. The higher temperature region of the data is not affected by impurity and is called intrinsic region, while the lower temperature region is called extrinsic region. k= 1.38 x10 -23 J/atom-K a) What type of point defect will be created by the impurity CaCl 2 ? Since Ca ++ replaces Na + , Na + vacancy is created. V Na+ Ca Na . b) Explain the existence of the intrinsic and extrinsic regions. At low temperature, impurity Ca ++ controls the sodium vacancy concentration, which is independent of temperature. Na diffusion by vacancy mechanism requires
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Structure.homework-#8 answer - MTLE-2100 Structure of...

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