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Homework 9 - MSE 230 HW9(due 03/25 03/26 Spring 2010 1 The...

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MSE 230 HW9 (due 03/25, 03/26) Spring 2010 1. The important refractory ceramic MgO has the NaCl crystal structure. (a) Neglecting the small effect of CN on ionic radius, how much larger would the Mg 2+ cation have to be in order for MgO to possibly have the CsCl crystal structure? (b) Based on equation 2.8 (including associated footnote), which factor, ion valence or ion separation ( r = r cation + r anion ), contributes more to the higher melting temperature of MgO compared to NaCl (Table 2.3)? Do not calculate absolute E A values, but show the relative effects using ratios. 2. One form of the important abrasive ceramic SiC has the ZnS crystal structure (see Fig. 12.4). (a) From an electronic structure standpoint, explain briefly why the coordination number in SiC is 4. (b) Calculate the lattice parameter of this structure based on the true density (pore-free) of 3.22 g/cm 3 . (c) Bulk SiC, like many bulk ceramics, often contains residual porosity (small voids) due to incomplete sintering. What volume fraction (i.e., 0 to 1) of porosity would be in a sintered SiC ceramic having a bulk density of 3.10 g/cm
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