# Structure.questions_III-answer - In general a = 2 r(anion...

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MTLE-2100 Structure of Engineering Materials, Questions III-Answers 1. A glass can have different properties depending upon its cooling rate. Referring to the schematic diagram of the volume-temperature relation, predict the changes of density, and refractive index of a glass with the cooling rate. Usually, glasses prepared by faster cooling rate have larger specific volume and consequently lower density, lower refractive index. (exception SiO 2 glass). 2. Calculate the lattice parameter of the rocksalt structure in terms of the cation and anion radii, r c and r a . Using the ionic radii, r Mg2+ = 0.72 Å , r O2- = 1.40 Å , calculate the lattice parameter and density of MgO.
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Unformatted text preview: In general, a = 2 r (anion) + 2r (cation) For MgO there are four Mg ions and four O ions. a = 2 r O2-+ 2r Mg2+ = 2(1.40 + 0.72)A =4.24A density = 4 (24.31+16)g/(6.02x10 23 )(4.24x10-8 cm) 3 = 161.24/45.89 = 3.51 g/cm 3 3. For MgO, draw the (111) and (110) planes, identifying atoms and empty site locations. 4. ZrO 2 takes various crystal structures. At the highest temperature, it takes cubic; at intermediate temperature, tetragonal and at the low temperature, monoclinic structure. Sketch a schematic Gibbs free energy vs. temperature for all three structures. 5. Would polymer crystals be isotropic? No. They would be anisotropic....
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