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# MT1solutions - MIDTERM I SOLUTIONS MASC 310 Closed book...

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1 MIDTERM I - SOLUTIONS MASC 310 September 25, 2008 Closed book, closed notes Scientific calculators allowed __________________________ Name

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2 Useful Data and Formulas % ionic character () ( ) [ ] { } 100 25 . 0 exp 1 2 × = B A X X APF=( volume of atoms in a unit cell)/(total unit cell volume) ρ = nA V c N A = n ' A C + A A V C N A n λ = 2 d hkl sin θ d hkl = a h 2 + k 2 + l 2 M n = x i M i M w = w i M i DP = M n m
3 1.. Atomic bonding. a. Name three material properties that can be inferred from the shape of the bond-energy and bond-force curves. b. Explain how the bond energy curve might differ for two materials & one with a positive coefficient of thermal expansion, and one with a negative coefficient of thermal expansion. Sketch curves for both materials. c. Both SiC and GaAs exhibit the zinc blende structure. Determine the % ionic character for each substance. a. Melting point, elastic modulus, and coefficient of thermal expansion b. Bond energy curves for most materials are typically asymmetric, with steeper ascents on the LHS (since the repulsive energy between atom cores scales with 1/ r n , while the attractive energy scales with 1/ r ). Thus, the average atomic separation distance increases with energy (temperature), as indicated by the dashed line in the plot on the left. This results in a positive CTE. For a material to have a negative CTE, the bond energy curve must be asymmetric also, but the steeper ascent must be on the RHS, as indicated by the plot on the right. This is rare, but not impossible.

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MT1solutions - MIDTERM I SOLUTIONS MASC 310 Closed book...

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