eo1mtm - (a Label the two-phase fields in the diagram(b Let...

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Engineering 45 The Structure and Properties of Materials Midterm Exam March 21, 2001 Problem 1: (a) Describe the diamond cubic structure and explain why it is a natural structure for group IV semiconductors and insulators (e.g., C, Si, Ge). (b) If a small concentration of P substitutes for Si in the diamond cubic lattice the conductivity increases significantly. Why? (c) If diamond transforms to graphite its conductivity increases significantly (in fact, it becomes a metallic conductor). Why? Problem 2: (a) What is meant by the phrase “cross-linking” in a polymer? (b) Good elastomers, like vulcanized rubber, are cross-linked. Why? (c) Thermoplastic polymers are not cross-linked (or very lightly cross-linked). Why? Problem 3: A binary system of atoms A and B has the complex phase diagram drawn below.
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Unformatted text preview: (a) Label the two-phase fields in the diagram. (b) Let a liquid with the composition indicated by the vertical line on the phase diagram be cooled from the liquid slowly enough to preserve equilibrium. Describe the evolution of the system as it is cooled. Page 1 Engineering 45: Spring, 2000 Midterm Exam (c) Suppose that you are given the assignment of purifying a B-rich solution until it is almost pure B. How would you proceed? Problem 4: (a) Silica glass is a good thermal insulator, while Ag is not (have you ever drunk chilled wine from a silver goblet?). Why? (b) BCC sodium has a higher Debye temperature than HCP sodium. Which of the two phases is likely to be preferred at high temperature? Why? Page 2...
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eo1mtm - (a Label the two-phase fields in the diagram(b Let...

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