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Unformatted text preview: 3.091 Fall Term 2009 Test #3 Monday, November 23, 2009 11:05 a.m. — 11:55 a.m. Name: solution outlines Recitation Instructor: Donald R. Sadoway A complete test consists of 6 questions. Write your answers on these pages. State your assumptions and show calculations that support your conclusions. RESOURCES PERMITTED: PERIODIC TABLE OF THE ELEMENTS, TABLE OF CONSTANTS, AN AID SHEET (ONE PAGE 8 ¡ " ¢ 11"), AND A CALCULATOR. NO BOOKS OR OTHER NOTES ALLOWED. USE OF WIRELESS COMMUNICATIONS DEVICES STRICTLY FORBIDDEN. #1 11% #2 17% #3 18% #4 19% #5 13% #6 22% TOTAL 100% 3.091 Fall Term 2009 Test #3 page 2 Problem #1 (11%) Silver bromide (AgBr) has rock salt crystal structure, i.e., FCC Bravais lattice with the ion pair, Ag + and Br ¡ as basis. The dominant defect in AgBr is the Frenkel disorder. (a) Does the Frenkel disorder in AgBr create vacancies of Ag + , vacancies of Br ¡ , or both? Explain. The ionic radii are 0.67 Å for Ag + and 1.96 Å for Br ¡ . (4%) - expect the smaller ion to form interstitials and create vacancies - here, Ag + , is the smaller ion, so expect Ag + interstitials and vacancies on the Ag + sublattice (b) Calculate the temperature at which the fraction of Frenkel defects in a crystal of AgBr exceeds 1 part per billion = 1 ppb = 10 –9 . The enthalpy of Frenkel defect formation, ¢ H F , has a value of 1.16 eV / defect, and the entropic prefactor, A , has a value of 3.091. (7%) Problem #2 (17%) (a) For a given alloy composition, explain why the yield strength of the amorphous form (metallic glass) is greater than that of the crystalline form. - in metals, plastic deformation occurs by slip. This involves closest-packed planes moving relative to one another along closest-packed directions (5%)- in metallic glasses, owing to the lack of long-range order, there are no slip systems operative - hence, the threshold for slip is higher in metallic glasses than in crystals of the same...
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- Spring '09
- Chemistry, Chemical reaction, Void, benzoyl peroxide