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Unformatted text preview: FINAL Name ENSC 3313, Spring 2002 Answer anv 20 Questions oQly, 2 points each. 36 1. Actinium (Ac) is Element #89 and is naturally occurring, but was not discovered until the beginning of the 20th century. The most stable isotope is Ac227 which has a halflife of21.6 years. Please calculate the proportion of a 100 ~g sample that decays in 20 years. , A = AoetlA. In ( ~ ) = tlA 1n(O.5) = 21.6 yr/A => A = 31.2 yr. A ) = e20/31.2 Ao = 0.5268 This is the proportion remaining. Question asks for the proportion that decays, hence 1 0.5268 = ~4732. Wavelengths shorter than 225 nanometers are strongly absorbed by the valence electrons of diamond's carbon atoms, which then move to the conduction band and can produce a "response" signal. What band gap, E, (energy change LlE) does this correspond to? Answer in electron volts please. Know that Planck's constant, h = 0.662 x 1033 J.s, and leV = 0.160 X 1018 J. The velocity of light = 0.299 X 109 m/s. 2. .. ~ ~=~ A Substituting ~ = = 5.5 eV 32 fold \ I ...i".. /' ~ I \ \ . \ and explanatory comments, as necessary. 3 fold * 4 fold, through middle of opposite faces .J;: fOld * 3 fold, cube diagonals * 2 fold, through middle of opposite edges 1 fold * 1 fold elsevvhere \ \ ... ~...J... Identify the lattice plane and lattice direction sketched. 4. t 37 ~ with origin at O * Extending the 'x' axis, the intercepts are 1/3, 1/4, 1. * Taking reciprocals, we have (3 4 1 ) Direction * From start to end of line within cell, we go 1, 1/2, 1 * Multiplying by 2 to clear fractions, we have [212 ] Checking line lies in plane hu + kv + f!w = 0 3.2+4.1+1.2=0 @ How many directions of the form <341> are there in an orthorhombic unit cell (a :;t: b :;t: c; a = 13 = y = 900)? Do not count traveling along the same line oppositely! 5. . If a * b * c, then we cannot change order, only signs. [341] [341 ~ Hence, r ] rTotal 4 341 341other sign changes would involve traversing the same lines appropriately, e.g. [341] J [3411 Bronze is a solid solution alloy of tin (Sn) dissolved in copper (Cu). The tin content is quite low. If a bronze contains 3 atomic % Sn, what weight % Sn does this correspond to? The amu are Sn = 118.69 and Cu = 63.54. 6. Consider 100 atoms total; we have 97 of Cu and 3 of Sn Weight of97 Cu atoms =97(63.54)/AN AN = Avogadro's Number Weight of 3 Sn atoms = 3(118.69)/ AN w/o Sn = 3(118.69) 3(118.69)+97(63.S4)x 100 = ~ The activation energy for the diffusion of carbon in BCC iron is 122.5 kJ/mole. At what temperature will the diffusion rate be one hundredth (1/100x) that at 20C? The gas constant, R, is 8.31 J/mole.K. 7 3J D=D eQ/RT o Substituting 1 293 Solving T~ The stressstrain plot is from a cylindrical specimen of 50 mm gauge length and 12.5 'mm diameter....
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 Fall '11
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