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hw9sol - H omework#9 Solutions 1(a formers modifiers...

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3.091 Fall Term 2004 Homework #9 Solutions 1. (a) formers: SiO 2 , P 2 O 5 , B 2 O 3 , GeO 2 modifiers: CaO, MgO, Na 2 O ... intermediates: Al 2 O 3 , TiO 2 (b) V molar crystallization slow cooling fast cooling T T g, slow T g, fast T melting point (c) - how extreme the change is in atomic arrangement required upon transforming liquid into solid, i.e., complexities of both structures - cooling rate/viscosity 2. Al will not form a stable glass because atomic scale (re)ordering into an FCC structure requires only atomic jumps of short distance and thus small amounts of activation energy (available to some extent already at low temperatures). Ordering of Se (transition of Se from glassy to crystalline state) is impeded by the complexity of unit species on lattice sites (Se 8 rings) and by inadvertent chain growth (Se 8 – Se 20 ). These result in steadily increasing viscosity (with decreasing temperature) and, thus, in decreasing probability of stabilizing an ordered atomic structure. 3. Glasses are brittle because under stress they cannot plastically deform since dislocations, which are necessary for plastic deformation, are absent. Glasses are disordered and thus are inherently incapable of forming dislocations. 4. (a) The simplest way to differentiate a glass from a crystal is exposure to x-rays. In glass we encounter only scattering; in crystalline material X-rays can give rise to diffraction. (b) We can also heat up the substances. Glass will soften gradually; a crystalline material will likely melt at one particular temperature, or a solid and liquid phase will coexist over a temperature range.
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