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ps2soln - Problem Set#2 BENG 434/ENAS 534 Biomaterials Due...

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Problem Set #2 BENG 434/ENAS 534 Biomaterials Due: Tuesday, Sept. 28 at the beginning of class 1. Why do ceramic materials (SiO 2 for example) form glasses so readily, but metals do not? Explain focusing on the structures and bonding of the materials. Ceramic materials require a great deal of organization to go from the liquid to solid  (complex crystalline forms). Metals have more simple crystal structures. Glasses form  because of kinetic constraints. The solution cools to the point where the atoms cannot  move before the atoms are in their crystal structure. 2. How might one get a metallic glass? Metallic glasses may be formed by melt spinning. A Cu wheel (potentially water cooled)  spins very quickly and molten metal is poured on it. By controlling the spinning and  cooling of the wheel, the metal essentially flies off the wheel and cools before it can  crystallize forming a glass. 3. Solidification Theory. Solidification, a phase transformation, involves the nucleation of the solid phase followed by the growth of the solid phase. (20 pts. Total) (a) Let’s start with nucleation. We cool a beaker of water to -40 ˚C. What is the critical radius of the solid water nuclei at this temperature? What is the Gibbs Free Energy of formation of a spherical crystalline cluster of solid? Estimate the concentration of these nuclei. DATA: Interfacial energy between solid and liquid water is 25 ergs/cm 2 . The latent heat of fusion of ice is 335 joules/gram, and the density of ice is 0.92 grams/cm 3 .
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