L30 - SUMMARY FROM LAST CLASS Glass formation The crystal...

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Dr. P. Lucas U of A MSE 110 Glass formation The crystal growth rate U depends on the ability of the atoms to diffuse to the surface of the nuclei and therefore decreases with increasing viscosity η and decreasing temperature T. The nucleation rate I increases with G cryst and decreases when viscosity is too high at low T. Crystallization takes place in the critical region where the U and I curve overlap and both I and U are sizeable. The relative overlap between the I and U curves defines the ability of a material to crystallize. Metals are very hard to vitrify as I and U largely overlap, while SiO 2 is very hard to crystallize as I and U barely overlap. Glasses are formed by quenching the liquid through the critical region The glass can be viewed as a frozen liquid or a liquid with infinite viscosity. Time-Temperature-Transition curves give the amount of time necessary to crystallize a sample by a certain % at a given temperature. SUMMARY FROM LAST CLASS
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Dr. P. Lucas U of A MSE 110 Relaxation time GLASS TRANSITION When a liquid is cooled down, the thermal energy of atoms decreases and the attractive forces bring the atoms closer together, hence the volume decreases with temperature. T 1 >T 2 , V 1 >V 2 . When the temperature is decreased from T 1 to T 2 , the atoms rearrange into a smaller volume configuration. The rearrangement from V 1 to V 2 takes an amount of time τ . τ is the relaxation time .
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Dr. P. Lucas U of A MSE 110 Relaxation time GLASS TRANSITION
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