2.2材料概论-玻璃双语.ppt

* Properties of Glass The glass transition The glass transition is the region of temperatures where the properties of a cooled liquid continuously change from being “liquid-like” to “solid-like” Structure is continuous with the liquid state Properties are continuous with the liquid state Yet, the Glass Transition occurs over a relatively narrow range of temperatures and can be a “sharp” transition in some cases * The glass transition The glass transition results from the slowing down of the internal relaxation time necessary to achieve equilibrium properties as the temperature is lowered Rapidly rising viscosity and the associated viscosity relaxation time are at root of the slowing down of the liquids’ response to temperature change Most liquids have a weakly temperature dependent viscosity Glass forming liquids have a strongly temperature dependent viscosity What is the viscosity? Elastic solids deform elastically when stressed They relax back to their original state when stress is released Elastic modulus is the proportionality constant s = Ee Liquids have no restoring force They have no Elastic modulus It does require force to move the liquid though Rate of strain, Stress = viscosity ? strain rate * Area A Force, F Viscosity: Shear stress required to produce fixed strain rate: The higher the viscosity The higher the required stress Velocity * Temperature dependence of viscosity Soda lime glass as a common example Viscosity decreases rapidly above the annealing point, Tg Decreases less rapidly at higher temperatures * Phase separation (分相): Nucleation (成核) and Growth Spinodal Decomposition (螺旋线分解) * Why is phase separation so important ? Materials properties change and become non-uniform Optical properties, transparency, are dramatically degraded Electrical properties can change from insulating to conducting Appearance can change from clear and colorless to dark and absorbing Processing can become difficult due to second high temperature liquid ph