2: Principles of Glass Formation

Virtually any material can be formed as a glass under the proper experimental conditions. Kinetic theories describing nucleation and crystal growth rates allow the prediction of these conditions. Examination of the results of kinetic studies allows more general statements regarding the effects of crystal structures and the structures of the melts derived from these crystals, on the ease of formation of glasses from melts. These general trends correlate quite well with earlier theories of glass formation, which were based on structural and bonding considerations. In particular, the temperature-dependence of the viscosity is shown to be a major factor in determining the glass forming ability of a given melt. The use of TTT curves and the concept of critical cooling rate provide direct experimental methods for comparisons of glass forming ability among various materials.