Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 60

The classical glass transition is the transformation of a material from a hard, brittle state into a soft, rubbery or viscous state as temperature increases. The transition is characterized by the glass transition temperature, T _g. However, T _g is defined as a transition temperature by convention and actually represents a range of temperatures. The reverse transition is called vitrification and is achieved by cooling a viscous liquid into the glassy state.

The different uses of amorphous materials are often characterized by their operating temperature relative to their glass transition. For example, oxide glasses used in windows and hard plastics, like polystyrene used in food packaging, are utilized well below their glass transition temperature, i.e. when they are in a hard phase. On the other hand, rubber elastomers, like polyisoprene used in rubber bands, are utilized above their T _g.

The glass transition is not a true phase change. It is not generally characterized by a clear change in material structure. Rather than an abrupt discontinuous change in properties, the glass transition is a smooth change over the temperature range. For the glass–liquid transition, the viscosity of the material can smoothly change with temperature.

During a heating cycle, the glass transition can also refer to a devitrification – an amorphous to crystalline transition. The T _g may represent the temperature range over which nucleation and crystal growth starts to occur from an amorphous material. The T _g is always lower than the melting temperature, T _m, of the crystalline phase. The amorphous phase in question is not necessarily a thermodynamically stable state and the history of the material is critical to predict structure and properties. For example, an amorphous state that was initially achieved by quenching a disordered state (e.g. an amorphous metal) may devitrify upon heating to the T _g, but the transition is not reversible. It will not vitrify, i.e. will not become amorphous again, upon cooling.