Real Materials

· The diagrams opposite show how the volume of a glass-forming material depends upon the rate of cooling.
· The crystal that is formed at low cooling rates shows a distinct melting point, T_mp, whereas the glass formed during rapid cooling does not.
· At a temperature below the melting temperature of the crystal, the slope of the volume versus temperature curve of the glass shows a change in slope. This transition is the glass transition at temperature, T_g.
· The glass transition temperature for a given material depends upon the cooling rate, as shown in the lower diagram.

From: Kingery, Bowen, & Uhlmann, "Introduction to Ceramics," Wiley (1976)