Saturday, 5 March 2011

Formation of Glass

There are a lot of glasses – natural and laboratory created – in addition to the silica based one that we work with. However understanding how glasses in general are created helps to understand “our own”. In general, when the liquid phase of a material is cooled below its freezing temperature it usually transforms into a crystalline solid. But some materials do not crystallise when cooled to their freezing temperatures. Instead they create a rigid network which is known as glass. It is very similar in structure to a liquid – hence super cooled liquid.

At temperatures just above their freezing points, most materials have viscosities that are similar to water at room temperature. They are so fluid that the molecules can rapidly form crystalline structures. But many inorganic silica materials form glasses on cooling because their viscosity at and above their freezing points is very high. There are also high energy bonds between the silicon and oxygen molecules. The viscosity increases very rapidly as the temperature is reduced. These prevent the flow required for crystallisation. In organic glasses, e.g. resin, crystallisation is difficult because of the long chain molecules that the material is composed of, preventing the molecules from sliding past one another, i.e., the difficult structural re-arrangement that would be required to form crystals.

Based on MIT Solid State Chemistry Notes, 7, pp.5-6