information:filtering
Glasses

Glasses are amorphous solids, which are usually fabricated when the molten material cools very rapidly to below its glass transition temperature, without sufficient time for a regular crystal lattice to form. Solids are characterised by a high degree of connectivity between their molecules, and fluids have lower connectivity of their structural blocks. Melting of a solid material can also be considered as a percolation via broken connections between particles e.g. connecting bonds. In this approach melting of an amorphous material occurs, when the broken bonds form a percolation cluster with T dependent on quasi-equilibrium thermodynamic parameters of bonds e.g. on enthalpy (H) and entropy (S) of formation of bonds in a given system at given conditions:

where f is the percolation threshold and R is the universal gas constant.

Although H and S are not true equilibrium thermodynamic parameters and can depend on the cooling rate of a melt, they can be found from available experimental data on viscosity of amorphous materials.

Even below its melting point, quasi-liquid films can be observed on crystalline surfaces. The thickness of the film is temperature-dependent. This effect is common for all crystalline materials. This pre-melting shows its effects in e.g. frost heave, the growth of snowflakes, and, taking grain boundary interfaces into account, maybe even in the movement of glaciers.