Melting - Melting of Amorphous Solids (glasses)

Melting of Amorphous Solids (glasses)

Glasses are amorphous solids (e.g. amorphous materials that are at temperatures below the glass transition temperature) which are usually fabricated when the viscous molten material cools very rapidly to below its glass transition temperature, without sufficient time for a regular crystal lattice to form. Whether a material is liquid or solid depends primarily on the connectivity between its elementary building blocks so that solids are characterised by a high degree of connectivity where as fluids occur at lower connectivity of the 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_g dependent on quasi-equilibrium thermodynamic parameters of bonds e.g. on enthalpy (H_d) and entropy (S_d) of formation of bonds in a given system at given conditions:

where f_c is the percolation threshold and R is the universal gas constant. Although H_d and S_d 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.