The molecular arrangement of glass is irregular and its molecules are statistically uniform in space.In an ideal state, the physical and chemical properties of homogeneous glass (such as refractive index, hardness, elastic modulus, coefficient of thermal expansion, thermal conductivity, electrical conductivity, etc.) are the same in all directions.
No fixed melting point
Because glass is a mixture, amorphous, there is no fixed boiling point.The transition of glass from solid to liquid takes place within a certain temperature range (i.e., softening temperature range). Unlike crystalline materials, it has no fixed melting point.The softening temperature ranges from Tg to T1, where Tg is the transition temperature and T1 is the liquidus temperature. The corresponding viscosity is 1013.4 dPa· S and 104~6dPa·s, respectively.
The stability of
Glassy substance is usually by the molten body fast cooling and get, from molten glass state transition, the cooling process of viscosity increases sharply, too late to do regular arrangement of particles and form crystal, no release of latent heat of crystallization, therefore, glassy state substance than amorphous contained more internal energy, the energy between molten and amorphous, belongs to the metastable state.From the point of view of mechanics, glass is a kind of unstable high-energy state. For example, there is a tendency of low-energy state transformation, that is, there is a tendency of crystal disintegration. Therefore, glass is a metastable solid material.
The process of the glassy substance from the molten state to the solid state is gradual, and the change of its physical and chemical properties is continuous and gradual.This is obviously different from the crystallization process of the melt, in which new phases are bound to appear and many properties will change around the crystallization temperature point.The transition from the molten state to the solid state is completed within a wide temperature range. As the temperature gradually decreases, the glass melt viscosity gradually increases and finally forms solid glass, but no new phase is formed in the process.In contrast, the process of heating glass to melt is gradual.