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Properties of glass
- Feb 23, 2018 -

Isotropic

The molecular arrangement of glass is irregular and its molecules have statistical homogeneity in space. Ideally, the physical and chemical properties (such as refractive index, hardness, modulus of elasticity, coefficient of thermal expansion, thermal conductivity, electrical conductivity, etc.) of homogeneous glasses are the same in all directions.


No fixed melting point

Because the glass is a mixture of non-crystalline, so there is no fixed melting point. Glass from the solid into a liquid is a certain temperature range (ie softening temperature range) carried out, it is different from the crystalline material, there is no fixed melting point. The softening temperature range Tg ~ T1, Tg is the transition temperature, T1 is the liquidus temperature, the corresponding viscosity is 1013.4 dPa · s, 104 ~ 6dPa · s respectively.


Metastable

Glassy material is generally obtained by the rapid cooling of the melt obtained from the molten state to the glass transition, the viscosity of the sharp increase in the cooling process, the particles too late to form a regular arrangement of crystals, did not release the latent heat of crystallization, the glassy state Material than the crystalline material contains a high internal energy, the energy between the melting state and the crystalline state, belongs to the metastable state. From a mechanistic point of view, glass is an unstable high-energy state, such as the tendency of low-energy state transformation, that is, the tendency of crystallization, so glass is a metastable solid material.


Gradient reversibility

Glassy material from the molten state to the solid state process is gradual, and its physical and chemical changes are also continuous and gradual change. This is significantly different from the crystallization process of the melt, the crystallization process will inevitably appear new phase, in the vicinity of the crystallization temperature, many of the properties will change. The glassy material from the molten state to the solid state is completed in a wide temperature range, with the temperature decreases, the viscosity of the glass melt gradually increased, and finally the formation of solid glass, but no new phase in the process of formation. Instead, the process of heating the glass into a melt is also gradual.

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