The standard definition of a glass (or vitreous solid) is a solid formed by rapid melt quenching।[ If the cooling is sufficiently rapid (relative to the characteristic crystallization time) then crystallization is prevented and instead the disordered atomic configuration of the supercooled liquid is frozen into the solid state at the glass transition temperature Tg. Generally, the structure of a glass exists in a metastable state with respect to its crystalline form, although in certain circumstances, for example in atactic polymers, there is no crystalline analogue of the amorphous phase.[ As in other amorphous solids, the atomic structure of a glass lacks any long range translational periodicity. However, due to chemical bonding characteristics glasses do possess a high degree of short-range order with respect to local atomic polyhedra.[ It is deemed that the bonding structure of glasses, although disordered, has the same symmetry signature (Hausdorff-Besicovitch dimensionality) as for crystalline materials.[
Glass versus a supercooled लिकुइड
Glass is generally classed as an amorphous solid rather than a liquid.[ Glass displays all the mechanical properties of a solid. The notion that glass flows to an appreciable extent over extended periods of time is not supported by empirical research or theoretical analysis (see viscosity of amorphous materials). From a more commonsense point of view, glass should be considered a solid since it is rigid according to everyday experience.[
Some people consider glass to be a liquid due to its lack of a first-order phase transition[ where certain thermodynamic variables such as volume, entropy and enthalpy are discontinuous through the glass transition range. However, the glass transition may be described as analogous to a second-order phase transition where the intensive thermodynamic variables such as the thermal expansivity and heat capacity are continuous.[ . Despite this, the equilibrium theory of phase transformations in solids does not entirely hold for glass, and hence the glass transition cannot be classed as one of the classical equilibrium phase transformations in solids.[
Although the atomic structure of glass shares characteristics of the structure in a supercooled liquid, glass tends to behave as a solid below its glass transition temperature। A supercooled liquid behaves as a liquid, but it is below the freezing point of the material, and will crystallize almost instantly if a crystal is added as a core. The change in heat capacity at a glass transition and a melting transition of comparable materials are typically of the same order of magnitude, indicating that the change in active degrees of freedom is comparable as well. Both in a glass and in a crystal it is mostly only the vibrational degrees of freedom that remain active, whereas rotational and translational motion is arrested. This helps to explain why both crystalline and non-crystalline solids exhibit rigidity on most experimental time scales.
