Rapid cooling: When the liquid is
cooled rapidly there is no abrupt change in the specific volume at any temperature,
instead the slope of the specific volume-temperature curve changes continuously
over a range of temperature.

At low temperatures the rate of
change of volume with temperature (i.e. the thermal expansivity ? = V-1dV/dt)
is similar for both the glass and the crystalline material, but the absolute
magnitude of the specific volume of the glass is much larger and it also varies
with the rate at which the original liquid was cooled. The transition from the
liquid to the glassy state takes place over a range of temperature 6.

Liquidus and solidus: We can
identify two specific temperatures (at a given pressure) which would be the
characteristic of the composition of the mixture: the liquidus temperature,
above which no crystals exist and the
solidus temperature, below which no liquid exists.

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Glass transition
temperature: The crystallization process is characterized
by an abrupt change in volume at the melting temperature, Tm,
whereas, glass formation is characterized by a gradual change in slope of specific
heat or volume changes. ‘Glass transition temperature’, Tg is the
region over which the change of slope occurs. It is a continuous transition to
the glassy state and is not well defined. Thermal history affects the glass
transition temperature.

e)       Devitrification: If a glass is held at
a temperature in the transformation range and below the liquidus temperature
for a very long time, crystals will begin to grow. The glass is said to
devitrify 2.

f)        Fictive Temperature: The temperature
in the transformation range at which the structure of a given piece of glass
would be stable is called the fictive temperature. The higher the fictive
temperature the greater the difference between the physical properties of the
sample of glass and crystalline form of the material. Thus a general concept of
“glass” relates to systems, which have some degree of freedom that:

fluctuates at a rate, which depends strongly
on temperature and pressure, and

becomes so slow at low T or high P that the
fluctuations become frozen. At this point, properties determined by the slow
degree of freedom change value more or less abruptly, giving the “glass

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