We
often treat glass as a simple material. However it is a very complex
and as yet not fully understood material. One of the most curious
aspects is the transition between plastic and solid states. This is
the temperature range of glass annealing – called the glass
transition by scientists. This note comes largely from "Glass Properties" produced by Schott. The text in brackets [ ] is my additional
explanation.
The
glass transition comprises a smooth but very large increase in the
viscosity of the material. Despite the massive change in the
physical properties of a material through its glass transition, the
transition is not itself a phase transition of any kind [in this case from a liquid to a solid] and involves
discontinuities in thermodynamic and dynamic properties such as
volume, energy, and viscosity.
Below
the transition temperature range, the glassy structure does not relax
in accordance with the cooling rate used. The expansion coefficient
for the glassy state is roughly equivalent to that of the crystalline
solid. [Thus the CoE, which is taken as an average of expansion per
degree Celsius over the range of 0C to 300C, is an inadequate guide
to how the glass will behave at the glass transition and higher
temperatures.]
Glass
is believed to exist in a kinetically locked state, and its entropy,
density, and so on, depend on the thermal history. Therefore, the
glass transition is primarily a dynamic phenomenon. Time and
temperature are interchangeable quantities (to some extent) when
dealing with glasses.
[Viscosity
shows a relatively regular change with temperature changes.] In
contrast to viscosity, the thermal expansion, heat capacity,
shear modulus, and many other properties of inorganic glasses show a relatively sudden change at the glass transition temperature.
Any such step or kink can be used to define Tg
[the transition phase of glass]. To make this definition reproducible,
the cooling or heating rate must be specified.
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