Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 11
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Volume
Glass
Crystal
d
Liquid
Supercooled
liquid
T Tg
e
Temperature
Fig. (2.1): Variation in volume with temperature for a glassy and crystalline solid [1].
On cooling from the melt along the line ab, the volume of the liquid decreases at a steady
rate. If cooling is slow enough and nuclei are present in the melt, the liquid will
crystallise at temperature Tf, with an abrupt change in volume represented by the line bc.
When cooled further the crystalline material contracts along the line cd. However, if the
rate of cooling is high enough, crystallisation does not occur at Tf, and the volume of the
supercooled liquid decreases along be, which is a continuation of ab. At the temperature,
Tg, the glass transition temperature, the volume-temperature curve undergoes a change in
gradient and continues almost parallel to the curve of the crystalline form of the material.
Only below Tg is it correct to describe the material as a glass, and at this point the
viscosity is extremely high, ≈ 10 12 Pa.s. Between Tg and Tf (line be) the material is a
supercooled liquid. If the glass is held at temperature T, just below Tg, the volume will
decrease along the vertical arrow, until it reaches a point which is a continuation of the
Tf
b
c
a