Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 12
supercooled liquid contraction line, be. Other properties of the glass also change with
time in the region of the glass-transition temperature, a process known as ‘stabilisation’.
Above Tg, no such time-dependent changes are observed as the supercooled liquid cannot
reach a more stable state without crystallising. A glass on the other hand, at a temperature
well below Tg could theoretically achieve a more stable (glassy) state, given long enough.
Due to these stabilisation effects, the properties of glasses depend to a certain degree on
the rate of cooling, particularly in the temperature range near Tg. The exact value of Tg
depends on the rate of cooling (being higher, the higher the rate of cooling used) [1].
2.2. The structure of glass
2.2.1. Atomic structure
The discontinuity on cooling which occurs at Tg (see point e on fig. (2.1)) is related to the
failure of the material to adjust itself to the changing temperature, i.e. crystallise.
Therefore, it would be expected to find a similar structure between the liquid and glassy
forms of a material. X-ray diffraction (XRD) patterns of the glassy and liquid states show
broad diffuse diffraction rings (or an amorphous ‘halo’) at approximately the same
positions. This is in contrast to the sharply defined rings characteristic of the crystalline
form. This is due to the lack of long range periodic order in the glass and liquid, as
opposed to the systematic repetition over (relatively) long distances of atoms in unit cells
found in crystals. Crystalline materials, when finely divided also show broad diffuse
XRD patterns, which led Randall et al. [3] to believe glasses consisted of a random