Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 10
one of these liquids is held at a temperature just below the freezing point, it will slowly
crystallise, as the crystalline form is thermodynamically more stable than the liquid state.
However, if the liquid is continuously cooled from above the freezing point,
crystallisation may, or may not occur, depending on the rate of cooling 1 . If the rate of
cooling is slow, the material will more than likely crystallise to some extent, but at high
cooling rates, it is possible to avoid crystallisation altogether. During this cooling
process, the viscosity of the liquid increases to such a degree that atomic rearrangement
becomes difficult and the material has become a solid without long-range periodic order,
but with a structure closer to the liquid state. This metastable solid is called a glass. This
phenomena is not limited to inorganic materials, there are many examples of organic
glasses, and glasses do not necessarily have to be formed from rapid melt cooling; sol-gel
glasses are prepared using organic-precursors and heat treated at conditions close to
ambient. The relatively recent achievement of cooling rates of the order of 10 6 K.sec. -1
and greater has resulted in the formation of low volume wires or ribbons of glasses
formed from very fluid metal melts, i.e. metallic glasses [2].
A better appreciation of the relationship between the liquid, crystalline and glassy
forms of a material from melt-cooling can be gained by considering the volume-
temperature relationship of a glass-forming substance as shown in fig. (2.1) [1].
1 It is possible to form glass relatively easily from some systems, such as fluorozirconates (e.g. ZBLAN),
which have viscosities of the order of water (≈ 10 -2 Pa.s) at their melting points; this phenomenon will be
addressed in section 2.2.3, on the kinetic theory of glass formation.