Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 18
tendency to form a glass. For a binary system, glass formation is likely for ∆Fs > 1.33 [2].
Again, this theory can usefully categorise glass forming ability in conventional systems,
but not universally.
2.2.3. Kinetic theory of glass formation
Glass formation has been shown in materials of a wide variety of compositional, bonding,
and structural types. Therefore, considering how rapidly a vapour or liquid must be
cooled to avoid a detectable volume fraction of crystallisation (10 -6 0.0001 %) can be
a useful way of characterising its glass forming ability [2]. If nucleation frequencies, i (in
s -1 ), and growth rates, u (in cm.s -1 ), are known as functions of temperature, equation (2.2)
can be used to plot a time-temperature-transformation (TTT) diagram.
π
3
−6
3 4
10 = iu t
(2.2)
where it is the frequency of nucleation with time, and u 3 t 3 the growth in three dimensions
with time [2]. However, it is important to note that current theories of homogeneous
nucleation in glasses are not able to predict accurately the observed homogeneous rates,
and can be many orders of magnitude disparate [7]. From the measured TTT plot, it is
possible to obtain the time at each temperature before a significant fraction of the
undercooled melt has devitrified. These plots have a ‘nose’ shape with the temperature at
the apex of the nose where crystallisation is most rapid. This nose shape arises, as the
tendency for crystallisation will be initially enhanced thermodynamically on melt