Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 97
access. This results in a time delay in the onset of transformation of these large particles
[3]. The heat released from transformations within more massive particles would also be
more insulated from escape, further delaying detection of the transformation. The latent
heat of the transformation would then tend to increase the temperature of the large
particle than small particles. The greater temperature rise accelerates the reaction rate
resulting in early termination of the reaction in large particles. Therefore, for a fixed
overall sample mass, samples composed of one large piece (as opposed to powder) will
tend to transform initially more sluggishly, but ultimately more quickly for irreversible
transformations such as crystallisation [3]. The distance heat penetrates a particle, x, is
proportional to the square root of the product of thermal diffusivity, κd (κd=ktc/(Cpρ)
where ktc = thermal conductivity, Cp = heat capacity and ρ = density) and time (x=√(κdt)).
The effect of heating rate on sample transitions such as melting in the DTA is
significant. The onset of a transformation occurs at a higher temperature with increasing
heating rate due to a lag from the sample interior compared to the thermocouple [3]. Peak
intensity also increases with increasing heating rate as the reference temperature increase
is more rapid (therefore ∆T larger) whilst the sample attempts to remain at melting
temperature. Melting also exhibits a broader temperature range for increasing heating
rates since less time is allowed per degree of temperature increase for the reaction to
proceed [3].
However, low heating rates more accurately represent the onset temperature of a
transformation and reduce the accelerating effects of ‘self-feeding’ reactions. Also,
transformations which are very close in temperature-range may be more clearly resolved
with slower rates [3]. Ultimately, to obtain true thermodynamic data, experiments should