Tellurite And Fluorotellurite Glasses For Active And Passive

4. Thermal properties and glass stability; MDO 82
4. Thermal properties and glass stability
Characteristic temperatures – differential thermal analysis (DTA) and differential
scanning calorimetry (DSC)
Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC) are
techniques which can be used to determine temperatures at which changes of phase or
state (e.g. glass ↔ supercooled liquid, around the glass transition temperature, Tg) occur
in a material. These techniques are particularly important to the glass technologist, as key
characteristic temperatures can be accurately obtained, such as Tg, the onset of
crystallisation (Tx), the liquidus (Tl, the lowest temperature at normal pressure at which
the system is entirely in the liquid state) and the solidus (Ts, the highest temperature at
normal pressure at which the system is entirely in the solid state). From this information
the Tx–Tg gap can be calculated, which is a rough estimate of glass stability on reheating,
and an important consideration for fibredrawing, as this is the temperature region of
greatest stability against crystallisation. For fibredrawing, a low enough viscosity (≈ 10 4.5
Pa.s) must be exhibited in this temperature region. Therefore, this gap should be
maximised so the fibredrawing viscosity occurs as far below Tx as possible, as even a
small degree of crystallinity in the glass will result in optical absorption and scattering
losses over long optical pathlengths. However, for a glass to have a fibredrawing
viscosity as far below Tx as possible, and a large Tx-Tg gap, would usually require a
shallow viscosity-temperature relationship, i.e. a non-fragile glass-forming liquid [1, 2].