Tellurite And Fluorotellurite Glasses For Active And Passive

8. Fibre drawing; MDO 360
It can be seen the activation energy for viscous flow generally increases as Tg is
approached, as expected.
Braglia et al. [6] showed TeO2-Na2O-ZnO glasses behave in a Newtownian manner
(i.e. viscosity-temperature behaviour independent of the applied force). This is
advantageous for fibre drawing, as the viscosity will only depend on temperature, rather
than also draw speed, as seen with fluorozirconates and chalcogenides.
8.3.3. ESEM of crystallised fibres
Fig. (8.7) shows an electron micrograph of triangular shaped crystals near the surface (10
to 20 µm) of the glass fibre of composition MOF005ii (70TeO2-10Na2O-20ZnF2 mol. %),
mounted length-ways in epoxy resin, and cross-sectioned. These crystals have grown
close to the fibre surface, and are around 2 µm in diameter. This surface crystallisation,
could be due to degradation of the fibre surface during heating in the fibre drawing tower.
The nitrogen atmosphere in the fibre drawing furnace was not passed through a liquid
nitrogen dewar for preform MOF005ii. Therefore, there could have been sufficient water
vapour present in the atmosphere to attack and degrade the surface of the fibre, and
provide sites for crystal nucleation. OH groups lower the local viscosity of the glass [26],
increasing the likelihood of atomic rearrangement, and crystallisation. Physical
volatilisation from the fibre surface, could have changed the surface composition,
destabilising the glass and causing local crystallisation. Organic contaminants (from the
internal surface of the brass mould or preform handling) also could have been present on