Ultrasonic viscosity measurements, containerless processing and their significance in understanding the influence on devitrification of fluoride glasses

Heavy-metal fluoride glasses have become very promising optical fibre materials due to their predicted ultra-low attenuation and near ultraviolet (UV) to mid-infrared (IR) optical transparency range. This thesis examines the devitrification process(es) of ZBLAN glass by critically evaluating the current hypotheses of the role of gravity in the devitrification process and how the devitrification might be suppressed when processed terrestrially. Neither the shear thinning nor the density segregation explanations fully account for the role of gravity in devitrification and so an alternate explanation is proposed. A piezoelectric viscometer has been designed, and constructed to measure the low temperature viscosity of ZBLAN glass. However, due to the constraints of the current equipment, viscosity measurements could not be determined explicitly, but rather these are estimated from ultrasonic velocities. The viscosity estimates suggest that the optimum fibre drawing temperature region likely resides just inside the crystallisation temperature region, but below previously estimated values. A non-operational gas film levitator (GFL) was modified for the use with fluoride glasses and evaluated with test stainless steel and flint glass preforms. (Abstract shortened by UMI.)...