Effect of gravity on crystallization in heavy metal fluoride glasses processed on the T-33 parabolic flight aircraft

Heavy metal fluoride (HMF) glasses are prone to micro-crystal formation and resulting optical degradation during their high-temperature processing for fibre drawing. It is believed that the absence of gravity-driven density segregation in microgravity can reduce this undesired micro-crystallization during the processing of HMF glasses. Experiments were conducted on the T-33 parabolic flight aircraft in microgravity and under 2-g acceleration to study the effect of gravity on crystallization in HMF glasses. These preliminary experiments indicated that gravity enhances crystallization in HMF glasses during their processing at 370–400°C. However, these results were not considered conclusive due to the short duration of 20 seconds available on the parabolic flight aircraft. Subsequent ground-based experiments were conducted on the T-33 payload using statistical design of experiments to simultaneously study the effect of glass composition, processing temperature, processing time and mode of heating (continuous or pulsed). These experiments indicated that a continuous processing time of over two minutes at crystallization temperatures is required to observe a statistically significant amount of crystallization in HMF glasses. This established the need for longer duration experiments on the sounding rocket, space shuttle or space station.