Effect of microgravity on crystallization in heavy metal fluoride glasses processed on the CSAR-I sounding rocket

Gravity-driven density segregation in viscous glass is believed to trigger homogeneous nucleation during the high-temperature processing of heavy metal fluoride (HMF) glasses. Processing of HMF glasses in microgravity could, therefore, minimize commonly observed micro-crystal formation in these glasses during their heat treatment for fibre drawing. Although, preliminary experiments on parabolic flight aircraft had earlier indicated that gravity enhances and microgravity suppresses crystallization during the processing of HMF glasses, these results were considered inconclusive due to the short processing time of 20 seconds. The CSAR-I sounding rocket provided an opportunity to process HMF glasses over a longer duration of five minutes in microgravity. These experiments indicated that microgravity helps in reducing crystallization in HMF glasses during their heat treatment at 325°C, which is very close to their fibre drawing temperature range of 300–320°C.