Effect of microgravity on optical degradation in heavy metal fluoride glasses processed on the CSAR-II sounding rocket

Abstract

The optical properties of heavy metal fluoride (HMF) glasses degrade during the fibre drawing process due to undesired micro-crystal formation. Since gravity-driven density segregation is believed to be a major cause for homogeneous nucleation in viscous glasses during their ground-based processing, use of microgravity during such processes should minimize micro-crystal formation and resulting optical degradation. Our research on HMF glasses has been aimed at seeking experimental evidence to support this hypothesis. Earlier results from our experiments on the T-33 aircraft and the CSAR-I sounding rocket had indicated that microgravity helps in reducing crystallization in HMF glasses during their heat treatment at temperatures above 325°C. This temperature is very close to the fibre drawing temperature range of 300–320°C in these glasses, and implies that crystal nucleation and optical degradation may occur during fibre drawing. Therefore, the CSAR-II sounding rocket experiments were conducted on HMF glass samples at processing temperatures within the fibre drawing region. The results of CSAR-II sounding rocket experiments are in tune with the earlier findings and indicate that microgravity helps in reducing optical degradation in HMF glasses during their processing at temperatures in the fibre drawing region.