Suppression effects of cooling rate on crystallization in ZBLAN glass

Abstract

ZBLAN glass is a heavy metal fluoride glass that shows great potential in the application of long-haul telecommunication cables. However, during processing in the fiber-drawing temperature region, the material tends to undergo heavy devitrification, resulting in a crystalline fiber that is not usable as an optical waveguide. In this study, ZBLAN glass was processed with different cooling rates to discern how the rate of cooling affects the process of crystallization. Rapidly cooled ZBLAN glass test samples were then analysed with a wide range of spectroscopy and imaging techniques including EDS, SEM, TEM and XRD. These techniques revealed there are two predominant crystal phases, one rich in zirconium and barium, the other in zirconium and sodium. Ultimately, a critical cooling rate was established to be somewhere between 900°C/min and 4000°C/min, with a theoretically predicted value of 1081°C/min for a test sample volume size of 9.4×10−8m3. Cooling above this critical cooling rate yielded fully amorphous ZBLAN test samples that were completely free of nano-crystalline inclusions. Using Kramers-Kronig relations, an attenuation loss for the fully amorphous ZBLAN test samples was estimated to be 0.09dB/Km at a wavelength of 1800nm.