Dehydroxylation mechanisms of polyphosphate glasses in relation to temperature and pressure

Abstract

The mechanisms of dehydroxylation of some polyphosphate glasses have been studied between ambient temperature and 1000°C while the residual pressure of water vapor above the sample was controlled. The results indicate that devitrification of the glass affects the water release. Only one mechanism takes place both under atmospheric pressure with an inert flowing gas and under a residual water pressure of 5 mbar. Until the fixed residual water vapor pressure is larger than 10 −3 mbar, only one mechanism is present. But when the residual pressure of water vapor is decreased to a value of 10 −3 mbar, several mechanisms occur for both a Graham's salt and a sodiumcalcium polyphosphate glass. We observe water release governed by a mechanism of nucleation-growth of nuclei. We have determined the activation energy of this reaction and proposed mechanisms for the dehydroxylation of polyphosphate glasses.