Effects of hydration, annealing, and melting on heat transport properties of fused quartz and fused silica from laser-flash analysis

Abstract Thermal diffusivity (D) at high temperature (T) was measured from 15 samples of commercial SiO2 glasses (types I, II, and III with varying hydroxyl contents) using laser-flash analysis (LFA) to isolate vibrational transport, in order to determine effects of impurities, annealing, and melting. As T increases, Dglass decreases, approaching a constant (~ 0.69 mm2s− 1) above ~ 700 K. From ~ 1000 K to the glass transition, the slope of D is small but variable. Increases of D with T of up to 6% correlate with either low water and/or low fictive temperature and are attributed to removal of strain and defects during annealing. Upon crossing the glass transition, D substantially decreases to 0.46 mm2s− 1 for the anhydrous melt. Hydration decreases Dglass, makes the glass transition occur over wider temperature intervals and at lower T, and promotes nucleation of cristobalite from supercooled melt. Due to the importance of thermal history, a spread in D of about 5% occurs for any given chemical type. Combining prior steady-state, cryogenic data with our average results on type I glass provides thermal conductivity (klat = ρCPD) for type I: klat increases from ~ 0 K, becoming nearly constant above 1500 K, and drops by ~ 30% at Tg. We find that D− 1(T) correlates with thermal expansivity times temperature from ~ 0 K to melting due to both properties arising from anharmonicity.