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Viscosities of phase-separated glasses Bernheim, Philippe

Abstract

The effects of heat treatments on viscosity were analysed for three types of glasses. "Pyrex" glass could be approximated to a Newtonian liquid in the range of temperature 470 to 590°C. The activation energy for viscous flow varied from 65 to 85 kcal/mole according to the previous thermal history. All glasses exhibited phase separation to different degrees according to the heat treatments to which they had been submitted. Phase separation and different degrees of devitrification could account for the variation in viscosity which have been encountered. Several mathematical expressions were tested in an attempt to correlate viscosity change with time. The best fit was obtained with a relationship of the type n = (a + bt)[superscript]c where c varied in the range 0.2 to 0.5. In the case of a ternary borosilicate glass, the value was found to be 0.5. This may suggest that phase separation is a diffusion controlled process.

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