Pressure Dependence of Tg in Silicate Glasses From Electrical Impedance Measurements

Abstract

The pressure dependence of the glass transition temperature (T g ) has been estimated from the pressure effect on the activation energy of electric conductivity in alkaline (albite, haplogranite), alkaline earth (anorthite) silicate and SiO 2 glasses. The dc conductivity (σ) has been determined from the electrical impedance spectroscopy in the frequency range 10 5 -10 - 2 Hz. The impedance measurements have been performed in a controlled atmospheric furnace and in three types of high pressure apparatuses: piston cylinder, belt and multi-anvil presses in the pressure range 0.3-6 GPa. Below T g , the activation energy of σ is less than that at T>T g . The inflection point on the dependency of ln(σ) versus l/T defines T g . The T g in anorthite glass varies with pressure as T g =848°C+5 3°/GPa P (P is in GPa), in albite as T g =688°C-9.4°/GPaP, in haplogranitic glass as T g =777°C-45°/GPaP and in SiO 2 glass as T g =1050°C+50°/GPa P. The measured at T g dielectric relaxation times are several orders of magnitude smaller than the structural relaxation times and become slower with the increasing pressure. In Na bearing glasses, T g estimated from the electrical conductivity is a 'sodium ion mobility' T N a g, corresponding to the temperature range of the overlapped α- and β-relaxation processes and is, therefore, shifted to lower temperatures in comparison with calorimetric and dilatometric T g . The activation energy of the dielectric relaxation in anorthite increases with pressure having an activation volume of +10.5′5 cm 3 /mol, and in albite glass the activation volume is negative -6.5′2 cm 3 /mol.