Effect of Particle Size and Pretreatment on the Conductivity of Glass Powder during Compaction

Physical properties of solid materials can be strongly modified by pressure treatment at elevated temperatures. This study focuses on the compaction-induced behavior of powdered amorphous solids using Li2Si3O7-glass as an example. Experiments were carried out on distinct fractions with particle sizes from <25 mu m to 224-250 mu m. Measurements of electrical conductivity using impedance spectroscopy were carried out in situ at pressures up to 930 MPa and at temperatures from 373 K to 667 K. Simultaneous monitoring of volume changes allows correlating conductivity and porosity of samples. To study the effect of adsorbed water on surfaces, the material was pretreated by flushing with water-bearing nitrogen before the experiment. Continuous increase of electrical conductivity upon pressurization was observed for all particle size fractions both in the brittle and in the plastic deformation regimes. The pressure derivative of DC conductivity strongly increases with grain size at low T (373 K). At high T (608-665 K) the effect is less pronounced due to the onset of welding of particles forming continuous pathways for charge transport without grain boundaries as barriers. Welding of particles occurs already at temperatures significantly below the glass transition temperature, induced by strong local forces at grain-grain contacts. No effect by pretreatment of glass powder with water vapor was observed at low temperature, while at high temperature surface modification by adsorbed water resulted in enhancement of electrical conductivity, probably caused by lowering of viscosity at grain surfaces, which facilitates welding of particles.