Measurements of transient electric current associated with indentation fracture and deformation of brittle solids

In order to understand the fundamental behaviour of transient electrical activities associated with localized fracture and deformation of brittle solids, an electrically conductive diamond indenter, which is connected to a high-sensitivity charge detector, was indented on various brittle solids such as Al2O3 (sapphire), CaF2, LiF, MgO, NaCl, NaF, NaCl, SiO2 (synthetic quartz and natural quartz) and TiO2 at temperatures up to 700 degrees C under an ambient atmospheric condition. As a result, a transient electric charge (TEC) signal was detected during loading half-cycles when fracture and deformation occurred around the indent. It was found that the behaviour of the TEC signal from as-received crystals is characterized by two different critical temperatures: that is firstly, below the critical temperature T-c(f) of less than 0.3 T-m, where T-m is the melting temperature of solids, a fractoelectron emission current was detected, the intensity of which decreased as the temperature decreased and secondly, above the critical temperature T-c(l) of 0.6 T-m, an ionic conduction current was detected related to the thermal activation process concerned. No signal was detected at a temperature between T-c(f) and T-c(i). Further, thermally stimulated exoelectron emission current was also detected in crystals, coloured by the irradiation of a high-energy ray. This paper discusses the origin of these characteristic phenomena.