Role of water impurity in impact fracture of quartz in the vicinity of the phase transition at 573A°C

Synthetic quartz single crystals are subjected to fracture by a falling load in the temperature range from 20 to 650A degrees C (i.e., including the region of the alpha -> beta phase transition). The intensity of integrated acoustic emission (AE) generated during the impact is recorded in the frequency range from 80 kHz to 1 MHz. In the temperature range 20-300A degrees C and at temperatures above the phase transition temperature (573A degrees C), the energy distributions in temporal AE series are correctly described by the exponential function typical of random events, but at 400 and 500A degrees C, the energy distributions follow the power law typical of correlated accumulation of microcracks in heterogeneous materials. The temperature effect is explained by the presence of submicrometer inclusions of a vapor-water mixture in the material, which exist as a rule in natural and synthetic quartz single crystals. Upon heating of the material to a certain critical temperature, the internal pressure in the bubbles of liquid attains a value for which the shock wave causes cracking around a large number of uniformly distributed inclusions. As a result, a correlated improper process of accumulation of microscopic defects, which is obviously observed only in heterogeneous materials, evolves in the bulk of deformed quartz heated to 400-500A degrees C.