SURFACE AND GRAIN-BOUNDARY AMORPHIZATION - THERMODYNAMIC MELTING OF COESITE BELOW THE GLASS TRANSITION TEMPERATURE

摘要

Coesite, a high-pressure SiO2 polymorph, becomes amorphous during isothermal annealing below the glass transition temperature, T-g, at one-bar pressure. Transmission electron microscopy was used to examine this fusion process (vitrification) below T-g. The transformation is dominated by a heterogeneous nucleation-and-growth controlled process above the thermodynamic melting temperature, T-m (875 K), but below T-g (1480 K). Amorphous domains nucleate at free surfaces and grain boundaries, and the amorphous-crystalline interface propagates into the interior of the crystal: This ''interface-mediated'' amorphization (vitrification) is the same as ''interface-mediated'' melting, based on the thermodynamic, microstructural,and mechanistic aspects of the transformation. This amorphization process parallels electron-irradiation and pressure-induced amorphization in coesite. [References: 54]