Crystallization Behavior Of SiO₂-Al₂O₃-MgO-F Glass Ceramics

In this paper, the effects of glass compositions, heat-treatment processes on the crystallization behavior and properties in the SiO2-Al2O3-MgO-F glass system have been investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calorimeter (DSC) techniques.The typical glass was identified by differential scanning calorimeter (DSC) that the nucleation temperature of the glass is670℃and the crystallization temperature of950℃. There was no crystals precipitated only swelling in the nucleation temperature for1h. But the types, contents and morphology of crystals were significantly affected by crystallization temperature. With the increase of crystallization temperature, the content of mullite and kornerupine increased firstly and then decreased, and the content of mica crystals had been increasing. Increasing the crystallization temperature was availing to the mica precipitation. At the crystallization temperature of730℃, there was only ZrO2precipitated, while at the crystallization temperature of790℃, the droplet-like phase separations in base glass were observed, which promoted the precipitation of flower-shaped crystals and rod-like crystals. At the crystallization temperature of1030℃, the mica crystals were becoming orientated.mainly played the role as an intermediate network, but a small amount of was also used as nucleation. The crystals precipitated in the base glass were mainly mica, ZrO2, mullite and a small amount of kornerupine. Mullite was disappeared when the contents of Fe2O3additions were above0.5mass%; The contents of mica phase were increased and the crystal sizes of mica were become larger with the increase of the Fe2O3content. However, as the contents of Fe2O3additions were above0.8mass%, the amount of mica crystals were decreased and the sizes were become smaller.The crystallization behaviour of glass particals was investigated in the sintering processes. The results show that mica, KFeSi2O6and mullite were mainly precipitated in the glass. At the nucleation temperature, prolonged the swelling time is helpful for increasing the number of nucleus, but the crystals were no aligned. When imposed30MPa pressure at nucleation temperature670℃, the crystals appeared orientation. It was appeared that bulk crystallization occured with the increasing of the crystallization time. The surface crystallization was dominated in sintering processes, which always caused inter-granular fracture. The preferred growth of crystallographic planes was not always come into being.The morphology and size of mica crystals were affected by the particle size. When the particle size was less than16.45μm, the morphology of mica crystals changed from flake to spherical with the decreasing of particle size, and the crystal size of spherical-like mica was0.2μm. In different pressure directions, there was no crystallizating orientation occured, and the crystals were distributed uniformly in the residual glasses.In addition, the microstructure of mica glass-ceramics by hot-pressing was investigated. Mica and mullite were mainly crystals phase before and after hot-pressing, and the type of crystals did not vary with the temperature of heat-pressing. With the increasing of the heat pressing temperature, the content of mica and mullite were decreased, while the hardness of the material increased. The hardness of the materials were significantly influenced by the crystal orientation, and the maxmium gap of micro-hardness of the orientated marerials in different directions was up to0.5GPa.