Formation of glass-ceramic materials based on pyroxene solid solution—fluorapatite phases and their thermal expansion properties

Abstract

The crystallization characteristics of some calcia–magnesia–alumino–silicate glasses containing P 2O 5 and CaF 2 have been investigated by differential thermal analysis (DTA), X-ray analysis (XRD) and scanning electron microscope (SEM). Pyroxene solid solution and fluorapatite were the main phases crystallized by the controlled heat treatment of the glasses. The crystallization tendency of glasses was improved by increasing (CaO + P 2O 5 + CaF 2) at the expense of (Al 2O 3 + SiO 2) and also improved by adding minor additives of (Na 2O + B 2O 3 + TiO 2). Pyroxene solid solution is diopside-Ca-Tschermak's type solid solution. There was no solid solution phase formed between pyroxene and fluorapatite phase, therefore fluorapatite phase was increased by increasing its components in the glasses. Fine microstructure of fibrous growth was developed by increasing the fluorapatite in the glass-ceramic materials obtained. The thermal expansion coefficients ( α-values) of the glasses and glass-ceramics were determined. The α-values of the studied glasses ranged between 78 × 10 −7 and 99 × 10 −7 °C −1 in the (25–500 °C) temperature range and those of crystalline products ranged from 66 × 10 −7 to 100 × 10 −7 °C −1 in the (25–700 °C) temperature range. The data of the glasses were correlated to the internal structure of the glasses, nature and role played by glass forming cations. However, the expansivity of the crystalline glasses was mainly attributed to different factors including the crystalline phases formed and residual glassy phase.