Study of devitrification of Dicor glass

The nucleation and crystallization of tetrasilicic fluormica crystals in the Dicor group of glass ceramics is not as well understood as the crystallization sequence of trisilicic mica in the Macor group of glass ceramics. The main aim of this investigation was to obtain kinetic data of crystallization and to examine the devitrification process at different stages of heat treatment during non-isothermal heating. The kinetic parameters, such as the activation energy, Ec, the reaction order, n, and the frequency factor, v, for the devitrification reaction were computed using the Johnson-Mehl-Avrami (JMA) equation and the data from differential thermal analysis (DTA). The crystallization at different stages of devitrification was evaluated by scanning electron microscopy (SEM) observations, and Fourier transform infrared (FTIR) spectroscopy with KBr-reflectance technique. All DTA curves at different heating rates showed a single exothermic crystallization peak, just above the glass transition temperature in the temperature range of 745-850 degrees C. Linear relationships between the logarithm of heating rate and the reciprocal of temperature, and the logarithm of temperature rise and the reciprocal of temperature were observed. The computation of kinetic parameters gave the values for activation energy, Ec = 203 kJ mol-1, the reaction order n = 3.4 +/- 0.20 and the frequency factor v = 2.88 x 10(11) s-1. SEM observation showed precipitation of crystals in the DTA run which were considerably finer (approximately 0.1 micron) than the block shaped mica plates (approximately 4 microns) observed in the sample subjected to recommended non-isothermal-isothermal heating cycle. The FTIR spectra of beyond the exothermic peak DTA run samples, and the completely cerammed samples showed two additional peaks at wave numbers of 705 and 671 cm-1 compared with the ascast glass and the samples heated before the onset of the exothermic peak. Similar peaks in FTIR spectra also appeared from MgF2- powder at the same wave numbers. Based upon these results, it is proposed that during non-isothermal heating a spinodal decomposition of glass matrix occurs, which is followed by the exothermal precipitation of round tetrasilicic fluormica crystals by a cellular precipitation mechanism.