Effect of Cr2O3, ZrO2 and LiF on nucleation and crystallization of nepheline syenite-dolomite glass-ceramic compositions

Abstract

Inexpensive nepheline-pyroxene glass-ceramics of superior mechanical, and thermal properties could be easily prepared from nepheline syenite and dolomite rock mixture by using the conventional glass preparation and heat treatment methods. The parent glass composition, to yield ≈35 % nepheline ss (solid solution) and 65 % diopside ss, was formulated by a modified chemico-mineralogical calculation method after CIPW norm. The effect of the nucleating agents Cr2O3, ZrO2 and LiF on the mechanisms of nucleation and crystallization of these phases were determined by DTA, XRD, as well as optical and electron microscopy. In the absence of these nueleators, crystallization was difficult and semi-crystalline coarse microstructure of diopsidic ss was obtained. LiF showed maximum effect on crystal growth rates, while sufficient nucleation density for glass-ceramic of good quality was achieved by adding Cr2O3 even in the amount as small as 0.5 wt%. Both Cr2O3- and ZrO2-containing samples hindered the crystallization of nepheline but greatly stimulated the epitactic formation of diopside ss of holocrystalline microstructures. LiF-containing samples stimulated the crystallization of the framework silicates, nepheline ss and carnegieite, which progressively increased with the LiF concentration. The thermal expansion coefficient and Vickers hardness of the obtained glass-ceramic reached values of 9.0 × 10^-6 K-1 (20 to 300 °C) and (950 × 10^7 ± 25) N/m2, respectively.