Effect of Addition of Fluorine on Crystallization of Li2O-MgO-Al2O3-SiO2 Glass

Abstract

The effect of addition of fluorine on the crystallization of the glass having the composition Li3Mg1.75Al3Si8O23.75-x/2Fx, where x=0 to 1.84, was studied. The results obtained are summarized as follows:1) For a small amount of fluorine content (x=0-1.34), the tendency of the glass to crystallization increased with addition of fluorine. For more amount of fluorine content (x=1.43-1.84), however, the tendency of the glass to bulk crystallization and surface crystallization at relatively low temperatures decreased with addition of fluorine and the crystallization occured throughout the samples at higher temperatures. The cause for this effect of the addition of the fluorine was considered as follows. If it is assumed that fluorine atoms exist in the form of SiFO3 in the glass, addition of fluorine would weaken the structure of the glass, promoting its crystallization. When the fluorine content is over a certain value (x=1.43), however, the process of the exclusion of fluorine from the formation of the crystalline lattice, and the formation of β-quartz solid solution might be suppressed at relatively low temperatures. It was inferred that the precipitation of tiny MaF2 crystals in this step might have facilitated the subsequent crystallization of β-quartz solid solution in the bulk of the glass.2) β-quartz solid solution precipitated in the glass at relatively low temperatures transformed to β-spodumene solid solution at relatively high temperatures. The stability of the former depended on the content of fluorine in the glass. When the glass was heated once at various temperatures, the temperature of transformation decreased with increasing fluorine content. When the glass was heated at a higher temperature after the heat-treatment at lower temperatures, however, β-quartz solid solution was more stable in the glass with richer fluorine content (x=1.84) than in the glass with smaller fluorine content (x=1.34, 1.59). This was attributed to the smaller fluorine content of the residual glass in the sample with x=1.84 than those in the samples with x=1.34 and 1.59.3) The effects of fluorine content on the bending strength and the thermal expansion coefficient of the crystallized glsses were explained in terms of the state and the thermal expansion coefficient of the precipitated crystals.