Nucleation and Crystal Growth in Phase Separated Glasses in the Lithium Silicate System

Abstract

The glass forming systems may be regarded as model systems for technical glass ceramics. Many papers on investigation of nucleation in oxide glasses are known. Table 1 shows the compound glass-forming systems, for which the temperature dependence of the steady and unsteady crystal nucleation in the bulk nucleation were studied. As can be seen from Table 1, the greatest attention was paid by researchers to the system Li2OSiO2 [1-68] because of its practical importance. A specific feature of the system Li2O SiO2 is that, in the region of compositions from 0 to 32 mol % Li2O, the glasses have a tendency towards the phase separation. Many authors [1, 2, 3, 4] have believed that the process of phase separation of glasses into two vitreous phases favors a homogeneous crystallization of a glass with the formation of small crystals. As follows from [5, 6] the role of the phase separation is reduced to the appearance of phase boundaries that serve as a particular catalyst of the crystallization. According to [93], the crystallization of phase separated glasses cannot be referred to as catalytic. It is necessary to speak about the influence of the phase separation on a further crystallization process rather than about catalysis. Actually, a review of a number of experimental works [7, 8, 9, 10, 81, 11] demonstrates that the liquid–liquid phase separation before crystallization affects the crystal structure of glass-ceramic materials through at least three different ways. 1. The boundary between phases plays a role of an initiating agent. 2. Since, after the phase separation, the composition of a matrix or dispersed phase becomes closer to that of a crystallizing phase, the formation and growth of crystals progress more easily. 3. In one of the phases, the metastable phase precipitates and plays a role of a catalyst for the main crystalline phase. 4. The purpose of this work is to investigate the phase separation and its effect on the crystallization process in lithium silicate glasses that serve as a basis for the preparation of glass ceramic materials.