Effects of Al<sub>2</sub>O<sub>3</sub> Addition on Glassy Phase Separation and Crystallization of a PbO-TiO<sub>2</sub>-SiO<sub>2</sub> Glass

Abstract

Some of PbO-TiO2-SiO2 glasses are converted into polycrystalline materials consisting mainly of perovskite-type PbTiO3 crystals by heat treatment. However, the kind of crystals first precipitated in the glasses on heating was found to be markedly affected by the amount of Al2O3 in the glasses. For its reasoning, the crystallization processes of the PbO 40, TiO2 25, SiO2 35mol% glass (glass 1) and PbO 40, TiO2 25, Al2O3 10, SiO2 25mol% glass (glass 2) were investigated by means of DTA, electrical resistivity measurement, X-ray diffraction analysis, electron microscopic observation, infrared spectroscopic analysis and fluorescent X-ray spectroscopic analysis. The results are summarized as follows:1) It is concluded that glass 1 as-annealed has a homogeneous structure being composed of a network of uniformly distributed SiO4, TiO4 and PbO4 tetrahedra and modifier ions of Ti4+ and Pb2+ sitting in holes of the network. On heat treatment, the glass crystallizes directly forming a metastable pyrochlore-type lead titanate dissolving a small amount of SiO2, without going through any precursory structural change such as glassy two-phase separation. At an elevated temperature, the pyrochlore-type crystals transform to the perovskite-type PbTiO3 crystals.Glass 2 as-annealed has a two-phase structure being composed of a large number of minute glassy droplets rich in PbO and TiO2 and a continuous glassy matrix rich in SiO2. On heat treatment, the droplets first grow in size at the expence of the matrix, so rapidly as a sharp exotherm peak can be observed in DTA, and then the perovskite-type PbTiO3 crystals precipitate in the droplets.2) It is concluded that addition of the Al2O3 to a PbO-TiO2-SiO2 glass induces a glassy two-phase separation, and as the result promotes precipitation of the perovskite-type PbTiO3 crystals in the PbO- and TiO2-rich phase, thus suppressing formation of the metastable crystal. The effect of the Al2O3 on the glassy two-phase separation is interpreted in terms of “tricluster” model proposed by Lacy and field strengths of the constituents ions.