Raman studies of the phase separation mechanism of CaO-Al 2 O 3 -SiO 2 glasses in an electric field
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I. INTRODUCTION
IN recent years, there have been numerous studies of the nucleation and crystallization of glasses, and many glassceramic materials prepared by controlled crystallization have been developed, with a variety of properties and applications, including in the medical and dental fields, in the sealingmaterials field, and in the structural-materials field.[1,2,3] Much progress has also been made in the area of glassceramics, in order to improve their properties. Glass-ceramics are able in many instances, to tailor the properties of the materials through the control of phase separation, the completeness of crystallization, and the optimizing of phase morphology. It is well established that liquidliquid phase separation can substantially influence the course of crystallization in glasses. Such effects are particularly important in the formation of many glass-ceramic bodies; phase separation has long been considered an important precursor step in promoting the required copious nucleation. For this reason, the influence of phase separation on crystallization has been extensively studied, with particular attention directed to its effect on crystal nucleation.[4–7] Evidence of a pronounced effect by the electric field on the kinetics of phase separation has been provided by other studies.[8,9] In those studies, a marked acceleration of separation in a complex silicate glass is noted for low-frequency fields of modest intensities. The authors also studied the effect of an electric field on the phase separation of glasses.[10] The results showed that the free energy may be reduced or enhanced by the electric field, depending on the ratio of the dielectric constants of the old and new phases. This means that the electric field can either stimulate or inhibit the nucleation process, but no attempt was made to systematically investigate the effect of an electric field on phase separation. In the present work, phase separation of glasses by applying an electric field is investigated by Raman spectroscopy. The objectives are to study the mechanism of phase separation under an electric field, then to further preW. LIU, Assistant Professor, X.M. GU, Graduate Student, K.M. LIANG, Y.K. ZHENG, and S.R. GU, Professors, and H. CHEN, Postdoctor, are with the Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China. Manuscript submitted October 8, 1998.
METALLURGICAL AND MATERIALS TRANSACTIONS B
pare nanometer glass-ceramics by controling the phase separation of glasses. II. EXPERIMENTS The CaO2Al2O32SiO2 (CAS) glasses used in present study were made from analytically pure ingredients. The content ranges of the compositions are (in wt pct) 15 to 20 pct CaO, 10 to 15 pct Al2O3 and 60 to 70 pct SiO2. TiO2 (5 to 10 pct) had been added to the mixtures as a nucleating agent. After being uniformly mixed, the compositions of the glasses were melted at 1450 8C for 1 hour. The glasses were poured onto a stainless steel plate and cooled to room temperature in air. Then the glass
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