Glass-Crystal Boundaries in Liquid-Phase Sintered Ceramics
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Glass-Crystal Boundaries in Liquid-Phase Sintered Ceramics
N. Ravishankar and C. Barry Carter Dept. of Chemical Engineering and Materials Science, University of Minnesota 421 Washington Ave. S.E., Minneapolis MN 55455 USA
Report # S0012 (version 2) (To be submitted to MRS Proceedings Spring 2000)
19 April 2000
Please address all correspondence to: Prof. C. Barry Carter Ph: 612-625-8805
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Glass-Crystal Boundaries In Liquid-Phase Sintered Ceramics N. Ravishankar and C. Barry Carter Department of Chemical Engineering and Materials Science, University of Minnesota 421 Washington Ave SE, Minneapolis, MN 55455-01432 ABSTRACT The interface between dewet glass droplets and the free surface of a crystal and the interface between the intergranular glass and adjoining crystalline grains have been examined with particular emphasis on the influence of the crystallography of the free surface and the grain boundary. The wetting of liquid on the free surface has been shown to depend on the surface structure. The migration of boundaries containing a liquid phase has been studied. The migration is initiated by the difference in surface energy of the bounding planes. Faceting of the grain boundary planes has been examined. It is proposed that the boundary migrates by the motion of the facets. INTRODUCTION Liquid-phase sintering (LPS) is a common route for processing many of the commercial ceramic materials [1]. The liquid phase that forms at the annealing temperature enhances the kinetics of the process, thus making it commercially viable. The liquid that aids in the sintering process is usually retained as a glassy phase in the microstructure. The glass is distributed along various grain boundaries and multi-grain junctions. It has been reported for oxide ceramics that there is a variation in the thickness of the intergranular glass layer depending on the energy of the grain boundary [2, 3] while it has been reported to be independent of boundary orientation in case of Si3N4 [4]. In either case, the glass-crystal interface thus formed controls the properties of the sintered product and thus it is important to understand the structure and behavior of this interface. In spite of its importance, very little is known about this interface. The present paper reviews the glass-crystal interface with particular reference to the liquid-phase sintering process. In particular, the interface between dewet glass droplets and free surface of the crystal and the migration of grain boundaries containing intergranular glass layer will be discussed. The interface between the glass and the free surface of a crystalline material provides a basis for understanding the interface between intergranular glass and the crystalline grains. Detailed examination of the solid/liquid/vapor (SLV) triple junction reveals many important aspects of the liquid-solid interactions [5-8]. The wetting behavior of liquids on reconstructed high-index surfaces provides useful insight on the importance of surface energy
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