Exudation of Silicate Liquid from Polycrystalline Alumina
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Exudation of Silicate Liquid from Polycrystalline Alumina N. Ravishankar, Jeffrey K. Farrer and C. Barry Carter Department of Chemical Engineering and Materials Science, University of Minnesota 421 Washington Ave SE, Minneapolis, MN 55455-0132 ABSTRACT The presence of a liquid film at grain boundaries affects the mechanical properties of the material. One of the ways to eliminate the liquid film is to exude it from the grain boundaries to the free surface of the material. The present paper presents results on the exudation of a liquid silicate from grain boundaries in polycrystalline alumina. The results are compared to exudation behavior from bicrystals of alumina. This approach is very useful in understanding the mechanism of the exudation process. INTRODUCTION Glass films along grain boundaries in ceramics may be the result of the additives used for liquid-phase sintering (LPS) or may be due to the presence of unavoidable impurities in these materials. In either case, the presence of the film affects the properties of the material [1, 2]. For instance, the varistor action of ZnO ceramic depends on the presence of the intergranular film [3] while the liquid formed at grain boundaries in AlN ceramics getter the oxygen that is present in the AlN lattice, thus leading to higher thermal conductivity [4]. The mechanical properties of alumina ceramics at high temperatures are adversely affected by the presence of a silicate glass film. Thus, a post-processing treatment may be required to optimize properties of the material. The post-fabrication treatment may be used to crystallize the grain-boundary glass [5-7] or to eliminate it from the grain boundaries by exuding it to the surface [8-10]. It is also possible to use a fugitive sintering aid (e.g., LiF in spinel) which volatilizes during or after sintering [11]. In general, it is not possible to crystallize the glassy phase completely [12], so the exudation of the liquid phase is an important alternative. There have been few systematic studies of the elimination of the grain-boundary liquid by its migration to the free surface. The process has been reported to occur in cases where highly reducing conditions were used. The elimination of yttrium-aluminum oxide from polycrystalline AlN under reducing conditions was demonstrated [13]. The migration of liquid phase to the free surface during the sintering of Mg-PSZ with SrO and SiO2 additions has been shown [14]. In the present paper, exudation of anorthite liquid from grain boundaries in polycrystalline alumina has been reported. The infiltration of liquid into bulk and dimpled samples has been carried out using a similar method that was used for MgO samples [15]. While the studies of exudation from the polycrystalline samples illustrate the behavior of a large number of grains, a bicrystal approach is more suited to obtaining specific crystallographic information [10]. Hence studies of exudation from bicrystal boundaries have also been carried out. The results have been analyzed in terms of a thermodynamic model [16]. EX
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