Characterization of Grain Boundaries in Alumina

Abstract

Alpha alumina is used in such applications as substrates in electronic packaging or envelopes for Na vapor lamps. Thus, a large amount of research has been devoted to improving the sintering of alpha alumina. MgO is routinely used as an additive in alumina to enhance the densification of alpha alumina compacts (1). This additive not only enhances densification but also inhibits grain growth: exaggerated grain growth is detrimental to product performance. However, it is still not fully understood how this additive affects the grain boundaries of the compacts (2,3). It has previously been reported that the combination of this additive and impurities which may be present in concentrations exceeding the solid solubility limit, causes the formation of second-phase particles (2). The purpose of the present paper is to review some recent studies on alpha alumina grain boundaries and to report some new results. Reports on TEM observations on grain boundaries in alumina in most cases have not related the structure at the boundaries, for example (3,4). Of the studies that did report boundary structure, the most frequently discussed boundaries were twins, specifically basal and rhombohedral twins (8).