EFFECTS OF MAGNESIUM-OXIDE ON GRAIN-BOUNDARY SEGREGATION OF CALCIUM DURING SINTERING OF ALUMINA

The advantage of certain amounts of MgO addition in alumina sintering has been realized, and it is common practice. In an attempt to understand the role of MgO in the presence of CaO in commerical-grade alumina, grain-boundary segregation of Ca was investigated by scanning Auger electron microprobe (SAM) using an ultrapure alumina after controlled doping of CaO and/or MgO. The commercial-grade alumina, which usually contains a small amount of CaO, is difficult to sinter to high density. The pure alumina composition (> 99.999%) gives "clean" boundaries when it is sintered under "clean" conditions. As the power was doped with 100 ppm of CaO and sintered at 1300-degrees to 1500-degrees-C, all of the grain boundaries were enriched by Ca as observed by others. However, it was also discovered that some of the grain boundaries are enriched by an exceptionally high concentration of Ca. Such a large variation of Ca contents depending on the grain-boundary facets disappeared when samples were codoped with small amounts of MgO. The results suggest that MgO plays a beneficial role in controlling the anisotropic segregation of Ca to various interfaces including grain boundaries and pore surfaces during sintering of alumina. MgO thus enhances chemical homogeneity of commercial-grade alumina.