A translucent polycrystalline MgAI,O, ceramic was prepared from finely divided coprecipitated spinel in which a small amount of CaO added as a sintering aid was uniformly distributed. The CaO promotes densification through the formation of a liquid phase at the sintering temperatures. Depending on the sintering treatment, the relative density of the sintered spinel was 99.7 to -100% of theoretical. The in-line optical transmission was > 10% from 0.3 to 6.5 pm. Total transmission in the visible region was between 67 and 78%.

Solid solution formation in the systemBaTiOr gabor was established by X-ray diffraction and dielectric measurements. Solid solutions with cubic symmetry were observed in the composition range from 4 to 90 mole yo I u y b o a at room temperature. The lattice parameter for the BaTiOp solid solutions increased with increasing -boa;that for the KNbOa solid solutions decreased with the addition of BaTiOa. A distinct discontinuity in lattice parameter was observed at the composition containing about 65 mole yo BaTiOs. Dielectric measurements were made from -195O to 400OC. The cubic-tetragonal transition temperature of BaTiOs was rapidly lowered with increasing addition of KNb03, whereas the two lower phase transition temperatures were raised. All three phase transitions of KNbOI were rapidly lowered with increasing addition of BaTi08. The observed phase transitions, lattice parameters, and electron probe data suggest a complex region in the subsolidus which extends from 35 to about 75 mole % KNb03.

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The sintering and grain-growth kinetics of finely divided MgAl,OI were determined from 1300" to 1600°C for densities up to 96% of theoretical. These results show that sintering is governed by volume diffusion and that the temperature dependence of the diffosivities is 157 exp [( -118 kcal/mol)/RT] cm'/s. Grain growth follows the expression (G'-GGo') =Kt, where K=51.3 exp [(-I10 kcal/mol)/RT] cm2/s.