Hot pressing and physical properties of Na beta alumina

Dense transparent/translucent Na-beta-alumina was hot pressed using commercial powders having low SiO2 contents, provided that agglomerates could be broken down by milling. Partial (up to 60 vol %) transformation to alpha alumina during hot pressing was caused by SiO2 either present in some powders or introduced by excessive milling or present through the addition of Na2O-SiO2 glass. Such transformation also results from increased storage time or from direct water contamination. Two wt% MgO was used to aid densification, and 2 wt% Na2O to aid conductivity. Flexure strengths averaging 290 MN/sq m, a measured Young's modulus of 21,300 MN/sq m, and the observed pores or flaws at the fracture origins gave calculated values for the fracture energy in good agreement with the directly measured value of 13 J/sq m, for larger pores and flaws. The ionic resistivity was low, e.g., about 3 ohm-cm at 350 C. Effects of surface finish, limited porosity, and hot-pressing texture varied resistivity by up to a factor of 10.