Synthesis of aluminum borate powder, fabrication and characterization of aluminum borate-based ceramics

Al2O3/B2O3 powder mixtures prepared at various ratios from 10.7 to 54.1 wt% B2O3 were used to synthesize Al18B4O33 powders by solid state calcination at 1300 °C for 1 h. Al18B4O33 formation was completed at 44 wt% B2O3 instead of stoichiometric ratio of 13.17 wt% B2O3. In other words, B2O3-rich liquid phase accelerated Al18B4O33 crystallization. Al18B4O33 particles were not acicular in shape when first formed, but they were fully acicular for the 44 wt% and the 54.1 wt% B2O3 mixtures. Anorthite-based glass and CaO were selected to induce liquid phase sintering. CaO considerably improved densification to 95.4 % relative density at 1350 °C. Both glass and mullite also enhanced densification to 94.8 %. Epitaxial nucleation of mullite on Al18B4O33 particles resulted in a solid solution as well. Dielectric, mechanical and thermal properties of the samples were investigated and evaluated for possible applications.