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

Aluminum borate is a suitable ceramic for structural applications due to its low thermal expansion coefficient (4.5×10-6/°C), high elastic modulus (400 GPa), low density (2.93 gr/cm3). In particular, it is used as thermal insulator due to its low thermal conductivity (4 - 6 W/m.K up to 1000°C). However, its acicular particle shape makes densification difficult during sintering. In this thesis, Al2O3 and domestic B2O3 source were utilized to synthetize Al18B4O33 powders by solid state calcination technique and then basic characterizations (phase formation, microstructural evolution, mechanical, thermal, dielectric properties) were completed. Seven formulations (10.7, 13.4, 18.1, 27.9, 34.4, 44, 54.1 wt% B2O3) were prepared and then calcined at 1300°C for 1h to synthesize Al18B4O33 powders. Full Al18B4O33 formation was obtained at 44 wt% B2O3 (e.g., rather than the stoichiometric ratio of 13.17 wt% B2O3). Our results show that 44 wt% B2O3 (e.g., Al2O3/ B2O3 mole ratio of 0.87) was required to obtain a melt that was required to induce nucleation or crystallization of Al18B4O33 phase at calcination temperatures higher than 1200°C. Acicular Al18B4O33 particles were fully obtained for the 44 wt% and 54.1 wt% B2O3 mixtures when Al2O3 was all consumed. However, its acicular particle shape made densification difficult during sintering. Therefore, its densification was controlled by a composite approach with mullite due to its similar crystal structure. AB, PAB, ABG and AMG samples were prepared by gel casting method. The PAB sample sintered at 1350°C for 1h had a relative density of 59.7 %, but 2 wt% CaO addition improved densification to 95.4% (the AB sample). The AMG ceramics reached 94.8% relative density with near-zero open porosity and water absorption levels. The ABG ceramics reached a limited densification of 89.4%. Dielectric constant of the samples varied between the 4 and 6.6 at 5 MHz. The AB sample had a thermal conductivity of 7.26 W/m.K but the PAB sample had 2.7 W/m.K due to its low densification. In this study, Al borate-glass (AB) and Al borate-mullite-glass (AMG) composites were investigated for the first time in literature. These results suggest that Al borate-based ceramics can be used for such applications as reinforcement material in metal matrix composites, filter, thermal insulator, substrate and radome materials. Keywords: Al borate powder synthesis, densification, dielectric properties, mechanical properties, thermal properties