Low-temperature synthesis, pyrolysis and crystallization of tantalum oxide gels

Abstract

Tantalum oxide gels in the form of transparent monoliths and powders have been prepared from hydrolysis of tantalum pentaethoxide under controlled conditions using different mole ratios of Ta(OC₂H₅)₅∶C₂H₅OH∶H₂O∶HCl. Alcohol acts as the mutual solvent and HCl as the deflocculating agent. For a fixed alkoxide∶ water∶ HCl ratio, the time of gel formation increased with the alcohol to alkoxide molar ratio. Thermal evolution of the physical and structural changes in the gel has been monitored by differential thermal analysis, thermogravimetric analysis, X-ray diffraction, and infrared spectroscopy. On heating to ∼ 400 °C, the amorphous gel crystallized into the low-temperature orthorhombic phase β-Ta₂O₅, which transformed into the high-temperature tetragonal phase α-Ta₂O₅ when further heated to ∼ 1450 °C. The volume fraction of the crystalline phase increased with the firing temperature. The α-Ta₂O₅ converted back into the low-temperature phase, β-Ta₂O₅, on slow cooling through the transformation temperature of 1360 °C, indicating a slow but reversible transformation.