LOW-TEMPERATURE SYNTHESIS, PYROLYSIS AND CRYSTALLIZATION OF TANTALUM OXIDE GELS

Tantalum oxide gels in the form of transparent monoliths and powders h ave been prepared from hydrolysis of tantalum pentaethoxide under cont rolled conditions using different mole ratios of Ta(OC2H5)(5):C2H5OH:H 2O:HCl. Alcohol acts as the mutual solvent and HCl as the deflocculati ng agent. For a fixed alkoxide:water:HCl ratio, the time of gel format ion increased with the alcohol to alkoxide molar ratio. Thermal evolut ion of the physical and structural changes in the gel has been monitor ed by differential thermal analysis, thermogravimetric analysis, X-ray diffraction, and infrared spectroscopy. On heating to similar to 400 degrees C, the amorphous gel crystallized into the low-temperature ort horhombic phase beta-Ta2O5, which transformed into the high-temperatur e tetragonal phase alpha-Ta2O5 when further heated to similar to 1450 degrees C. The volume fraction of the crystalline phase increased with the firing temperature. The alpha-Ta2O5 converted back into the low-t emperature phase, beta-Ta2O5, on slow cooling through the transformati on temperature of 1360 degrees C, indicating a slow but reversible tra nsformation.