JOURNAL OF MATERIALS SCIENCE 29 (1994) 5065-5070 Low-temperature synthesis, pyrolysis and crystallization of tantalum oxide gels N. P. BANSAL National Aeronautics and Space Administration, Lewis Research Center, Cleveland, OH 44135, USA 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(OC2Hs)5C2HsOH' H20' HCI. Alcohol acts as the mutual solvent and HCI as the deflocculating agent. For a fixed alkoxide:water: H CI 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 ~ the amorphous gel crystallized into the low-temperature orthorhombic phase J]-Ta20w which transformed into the high-temperature tetragonal phase ~-Ta205 when further heated to --~ 1450 ~ The volume fraction of the crystalline phase increased with the firing temperature. The ~-Ta20 ~ converted back into the low-temperature phase, ~-Ta20~, on slow cooling through the transformation temperature of 1360 ~ indicating a slow but reversible transformation. 1. Introduction Ta20 5 films have received attention [1] recently for applications as storage capacitors in very large-scale integrated memory cells and as a gate insulator in MOS devices because of their large dielectric constant. Tantalum oxide dielectric films are also frequently used [2] in display technologies, such as thin-film electroluminescence and thin-film transistor liquid- crystal displays. Ta20 s thin films have also been used for optical waveguides [3, 4] and antireflection coat- ings [5] on solar cells due to the high chemical stability of this oxide and its optimum optical proper- ties, such as a high refractive index and a very low absorption coefficient for light. Ta20 5 films have also been used as solid-state oxygen sensors [6] because of their high ionic conductivity. TazO 5 coatings depos- ited [7] from tantalum pentaethoxide solutions using the sol-gel technique, are also being investigated [7] as interracial barrier layers to prevent chemical inter- action between high-temperature superconductor YBa2Cu3Ov_ = films and ceramic substrates. Because of its excellent chemical stability and high thermal stability, Ta20 5 may also be useful as compliant/pro- tective layers for the fibre-matrix interface in fibre- reinforced ceramic-matrix composites which are being developed for use as high-temperature struc- tural materials in advanced high-efficiency, high-per- formance engines. TazOs is the only thermodynamically stable oxide of tantalum [8, 9"]. It exists in two crystallographic forms, a low-temperature orthorhombic phase, termed 13- or L-Ta20 5, and a high-temperature tetragonal phase, called ~- or H-Ta2Os. The transformation occurs reversibly and slowly at --~ 1360 __+10 ~ [8]. 0022-2461 9 1994 Chapman & Hall In addition, several metastable polymorphic modifica- tions have been reported [10-12]. The objective of the present work was to synthesize tantalum pentoxide by the sol-gel technique and to study the thermal evolution of the gel structure. Transparent monolithic and powder gels have been prepared from the hydrolysis of tantalum pentaetho- xide under various experimental conditions. The structural changes occurring in the gel as a function of heat-treatment temperature have been investigated using differential thermal analysis, thermogravimetric analysis, X-ray diffraction, and infrared spectroscopy. 2. Experimental procedure Tantalum ethoxide, Ta(OC2Hs)5, from Alfa Products and 200 proof ethyl alcohol were used without any further treatment. The relative amounts of various chemicals used are given in Table I. The handling of Ta(OCzHs) 5 was carried out inside a glove box be- cause of its high reactivity with atmospheric moisture. After appropriate quantities of tantalum pentaethox- ide and ethanol were mixed, the container was sealed with parafilm and taken out of the dry box. A 50 % (vol/vol) HC1 solution (prepared by mixing equal volumes of concentrated hydrochloric acid and water) was then slowly added dropwise with a burette under brisk stirring. A homogeneous clear solution having a light-yellow colour resulted. The sealed containers were stored under ambient conditions to allow com- pletion of the alkoxide hydrolysis and polymerization reactions. It took anywhere from a few hours to several days for gel formation, depending upon the relative concentrations of the various reactants in the 5065