Sol-Gel Synthesis of Highly Dispersed Tantalum Hafnium Carbide Ta4HfC5

Powders of nanocrystalline carbide Ta4HfC5 were produced by the sol-gel synthesis of a finely divided Ta2O5-HfO2-C composite powder (with tantalum pentaamyloxide and hafnium acetylacetonate as precursors and a phenolformaldehyde resin as a carbon source) with subsequent low-temperature carbothermal synthesis (1200-1300 degrees C, 2-4 h, dynamic vacuum). The composition of the initial Ta2O5-HfO2-C composite powder was optimized by decreasing excess carbon content. The elemental and phase compositions of the obtained carbide product were found. It was shown that, at low temperature of the carbothermal synthesis, single-phase carbide Ta4HfC5 can decompose into a mixture of phases of tantalum hafnium carbide and hafnium tantalum carbide; however, an unambiguous conclusion was impossible to draw because the produced powders were finely divided (the crystallite size was similar to 23-40 nm, and the particle size (as determined by scanning electron microscopy) was similar to 37-45 nm). The thermal behavior of the obtained finely divided Ta4HfC5 powders in an air flow at 20-1000 degrees C was studied, and the effect of the crystallite and particle sizes in the samples on the positions of exothermic peaks related to oxidation was established, It was noted that a product of the oxidation of the produced Ta4HfC5 powders is not only the Ta2O5 phase, but also the complex oxide Hf6Ta2O17.