Sol–Gel Synthesis of Highly Dispersed Tantalum Hafnium Carbide Ta4HfC5

Abstract

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°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 ~23–40 nm, and the particle size (as determined by scanning electron microscopy) was ~37–45 nm). The thermal behavior of the obtained finely divided Ta₄HfC₅ powders in an air flow at 20–1000°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.