Tantalum hafnium carbide is a refractory chemical compound with a general formula , which can be considered as a solid solution of tantalum carbide and hafnium carbide. It was originally thought to have the highest melting point of any known substance, but new research has proven that hafnium carbonitride has a higher melting point.
Properties Individually, the tantalum and hafnium carbides have the highest melting points among the binary compounds, and , respectively, and their "alloy" with a composition has a melting point of .
Very few measurements of melting point in tantalum hafnium carbide have been reported, because of the experimental difficulties at extreme temperatures. A 1965 study of the TaC-HfC solid solutions at temperatures 2,225–2,275 °C found a minimum in the vaporization rate and thus maximum in the thermal stability for . This rate was comparable to that of tungsten and was weakly dependent on the initial density of the samples, which were sintered from TaC-HfC powder mixtures, also at 2,225–2,275 °C. In a separate study, was found to have the minimum oxidation rate among the TaC-HfC solid solutions. was manufactured by Goodfellow company as a 45 μm powderGoodfellow catalogue, February 2009, p. 102 at a price of $9,540/kg (99.0% purity).NIAC 7600-039 FINAL REPORT, NASA Institute for Advanced Concepts – A Realistic Interstellar Explorer, 14 October 2003, p. 55
In 2015, atomistic simulations predicted that hafnium carbonitride could have a melting point exceeding by 200 K. This was later verified by experimental evidence in 2020.
Structure Individual tantalum and hafnium carbides have a rocksalt cubic lattice structure. They are usually carbon deficient and have nominal formulas and , with x = 0.7–1.0 for Ta and x = 0.56–1.0 for Hf. The same structure is also observed for at least some of their solid solutions. The density calculated from X-ray diffraction data is 13.6 g/cm3 for . Hexagonal NiAs-type structure (space group P63/mmc, No. 194, Pearson symbol hP4) with a density of 14.76 g/cm3 was reported for .
See also Tantalum carbide Hafnium carbide Hafnium carbonitride
References