Tantalum hafnium carbide
|Melting point||3,942 °C (7,128 °F; 4,215 K)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Tantalum hafnium carbide is a refractory chemical compound with a general formula TaxHf1-xCy, which can be considered as a solid solution of tantalum carbide and hafnium carbide. Individually, these two carbides have the highest melting points among the binary compounds, 3,983 °C (7,201 °F) and 3,928 °C (7,102 °F), respectively, and their "alloy" with a composition Ta4HfC5 is believed to have a melting point of 3,942 °C (7,128 °F). This value was reported in 1930 and included in the Encyclopædia Britannica as the highest melting point for any solid. This attribution was reprinted in Britannica up to at least 1995, and from about 1998 was changed to "one of the highest".
Very few, if any, other measurements of melting point in tantalum hafnium carbide have been reported, because of the obvious experimental difficulties at extreme temperatures. A 1965 study of the TaC-HfC solid solutions at temperatures 2225–2275 °C found a minimum in the vaporization rate and thus maximum in the thermal stability for Ta4HfC5. 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 2225–2275 °C. In a separate study, Ta4HfC5 was found to have the minimum oxidation rate among the TaC-HfC solid solutions. Ta4HfC5 was manufactured by Goodfellow company as a 45 µm powder at a price of $9,540/kg (99.0% purity).
Individual tantalum and hafnium carbides have a rocksalt cubic lattice structure. They are usually carbon deficient and have nominal formulas TaCx and HfCx, 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 Ta0.5Hf0.5C. Hexagonal NiAs-type structure (space group P63/mmc, No. 194, Pearson symbol hP4) with a density of 14.76 g/cm3 was reported for Ta0.9Hf0.1C0.5.
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