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 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 point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...
s 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 obvious 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 powder at a price of $9,540/kg (99.0% purity).
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
X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...
data is 13.6 g/cm
3 for .
Hexagonal
NiAs-type structure (
space group P63/mmc, No. 194,
Pearson symbol hP4) with a density of 14.76 g/cm
3 was reported for .
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See also
* Tantalum carbide
* Hafnium carbide
* Hafnium carbonitride
References
{{hafnium compounds
Refractory materials
Carbides
Tantalum compounds
Hafnium compounds