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Identifiers | |
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3D model (JSmol) | |
ChemSpider | |
ECHA InfoCard | 100.031.910 |
EC Number |
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PubChem CID | |
CompTox Dashboard (EPA) | |
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Properties | |
HfC | |
Molar mass | 190.50 g·mol−1 |
Appearance | black odorless powder |
Density | 12.2 g/cm3 [1] |
Melting point | 3,958 °C (7,156 °F; 4,231 K) [2] |
insoluble | |
Structure | |
Cubic crystal system, cF8 | |
Fm3m, No. 225 | |
Hazards | |
GHS labelling: | |
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Warning | |
H228 | |
NFPA 704 (fire diamond) | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). |
Hafnium carbide ( Hf C ) is a chemical compound of hafnium and carbon. Previously the material was estimated to have a melting point of about 3,900 °C. [2] More recent tests have been able to conclusively prove that the substance has an even higher melting point of 3,958 °C exceeding those of tantalum carbide and tantalum hafnium carbide which were both previously estimated to be higher. [3] However, it has a low oxidation resistance, with the oxidation starting at temperatures as low as 430 °C. [4] Experimental testing in 2018 confirmed the higher melting point yielding a result of 3,982 (±30°C) with a small possibility that the melting point may even exceed 4,000°C. [5]
Atomistic simulations conducted in 2015 predicted that a similar compound, hafnium carbonitride (HfCN), could have a melting point exceeding even that of hafnium carbide. [6] Experimental evidence gathered in 2020 confirmed that it did indeed have a higher melting point exceeding 4,000 °C, [7] with more recent ab initio molecular dynamics calculations predicting the HfC0.75N0.22 phase to have a melting point as high as 4,110 ± 62 °C, highest known for any material. [8]
Hafnium carbide is usually carbon deficient and therefore its composition is often expressed as HfCx (x = 0.5 to 1.0). It has a cubic (rock-salt) crystal structure at any value of x. [9]
Hafnium carbide powder is obtained by the reduction of hafnium(IV) oxide with carbon at 1,800 to 2,000 °C. A long processing time is required to remove all oxygen. Alternatively, high-purity HfC coatings can be obtained by chemical vapor deposition from a gas mixture of methane, hydrogen, and vaporized hafnium(IV) chloride.
Because of the technical complexity and high cost of the synthesis, HfC has a very limited use, despite its favorable properties such as high hardness (greater than 9 Mohs [10] ) and melting point. [2]
The magnetic properties of HfCx change from paramagnetic for x ≤ 0.8 to diamagnetic at larger x. An inverse behavior (dia-paramagnetic transition with increasing x) is observed for TaCx, despite its having the same crystal structure as HfCx. [11]