Hafnium(IV) carbide

Hafnium carbide (HfC) is a chemical compound of hafnium and carbon. With a melting point of about 3900 °C it is one of the most refractory binary compounds known.[2] However, it has a low oxidation resistance, with the oxidation starting at temperatures as low as 430 °C.[3] This compound may be seen on future spacecraft as part of the heat shield.

Hafnium carbide
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.031.910
Properties
HfC
Molar mass 190.50 g/mol
Appearance black odorless powder
Density 12.2 g/cm3[1]
Melting point 3,900 °C (7,050 °F; 4,170 K)[2]
insoluble
Structure
Cubic crystal system, cF8
Fm3m, No. 225
Hazards
not listed
NFPA 704 (fire diamond)
Flammability code 2: Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur. Flash point between 38 and 93 °C (100 and 200 °F). E.g. diesel fuelHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformReactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazards (white): no code
2
2
1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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.[4]

Hafnium carbide powder is obtained by the reduction of hafnium(IV) oxide with carbon at 1800 to 2000 °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 (>9 Mohs[5]) 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.[6]

References
  1. Physical Constants of Inorganic Compounds in Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. pp. 4–44 ff. ISBN 0-8493-0486-5.
  2. Harry Julius Emeléus (1968). Advances in Inorganic Chemistry and Radiochemistry. Academic Press. pp. 169–170. ISBN 978-0-12-023611-4.
  3. Shimada, Shiro (1992). "Oxidation Kinetics of Hafnium Carbide in the Temperature Range of 480o to 600oC". Journal of the American Ceramic Society. 75 (10): 2671–2678. doi:10.1111/j.1151-2916.1992.tb05487.x.
  4. Lavrentyev, A; Gabrelian, B; Vorzhev, V; Nikiforov, I; Khyzhun, O; Rehr, J (2008). "Electronic structure of cubic HfxTa1–xCy carbides from X-ray spectroscopy studies and cluster self-consistent calculations". Journal of Alloys and Compounds. 462 (1–2): 4–10. doi:10.1016/j.jallcom.2007.08.018.
  5. CRC Materials Science and Engineering Handbook (2001).
  6. Aleksandr Ivanovich Gusev; Andreĭ Andreevich Rempel; Andreas J. Magerl (2001). Disorder and order in strongly nonstoichiometric compounds: transition metal carbides, nitrides, and oxides. Springer. pp. 513–516. ISBN 978-3-540-41817-7.


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