Tantalum hafnium carbide

Tantalum hafnium carbide
Identifiers
71243-79-3 Yes
Properties
Ta4HfC5
Melting point 4,215 °C (7,619 °F; 4,488 K)
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

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,[1][2] and their "alloy" with a composition Ta4HfC5 is believed to have a melting point of 4,215 °C (7,619 °F). This value was reported in 1930[3] and included in the Encyclopædia Britannica as the highest melting point for any solid.[4] This attribution was reprinted in Britannica up to at least 1995,[5] and from about 1998 was changed to "one of the highest".[6][7]

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.[8] Ta4HfC5 was manufactured by Goodfellow company as a 45 µm powder[9] at a price of $9,540/kg (99.0% purity).[10]

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.[1] The density calculated from X-ray diffraction data is 13.6 g/cm3 for Ta0.5Hf0.5C.[11][12] 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.[11]

References

  1. ↑ 1.0 1.1 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: 4–10. doi:10.1016/j.jallcom.2007.08.018.
  2. ↑ Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. ISBN 0-8493-0486-5.
  3. ↑ Agte, C.; Alterthum, H. (1930). "Researches on Systems with Carbides at High Melting Point and Contributions to the Problem of Carbon Fusion". Zeitschrift für technische Physik 11: 182–191. ISSN 0373-0093.
  4. ↑ Walter Yust (1954). Encyclopædia Britannica: a new survey of universal knowledge. Encyclopædia Britannica. Retrieved 28 April 2011.
  5. ↑ Hafnium in Encyclopædia Britannica, 1995 print edition.
  6. ↑ Encyclopædia Britannica; inc (1998). The New Encyclopædia Britannica. Encyclopædia Britannica. ISBN 978-0-85229-633-2. Retrieved 28 April 2011.
  7. ↑ "Hafnium". Encyclopædia Britannica. Encyclopædia Britannica, Inc. Retrieved 17 December 2010.
  8. ↑ Deadmore, D. L. (1965). "Vaporization of Tantalum Carbide-Hafnium Carbide Solid Solutions". Journal of the American Ceramic Society 48 (7): 357–359. doi:10.1111/j.1151-2916.1965.tb14760.x.
  9. ↑ Goodfellow catalogue, February 2009, p. 102
  10. ↑ NIAC 7600-039 FINAL REPORT, NASA Institute for Advanced Concepts – A Realistic Interstellar Explorer, 14 October 2003, p. 55
  11. ↑ 11.0 11.1 Rudy, E.; Nowotny, H. (1963). "Untersuchungen im System Hafnium-Tantal-Kohlenstoff". Monatshefte für Chemie 94 (3): 507–517. doi:10.1007/BF00903490.
  12. ↑ Rudy, E.; Nowotny, H.; Benesovsky, F.; Kieffer, R.; Neckel, A. (1960). "Über Hafniumkarbid enthaltende Karbidsysteme". Monatshefte für Chemie 91: 176–187. doi:10.1007/BF00903181.