Tantalum carbide

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Tantalum carbide
γ-tantalum carbide in cubic phase
Powder of tantalum carbide
Identifiers
CAS number 12070-06-3 YesY, 12070-07-04 (TaC0.5)
EC-number 235-118-3
Properties
Molecular formula CTa
Molar mass 192.96 g mol−1
Appearance Brown-gray powder
Odor Odorless
Density 14.3–14.65 g/cm3 (TaC)
15.1 g/cm3 (TaC0.5)[1]
Melting point 3,850–3,880 °C (6,960–7,020 °F; 4,120–4,150 K)
(TaC)[2]
3,327 °C (6,021 °F; 3,600 K)
(TaC0.5)[1]
Boiling point 4,780–5,470 °C (8,640–9,880 °F; 5,050–5,740 K)
(TaC)[1][2]
Solubility in water Insoluble
Solubility Soluble in HF-HNO3 mixture[1]
Thermal conductivity 21 W/m·K[2]
Thermochemistry
Specific
heat capacity
C
36.71 J/mol·K[3]
Std molar
entropy
So298
42.29 J/mol·K
Std enthalpy of
formation
ΔfHo298
−144.1 kJ/mol
Related compounds
Related refractory ceramic materials Zirconium nitride
Niobium carbide
Zirconium carbide
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Tantalum carbides form a family of binary chemical compounds of tantalum and carbon with the empirical formula TaCx, where x usually varies between 0.4 and 1. They are extremely hard, brittle, refractory ceramic materials with metallic electrical conductivity. They appear as brown-gray powders which are usually processed by sintering. Being important cermet materials, tantalum carbides are commercially used in tool bits for cutting applications and are sometimes added to tungsten carbide alloys.[4] The melting points of tantalum carbides peak at about 3880 °C depending on the purity and measurement conditions; this value is among the highest for binary compounds.[5][6] Only tantalum hafnium carbide has a distinctly higher melting point of about 4215 °C, whereas the melting point of hafnium carbide is comparable to that of TaC.

Preparation[edit]

TaCx powders of desired composition are prepared by heating a mixture of tantalum and graphite powders in vacuum or inert gas atmosphere (argon). The heating is performed at temperature of about 2000 °C using a furnace or an arc-melting setup.[7][8] An alternative technique is reduction of tantalum pentoxide by carbon in vacuum or hydrogen gas atmosphere at a temperature of 1500–1700 °C. This method was used to obtain tantalum carbide back in 1876,[9] but it lacks control over the stoichiometry of the product.[6]

Crystal structure[edit]

β-TaC0.5 with the unit cell, blue color is tantalum.

TaCx compounds have a cubic (rock-salt) crystal structure for x = 0.7–1.0;[10] the lattice parameter increases with x.[11] TaC0.5 has two major crystalline forms symmetry. The more stable one has an anti-cadmium iodide-type trigonal structure which transforms upon heating to about 2000 °C into a hexagonal lattice with no long-range order for the carbon atoms.[7]

Formula Symmetry Type Pearson symbol Space group No Z ρ (g/cm3) a (nm) c (nm)
TaC Cubic NaCl[11] cF8 Fm3m 225 4 14.6 0.4427
TaC0.75 Trigonal[12] hR24 R3m 166 12 15.01 0.3116 3
TaC0.5 Trigonal[13] anti-CdI2 hP3 P3m1 164 1 15.08 0.3103 0.4938
TaC0.5 Hexagonal[8] hP4 P63/mmc 194 2 15.03 0.3105 0.4935

Here Z is the number of formula units per unit cell, ρ is the density calculated from lattice parameters.

Properties[edit]

The bonding between tantalum and carbon atoms in tantalum carbides is a complex mixture of ionic, metallic and covalent contributions, and because of the strong covalent component these carbides are very hard and brittle materials. For example, TaC has a microhardness of 1600-2000 kg/mm2[14] (~9 Mohs) and an elastic modulus of 285 GPa, whereas the corresponding values for tantalum are 110 kg/mm2 and 186 GPa. The hardness, yield stress and shear stress increase with the carbon content in TaCx.[15] Tantalum carbides have metallic electrical conductivity, both in terms of its magnitude and temperature dependence. TaC is a superconductor with a relatively high transition temperature of TC = 10.35 K.[11]

The magnetic properties of TaCx change from diamagnetic for x ≤ 0.9 to paramagnetic at larger x. An inverse behavior (para-diamagnetic transition with increasing x) is observed for HfCx, despite it has the same crystal structure as TaCx.[16]

See also[edit]

References[edit]

  1. ^ a b c d Lide, David R., ed. (2009). CRC Handbook of Chemistry and Physics (90th ed.). Boca Raton, Florida: CRC Press. ISBN 978-1-4200-9084-0. 
  2. ^ a b c 5196273 
  3. ^ Tantalum carbide in Linstrom, P.J.; Mallard, W.G. (eds.) NIST Chemistry WebBook, NIST Standard Reference Database Number 69. National Institute of Standards and Technology, Gaithersburg MD. http://webbook.nist.gov (retrieved 2014-07-02)
  4. ^ John Emsley (11 August 2003). Nature's building blocks: an A-Z guide to the elements. Oxford University Press. pp. 421–. ISBN 978-0-19-850340-8. Retrieved 2 May 2011. 
  5. ^ The claim of melting point of 4000 °C in TaC0.89 is based not on actual measurement but on an extrapolation of the phase diagram, using an analogy with NbC, see Emeléus
  6. ^ a b Harry Julius Emeléus (1968). Advances in Inorganic Chemistry and Radiochemistry. Academic Press. pp. 174–176. ISBN 978-0-12-023611-4. Retrieved 3 May 2011. 
  7. ^ a b Lonnberg, B; Lundstrom, T; Tellgren, R (1986). "A neutron powder diffraction study of Ta2C and W2C". Journal of the Less Common Metals 120 (2): 239–245. doi:10.1016/0022-5088(86)90648-X. 
  8. ^ a b Rudy, Erwin; Brukl, C. E.; Windisch, Stephan (1968). "Constitution of Ternary Ta-Mo-C Alloys". Journal of the American Ceramic Society 51 (5): 239–250. doi:10.1111/j.1151-2916.1968.tb13850.x. 
  9. ^ Joly, A. (1876). Compt. Rend. 82: 1905. 
  10. ^ 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. 
  11. ^ a b c Valvoda, V. (1981). "X-ray diffraction study of Debye temperature and charge distribution in tantalum monocarbide". Physica Status Solidi (a) 64: 133–142. doi:10.1002/pssa.2210640114. 
  12. ^ Yvon, K.; Parthé, E. (1970). "On the crystal chemistry of the close-packed transition-metal carbides. I. The crystal structure of the [zeta]-V, Nb and Ta carbides". Acta Crystallographica Section B Structural Crystallography and Crystal Chemistry 26 (2): 149–153. doi:10.1107/S0567740870002091. 
  13. ^ Bowman, A. L.; Wallace, T. C.; Yarnell, J. L.; Wenzel, R. G.; Storms, E. K. (1965). "The crystal structures of V2C and Ta2C". Acta Crystallographica 19: 6–9. doi:10.1107/S0365110X65002670. 
  14. ^ Kurt H. Stern (1996). Metallurgical and Ceramic Protective Coatings. Chapman & Hall.
  15. ^ Oyama, S. Ted (1996). The chemistry of transition metal carbides and nitrides. Springer. pp. 29–30. ISBN 978-0-7514-0365-7. Retrieved 3 May 2011. 
  16. ^ 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. Retrieved 3 May 2011.