Tantalum carbide

Tantalum carbides (TaC)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.

Tantalum carbide
Names
IUPAC name
Tantalum carbide
Other names
Tantalum(IV) carbide
Identifiers
3D model (JSmol)
ECHA InfoCard 100.031.914
EC Number
  • 235-118-3
UNII
Properties
TaC
Molar mass 192.96 g/mol
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]
Insoluble
Solubility Soluble in HF-HNO3 mixture[1]
Thermal conductivity 21 W/m·K[2]
Thermochemistry
36.71 J/mol·K[3]
42.29 J/mol·K
−144.1 kJ/mol
Related compounds
Related refractory ceramic materials
 Zirconium nitride
Niobium carbide
Zirconium carbide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

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 may have a slightly higher melting point of about 3942 °C,[7] whereas the melting point of hafnium carbide is comparable to that of TaC.

Preparation

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 a temperature of about 2000 °C using a furnace or an arc-melting setup.[8][9] An alternative technique is reduction of tantalum pentoxide by carbon in vacuum or hydrogen atmosphere at a temperature of 1500–1700 °C. This method was used to obtain tantalum carbide in 1876,[10] but it lacks control over the stoichiometry of the product.[6] Production of TaC directly from the elements has been reported through self-propagating high-temperature synthesis.[11]

Crystal structure
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;[12] the lattice parameter increases with x.[13] TaC0.5 has two major crystalline forms. 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.[8]

FormulaSymmetryTypePearson symbolSpace groupNoZρ (g/cm3)a (nm)c (nm)
TaCCubicNaCl[13]cF8Fm3m225414.60.4427
TaC0.75Trigonal[14]hR24R3m1661215.010.31163
TaC0.5Trigonal[15]anti-CdI2hP3P3m1164115.080.31030.4938
TaC0.5Hexagonal[9]hP4P63/mmc194215.030.31050.4935

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

Properties

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[16] (~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.[17]

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.[13]

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 that it has the same crystal structure as TaCx.[18]

Application

Tantalum carbide is widely used as sintering additive in ultra-high temperature ceramics (UHTCs) or as a ceramic reinforcement in high-entropy alloys (HEAs) due to its excellent physical properties in melting point, hardness, elastic modulus, thermal conductivity, thermal shock resistance, and chemical stability, which makes it a desirable material for aircraft and rockets in aerospace industries.

Wang et al. have synthesized SiBCN ceramic matrix with TaC addition by mechanical alloying plus reactive hot-pressing sintering methods, in which BN, graphite and TaC powders were mixed with ball-milling and sintered at 1900 °C to obtain SiBCN-TaC composites. For the synthesis, the ball-milling process refined the TaC powders down to 5 nm without reacting with other components, allowing to form agglomerates that are composed of spherical clusters with a diameter of 100 nm-200 nm. TEM analysis showed that TaC is distributed either randomly in the form of nanoparticles with sizes of 10-20 nm within the matrix or distributed in BN with smaller size of 3-5 nm. As a result, the composite with 10 wt% addition of TaC improved the fracture toughness of the matrix, reaching 399.5MPa compared to 127.9MPa of pristine SiBCN ceramics. This is mainly due to the mismatch of thermal expansion coefficients between TaC and SiBCN ceramic matrix. Since TaC has a larger coefficient of thermal expansion than that of SiBCN matrix, TaC particles endures tensile stress while the matrix endures tensile stress in radial direction and compressive stress in tangential direction. This makes the cracks to bypass the particles and absorbs some energy to achieve toughening. In addition, the uniform distribution of TaC particles contributes to the yield stress explained by Hall-Petch relationship due to a decrease in grain size.[19]

Wei et al. have synthesized novel refractory MoNbRe0.5W(TaC)x HEA matrix using vacuum arc melting. XRD patterns showed that the resulting material is mainly composed of a single BCC crystal structure in the base alloy MoNbRe0.5W and a multi-component (MC) type carbide of (Nb, Ta, Mo, W)C to form a lamellar eutectic structure, with the amount of MC phase proportional to TaC addition. TEM analysis showed that the lamellar interface between BCC and MC phase presents a smooth and curvy morphology which exhibits good bonding with no lattice misfit dislocations. As a result, the grain size decreases with increasing TaC addition which improves the yield stress explained by Hall-Petch relationship. The formation of lamellar structure is because at elevated temperature, the decomposition reaction occurs in the MoNbRe0.5W(TaC)x composites: (Mo, Nb, W, Ta)2C → (Mo, Nb, W, Ta) + (Mo, Nb, W, Ta)C in which Re is dissolved in both components to nucleate BCC phase first and MC phase in the following, according to the phase diagrams.[20] In addition, the MC phase also improves the strength of composites, due to its stiffer and more elastic property compared to BCC phase.[21]

Wu et al. have also synthesized Ti(C, N)-based cermets with TaC addition with ball-milling and sintering at 1683K. TEM analysis showed that TaC helps dissolution of carbonitride phase and converts to TaC-binder phase. The resulting is a formation of “black-core-white rim” structure with decreasing grain size in the region of 3-5 wt% TaC addition and increasing transverse rupture strength (TRS). 0-3 wt% TaC region showed a decrease in the TRS because the TaC addition decreases the wettability between binder and carbonitride phase and creates pores. Further addition of TaC beyond 5 wt% also decreases TRS because TaC agglomerates during sintering and porosity again forms. The best TRS is found at 5wt% addition where fine grains and homogeneous microstructure are achieved for less grain boundary sliding.[22]

Natural occurrence

Tantalcarbide is a natural form of tantalum carbide. It is a cubic, extremely rare mineral.[23]

See also

References
  1. 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. 5196273
  3. Tantalum carbide in Linstrom, Peter J.; Mallard, William 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. Emsley, John (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. Emeléus, Harry (1968). Advances in Inorganic Chemistry and Radiochemistry. Academic Press. pp. 174–176. ISBN 978-0-12-023611-4. Retrieved 3 May 2011.
  7. 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.
  8. 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.
  9. 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.
  10. Joly, A. (1876). "Sur les azotures et carbures de niobium et de tantale". Compt. Rend. (in French). 82: 1195.
  11. Shuck, Christopher E.; Manukyan, Khachatur V.; Rouvimov, Sergei; Rogachev, Alexander S.; Mukasyan, Alexander S. (January 2016). "Solid-flame: Experimental validation". Combustion and Flame. 163: 487–493. doi:10.1016/j.combustflame.2015.10.025.
  12. 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.
  13. 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.
  14. 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. 26 (2): 149–153. doi:10.1107/S0567740870002091.
  15. 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.
  16. Kurt H. Stern (1996). Metallurgical and Ceramic Protective Coatings. Chapman & Hall.
  17. 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.
  18. Gusev, Aleksandr; Rempel, Andrey; Magerl, Andreas (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.
  19. Wang, Bingzhu, et al. "Effects of TaC addition on microstructure and mechanical properties of SiBCN composite ceramics." Ceramics International 45.17 (2019): 22138-22147
  20. E. Rudy, S. Windisch, C.E. Brukl, Technical Report No. AFML-TR-65-2, Part II, Ternary Phase Equilibria in Transition Metal Boron-carbon-silicon Systems, vol. XVII, 1967
  21. Wei, Qinqin, et al. "Microstructure evolution, mechanical properties and strengthening mechanism of refractory high-entropy alloy matrix composites with addition of TaC." Journal of Alloys and Compounds 777 (2019): 1168-1175
  22. Wu, Peng, et al. "Effect of TaC addition on the microstructures and mechanical properties of Ti (C, N)-based cermets." Materials & Design 31.7 (2010): 3537-3541
  23. Mindat, http://www.mindat.org/min-7327.html
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