Tantalum carbide
γ-tantalum carbide in cubic phase | |
Powder of tantalum carbide | |
Names | |
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IUPAC name
Tantalum carbide
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Other names
Tantalum(IV) carbide
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Identifiers | |
12070-06-3 12070-07-04 (TaC0.5) |
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EC Number | 235-118-3 |
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] | |
Std molar
entropy (S |
42.29 J/mol·K |
Std enthalpy of
formation (ΔfH |
−144.1 kJ/mol |
Related compounds | |
Related refractory ceramic materials
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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).
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verify (what is ?) | |
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 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 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 gas atmosphere at a temperature of 1500–1700 °C. This method was used to obtain tantalum carbide back in 1876,[10] but it lacks control over the stoichiometry of the product.[6]
Crystal structure
TaCx compounds have a cubic (rock-salt) crystal structure for x = 0.7–1.0;[11] the lattice parameter increases with x.[12] 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.[8]
Formula | Symmetry | Type | Pearson symbol | Space group | No | Z | ρ (g/cm3) | a (nm) | c (nm) |
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TaC | Cubic | NaCl[12] | cF8 | Fm3m | 225 | 4 | 14.6 | 0.4427 | |
TaC0.75 | Trigonal[13] | hR24 | R3m | 166 | 12 | 15.01 | 0.3116 | 3 | |
TaC0.5 | Trigonal[14] | anti-CdI2 | hP3 | P3m1 | 164 | 1 | 15.08 | 0.3103 | 0.4938 |
TaC0.5 | Hexagonal[9] | 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
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[15] (~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.[16] 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.[12]
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.[17]
See also
References
- ↑ 1.0 1.1 1.2 1.3 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 2.0 2.1 2.2 5196273
- ↑ 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)
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 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
- ↑ 6.0 6.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 8.0 8.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 9.0 9.1 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ 12.0 12.1 12.2 Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Kurt H. Stern (1996). Metallurgical and Ceramic Protective Coatings. Chapman & Hall.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
- ↑ Lua error in package.lua at line 80: module 'strict' not found.
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