Tantalum carbide

Tantalum carbides (TaC) form a family of binary chemical compounds of tantalum and carbon with the empirical formula Template:Chem2, 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.<ref>Template:Cite book</ref>

The melting points of tantalum carbides was previously estimated to be about Template:Convert depending on the purity and measurement conditions; this value is among the highest for binary compounds.<ref>The claim of melting point of Template:Convert in TaC_0.89 is based not on actual measurement but on an extrapolation of the phase diagram, using an analogy with NbC, see Emeléus</ref><ref name=b1>Template:Cite book</ref> And only tantalum hafnium carbide was estimated to have a higher melting point of Template:Convert.<ref>Template:Cite journal</ref> However new tests have conclusively proven that TaC actually has a melting point of 3,768 °C and both tantalum hafnium carbide and hafnium carbide have higher melting points.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Preparation

Template:Chem2 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 Template:Convert using a furnace or an arc-melting setup.<ref name=j1/><ref name=j2/> An alternative technique is reduction of tantalum pentoxide by carbon in vacuum or hydrogen atmosphere at a temperature of Template:Convert. This method was used to obtain tantalum carbide in 1876,<ref>Template:Cite journal</ref> but it lacks control over the stoichiometry of the product.<ref name=b1/> Production of TaC directly from the elements has been reported through self-propagating high-temperature synthesis.<ref>Template:Cite journal</ref>

Crystal structure

File:Β-Ta2C-bas.png

β-Template:Chem2 with the unit cell, blue color is tantalum

Template:Chem2 compounds have a cubic (rock-salt) crystal structure for x = 0.7–1.0;<ref name=j5>Template:Cite journal</ref> the lattice parameter increases with x.<ref name=j3/> Template:Chem2 has two major crystalline forms. The more stable one has an anti-cadmium iodide-type trigonal structure, which transforms upon heating to about 2,000 °C into a hexagonal lattice with no long-range order for the carbon atoms.<ref name=j1>Template:Cite journal</ref>

Formula	Symmetry	Type	Pearson symbol	Space group	No	Z	ρ (g/cm3)	a (nm)	c (nm)
TaC	Cubic	NaCl<ref name=j3>Template:Cite journal</ref>	cF8	FmTemplate:Overlinem	225	4	14.6	0.4427
Template:Chem2	Trigonal<ref>Template:Cite journal</ref>		hR24	RTemplate:Overlinem	166	12	15.01	0.3116	3
Template:Chem2	Trigonal<ref>Template:Cite journal</ref>	anti-CdI2	hP3	PTemplate:Overlinem1	164	1	15.08	0.3103	0.4938
Template:Chem2	Hexagonal<ref name=j2>Template:Cite journal</ref>		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 1,600–2,000 kg/mm²<ref>Kurt H. Stern (1996). Metallurgical and Ceramic Protective Coatings. Chapman & Hall.</ref> (~9 Mohs) and an elastic modulus of 285 GPa, whereas the corresponding values for tantalum are 110 kg/mm² and 186 GPa.<ref>Template:Cite book</ref>

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 T_C = 10.35 K.<ref name=j3/>

The magnetic properties of Template:Chem2 change from diamagnetic for x ≤ 0.9 to paramagnetic at larger x. An inverse behavior (para-diamagnetic transition with increasing x) is observed for Template:Chem2, despite that it has the same crystal structure as Template:Chem2.<ref>Template:Cite book</ref>

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 Template:Convert 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 399.5 MPa compared to 127.9 MPa 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.<ref>Wang, Bingzhu, et al. "Effects of TaC addition on microstructure and mechanical properties of SiBCN composite ceramics." Ceramics International 45.17 (2019): 22138-22147</ref>

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:

Template:Chem2

in which Re is dissolved in both components to nucleate BCC phase first and MC phase in the following, according to the phase diagrams.<ref>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</ref> In addition, the MC phase also improves the strength of composites, due to its stiffer and more elastic property compared to BCC phase.<ref>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</ref>

Wu et al. have also synthesized Ti(C, N)-based cermets with TaC addition with ball-milling and sintering at Template:Convert. 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.<ref>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</ref>

Natural occurrence

Tantalcarbide is a natural form of tantalum carbide. It is a cubic, extremely rare mineral.<ref>Mindat, http://www.mindat.org/min-7327.html</ref>

See also

• Tantalum hafnium carbide
• Hafnium carbide
• Hafnium carbonitride