Niobium monocarbide

Sim Simard, B., Presunka, P.I., Loock, H.P., Berces, A., Launila, O. Laser spectroscopy and density functional calculations on niobium monocarbide, J. Chem. Phys. 107 (1997) 307-318. 

Cuboid and cycloid niobium monocarbide (NbC) whiskers, 0.1-2.0 pm in diameter and 5-100 pm in length, and having a square-shaped tip, were recently synthesized by heating mixtures of niobium oxide (Nb203) and carbon black at temperatures over 1100°C [38]. Silicon carbide nano-whiskers, 20-50 nm in diameter and 2-5 pm in length, were carbothermally synthesized by reducing ultrafine precipitated silica powders with ultrafine carbon black by microwave heating [29]. These processes proceed without addition of metal particle catalysts, and therefore by a VS phase transformation [14] [29] [38]. 

Niobium carbide, also known as columbium carbide, is a important refractory material with a high melting point. It is used as a CVD coating mostly on an experimental basis. Niobium carbide has two phases Nb2C and the monocarbide NbC. The latter is the only phase of industrial importance and the only one reviewed here. Its characteristics and properties are summarized in Table 9.5. 

The transition metal carbides and nitrides have often been called interstitial compounds [70] however, this is somewhat misleading. The small boron, carbon, or nitrogen atoms certainly occupy octahedral or trigonal prismatic voids of the metal sublattice, but the arrangement of the metal atoms themselves is different from that of the element. In the monocarbides the transition metal atoms show cubic close packing. However, titanium, zirconium, and hafnium are packed hexagonally and vanadium, niobium, and tantalum are body centered cubic [1]. Thus, these monocarbides are inorganic compounds with their individual crystal structures and they should not be considered as an interstitial compound of a transition metal host lattice. 

Lavender-gray powder, soluble in HF-HNO3 mixture. Wear-resistant Niobium (HM > 9) film, used for coating graphite in nuclear reactors. Oxidation in air monocarbide becomes severe only above 1000 °C. 

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