Niobium carbide
Chemical compound
From Wikipedia, the free encyclopedia
Niobium carbide (NbC and Nb2C) is an extremely hard refractory ceramic material, commercially used in tool bits for cutting tools. It is usually processed by sintering and is a frequent additive as grain growth inhibitor in cemented carbides. It has the appearance of a brown-gray metallic powder with purple lustre. It is highly corrosion resistant.
| Identifiers | |
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3D model (JSmol) |
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| ECHA InfoCard | 100.031.913 |
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PubChem CID |
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CompTox Dashboard (EPA) |
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| Properties | |
| NbC | |
| Molar mass | 104.917 g/mol |
| Density | 7.820 g/cm3 |
| Melting point | 3,608 °C (6,526 °F; 3,881 K) |
| Structure | |
| Cubic (rock salt structure), Fm3m, cF8) | |
| Related compounds | |
Related Refractory ceramic materials |
zirconium nitride, tantalum 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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Synthesis
Niobium carbide can be produced by the heating of niobium oxide in a vacuum at 1800 °C and adding coke.
Properties and uses
Niobium carbide has a Young's modulus of approximately 452 GPa, and a shear modulus of 182 GPa.[1] It has a Poisson's ratio of 0.227.[1]
Niobium carbide is a frequent intentional product in microalloyed steels due to its extremely low solubility product in austenite, the lowest of all the refractory metal carbides. This means that micrometre-sized precipitates of NbC are virtually insoluble in steels at all processing temperatures and their location at grain boundaries helps prevent excessive grain growth in these steels. This is of enormous benefit, and the cornerstone of microalloyed steels, because it is their uniform, very fine grain size that ensures both toughness and strength. The only commonly occurring compound with a lower solubility and hence, greater potential for restricting the grain growth of steels is titanium nitride.
Depending on grain size, niobium carbide may burn at 200-800 °C in air. A layer of niobium carbide can be created by chemical vapor deposition. Zirconium carbide and niobium carbide can be used as refractory coatings in nuclear reactors.
Applications
Niobium Carbide (NbC) has shown promise for applications in nuclear thermal propulsion (NTP) reactor cores. NTP is a promising technology for space propulsion applications, providing a high specific impulse compared to conventional chemical engines, and high thrust compared to nuclear electric propulsion (NEP). One significant challenge lies in the fuel materials that make up NTP reactor cores [2] which require extremely high melting temperatures and corrosion resistance, as well as high thermal conductivity, fracture toughness, and good neutronic behavior. Niobium Carbide, a Transition Metal Carbide (TMC), exhibited aptitude for these desired characteristics in past studies. Solid solution matrix materials composed of NbC in combination with other TMCs, such as Zirconium Carbide (ZrC), Tantalum Carbide (TaC), and Titanium Carbide (TiC), demonstrate potential due to their extremely high melting temperatures, compatibility with hot hydrogen, and theoretical capacity to form solid solutions with Uranium Carbide (UC), a reactor core fuel candidate[3][4]. Multi-component carbide solid solution materials combining two or more TMCs are promising for their hypothetical capacity to increase fracture toughness when in solid solution.
Natural occurrence
Niobium carbide occurs naturally as Niobocarbide. This is an extremely rare mineral, and has been found in Sverdlovsk Oblast, Russia.[5]
