Niobium diboride (NbB<sub>2</sub>) is a highly covalent refractory ceramic material with a hexagonal crystal structure.
NbB<sub>2</sub> can be synthesized by stoichiometric reaction between constituent elements, in this case Nb and B. This reaction provides for precise stoichiometric control of the materials. Reduction of Nb<sub>2</sub>O<sub>5</sub> (or NbO<sub>2</sub>) to niobium diboride can also be achieved via metallothermic reduction. Inexpensive precursor materials are used and reacted according to the reaction below:
Mg is used as a reactant in order to allow for acid leaching of unwanted oxide products. Stoichiometric excesses of Mg and B<sub>2</sub>O<sub>3</sub> are often required during metallothermic reductions in order to consume all available niobium oxide.
Borothermal reduction of NbO<sub>2</sub> with elemental boron via solidâÂÂstate reaction results in nanorods (40 à800 nm<sup>2</sup>). A variation of the borothermal reduction in molten salt using Nb<sub>2</sub>O<sub>5</sub> with elemental boron produces nanocrystals (61 nm).
Nanocrystals of NbB<sub>2</sub> were successfully synthesized by Zoli's reaction, a reduction of Nb<sub>2</sub>O<sub>5</sub> with NaBH<sub>4</sub> using a molar ratio M:B of 1:4 at 700 ðC for 30 min under argon flow.
NbB<sub>2</sub> is an ultra high temperature ceramic (UHTC) with a melting point of 3050 ðC. This along with its relatively low density of ~6.97 g/cm<sup>3</sup> and good high temperature strength makes it a candidate for high temperature aerospace applications such as hypersonic flight or rocket propulsion systems. It is an unusual ceramic, having relatively high thermal and electrical conductivities (electrical resistivity of 25.7 üéâ cm, CTE of 7.7â 10<sup>âÂÂ6</sup>/ðC), properties it shares with isostructural titanium diboride, zirconium diboride, hafnium diboride and tantalum diboride.
NbB<sub>2</sub> parts are usually hot pressed or spark plasma sintering (mechanical pressure applied to the heated powder) and then machined to shape. Sintering of NbB<sub>2</sub> is hindered by the material's covalent nature and presence of surface oxides which increase grain coarsening before densification during sintering. Pressureless sintering of NbB<sub>2</sub> is possible with sintering additives such as boron carbide and carbon which react with the surface oxides to increase the driving force for sintering but mechanical properties are degraded compared to hot pressed NbB<sub>2</sub>.