Niobium diboride (NbB₂) is a highly covalent refractory ceramic material with a hexagonal crystal structure.

Preparation

NbB₂ 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 Nb₂O₅ (or NbO₂) to niobium diboride can also be achieved via metallothermic reduction. Inexpensive precursor materials are used and reacted according to the reaction below: Nb₂O₅ + 2 B₂O₃ + 11 Mg → 2 NbB₂ + 11 MgO Mg is used as a reactant in order to allow for acid leaching of unwanted oxide products. Stoichiometric excesses of Mg and B₂O₃ are often required during metallothermic reductions in order to consume all available niobium oxide. Borothermal reduction of NbO₂ with elemental boron via solid‐state reaction was proposed by Jha and coworker to obtain nanorods (40 × 800 nm²), A variation of the borothermal reduction in molten salt was proposed by Ran and co‐worker using Nb₂O₅ with elemental boron to produce nanocrystals (61 nm). Nanocrystals of NbB₂ were successfully synthesized by Zoli's reaction, a reduction of Nb₂O₅ with NaBH₄ using a molar ratio M:B of 1:4 at 700 °C for 30 min under argon flow. Nb₂O₅ + 13/2 NaBH₄ → 2 NbB₂ + 4Na(g,l) + 5/2 NaBO₂ + 13 H₂(g)

Properties and use

NbB₂ is an ultra high temperature ceramic (UHTC) with a melting point of 3050 °C. This along with its relatively low density of ~6.97 g/cm³ 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 µΩ⋅cm, CTE of 7.7⋅10⁻⁶ °C⁻¹), properties it shares with

isostructural Isostructural chemical compounds have similar chemical structures. "Isomorphous" when used in the relation to crystal structures is not synonymous: in addition to the same atomic connectivity that characterises isostructural compounds, isomorphous s ...

titanium diboride Titanium diboride (TiB2) is an extremely hard ceramic which has excellent heat conductivity, oxidation stability and wear resistance. TiB2 is also a reasonable electrical conductor,J. Schmidt et al. "Preparation of titanium diboride TiB2 by spark p ...

zirconium diboride Zirconium diboride (ZrB2) is a highly covalent refractory ceramic material with a hexagonal crystal structure. ZrB2 is an ultra high temperature ceramic (UHTC) with a melting point of 3246 °C. This along with its relatively low density of ...

hafnium diboride Hafnium diboride belongs to the class of ultra-high-temperature ceramics, a type of ceramic composed of hafnium and boron. It has a melting temperature of about 3250 °C. It is an unusual ceramic, having relatively high thermal and electrical ...

and

tantalum diboride Tantalum borides are compounds of tantalum and boron most remarkable for their extreme hardness. Properties The Vickers hardness test, Vickers hardness of TaB and TaB2 films and crystals is ~30 GPa. Those materials are stable to oxidation below 7 ...

. NbB₂ parts are usually hot pressed or

spark plasma sintering Spark plasma sintering (SPS), also known as field assisted sintering technique (FAST) or pulsed electric current sintering (PECS), or plasma pressure compaction (P2C) is a sintering technique. The main characteristic of SPS is that the pulsed or u ...

(mechanical pressure applied to the heated powder) and then machined to shape. Sintering of NbB₂ is hindered by the material's

covalent A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...

nature and presence of surface oxides which increase grain coarsening before

densification Urban density is a term used in urban planning and urban design to refer to the number of people inhabiting a given urbanized area. As such it is to be distinguished from other measures of population density. Urban density is considered an importan ...

during

sintering Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing ...

Pressureless sintering Clinker nodules produced by sintering Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing ...

of NbB₂ is possible with sintering additives such as

boron carbide Boron carbide (chemical formula approximately B4C) is an extremely hard boron–carbon ceramic, a covalent material used in tank armor, bulletproof vests, engine sabotage powders, as well as numerous industrial applications. With a Vickers hard ...

and

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

which react with the surface oxides to increase the driving force for sintering but mechanical properties are degraded compared to hot pressed NbB₂.

References

{{Borides Ceramic materials Borides Niobium(II) compounds