Niobium diboride
Niobium diboride(NbB2) is a highly covalent refractory ceramic material with a hexagonal crystal structure.
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| Names | |
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| IUPAC name
niobium diboride
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| Systematic IUPAC name
boron; niobium | |
| Other names
NbB2
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| Identifiers | |
3D model (JSmol)
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| ChemSpider | |
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PubChemCID
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| Properties | |
| NbB2 | |
| Molar mass | 114.526 g/mol |
| Appearance | grey powder |
| Density | 6.97 g/cm3 |
| Melting point | ~3050°C |
| Boiling point | N/A |
| Insoluble | |
| Structure | |
| Hexagonal,hP3a = 3.085 Å, c = 3.311 Å and c/a = 1.071 Å | |
| P6/mmm, No. 191 | |
| Hazards | |
| Occupational safety and health(OHS/OSH): | |
Main hazards
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Uninvestigated |
Except where otherwise noted, data are given for materials in theirstandard state(at 25 °C [77 °F], 100 kPa).
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Preparation
[edit]NbB2can be synthesized by stoichiometric reaction between constituent elements, in this caseNbandB.This reaction provides for precise stoichiometric control of the materials.[2]Reduction ofNb2O5(orNbO2) 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 B2O3are often required during metallothermic reductions in order to consume all available niobium oxide.
Borothermal reduction of NbO2with elemental boron via solid‐state reaction was proposed by Jha and coworker to obtain nanorods (40 × 800 nm2),[3]
A variation of the borothermal reduction in molten salt was proposed by Ran and co‐worker using Nb2O5with elemental boron to produce nanocrystals (61 nm).[4]
Nanocrystals of NbB2were successfully synthesized by Zoli's reaction, a reduction of Nb2O5with NaBH4using a molar ratio M:B of 1:4 at 700 °C for 30 min under argon flow.[5]
Nb2O5+ 13/2 NaBH4→ 2 NbB2+ 4Na(g,l) + 5/2 NaBO2+ 13 H2(g)
Properties and use
[edit]NbB2is anultra high temperature ceramic(UHTC) with a melting point of 3050 °C.[6]This along with its relatively low density of ~6.97 g/cm3and 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), properties it shares withisostructuraltitanium diboride,zirconium diboride,hafnium diborideandtantalum diboride.[7]
NbB2parts are usuallyhot pressed[8]orspark plasma sintering[9](mechanical pressure applied to the heated powder) and then machined to shape. Sintering of NbB2is hindered by the material'scovalentnature and presence of surface oxides which increasegrain coarseningbeforedensificationduringsintering.Pressureless sinteringof NbB2is possible with sintering additives such asboron carbideandcarbonwhich react with the surface oxides to increase the driving force for sintering but mechanical properties are degraded compared to hot pressed NbB2.
References
[edit]- ^"Diborylidyneniobium B2Nb | ChemSpider".www.chemspider.com.
- ^Çamurlu, H. Erdem & Filippo Maglia. (2009). "Preparation of nano-size ZrB2powder by self-propagating high-temperature synthesis ".Journal of the European Ceramic Society.29(8): 1501–1506.doi:10.1016/j.jeurceramsoc.2008.09.006.
- ^Jha, Menaka; Ramanujachary, Kandalam V.; Lofland, Samuel E.; Gupta, Govind; Ganguli, Ashok K. (2011-07-26). "Novel borothermal process for the synthesis of nanocrystalline oxides and borides of niobium".Dalton Transactions.40(31): 7879–88.doi:10.1039/c1dt10468c.ISSN1477-9234.PMID21743887.S2CID45554692.
- ^Ran, Songlin; Sun, Huifeng; Wei, Ya'nan; Wang, Dewen; Zhou, Niming; Huang, Qing (2014-11-01). "Low-Temperature Synthesis of Nanocrystalline NbB2Powders by Borothermal Reduction in Molten Salt".Journal of the American Ceramic Society.97(11): 3384–3387.doi:10.1111/jace.13298.ISSN1551-2916.
- ^Zoli, Luca; Galizia, Pietro; Silvestroni, Laura; Sciti, Diletta (23 January 2018)."Synthesis of group IV and V metal diboride nanocrystals via borothermal reduction with sodium borohydride".Journal of the American Ceramic Society.101(6): 2627–2637.doi:10.1111/jace.15401.
- ^Perry, Dale L. (2011).Handbook of inorganic compounds(2nd ed.). Boca Raton: Taylor & Francis.ISBN9781439814611.OCLC587104373.
- ^Kovenskaya, B.; Serebryakova, T. I. (May 1970)."Physical properties of niobium boride phase".Soviet Powder Metallurgy and Metal Ceramics.9(5): 415–417.doi:10.1007/BF00796512.S2CID91501914– via SpringerLink.
- ^IWASA, Mikio; KINOSHITA, Makoto; HAYAMI, Ryozo; YAMAZAKI, Tatsuo (1979-06-01)."ホウ hóa ニオブのホットプレス".Journal of the Ceramic Association, Japan(in Japanese).87(1006): 284–290.doi:10.2109/jcersj1950.87.1006_284.ISSN0009-0255.
- ^Sairam, K.; Sonber, J.K.; Murthy, T.S.R.Ch.; Subramanian, C.; Fotedar, R.K.; Hubli, R.C. (2014). "Reaction spark plasma sintering of niobium diboride".International Journal of Refractory Metals and Hard Materials.43:259–262.doi:10.1016/j.ijrmhm.2013.12.011.