Tungsten borides
Image:Magnesium-diboride-3D-balls.png WB2]]
Image:TlI_structure.png β-WB]]
Tungsten borides are compounds of tungsten and boron. Their most remarkable property is high hardness. The Vickers hardness of WB or WB2 crystals is ~20 GPa{{Cite journal | doi = 10.1143/JJAP.34.226| title = Preparations and Some Properties of W2B, δ-WB and WB2 Crystals from High-Temperature Metal Solutions| journal = Japanese Journal of Applied Physics| volume = 34| pages = 226–231| year = 1995| last1 = Okada | first1 = S. | last2 = Kudou | first2 = K. | last3 = Lundström | first3 = T. | issue = 1| bibcode = 1995JaJAP..34..226O| s2cid = 95651766}} and that of WB4 is ~30 GPa for loads exceeding 3 N.
Synthesis
Single crystals of WB2−x, x = 0.07–0.17 (about 1 cm diameter, 6 cm length) were produced by the floating zone method,{{Cite journal | doi = 10.1016/0022-0248(95)00155-7| title = Preparation of WB2−x single crystals by the floating zone method| journal = Journal of Crystal Growth| volume = 154| pages = 81–84| year = 1995| last1 = Otani | first1 = S. | last2 = Ishizawa | first2 = Y. | issue = 1–2| bibcode = 1995JCrGr.154...81O}} and WB4 crystals can be grown by arc-melting a mixture of elemental tungsten and boron.{{Cite journal | doi = 10.1073/pnas.1102636108|pmid=21690363| title = Tungsten tetraboride, an inexpensive superhard material| journal = Proceedings of the National Academy of Sciences| volume = 108| issue = 27| pages = 10958–62| year = 2011| last1 = Mohammadi | first1 = R.| last2 = Lech | first2 = A. T.| last3 = Xie | first3 = M.| last4 = Weaver | first4 = B. E.| last5 = Yeung | first5 = M. T.| last6 = Tolbert | first6 = S. H.| last7 = Kaner | first7 = R. B. | pmc=3131357| bibcode = 2011PNAS..10810958M|doi-access=free}}
Structure
WB2 has the same hexagonal structure as most diborides (AlB2, MgB2, etc.).{{Cite journal | doi = 10.1126/science.151.3706.75| title = Tungsten Diboride: Preparation and Structure| journal = Science| volume = 151| issue = 3706| pages = 75| year = 1966| last1 = Woods | first1 = H. P.| last2 = Wawner | first2 = F. E.| last3 = Fox | first3 = B. G.| bibcode = 1966Sci...151...75W | pmid=17842093| s2cid = 7686903}} WB has several forms, α (tetragonal), β (orthorhombic) and δ (tetragonal).
Properties
δ-WB and WB2 crystals have metallic resistivities of 0.1 and 0.3 mΩ·cm, respectively. The oxidation of W2B, WB and WB2 is significant at temperatures above 600 °C. The final oxidation products contain WO3 and probably amorphous B2O3 or H3BO3. The melting temperatures of W2B, WB and WB2 are 2670, 2655 and 2365 °C, respectively.
| class="wikitable" style="align="center"
|+ Properties | |||
| Material
! Vickers hardness (GPa) ! Bulk Modulus (GPa) ! Melting point (°C) | |||
|---|---|---|---|
| W2B | 2670 | ||
| WB | ~20 | 2655 | |
| WB2 | ~20 | 2365 | |
| WB4 | ~30 |
References
{{reflist}}
{{Tungsten compounds}}
{{Borides}}