Chemistry:Tungsten borides
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[1][2] and that of WB4 is ~30 GPa for loads exceeding 3 N.[3]
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,[1] and WB4 crystals can be grown by arc-melting a mixture of elemental tungsten and boron.[3]
Structure
WB2 has the same hexagonal structure as most diborides (AlB2, MgB2, etc.).[4] WB has several forms, α (tetragonal), β (orthorhombic) and δ (tetragonal).[2]
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.[2]
| Material | Vickers hardness (GPa) | Bulk Modulus (GPa) | Melting point (°C) |
|---|---|---|---|
| W2B | 2670 | ||
| WB | ~20 | 2655 | |
| WB2 | ~20 | 2365 | |
| WB4 | ~30 |
References
- ↑ 1.0 1.1 Otani, S.; Ishizawa, Y. (1995). "Preparation of WB2−x single crystals by the floating zone method". Journal of Crystal Growth 154 (1–2): 81–84. doi:10.1016/0022-0248(95)00155-7. Bibcode: 1995JCrGr.154...81O.
- ↑ 2.0 2.1 2.2 Okada, S.; Kudou, K.; Lundström, T. (1995). "Preparations and Some Properties of W2B, δ-WB and WB2 Crystals from High-Temperature Metal Solutions". Japanese Journal of Applied Physics 34 (1): 226–231. doi:10.1143/JJAP.34.226. Bibcode: 1995JaJAP..34..226O.
- ↑ 3.0 3.1 Mohammadi, R.; Lech, A. T.; Xie, M.; Weaver, B. E.; Yeung, M. T.; Tolbert, S. H.; Kaner, R. B. (2011). "Tungsten tetraboride, an inexpensive superhard material". Proceedings of the National Academy of Sciences 108 (27): 10958–62. doi:10.1073/pnas.1102636108. PMID 21690363. Bibcode: 2011PNAS..10810958M.
- ↑ Woods, H. P.; Wawner, F. E.; Fox, B. G. (1966). "Tungsten Diboride: Preparation and Structure". Science 151 (3706): 75. doi:10.1126/science.151.3706.75. PMID 17842093. Bibcode: 1966Sci...151...75W.
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