Tungsten borides

Structure of hexagonal WB2
Structure of orthorombic 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[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 temnperatures of W2B, WB and WB2 are 2670, 2655 and 2365 °C, respectively.[2]

References

  1. 1 2 Otani, S.; Ishizawa, Y. (1995). "Preparation of WB2−x single crystals by the floating zone method". Journal of Crystal Growth. 154: 81. Bibcode:1995JCrGr.154...81O. doi:10.1016/0022-0248(95)00155-7.
  2. 1 2 3 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: 226. Bibcode:1995JaJAP..34..226O. doi:10.1143/JJAP.34.226.
  3. 1 2 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.
  4. ↑ Woods, H. P.; Wawner, F. E.; Fox, B. G. (1966). "Tungsten Diboride: Preparation and Structure". Science. 151 (3706): 75. Bibcode:1966Sci...151...75W. doi:10.1126/science.151.3706.75.
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