Tantalum boride explained

Tantalum borides are compounds of tantalum and boron most remarkable for their extreme hardness.

Properties

The Vickers hardness of TaB and TaB₂ films and crystals is ~30 GPa. Those materials are stable to oxidation below 700 °C and to acid corrosion.

TaB₂ has the same hexagonal structure as most diborides (AlB₂, MgB₂, etc.).^[1] The mentioned borides have the following space groups: TaB (orthorhombic, Thallium(I) iodide-type, Cmcm), Ta₅B₆ (Cmmm), Ta₃B₄ (Immm), TaB₂ (hexagonal, aluminum diboride-type, P6/mmm).

Preparation

Single crystals of TaB, Ta₅B₆, Ta₃B₄ or TaB₂ (about 1 cm diameter, 6 cm length) can be produced by the floating zone method.^{[2] [3]}

Tantalum boride films can be deposited from a gas mixture of TaCl₅-BCl₃-H₂-Ar in the temperature range 540–800 °C. TaB₂ (single-phase) is deposited at a source gas flow ratio (BCl₃/TaCl₅) of six and a temperature above 600 °C. TaB (single-phase) is deposited at BCl₃/TaCl₅ = 2–4 and T = 600–700 °C.^[4]

Nanocrystals of TaB₂ were successfully synthesized by the reduction of Ta₂O₅ with NaBH₄ using a molar ratio M:B of 1:4 at 700-900 °C for 30 min under argon flow.^[5]

Ta₂O₅ + 6.5 NaBH₄ → 2 TaB₂ + 4 Na(g,l) + 2.5 NaBO₂+ 13 H₂(g)

Notes and References

1. Chen . Xing-Qiu . Fu . C. L. . Krčmar . M. . Painter . G. S. . Electronic and Structural Origin of Ultraincompressibility of5dTransition-Metal DiboridesMB2(M=W, Re, Os) . Physical Review Letters . American Physical Society (APS) . 100 . 19 . 2008-05-16 . 0031-9007 . 10.1103/physrevlett.100.196403 . 196403. 18518467 . 2008PhRvL.100s6403C .
2. Otani . S . Korsukova . M.M . Mitsuhashi . T . Floating zone growth and high-temperature hardness of NbB2 and TaB2 single crystals . Journal of Crystal Growth . Elsevier BV . 194 . 3–4 . 1998 . 0022-0248 . 10.1016/s0022-0248(98)00691-5 . 430–433. 1998JCrGr.194..430O .
3. Okada . Shigeru . Kudou . Kunio . Higashi . Iwarni . Lundström . Torsten . Single crystals of TaB, Ta5B6, Ta3B4 and TAB2, as obtained from high-temperature metal solutions, and their properties . Journal of Crystal Growth . Elsevier BV . 128 . 1–4 . 1993 . 0022-0248 . 10.1016/s0022-0248(07)80109-6 . 1120–1124. 1993JCrGr.128.1120O .
4. Motojima . Seiji . Kito . Kazuhito . Sugiyama . Kohzo . Low-temperature deposition of TaB and TaB2 by chemical vapor deposition . Journal of Nuclear Materials . Elsevier BV . 105 . 2–3 . 1982 . 0022-3115 . 10.1016/0022-3115(82)90383-x . 262–268. 1982JNuM..105..262M .
5. Zoli. Luca. Galizia. Pietro. Silvestroni. Laura. Sciti. Diletta. 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. 23 January 2018. 10.1111/jace.15401. free.