Tantalum boride

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Structure of TaB2
Structure of TaB

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

Properties[edit]

The Vickers hardness of TaB and TaB2 films and crystals is ~30 GPa.[1][2][3] Those materials are stable to oxidation below 700°C and to acid corrosion.[1][3]

TaB2 has the same hexagonal structure as most diborides (AlB2, MgB2, etc.).[4] 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).[3]

Preparation[edit]

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.[1]

References[edit]

  1. ^ a b c S. Motojima et al. "Low-temperature deposition of TaB and TaB2 by chemical vapor deposition" J. Nucl. Mater. 105 (1982) 262
  2. ^ a b S. Otani et al. "Floating zone growth and high-temperature hardness of NbB2 and TaB2 single crystals" J. Cryst. Growth 194 (1998) 430
  3. ^ a b c d S. Okada et al. "Single crystals of TaB, Ta5B6, Ta3B4 and TAB2, as obtained from high-temperature metal solutions, and their properties" J. Cryst. Growth 128 (1993) 1120
  4. ^ X. Chen et al. "Electronic and Structural Origin of Ultraincompressibility of 5d Transition-Metal Diborides" Phys. Rev. Lett. 100 (2008) 196403