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Open Access Publications from the University of California

Origin of Superhardness of Metallic Tungsten Monoboride

  • Author(s): Yeung, Michael Tyrone
  • Advisor(s): Kaner, Richard B
  • et al.

Conventional superhard materials (Vicker’s hardness Hv ≥ 40 GPa) are typically found in highly covalent systems such as diamond and cubic boron nitride. Here, we demonstrate that even materials dominated by metallic bonding can be made to be superhard. A solid solution between tungsten monoboride and tantalum monoboride, i.e. W1-xTaxB, produces a material that is both superhard (Hv = 42.8 � 2.6 GPa) and ultra-incompressible (bulk modulus = 337 � 3 GPa). Note that this new superhard material is derived from two non-superhard parents. The hardness of W1-xTaxB increases linearly with increasing tantalum concentration up to 50%, strongly suggesting that the increased hardness comes from solid-solution hardening of the metallic bilayer. Further evidence for this hypothesis comes from high-pressure radial diffraction. Tantalum-substituted tungsten monoboride represents the newest superhard member in the tungsten-boron system.

Next, we demonstrate that the superhard metal W0.5Ta0.5B can be prepared as nanowires through flux growth. The primary focus of superhard materials development has relied on chemical tuning of the crystal structure. While these intrinsic effects are invaluable, there is a strong possibility that hardness can be dramatically enhanced using extrinsic effects. The aspect ratios of the nanowires are controlled by the concentration of boride in molten aluminum, and the nanowires grow along the boron-boron chains, confirmed via electron diffraction. This morphology inherently results from the crystal habit of borides and can inspire the development of other nanostructured materials.

Finally, we study the role of inorganic crystal structure towards surface area through the model system, tungsten trioxide. High surface area in h-WO3 has been verified from the intracrystalline tunnels. This bottom-up approach differs from conventional top-down templating-type methods. The 3.67 � diameter tunnels are characterized by low-pressure CO2 adsorption isotherms with non-local density functional theory fitting, transmission electron microscopy, and thermal gravimetric analysis. These open and rigid tunnels absorb H+ and Li+, but not Na+ in aqueous electrolytes without inducing a phase transformation, accessing both internal and external active sites. Moreover, these tunnel structures demonstrate high specific pseudocapacitance and good stability in an H2SO4 aqueous electrolyte. Thus, the high surface area created from 3.67 � diameter tunnels in h-WO3 shows potential applications in electrochemical energy storage, selective ion transfer and selective gas adsorption.

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