Shanmin Wang

A New Molybdenum Nitride Catalyst with Rhombohedral MoS2 Structure for Hydrogenation Applications

Nitrogen-rich transition-metal nitrides hold great promise to be the next-generation catalysts for clean and renewable energy applications. However, incorporation of nitrogen into the crystalline lattices of transition metals is thermodynamically unfavorable at atmospheric pressure; most of the known transition metal nitrides are nitrogen-deficient with molar ratios of N:metal less than a unity. In this work, we have formulated a high-pressure route for the synthesis of a nitrogen-rich molybdenum nitride through a solid-state ion-exchange reaction. The newly discovered nitride, 3R-MoN2, adopts a rhombohedral R3m structure, isotypic with MoS2. This new nitride exhibits catalytic activities that are three times more active than the traditional catalyst MoS2 for the hydrodesulfurization of dibenzothiophene and more than twice as high in the selectivity to hydrogenation. The nitride is also catalytically active in sour methanation of syngas with >80% CO and H2 conversion at 723 K. Our formulated route for the synthesis of 3R-MoN2 is at a moderate pressure of 3.5 GPa and, thus, is feasible for industrial-scale catalyst production.

Pressure calibration for the cubic press by differential thermal analysis and the high-pressure fusion curve of aluminum

Cell pressures in the cubic press have been determined by means of differential thermal analysis (DTA) based on the known fusion curve of aluminum. The results compared favorably with those of the well-known fixed-point method, based on pressure-induced phase transitions in bismuth (I–II, 2.55 GPa) and thallium (II–III, 3.68 GPa). The high-pressure fusion curve of lead was measured using our calibrated results, which agreed well with those of the previous work. All of this was made possible by the development of a successful cell assembly for the DTA analyses in the cubic press. Details of the design are described in the present work.

The Hardest Superconducting Metal Nitride

Transition–metal (TM) nitrides are a class of compounds with a wide range of properties and applications. Hard superconducting nitrides are of particular interest for electronic applications under working conditions such as coating and high stress (e.g., electromechanical systems). However, most of the known TM nitrides crystallize in the rock–salt structure, a structure that is unfavorable to resist shear strain, and they exhibit relatively low indentation hardness, typically in the range of 10–20 GPa. Here, we report high–pressure synthesis of hexagonal δ–MoN and cubic γ–MoN through an ion–exchange reaction at 3.5 GPa. The final products are in the bulk form with crystallite sizes of 50 – 80 μm. Based on indentation testing on single crystals, hexagonal δ–MoN exhibits excellent hardness of ~30 GPa, which is 30% higher than cubic γ–MoN (~23 GPa) and is so far the hardest among the known metal nitrides. The hardness enhancement in hexagonal phase is attributed to extended covalently bonded Mo–N network than that in cubic phase. The measured superconducting transition temperatures for δ–MoN and cubic γ–MoN are 13.8 and 5.5 K, respectively, in good agreement with previous measurements.

Ultrahigh-pressure densification of nanocrystalline WB ceramics

Phase-pure nanostructured WB ceramics are hot pressed at ultrahigh pressures of 1.0 to 3.0 GPa and high temperatures of 700 to 1000 °C (UHPHT) for 60 min. The UHPHT samples are nanograin size from 15 to 40 nm. Our experimental observation shows that ultrahigh pressure could improve densification, and the density of WB samples could reach 99.4% of theoretical. The comparative experiments carried out at ambient pressure and temperatures of 550 to 1100 °C for 60 min indicate that the external pressure was favorable for phase-pure and highly dense WB formation. In addition, the UHPHT samples give a high hardness value of 28.9 ± 0.8 GPa.

Synthesis of Stoichiometric and Bulk CrN through a Solid-State Ion-Exchange Reaction

Chromium mononitride (CrN) exhibits interesting magnetic, structural, and electronic properties for technological applications. Experimental reports on these properties are often inconsistent owing to differences in the degree of nonstoichiometry in CrN(x). To date, the preparation of bulk and stoichiometric CrN has been challenging; most products are in the form of a thin film produced by non-equilibrium processes, and are often nonstoichiometric and poorly crystallized. In this work, we formulated a solid-state ion-exchange route for the synthesis of CrN under high pressure. The final CrN product is phase-pure, stoichiometric, and well-crystallized in the bulk form. Near-stoichiometric and well-crystallized CrN can be synthesized using the same route at atmospheric pressure, making massive and industrial-scale production technologically feasible. The successful synthesis of stoichiometric and bulk CrN is expected to open new opportunities in diverse areas of fundamental research.

Hardness, elastic, and electronic properties of chromium monoboride

We report high-pressure synthesis of chromium monoboride (CrB) at 6 GPa and 1400 K. The elastic and plastic behaviors have been investigated by hydrostatic compression experiment and micro-indentation measurement. CrB is elastically incompressible with a high bulk modulus of 269.0 (5.9) GPa and exhibits a high Vickers hardness of 19.6 (0.7) GPa under the load of 1 kg force. Based on first principles calculations, the observed mechanical properties are attributed to the polar covalent Cr-B bonds interconnected with strong zigzag B-B covalent bonding network. The presence of metallic Cr bilayers is presumably responsible for the weakest paths in shear deformation.

Crystal structures, elastic properties, and hardness of high-pressure synthesized CrB2 and CrB4

Chromium tetraboride (CrB4), a recently proposed candidate for superhard materials, has been synthesized at high pressure and temperature by a solid-state reaction. As a byproduct, chromium diboride (CrB2) also forms and co-exists with CrB4 in the final product. The comparative studies of crystal structure, elastic property, and hardness of both phases have been conducted at the same sample environment conditions. The crystal structure of CrB4 has been refined with an orthorhombic symmetry of Immm(space group no. 71) or Pnnm (space group no. 58) using X-ray diffraction data. Further simulations indicate that the structural distinction between Immm and Pnnm can be resolved by neutron diffraction, due to the high scattering cross-section of boron (11B) by neutrons. Although CrB2 and CrB4 have close bulk modulus at about 230 GPa, the measured asymptotic Vickers hardness yields 16 GPa for CrB2 but 30 GPa for CrB4, which is nearly two times that of CrB2. The dramatic enhancement in hardness in CrB4 is attributed to the strong three-dimensional Cr-B network, in contrast to the layered lattice structure of hexagonal CrB2.

Phase-transition induced elastic softening and band gap transition in semiconducting PbS at high pressure.

We have investigated the crystal structure and phase stability, elastic incompressibility, and electronic properties of PbS based on high-pressure neutron diffraction, in-situ electrical resistance measurements, and first-principles calculations. The refinements show that the orthorhombic phase is structurally isotypic with indium iodide (InI) adopting a Cmcm structure (B33). The cubic-to-orthorhombic transition occurs at ∼2.1(1) GPa with a 3.8% volume collapse and a positive Clausius-Clapeyron slope. Phase-transition induced elastic softening is also observed, which is presumably attributed to the enhanced metallic bonding in the B33 phase. On the basis of band structure simulations, the cubic and orthorhombic phases are typical of direct and indirect semiconductors with band gaps of 0.47(1) and 1.04(1) eV, respectively, which supports electrical resistivity measurements of an abrupt jump at the structural transition. On the basis of the resolved structure for B33, the phase transition paths for B1→B33→B2 involve translation of a trigonal prism in B1 and motion of the next-nearest neighbor Pb atom into {SPb7} coordination and subsequent lattice distortion in the B33 phase.

High Pressure Phase-Transformation Induced Texture Evolution and Strengthening in Zirconium Metal: Experiment and Modeling.

We studied the phase-transition induced texture changes and strengthening mechanism for zirconium metal under quasi-hydrostatic compression and uni-axial deformation under confined high pressure using the deformation-DIA (D-DIA) apparatus. It is shown that the experimentally obtained texture for ω-phase Zr can be qualitatively described by combining a subset of orientation variants previously proposed in two different models. The determined flow stress for the high-pressure ω-phase is 0.5–1.2 GPa, more than three times higher than that of the α-phase. Using first-principles calculations, we investigated the mechanical and electronic properties of the two Zr polymorphs. We find that the observed strengthening can be attributed to the relatively strong directional bonding in the ω phase, which significantly increases its shear plastic resistance over the α-phase Zr. The present findings provide an alternate route for Zr metal strengthening by high-pressure phase transformation.

Ultrastrong Boron Frameworks in ZrB12: A Highway for Electron Conducting

ZrB12 , with a high symmetrical cubic structure, possesses both high hardness ≈27.0 GPa and ultralow electrical resistivity ≈18 µΩ cm at room temperature. Both the superior conductivity and hardness of ZrB12 are associated with the extended BB 3D covalent bonding network as it is not only favorable for achieving high hardness, but also provides conducting channels for transporting electrons.