In this work, the structure of WB synthesized at high pressure and high temperature (HPHT) was accurately determined by X-ray diffraction and Rietveld refinement. Its asymptotic Vickers hardness (H) value is 25.5 GPa which is much lower than the previous theoretical results (36-40 GPa). It is worth noting that the chemical bonds between the W layers and two different kinds of B layers show obvious polarization character based on the results obtained from X-ray photoelectron spectroscopy (XPS) and electron localization functions (ELFs), density of states (DOS), topological analysis of the static electron density and Mulliken population. This result can well clarify that WB is only a hard but not superhard material. Thus, a 3D network structure can not be formed between the W layers and the B layers which is previously predicted by theoretical calculations. Our results are helpful to understand the hardness mechanism and design superhard materials in TMBs.
An in situ energy dispersive x-ray diffraction study on nanocrystalline ZnS was
carried out under high pressure up to 30.8 GPa by using a diamond anvil cell. The
phase transition from the wurtzite to the zinc-blende structure occurred at
11.5 GPa, and another obvious transition to a new phase with rock-salt
structure also appeared at 16.0 GPa—which was higher than the value
for the bulk material. The bulk modulus and the pressure derivative of
nanocrystalline ZnS were derived by fitting the Birch–Murnaghan equation. The
resulting modulus was higher than that of the corresponding bulk material,
indicating that the nanomaterial has higher hardness than the bulk material.
In this work, high-quality bulk WC-structured WN (δ-WN) was synthesized via an untraditional method and the structure was accurately determined by X-ray diffraction and Rietveld refinement. In the process of synthesizing δ-WN, WN and melamine were used as tungsten source and nitrogen source, respectively. The result of successfully synthesized high-quality δ-WN indicates that our method is an effective route for synthesizing high-quality bulk δ-WN and melamine is a pure nitrogen source for introducing the nitrogen to the metal precursor. The mechanical properties, bulk modulus, and Vickers hardness (HV) were first investigated by in situ high-pressure X-ray diffraction and Vickers microhardness tests, respectively. It is worth noting that the bulk modulus of δ-WN is 373 ± 8.3 GPa, which is comparable to that of c-BN. The Vickers hardness is 13.8 GPa under an applied load of 4.9 N. It is worth noting that W-W metallic bond and W-N ionic bond are mainly chemical bond in δ-WN based on the analysis of electron localization function (ELF), density of states (DOS), and Mulliken population. This result can well clarify that δ-WN is only a hard material for the lack of strong W-N covalent bonds to form 3D network structure. Our results are helpful to understand the hardness mechanism and design superhard materials in transition-metal nitrides.
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