A. S. Kurlov

Band Structure and Properties of Polymorphic Modifications of Lower Tungsten Carbide W 2 C

The structural and electronic properties are investigated and the relative stabilities of all the known polymorphic modifications (α, β, γ, ɛ) of the lower tungsten carbide W2C are numerically estimated using the ab initio full-potential linearized augmented plane wave (FLAPW) method within the generalized gradient approximation (GGA) of the local spin density. The equilibrium parameters of the crystal lattices, the band structures, and the total and partial densities of states are determined for the first time within a unified approach. The energies of formation of the α, β, γ, and ɛ polymorphic modifications of the lower tungsten carbide [in the reactions W2C ↔ 2W + C (graphite)] are calculated and used to discuss their relative stabilities.

Neutron Diffraction Study of Nanocrystalline NbC 0.93 Powders and the Anisotropy of Deformation Distortions

Broadening of the diffraction reflections in nanocrystalline powders of nonstoichiometric niobium carbide NbC0.93 with the average particle size from ∼200 to ∼2000 Å has been studied by the neutron diffraction method. The functional dependence of the broadening W2 on the interplanar distance d2 has been obtained including size, deformation, and inhomogeneous broadening. The average size of the coherent scattering regions and microstrains in crystallites with allowance for the anisotropy of the deformation distortions have been estimated.

V8C7–δ superstructure in nonstoichiometric vanadium carbide powders

Neutron and X-ray diffraction methods have been applied jointly to study the crystal structure of the ordered phase of V8C7–δ in macro- and nanocrystalline vanadium carbide powders. The deviation of the composition of the ordered V8C7–δ phase from the stoichiometric composition V8C7 has been established. The magnitudes and directions of the displacements of C and V atoms have been determined. It has been shown that vanadium atoms are displaced towards a vacancy, increasing the electron density of the vacancy position of the carbon sublattice.

Accounting for nonstoichiometry of niobium carbide NbC y upon milling to a nanocrystalline state

The dependence of the size of particles in the prepared nanocrystalline powders on the composition of nonstoichiometric compounds within their homogeneity intervals has been considered in terms of the high-energy ball milling model. It has been shown that the effect of nonstoichiometry on the milling manifests itself in the concentration dependences of the main characteristics (parameters of the crystal structure, energy of interatomic bonds, elastic properties) of the milled nonstoichiometric compound. The results of model calculations performed for nonstoichiometric cubic niobium carbides NbCy have been compared with the experimental data on milling of the NbC0.93 carbide.

Effect of the milling energy on the anisotropy of deformation distortions in nanocrystalline powders of nonstoichiometric tantalum carbide TaC y

The effect of the milling energy and nonstoichiometry of cubic tantalum carbide TaCy (0.81 ≤ y ≤ 0.96) on the strain anisotropy of the crystal and the size of particles in nanocrystalline powders has been experimentally investigated using X-ray diffraction. The functional dependence of the reduced broadening of diffraction reflections on the scattering vector, which takes into account the contributions of the size, strain, and inhomogeneous broadenings, has been obtained. The average size of coherent scattering regions and the crystallite microstrain accounting for the anisotropy of deformation distortions have been estimated. It has been shown that a more precise description of the experimental data on the broadening of diffraction reflections is achieved by the inclusion of the microstrain anisotropy and inhomogeneous broadening in the analysis.

Effect of small particle sizes on the measured density of nanocrystalline powders of nonstoichiometric tantalum carbide TaCy

Nanocrystalline powders of the nonstoichiometric tantalum carbide TaCy (0.81 ≤ y ≤ 0.96) with an average particle size in the range from 45 to 20 nm have been prepared using high-energy ball milling of coarse-grained powders. The density of the initial coarse-grained and prepared nanocrystalline powders of TaCy has been measured by helium pycnometry. The sizes of particles in tantalum carbide powders have been estimated using the X-ray diffraction analysis and the Brunauer–Emmett–Teller (BET) method. The density of TaCy nanopowders measured by helium pycnometry is underestimated as compared to the true density due to the adsorption of helium by the highly developed surface of the nanocrystalline powders. It has been shown that the difference between the true and measured densities is proportional to the specific surface area or is inversely proportional to the average particle size of the nanopowders. The large difference between the true and measured pycnometric densities indicates a superhydrophobicity of the tantalum carbide nanopowders.

Effect of the nonstoichiometry of tantalum carbide TaC y on the particle size of nanopowders prepared by milling

The effect of the nonstoichiometry of tantalum carbide TaCy on the size of particles in nanocrystalline powders prepared by milling has been investigated experimentally for the first time. It has been shown that the effect of the nonstoichiometry on the milling manifests itself in the concentration dependences of the parameters of the crystal structure, energy of interatomic bonds, and elastic properties of the milled nonstoichiometric compound. The experimental data on milling of the tantalum carbide TaCy (0.81 ≤ y ≤ 0.96) have been compared with the theoretical dependences of the particle size D of nanopowders on the milling time t and the composition y of the nonstoichiometric cubic carbides TaCy. It has been established that, under otherwise equal conditions, the milling for 15 h makes it possible to obtain carbide powders with an average particle size of ∼20 nm and a specific surface area of 25 m2 g−1.

Domains of the phases V8C7 and V3C2 in bulk carbide VCy

The formation of cubic (V8C7) and orthorhombic (V3C2) superstructures at the upper and lower boundaries of the homogeneity interval of nonstoichiometric vanadium carbide VCy has been studied. It has been found that the composition of the ordered phase V8C7 − δ deviates from the composition of the ideal superstructure V8C7 because of the reduced filling factor of 4(a) site of the nonmetal sublattice of carbon atoms. Vanadium atoms forming the octahedral environment □V6 of vacant sites in the phases V8C7 and V3C2 are displaced toward a vacancy. Early stages of the formation of domains of ordered phases have been studied. The effect of the quenching and annealing temperatures on the dimension of domains has been determined. In bulk vanadium carbide, the formation of domains of ordered phases begins at the grain boundaries of disordered carbide VCy and results in the appearance of a domain nanostructure.

Milling of nonstoichiometric niobium carbide powder to a nanocrystalline state

Using high-energy ball milling, we obtained nanocrystalline nonstoichiometric NbCy niobium carbide powders. The crystal structure, microstructure, and particle size distribution of the starting and milled powders were determined using X-ray diffraction, laser diffraction, and scanning electron microscopy. Experimental data on the milling of NbCy (0.77 ≤ y ≤ 0.96) niobium carbides were compared to theoretical model dependences of the particle size D of the nanopowders on milling time τ and composition y for nonstoichiometric cubic NbCy carbides. Our results demonstrate that 15-h milling of niobium carbide powder with an average particle size of ≃6 μm yields nanopowder with an average particle size of 20–30 nm and a specific surface area of 25–30 m2/g.

Symmetry analysis of ordered phases of the lower tungsten carbide W2C

AbstractThe channels of disorder-order phase transitions through which there can arise orthorhombic (space group Pbcn) β′-W2C, rhombohedral (space group P