V. I. Vershinnikov

Ordering of carbon atoms in boron carbide structure

Boron carbide crystals have been obtained in the entire compositional range according to the phase diagram by self-propagating high-temperature synthesis (SHS). Based on the results of X-ray diffraction investigations, the samples were characterized by the unit-cell metric and reflection half-width in the entire range of carbon concentrations. A significant spread in the boron carbide unit-cell parameters for the same carbon content is found in the data in the literature; this spread contradicts the structural concepts for covalent compounds. The SHS samples have not revealed any significant spread in the unit-cell parameters. Structural analysis suggests that the spread of parameters in the literary data is related to the unique process of ordering of carbon atoms in the boron carbide structure.

SHS-produced boron carbide: Some special features of crystal structure

Boron carbides with carbon contents (σ) of 7–24 at. % were prepared by SHS method and their lattice parameters were determined by high-precision XRD analysis. An unusually wide spread in the literature data on lattice parameters of boron carbide as a function of σ was associated with the process of crystal ordering caused by gradual replacement of boron by carbon atoms. For SHS-produced boron carbide, the above spread turned out minimal. Lattice parameter c was found to attain the unusually high values of 12.20–12.31 s was observed at σ = 13.2%. The widths of diffraction lines from boron carbide were found to depend on σ and attain their maximum values at σ = 13.2% when the lattice is most disordered. Our results can make a basis for elaborating the means for regulating the structure/properties of boron carbide.

SHS of boron carbide: Influence of combustion temperature

Boron carbides of varied composition were synthesized through magnesiothermic SHS reaction at temperatures between 1500 and 2500°C and characterized by XRD. Variation in synthesis temperature was found to change the lattice parameters of boron carbide, which was associated with the disordering of the crystal structure.

Preparation of ultrafine and nanosized MoSi2 particles by self-propagating high-temperature synthesis with a reduction step

We have developed technological principles of the preparation of ultrafine and nanosized MoSi2 particles by self-propagating high-temperature synthesis (SHS) with a reduction step. The effect of synthesis conditions (starting-mixture composition, relative amounts of reactants, and the presence and amount of an inert diluent) on the composition, structure, and particle size of the powders has been studied. The results demonstrate that inert additives reduce the adiabatic temperature. The crystallite size of MoSi2 decreases with increasing additive concentration. The MoSi2 powders obtained by SHS with a reduction step have the form of agglomerates consisting of spherical particles ranging widely in size: from large (several microns) to ultrafine and nanosized. The composition of the powders was checked by chemical analysis, microstructural examination, and X-ray diffraction.

High-quality cemented carbides on the basis of near-nano and coarse-grain WC powders obtained by self-propagating high-temperature synthesis (SHS)

In the present paper cemented carbides with different grain sizes produced with WC powders obtained by the self-propagating high-temperature synthesis (SHS) were examined. Morphology, particle size distribution and chemical composition and of the SHS-powders were studied in detail. Microstructures of a submicron grade with 5% Co made from the near-nano WC powder obtained by the SHS and a standard submicron grade are similar with slightly more large WC grains in the sample made from the SHS WC powder. The mechanical properties of the ultrafine WC–5% Co grade made from the near-nano WC powder obtained by the SHS and its wear-resistance are comparable with those of the standard ultrafine grade. The microstructure of a medium-coarse WC–6% Co grade made from the medium-coarse SHS WC powder and its properties are comparable with those of the convenient medium-coarse carbide grade for percussive drilling. Results of laboratory performance tests on percussive drilling of the medium-coarse WC–6% Co grade obtained from both the SHS WC powder and conventionally fabricated WC powder indicate that their wear-resistance and performance toughness are very similar. Thus, it is established that high-quality WC–Co cemented carbides with different WC grain sizes varying from submicron to medium-coarse can be produced from the SHS WC powders.

SHS of single crystals in the B-C-Mg system: Crystal structure of new modification of B25C4Mg1.42 = [B12]2[CBC][C2]Mg1.42

A new modification of B25C4Mg1.42, [B12]2[CBC][C2]Mg1.42, was prepared by magnesiothermic SHS and characterized by XRD. This compound was found to have the following crystallographic parameters: a = 9.626(1), b = 11.329(1), c = 8.966(1) Å, β = 105.80(3)°, V = 940.8(2) Å3, space group P21/b, Z = 4, R = 0.032. SHS-produced crystals exhibited high acid resistance and hardness and can be recommended as a starting compound for synthesis of other modifications of carboboride.

Extraction of TiAl powder from SHS-produced TiAl–MgO semiproduct by treatment in different solutions

Explored was the extraction of target TiAl powder from SHS-produced TiAl–MgO semiproduct by treatment in acids (H2SO4, HCl,) and NH4Cl solution. Acid leaching was found to be accompanied by marked elution of target TiAl: the yield of TiAl powder was 21 and 33% in case of leaching with H2SO4 and HCl, respectively. But upon leaching with enriched NH4Cl solution at 70–80°C for 30 min, the yield of purified TiAl powder attained a value of 60%.

Ultrafine and nanosized MoSi2 powders by SHS process with a reduction stage

Suggested are basic principles for fabrication of fine MoSi2 powders by SHS process with a reduction stage. Investigated was the influence of green composition, stoichiometric ratio, inert NaCl additive, and synthesis conditions on the structure/properties of thus fabricated MoSi2 powders. With increasing amount of added NaCl, the mean size of MoSi2 crystallites was found to decrease. The fabricated MoSi2 powders represented the agglomerates formed by the crowds of smaller (200 nm) and larger (up to 1–3 μm) particles. The powders were characterized by SEM, XRD, and chemical analysis.

Extraction of Ti Powder from Ti–MgO–Mg(–CaO) Cakes Produced by Magnesiothermic Reduction

The extraction of Ti powder from SHS-produced Ti–MgO–Mg(–CaO) cakes by treatment in leaching solutions (HNO3, HCl, and NH4Cl) was explored and optimized in relation to such factors as concentration of leaching agent, leaching temperature, chemical resistance of target Ti powder, and extent of byproducts extraction. The type of leaching solution was found to affect the size, structure, and morphology of resultant Ti powder. Best results were obtained at 70°C with aqueous solutions of: (1) the nitric acid taken in a 6-fold excess to the Mg content of combustion product and (2) the ammonium chloride taken in a 20-fold excess to nominal Mg content.

Fine Ti Powders Through Metallothermic Reduction in TiO2–Mg–Ca Mixtures

Fine Ti powders were prepared through magnesiothermic reduction in TiO2–Mg–Ca mixtures under 4 MPa of Ar followed by acid leaching (with HCl or HNO3) and characterized by XRD, SEM/EDS, and chemical analysis. Thus prepared Ti powders exhibited the specific surface ranging between 7.0 and 30.0 m2/g. The produced Ti powders can find their application in pyrotechnics, powder metallurgy, and as raw material for SHS of inorganic compounds.