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The influence of the ratio of the initial components on the combustion parameters and modes of mixtures of aluminum with copper oxide has been studied. It has been shown that under normal conditions, such mixtures can burn stably when they contain not less than 30% copper oxide. Moreover, with an increase in the content of copper oxide to the stoichiometric ratio, there is a regular change in combustion mode: self-oscillatory, spin, combined convective and multi-hotspot, flame, and fireball combustion modes. In addition, the effect of the ratio of the components on the combustion of copper oxide–aluminum–titanium ternary mixtures was studied, and concentration regions were determined for four main modes of their combustion: the self-oscillatory, hotspot, flame, and fireball modes. It has been shown that the hotspot combustion of such mixtures can proceed in five different modes: a spin mode, a multi-hotspot mode, a combined convective and multi-hotspot mode, a multi-hotspot mode with the formation of a counter front, and a multi-hotspot mode with periodic ejection of combustion products. Depending on the ratio of the initial components, the condensed combustion products of mixtures of copper oxide with aluminum and titanium were found to contain copper, Al3Ti, Ti3Al, and CuAl5Ti2 intermetallic compounds, and Al2O3, TiO2, TiO, Cu2O, Al2TiO5, and Cu3Ti3O oxides.

The effect of the ratio of the components of mixtures of nickel (II) oxide with titanium on the combustion modes and burning rate of compositions based on them is studied. It is found that under normal conditions, a change in the mass content of nickel oxide from 75 to 30% leads to a regular change in combustion mode: a flame, a multi-hotspot mode, and a self-oscillating mode with a periodic separation of combustion products from the burning surface. It is shown that the maximum burning rate of such mixtures (82 mm/s) is achieved at equal mass proportions of nickel oxide and titanium. In the condensed combustion products of a mixture of nickel oxide with titanium, Ti2Ni intermetallic and Ni2Ti4O $$_{x}$$ binary oxide were identified.

We have studied the self-propagating high-temperature synthesis (SHS) process due to the combustion of a WO3 + ZrO2 + TiO2 + Mg multicomponent mixture. The ratio of the oxide phases and excess Mg content have been shown to have a significant effect on the phase composition of the synthesized material. Increasing the WO3 content of the starting mixture from 7.4 to 59.6% raises the adiabatic combustion temperature from 1560 to 3099 K and reduces the percentage of Zr and Ti metals in the synthesis product, which pass into the W〈Zr,Ti〉 solid solution. We have optimized the composition of the starting mixture, 50.6% WO3 + 6.9% ZrO2 + 4.5% TiO2 + 38% Mg, so that its combustion yields a semifinished product consisting of W〈Zr,Ti〉 alloy, MgO, and Mg. After acid leaching, we have obtained a single-phase W〈Zr,Ti〉 substitutional solid solution. Chemical analysis and X-ray diffraction data demonstrate that the alloy thus prepared is similar in composition to W0.66Zr0.17Ti0.17.

Technologically viable principles have been developed for the preparation of the MAX phase Ti3SiC2 by self-propagating high-temperature synthesis (SHS) with a reduction step, using titanium dioxide. We have studied the influence of synthesis conditions (starting-mixture composition and ratio of reactants) on the composition, structure, and particle size of the Ti3SiC2 powder. The results demonstrate that the addition of 10 to 20 wt % excess magnesium to the starting mixture leads to a decrease in the percentage of the TiSi2 and Ti5Si3 silicides in the Ti3SiC2 powder (from 8 to 5 wt %), with the smallest percentage of titanium carbide corresponding to a 10% excess of magnesium in the starting mixture. Raising the amount of silicon in the starting mixture by 10% leads to a decrease in the percentages of the TiSi2 and Ti5Si3 silicides and titanium carbide in the Ti3SiC2 powder. The addition of sodium chloride and magnesium perchlorate to the starting mixture has been shown to influence the percentages of the TiSi2 and Ti5Si3 silicides and TiC in the Ti3SiC2 powder. We have optimized conditions for the synthesis of powder containing 89.1 wt % Ti3SiC2. The powders prepared by SHS with a reduction step consist of agglomerates of layered particles ranging in size from 130 to 770 nm.

This work is devoted to the preparation of powders of Ti2AlC and Ti3AlC2 MAX phases by self-propagating high-temperature synthesis (SHS) using the magnesium-thermal reduction from oxide feedstock. Oxide TiO2 is used as the titanium source, and magnesium is used as the reducing agent. Purification with the removal of magnesium oxide is performed in diluted hydrochloric acid at a temperature of 70°C and concentration of 1 : 3. The target product yield with the magnesium-thermal reduction is 35–40%. It is revealed that the synthesis product with the stoichiometric component ratio after chemical leaching in hydrochloric acid consists of Ti2AlC, MgAl2O4, and TiC. The formation of spinel MgAl2O4 is associated with the deficiency of the magnesium reducing agent in the charge; herewith, aluminum partially enters the reduction reaction of titanium from its oxide with the formation of Al2O3. This leads to the formation of spinel MgO ⋅ Al2O3. An increase in the content of excess magnesium in the charge from 20 to 30 wt % conditions the complete reduction of titanium from its oxide by magnesium with the formation of the Ti2AlC MAX phase and titanium carbide. A decrease of 10 wt % in the carbon content causes a decrease in the titanium carbide fraction to 4%. The product, containing Ti3AlC2 and Ti2AlC MAX phases, as well as TiC, is formed at an excess soot content from 20 to 35%, and the Ti3AlC2 weight fraction increases from 86 to 89%, respectively. These powders comprise agglomerates (87% of them are smaller than 65 μm) consisting of thin plates of MAX phases 70–100 nm in thickness.

The paper focuses on obtaining Ti 2 AlC and Ti 3 AlC 2 MAX phase powders by self-propagating high-temperature synthesis (SHS) from oxide raw materials using magnesium-thermal reduction. The source of titanium was its oxide TiO 2 with magnesium used as a reducing agent. Cleaning from magnesium oxide was conducted in hydrochloric acid solution with a concentration of 1:3 at t = 70 °C. The yield of the target product in magnesium thermal reduction is 35–40 %. It was found that the synthesis product consisted of Ti 2 AlC, MgAl 2 O 4 and TiC after chemical leaching in hydrochloric acid at the stoichiometric ratio of components. MgAl 2 O 4 spinel was formed due to the lack of magnesium reducing agent in the green mixture, while some part of aluminum reacted with titanium oxide reducing it and forming Al 2 O 3 . It led to MgO·Al 2 O 3 formation. An increase in the excess magnesium content in the green mixture from 20 wt.% to 30 wt.% leads to the complete reduction of titanium from its oxide by magnesium with the formation of Ti 2 AlC MAX phase and titanium carbide. A decrease in carbon content by 10 wt.% in the green mixture leads to a decrease in titanium carbide content to 4 %. With an excess content of soot from 20 % to 35 %, a product containing Ti 3 AlC 2 , Ti 2 AlC and TiC MAX phases is formed, and the mass fraction of Ti 3 AlC 2 increases from 86 % to 89 %, respectively. The resulting powders are agglomerates consisting of thin plates of 70–100 nm thick MAX phases. 87 % of such agglomerates are less than 65 μm in size.

This work is devoted to the fabrication of ZrB2–ZrSi2–MoSi2-based composite powder ceramics by self-propagating high-temperature synthesis (SHS) according to the scheme of magnesium thermal reduction from oxide feedstock and its subsequent hot-pressing (HP) consolidation. The combustion of reaction mixtures has rather high adiabatic temperatures in a range of 2060–2120 K and burning rates in a range of 8.3–9.4 g/s. The end product yield of the magnesiothermal reduction is 34–38%. The resulting powder contains 13–47% ZrB2, 21–70% ZrSi2, 2–32% ZrSi, and 10–18% MoSi2, depending on the composition of the initial reaction mixture; has high structural homogeneity; and consists of composite polyhedral particles with an average size on the order of 8 μm. The structure of ceramics consolidated by HP from the SHS powder is homogeneous and includes needle ZrB2 grains distributed in the ZrSi2 matrix, MoSi2 inclusions of various morphologies, and inclusions of ZrSiO4 silicate distributed over the ZrSi2 grain boundaries. The HP-formed samples have a high degree of homogeneity of the chemical composition and residual porosity of 2.5–7.4%.

The results of a study of the kinetics and mechanism of heterogeneous and compact ceramics oxidation in the ZrSi2-MoSi2-ZrB2 system at air temperature of 1400 °C are presented. The ceramics were obtained by the hot-pressing of composite powders that were manufactured by self-propagating high-temperature synthesis following the magnesiothermal recovery approach. Oxidation kinetics are described using a power function with index n > 2 , confirming the significant influence on the evolution process in the structure of the formed oxidation coating. The oxidation mechanism includes the formation of a two-layer structure consisting of a continuous silicate film, of which the outer part contains magnesium and a sublayer based on the ZrSiO4 phase, with the scheelite structure encapsulating the ZrB2 and MoSi2 grains. The influence of the ZrSi2, MoSi2 and ZrB2 phases on the structural-morphological peculiarities of the appearing oxide films and the effectiveness of its protective action are revealed.

A Ti–Zr alloy was prepared by magnesiothermic reduction from TiO2–ZrO2–Mg mixtures followed by acid leaching and the influence of leaching conditions on the quality of final product was explored. For nitric and hydrochloric acids as leaching agents, conditions for complete removal of MgO from crude TiZr–MgO combustion product had been optimized. The resistance of combustion-synthesized Ti–Zr alloy to elution with nitric acid was higher than that with hydrochloric acid.

This work is devoted to the fabrication of heterophase powder and consolidated ceramics based on hafnium and molybdenum borides and silicides by combining self-propagating high-temperature synthesis (SHS) and hot pressing (HP). Composite ceramic HfB2–HfSi2–MoSi2 SHS powders are fabricated according to the magnesium-thermal reduction flowsheet from oxide raw materials, in which the combustion wave has temperatures of 1750–2119 K and rather high mss combustion rates of 8.4–9.3 g/s. The structure of synthesized SHS powders consists of relative coarse MoSi2 grains up to 10 μm in size, submicron elongated HfB2 grains mainly located inside the MoSi2 grains, and rounded Si precipitates. The composition with a lower boron concentration contains numerous polyhedral HfSi2 grains smaller than 10 μm in size. The resulting powders have an average particle size of ~6 μm with a maximal size up to 26 μm. The phase compositions of the ceramics consolidated by the HP method and synthesized SHS powders are identical. The microstructure of compact samples consists of faceted elongated HfB2 grains 0.5–10.0 μm in size, polyhedral HfSi2 and MoSi2 grains up to 8–10 μm in size, and silicon interlayers. The consolidated ceramics have a high structural and chemical homogeneity, low residual porosity of 1.1–1.7%, high hardness of 11.7–12.6 GPa, and thermal conductivity of 62–87 W/(m K).

The paper focuses on obtaining heterophase powder ceramics and consolidated ceramics based on borides and silicides of hafnium and molybdenum by combining the methods of self-propagating high-temperature synthesis (SHS) and hot pressing (HP). Composite ceramic SHS powders HfB2–HfSi2–MoSi2 were obtained according to the scheme of magnesium-thermal reduction from oxide raw materials where the combustion wave is characterized by temperatures of 1750–2119 K and high mass combustion rates of 8,4– 9,3 g/s. The structure of synthesized SHS powders consists of relatively large MoSi2 grains up to 10 μm in size and submicron elongated HfB2 grains located mainly inside the MoSi2 grains and rounded Si precipitates. The composition with a lower concentration of boron contains a large number of polyhedral HfSi2 grains with a size of less than 10 μm. The resulting powders are characterized by an average particle size of ~6 μm with a maximum size up to 26 μm. Phase compositions of ceramics consolidated by the HP method and SHS synthesized powders are identical. The microstructure of compact samples consists of faceted HfB2 elongated grains 0,5– 10,0 μm in size, polyhedral HfSi2 and MoSi2 grains up to 8–10 μm in size and silicon interlayers. Consolidated ceramics has a high structural and chemical homogeneity, low residual porosity of 1,1–1,7 %, high hardness of 11,7–12,6 GPa and thermal conductivity of 62–87 W/(m·K).

The kinetics and mechanism of high-temperature oxidation of dense heterophase ZrSi2-MoSi2-ZrB2 ceramics at a temperature of 1650 °C are studied. The ceramics were fabricated by hot pressing of composite powders prepared by self-propagating high-temperature synthesis with magnesiothermic reduction. The oxidation kinetic curve is described by the power-law function, which indicates that the evolutionary changes in the structure of the formed oxide films significantly affect the course of the oxidation process. The oxidation mechanism involves formation of a multi-layered structure of heterogeneous oxide film, partial dissociation of the ZrSiO4 phase, and formation of secondary MoB and Mo5Si3 compounds. The influence of the ZrSi2, MoSi2 and ZrB2 phases on the structural and morphological features of the formed oxide films and the efficiency of their protective action are shown. Silicon is reduced and zirconium is simultaneously oxidized to ZrO2 in the ZrSi2-ZrSiO4 system at temperatures above 1620 °C in the absence of oxygen or in low-oxygen environment.

Technologically viable principles have been developed for the preparation of titanium and zirconium powders via reduction of metal oxides. We have studied the effect of synthesis conditions (starting-mixture composition and relationships between reactants) on the composition, structure, and particle size of the powders. The results demonstrate that the addition of excess magnesium to the starting mixture leads to the complete reduction of the oxides and that reducing the combustion temperature of the starting mixture leads to an increase in the percentage of zirconium oxide in the final product. The titanium and zirconium powders prepared in the combustion regime with a reduction stage consist of agglomerates of particles ranging widely in size: from large (several microns) to ultrafine and nanoscale particles. The use of magnesium perchlorate as a heating additive leads to the formation of a titanium–zirconium solid solution. The composition of the powders has been determined by chemical analysis, microstructural analysis, and X-ray diffraction.

Technologically viable principles have been developed for the preparation of the MAX phase Ti2AlC by self-propagating high-temperature synthesis (SHS) with a reduction step, using titanium dioxide. We have studied the influence of synthesis conditions (starting-mixture composition and ratio of reactants) on the composition, structure, and particle size of Ti2AlC powders. The results demonstrate that an excess of magnesium in the starting mixture leads to a decrease in the percentage of MgAl2O4 (spinel), and carbon deficiency in the starting mixture reduces the percentage of titanium carbide in the final product. The Ti2AlC powders prepared by SHS consist of agglomerates of layered particles differing in size: from coarse (several microns) to ultrafine and nanometer-sized particles. The composition of the powders was confirmed by chemical analysis, microstructural examination, and X-ray diffraction.

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.

This paper describes the study of the effect of physical and chemical properties of powder titanium and the equivalence ratio on the burning parameters of its mixtures with copper oxide. It is shown that the linear burning rate of such mixtures is significantly dependent on the brand of used titanium. Under normal conditions, the mixtures can stably burn with the content of copper oxide of not smaller than 13%. With an increase in the content of copper oxide to a stoichiometric ratio, there is a change of combustion regimes: multifocal, multifocal with detachment of solid residue, combination of convective and multifocal, multifocal with melt formation, flaming combustion, and fireball. It is determined that, in this system, there may be convective burning under atmospheric pressure in the absence of outer reinforcement.

Boron carbide is prepared by self-propagating high-temperature synthesis (SHS) in a composition range from 5 to 30 at % carbon. The introduction of an inert (MgO) and active (Mg(ClO4)2) additives into the reaction mixture leads to a variation in process parameters such as the temperature and combustion rate. It is established that the unit cell metrics of boron carbide substantially vary depending on the synthesis conditions. The degree of the effect of the SHS mode on the crystal structure rises with an increase in the carbon fraction in the boron carbide structure. This regularity is associated with the ordering diversity of carbon atoms in nonstoichiometric boron carbide. No influence of the synthesis conditions on the unit cell parameters is observed for stoichiometric boron carbide, which is associated with the structure saturation with carbon. It is shown that the variation in the combustion temperature during SHS of boron carbide of the same composition leads to the variability of the structural parameters, thus reflecting the influence of the synthesis conditions on the material crystal structure.

Boron carbide crystals of various compositions were studied by X-ray powder diffraction in the temperature range of 25-700 ​°C. The formation of a metastable structure of boron carbide under the conditions of self-propagating high-temperature synthesis was detected. Its metastability manifested in a decrease of the unit cell volume after the first heating-cooling cycle. The cell volume change disappeared in all the subsequent heating-cooling cycles. For B12C3 crystals this effect did not occur. The metastability of boron carbide crystals was explained by the presence of C and/or B atoms in crystalline structure channels and their motion along the channels and up to position of structural vacancies under the temperature increasing.

Magnesiothermic SHS from reactive mixtures containing energy-producing additive, Mg(ClO4)2, in green mixtures was used to explore crystallization of boron carbide and its derivatives in a large SHS reactor in conditions of strong temperature gradients within the sample bulk. The addition of Mg(ClO4)2 increased a maximum combustion temperature (Tc) up to 2500°C (instead of 2000°C without the additive). The presence of gasifying agent resulted in a non-uniform distribution of combustion products. Except for the central sample area with a maximum value of Tc, the product exhibited the cell parameters corresponding to stoichiometric B4C. A two-phase material formed in the central zone was found to contain B13C2 and B25C4Mg1.42. This was explained by different temperature conditions in different areas of the sample bulk.

Near-nano WC powders with mean grain sizes of about 200 nm were prepared by the SHS method including the reduction of WO 3 by Mg in the presence of carbon and regulating additives. The chemical leaching and refinement of the SHS reaction products allowed one to obtain stoichiometric WC containing only traces of oxygen and magnesium. The thermal reduction of WO 3 and V 2 O 5 by magnesium in the presence of carbon resulted in obtaining two carbide phases of WC and complex carbide (W,V)C with the fcc crystal lattice having a grain size of less than 300 nm. It was established that the tungsten oxide reduction by magnesium in the presence of carbon cannot be used to synthesize coarse-grain WC powders. Coarse-grained WC powders were obtained using the W + C mixture heated to high temperatures by a simultaneous exothermic reaction of interaction between magnesium perchlorate Mg(ClO 4 ) and magnesium. The coarse-grain WC powder synthesized in such a way is nearly stoichiometric and consists of sintered round-shaped agglomerates with the average grain size of up to 16 μm and containing only traces of magnesium and oxygen. The agglomerates comprise WC single-crystals of roughly 1 μm to 8 μm in size.

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.

The features of sintering of the WC-Co hard alloy of the VK5 brand using the feedstock of tungsten carbide powder fabricated by self-propagating high-temperature synthesis (SHS) are investigated. It is established that the physicochemical processes occurring when sintering hard alloys are similar to sintering processes of standard alloys made of traditional tungsten carbide powders. The hard alloy produced in this study possesses high hardness, strength, and crack resistance, which are characteristic of quasi-nanocrystalline materials.

Splitting of reflections from boron carbide has been found for the first time by an X-ray diffraction study of polycrystalline mixture of boron carbide В15–хСх, (1.5 ≤ x ≤ 3) and its magnesium derivative C4B25Mg1.42. An analysis of reflection profiles shows that this splitting is due to the presence of boron carbide phases of different compositions in the sample, which are formed during crystal growth. The composition changes from В12.9С2.1 to В12.4С2.6.

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.

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.

The possible application of self-propagating high-temperature synthesis (SHS) for preparing nanosized powders of refractory compounds is examined. The structurization of tungsten carbide and titanium carbide powders produced by SHS with a reduction stage is studied. The synthesis is based on exothermic reactions between tungsten oxide or titanium oxide, carbon, and magnesium metal. The influence of starting-mixture composition, ratio of components, and nature of adjusting additives on the particle size, morphology, and phase composition of WC and TiC powders is analyzed. Procedures are described for recovering tungsten carbide or titanium carbide from intermediate products using chemical dispersion, i.e., chemicothermal treatment of the ground cake in various solutions. As a result, impurities are removed and defect-rich intercrystalline layers are dissolved. Thus the sinter cake breaks into homogenous hexagonal WC or tetragonal TiC particles. The powder is additionally refined when the final product is treated in various solutions: the uniform shrinkage of the particles is observed because of their partial dissolution in acid and alkaline media, while the structure and properties in the central part of the substance or phase remain unchanged. The influence of the dispersion agent on the morphology and particle size of WC and TiC powders is examined. Conditions are determined for producing TiC and WC powders containing up to 80% of particles smaller than 30 nm using SHS with subsequent chemical dispersion. Based on the research, an SHS process is developed for producing ultrafine and nanosized TiC and WC powders on a commercial scale.

The feasibility of preparing ultrafine and nanometer-sized titanium carbide particles by self-propagating high-temperature synthesis (SHS) has been studied. Data are presented on the structure formation of TiC powders during SHS with a reduction step. Basic to this process is an exothermic reaction between titanium dioxide, magnesium metal, and carbon. The effects of the composition of the starting mixture, relationship between its components, and the morphology and particle size of the starting TiO2 powder on the particle size of the forming material have been investigated. The TiC powder was recovered from the sinter cake by chemical dispersion, a chemothermal treatment of the synthesis product in different solutions. The results demonstrate that treatment of the sinter cake with appropriate solutions removes impurities and causes imperfect intergranular layers to dissolve. As a result, the cake breaks down into homogeneous single-crystal particles. Subsequent treatment in different solutions further reduces the particle size of the powder. The effect of the composition of the dispersing solution on the particle size of the TiC powder has been studied. Our results made it possible to identify conditions for the preparation of titanium carbide powders containing up to 70% of particles less than 0.3 μm in size by SHS followed by chemical dispersion.

Fine-particle tungsten carbide powders are produced from tungsten(VI) oxide by self-propagating high-temperature synthesis. The starting-mixture composition is shown to have a significant effect on the phase composition, morphology, and particle size of the WC powder. A procedure is described for recovering tungsten carbide from intermediate products using chemical dispersion. The influence of the dispersion agent on the morphology and particle size of the resultant powder is examined. The results indicate that the use of organic solvents for chemical dispersion prevents the formation of solid agglomerates, enhances the dissolution rate of impurities and defect-rich intercrystalline layers, and makes it possible to obtain tungsten carbide powders with a predominant particle size no greater than 0.05–0.2 μm.

The possibility of preparing ultrafine boron nitride powders by self-propagating high-temperature synthesis (SHS) is examined. The results demonstrate that, by varying SHS conditions (starting-mixture composition, combustion rate, cooling rate, and intermediate grinding time), one can control the morphology and particle size of the resultant hexagonal BN powder. Systematic data are presented on the chemical dispersion of SHS products for separating nanometer-sized particles. The effect of chemical dispersion (composition of the dispersion medium, process temperature, and dispersion time) on the particle size, morphology, and chemical and phase compositions of boron nitride powders is analyzed. Hexagonal boron nitride powders are obtained with a purity of 99.5+%, particle size of 0.1–0.2 μm, and specific surface of up to 65 m2 /g.

The new title modification of B25C4Mg1.42 is synthesized from powders of magnesium, boron oxide, carbon black, and magnesium perchlorate by magnesiothermic self-propagating high-temperature synthesis.

Transition metal carbides, particularly tungsten carbide, are rather attractive due to their physical and mechanical properties [1]. They are characterized by the high melting point, unusual hardness, low friction coefficient, chemical inertness, oxidation resistance, and excellent electric conductivity. Nowadays, highly dispersed tungsten carbide powders appear to be very important for production of wear-resistant parts, cutters, non-iron alloys, etc.

Boron carbide with compositions corresponding to the homogeneity region in its phase diagram was obtained by self-propagating high temperature synthesis (SHS). The received samples were characterized by X-ray diffraction (XRD) analysis, lattice parameters and half-width of reflections have been investigated. The lattice parameters of boron carbide reported in the literature exhibit a too wide spread in their values for equal carbon content. It is unusually for covalent compounds such as boron carbide which has strong chemical bonds. The largest spread in lattice parameters was observed at 13.2 % (atomic) of carbon in boron carbide. In other hand, for SHS-produced samples the spread was minimal. Using XRD analysis cell parameters of boron carbide in our experiments was found to depend on carbon concentration in non-linear way. Lattice parameter was found to reach the unusually high value of 12.31 Å. The half-widths of boron carbide diffraction lines were found to depend on carbon concentration and reach their maximum values at 13.2 atomic % of carbon when the lattice is most disordered. The structure analysis allows to associate it with the process of crystal structure ordering caused by replacement of boron atoms by carbon ones during formation of boron carbide structure. The carbon atoms can be incorporated into different positions in both the linear groups and the icosahedra. In other words, some certain composition can correspond to different structure. Therefore, both experimental data and crystal-chemical considerations allow to conclude the possibility of different kinds of ordering in boron carbide structure, resulting in instability of lattice parameters and consequently in properties.