The kinetics and mechanism of hydrogen evolution on the MoSi2 electrode in x M H2SO4 + (0.5 - x) M Na2SO4 (x = 0.50; 0.35; 0.20) solutions have been studied. The cathodic polarization curves of MoSi2 in the studied solutions are characterized by Tafel region with a slope of (-0.070)+/- 0.002 V. The reaction order of the cathodic process with respect to hydrogen ions at the potentials of Tafel region is similar to 1.0; the derivative of the electrode potential with a change in electrolyte acidity is similar to 0.072 V. The impedance spectra of the MoSi2 electrode in the studied potential range have the shape of a semicircle located in the capacitive half-plane, with the centre in the region of positive values of the imaginary impedance component; in the region of the highest frequencies, a short straight section is recorded on the impedance plots, indicating the presence of pores in the surface layer of the electrode. To describe the hydrogen evolution reaction on MoSi2, we used an equivalent electrical circuit, the Faraday impedance of which consists of series-connected charge transfer resistance (R-1) and a parallel R(2)C(2)Z(d)-chain responsible for the adsorption of atomic hydrogen on the surface and its diffusion into the depth of the electrode material. The equivalent circuit also includes the solution resistance (R-s) and the double layer capacitance impedance, which is modeled by the constant phase element CPE1. It is shown that the hydrogen evolution reaction on molybdenum disilicide in a sulphuric acid electrolyte proceeds along the discharge - recombination route with a quasi-equilibrium discharge stage when the Temkin logarithmic isotherm for adsorbed atomic hydrogen is fulfilled. The hydrogen evolution reaction is complicated by hydrogen absorption proceeding in the mode of solid-phase diffusion kinetics.

This paper presents experimental results concerning the one-step preparation of cast divanadium nitride and aluminum oxynitride (AlON) by high -temperature synthesis under 5 MPa of N 2 from green mixtures containing vanadium oxide, aluminum, and aluminum nitride. It was shown that the mixtures with a wide range of component ratios are capable to burn with the temperature exceeding the melting point of the final products, divanadium nitride and aluminum oxynitride, that makes it possible to obtain them in the form of a melt with subsequent separation of vanadium nitride (lower ingot) and aluminum oxynitride (upper ingot). The resulting products had no cohesion and were easily separated from each other. Variation in the content of aluminum nitride in the charge markedly affected the synthesis process, phase composition, and microstructure of the target products. The content of excess aluminum nitride in the starting mixture alpha = 10% was found to be optimum on a yield/composition of the target products.

A cast oxide ceramic material consisting of Al2O3–Cr2O3 solid solution and ZrO2 inclusions uniformly distributed in it was prepared through metallothermic SHS from highly exothermic thermite-type mixtures containing molybdenum (VI) and chromium (III) oxides and Al/Zr as a combined reducing agent. The influence of green mixture mass on the synthesis parameters, composition, and microstructure of the final products was studied. The optimal conditions for high-temperature synthesis of cast composite oxide material consisting of 80 wt

Изучены кинетика и механизм реакции выделения водорода на MoSi2-электроде в растворах x M H2SO4 + (0.5-x) M Na2SO4 (x = 0.50, 0.35, 0.20). Катодные поляризационные кривые MoSi2 в исследованных растворах характеризуются тафелевским участком с наклоном, равным (-0.070)±0.002 В. Порядок реакции катодного процесса по ионам водорода при потенциалах тафелевской области составляет ~1.0; производная электродного потенциала при изменении кислотности электролита равна ~0.072 В. Спектры импеданса MoSi2-электрода в изученном диапазоне потенциалов имеют вид полуокружности, расположенной в емкостной полуплоскости, с центром в области положительных значений мнимой составляющей импеданса; в области наиболее высоких частот на графиках импеданса регистрируется короткий прямолинейный участок, свидетельствующий о присутствии пор в поверхностном слое электрода. Для описания реакции выделения водорода на MoSi2 использована эквивалентная электрическая схема, фарадеевский импеданс которой состоит из последовательно соединенных сопротивления переноса заряда (R1) и параллельной R2C2Zd-цепочки, отвечающей за адсорбцию атомарного водорода на поверхности и его диффузию вглубь материала электрода. Эквивалентная схема также включает сопротивление раствора (Rs) и импеданс емкости двойного электрического слоя, который моделируется элементом постоянной фазы СРЕ1. Показано, что реакция выделения водорода на дисилициде молибдена в сернокислом электролите протекает по маршруту разряд - рекомбинация с квазиравновесной стадией разряда при выполнении логарифмической изотермы Темкина для адсорбированного атомарного водорода. Реакция выделения водорода осложнена процессом абсорбции водорода, протекающим в режиме твердофазно-диффузионной кинетики. The kinetics and mechanism of hydrogen evolution on the MoSi2 electrode in x M H2SO4 + (0.5-x ) M Na2SO4 ( x = 0.50; 0.35; 0.20) solutions have been studied. The cathodic polarization curves of MoSi2 in the studied solutions are characterized by Tafel region with a slope of (-0.070)±0.002 V. The reaction order of the cathodic process with respect to hydrogen ions at the potentials of Tafel region is ~1.0; the derivative of the electrode potential with a change in electrolyte acidity is ~0.072 V. The impedance spectra of the MoSi2 electrode in the studied potential range have the shape of a semicircle located in the capacitive half-plane, with the centre in the region of positive values of the imaginary impedance component; in the region of the highest frequencies, a short straight section is recorded on the impedance plots, indicating the presence of pores in the surface layer of the electrode. To describe the hydrogen evolution reaction on MoSi2, we used an equivalent electrical circuit, the Faraday impedance of which consists of series-connected charge transfer resistance ( R1) and a parallel R2C2Zd-chain responsible for the adsorption of atomic hydrogen on the surface and its diffusion into the depth of the electrode material. The equivalent circuit also includes the solution resistance (Rs) and the double layer capacitance impedance, which is modeled by the constant phase element CPE1. It is shown that the hydrogen evolution reaction on molybdenum disilicide in a sulphuric acid electrolyte proceeds along the discharge - recombination route with a quasi-equilibrium discharge stage when the Temkin logarithmic isotherm for adsorbed atomic hydrogen is fulfilled. The hydrogen evolution reaction is complicated by hydrogen absorption proceeding in the mode of solid-phase diffusion kinetics.

Ceramic composite material Al2O3–Cr2O3 + TiC was prepared by metallothermic self-propagating high-temperature synthesis (SHS) under 5 MPa of Ar pressure from green mixtures containing TiO2, Al, and C powders, and additive of CrO3 + Al blend. An increase in the mass fraction α of CrO3 + Al in the general charge was found to raise the synthesis temperature. For α < 0.2, sintered multiphase products were formed. In case of high additive content (α > 0.2), the combustion products were separated into two layers: carbide and oxide phases. Optimum synthesis conditions for forming the cast product consisting of Al2O3–Cr2O3 solid solution ("ruby") and uniformly distributed TiC particles were found. In order to evaluate the possibility of using this material as a cutting tool, physical and mechanical properties were determined. SHS-prepared Al2O3–Cr2O3 + TiC composite after 24-h milling in a planetary mill, pressing under a pressure of 200 MPa, and sintering at 1460°С was found to possess density of 4.55 g/cm3, hardness of 20 GPa, flexural strength of 680 MPa, and crack resistance of 4.2 MPa m1/2.

Materials based on molybdenum-aluminium-carbon compounds have a considerable potential for use under intense wear conditions at elevated temperatures. This paper presents the experimental results of self-propagating high-temperature synthesis of compounds within the Mo-Al-C system. By combining two processes: SHS of the elements and SHS-metallurgy, cast materials containing the Mo 3 Al 2 C, Mo 2 C, Mo 3 Al, and Mo 3 Al 8 phases were obtained. The experiments used mixtures with compositions calculated according to the ratio (1 - α)(3MoO 3 -8Al-C)/α(3Mo-2Al-C), where a varied in the range from 0 to 1. The synthesis was carried out in a laboratory reactor of 3 L volume at an initial argon pressure of 5 MPa. The mass of the initial mixtures in all experiments was 20 g. The process of combustion was initiated by a 0.5 mm diameter molybdenum wire spiral by applying 28 V voltage to it. The resulting end products were studied by X-ray diffraction and local microstructural analysis. A significant influence of the ratio of the initial reagents on the synthesis parameters, phase composition, and microstructure of the target products was established. Introduction into the high-exothermic mixture 3MoO 3 -8Al-C inert “cold” mixture 3Mo-2Al-C leads to an increase in the content of carbide phases in the ingots. The possibility of obtaining cast materials based on the triple phase Mo 3 Al 2 C, the maximum content of which is 87 wt. % at the content of the “cold” mixture in the charge α = 0.4 is shown. The presence of secondary phases of molybdenum carbide (Mo 2 C) and molybdenum aluminides (Mo 3 Al 8 , Mo 3 Al) in the final products is due to a change in the composition of the initial mixture caused by the ejection of components during combustion and insufficient existence time of the melt formed in the combustion wave.

Cast four-component МАХ compounds (V 1– x Nb x ) 2 AlC were prepared from V 2 O 5 –Nb 2 O 5 –Al–C powder mixtures by metallothermic SHS under 5 MPa of Ar and characterized by XRD, SEM/EDS, and video filming. Target product were found to contain (V 1– x Nb x ) 2 AlC compounds and Al 3 Nb. Synthesized materials can find their application as structural materials in high-temperature applications in aggressive media.

High-temperature synthesis of cast composite materials in the Cr–Mn–Al–C system with various ratios between the Cr2AlC MAX phase and chromium and manganese aluminides and carbides was studied. Experiments were performed in a universal 3-L reactor under argon pressure Р = 5 MPa. Mixtures of powders of chromium(III) and manganese(II, IV) oxides and calcium peroxide with aluminum (ASD-I) and carbon were used as a charge. The synthesis features and the phase composition and microstructure of the target products are significantly influenced by the reactant ratio in the charge. At the stoichiometric ratio of the components corresponding to the Cr2AlC phase, a cast composite material consisting of the Cr2AlC MAX phase, chromium carbides Cr7C3 and Cr3C2, and chromium aluminide Cr5Al8 is formed. In the course of combustion of the charge with the component ratio corresponding to the Mn2AlC phase, a cast composite material consisting of manganese carbides (Mn3AlC, Mn0.545Al0.42C0.035) and aluminide (MnAl) is formed. When these compositions are combined in an 0.75 : 0.25 ratio, the final product is a composite material consisting of a solid solution based on the Cr2AlC MAX phase, manganese carbides (Mn3AlC), and chromium aluminides (Cr2Al). When these compositions are combined in an 0.5 : 0.5 ratio, the final product is a composite material consisting of the manganese-doped Cr2AlC MAX phase and of manganese (Mn14Al86, Mn22.5Al77.5) and chromium (Cr5Al8) aluminides. The final products were characterized by X-ray diffraction and local microstructural analysis.

The possibility of obtaining a composite material based on titanium and chromium borides with a nickel binder by metallothermic SHS was considered. In the experiments, calcium chromates CaCrO4 and CaCr2O7 were used as the chromium-containing component of green mixtures. Thermodynamic calculations showed that the combustion temperature of the mixture based on calcium dichromate (CaCr2O7) exceeded by 200°C the combustion temperature of the mixture based on calcium chromate (CaCrO4). The use of calcium dichromate allowed us to expand the limits of combustion and phase separation of the mixture, as well as to increase the yield of the target product. XRD analysis and SEM/EDS data showed that the obtained composite material contains triple boride Ni20AlB14 as a basis and chromium and titanium borides and titanium–chromium boride, which are uniformly distributed over the material.

— Experimental data are presented on the high-temperature synthesis of cast composite materials in the Cr–Al–C system with different relative amounts of the MAX phase Cr 2 AlC and chromium carbides and aluminides. The experiments were carried out in multipurpose self-propagating high-temperature synthesis (SHS) reactors at an argon pressure p = 5 MPa. The starting mixtures consisted of calcium chromate (CaCrO 4 ), aluminum (ASD-1), and carbon powders. It has been shown that varying the percentage of carbon in the starting mixture may have a significant effect on the synthesis process and the phase composition and microstructure of the final products. It has been found that, in the case of the stoichiometric starting mixture composition, the synthesis yields cast composite materials consisting predominantly of the MAX phase Cr 2 AlC and containing the lower chromium carbide Cr 7 C 3 and the chromium aluminide Cr 5 Al 8 . The addition of excess (superstoichiometric) carbon to the starting mixture leads to an increase in the percentage of the MAX phase Cr 2 AlC in the synthesis product, disappearance of the chromium aluminide Cr 5 Al 8 , and the formation of the higher chromium carbide Cr 3 C 2 instead of the lower carbide. The final synthesis products have been characterized by X-ray diffraction and local microstructural analysis. The structure and composition of the synthesis products obtained under various conditions have been determined.

The possibility of producing cast alloyed nitrided steel (up to 1.3 wt % N) by SHS metallurgy is shown. The initial mixture for making alloy cast steel includes iron, chromium, manganese, nickel, molybdenum, vanadium, silicon, and aluminum oxides. Chromium nitride is used as the main nitrogen source. Initial mixtures are shown to be capable of burning over a wide range of reagent ratios, and the combustion temperature exceeds the melting temperature of the condensed combustion products (steel, reducing-metal oxide), which makes it possible to perform their gravitational separation and to make steel in an as-cast form. Experimental studies are carried out in a reactor under nitrogen pressure and in a centrifugal installation under the combined action of overload and pressure to exclude the spread of a mixture during combustion. When the pressure in the reactor is increased from 0.1 to 5.5 MPa, the nitrogen content in steel alloyed with chromium, manganese, nickel, molybdenum, vanadium and silicon is found to increase from 0.4 to 1.3 wt %. Under overload (150 g ) and an increase in the pressure from 0.1 to 1 MPa, the nitrogen content increases from 0.4 to 0.55 wt %. According to X-ray diffraction and electron-probe microanalysis data, the base of the steel has the γ-Fe lattice and is a solution of alloying elements in iron. The shift of the diffraction peaks and their broadening indicate the dissolution of alloying elements in iron. α-Fe precipitates and point inclusions of vanadium, chromium, and impurity-aluminum nitrides are also detected.

Using the method of self-propagating high-temperature synthesis (SHS metallurgy), cast composite materials were produced in the Cr–Ti–B system. The experiments were carried out in universal SHS reactors with an initial argon pressure of Р in = 5 MPa. Mixtures of CaCrO 4 , TiO 2 , Al, and B powders were used as batch. It was shown that by varying the mass ratio α of CaCrO 4 /2Al/2B and 3TiO 2 /4Al/6B mixtures in a batch, it is possible to significantly affect the synthesis patterns, phase composition, and microstructure of the target products. The initial mixtures are capable of burning in the range of α 0–20%. The phase separation limit occurs at α = 15%. The introduction of a highly exothermic CaO 2 + Al additive into the mixture made it possible to expand the phase separation limit to α = 20%. As α increases, the amount of titanium boride in the final product increases. The resulting composite material consists of titanium-chromium boride distributed in a matrix of chromium boride. The synthesized materials were characterized by X-ray and local microstructural analysis. The structural phase states of the target products produced under various conditions were studied.

Cast composite material based on Cr2AlC MAX phase has been prepared by metallothermic self-propagating high-temperature synthesis (SHS) from a mixture of powders of chromium oxide, aluminum, and carbon. The experiments have been performed using an SHS reactor with a volume V of 3 L under an excess inert gas (Ar) pressure (P = 5 MPa). The prepared material has been studied by X-ray diffraction analysis, scanning electron microscopy, and local microstructural analysis. The quantitative analysis has been performed by the Rietveld method. The electrical resistivity was measured in the 100–1300 K temperature range. The resulting material is an electrical conductor with metallic conductivity in the 100–1300 K temperature range, and has the electrical resistivity of the same order as the samples containing 100% Cr2AlC.

A cast material based on the Nb2AlC MAX phase was obtained by SHS metallurgy. Synthesis was carried out from the Nb2O5– Al–C mixture with a high-energy CaO2–Al additive. Thermodynamic calculation results correlate well with experimental data. It was found that the CaO2–Al additive content has a substantial effect on the thermodynamic parameters and phase composition of the final product. It was shown that synthesis from the specified mixtures passed in a stationary mode with steady combustion wave. Increasing the additive content leads to increasing combustion rate (from 6 to 12 mm/s), and product yield to ingot increases (from 30 to 47 %) up to 15 wt.% of the additive and then decreases. Variation in the composition of initial mixtures can provide a significant impact on both synthesis parameters and final product phase composition. Optimal conditions of material synthesis to ensure maximum yield of the Nb2AlC MAX phase in the ingot composition were determined. The liquid phase lifetime during synthesis is a determining factor influencing the Nb2AlC content in the final product. It is shown that the maximum Nb2AlC phase amount (67 wt.%) is reached with 15 wt.% of the high-energy additive in the initial charge.

In this paper, we studied the reaction mechanism of chemical transformations of the initial components in the combustion wave of MoO3/TiO2/Al/Si mixtures of the thermite type in the synthesis of molded molybdenum disilicide (MoSi2) and binary molybdenum titanium silicates ((MoTi)Si2). Syntheses were carried out in a high-temperature synthesis reactor at an initial argon pressure of P0 = 5 MPa. The effect of the ratio of initial reagents and geometric factors on the laws of synthesis of these materials was experimentally studied. The intervals of the component ratios, at which molybdenum and titanium silicides can be synthesized with specified compositions, are determined. We carried out experiments on stopping the combustion front. The chemical transformation of the components of the initial MoO3/TiO2/Al/Si mixtures in the combustion wave is shown to proceed in stages, and chemical reactions can be considered as chemically conjugated processes. The obtained results provide the scientific basis for the creation of promising molded silicide ceramics with high performance properties.