The synthesis and study of the characteristics of β-phase silicon carbide powder is carried out. The compound is obtained by combining endothermic and exothermic reactions (carbothermic reduction of silicon dioxide and synthesis from simple substances) in an induction furnace in an atmosphere of nitrogen and carbon monoxide (CO). Nanofibrous carbon (NFC) is used as the reducing agent and carbide-forming reagent. This carbon agent is obtained as a result of the decomposition of light hydrocarbons. NFC is characterized by a high specific surface area ( 150 m2/g) in comparison, for example, with carbon black ( 50 m2/g). NFC is a fairly pure material and the impurity content in it does not exceed 1 wt

Composite ceramic materials based on B4C with the addition of TiB2 in amounts of 0, 10, 20, 25 and 30 mol. % have been studied. Titanium diboride was synthesized from TiO2 powder and nanofibrous carbon using the boron carbide method in an induction furnace at 1650 °C in an argon atmosphere. The samples were produced by hot pressing at 2100 °C and 25 MPa in an argon environment. The phase composition was determined, and the apparent density and open porosity of the experimental materials were measured. The microstructure was assessed using optical and scanning electron microscopy. The investigations revealed that an increase in the TiB2 content reduces the open porosity while concurrently enhancing the relative density of the boron carbide ceramics. For a sample containing 30 mol. % TiB2 , the open porosity and relative theoretical density were 1.6 and 99 %, respectively. Using XRD and XRS analyses established that the synthesized materials are comprised of two phases: B4C and TiB2 . The average grain size of TiB2 was 0.85 ± 0.02 µm for the sample with 10 mol. % TiB2 and 8.90 ± 0.25 µm for the material with 30 mol. % TiB2 . It was found that at higher TiB2 concentrations, large clusters of grains are formed. The destruction pattern of B4C grains is intragranular, while TiB2 grains are characterized by intergranular destruction. For a sample containing 30 mol. % TiB2 , the fracture toughness was 4.97 ± 0.23 MPa∙m0.5, and the hardness was 3320 ± 120 HV0.5 . Therefore, the addition of TiB2 at these specified concentrations facilitated a 30 % enhancement in fracture toughness relative to single-phase B4C while preserving a high level of hardness.

Composite ceramic materials based on B 4 C with the addition of TiB 2 in amounts of 0, 10, 20, 25 and 30 mol. % have been studied. Titanium diboride was synthesized from TiO 2 powder and nanofibrous carbon using the boron carbide method in an induction furnace at 1650 °C in an argon atmosphere. The samples were produced by hot pressing at 2100 °C and 25 MPa in an argon environment. The phase composition was determined, and the apparent density and open porosity of the experimental materials were measured. The microstructure was assessed using optical and scanning electron microscopy. The investigations revealed that an increase in the TiB 2 content reduces the open porosity while concurrently enhancing the relative density of the boron carbide ceramics. For a sample containing 30 mol. % TiB 2 , the open porosity and relative theoretical density were 1.6 and 99 %, respectively. Using XRD and XRS analyses established that the synthesized materials are comprised of two phases: B 4 C and TiB 2 . The average grain size of TiB 2 was 0.85 ± 0.02 µm for the sample with 10 mol. % TiB 2 and 8.90 ± 0.25 µm for the material with 30 mol. % TiB 2 . It was found that at higher TiB 2 concentrations, large clusters of grains are formed. The destruction pattern of B 4 C grains is intragranular, while TiB 2 grains are characterized by intergranular destruction. For a sample containing 30 mol. % TiB 2 , the fracture toughness was 4.97 ± 0.23 MPa∙m 0.5 , and the hardness was 3320 ± 120 HV 0.5 . Therefore, the addition of TiB 2 at these specified concentrations facilitated a 30 % enhancement in fracture toughness relative to single-phase B 4 C while preserving a high level of hardness.

Ceramic composites based on B4C with a CrB2 mole fraction of 0–30

The results of studying the process of obtaining B 4 C/CrB 2 powder mixtures by the boron-carbide reduction of chromium oxide Cr 2 O 3 in the presence of nanofiber carbon and the results of studying some properties of ceramics made using the synthesized powder are presented. By the method of thermodynamic modeling, it is found that the minimum temperature of the complete reduction of Cr 2 O 3 by the boron-carbide reduction method is 1540°С at a pressure of ⁓0.08 MPa. The characteristics of powders containing 10–70 mol % of the CrB 2 phase are studied. The average size of 50% of the powder particles for the studied compositions does not exceed 11 µm. The specific surface area of the samples does not exceed 6 m 2 /g. The oxidation of the resulting mixtures with oxygen in air begins at a temperature of ~550°С. At the same time, when the temperature reaches 1000°С, no more than 65 wt % of powders is oxidized. Ceramics made using the synthesized powder mixture B 4 C–10 mol % CrB 2 by hot pressing have a relative density of 95% and fracture toughness of 5.25 ± 0.15 MPa m 0.5 .

The results of the researching process of obtaining composition powder material B4C–TiB2 by carbide reduction of titanium dioxide, using carbon reducing agent – carbon nanofibers, are presented. Furthermore, the results of studying of some properties of ceramics made using the synthesized powder are presented. The synthesis of composite materials was carried out in an induction crucible furnace for 20 min in the temperature range of 1200–1900 °C in an argon atmosphere. It has been established that the optimum temperature of the synthesis is 1650 °C, irrespective of the batch composition. The characteristics of the composite powders containing 10–30 mol. % of the TiB2 phase have been studied. X-ray electron microscopy has revealed that the particles of the powder are predominantly aggregated. There are two peaks in the particle size distribution histograms. The part of the histogram with a smaller particle size mainly characterizes the B4C phase. The part of the histogram with a larger particle size characterizes the TiB2 phase. The average particle size of the B4C phase is in the range of 5.3–5.5 µm, and that of the TiB2 phase is in the range of 33.6–41.9 µm. The average size of 50 % of composite powder’s particles for these contents does not exceed 13.4 μm. The surface area of the samples does not exceed 5 m2/g. The oxidation of the composite powder materials by atmospheric oxygen begins at a temperature of approximately 500 °C. At the same time, when the temperature reaches 1000 °C, no more than 45 wt. % of the studied powders is oxidized. Ceramics made with the synthesized powder mixture B4C + 30 mol. % TiB2 by hot pressing has shown rather high values of relative density (99.0±1.1 %) and fracture toughness (5.0±0.2 MPa∙m0.5).

The properties, application, and methods for producing chromium and zirconium carbides are considered. These carbides are oxygen-free refractory metal-like compounds. As a result, they are characterized by high values of thermal and electrical conductivity. Their hardness is relatively high. Chromium and zirconium carbides exhibit significant chemical resistance in aggressive environments. For these reasons, they have found application in modern technology. Chromium carbide is used mainly as component of surfacing mixtures to create protective coatings that resist intensive abrasive wear, including at elevated temperatures (up to 800 °C) in oxidizing environments. This compound is also used in the manufacture of tungsten-free hard alloys and carbide steels. Chromium carbide, along with vanadium carbide, is used as a grain growth inhibitor in WC – Co hard alloys. Powdered zirconium carbide can be used to polish the surface of items made of ferrous and non-ferrous metals. The properties of refractory compounds depend on the content of impurities and dispersion (particle size). To solve a specific problem associated with the use of refractory compounds, it is important to choose the right method for their preparation, to determine the permissible content of impurities in the initial components. This leads to the existence of different methods for the synthesis of carbides. The main methods for their preparation are: synthesis from simple substances (metals and carbon), metallothermal and carbothermal reduction. Plasma-chemical synthesis (vapor-gas phase deposition) is also used to obtain carbide nanopowders. A characteristic is given to each of these methods. Information on the possible mechanism of the processes of carbothermal synthesis is presented.

The properties, application, and methods for producing titanium and vanadium carbides are considered. These carbides are oxygen-free refractory metal-like compounds. As a result, they are characterized by high values of thermal and electrical conductivity. Their hardness is relatively high. Titanium and vanadium carbides exhibit significant chemical resistance in aggressive environments. For these reasons, they have found application in modern technology. These carbides are used as surfacing materials for the application of wear-resistant coatings to steel products. It is possible to use them as catalysts in organic synthesis. Titanium carbide is used in tungsten-free hard alloys, carbide steels. Due to its high hardness, it is used as an abrasive and as a component of ceramic cutting tools. Vanadium carbide serves as an inhibitor of the growth of tungsten carbide grains in hard alloys. The properties of refractory compounds depend on the content of impurities and dispersion (particle size). To solve a specific problem associated with the use of refractory compounds, it is important to choose the right method for their preparation and to determine the permissible content of impurities in the initial components. This leads to existence of different methods for the synthesis of carbides. The main methods for their preparation are: synthesis from simple substances (metals and carbon), metallothermal and carbothermal reduction. Plasma-chemical synthesis (vapor-gas phase deposition) is also used to obtain carbide nanopowders. A characteristic is given to each of these methods. Information on the possible mechanism of the processes of carbothermal synthesis is presented.

The basic element of the design of a power module is a metallized ceramic substrate. In this work, the formation of metallization coatings by the method of thermal transfer of metallization pastes (Mo-Mn-Si + binder) for alumina and aluminum nitride ceramics was carried out. The fixing of the metallization coating on the ceramic substrate was performed by firing at a temperature of 1320 °C. The subsequent deposition of the copper layer was carried out by the method of cold gas-dynamic spraying (CGDS) followed by annealing of the deposited coating. For high-quality adhesion, the optimum annealing temperature was 1000 °C.

Федеральное государственное бюджетное учреждение наукиИнститут неорганической химии им.А. В. Николаева Сибирского отделения Российской академии наук

Graphite oxides obtained by a modified Hummers method were studied using scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, and the Boehm titration method. It was proposed to use ice instead of water to ensure the best hydrolysis and conduct the process at low temperatures without overheating of the reaction mixture. The effect of different ice content of the reaction mixture on the phase composition, chemical composition, and composition of surface functional groups in graphite oxide was studied. The use of frozen hydrogen peroxide in the synthesis was also proposed to decrease the number of stages.

In this paper, we report the preparation of a B4C/ZrB2 composite powder material via boron carbide reduction of zirconium oxide in the presence of carbon nanofiber as a carbon reducing agent. The material was synthesized in the temperature range 1200–1900°C in 20 min. The optimal synthesis temperature was 1650°C, independent of the starting-mixture composition. We have studied characteristics of the composite powders containing 10–30 mol

Boron carbide is characterized by a unique combination of low density (2.52 g/cm3), high hardness (up to 40 GPa), chemical inertness, the high melting point (2450 °C); for these reasons, the ceramics based on this compound have found application in a number of areas of state-of-the-art technologies. However, it is difficult to obtain dense B4C-based ceramics because of a low value of the self-diffusion coefficient, low plastic deformation of this compound, and high sliding resistance between its grains. The use of modifying additives of transition metal diborides appears to be a promising approach to improving the operational characteristics of B4C-based ceramics. They tend to activate the sintering process by means of activation energy reduction, which leads to a decrease in a grain size, an increase in density, strength, and fracture strength of sintered compositions. Zirconium diboride is often used for this purpose. The objective of the work is to study the changes occurring in the charge of boron carbide, zirconium dioxide and carbon when it is heated to determine the temperature of the complete reagents transformation into B4C –ZrB2 composite mixture.

Introduction. Titanium carbide and diboride are characterized by high values of hardness, chemical inertness and for this reason are widely used in modern technology. This paper provides information on the synthesis of titanium carbide and diboride by carbothermal and carbide-boron methods, respectively, on the use of titanium carbide as an abrasive and in the manufacture of tungsten-free hard alloys, carbide steels, wear-resistant coatings, as well as titanium diboride in the production of cutting tools and ceramics based on boron carbide The aim of this work is to study the processes of synthesis of highly dispersed powders of titanium carbide and diboride, which are promising for the manufacture of cutting tools, wear-resistant coatings, abrasives and ceramics. Research methods. Titanium oxide TiO2, nanofibrous carbon (NFC), and highly dispersed boron carbide were used as reagents for the synthesis of titanium carbide and diboride. Experiments to obtain titanium carbide were carried out in a resistance furnace, and titanium diboride in an induction furnace. X-ray studies of the phase composition of titanium carbide and diboride samples were carried out on an ARL X-TRA diffractometer (Thermo Electron SA). The determination of the content of titanium and impurities in the samples of titanium carbide and diboride was carried out by the X-ray spectral fluorescence method on an ARL-Advant'x analyzer. The total carbon content in the titanium carbide samples was determined on an S-144 device from LECO. The content of boron and other elements for titanium diboride samples was determined by inductively coupled plasma atomic emission spectrometry (ICP AES) on an IRIS Advantage spectrometer (Thermo Jarrell Ash Corporation). The surface morphology and particle sizes of the samples were studied using a Carl Zeiss Sigma scanning electron microscope (Carl Zeiss). The determination of the particle/aggregate size distribution was performed on a MicroSizer 201 laser analyzer (BA Instruments). Results. The paper proposes technological processes for obtaining highly dispersed powders of titanium carbide and diboride. The optimum synthesis temperature for titanium carbide is 2,000…2,100 oC, and for titanium diboride 1,600…1,700 oC. The content of the basic substance is at the level of 97.5…98.0 wt. %. Discussion. A possible mechanism for the formation of titanium carbide and diboride is proposed, which consists in the transfer of vapors of titanium oxides to the surface of solid carbon (synthesis of titanium carbide) and vapors of boron and titanium oxides to the surface of solid carbon (synthesis of titanium diboride). Due to the high purity and dispersion values, the resulting titanium carbide powder can be used as an abrasive material and for the manufacture of tungsten-free hard alloys, carbide steels, wear-resistant coatings, and titanium diboride powder can be used for the preparation of cutting tools and ceramics based on boron carbide.

Based on analysis of phase diagram data and thermodynamic modeling, we have evaluated the optimal temperature ranges of the processes underlying the preparation of B4C–TiB2 and B4C–ZrB2 composite powders via boron carbide reactions in the presence of excess boron carbide: 2MO2 + (n + 1)B4C + 3C = 2MB2 + 4CO + nB4C (M = Ti, Zr). The values of n have been taken so as to obtain composite powders with the following compositions (mol %): 90B4C–10MB2 (n = 19), 80B4C–20MB2 (n = 9), 75B4C–25MB2 (n = 7), and 70B4C–30MB2 (n = 5.67). We have found temperatures that ensure the preparation of composite powders with tailored composition at various CO pressures. At a CO pressure of 0.0773 MPa, these temperatures for both reactions are 1816 K (~1540°C), independent of the composition of the synthesized powders. The eutectic temperature in the B4C–TiB2 system is ~2200°C and that in the B4C–ZrB2 system is ~2280°C. Thus, at a nearly atmospheric pressure in the reactor, the optimal synthesis temperature of B4C–TiB2 composite powder lies in the range 1540–2200°C and that of B4C–ZrB2 composite powder lies in the range 1540–2280°C. Such powders are potentially attractive for the fabrication of ceramics with improved performance parameters.

Part two of the review considers the properties, applications, and methods of producing chromium and zirconium diborides. These diborides are oxygen-free, refractory metal-like compounds characterized by high values of thermal and electrical conductance and relatively high hardness. Chromium and zirconium diborides exhibit significant chemical resistance in aggressive environments. Thus, they have found application in modern engineering. Chromium diboride is used as a sintering additive to improve the properties of boron carbide and titanium diboride ceramics. Zirconium diboride is a component of advanced ultrahigh temperature ZrB 2 –SiC ceramics (UHTC) used in supersonic aircrafts and gas turbine assemblies. B 4 C–CrB 2 , and B 4 C–ZrB 2 ceramics have high-quality performance characteristics, in particular, increased crack resistance. The properties of refractory compounds depend on the content of impurities and dispersion. Therefore, to solve a specific problem associated with the use of refractory compounds, it is important to choose the method of their preparation correctly and determine the admissible content of impurities in the primary components. This leads to the diversity of existing methods of synthesizing borides. The main methods of their preparation are synthesis from elements, boron thermal reduction of oxides, carbothermal reduction (carbon reduction of mixtures of metal oxides and boron), metallothermic reduction of metal oxides and boron mixtures, and boron-carbide reduction. There is also plasma-chemical synthesis (deposition from the vapor-gas phase) used to obtain diboride nanopowders. Each of these methods is described.

The properties, applications and methods for producing titanium and vanadium diborides are considered. These diborides are oxygen-free refractory metal-like compounds. As a result, they are characterized by high values of thermal and electrical conductivity. Their hardness is relatively high. Titanium and vanadium diborides exhibit significant chemical resistance in aggressive environments. For these reasons, they have found application in modern technics. So, they are used as surfacing materials when applying wear-resistant coatings on steel products. It is also possible to use vanadium diboride as a catalyst in organic synthesis and the anode in renewable electrochemical current sources. Perspective are ceramics B 4 C – TiB 2 and B 4 C – VB 2 , which make it possible to obtain products based on boron carbide with high-quality performance characteristics, in particular, with increased crack resistance. Such composite ceramics are obtained by means of hot pressing, spark plasma sintering and pressureless sintering. The properties of refractory compounds depend on the content of impurities and dispersion. Therefore, to solve a specific problem associated with the use of refractory compounds, it is important to choose the method of their preparation correctly, to determine the admissible content of impurities in the starting components. This leads to the presence of different methods for the synthesis of borides. The main methods for their preparation are: synthesis from simple substances (metals and boron); borothermal reduction of oxides; carbothermal reduction (reduction of mixtures of metal oxides and boron with carbon; metallothermal reduction of mixtures of metal oxides and boron; carbide-boron reduction. Plasma-chemical synthesis (deposition from the vapor-gas phase) is also used to obtain diboride nanopowders. Each of these methods is characterized in the article.

The study presents a possible mechanism to produce carbides and diborides of transition metals, such as titanium, vanadium, chromium and zirconium. The carbothermal synthesis of transition metal carbides has defined the direct dependence between the thermodynamic stability of oxides and the temperature range of the reduction onset (the stronger the oxide, the higher the value of the temperature is). It reaches 2000-2100, 1500-1600, 1300-1400 and 2100-2200°C for such carbides as TiC, VC0,88, Cr3C2 and ZrC respectively. The same dependence has not been found for the diborides of these metals. Optimum synthesis temperatures for all these compounds lie in the range of 1600-1700 °C. This viable method to produce transition metal carbides consists in the transfer of vaporous higher and lower oxides. Diborides preparation involves the transfer of oxides and boron vapors onto the surface of the carbon material with the subsequent chemical interaction. In the case of carbide-boron reduction of zirconium oxide in excess of boron carbide, the reaction product will be a composite material (B4C – ZrB2). The ceramics based on this composite possesses high performance properties.