Search

Oxidation under exposure to a supersonic dissociated air jet (with heat fluxes in the range 363–779 W/cm2, total exposure time: 2000 s) was studied for HfB2–30 vol % SiC ultra-high-temperature ceramics (UHTC) doped with a lowered amount (1 vol %) of reduced graphene oxide (GO). Doping the ceramics with a relatively low amount of reduced GO (1 vol %) did not prevent a dramatic increase in the average surface temperature to 2300–2400°С. However, the existence time of surface temperatures below 1800–1850°С increased considerably, probably due to an increase in the thermal conductivity of the ceramics. The ablation rate of the material was determined as 6.5 × 10–4 g/(cm2 min), which is intermediate between the respective values for HfB2–SiC ceramics and the ceramics doped by 2 vol % graphene. The microstructure features and elemental composition of the oxidized surface and chips of the material were studied. The structure and thickness of the oxidized near-surface region were determined.

Ti3SiC2 was synthesized by hot pressing at 1400°С for 15 and 30 min at a pressure of 30 MPa from powder mixtures, mainly from Ti/Si/TiC and Ti/TiSi2/TiC. The amount of the Ti3SiC2 phase synthesized from these mixtures was 94.0 and 95.9 vol %, respectively (1400°С, 30 min). The Ti3SiC2 phase crystallized as elongated grains. The effect of the hot pressing time on the formation of the Ti3SiC2 phase from the starting Ti/Si/TiC powders was determined. The final products contained titanium carbide and a small amount of silicon carbide as intermediate phases (for composites containing the starting silicon carbide powders). An excess of silicon leads to the formation of the largest amount of the Ti3SiC2 phase.

The conditions for β-SiAlON to be prepared by reacting highly dispersed mixed silicon and aluminum alkoxide xerogels with molecular nitrogen at 1600–1700°С are determined. The nitrogenation of mixed xerogels is shown to be due to the catalytic pyrolytic decomposition of hydrolysis products of silicon and aluminum alkoxides in a nitrogen atmosphere, namely, a mixture of alcohols that are adsorbed or chemisorbed on aluminosilicate xerogel particles. The pyrolysis of alcohols in a nitrogen atmosphere catalyzed by highly dispersed alumina yields hydrogen and carbon monoxide, which activate nitrogenation and affect the phase composition of the samples and change the sialon stoichiometry from β-Si3Al3O3N5 to the 15R polytype.

The preceramic oligomer organomagnesiumoxanealumoxane was synthesized by the co-condensation of organoaluminoxane and magnesium acetylacetonate. Powders of magnesium aluminate spinel (MAS) were obtained by the pyrolysis of the oligomer at 700 and 1250°C. The effect of the pyrolysis temperature on the physicochemical and rheological properties of the powder was studied. With increasing pyrolysis temperature, the particle shape becomes closer to isometric, which leads to a decrease in the limiting length of the linear portion of the compaction curves. A MAS powder containing 3 wt % Sm2O3 was produced to study the effect of the sintering additive. The sintering kinetics was investigated by dilatometry, and the effect of the sintering additive Sm2O3 on the process and the mass-transfer mechanism during the sintering of spinel was estimated. According to the quantitative analysis data, the activation energies for the pure MAS and the MAS doped with Sm2O3 are Q1250 = 481 kJ/mol and QSm2O3 = 775 kJ/mol, respectively. The sintering additive does not affect the mass-transfer mechanism; however, it increases the activation energy by 60%, doubles the shrinkage, and decreases the temperature of the beginning of sintering from 1546 to 1494°C.

Powders of cerium-containing hydroxyapatites (Ce–HA) with an estimated cerium concentration of up to 0.5 mol % have been synthesized. The phase formation and luminescent properties of the compounds after heat treatment in oxidizing and reducing atmospheres have been investigated. It has been shown that a high HA luminescence intensity can be achieved by preventing the cerium(III) → cerium(IV) transition in a reducing atmosphere at 900–1200°C, as well as by cosubstitution of Ce3+ and Li+ ions for Ca2+ ions to compensate for the excess charge of the additive. The dependence of the luminescence intensity of materials on the heat treatment temperature and cerium concentration is shown.

A new approach to the fabrication of C/SiC core–shell composite fibers through the high-temperature siliconization of carbon fibers with SiO gas in a laboratory-made semi-batch reactor using a granulated 9Si + SiO2 mixture as a solid-phase source of SiO reactive gas has been proposed. The flowsheet of the process provides a uniform siliconization of the material throughout the entire volume of the reactor. It has been shown that C/SiC composite fibers can thus be prepared. Carbon fiber conversion can vary over a wide range depending on the reactor load parameters. The core–shell composite structure is a result of the growth of the SiC layer inward the fiber, which can give rise to deep longitudinal–radial cracks when the SiC shell thickness exceeds 0.7–0.8 µm. At lower conversions, no cracking of the SiC shell occurs, and the produced C/SiC core–shell composite fibers retain their integrity.

Experimental data on the effect of the ambient atmosphere (a vacuum or argon) on the phase composition of silicon carbide fibers formed upon the siliconization of carbon felt are presented. The major phases produced in the siliconization of carbon felt are β-SiC (3C) in vacuo and α-SiC (6H) in the argon atmosphere. When carbon felt is siliconized in vacuo, residual silicon is present on the surface of the silicon carbide felt fibers. The ambient atmosphere determines the nature of the siliconization process at 1650°C: in vacuo, SiC is formed due to the reaction of carbon fibers with the silicon melt wetting them, while in argon, it is formed due mainly to the reaction with vaporous silicon.

The compaction of initial powders, the bending strength, and the microhardness of 21R sialon ceramics obtained by hot pressing have been studied experimentally. It is found that the Sm2O3 sintering additive substantially reduces the annealing temperature and enhances the properties of the ceramics. The samples of 21R sialon ceramics without additives, which are prepared by annealing at 1950°C, have a density of 3.01 g/cm3, a bending strength of 240 ± 15 MPa, and a Vickers microhardness of 16.2 ± 0.4 GPa; the samples of 21R sialon ceramics with 2.5 wt % Sm2O3, which are annealed at 1750°C, are characterized by a density of 3.39 g/cm3, a bending strength of 315 ± 16 MPa, and a Vickers microhardness of 21.9 ± 0.2 GPa. It is shown that Sm2O3 reacts with 21R sialon in the temperature range of 1600–1700°C to form the 27R sialon and SmAlO3 impurity phases. In the samples annealed at 1750°C, the 27R sialon and Sm–sialon (Sm3Si2.5Al3.5O12.5N1.5) impurity phases are detected.

Specific features of the synthesis of sialons (SiAlON) in the Si–Al–O–N system by firing at 1600–1700°C in a nitrogen atmosphere of mixtures of different initial compositions corresponding to the same composition of the final product Si3Al3O3N5 have been studied. β-Sialon was synthesized from reaction mixtures based on crystalline silicon and aluminum nitrides and amorphous highly dispersed hydrated oxides—(1) AlN + SiO2(sol), (2) Si3N4 + Al2O3(sol), and (3) AlN + Si3N4 + Al2O3(sol)—obtained by applying sols based on silicon or aluminum alkoxides (based on the β-sialon stoichiometry) to the surface of nitride powders by the sol-gel method. It has been demonstrated that β-Si3Al3O3N5 can form in the reaction at 1600°C of highly dispersed mixed xerogels made of silicon and aluminum alkoxides—(4) (SiO2 + Al2O3) (mixed sol)—with molecular nitrogen without the participation of the initial nitrides. The influence of the starting composition of xerogel mixtures and synthesis conditions on the phase composition and morphological features of sialons obtained in one stage of calcination in a nitrogen atmosphere has been studied by thermogravimetry, X-ray powder diffraction analysis, and electron microscopy. It has been shown that the proposed reactions of the formation of sialons involving crystalline nitrides, xerogels, and molecular nitrogen are related to reduction–nitridation reactions, in which the reducing agents are organic products of hydrolysis of silicon and aluminum alkoxides and their pyrolytic decomposition in a nitrogen atmosphere.

Ultra-high-temperature ceramic (UHTC) materials with a density of 94.5 ± 1.3% have been manufactured by hot pressing a (HfB2–30 vol % SiC)–5 vol % Y3Al5O12 composite powder, which has been prepared sol–gel method, at a moderate temperature of 1850°C (holding time, 30 min; pressure, 30 MPa). The oxidation resistance of this ceramic has been studied at elevated temperatures under the effect of a supersonic dissociated air flow (on a high-frequency induction plasmatron). The maximum surface temperature has been ~2550°C. An analysis of the kinetics of temperature changes depending on the heat load indicate that, when five percent of Y3Al5O12 by volume of HfB2–30 vol % SiC is introduced into the ceramics, the thermal conductivity of the material decreases. This is not critical from the point of view of the stability of the obtained sample to single temperature drops of ~700–1400°C in a few seconds with sharp heating and cooling of the sample. A decrease of ~35–40% in weight loss of the (HfB2–30 vol % SiC)–5 vol % Y3Al5O12) sample has been noted as the result of exposure as compared with unmodified analogs. Due to the presence of Y3Al5O12 in the initial ceramics, there is also a certain amount of the stabilized phase of cubic HfO2 in the oxidized layer in addition to the monoclinic.

Samples of a solid solution aluminum oxonitride (γ-AlON) of spinel structure with different ratios of components (Al2O3 and AlN) were prepared by the sol–gel technique combined with high-temperature solid-state synthesis at 1750°C in a nitrogen atmosphere. The X-ray diffraction study of polycrystalline samples was performed by the Rietveld method. It was shown that the crystal structure of γ-AlON can be described by the model of permanent anions. The occupancies of tetrahedral and octahedral positions by aluminum atoms in the cationic sublattice of spinel are less than one, and the vacancies are predominantly located in octahedral positions. The lattice constant of the spinel phase (7.9499 A) in the sample with 67 mol % of Al2O3 and the near-stoichiometric composition Al23O27N5, almost coincides with early published data. The spinel parameter u slightly depends on the concentration and is equal to ∼0.3810.

Porous HfB2–65 vol % SiC samples (porosity 34.5%) were produced by reactive hot pressing of HfB2–(SiO2–C) composite powder at 1800°C (heating rate 10 deg/min, holding duration 15 min) and 30 MPa. Using a high-temperature induction plasmatron, their resistance to oxidation by a supersonic dissociated air flow was studied (the heat fluxes in the course of the experiment were varied from 363 to 779 W/cm2). The observation of the temperature distribution over the surface of the sample during the experiment showed that a sharp increase in temperature from ~1770–1850 to ∼2600°C in the samples under investigation occurred at lower heat fluxes and shorter treatment times than that in denser HfB2–30 vol % SiC samples (porosity 9–11%). This indicated that increasing the density of the HfB2–SiC material and also increasing the silicon carbide content reduced the oxidation resistance. However, the fact that the studied sample withstood 37-min exposure to a high-enthalpy dissociated air flow (including 27 min at a surface temperature of 2560–2620°C) without destruction or complete oxidation makes it possible to assign it to ultra-high-temperature materials.

Highly disperse and reactive (HfB2–SiC)@(Ta2O5–HfO2–C) composite powders were manufactured by sol–gel technology where Ta2O5–HfO2–C amorphous components were nanostructured and distributed in each other as uniformly as possible. The reactive sintering of the prepared composite powders at a relatively low temperature (1800°С) with an exposure time of 30 min and the pressure 30 MPa yielded (HfB2–30 vol % SiC)–xTa4HfC5 ultra-high temperature ceramics (UHTCs), where x = 5, 10, and 15 vol %, with a relative density of 75–78%. X-ray powder diffraction proved the complete conversion of tantalum and hafnium oxides to complex carbide Ta4HfC5. The average grain size as determined by scanning electron microscopy did not exceed 2–3 µm in HfB2 and 30–60 nm for the Ta4HfC5 phase. Thermal analysis in flowing air showed that, in the temperature range 20–1400°С, the increasing percentage of tantalum–hafnium carbide (the least oxidation resistant phase) leads to a systematic increase in oxidation-induced weight gain; however, a tendency to saturation is observed. The microstructural specifics of the oxidized surface are noticed depending on the composition of HfB2–SiC–Ta4HfC5 ceramics.

Samples of an ultra-high-temperature HfB2–30 vol % SiC ceramic material with an SiC crystallite size of ~50 nm have been fabricated by hot pressing of an HfB2–(SiO2–C) composite powder synthesized through a sol–gel route. It has been found that the selected hot-pressing conditions (1800°C, 15 min, 30 MPa) ensure complete conversion of silica to silicon carbide and afford a material with a relative density of 96.9%. The effect of a relief applied to the sample surface in the form of ring grooves with a depth of

The main results of scientific sessions at the Sixth Interdisciplinary Scientific Forum with international participation “New Materials and Advanced Technologies” were presented; the sections included: “Nanomaterials and nanotechnology”; “Inorganic functional materials”; “Construction materials”; “Biomaterials and technologies”; “Materials and technologies for green chemistry”; “New materials and technologies in oil and gas industry (gas, oil, energetics).” More than 700 scientists from Russia, Azerbaijan, Belarus, Kazakhstan, Moldova, Tajikistan, Uzbekistan, France, and Czech Republic took part online in the forum, which gave rise to both national and international contacts. Two hundred and twenty six oral reports were presented at the forum, and lectures by famous scientists were organized. Scientific sessions and round tables with participation of representatives of foundations, industries, and scientific journals were held; the foresight sections were devoted to problems of the development of new materials and their commercialization and coverage. The topics included in the program of the forum gathered members of various interdisciplinary research groups concerned with the creation of materials and their use in various sectors of economy, and also the representatives of industrial enterprises interested in the latest developments. The high level of scientific training demonstrated by Russian and foreign scientists at the forum and wide discussion of their original and significant results is an important basis for further development of materials sciences, introduction of advanced technologies, and successful implementation of important priority programs for the development of science and technology.