V. G. Sevastyanov

Synthesis of highly dispersed super-refractory tantalum-zirconium carbide Ta4ZrC5 and tantalum-hafnium carbide Ta4HfC5 via sol-gel technology

Nanocrystalline powders of super-refractory complex carbides Ta4HfC5 and Ta4ZrC5 were synthesized using a hybrid method comprising sol-gel technology for preparing highly dispersed metal oxidescarbon starting mixtures and a relatively low-temperature (1300–1500°C) carbothermal synthesis under a dynamic vacuum (P = 1 × 10−3 to 1 × 10−5 MPa). The elemental and phase compositions of the products and average crystallite sizes were determined. TEM was used to study particle morphology and dispersion. Microstructures were observed by SEM. BET specific surface areas were determined for powders prepared at 1400°C.

Low-temperature synthesis of nanodispersed titanium, zirconium, and hafnium carbides

Nanosized refractory titanium, zirconium, and hafnium carbides were manufactured from highly dispersed metal dioxide-carbon starting mixtures at moderate temperatures of 1200°C or lower. The products were characterized by powder X-ray diffraction, elemental analysis, and transmission electron microscopy. The average size of particles (in nanometers) manufactured at 1200°C was as follows: for TiC, 13 ± 4; for ZrC, 17 ± 3; and for HfC, 16 ± 3; the average crystallite size (in nanometers) was as follows: for TiC, 8 ± 2; for ZrC, 5 ± 2; and for HfC, 8 ± 3. Thermodynamic modeling was performed for the synthesis of Group IVB carbides via carbothermal reduction of the corresponding oxides. The results show that in the titanium dioxide-carbon system, for example, titanium monocarbide formation is possible at a temperature as low as 850°C (p = 10−4 MPa). Highly dispersed metal dioxide-carbon starting mixtures were prepared using solgel technology from metal alkoxyacetylacetonates in the presence of a polymeric carbon source.

Synthesis of ultrafine yttrium aluminum garnet using sol-gel technology

Mesoporous yttrium aluminum garnet Y3Al5O12 powders were prepared using sol-gel technology proceeding from solutions of metal alkoxoacetylacetonates. Xerogel microstructure was studied by SEM, and the fact of mesopores being formed was established. The temperature range within which Y3Al5O12 crystallizes in a dynamic mode from the xerogel was determined to be 850–950°C using an SDT Q600 TGA/DTA/DSC analyzer. A 1-h isothermal treatment of the xerogel was shown to reduce the garnet phase formation temperature to 800°C. At lower temperatures (400, 450 or 500°C), even long-term (6-h) calcination yielded X-ray amorphous powders with developed surfaces (specific surface areas were 230–350 m2/g). Powder particle coarsening was studied upon sintering for 2 and 4 h at 1000, 1200, and 1400°C.

HfB2-SiC (45 vol %) ceramic material: Manufacture and behavior under long-term exposure to dissociated air jet flow

Ultra-high-temperature composite materials HfB2-SiC containing 45 vol % SiC were prepared by spark plasma sintering. The behavior of a sample of this composition under exposure to a subsonic jet of dissociated air of a high-frequency induction plasmatron was studied; the total time was more than 30 min. Under certain test conditions, some regions of the sample were found to experience a rapid increase in temperature up to 2700°C. So, most of the surface area of the sample experienced exposure to temperatures up to 2500–2700°C for more than 15–18 min, while the rest of the surface had a temperature of 1700–1800°C during almost the entire duration of the experiment. The joint use of optical microscopy, scanning electron microscopy (with EDX analysis), X-ray powder diffraction, and X-ray computed microtomography enabled us to study the microstructure and composition of a structurally complex oxidized layer.

Synthesis, vaporization, and thermodynamics of ultrafine Nd2Hf2O7 powders

Ultrafine Nd2Hf2O7 powders with the pyrochlore structure are prepared by self-propagating hightemperature synthesis (in the Pechini version). The elemental and phase composition of the powder are studied. Microstructure is studied by scanning electron microscopy. BET specific surface areas are determined. Thermal behavior is studied by TGA/DSC/DTA up to 1473 K. The sintering kinetics of as-synthesized Nd2Hf2O7 powder at various temperatures is studied. Nd2Hf2O7 vaporization in the range 2400–2600 K is studied by Knudsen effusion/mass spectrometry, and the thermodynamic characteristics of this compound are determined.

HfB2-SiC (10–20 vol %) ceramic materials: Manufacture and behavior under long-term exposure to dissociated air streams

Ultra-high-temperature composite materials HfB2-SiC containing 10, 15, and 20 vol % SiC were prepared by spark plasma sintering. The behavior of the samples prepared under long-term exposure to subsonic dissociated airstreams of a high-frequency induction plasmatron was studied. The total test time per sample was 35–42 min. Under certain exposure conditions (which were dependent on the composition of a sample), some regions of the sample were found to experience a rapid increase in temperature up to 2700°C. These regions enlarged over time, so that most of the surface area of the sample experienced exposure to temperatures of up to 2500–2700°C for 19–38 min, while the rest of the surface had a temperature of up to 1800–1900°C during almost the entire duration of the experiment. The joint use of optical microscopy, scanning electron microscopy (with EDX analysis), and X-ray powder diffraction enabled us to study the microstructure and composition of a structurally complex oxidized layer.

Synthesis of nanocrystalline silicon carbide using the sol-gel technique

Highly disperse silicon carbide is synthesized using a hybrid method comprising the sol-gel step to provide the SiO2-C starting mixture involving the formation of a transparent gel and carbothermal synthesis under relatively soft conditions, namely, at temperatures of 1200–1500°C under a dynamic vacuum. The elemental and phase compositions of the products and their thermal behavior in air are studied. A relationship is found to exist between the microstructure of the product, on the one hand, and the temperature and time of heat treatment, on the other.

Production of 8%Y2O3-92%ZrO2 (8YSZ) thin films by sol-gel technology

Hydrolytically active heteroligand coordination compounds [M(C5H7O2)x(iOC5H11)y] (M = Zr, Y) were synthesized from zirconium and yttrium acetylacetonates, and the kinetics of gelation of their solutions by hydrolysis and polycondensation was studied. Silicon substrates were dip-coated with thin films of solutions of these precursors, and their bulk hydrolysis was investigated. In heat treatment of xerogel coatings under various conditions, the crystallization of nanostructured films of oxide of the composition 8%Y2O3-92%ZrO2 (8YSZ) was examined, and the dependence of the particle phase composition, microstructure, and size on the synthesis conditions was described.

Behavior of a sample of the ceramic material HfB2–SiC (45 vol %) in the flow of dissociated air and the analysis of the emission spectrum of the boundary layer above its surface

Ceramic HfB2–SiC samples with a silicon carbide content of 45 vol % have been obtained by spark-plasma sintering, their density and calculated porosity determined, and certain studies performed using IR spectroscopy, XRD, and other methods. The behavior of the HfB2–SiC (45 vol %) material has been studied using the VGU-4 high-frequency induction plasmatron with heating by a subsonic flow of dissociated air. It has been shown that the average temperature of the surface of the samples in the process of heating rises up to 2680–2690°C, which is connected with the formation on the surface, which has the temperature of ∼1700–1900°C, of local regions with the temperature of 2600–2700°C with gradually expanding areas. The total time during which the average surface temperature is higher than 2000°C is ∼30 min. The change in the composition of the gas phase above the sample surface in the course of the experiment was investigated using optical emission spectroscopy. Assumptions, which explain the changes in the concentration of boron and silicon under the action of a high-enthalpy flow, have been advanced. The elemental composition and the phase composition have been determined, and the microstructure of the surface and of various cuts of the samples have been studied.

Preparation of nanostructured thin films of yttrium iron garnet (Y3Fe5O12) by sol–gel technology

The synthesis of hydrolytically active heteroligand coordination compounds [M(C5H7O2)3–x(C5H11Oi)x] (where M = Al3+ and Y3+) using aluminum and yttrium acetylacetonates has been studied. The gel formation kinetics in their solutions upon hydrolysis and polycondensation has also been studied. Thin films of a solution of these precursors have been applied to polished sapphire substrates by dip coating. The crystallization of nanostructured yttrium aluminum garnet (Y3Al5O12) films during heat treatment of xerogel coatings under various conditions has been studied. How the phase composition, microstructure, and particle size depend on the synthesis parameters has been recognized.