V. G. Sevast’yanov

Promising ultra-high-temperature ceramic materials for aerospace applications

Some aspects of heat transfer upon the interaction between components with a sharp leading edge and high-enthalpy high-speed flow of dissociated air have been considered; some material characteristics, which should be primarily taken into account when prognosticating the behavior of materials that are promising for using as components of hypersonic flight vehicles, have been substantiated; specific features of the oxidation of materials based on zirconium and hafnium diborides have been touched briefly; the methods of increasing oxidation resistance of these materials that have been developed by various groups of researchers have been demonstrate; some works concerning the behavior of samples under the effect of high-enthalpy flows of dissociated air have been described, including those that simulate sharp leading domes and edges of wings of hypersonic flight vehicles.

Low-temperature synthesis of TaC through transparent tantalum-carbon containing gel

Thermodynamic modeling of the synthesis of tantalum monocarbide via the carbothermic reduction of tantalum(V) oxide indicates that TaC synthesis is thermodynamically plausible below 1000°C at pressures from 1 × 10−5 to 1 × 10−4 MPa. Using a Ta(OC5H11)5 solution in n-pentanol, we prepared a transparent tantalum-carbon containing gel and then a fine-particle Ta2O5 + C mixture, which was used to synthesize tantalum monocarbide at temperatures from 850 to 1200°C and pressures from 10−5 to 10−4 MPa. The elemental and phase compositions of the samples were determined, and the morphology of the TaC particles was examined by electron microscopy.

Production of ultrahigh temperature composite materials HfB2-SiC and the study of their behavior under the action of a dissociated air flow

Ultrahigh temperature composite materials HfB2-SiC containing 25, 35, and 45 vol % SiC were produced by spark plasma sintering. Modeling of heating under the action of a dissociated air flow for selected samples using a VGU-4 induction plasma generator showed that these materials do not degrade even while keeping at a surface temperature of more than 2000°C (up to 2600°C) for 11 min. A combination of optical microscopy, scanning electron microscopy (with EDX analysis), and X-ray computed microtomography were used for investigating the microstructure and composition of the oxidized layer before and after heating.

Vaporization of molecular titanium coordination compounds: a structural-thermochemical approach

Abstract A semi-empirical structural–thermochemical approach was employed to study the vaporization properties of new molecular titanium coordination compounds. Examination of intramolecular steric shielding of individual atoms, and analysis of intermolecular close contacts, identified which groups of atoms in the molecules were important. Evaporation enthalpies were calculated by summing the contributions from the individual groups. Addition of the measured melting enthalpies gave sublimation enthalpies agreeing closely with data determined experimentally using high temperature mass spectrometry.

Synthesis of Ultrafine Refractory Oxides Zirconia-Hafnia-Yttria by Sol-Gel Technology

A transparent gel was prepared using a hydrolytically active solution of zirconium, hafnium, and yttrium alkoxoacetylacetonates. This gel served to synthesize ultrafine zirconia-hafnia-yttria complex oxide. The product dispersity was studied as affected by the parameters of xerogel heat treatment. The thermal behavior of the xerogel was studied by DSC-TGA. Elemental analysis (laser mass spectrometry) and phase analysis of the synthesized materials were carried out. Average crystallite sizes were calculated by the Scherrer relationship. The particle morphology was studied by TEM. Specific surface areas were determined. Sintering of ultrafine oxide powders was studied at 1000, 1200, and 1400°C.

Experimental and theoretical determination of the saturation vapor pressure of silicon in a wide range of temperatures

Reference books and original studies devoted to the determination of the saturation vapor pressure of silicon in a wide range of temperatures have been analyzed. It has been established that no reliable experimental data in the range of high temperatures (above 2000 K) are available in the literature. It has been demonstrated that there is a need to perform additional theoretical and experimental investigations with the use of different methods. The total pressure and partial pressures of Sin (n = 1−6) molecules over liquid silicon are calculated in the temperature range 1700–3400 K. The calculation of the composition of the gas phase over the “Si(l)-container” systems is performed. Materials of the crucibles intended for the use in experimental investigations of the temperature dependence of the saturation vapor pressure of silicon over the liquid phase are recommended.

Coordination compounds with the general formula trans-[M(18-crown-6)(C5HO2F6)2] as structural-thermochemical analogs. The complexes trans-[Pb(18-crown-6)(C5HO2F6)2] and trans-[Ba(18-crown-6)(C5HO2F6)2]

The molecular geometries of the complexes trans-[M(18-crown-6)(C5HO2F6)2] (where M = Ca, Sr, Ba (I), Zn, Cd, Sn, Pb (II), Fe, Co, Eu, and Yb) were modeled by the molecular mechanics method with fixed R(M-O) distances. The shielding degrees of the central metal atom in these complexes were calculated and the number and types of possible intermolecular contacts between their molecules in the structure were determined. The intermolecular interactions involve identical fragments (atoms) of the ligands: the CF3 groups of the hexafluoroacetylacetonate ligands and the methylene fragments of the crown ether. Previously unknown complex II and complex I were synthesized according to an original procedure. The structure and thermochemical properties (including sublimation by the Knudsen method) of complex II were studied. As in complex I, the metal cation in complex II is in the cavity of the macrocycle of the crown ether; the hexafluoroacetylacetonate ligands are trans relative to that cation. The presumed similarity of complexes I and II in thermochemical characteristics was confirmed experimentally. Both the complexes melt in close temperature intervals and sublime at the same temperature (∼10−2 mm Hg) without decomposition. The enthalpies of sublimation of complexes I and II, as well as the entropy contributions to their volatilities, are equal to within the experimental error.

Studies of thermal stability of nanocrystalline SnO2, ZrO2, and SiC for semiconductor and thermocatalytic gas sensors

The thermal stability of synthesized and commercial SnO2, ZrO2, and SiC nanopowders is compared. The crystallite growth rate during the isothermal annealing of the materials at 700°C for 30 h is evaluated. The crystallites’ average size was determined by X-ray phase analysis (using the Scherrer method). The effect of impurity content on the kinetics of crystallite growth is studied for the synthesized SnO2 and ZrO2. Semiconductor and thermocatalytic sensors, based on the synthesized and commercial materials, are manufactured. The long-term stability of the sensors’ signal is compared with the thermal stability of the nanopowders.

Silicon carbide transport during carbothermic reduction of SiO2: Thermodynamic evaluation and experimental study

Mass-transfer processes during the high-temperature carbothermic reduction of silicon dioxide have been studied using thermodynamic modeling. The chemical vapor transport of silicon carbide has been investigated using SiO2 + xSiC mixtures—major reaction products in the SiO2-C system—as examples. Thermodynamic modeling results indicate that the vapor transport of silicon carbide is possible at temperatures from 1300 to 1500°C, and that the major gaseous species involved are Si and CO. Vapor transport processes have been studied experimentally. It is shown that the thermal reaction between carbon monoxide and silicon leads not only to direct conversion of silicon particles to silicon carbide but also to the growth of silicon oxycarbide fibers. The synthesized material has been characterized by x-ray diffraction and high-resolution optical microscopy.

Computer-aided method of analysis of nanocomposite structure on the basis of calculations of isolines of fractal dimensionality

A computer-aided method of analysis of nanocomposite textures on the basis of calculation of the fractal dimensions of nanostructures using microphotographs of the nanocomposites is proposed. The computer-aided method of assessment of the fractal dimensions is notable for the use of an algorithm of calculation of the self-similarity indices of the power spectra of two-dimensional luminance fields of the nanostructure imaging on microphotographs and the use of the box-counting algorithm for measurement of the fractal dimensions. This method makes it possible to assess the rates and directions of cluster growth in the nanocomposite texture. On the basis of numerous computer experiments using the method proposed, we have ascertained that dark zones of two types are present on microphotographs of the texture of the nanocomposites obtained by sedimentation of tin oxides and formation of fine-particle silicon carbide: homogeneous zones, where there is no manifestation of the fine structure of the nanocomposites, and indistinctly contoured structured zones that hypothetically can correspond to the formation of new clusters or ensembles of nanoparticles.