E. P. Simonenko

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.

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.

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.

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.

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.

Thermodynamic Analysis of the Production of Silicon Carbide via Silicon Dioxide and Carbon

The yield of silicon carbide strongly depends on the synthetic conditions. As it results from the literature search, thermodynamic modeling of SiC production using correct values of thermodynamic functions has not been carried out. In order to select the optimum synthetic conditions, the calculation of thermodynamic equilibrium in the system SiO2+nC has been performed under different conditions (р=const; V=const; n=2,3,5). Thermodynamic functions were evaluated on the basis of molecular constants of SiC, SiC2, Si3C, Si2C, Si2C2, selected during comprehensive analysis of the modern data on the structure of these molecules. The calculation of equilibrium compositions was made in the temperature range 1400-3000K. Here all possible components of condensed phases and gas phase have been taken into account. Isochoric process (V=7m/kg) occurs in larger temperature range (1600-2000K); in addition, the decomposition of silicon carbide takes place to a lesser degree, and the concentration of silicon monoxide is considerably higher than that of other components (except CO). It was shown that in respect to SiC production, the isochoric process with n ≥ 3 is the best choice. Introduction and formulation of the problem The engineering of new C/AaBb (A = metal; B = O, C, B) ceramic composites and advanced hightemperature materials (in particular structurally modified, functionally graded materials) based on such composites is a challenging problem of practical importance. Advances in this area are impossible without combining various techniques and approaches, including high-temperature chemistry, sol—gel processing and chemical thermodynamics. A key role in determining the performance of the resulting material is played by the final, high-temperature step of the fabrication process. Fundamentally, fine-particle materials dispersed in a carbon—graphite or graphite matrix may exhibit a number of new spectroscopic properties (modified vibrational, electronic, X-ray fluorescence and other spectra), occasionally similar to those of fullerene-like clusters. At the Materials Week 2002 (Munich) [1], we outlined an approach to preparing carbon—graphite matrix composites containing zirconium (hafnium) carbides, suitable for the synthesis of fullerene-like clusters by the arc discharge method. Attention was given mainly to thermodynamic analysis and the design of precursors [2], since the preparation of fully graphitized materials presents a number of technical problems (heating in a controlled reducing atmosphere for several weeks at temperatures of up to 3000°C), which impedes detailed experimental studies. In spite of the fact that the process of carbothermal reduction of silicon dioxide is one of the main and “old” processes in silicon chemistry and metallurgy, and the results of the synthesis of silicon carbide strongly depend on the synthetic conditions, thermodynamics of this process has not been studied thoroughly yet. In particular, we couldn’t find in literature information concerning thermodynamic modeling of SiC production using correct values of thermodynamic functions. Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 59-62 doi:10.4028/www.scientific.net/MSF.457-460.59

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.