V. L. Stolyarova

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.

Thermodynamic properties of silicate glasses and melts: I. System BaO-SiO2

Gibbs energies, activities, and chemical potentials of BaO and SiO2 in melts of the BaO-SiO2 system within the temperature range 1840–1970 K were determined. A significant negative deviation from the ideal behavior was found in the system studied.

High-temperature mass spectrometric study of the vaporization processes of V2O3 and vanadium-containing slags

A Knudsen effusion mass spectrometric method was used to study the vaporization processes and thermodynamic properties of pure V(2)O(3) and 14 samples of vanadium-containing slags in the CaO-MgO-Al(2)O(3)-SiO(2) system in the temperature range 1875-2625 K. The system was calibrated using gold in the liquid state as the standard. Vaporization was carried out from double tungsten effusion cells. First it was shown that, in vapor over V(2)O(3) and the vanadium-containing slags in the temperature range 1875-2100 K, the following vapor species were present: VO(2), VO, O, WO(3) and WO(2), with the latter two species being formed as a result of interaction with the tungsten crucibles. The temperature dependencies of the partial pressures of these vapor species were obtained over V(2)O(3) and the slags. The ion current comparison method was used for the determination of the V(2)O(3) activities in slags as a function of temperature with solid V(2)O(3) as a reference state. The V(2)O(3) activity coefficients in the slags under investigation indicated positive deviations from ideality at 1900 K and a tendency to ideal behavior at 2100 K. It was shown that the V(2)O(3) activity as a function of the slag basicity decreased at 1900 K and 2000 K and was practically constant in the slag melts at 2100 K. The results are expected to be valuable in the optimization of slag composition in high-alloy steelmaking processes as well as for their environmental implications.

On the fluctuation structure of single-phase glasses in the SrO-B2O3-SiO2 system

The structure of single-phase glasses in the SrO-B2O3-SiO2 system has been studied by the small-and large-angle X-ray scattering technique. The glasses containing 35, 40, and 45 mol % SrO upon equimolar replacement of B2O3 by SiO2 have been investigated. It has been demonstrated that the glasses do not contain chemical inhomogeneity regions. The inhomogeneity of the glasses is determined only by thermal density fluctuations. The isothermal compressibility varies insignificantly upon replacement of B2O3 by SiO2 and decreases with an increase in the SrO content. The glass structure is consistent with the model of ideal associated solutions.

Mass-spectrometric study of vaporization of high refractory ceramics

150 Development of new approaches to modification of casting technology for new generation gas turbine engine (GTE) blades using advanced refractory alloys, which would allow the working gas temperature at the turbine inlet to be increased to 2273 K, contributes to the design of almost “stoichiometric” gas turbine engine [1, 2]. However, the practical manufacture of the GTE blade cast products from high temperature composites requires, first of all, choosing ceramic materials for subsequent development of the manufac ture of ceramic rods and molds with high thermal sta bility and chemical inertness to the melts during molding by directional crystallization at temperatures above 2000 K. The successful solution of this problem is highly important regarding the strategic trends of development of materials and material processing technologies for the period up to 2030, as was shown in detail in a review [3].

Mass spectrometric study of thermodynamic properties in the Yb2O3-ZrO2 system at high temperatures

RATIONALE Materials based on the Yb2O3-ZrO2 system have many industrial applications such as high-temperature solid electrolytes, ceramics with special properties and most importantly for thermal barrier coatings. As their synthesis and use take place at high temperatures, information on the vaporization processes, thermodynamic properties and phase equilibria of this system at high temperatures is of great importance. METHODS Measurements were performed by high-temperature Knudsen effusion mass spectrometry with a MS-1301 mass spectrometer. Vaporization was carried out using two tungsten effusion cells containing the sample under study and pure Yb2O3. The values of component activities in the Yb2O3-ZrO2 system were also calculated using the CALPHAD approach. RESULTS The Yb and O vapor species were identified over the samples studied at 2400 K. Using these data the ZrO2 activities, chemical potentials of components and the Gibbs energies of the solid solution formation were calculated in this system. The thermodynamic values were also obtained as the result of modeling of the Yb2O3-ZrO2 system based on the CALPHAD approach using the data available on the phase diagram of this system and calorimetric measurements only. CONCLUSIONS The thermodynamic functions found in the Yb2O3-ZrO2 system at 2400 K, such as the activities of components and the Gibbs energy of formation, displayed negative deviation from ideality. Mutual agreement was observed between the experimental thermodynamic values and the results of calculations based on the CALPHAD approach.

High-temperature mass spectrometric study and modeling of thermodynamic properties of binary glass-forming systems containing Bi2O3.

RATIONALE Binary glass-forming systems containing bismuth(III) oxide, especially the Bi2O3-SiO2 system, are of great importance in modern materials science: preparation of thin films, fiber optics, potential solar converters, and radiation shields in nuclear physics. Information on vaporization processes and thermodynamic properties obtained in the present study and the results of modeling of this system will be useful for optimization of the synthesis and applications of Bi2O3-containing materials at high temperatures. METHODS High-temperature Knudsen effusion mass spectrometry was used to study the vaporization processes and to determine the partial pressures of components of the Bi2O3-SiO2 system. Measurements were performed with a MS-1301 mass spectrometer. Vaporization was carried out using two iridium-plated molybdenum effusion cells containing the sample under study and pure bismuth(III) oxide (reference substance). Modeling of the thermodynamic properties and structure of glasses and melts in the Bi2O3-SiO2 and Bi2O3-B2O3 systems was performed using a modified approach based on the generalized lattice theory of associated solutions (GLTAS). RESULTS At a temperature of 1000 K, Bi and O2 were found to be the main vapor species over the samples studied. The Bi2O3 activity as a function of composition of the Bi2O3-SiO2 system was obtained from the measured partial pressures of the vapor species. The thermodynamic properties of mixing from oxides in this system were calculated. The advantages of GLTAS for modeling of glasses and melts in the binary systems containing Bi2O3 were demonstrated. CONCLUSIONS The thermodynamic functions of mixing in glasses and melts of the Bi2O3-SiO2 system determined at 1000 K in the present study, as well as in the Bi2O3-B2O3 system, demonstrated negative deviations from ideality. Modeling of the obtained experimental data using GLTAS allowed a correlation to be found between the thermodynamic properties and the relative number of bonds of various types formed in the glasses and melts of these systems.

On the structure of glasses in the BaO-B2O3-SiO2 system

The structure of single-phase glasses in the BaO-B2O3-SiO2 system has been studied by the large- and small-angle X-ray scattering techniques. The glasses containing 40 mol % BaO upon equimolar replacement of B2O3 by SiO2 have been investigated. It has been demonstrated that the incorporation of barium ions into structural groupings fixes their position and provides ordering in the distribution of barium ions at interatomic distances up to at least 5 Å. The glasses under investigation are homogeneous, and their inhomogeneity is determined by thermal density fluctuations and fluctuations of the concentration of a part of barium ions distributed in a statistically random manner in the volume of the glass. The observed ordering in the distribution of barium ions is not reduced to the formation of local clusters with an increased concentration of barium ions but is most likely a characteristic feature of the bulk glass structure. The glass structure is consistent with the model of ideal associated solutions.

Kinetics of early stages of phase separation in glasses of the PbO-B2O3 system

Kinetics of changes in sizes of phase regions and volumes at the early stages of the phase separation of glasses in the PbO-B2O3 system has been studied using small-angle X-ray scattering (SAXS). The existence of the interval of heating durations in which the changes in the sizes of the phase regions and volumes are small so that it is possible to approximately consider that the structure is in a relatively stable state has been experimentally established. The relative phase volumes and compositions in glasses of the studied system are close to the equilibrium ones already for the sizes (radii) of the regions of the separated phase of about 15–20 Å and the Ostwald ripening (coalescence) stage begins. Maxima due to the interparticle interference have been observed in the angular dependences of the SAXS intensities. This indicates ordering in the distribution of the phase regions.

Thermodynamic properties of silicate glasses and melts: V. Systems CaB2O4-CaSiO3 and Ca2B2O5-CaSiO3

Thermodynamic properties of melts of the CaB2O4-CaSiO3 and Ca2B2O5-CaSiO3 systems were determined by the method of high-temperature mass-spectrometry. The melts of these systems are characterized by negative deviations from the ideal behavior at 1800 K.