S. M. Shugurov

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.

Gaseous vanadium molybdate and tungstates: thermodynamic properties and structures.

The stability of gaseous vanadium molybdate and vanadium tungstates was confirmed by high-temperature mass spectrometry. A number of gas-phase reactions involving vanadium-containing salts were studied. On the basis of equilibrium constants, the standard formation enthalpies of gaseous VMoO(4) (-676 ± 27 kJ/mol), VWO(3) (-331 ± 29 kJ/mol), and VWO(4) (-706 ± 23 kJ/mol) at 298 K were determined. A theoretical study of these salts revealed the structure with bidentate binding of the vanadium cation to the anion part to be the lowest-lying isomer, with a quartet spin state for VMoO(4) and VWO(4) molecules as well as a sextet spin state for the VWO(3) molecule. On the basis of critical analysis of the literature data concerning standard formation enthalpies of gaseous VO and VO(2), we adopted new values of Δ(f)H°(298) = 135 ± 10 kJ/mol for VO(g) and -185 ± 15.0 kJ/mol for VO(2)(g). Overall, the results obtained allowed us to estimate the standard formation enthalpy of VMoO(3) to be -318 kJ/mol with an accuracy near 40 kJ/mol.

Thermodynamic properties of the gaseous gallium molybdates and tungstates.

A number of gaseous oxyacid salts have been identified by Knudsen effusion mass spectrometry by vaporizing Ga(2)O(3) from molybdenum and tungsten cells. The stability of gaseous molecules Ga(2)MoO(4), Ga(2)WO(4), Ga(2)Mo(2)O(7), and Ga(2)W(2)O(7) was deduced from the measurements. The structures and molecular parameters of all salts investigated were obtained using quantum chemical calculations. On the basis of equilibrium constants measured for gas-phase reactions, the standard formation enthalpies were determined to be -827 +/- 26, -843 +/- 26, -1578 +/- 32, and -1525 +/- 34 kJ.mol(-1) for Ga(2)MoO(4), Ga(2)WO(4), Ga(2)Mo(2)O(7), and Ga(2)W(2)O(7), respectively.

Thermodynamic study of gaseous vanadium phosphates by high‐temperature mass spectrometry

Knowledge of the structures of gaseous oxyacid salts of the M(m) XO(n) type is of interest for understanding the nature of chemical bonds. Gaseous VPO(2) and VPO(3) have been identified by Knudsen effusion mass spectrometry during the vaporization of mixtures of V(2)O(3) and alkali earth phosphates from molybdenum and tungsten effusion cells. The structures and molecular parameters of the gaseous vanadium phosphates under study were determined using quantum chemical calculations. On the basis of equilibrium constants measured for gas-phase reactions, the standard formation enthalpies were determined to be -273 ± 17 and -615 ± 16 kJ·mol(-1) for VPO(2) and VPO(3), respectively.

Thermochemical study of gaseous salts of oxygen-containing acids: XIX. Tin salts

Reactions of the gas-phase synthesis of tin vanadates, borates, and molybdates were studied. Standard enthalpies of formation and atomization of gaseous salts SnV2O6, SnB2O4, Sn2B2O5, SnMoO4, Sn2MoO5, and SnMo2O7 were determined.

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.

Thermodynamic properties of the La2O3-ZrO2 system by Knudsen effusion mass spectrometry at high temperature

RATIONALE Zirconia doped with a lanthanum oxide system is of high interest due to its exceptional thermal stability for the development of high-performance ceramics. It possesses the beneficial properties of pure zirconia, such as heat resistance, mechanical strength and inertness, but also eliminates its main disadvantage, i.e. brittleness. At high temperatures, components of such systems may vaporize selectively, leading to significant change in composition, and hence, the thermal resistance, phase stability and performance of the ceramic materials. Therefore, information on the vaporization processes and thermodynamic properties of the La2 O3 -ZrO2 system is of great importance. METHODS Knudsen effusion mass spectrometry was used to study the vaporization processes and thermodynamic properties of solid solutions in the La2 O3 -ZrO2 system at 2110 K. Pure La2 O3 was used as a reference substance. Comprehensive characterization of the precursors and ceramics was performed via SEM, STA, XRD and PSD analysis. RESULTS Zirconia-doped lanthania precursor powders and ceramics with La2 O3 of 33.3, 50, 70 and 90 mol.% content were manufactured by solid-state synthesis and the original cryochemical technique. La, LaO, ZrO and ZrO2 were found to be the main vapor species over the samples studied. The activities and thermodynamic properties of La2 O3 were calculated. Via XRD analysis it was shown that the phase composition of xLa2 O3 -(100-x)ZrO2 powders (x = 0.33, 0.5, 0.7 and 0.9 mol. fraction) significantly depends on the synthesis technique chosen. CONCLUSIONS According to the XRD results combined with Rietveld refinement of the patterns, 33.3 La2 O3 -66.7 ZrO2 ceramics after solid-state synthesis are composed of well-formed cubic pyrochlore-type La2 Zr2 O7 with 5 wt.% admixture of monoclinic and cubic ZrO2 whereas 33.3 La2 O3 -66.7 ZrO2 precursor powders after cryochemical synthesis correspond to low-crystalline La(OH)3 . The components of the La2 O3 -ZrO2 system evaporate separately: there is no temperature range where lanthanum and zirconium gaseous species are present together. It was found that the activities of lanthania have low negative deviation from the ideal case.

Gaseous titanium molybdates and tungstates: Thermodynamic properties and structures

RATIONALE Titanium is a component of various construction materials, which is often used at high temperature and in an oxidizing atmosphere. Thermally stable even at high temperatures, titanium compounds may appear in the condensed phase. To predict the possibility of existence of gaseous associates formed by titanium oxides it is important to know their thermodynamic characteristics. Until the present investigation no gaseous salts of titanium were known. METHODS Measurements were performed by high-temperature Knudsen effusion mass spectrometry with a MS-1301 mass spectrometer. Vaporization was carried out using molybdenum and tungsten effusion cells containing samples of pure Au, Ti3O5 and SiO2. A theoretical study of gaseous titanium molybdates and tungstates in different spin states was performed by quantum chemical density functional theory (DFT) B3LYP and M06 methods. RESULTS On the basis of the equilibrium constants of gaseous reactions, the standard formation enthalpies of gaseous TiMoO3 (-424 ± 28 kJ/mol), TiWO3 (-400 ± 22 kJ/mol), TiMoO4 (-795 ± 29 kJ/mol), TiWO4 (-750 ± 24 kJ/mol), TiMoO5 (-1146 ± 23 kJ/mol) and TiWO5 (-1125 ± 22 kJ/mol) at 298 K were determined. Energetically favorable structures were localized and vibrational frequencies were evaluated in the harmonic approximation. Natural atomic charges, bond orders, and valence indices were calculated for all relevant structures. CONCLUSIONS The stability of gaseous species TiMoOn and TiWOn (n = 3, 4, 5) was confirmed by high-temperature mass spectrometry. A number of gas-phase reactions involving titanium-containing gaseous salts were studied. Enthalpies of reactions of gaseous TiXOn (X = Mo, W; n = 3, 4, 5) formation were evaluated theoretically and the obtained values are in agreement with the experimental ones.

Thermal stability and structures of gaseous GeB2O4 and GeMo2O7

The stability of gaseous species GeB2O4 and GeMo2O7 was confirmed by Knudsen effusion mass spectrometry. The gas-phase reactions involving B2O3, Mo2O6, MoO3, GeB2O4 and GeMo2O7 were studied. On the basis of equilibrium constants, the standard formation and atomization enthalpies of gaseous GeB2O4 (−1101 ± 18 kJ mol−1 and 3600 ± 20 kJ mol−1) and GeMo2O7 (−1449 ± 41 kJ mol−1 and 4810 ± 43 kJ mol−1) at 298 K were determined. To get structure parameters and vibrational frequencies of gaseous GeB2O4 and GeMo2O7 quantum chemical investigation of these molecules was undertaken. Formation enthalpies of gaseous GeTaO3, GeTiO3, GeVO3 and GePO2 were estimated.