G. F. Voronin

Thermodynamics of superconducting phases in the Y-Ba-Cu-O system

A thermodynamic phenomenological model is presented to describe thermodynamic properties, oxygen content and ordering in solid solutions from the family of phases Y2Ba4Cu6+nO14+n. All available thermodynamic and structural data on YBa2Cu3O6+z (123) are critically evaluated with the help of this model resulting in a self-consistent set of thermodynamic functions of the 123 tetragonal (t) and orthorhombic (o) phases. This can be used for T<100 K and for all compositions. The data phase diagram of the pseudobinary system YB2Cu3O6−YBa2Cu3O7 is calculated. The orthorhombic-to-tetragonal second-order phase transition curve terminates in a tricritical point where it bifurcates into two solvus lines that encircle the low-temperature two-phase (t+o) field. Thermodynamic properties of YBa2Cu4O8 (124), Y2Ba4Cu7O14+z (247) and some other phases from the homologous series Y2Ba4Cu6+nO14+n are estimated. The phase boundaries between the superconducting phases are calculated. They are plotted on an Ellingham and compared with data from the literature. The present thermodynamic data can be readily used for computing the conditions for thermodynamic stability of superconductors in various chemical environments.

Phase equilibria and stability of superconductors in the Y-Ba-Cu-O system

Abstract The thermodynamic properties of nonsuperconducting phases in the Y-Ba-Cu-O system are critically reviewed and those of YBa 4 Cu 3 O 8.5 are estimated. With the aid of these data, subsolidus phase equilibria are calculated for a wide range of oxygen pressures and temperatures, and stability fields of the 123, 124, and 247 superconductors are constructed. It is shown that the orthorhombic 123 phase is thermodynamically unstable at all temperatures and compositions. The stability diagrams obtained in this study can be readily used for optimization of synthesis conditions of the superconducting phases in the Y-Ba-Cu-O system.

Simultaneous assessment of the YBa2Cu3O6+z thermodynamics under the linear error model

About 3000 experimental points obtained in 220 miscellaneous experiments published in 57 papers have been processed simultaneously in order to obtain the most reliable Gibbs energy of the YBa2Cu3O6+z solid solution in the temperature range from 250 to 1300 K. A part of this solution is well-known as the “Hi-Tc Y123” phase. All other thermodynamic properties of the solution including the conditions for the tetragonal-orthorhombic phase transition and the miscibility gap at lower temperatures, are derived from the assessed Gibbs energy. The linear error model introduced recently by one of the authors has been employed for the simultaneous assessment. The results obtained are compared with those of the conventional weighted least squares method and the benefit of the new approach is discussed. Another problem in simultaneous assessment that is also considered is visualizing the quality of the fit. New types of graphs (partly based on the linear error model) that facilitate visualizing the quality of the fit ...

Solution of ill-posed problems in thermodynamics of phase equilibria. The Zro2 — Y2o3 system

Abstract Problems in thermodynamics of phase equilibria are classified into well-posed and ill-posed. The principles underlying solution of ill-posed problems are explained with special reference to calculation of thermodynamic properties of all phases in the ZrO 2 — Y 2 O 3 system whose phase diagram is plotted.

Effects of CO2 on thermodynamic stability of superconductors in the Y-Ba-Cu-O system

Abstract Regions for thermodynamics stability of superconductors from the family of phases Y 2 Ba 4 Cu 6+ n O 14+ n ( n =0, 1, 2) in CO 2 +O 2 gas mixtures have been established for a wide range of temperatures and partial pressures based on the thermodynamic study of the Y-Ba-Cu-O system. The results show that CO 2 has a crucial effect on the stability of the superconductors, especially Y 2 Ba 4 Cu 7 O 14+ w and YBa 2 Cu 4 O 8 . The reactions of the superconductors with CO 2 , that limit their stability regions, always yield the Y 2 BaCuO 5 phase. Nevertheless, Y 2 Cu 2 O 5 often becomes a more stable product at higher CO 2 partial pressures, some distance away from the stability range. Furthermore, metastable equilibria corresponding to decomposition into Y 2 Cu 2 O 5 or Y 2 O 3 are important under some conditions because of kinetic effects.

A universal method for calculating isobaric-isothermal sections of ternary system phase diagrams

A method for calculating and constructing isobaric-isothermal sections of ternary system phase diagrams with the use of convex hulls was developed. The method is based on the projection of the singularities of the lower part of the Gibbs energy convex hull onto the plane of component mole fractions followed by a geometric analysis of the properties of this projection. The method is applicable to a wide range of ternary systems, it can be used to find tie-line coordinates and determine the phase compositions of all the diagram regions. The quality of the suggested algorithm was estimated, and examples of the construction of several phase diagrams are given.

THERMODYNAMIC CHARACTERIZATION OF HIGH-TEMPERATURE SUPERCONDUCTORS IN THE YTTRIUM-BARIUM-COPPER-OXYGEN SYSTEM. THE Y123 SOLID SOLUTION (Technical Report)

The aim of this report is to inform the chemical community about a self-consistent thermodynamic data set for the YBa2 Cu3 O6+z (1 ≥ z ≥ 0) solid solution, that is well known as the Y123 phase and possesses superconducting properties at z~1 and low temperatures. About 3300 experimental points obtained in 240 miscellaneous experiments published in 78 papers have been processed simultaneously in order to obtain the most reliable Gibbs energy function of the Y123 phase in the temperature range from 250 to 1300 K and pressures up to 100 kbar. A function is recommended for approximation of the Gibbs energy, which has 16 adjustable parameters. All other thermodynamic properties of the Y123 solution, including the conditions for its internal stability, can be derived from the assessed Gibbs energy. Brief descriptions of the thermodynamic model, experimental and data assessment methods as well as examples of self-consistent thermodynamic data applications are given.

Thermodynamic properties of AI + BV intermetallic compounds

Abstract The vapour pressure of alkali metal over the alloys Na + Sb, K + Sb, and Rb + Sb has been measured by the method of molecular beams with the help of a surface ionization detector. Enthalpies of formation and entropies of the 11 intermediate phases have been calculated. Some regularities in the thermodynamic properties in the series of alkali antimonides are discussed and the use of certain models for chemical bonds in these substances is demonstrated.

Thermodynamics of the Pb–Pd system

Abstract In the ranges 590 T /K x Pd E =f( T , x Pd ) using the reversible concentration cell: − W,Pb(pure)/Pb 2+ in liquid electrolyte / Pb x Pd 1−x , W + . Activities of lead in the liquid and solid [Pb+Pd] alloys derived from the measured cell potentials allowed the molar partial (Δ mix G Pb ° , Δ mix H Pb ° and Δ mix S Pb ° ) and integral (Δ mix G m °,xs , Δ mix H m °,xs and Δ mix S m °,xs ) excess thermodynamic functions to be calculated in the one- and two-phase regions (reference state, pure liquid Pd and Pb). The integral molar excess Gibbs energy of formation of the liquid alloy has been described by the following relation: Δ mix G °,xs m =x(1−x)[∑ i i=1 A i (1−2x) n−1 ] with x=x Pd and A i =h i −Ts i . The best description of this excess function was obtained with the following values: h 1 =−16 564; h 2 =13 270; h 3 =−8979; h 4 =2984; and s 1 =1.8554; s 2 =−0.7850; s 3 =s 4 =0. Assuming that all of the intermediate phases are stoichiometric, the excess functions of formation have been calculated. These data are as follows: Phase −Δ f G °(700 K) −Δ f H ° kJ/mol kJ/mol Pb 2 Pd 30.33±0.1 33.31±0.3 PbPd 42.43±0.2 42.7±0.6 Pb 9 Pd 13 48.31±0.3 47.3±1.2 (γ)-Pb 3 Pd 5 49.96±0.4 48.6±2 PbPd 3 55.76±0.5 55.2±4 The validity of this assumption will be discussed. Moreover, the liquidus and solidus lines of the equilibrium phase diagram, as well as the range of stoichiometry of the definite compounds, have been specified.

Calculation of phase equilibria and construction of phase diagrams by convex hull method

Recent findings for a new method developed to calculate phase equilibria are reviewed and analyzed. This method is based on the construction of the convex hulls for characteristic functions in heterogeneous systems. The theoretical basis of this method, as well as the possibilities and features of its practical application in scientific research and teaching chemical thermodynamics, is considered. Software packages developed at the Department of Chemistry of Moscow State University are briefly described.