[en] The authors put forward models and give basic approximations for a thermodynamic description of two-component and three-component isomorphous solid solutions. They consider the various contributions made to the free energy of solid solutions formed by isomorphous components, namely the chemical contribution (using Becker's model), the contribution made by the energy of elastic deformation of the crystal lattice, the vibrational contribution, and that of thermally excited electrons. Further, the authors demonstrate the predominant part played in shaping the decomposition curves by the elastic component of the free energy, and analyse the effect of the concentration and temperature dependence of the modulus of elasticity on the shape of the decomposition curves for the solid solutions. Results are given for the calculation of: (a) decomposition curves (solubility curves) for eight systems of γ-uranium (with β-Zr, β-Hf, V, Nb, Ta, Cr, Mo and W) and also for the system β-Zr-Nb; and (b) phase boundaries for the separation regions of body-centred cubic solution in the ternary systems U-Nb-Mo and U-Nb-Zr. A comparison of the results of calculations with experimental phase diagrams indicates, as a whole, that there is satisfactory quantitative concordance. The authors establish the interrelationship between the shape of the phase equilibrium boundaries for isomorphous solid solutions in two-component and three-component systems, on the one hand, and the concentration relationships for some of the simplest physical characteristics, such as the coefficient of electron thermal capacity, Debye temperature, moduluo of elasticity, and anharmonic characteristics, on the other. The difference in principle between the present calculations and the numerous computer calculations of phase diagrams is the maximum generality of the problem as formulated and its solution in the mathematical sense. Indeed, the phase equilibrium curves were calculated by direct computer plotting of the general tangent to the isothermal sections of the free energy surface, and computation of the points of tangency. The main factors responsible for the shape of the asymmetric curves computed for decomposition and solubility are, for the systems U-Zr, U-Hf, U-Nb and U-Ta, as follows: the elastic component of the free energy, which decreases as the temperature rises, the mixing energy at OK, and the configuration-entropy term. A further contribution is made by the vibration component of the free energy for the systems U-V, U-Cr, U-Mo, U-W and Zr-Nb; the system U-Mo exhibits an additional factor of electron origin

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[en] The authors present and discuss expressions for the partial free energies of three-component carbide interstitial phases with allowance for concentration variation in the heat of formation of vacancies. They analyse the congruent vaporization of the solutions and show that stable congruent vaporization can be expected in the case of solutions with short-range ordering. Expressions for the activities of the components in three-component systems are used to evaluate thermodynamic factors. The values of the latter are discussed in the light of the specific features of diffusion-controlled processes in three-component interstitial phases

[en] The paper describes the use of a quadrupole-filter residual-gas analyser and of an ultra-vacuum pumping unit for obtaining thermodynamic data on liquid-vapour systems such as Na-I and Na-Cs. It describes the apparatus and method used (Knudsen cell) as well as the first experiments performed with NaI, sodium and a Na-5at. % Cs alloy. These experiments were used in each case to determine: the nature and composition of the vapour phase (monomeric and dimeric types); the sublimation and vaporization heat. The monomeric (NaI) and dimeric (Na₂I₂) types of sodium iodide were investigated and their sublimation heats measured and found to be 43 kcal/mol and 46 kcal/mol, respectively. Assuming that the capture cross-section of the dimer is 1.5, the proportion of the latter is 4%. A vaporization heat of 23.5 kcal/mol and dimer proportion of 1.5% were recorded for sodium at 330degC. The literature on the subject gives 24.5 kcal/mol and 2%, respectively. A vaporization heat of 17 kcal/mol for caesium and 23 kcal/mol for sodium were found in the case of a Na-5at.percentCs alloy. In the next stage of the analysis ''twin'' cells will be used in order to determine directly the activity of each constituent in a metal alloy by making a comparison with the pressure above the pure metal. In practice a cell used with lithium and sodium has shown an interaction between the two holes of approximately 20%, which will make it necessary to align and focus the molecule beam more precisely. (author)

[en] The values given in the literature for the enthalpy of the formation of titanium diboride, as obtained experimentally and by theoretical estimation, range between -32 and -74.4 kcal/mol. In this paper the authors use the method of direct synthesis from elements in a Calvet calorimeter to determine the enthalpy of formation, ΔHsub(f)⁰, sub(298), of titanium diboride with the composition Tisub(1.000+-0.002)Bsub(2.056+-0.006)Csub(0.009)Nsub(0.003), which was found to be -76.78+-0.83 kcal/mol. They calculate that ΔHsub(f,298) (TiBsub(2.056)=-76.14+-0.85 kcal/mol. The procedure employed makes it possible to carry out the titanium diboride synthesis reaction with the calorimeter at room temperature

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[en] A new method has been devised for establishing the compositions of the lower phase boundary, the congruently vaporizing solution, and the upper phase boundary for high-temperature non-stoichiometric phases having significant ranges of homogeneity. The measurements are made at high temperature rather than on quenched samples. A two-phase sample of known composition is used. One of the phases is the non-stoichiometric one containing the particular composition at which congruent vaporization occurs; the other is the adjacent higher or lower phase. A weighed quantity is vaporized at constant temperature in a mass spectrometer until the congruently vaporizing state has been reached. The intensities of ions are measured during the entire experiment. The loss in mass, the initial composition, the intensities, and the times are used to obtain for the non-stoichiometric phase the composition of the phase boundary and of the congruently vaporizing solution. The experiment is repeated with another sample whose initial composition is on the other side of the non-stoichiometric phase. The method is illustrated with results for the titanium oxygen system in which the Ti₃O₅ phase contains the congruently vaporizing solution. (author)

[en] Some recent compilations of elementary thermodynamic properties of metal-water systems and derived quantities which are needed for optimum design and operation of nuclear steam generation equipment are summarized. Free energies of iron, cobalt, nickel, copper and aluminium metal, oxides and ions in dilute aqueous solution at temperatures up to 300degC are tabulated. Derived data on potential-pH relations, solubilities, H₂ and O₂ equilibrium pressures for reduction and oxidation processes and compound oxide formation are outlined. The practical significance is indicated. Needs for improved data are noted. (author)

No abstract available