Relative Partial Molar Free Energies Research Articles

Thermodynamic Properties of Dilute Solutions of Hydrogen in Glassy PD.80SI.20

ABSTRACTEquilibrium vapor pressures of dilute solutions of hydrogen in glassy Pd.80Si.20 have been measured from 10–90°C and at hydrogen pressures P of 1–100 torr. Under these conditions the ratio of hydrogen to alloy, x as determined by a volumetric method, reaches a maximum value of 0.0070. Over this range of x, the system exhibits a positive deviation from Sieverts' Law; isotherms were analyzed in terms of Lacher's modified statistical mechanical theory of hydrogen in palladium. The data were used to calculate relative partial molar enthalpies, excess entropies, and excess free energies for the formation of the solid solutions. The thermodynamic properties were found to vary with hydrogen content over the composition range studied.

Nonstoichiometry in uranium dioxide

Abstract Thermodynamic properties of uranium oxides with O/U atomic ratios between 2.04 and 2.34, principally in the nonstoichiometric UO 2+x single-phase region, were determined at temperatures in the range 500°C to 1100°C by electromotive force measurements with cells of the type Ni-NiO¦ZrO 2 (+CaO)¦uranium oxide. The relative partial molar free energies of oxygen for the single-phase region UO 2+x and the two two-phase coexisting regions UO 2+x -U 4 O 9−y and U 4 O 9 -U 3 O 8−z were obtained with adequate precision. Several statistical models for the defect structure of UO 2+x have been reviewed. The variation of the relative partial molar entropy of oxygen with composition in the UO 2+x phase was derived on the basis of the 2:2:2 defect complex model proposed from the neutron diffraction study of Willis. The entropies were computed to fit the experimental data assuming that the excess oxygen atoms entered into the fluorite structure in pairs and that the available sites for such oxygen pairs and U 5+ ions would be partially restricted for occupancy.

New Phase Transitions in Non-Stoichiometric U3O8-x at High Temperatures, (I)

The phase equilibrium of non-stoichiometric U3O8-x has been studied by thermogravimetry in the range 765°≦T≦995°C and 10−4≦Po2≦1 atm. The results suggest the presence of six phases within U3O8-x the phase, separated by second (or higher) order transitions. The relative partial molar free energies, enthalpies and entropies within an each of the six phases, as well as the standard free energies, enthalpies and entropies for the phase changes are calculated and compared with previous works.

Thermodynamic Study on the Oxides of Iron, Nickel and Cobalt by the Solid-State Electrolyte Cells

The electromotive force measurements were made on the stacked pellet type galvanic cells consisting of the 0.85 ZrO2-0.15 CaO electrolyte with the electrodes including Fe-FeO (wiistite), FeO-Fe2O4 (magnetite), Fe2O4-Fe2O3 (hematite), Ni-NiO and Co-CoO two-phase coexisting systems at temperatures of 630°to 1100°C, in order to obtain the relative partial molar free energies of oxygen for the two-phase oxide systems. The data obtained were in good agreement with the accepted values, which proved that the cells functioned satisfactorily.The feasibility of employing the solid electrolyte galvanic cell method, especially the open cell staccked pellet technique, was discussed. The careful consideration of several factors made it possible to obtain equilibrium values of the electromotive force.

Thermodynamic study of the Mg−Sb system

Reversible galvanic cell technique has been used to determine the activities, relative partial molar free energies, entropies, and enthalpies of magnesium in liquid Mg−Sb alloys. The potentials were measured in the temperature range 980° to 1250°K, employing the fused salt mixture NaCl+KCl (eutectic)+MgCl2 as the electrolyte. The heat and entropy changes accompanying the formation of Mg3Sb2(s) from liquid components, 3Mg(1)+2Sb(1)=Mg3Sb2(s) were derived from the experimental results; the values are ΔH o=−87,200 (±700) cal per gfw and ΔS o=−31.50 (±1.00) cal per gfw,oK.

High-Temperature Thermodynamic Properties of Oxygen-Deficient Urania

Measurements of oxygen partial pressures over oxygen-deficient urania compositions, including the difficult region very close to stoichiometry, have been made in the temperature range 2080–°2705°K. The transpiration method was used with flowing H2–H2O mixtures in order to fix the oxygen potential of the carrier gases. The shape of the isotherms are qualitatively in accord with expectations based on defect theory. At the lower temperatures (2080°–2390°K), the isotherms exhibit a sharp increase in oxygen partial pressure near the stoichiometric composition. At the higher temperatures (2495°–2705°K), large deviations from the stoichiometric composition are accompanied by relatively small changes in oxygen partial pressures. Relative partial molar free energies, entropies, and enthalpies of solution were calculated and discussed in relation to defect theory.

The activity of copper in solid copper-platinum alloys and some observations on the ordering of Cu 3Pt

The activities and relative partial molar free energies of copper-platinum alloys containing 6.9 to 49.2 at.% platinum were determined over the temperature range 550° to 800°C. The measurements were made with a galvanic cell employing a calcia stabilized zirconia electrolyte. The activity of copper exhibits negative deviations from Raoult's law over the entire range of composition and temperature investigated. The changes in activity on ordering indicate that both Cu 3Pt and CuPt are formed by first-order transitions. Where comparisons are possible, the thermodynamic quantities determined in this investigation are in good agreement with the previous work of Weibke and Matthes. The breaks observed in the cell potential-temperature curves indicate that ordering within the region of the Cu 3Pt superlattice occurs at much higher temperatures and that the maximum critical temperature is at a lower platinum content than shown by the electrical resistivity measurements of Schneider and Esch. High temperature X-ray diffraction studies confirmed this conclusion. The details of the X-ray diffraction and thermodynamic observations indicate rather complex ordering behavior within the Cu 3Pt region. For alloys containing more than 19 at.%Pt a second-order reaction appears to occur above the critical temperature. Based on circumstantial evidence, it is suggested that this reaction may be associated with the formation of periodic anti-phase domains similar to those observed in Cu 3Pd. At least one additional reaction, the character of which is unknown, can be detected at temperatures substantially below the critical temperatures.

The relative thermodynamic properties of solid nickel-palladium alloys

The relative thermodynamic properties of solid nickel-palladium alloys were determined in the temperature range 700–1200°C using a galvanic cell with a solid electrolyte. Within experimental error, the relative partial molar free energies of nickel were linear functions of the temperature, signifying that ΔC p , above 700°C is essentially zero and that the specific heats at elevated temperatures can be approximated by the Kopp-Neumann rule. Nickel-rich solutions were found to exhibit endothermic enthalpies of mixing and positive deviations from Raoult's law for the activity of the solvent and negative deviations for the activity of the solute. Palladium-rich solutions, on the other hand, exhibit exothermic enthalpies of mixing and negative deviations from Raoult's law for the activity of the solvent and solute alike. The excess entropies of mixing are positive for all compositions. This is considered to be primarily a result of the ferromagnetic properties of the alloys. From a consideration of magnetic, electronic, vibrational, and lattice distortional factors it has been concluded that nickel—palladium alloys, particularly those rich in palladium, exhibit significant short-range order.

Some observations on the uranium-niobium hydrogen system

The reaction of body-centered-cubic gamma phase U + 10 wt % Nb and U + 90 wt % Nb with hydrogen was investigated below one atmosphere of pressure between 264° and 571° C. Dissociation pressure hysteresis was found in the 264° C isotherm for the U + 10 wt % Nb alloy, but did not exist at 299° C. The maximum composition reached at 264dg C was U Nb 0.307 H 1.563- Even though UO 2 was present on the sample, it was felt that the hysteresis was an intrinsic property of the metal lattice. The formation of a reaction product on quenching in U + 10 wt % Nb — H alloys indicated a low temperature phase which could have formed by a martensitic mechanism. No hysteresis was found in the equilibrium isotherms for U + 90 wt % Nb — H alloys. The relative partial molar enthalpies and entropies decreased from a mole ratio, n H n U + n Nb , of 0.04 to 0.40 and then increased to n H n U + n Nb = 0.575. Relative partial molar free energies increased with increasing composition at a given temperature and increasing temperature at a given composition. A given percentage of hydrogen was less stable in a U + 90 wt % Nb alloy than in pure Nb.

Studies on polyoxypropylene glycols. Part I. Vapor pressures and heats of mixing in the systems: Polyglycols–methanol

AbstractThermodynamic studies have been made on various fractions of polyoxypropylene glycols of molecular weights from 150 to 3350 in methanol. Vapor pressures of the solvent and the relative lowering of vapor pressure of the solvent by the polymer have been determined in a special differential manometer over the temperature range −30°C. to 28°C. Heats of mixing for the systems have been determined in an isothermal phase‐change calorimeter empolying diphenyl ether as the dilatometric fluid. The relative partial molar free energies are derived for the solvent and solute.