M. Tetenbaum

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.

Experimental evidence of a miscibility gap in the YBa2Cu3Ox system

Abstract Based on the results of EMF measurements, plots of the logarithm of the oxygen fugacities versus stoichiometry in YBa 2 Cu 3 O x show an inflection around x ≅ 6.65 at temperatures between 400–500°C. There is no apparent inflection at higher temperatures (550–750°C). These new low-temperature data are consistent with the presence of a miscibility gap at lower temperatures. The presence of this gap is evidenced directly by the intersection of the values of RT ln p (O 2 )extrapolated to lower temperatures.

Heat capacity and thermodynamic functions of β‐242Pu2O3 from 8 to 350 K. Contributions to the excess entropy

The heat capacity of β‐242Pu2O3 was determined by a quasiadiabatic method from 8 to 350 K. The preparation of a single‐phase hexagonal plutonium sesquioxide by hydrogen reduction of 242PuO2 at 2250 K is described. A λ‐type heat capacity anomaly with its peak at 17.65 K was found and this anomaly is associated with an antiferromagnetic transition which is reported by McCart et al. in the preceding paper. An excess entropy of β‐242Pu2O3 at 298.15 K is evaluated and shown to approach Sexcess = 2Ru2009ln6 = 29.79 Ju2009K−1u2009mol−1. In the same manner the previously published experimental entropy of 242PuF3 at 298.15 K was shown to contain an excess entropy contribution approaching Sexcess = Ru2009ln6 at 298.15 K. For each compound it was concluded that the excess entropy associated with antiferromagnetic ordering is Ru2009ln2 per mol of Pu+3 ions. For β‐242Pu2O3 at 298.15 K the heat capacity C°p, entropy S°, enthalpy increment H°(T)−H°(0), and the Gibbs energy divided by temperature [G°(T)−H°(0)]/T are, respectively, (116.98±0...

Oxygen stoichiometry, phase stability, and thermodynamic behavior of the lead-doped Bi-2223 and Ag/Bi-2223 systems

Abstract Electromotive-force (EMF) measurements of oxygen fugacities as a function of stoichiometry have been made in the lead-doped Bi-2223 superconducting system in the temperature range 700–815°C by means of an oxygen titration technique that employs an yttria-stabilized zirconia electrolyte. The results of our studies indicate that processing or annealing lead-doped Bi-2223 at temperatures ranging from 750 to 815°C and at oxygen partial pressures ranging from ∼ 0.02 to 0.2 atm should preserve Bi-2223 as essentially single-phase material. Thermodynamic assessments of the partial molar quantities Δ S ( O 2 ) and Δ H ( O 2 ) indicate that the plateau regions in the plot of oxygen partial pressure versus oxygen stoichiometry (x) can be represented by the diphasic CuOue5f8Cu2O system. In accord with the EMF measurements, it was found that lead-doped Bi-2223 in a silver sheath is stable at 815°C for oxygen partial pressures between 0.02 and 0.13 atm.

Thermoelectric Properties of Uranium Monosulfide, Thorium Monosulfide, and US‐ThS Solid Solutions

The measurements on the temperature dependency of the Seebeck coefficient and resistivity of sintered specimens of uranium monosulfide, thorium monosulfide, and US‐ThS solid solutions indicate semimetallic and metallic behavior for US and ThS, respectively. The thermoelectric power of US exhibits a weak dependence on temperature and decreases with increasing ThS content; except for initial disordering effects, the resistivity of US shows a similar dependence on temperature and composition. The presence of higher sulfides in US results in a considerable lowering of the Seebeck coefficient with increasing temperature. The figures of merit for US are too low for useful high‐temperature power generation by a factor of approximately ten.

Some aspects of the thermodynamic behaviour of the lead-doped Bi-2223 system☆

Abstract A thermodynamic assessment of lead-doped Bi-2223 with emphasis on compositions and oxygen partial pressures within the homogeneity region prior to solid-state decomposition is presented. Equations for the variation of oxygen partial pressure with composition and temperature have been derived from our EMF measurements. Long-term metastability was indicated during cycling over a temperature range of ∼ 700–815°C of a lead-doped Bi-2223 sample having an oxygen-deficient stoichiometry of 9.64 prior to solid-state decomposition corresponding to the diphasic CuOue5f8Cu 2 O system. A trend of increasing negative values of the partial molar enthalpy Δ H ( O 2 ) and entropy Δ S ( O 2 with increasing oxygen deficiency of the condensed phase indicated an increase in ordering of the cuprate structure prior to solid-state decomposition.

Some aspects of the thermodynamic behavior of high-Tc oxide systems via EMF and related measurements

Abstract EMF measurements of oxygen fugacities as a function of stoichiometry have been made in the YBa2Cu3Ox, NdBa2Cu3Ox, and Nd1.81Ce0.19CuOx superconducting systems in the temperature range 400–750°C by means of an oxygen titration technique with a yttria-stabilized zirconia electrolyte. The object of our current measurements is to investigate the effect of ionic size of Y, and Nd on the immiscibility and other structural transitions in the LnBa2Cu3Ox system. The shape of the 400°C isotherm for Nd-Ba2Cu3Ox indicates that, if a miscibility gap is present, it exists at lower temperatures and at values of x that are higher than those for the miscibility gap we deduce in the YBa2Cu3Ox system. The relative locations of the miscibility gaps are consistent with the effects of ionic radii on the composition dependence of Tc for these systems. Our results explain the two plateaus in measured values of Tc as a function of composition for the YBa2Cu3Ox system an appear to be consistent with the less pronounced Tc plateau found for the NdBa2Cu3Ox system at higher values of stoichiometry, partial pressures of oxygen above NdBa2Cu3Ox are higher than for the YBa2Cu3Ox system. Results of limited measurements on the n-type (electron-doped) superconducting Nd1.81Ce0.19CuOx system will be presented, as well as a thermodynamic assessment and intercomparison of our oxygen partial pressure measurements with the results of other measurements.

Oxygen stoichiometry, phase stability, and thermodynamic behavior of the lead-doped and lead-free Bi-2212 systems

Abstract Electromotive-force (EMF) measurements of oxygen fugacities as a function of stoichiometry have been made on lead-doped and lead-free Bi2−zPbzSr2Ca1Cu2Ox superconducting ceramics in the temperature range ≈ 700–815°C by means of an oxygen-titration techique that employs an yttria-stabilized zirconia electrolyte. Equations for the variation of oxygen partial pressure with composition and temperature have been derived from our EMF measurements. Thermodynamic assessments of the partial molar quantities Δ H (O2) and Δ S (O2) for lead-doped Bi-2212 and lead-free Bi-2212 indicate that the solid-state decomposition of these bismuth cuprates at low oxygen partial pressure can be represented by the diphasic CuOue5f8Cu2O system.

Thermodynamic and nonstoichiometric behavior of promising Hi-Tc cuprate systems via electromotive force measurements: A short review

Electromotive force (EMF) measurements of oxygen fugacities as a function of stoichiometry have been made on the YBa2Cu3Ox, GdBa2Cu3Ox, NdBa2Cu3Ox, and bismuth cuprate systems in the temperature range ∼400 °C to 750 °C by means of an oxygen titration technique with an yttriastabilized zirconia electrolyte. The shapes of the 400 °C isotherms as a function of oxygen stoichiometry for the Gd and Nd cuprate systems suggest the presence of miscibility gaps at values of x that are higher than those in the YBa2Cu3Ox system. For a given oxygen stoichiometry, oxygen partial pressures above GdBa2Cu3Ox and NdBa2Cu3Ox the higher (above x=6.5) than that for the promising YBa2-Cu3Ox system.

Thermodynamic and nonstoichiometric behavior of the lead-doped and lead-free Bi-2212 systems

EMF measurements of oxygen fugacities as a function of stoichiometry were made on pb-doped and Pb-free superconducting ceramics at 700-815 C using oxygen titration. Equations of oxygen partial pressure vs composition and temperature were derived from the EMF measurements. Thermodynamic assessments of the partial molar quantities {Delta}{bar H}(O{sub 2}) and {Delta}{bar S}(O{sub 2}) for Pb-doped Bi- 2212 and Pb-free Bi-2212 indicate that the solid-state decomposition of these Bi cuprates at low oxygen partial pressure can be represented by the diphasic CuO-Cu{sub 2}O system. Comparison of these results with Pb-doped Bi-2223 in powder and silver sheath form is presented.