K. T. Jacob

Gibbs energies of formation of chromium carbides

The carbon potentials corresponding to the two-phase mixtures Cr + Cr23C6, Cr23C6 + Cr7C3, and Cr7C3 + Cr3C2 in the binary system Cr-C were measured in the temperature range 973 to 1173 K by using the methane-hydrogen gas equilibration technique. Special precautions were taken to prevent oxidation of the samples and to minimize thermal segregation in the gas phase. The standard Gibbs energies of formation of Cr23C6, Cr7C3, and Cr3C2 were derived from the measured carbon potentials. These values are compared with those reported in the literature. The Gibbs energies obtained in this study agree well with those obtained from solid-state cells incorporating CaF2 and ThO2(Y2O3) as solid electrolytes and sealed capsule isopiestic measurements reported in the literature.

Gibbs energy of formation of cobalt divanadium tetroxide

The Gibbs energy of formation of V2O3-saturated spinel CoV2O4 has been measured in the temperature range 900–1700 K using a solid state galvanic cell, which can be represented as Pt, Co + CoV2O4 + V2O3/(CaO) ZrO2/Co + CoO, Pt. The standard free energy of formation of cobalt vanadite from component oxides can be represented as CoO (rs) + V2O3 (cor) → CoV2O4 (sp), ΔG° = −30,125 − 5.06T (± 150) J mole−1. Cation mixing on crystallographically nonequivalent sites of the spinel is responsible for the decrease in free energy with increasing temperature. A correlation between “second law” entropies of formation of cubic 2–3 spinels from component oxides with rock salt and corundum structures and cation distribution is presented. Based on the information obtained in this study and trends in the stability of aluminate and chromite spinels, it can be deduced that copper vanadite is unstable.

Solid-state cells with buffer electrodes for accurate thermodynamic measurements: system Nd-Ir-O

Abstract A new design for the solid-state cell incorporating a buffer electrode for high-temperature thermodynamic measurements is presented. The function of the buffer electrode, placed between the reference and working electrodes, is to absorb the electrochemical flux of the mobile species through the solid electrolyte caused by trace electronic conductivity. The buffer electrode prevents polarization of the measuring electrode and ensures accurate data. The application of this novel design and its advantages are demonstrated by measurement of the standard Gibbs energies of formation of Nd6Ir2O13 (low-temperature form) and Nd2Ir2O7 in the temperature range from 975 to 1450 K. Yttria-stabilized zirconia is used as the solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode. For the design of appropriate working electrodes, phase relations in the ternary system Ndue5f8Irue5f8O were investigated at 1350 K. The two ternary oxides, Nd6Ir2O13 and Nd2Ir2O7, compositions of which fall on the join Nd2O3ue5f8IrO2, were found to coexist with pure metal Ir. Therefore, two working electrodes were prepared consisting of mixtures of Ir+Nd2O3+Nd6Ir2O13 and Ir+Nd6Ir2O13+ Nd2Ir2O7. These mixtures unambiguously define unique oxygen chemical potentials under isothermal and isobaric conditions. The standard Gibbs energies of formation (ΔG°f (ox)) of the compounds from their component binary oxides Nd2O3 and IrO2, obtained from the emf of the cells, can be represented by the equations: Nd 6 Ir 2 O 13 : Δ G° f (ox) / J mol −1 = −115u2008890 +7.67 T/ K ( ±2640 ) Nd 2 Ir 2 O 7 : Δ G° f (ox) / J mol −1 = −87u2008690 +6.23 T/ K ( ±1460 ) Based on the thermodynamic information, chemical potential diagrams for the system Ndue5f8Irue5f8O are developed.

Solute-solute and solute-solvent interactions in transition metal alloys : Pt-Ti system

Abstract The activity of Ti in solid Pt has been measured as a function of composition at 1573 K using a metal oxide–gas equilibration technique under controlled oxygen partial pressures. Thin foils of Pt were equilibrated with TiOx at constant oxygen chemical potentials. Oxygen partial pressures were established using Ar–H2–H2O gas mixtures of controlled composition. A solid state cell, based on yttria doped thoria as the solid electrolyte, independently determined the chemical potential of oxygen in the gas phase. The concentration of Ti in solid Pt was determined using spectrophotometric methods. The activities of Ti were computed from the oxygen potentials established by the gas phase coupled with independent data on the thermodynamic properties of titanium oxides. The excess chemical potential of titanium in solid Pt at 1573 K in J mol-1 can be represented as ΔμETi = —83 940 — 214 140 (1 — X Ti )2 with an error of ±2800 J mol-1. The activity coefficient of Ti at infinite dilution determined from this study and that of other elements of the first transition series in solid Pt obtained from previous work confirm the trend predicted by both Miedema’s model and Engel–Brewer theory. The attractive interaction between the solute and the solvent (Pt) increases with decreasing atomic number of the solute. The self-interaction parameters of the first transition series elements in solid Pt indicate an increase in solute–solute repulsion with decrease in 3d electron concentration of the solute. The standard Gibbs energy of formation of TiPt3 is —282·57 ± 4 kJ mol-1 at 1573 K. The large negative values of the Gibbs energy of formation of phases in the system Pt–Ti indicate that Pt is not phase compatible with nitrides and carbides of Ti at high temperatures.

Tie-lines and mixing properties of solid solutions in the system CaO-SrO-PbO-O at 1100 K

Phase relations in the system CaO-SrO-PbO-O-2 at 1100 K have been determined by equilibrating samples with different compositions in air, oxygen, or evacuated ampoules for 7 days and characterizing quenched specimens by optical and scanning electron microscopy, energy-dispersive analysis of x-rays (EDX), and x-ray diffraction (XRD). There is a solid-state miscibility gap in the pseudo-binary system CaO-SrO, and continuous solid solubility between Ca2PbO4 and Sr2PbO4 at 1100 K. Substitution of Ca for Sr occurs only to a limited extent (similar to 2 mol.%) in SrPbO3. The calcium-rich solid solutions (Ca1-ySry)(2)PbO4 characterized by y less than or equal to 0.255 are in equilibrium with PbO in air; compositions with y greater than or equal to 0.255 coexist with (Ca1-zSrz)PbO3. There is a three-phase region involving the two monoxide solid solutions (Ca1-xSrx)O on either side of the miscibility gap with x = 0.24 and 0.71 and (Ca1-ySry)(2)PbO4 with y = 0.96. Accurately determined are the locations of tie-lines between the solid solutions. Attainment of equilibrium was checked by the conventional tie-line rotation technique. The excess Gibbs energy of mixing of the solid solution with orthorhombic structure is obtained by an analysis of tie-line data; for the mixing of one mole of Ca and Sr represented by (Ca1-ySry)Pb0.5O2, Delta G(E) = y(1 - y) [15,840 - 2950 y] J/mol. The thermodynamic properties suggest the onset of immiscibility in this solid solution below 884 (+/-5) K. The miscibility gap is asymmetric with a critical composition at y = 0.43 (+/-0.02), Inside the triangle (Ca1-ySry)Pb0.5O2 - (Ca1-zSrz)PbO3 - PbO, a small liquid-phase region is present close to the PbO corner, surrounded by three two-phase fields. Each corner of the approximately triangular liquid-phase region is associated with a three-phase field