J. P. Hajra

Measurement of Gibbs energies of formation of CoF2 and MnF2 using a new composite dispersed solid electrolyte

Gibbs energies of formation of CoF2 and MnF2 have been measured in the temperature range from 700 to 1100 K using Al2O3-dispersed CaF2 solid electrolyte and Ni+NiF2 as the reference electrode. The dispersed solid electrolyte has higher conductivity than pure CaF2 thus permitting accurate measurements at lower temperatures. However, to prevent reaction between Al2O3 in the solid electrolyte and NiF2 (or CoF2) at the electrode, the dispersed solid electrolyte was coated with pure CaF2, thus creating a composite structure. The free energies of formation of CoF2 and MnF2 are (± 1700) J mol−1; {fx37-1} The third law analysis gives the enthalpy of formation of solid CoF2 as ΔH° (298·15 K) = −672·69 (± 0·1) kJ mol−1, which compares with a value of −671·5 (± 4) kJ mol−1 given in Janaf tables. For solid MnF2, ΔH°(298·15 K) = − 854·97 (± 0·13) kJ mol−1, which is significantly different from a value of −803·3 kJ mol−1 given in the compilation by Barinet al.

Alloy oxide equilibria in the Cr-Mn-O system

The phase boundaries of the Cr-Mn-O system have been investigated by alloy-oxide equilibria at 1173 and 1273 K and by isopiestic technique at 1323 K. The oxide phases which coexist in equilibrium with the Cr-Mn alloys are determined by x-ray diffraction studies. The results of the experiments indicate the presence of MnO in equilibrium with Mn-rich alloy whereas MnCr2O4 and Cr2O3 phases coexist with almost pure Cr. A three-phase equilibrium consisting of MnCr2O4 and MnO phases has been detected at the alloy composition XMn=0·252 at 1323 K. The composition of the alloy delineates the phase boundaries in the isothermal sections of the system. The results are interpreted by thermodynamic analysis of the Cr-Mn-O system using the data from the isopiestic measurements and those available in the literature.

Electrical transport in magnesium aluminate

The conductivity of MgAl2O4 has been measured at 1273, 1473 and 1673 K as a function of the partial pressure of oxygen ranging from 105 to 10−14 Pa. The MgAl2O4 pellet, sandwiched between two platinum electrodes, was equilibrated with a flowing stream of either Ar + O2, CO + CO2 or Ar + H2 + H2O mixture of known composition. The gas mixture established a known oxygen partial pressure. All measurements were made at a frequency of 1 kHz. These measurements indicate pressure independent ionic conductivity in the range 1 to 10−14 Pa at 1273 K, 10−1 to 10−12 Pa at 1473 K and 10−1 to 10−4 Pa at 1673 K. The activation energy for ionic conduction is 1·48 eV, close to that for self-diffusion of Mg2+ ion in MgAl2O4 calculated from the theoretical relation of Glyde. Using the model, the energy for cation vacancy formation and activation energy for migration are estimated.

Thermodynamic modelling of phase equilibria in Al-Ga-P-As system

A generalized thermodynamic expression of the liquid Al-Ga-P-As alloys is used in conjunction with the solid solution model in determining the solid-liquid equilibria at 1173 K and 1273 K. The liquid solution model contains thirtyseven parameters. Twentyfour of them pertain to those of the six constituent binaries, twelve refer to the specific ternary interactions. Additionally the liquid solution model also contains a specific quaternary interaction parameter. The latter has been evaluated here based on the experimental data available in the literature. The present research shows an excellent agreement between the derived and experimental values at 1173 K and 1273 K for the system. The article also presents a comparison between the evaluated values with those based on the regular solution model for the liquid alloys.

Potentiometric determination of the stability of BaCu2O2

Abstract The thermodynamic stability of the compound BaCu2O2 was determined using a solid-state galvanic cell: Pt, Ar + O 2 , Cu 2 O + BaCu 2 O 2 + BaF 2 ∥BaF 2 ∥BaF 2 + BaO, O 2 + Ar, Pt as a function of temperature in the range 970–1170 K. Single crystal BaF2 was used as the solid electrolyte. The partial pressure of oxygen in the argon gas flowing over the electrodes was 1.27 Pa. The reversible e.m.f. of the cell can be expressed by E = 224−0.016T(±1.8) mV . The Gibbs free energy of formation of barium cuprite from component oxides according to the reaction BaO(s) + Cu 2 O(s) → BaCu 2 O 2 (s) is ΔG° = −43 230 + 3.09T(±350) J mol −1 .

A new formalism for representation of heat capacities of metals

The applicability of a function involving geometrical progression of temperature in interpreting the heat capacities of metals has been studied. The constants of the function have been described in terms of vibrational, electronic and magnetic contributions to heat capacities. The equation may be useful in representing heat capacity of metals.