Thermodynamic Properties of the Oxides of Fe, Ni, Pb, Cu, and Mn, by EMF Measurements

Abstract

A "closed system" solid electrolyte electrochemical cell has been designed to investigate the thermodynamic properties of metal oxides. The measurements with this cell are free of mixed potentials arising from nonequilibrium oxygen pressure conditions in the electrode compartments. The oxide systems investigated include Ni-NiO, Pb-PbOr Cu-Cu20, Cu20-CuO, Fe-FexO, FeuO-Fe304, Fe304-Fe203, MnO-Mn304, and Mn~O4-Mn203. Calculation of equilibria involving oxide phases at high temperatures is often handicapped by the lack of reliable thermodynamic data on free energies. Most of the standard free energies of formation for oxides available in the literature have been calculated from enthalpies and heat capacities measured calorimetri-cally or from gas-solid equilibrium-type measurements. Quite often the high-temperature heat capacities are obtained by extrapolation from .lower temperature data or estimated using semi-empirical rules. Calculations based on calorimetric data are often contradictory and include the rather wide error limits of the various thermal quantities involved in the calculation. For example, the free energies of formation of the oxides PbO, NiO, FeO, and Cu20 given by Kubaschewski and Evans (1), have an estimated uncertainty of-+3, __2,-+3, and _+1, kcal/mole of oxide, respectively. Also, the free energies for these oxides found in thermodynamic tabulations by Wicks and Block (2) differ from the average values given by Kubaschewski and Evans (1) by about 1 kcal. More recently, electrochemical measurements with galvanic cells using solid oxide electrolytes made possible the direct determination of free energies at elevated temperatures with a higher degree of accuracy. Particularly, the pioneering work by Kiukkola and Wagner (3) has demonstrated the usefulness of the solid solutions of calcium and zirconium oxides as selective oxygen anion electrolytes. These elec-trolytes have been used in solid oxide electrochemical cells to separate the two electrode compartments, to screen the electronic conductivity of the oxides at high temperatures, and to establish electrochemical contacts which are free of junction potentials. The systems investigated so far by this technique include the oxides of Ni (3-15, 21, 23), Fe (3-7, 9-12, 14-22), Cu (3, 4, 8, 12, 23), Pb (4, 8, 12), Mn (6), and many others. These measurements have been conducted by the "open cell stacked pellet" technique described by Kiukkola and Wagner (3). In this method the indicating electrode consisted of a pelletized and sintered mixture of the metal-metal oxide system under investigation separated from the reference electrode by a pellet of the zirconia-calcia diaphragm. The reference electrode was another metal-metal oxide system of known free energy of formation. Most of these cells were operated under flow using an inert gas common for both electrodes. Blumenthal and Whitmore (6) used the common open cell stacked pellet technique in a stream of purified inert gas along with a ZrO2 based electrolyte to measure the systems Fe804-Fe203 and MnO-Mn304 between 750 ~ and 1050~ Their ranges of scatter amounted to-+ 5 my with the latter and-+ 7 mv with the former, approximately two to four times the scatter observed by workers working with more conventional oxide systems. Rezukhina et al. (24) modified the usual technique slightly and operated their cells under vacuum. Rapp (7) determined the free energy of formation of MoO2 from Mo using as reference half-cells the systems Fe-FexO and Ni-NiO along with ZrO2 (0.15 CaO) as the solid electrolyte. When his cell was constructed simply by pressing together flat metal oxide and electrolyte tablets, he noted that the cell voltage was about 35 mv too low and very dependent on helium flow rate. His later results were greatly improved by fabricating the electrolyte in an H-form and fitting the sintered metal-metal oxide cylinders into each side. Roeder and Smeltzer (9) mounted their stacked pellets of electrodes and electrolyte independently from the furnace assembly. Finally, Steele and Alcock (4) developed a cell whereby the two electrodes could be kept separate by pressing one side of the electro-lyte against the open polished end of an alumina tube.