Wolfgang Preis

Nonstoichiometry and transport properties of strontium-substituted lanthanum cobaltites

Abstract In La1−xSrxCoO3−δ (LSC), the oxygen nonstoichiometry and transport properties change with variations in Sr content, temperature and oxygen partial pressure. The purpose of this study was to investigate and correlate these parameters. The extent of oxygen deficiency was investigated by oxygen exchange measurements over the range from 300 to 900 °C and 1.9×10−2⩾pO2⩾1.0×10−4 bar. The oxygen deficiency was found to increase with decreasing oxygen partial pressure and/or increasing temperature. The chemical diffusion coefficient and the surface oxygen exchange coefficient were determined simultaneously by fitting transient re-equilibration curves to the respective diffusion equation at T=725 °C. The electronic conductivity was studied as a function of temperature and oxygen partial pressure at 1.9×10−2≤pO2≤1.0×10−4 bar. Furthermore, these data were correlated with those of the nonstoichiometry to yield conductivity values for constant oxygen content. In the case of La0.6Sr0.4CoO3−δ (LSC04), the temperature dependence of the isostoichiometric conductivities yields activation energies of 0.02≤Ea≤0.05 eV.

Critical evaluation of solubility data: enthalpy of formation of siderite

A general method for the critical evaluation of solubility data of sparingly-soluble metal carbonates has been proposed, which allows the optimization of the thermodynamic properties of the solid phase. This method was applied to siderite, FeCO3(s), leading to a new optimized value for the standard enthalpy of formation, ΔfH(FeCO3) = (−752.0 ± 1.2) kJ mol−1. The generally applicable Gibbs energy minimization program ChemSage has been used for the simultaneous analysis of all relevant literature data of the solubility of siderite valid at various temperatures. The activity coefficients of the ionic species as well as the osmotic coefficient of the solvent were calculated by means of the Davies approximation. Whereas the values for ΔfH(FeCO3) given in the published thermodynamic databases deviate from each other by more than 10 kJ mol−1, it was shown that the results of solubility measurements agree remarkably well with each other and with the most reliable values obtained by other methods. The large variety of literature data for ΔfH(FeCO3) can be attributed to a discrepancy in the auxiliary data for the iron(II) ion. The reliability of a recent selection of the thermodynamic properties of Fe2+ was confirmed by the present evaluation of solubility data.

Fast grain boundary diffusion and rate-limiting surface exchange reactions in polycrystalline materials

Analytical solutions to the diffusion equations for fast grain boundarydiffusion in polycrystalline solids of finite thickness bounded by two parallel surface planes are presented. The grain boundarydiffusion coefficient may be higher by several orders of magnitude than the bulk (volume) diffusion coefficient. The microstructure of the polycrystalline sample has been modeled by means of an isolated grain boundary as well as an array of parallel grain boundaries. The surface concentration has been assumed to vary continuously during the diffusion process, as the surfaceexchange reactions of the diffusing species are considered to be fairly slow and equilibrium between the surface and the constant diffusion source (gas phase) is not attained. Two different cases for surfacediffusion at the interfacepolycrystalline sample/diffusion source are taken into account, viz. negligible and fast surfacediffusion. The pertinent analytical solutions allow the calculation of two-dimensional diffusion profiles in thin films and average concentration profiles for semi-infinite diffusion systems. In addition, the time dependence of the total amount of diffusing species exchanged between the diffusion source and the solid sample has been calculated. The theoretical results are interpreted in terms of Harrison’s classification of diffusion regimes. The parallel grain boundary model enables the calculation of diffusion profiles for both type-A and type-B kinetics. In the case of type-A kinetics expressions for the effective diffusion coefficient as well as the effective surface exchange coefficient are proposed, which depend on the corresponding bulk values, the microstructure of the specimen, and the respective surface condition.

Solid-Solute Phase Equilibria in Aqueous Solution. XII. Solubility and Thermal Decomposition of Smithsonite

AbstractThe solubility constant of ZnCO3, smithsonite, in aqueous NaClO4 solutions hasbeen investigated as a function of temperature (288.15 ≤ T/K ≤ 338.15) atconstant ionic strength I = 1.00 mol-kg−1. In addition, the solubility of zinccarbonate has been determined at 2.00 and 3.00 mol-kg−1 NaClO4 (298.15 K).The solubility measurements have been evaluated by applying the Daviesapproximation, the specific ion-interaction theory, and the Pitzer model, respectively.The thermodynamic interpretation leads to an internally consistent set ofthermodynamic data for ZnCO3 (298.15 K): solubility constant log*Kp500 = 7.25 ± 0.10,standard Gibbs energy of formation Δi Gθ (ZnCO3) = (−777.3±0.6)kJ-mol−1, standard enthalphy of formation Δf Hθ (ZnCO3)= (−820.2±3.0) kJ-mol−1,and standard entropy Sθ (ZnCO3) = (77±10)J-mol−1 K.−1. Furthermore, the DSCcurve for the thermal decarbonation of zinc carbonate has been recorded in orderto obtain the enthalpy of formation ΔfHθ (ZnCO3) =(−820.2±2.0) from theheat of decomposition. Finally, our results are also consistent within theexperimental error limits with a recent determination of the standard entropy ofsmithsonite, leading to a recommended set of thermodynamic properties of ZnCO3:

Solubilities of metal carbonates

\begin{gathered} \Delta _f G^\Theta ({\text{ZnCO}}_{\text{3}} ) = ( - 737.3 \pm 0.6){\text{kJ - mol}}^{{\text{ - 1}}} \hfill \\ \Delta _f H^\Theta ({\text{ZnCO}}_{\text{3}} ) = ( - 818.9 \pm 0.6){\text{kJ - mol}}^{{\text{ - 1}}} \hfill \\ {\text{ }}S^\Theta ({\text{ZnCO}}_{\text{3}} ) = (81.2 \pm 0.2){\text{J - mol}}^{{\text{ - 1}}} - {\text{K}}^{{\text{ - 1}}} \hfill \\ \end{gathered}

Thermodynamic Investigation of Phase Equilibria in Metal Carbonate-Water-Carbon Dioxide Systems

La 0.4 Sr 0.6 CoO 3-δ was prepared by the glycine nitrate process and characterized in respect of purity and phase composition by powder X-ray diffraction and analytical transmission electron microscopy. The oxygen-nonstoichiometry, as a function of oxygen partial pressure (10 -4 bar <P O2 < 10 2 bar) and temperature (300°C < T < 900°C) as well as the chemical diffusion coefficient (725°C and 825°C), were determined by means of oxygen exchange measurements. Electronic conductivity as measured with the van der Pauw-technique is given as a function of temperature (between 300°C and 900°C) and oxygen partial pressure. With the obtained values for the oxygen-nonstoichiometry of La 0.4 Sr 0.6 CoO 3-δ , it is possible to take into account the change of the oxygen content of the sample with temperature and to calculate isostoichiometric (δ = constant) electronic conductivities. Additionally, chemical diffusion and surface exchange coefficients could be obtained from oxygen exchange measurements using simplified and exact fitting procedures.

Electrical properties of grain boundaries in interfacially controlled functional ceramics

Solubilities in aqueous media of sparingly-soluble metal carbonates play an important role in chemical processes whether they occur in a laboratory, on an industrial scale, or in the geologic environment. Solubility phenomena (i.e. dissolution and precipitation reactions) of metal carbonates control procedures for preparing, separating and purifying chemicals. Moreover, the interactions of the hydrological cycle with the cycle of carbonate rocks, the naturally occurring dissolution of carbonate minerals in water as well as their precipitation on the ocean floor and in the sediments of rivers and lakes can be described by the principles of solubility, although gigantic quantities of material may be involved. The experimental methods for the determination of carbonate solubilities are reviewed and the results are discussed within the frame-work of equilibrium thermodynamics. It will be shown that Gibbs functions of pure stoichiometric carbonate phases can be determined by accurately measuring solubilities in aqueous electrolyte systems. In cases where solid-solid phase transformations and recrystallisations are kinetically inhibited, the methods developed were successfully applied to metastable equilibria. Occasionally, solubility data were even employed to estimate activity coefficients of components forming a series of carbonate solid solutions. Whenever possible the activity coefficients of the reacting species were controlled by use of a constant ionic medium. Solubility constants obtained at various fixed ionic strengths were fitted to the Pitzer equations and extrapolated to infinite dilution. Provided the standard potentials of the respective metal ion/metal electrode are known over a range of ionic strengths the values of solubility constants at infinite dilution can be calculated independently. In any case the optimised thermodynamic constants were incorporated in a comprehensive computer model which permits a wide variety of solubility calculations. These are illustrated by a case study of a multicomponent system with industrial relevance.