Oxygen permeation rates are reported for 2 mm thick La1-xSrxCoO3-δ (0 ≤x≤0.8) in the temperature range 700 ≤T 1100 °C and oxygen partial pressure range 7.0 x 103 - 1.0 x 105 Pa. With the exception of x = 0.6 permeation rates increase with an increase in Sr doping level. The oxygen permeation can be described by Wagner theory. An anomaly is observed in the oxygen permeation between 750 - 775 °C, which is attributed to an order-disorder transition. It is further shown that the partial pressure at the low partial pressure side of the membrane has an important influence on the permeation rate. The (calculated) ionic conductivities and oxygen vacancy diffusion coefficients are among the highest reported in literature.

Oxygen permeation fluxes through dense SrCo0.80Fe0.20O3-δ discs have been measured in the temperature range of 620-920 °C under various oxygen partial pressure gradients. The permeation results are compared with the previous measurements. Below 800 °C, the apparent activation energy for the overall permeation is 22±4 kcal/mol. The permeation results are discussed in light of the phase diagram of SrCo0.80Fe0.20O3-δ. Based on experiments in which the membrane thickness is varied, we propose that the surface exchange process is the ratelimiting step in the overall permeation reaction. Preliminary catalytic studies of methane partial oxidation in a membrane reactor are reported.

The electrocatalytic behavior of the mixed electron - ion conductor SrCo0.8Fe0.2O3-δ, in the form of an electrode on YSZ, has been studied using impedance spectroscopy at low frequencies. The results suggest that the electrode process is dominated by the surface exchange at high temperatures and by the bulk oxygen diffusion at lower temperatures. No limiting current was observed in I-V measurements. The electrode polarization decreased with increasing current density. The creation of additonal oxygen vacancies in the electrode material is suggested to account for the better performance at the higher current densities.

Powder compositions in the series Lat-xAxCo1-yFeyO3-δ (A=Sr, Ca) have been prepared by a combustion synthesis method. Sintering of pressed powders produced high-density test specimens with the perovskite structure. The specimens exhibited high electrical conductivities with appreciable oxygen-ion conductivity that increased with Co content for the compositions studied. Oxygen permeation studies showed a significant flux of oxygen that increased with temperature for specimens in a P(O2) gradient with no applied field. Thermogravimetric studies of the Lat-xCo0.2Fe0.8O3-δ system indicated a reversible mass loss with increasing temperature that increased with Sr content.

Thin film SrCo0.8Fe0.2O3-δ were made by pulse laser deposition. The electrical conductivity is thermally activated in the temperature 25-500 °C with an activation energy of 0.17-0.19 eV and is temperature independant from 500-800 °C. The optical absorption shows characteristic features which are interpreted qualitatively in terms of a simple band structure diagram.

The blocking electrode method commonly used for the determination of the partial conductivities in a mixed ionic-electronic conductor has been suspected by several authors to give incorrect results in some cases. Experimental evidence illustrating the limitations of this method are presented. The resistance of three pellets of YSZ, the middle one playing the role of a “mixed” conductor, was studied by ac impedance spectroscopy. The imperfect contacts on a microscopic scale formed at the interface between the blocking electrodes and the sample were shown to give rise to additional resistances which cannot be separated from the total bulk resistance. The method was also used to study the conductivity of gold metal. Despite the presence of the ionically blocking electrodes an electronic current is observed. This electronic leakage current is due to the interaction with the gaseous environment, the mixed conductor playing the role of an electrode for the conversion of oxygen ions to electrons.

We solve the transport equations for diffusion-reaction, in the limit of a small O2 pressure gradient, in a mixed oxygen ion-electronic conductor (MIEC) solid oxide membrane consisting of a porous anode and cathode separated by a thin dense layer. The enhancement in ion current due to the porous electrodes calculated for MIEC perovskites is of order 100 for electrode surface area of 106 cm2/cm3.

The ac conductivity was studied for different compositions of CeO2 ceramics, undoped and doped with Y3+ ions in the composition range 1 to 12 atomic %Y over the temperature range from 50 K to 500 K. It was observed that in temperature range 300-400 K, the ac conductivity (as well as the real part of the dielectric constant, ε',) follows the universal power-law frequency dependence with exponent s ≈ 0.63. No systematic variation of s, either with temperature or with dopant concentration, was observed, although for nominally “pure” CeO2 no power-law regime was observed. The features of the ac behavior were explained on the basis of a percolation approach.

Water uptake and conduction have been studied in SrYb0.05Ce0.95O3-δ, a composition known to conduct protons, oxygen ions, and electrons, depending on temperature and environment. Water uptake kinetics evaluated by thermogravimetry occurred in two distinct stages: a rapid, initial weight gain (0.39±0.09 eV, attributed to grain boundary and near surface hydration) followed by a much slower uptake (2.8±0.4 eV, attributed to hydration of the bulk grains). From cyclic voltammetry and mass spectrometry measurements for a cell exposed to asymmetric conditions, currents and activation energies for electronic, oxygen ion, and proton conduction were determined. The activation energy for electronic conduction, 0.90±0.09 eV, is believed to be artificially high due to the increase in electron carrier concentration with increased temperature. The activation energy for oxygen ion conduction (0.97±0.10 eV) agrees well with other oxygen conductors. Proton conduction appeared to follow two different mechanisms: a low temperature process characterized by an activation energy of 0.42±0.04 eV, and a high temperature process, characterized by an activation energy of 1.38±0.13 eV. A possible explanation is that proton conduction at low temperatures is dominated by grain boundaries and is dominated by bulk conduction at higher temperatures.

In a randomly oriented polycrystalline ionically conducting material the total conductivity is reduced by the lengthening of the effective conduction pathway which is determined by the microstructure. It is, therefore, desirable to develop a better understanding of the relationship between the conductivity and the microstructure of the ceramic material.

This work focused on the quantification of the various contributions to the overall conductivity. Na-β” alumina ceramics with different microstructures but the same chemical composition were used as samples. Conductivity data were obtained by impedance spectroscopy measurements (IS) carried out in a temperature range from 350 to -30 °C at frequencies from 1 Hz to 500 KHz. An attempt has been made to calculate distinct geometric factors for the specific values of the grain and grain boundary contribution. These were inferred from the sample geometry, Laser Scanning Microscope (LSM) pictures, microstructural observations by image analysis, Transmission Electron Microscope (TEM) pictures, and considerations of the current pathways in the anisotropically conducting ceramic.

Various lithium phosphorus oxynitrides have been prepared in the form of glasses, polycrystals, and thin films. The structures of these compounds were investigated by X-ray and neutron diffraction, X-ray photoelectron spectroscopy (XPS), and high-performance liquid chromatography (HPLC). The ac impedance measurements indicate a significant improvement of ionic conductivity as the result of incorporation of nitrogen into the structure. In the case of polycrystalline Li2.88PO3.73N0.14 with the γ-Li3PO4 structure, the conductivity increased by several orders of magnitude on small addition of nitrogen. The highest conductivities in the bulk glasses and thin films were found to be 3.0 × 10-7 and 8.9 × 10-7 S-cm-1 at 25°C, respectively.

Computer-based atomistic simulation techniques have been used to investigate the crystal chemistry and phase relationships in alkaline earth hexa-aluminates. The lattice energies for hexa-aluminate-related structural models are calculated and successfully predict that barium prefers the β-alumina-type structures with charge-balancing defects, whilst calcium or strontium prefers the ideal magnetoplumbite structure. In the case of magnesium, it is predicted that it does not prefer a hexa-aluminate structure.

When a minor amount of HF is added to the SnF2 reacted with lead nitrate in aqueous solutions to prepare PbSnF4, a phase transition from tetragonal α-PbSnF4 to orthorhombíc o-PbSnF4 takes place. The transition is essentially bidimensional and takes place in the (a,b) plane of the unit-cell. The compactness of the structure increases at the transition. No essential structural change occurs: the transition is most likely displacive and it is driven by bidimensional nonuniform strain acting along the aand baxes of the unit-cell. This transition is similar to ferroic transitions (in this case, paraelastic → ferroelastic). No detectable change of chemical composition occurs at the transition, and the reason why the presence of HF in the reaction mixture causes the transition remains unknown.

Glasses of B2O3-Na2O-Na2I2 have been prepared over a wide range of compositions on the basis of a fixed O/B ratio and a fixed Na/B ratio, respectively. Raman spectra and the ionic conductivity have been measured to elucidate the role of the iodide. In the fixed O/B ratio, a drastic increase of the conductivity with the addition of iodide is observed. The conductivity results are found to be consistent with the Raman results. In the fixed Na/B ratio, the substitution of iodide does not influence significantly the glass structure as well as the ionic conductivity.

The highest-temperature polymorph of sodium sulfate, (I), has significant orientational disorder in its structure which allows it to readily accept substitution by di- and trivalent cations. Although Na2SO4 (I) exhibits reasonable ionic conductivity, it cannot be quenched to room temperature without changing phase. However, aliovalent solid solutions of (I) can be quenched to RT and resultant cation vacancies promote conduction via Na+ migration. The closely related but more ordered phase Na2SO4 (III) can also form aliovalent solid solutions which can be quenched to RT.

The europium(III) ion is an extremely sensitive and useful probe of its immediate local environment. The presence, location, and intensity of its fluorescence transitions (particularly the 5D0→7F0-2 emissions) can provide detailed information about the symmetry, nature, and multiplicity of the individual Eu3+ site which cannot be gained from X-ray or neutron diffraction techniques.

Our research (supported by the National Science Foundation and the State of Oklahoma) involves the structural characterization of two europium-doped sodium sulfate phases via fluorescence spectroscopy.

The calcium ferrite type structure of Na0.875Fe0.875Ti1.125O4[1] has double tunnels which are occupied by double rows of sodium atoms running along the b-axis. We have partially removed sodium from this compound at moderate temperatures with different oxidizing agents. Electrochemical studies show that the resulting materials, Na0.875Fe0.875Ti1.125O4 (0 ≤ δ ≤ 0.4), can reversibly intercalate lithium at potentials between 3.8 and 1 volt. At the lowest voltages, a compound containing ∼ 0.5 lithium per formula is formed. By chemical reaction of Na0.875Fe0.875Ti1.125O4 with n-butyllithium, the maximum lithium content also corresponds to Li0.5Na0.875Fe0.875Ti1.125O4. This suggests that many more than the one eighth of the empty sodium sites, per unit cell, of the parent phase are now being occupied by lithium.

Fluoride compounds with the tysonite (LaF3- type) crystal structure form an important class of materials which find diverse technological applications as fast ionic conductors. The results of investigations of the ionic conductivity process within tysonite structured R1-xMxF3-x single crystals (R=rare earths and M=alkali-rare earths) are presented using various techniques of electrical response spectroscopy. Influence of isomorphic substitutions of iso- and heterovalent cations on ion transport parameters was studied. By monitoring the composition of nonstoichiometric R1-xMxF3-x. crystals we can vary fluorine-ion conductivity within the range 10−7 - 10−4 S/cm at 293 K and 10−4 - 10−2 S/cm at 500 K. The studied nonstoichiometric crystals with the LaF3-structure are promising solid electrolytes which can be used in chemical sensors and fluorine generators.

The properties of some examples of new classes of highly conducting ionic membranes are described and evoluated in terms of applications in new-design electrochemical devices.

New realizations of the type of ambient temperature lithium ion conducting electrolyte introduced at this symposium two years ago are given and a 5 volt electrochemical window, absent from the earlier examples, is demonstrated. The new systems satisfy the criteria for “ionic oils” though their conductivities, - 10-3 S cm-1, are smaller than desirable.

New ion conductive polymers were obtained by dissolving polycations into room temperature molten salt consisting of methyltriethylammonium chloride (MEAC1) and aluminium chloride (A1C13). These complexes formed viscoelastic solids when the polycation content was increased more than 4 unit mol%. The ionic conductivity of polycation complexes was in the range of 10-4-10-3 Scm-1 at room temperature and were affected by their composition, types of polycations, and counter anion species.