Finn Willy Poulsen

Prospects and problems of dense oxygen permeable membranes

The prospects of using mixed ionic/electronic conducting ceramics for syngas production in a catalytic membrane reactor are analysed. Problems relating to limited thermodynamic stability and poor dimensional stability of candidate materials are addressed. The consequences for these problems, of flux improving measures like minimization of membrane thickness and minimization of the losses due to oxygen exchange over the membrane surfaces, are discussed. The analysis is conducted on two candidate materials: La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3- δ and SrFeCo 0.5 O x . Finally, experimental investigations of the dimensional stability of the latter material under reducing conditions are reported.

Conductivity and seebeck measurements on strontium ferrates

Abstract The Seebeck coefficient of nine strontium ferrate ceramics with the nominal compositions: SrFeyO3−x (where y=0 .909, 0.952, 1, 1.05, 1.10), and NazSr1−zFeO3−x and LazSr1−zFeO3−x (where z=0.05, 0.1) were measured in air from 300–1000°C. The deficiency in oxygen stoichiometry, x, adjusts itself in air from approximately 0.15 to 0.35 in this temperature range. Most of the materials have reasonably high electronic conductivities, and some samples of lanthanum-doped ferrate reached conductivities of 50–90 S/cm at 25°C. The total spread in conductivity between the 9 samples was 4 decades at 25°C. Negative temperature coefficients for the total conductivity were observed above 5–600°C. The thermopowers (−5 to +120 μV/K) show similar (strong) temperature dependencies for all materials and increase with increasing temperature. The temperature dependency of conductivity and thermopower can be explained by the strong variation of oxygen content (hole concentration decreases with increasing temperature). A simple defect model for SrFeO3−x at high PO2 was consistent with the observed oxygen partial pressure dependence of the conductivity at 800°C.

Ionic conductivity of solid lithium iodide and its monohydrate

Abstract The solid electrolytes LiI, LiI,H 2 O and LiI,D 2 O have been characterized by ac- and dc-conductivity measurements. LiI exhibits two conductivity regions: an extrinsic below ≈ 180°C and an intrinsic above, with activation energies of 0.43±0.04 eV and 0.81±0.05 eV respectively. The room temperature conductivities of the hydrates LiI,H 2 O and LiI,D 2 O are 6.6× 10 −6 and 6.1×10 −6 (Ω cm) −1 respectively. The activation energy for Li-ion motion in LiI,D 2 O is 0.66±0.05 eV.

Speculations on the existence of hydride ions in proton conducting oxides

Abstract The chemical and physical nature of the hydride ion is briefly treated. Several reactions of the hydride ion in oxides or oxygen atmosphere are given. A number of perovskites and inverse perovskites are listed, which contain the H − ion on the oxygen or B-anion sites in the archetype ABO 3 system. H − is stable with respect to oxide and halide anions but, among cations only with respect to oxides and halides of strongly electropositive metals such as alkaline, alkaline-earth and main group III metals. H − is only stable in combination with transition metal ions of certain elements in their lowest positive oxidation state. Mixed oxide/hydride containing perovskites may thus exist. Steinsvik et al. have recently suggested a defect model for a perovskite including substitutional hydride ions on the oxygen site, H O ⋅ , and protons associated with a lattice oxygen, OH O ⋅ . The defect equations for this acceptor doped A(II)B(IV)O 3 model compound are solved without using the conventional Brouwer approximations. One case is presented where hydride formation is suppressed, and another case where it is promoted. Plots of concentration versus water and oxygen partial pressures show new interesting features; these are discussed.

Thermoelectric power of YSZ

Thermoelectric power of an yttria stabilized zirconia (YSZ) thermocell, Pt|YSZ|Ptm has been determined at 1273 K in different atmospheres. The atmospheres used have been mixtures of O2-inert gases, CO-CO2 and H2-H2O. Oxygen partial pressures ranged between 1 and 10-21 atm. The yttria content in the YSZ was 8 mol%. The dependence on temperature of the thermoelectric power of YSZ in air has also been measured between 600 and 1100°C. The thermoelectric power was determined to be 0.486 mV/K in air at 1 atm, the colder end was positive relative to the hotter end. The transported entropy of O2- in yttria-stabilized zirconia is calculated from a combination of thermodynamic and measurement data and is found to be 33 JK-1 mol-1, independent of oxygen partial pressure and temperature. It is shown that the major contribution to the measured thermoelectric power comes from the entropy change of the electrode reactions. It is possible, from the thermoelectric power of a YSZ thermocell, experimentally to determine approximative values of the single electrode reversible heat evolution in an SOFC. These calculations give that 60 kJ/mol e- will be evolved at the cathode when air is the cathode gas and 41 kJ/mol e- will be consumed at the anode in a mixture of 98% H2 and 2% H2O, at 1273 K when the total gas pressure at both cathode and anode is 1 atm.

Thermoelectric power of doped cerium oxide

Abstract Thermoelectric power and electrical conductivity of CeO 2 doped with 2.5, 5 and 20 mole % CaO and 10 mole% Gd 2 O 3 were determined as function of oxygen partial pressure (Po 2 ) at 1000 °C and as function of temperature at three different Po 2 s. The conductivity measurements show ionic conduction at high Po 2 2 and n -type conduction in reducing atmospheres. The thermoelectric power is positive in the ionic conducting regime and negative for the n -type conduction. The thermoelectric power depends on temperature, except for very low Po 2 , and shows a contribution from electronic conduction already at very small electronic transference numbers.

Thermoelectric power of stabilized zirconia

Abstract It is shown how the thermoelectric power of an oxide with pure oxide ion vacancy conduction can be derived. The result differs somewhat from earlier works on the same subject. Measurements of the thermoelectric power of yttria-stabilized zirconia at 1000 °C were performed as a function of dopant concentration. The results of the measurement are presented together with literature data. They are found to be in acceptable agreement with the presented theory.

Thermoelectric Power of ErAl ( Mg ) O 3 − δ and LaAl ( Mg ) O 3 − δ

Thermoelectric power and electrical conductivity of LaAl 0.95 Mg 0.05 O 3-8 and ErAl 0.95 Mg 0.05 O 3-δ have been determined as a function of oxygen partial pressure between 1 and 10 -15 atm at 1000°C and as a function of temperature in air and in a hydrogen-water buffer. In oxidizing atmospheres these compounds are found to be p-type conductors and in reducing atmospheres ionic conductors. The thermoelectric power at 1000°C depends strongly on oxygen partial pressure. In both reducing and oxidizing atmospheres the thermoelectric power depends on temperature. In this work a new method for calculating ionic transference numbers directly from measurements of the thermoelectric power is discussed.

Super ionic conduction in alkali metal hexachloro niobates and tantalates

ac-conductivity, DSC, and neutron diffraction studies of the ionic conductors KNbCl 6 and KTaCl 6 are presented. The first order phase transitions to the super ionic state occur at temperatures in the range 299–334°C. Conductivities up to 0.35 S/cm are observed. The lowest transition is from a hexagonal to a cubic fcc structure, whereas no definite conclusions could be drawn concerning the structure of the super ionic phases, since only three diffraction peaks were observed in the diffractograms.

Evaluation of 2- and 4-point conductivity measurements on oxide ion conductors

A comaprison between different ac measuring techniques has been performed and evaluation methods for the determination of ionic conductivity in oxygen ion conductors are described. 2- and 4-point impedance measurements using two different frequency response analysers (Solartron FRA-1174 and FRA-1250) are compared. The materials studied were polycrystalline cubic ceriagadolinia, tetragonal zirconia-yttria ceramics and single crystallin YSZ. Empirical rules for the determination of conductivity by geometrical methods from “overlapping” semicircies in the complex plane representation of the immittance data were established. This was supported by model calculations and non-linear least squares fitting techniques.