Kjell Wiik

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.

Oxygen permeation in the system SrFeO3−x–SrCoO3−y

Abstract Oxygen flux through dense membranes with composition SrFeO 3− δ , SrFe 0.67 Co 0.33 O 3− δ and Sr 0.97 Fe 0.33 Co 0.67 O 3− δ (0, 33% and 67% Co) have been measured primarily at 1000 °C as a function of membrane thickness with synthetic air on the primary side and He on the secondary side. Maximum flux rate was found to increase with cobalt content with an observed maximum for a membrane with composition 67% cobalt and thickness 0.5 mm corresponding to 3.8×10 −6 mol/cm 2 s. Based on experimental data, the critical thickness has been estimated to decrease from 2.0 mm for SrFeO 3− δ to 0.7 mm for a compound with 67% cobalt added. A severe oxygen flux reduction is observed below approx. 900 °C for membrane samples with composition Sr 0.97 Fe 0.33 Co 0.67 O 3− δ , corresponding to a transition from a cubic perovskite with disordered oxygen vacancies to a rhombohedral brownmillerite structure with ordered oxygen vacancies.

Impurity diffusion of (141)Pr in LaMnO(3), LaCoO(3) and LaFeO(3) materials.

The impurity diffusion of Pr(3+) in dense polycrystalline LaMnO(3), LaCoO(3) and LaFeO(3) was studied at 1373-1673 K in air in order to investigate cation diffusion in these materials. Cation distribution profiles were measured by secondary-ion mass spectrometry and it was found that penetration profiles of Pr(3+) had two distinct regions with different slopes. The first, shallow region was used to evaluate the bulk diffusion coefficients. The activation energies for bulk diffusion of Pr(3+) in LaMnO(3), LaCoO(3) and LaFeO(3) were 126 +/- 6, 334 +/- 68 and 258 +/- 75 kJ mol(-1), respectively, which are significantly lower than previously predicted by atomistic simulations. The bulk diffusion of Pr(3+) in LaMnO(3) was enhanced compared to LaCoO(3) and LaFeO(3) due to higher concentrations of intrinsic point defects in LaMnO(3), especially La site vacancies. Grain-boundary diffusion coefficients of Pr(3+) in LaCoO(3) and LaFeO(3) materials were evaluated according to the Whipple-Le Claire equation. Activation energies for grain-boundary diffusion of Pr(3+) in LaCoO(3) and LaFeO(3) materials were 264 +/- 41 kJ mol(-1) and 290 +/- 36 kJ mol(-1) respectively. Finally, a correlation between activation energies for cation diffusion in bulk and along grain boundaries in pure and substituted LaBO(3) materials (B = Cr, Fe, Co) is discussed.

Phase equilibria and microstructure in Sr4Fe6−xCoxO13 0≤x≤4 mixed conductors

Abstract The densification, microstructure and phase evolution of Sr4Fe6−xCoxO13 (0≤x≤4) materials have been investigated by powder X-ray diffraction, electron microscopy and thermal analysis. Powders were prepared by the solid state reaction method or by the EDTA precursor method. Pure Sr4Fe6O13 is stable above 775±25°C in air until it melts peritectically at 1220±5°C. Below 775°C, Sr4Fe6O13 is unstable with respect to the formation of Sr1−xFeO3−δ and SrFe12O19. Co substituted Sr4Fe6O13 is only stable in a narrow temperature region near 900°C. At higher or lower temperature, the Co-content is reduced due to formation of the perovskite SrFe1−zCozO3−δ and the solid solutions Co3−yFeyO4 (below 900°C) or Co1−yFeyO (above 900°C). A plate-like morphology of Sr4Fe6−xCoxO13 grains was observed both in calcined powders and in sintered ceramics. Ball milling of the calcined powders was necessary prior to the sintering in order to achieve dense materials in the temperature region 1120–1170°C. Only pure Sr4Fe6O13 appeared as a single-phase material after sintering. Increasing amounts of the phases SrFe1−zCozO3−δ and Co1−yFeyO were observed with increasing sintering temperature and increasing Co-content due to the limited solubility of Co in Sr4Fe6−xCoxO13. The thermal expansion coefficient of the materials deviates from linear behavior due to the decreasing oxidation state of iron with increasing temperature. The present investigation demonstrates that Sr4Fe4Co2O13 materials with high oxygen permeability are not single-phase materials when sintered at high temperature.

Oxygen permeation of SrFe0.67Co0.33O3-d

Abstract Oxygen permeation fluxes through dense SrFe 0.67 Co 0.33 O 3− d membranes have been measured as a function of temperature, oxygen partial pressure gradient across the membrane, and membrane thickness. A maximum oxygen flux of 2.74 sccm/min cm 2 (1.87×10 −6 mol/s cm 2 ) was measured at 1000°C with air on the primary side, and He on the secondary side of the SrFe 0.67 Co 0.33 O 3− d membrane. Activation energies from 88 to 108 kJ/mol were found in the temperature range between 900 and 1000°C. At 1000°C with He on the secondary side of the membrane, a critical thickness of roughly 1.5 mm was found for SrFe 0.67 Co 0.33 O 3− d where the rate limiting step for oxygen permeation changed from surface exchange kinetics to bulk diffusion. The critical thickness appears to be dependent on the temperature and the p O 2 gradient across the membrane.

Reactions between La1−x CaxMnO3 and CaO-stabilized ZrO2 part I Powder mixtures

The chemistry and microstructure of the interface between calcium substituted lanthanum manganite and cubic calcia stabilized zirconia have been studied. The aim was to investigate the chemical stability of these materials as a model system for, respectively, the cathode and the electrolyte in solid oxide fuel cells. The relative amounts and time dependence of the formation of secondary phases (La2Zr2O7 and CaZrO3) and inter-diffusion between the primary phases were observed to depend on temperature, partial pressure of oxygen, and composition of the manganite. 30 mole % Ca on La-site and A-site deficiency of the manganite were shown to stabilize the heterophase interface in air. Reducing conditions were shown to destabilize the primary phases and increase the rate of formation of secondary phases. Pore-coarsening with increasing amount of Ca in the manganite was the most striking feature in the time dependence of the microstructure. The present findings are discussed in relation to the thermodynamic and kinetic stability of the cathode/electrolyte interface of conventional solid oxide fuel cells consisting of yttria stabilized zirconia and strontium substituted lanthanum manganite.

Reactions between La1−x CaxMnO3 and CaO-stabilized ZrO2 Part II Diffusion couples

The chemical stability of diffusion couples and coarse grain powder mixtures of calcium substituted lanthanum manganite and cubic calcia stabilized zirconia have been studied. The aim was to investigate the chemical stability of these materials as a model system for respectively the cathode and the electrolyte in solid oxide fuel cells. With increasing amount of Ca in lanthanum manganite, the major secondary phase was shifted from La2Zr2O7 to CaZrO3, and the thickness of the reaction layers of secondary phases was increasing with increasing heat treatment time. Precipitation of La2O3 had taken place in the perovskite containing low amounts of Ca (0 and 20 mol %). The transport mechanisms of the cations were strongly dependent on the interface geometry. La0.7Ca0.3MnO3 was observed to give the most stable interface to zirconia both in air and in reducing atmosphere (pO ∼ 10−6 atm). A-site deficiency of LaMnO3 was also observed to increase the stability. However, we conclude that a thin film of an electrode material consisting of lanthanum manganite on a zirconia substrate is unstable, regardless of A-site deficiency, because the solubility limit of Mn in the zirconia is not reached. From the experimental data, a reaction mechanism has been proposed, based on observations of relative diffusion rates.

LaFeO3 and LaCoO3 Based Perovskites: Preparation and Properties of Dense Oxygen Permeable Membranes

LaCoO3 and LaFeO3 based perovskites have been studied as model materials for oxygen permeable membranes. To avoid gas leakages it is of crucial importance to obtain dense membranes, hence the densification behavior in terms of temperature, stoichiometry (AB-ratio), presence of secondary phases and substituents (Ca and Sr) have been investigated. Only a narrow temperature window for the sintering is available and it is important to avoid the formation of secondary phases. The upper temperature to obtain high density materials is limited by a severe swelling. Mechanical properties such as bending strength and fracture toughness have been determined both at ambient- and elevated temperatures. Non elastic behaviour due to ferroelasticity is reported for pure LaFeO3, LaCoO3 and La0.8Ca0.2CoO3. The ferroelasticity is found to influence the behavior of the fracture toughness and fracture strength as a function of temperature. The electrical conductivity of pure LaFeO3 at 1000°C is determined in terms of partial pressure of oxygen. Cation diffusion is found to be rate controlling for the establishment of a time independent conductivity in the p-conductivity regime.

All-Oxide Thermoelectric Module with in Situ Formed Non-Rectifying Complex p–p–n Junction and Transverse Thermoelectric Effect

All-oxide thermoelectric modules for energy harvesting are attractive because of high-temperature stability, low cost, and the potential to use nonscarce and nontoxic elements. Thermoelectric modules are mostly fabricated in the conventional π-design, associated with the challenge of unstable metallic interconnects at high temperature. Here, we report on a novel approach for fabrication of a thermoelectric module with an in situ formed p–p–n junction made of state-of-the-art oxides Ca3Co4–xO9+δ (p-type) and CaMnO3–CaMn2O4 composite (n-type). The module was fabricated by spark plasma co-sintering of p- and n-type powders partly separated by insulating LaAlO3. Where the n- and p-type materials originally were in contact, a layer of p-type Ca3CoMnO6 was formed in situ. The hence formed p–p–n junction exhibited Ohmic behavior and a transverse thermoelectric effect, boosting the open-circuit voltage of the module. The performance of the module was characterized at 700–900 °C, with the highest power output of 5.7 mW (around 23 mW/cm2) at 900 °C and a temperature difference of 160 K. The thermoelectric properties of the p- and n-type materials were measured in the temperature range 100–900 °C, where the highest zT of 0.39 and 0.05 were obtained at 700 and 800 °C, respectively, for Ca3Co4–xO9+δ and the CaMnO3–CaMn2O4 composite.

Oxide-Based Thermoelectric Generator for High-Temperature Application Using p-Type Ca3Co4O9 and n-Type In1.95Sn0.05O3 Legs

Abstract A thermoelectric generator couples an entropy current with an electrical current in a way, that thermal energy is transformed to electrical energy. Hereby the thermoelectric energy conversion can be described in terms of fluxes of entropy and electric charge at locally different temperature and electric potential. Crucial for the function of a thermoelectric generator is the sign and strength of the coupling between the entropy current and the electrical current in the thermoelectric materials. For high-temperature application, tin-doped indium oxide (In1.95Sn0.05O3) and misfit-layered calcium cobalt oxide (Ca3Co4O9) ceramics were used as n- and p-type legs. The n-type material reaches a power factor of 6.8μW⋅cm−1⋅K−2