A.A. Yaremchenko

Oxygen Permeability of Ce0.8Gd0.2 O 2 − δ ‐ La0.7Sr0.3MnO3 − δ Composite Membranes

(CGO) and (LSM) possess similar thermal expansion coefficients and were thus combined in dual‐phase membranes for oxygen separation. Studies of oxygen permeation through CGO‐LSM composite ceramics, containing similar volume fractions of the phases, showed that the oxygen transfer is limited by the bulk ionic conductivity. The oxygen conduction in the composites depends strongly on processing conditions, decreasing with interdiffusion of the phase components. Blocking oxygen ionic conduction is assumed to be due to formation of layers with low ionic conductivity at the CGO grain boundaries, caused by diffusion of lanthanum and strontium into CGO. The permeation fluxes through CGO‐LSM membranes at high feed‐side oxygen pressures (1–50 atm) exhibit Wagner‐type behavior and exceed significantly the oxygen permeability at lower oxygen pressures.

Perovskite-like system (Sr,La)(Fe,Ga)O3−δ: structure and ionic transport under oxidizing conditions

Abstract The maximum solid solubility of gallium in the perovskite-type La 1− x Sr x Fe 1− y Ga y O 3− δ ( x =0.40–0.80; y =0–0.60) was found to vary in the approximate range y =0.25–0.45, decreasing when x increases. Crystal lattice of the perovskite phases, formed in atmospheric air, was studied by X-ray diffraction (XRD) and neutron diffraction and identified as cubic. Doping with Ga results in increasing unit cell volume, while the thermal expansion and total conductivity of (La,Sr)(Fe,Ga)O 3− δ in air decrease with gallium additions. The average thermal expansion coefficients (TECs) are in the range (11.7–16.0)×10 −6 K −1 at 300–800 K and (19.3–26.7)×10 −6 K −1 at 800–1100 K. At oxygen partial pressures close to atmospheric air, the oxygen permeation fluxes through La 1− x Sr x Fe 1− y Ga y O 3− δ ( x =0.7–0.8; y =0.2–0.4) membranes are determined by the bulk ambipolar conductivity; the limiting effect of the oxygen surface exchange was found negligible. Decreasing strontium and gallium concentrations leads to a greater role of the exchange processes. As for many other perovskite systems, the oxygen ionic conductivity of La 1− x Sr x Fe 1− y Ga y O 3− δ increases with strontium content up to x =0.70 and decreases on further doping, probably due to association of oxygen vacancies. Incorporation of moderate amounts of gallium into the B sublattice results in increasing structural disorder, higher ionic conductivity at temperatures below 1170 K, and lower activation energy for the ionic transport.

Ionic conductivity of La(Sr)Ga(Mg,M)O3−δ (M=Ti, Cr, Fe, Co, Ni): effects of transition metal dopants

Abstract Oxygen-ion conductivity of the perovskite-type solid solutions (La,Sr)Ga1−zM2O3−δ (M=Ti, Cr, Fe, Co; z=0–0.20), LaGa1−y−zMgyMzO3−δ (M=Cr, Fe, Co; y=0.10–0.20, z=0.35–0.60) and LaGa1−zNizO3−δ (z=0.20–0.50) was studied using the techniques of oxygen permeation, Faradaic efficiency, ion-blocking electrode and the e.m.f. of oxygen concentration cells. Oxygen-ion transference numbers vary from 2×10−6 to 0.98 throughout the series and p-type electronic conductivity increases with increasing transition metal content. Substitution of Ga with higher valence cations (Ti, Cr) decreases ionic conductivity whereas small amounts of Fe or Co (∼5%) increase ionic conductivity. For higher transition metal contents, lower levels of oxygen-ion conductivity and an increase in the activation energy, EA, for ionic transport, from 60 (5%-doped) to 230 kJ/mol (>40%-doped) are observed. In heavily doped phases, EA tends to decrease with temperature and, above 1170 K, values are similar to the undoped phase suggesting that an order–disorder transition takes place. Factors affecting the observed ionic conductivity trends are discussed.

Surface modification of La0.3Sr0.7CoO3−δ ceramic membranes

The dependence of oxygen permeability of dense La 0 . 3 Sr 0 . 7 CoO 3 - Φ ceramics on membrane thickness indicates significant surface exchange limitations to the permeation fluxes, which suggests a possibility to increasemembrane performance by surface activation. The cobaltite membranes with various porous layers applied onto the permeate-side surface were tested at 850-1120 K. Silver-modified La 0 . 3 Sr 0 . 7 CoO 3 membranes showed enhanced permeation at temperatures above 950 K; deposition of porous layers of PrO x and Pr 0 . 7 Sr 0 . 3 CoO 3 - Φ had no positive effect. The maximum oxygen permeability at 850-1120 K was observed in the case of porous La 0 . 3 Sr 0 . 7 CoO 3 - Φ layers with surface density about 10 mg cm - 2 . These results suggest that the surface exchange of lanthanum-strontium cobaltite membranes under an oxygen chemical potential gradient is limited by both oxygen sorption at the surface and ion diffusion through the surface oxide layers. Oxygen permeability of La 0 . 3 Sr 0 . 7 CoO 3 - Φ ceramics was found to increase with increasing grain size due to decreasing grain-boundary resistance to ionic transport.

Electron–hole conduction in Pr-doped Ce(Gd)O2−δ by faradaic efficiency and emf measurements

Abstract Non-negligible electrode polarisation in faradaic efficiency and emf electrochemical cells used for transport number determination results in apparent ion transference numbers which are lower than the true values. However, appropriate modifications of these techniques combined with use of electrodes having a high polarisation resistance, enable a precise determination of even minor electronic contributions to the total conductivity of solid electrolytes. Experimental modification of the faradaic efficiency method, taking into account the electrode polarisation resistance, is proposed and verified using Ce 0.80 Gd 0.18 Pr 0.02 O 2− δ ceramics. Substitution of 2% gadolinium cations in the lattice of Ce 0.80 Gd 0.20 O 2− δ solid electrolyte with praseodymium was found to increase the p-type electronic conductivity by 2.5–4 times, while the activation energy for the electron–hole transport decreases from 145 to 125 kJ mol −1 . The oxygen ion transference numbers of Ce 0.80 Gd 0.18 Pr 0.02 O 2− δ in air vary in the range 0.996–0.970 at 873–1223 K, decreasing with increasing temperature. No significant effect of co-doping with praseodymium on the ionic conductivity, crystal lattice and thermal expansion of ceria solid electrolyte was found.

Oxygen permeability and Faradaic efficiency of Ce0.8Gd0.2O2–δ–La0.7Sr0.3MnO3–δ composites

Composite Ce0.8Gd0.2O2−δ (CGO, solid electrolyte) and La0.7Sr0.3MnO3−δ (LSM, electronic conductor) ceramics were tested as dual-phase membranes for oxygen separation. Oxygen permeation through CGO–LSM composite ceramics containing similar percentages of both phases is limited by bulk ionic transport. In contrast to electronic transport, oxygen ion transport in these composites depends strongly on processing conditions, decreasing with interdiffusion of components. Oxygen ions are blocked by low ionic conductivity layers formed by diffusion of cations of LSM to the contacts between CGO grains. Testing of CGO–LSM membranes at high oxygen pressures (1–50 atm) showed that the composite ceramics are stable in these conditions and exhibit Wagner-type permeation fluxes.

Processing, microstructure and properties of LaCoO3-δ ceramics

Abstract Dense lanthanum cobaltite ceramics with different microstructures were prepared using several processing procedures, including chemical and ceramic synthesis routes. XRD, SEM, dilatometry, total electrical conductivity and oxygen permeability measurements were used for the characterization of these materials. Submicrometer size LaCoO 3− δ powders obtained via a cellulose-precursor technique or a combustion synthesis process showed much higher sinterability and poor compactability with respect to the powder prepared by the standard ceramic procedure. The influence of the processing route on crystal lattice, electronic conductivity and thermal expansion of LaCoO 3− δ ceramics was negligible. At the same time, the preparation technique significantly affects the ceramic microstructure and the oxygen ionic conductivity. LaCoO 3− δ membranes prepared via the standard ceramic technique, involving higher sintering temperatures, exhibit significantly higher oxygen permeation fluxes than ceramics prepared from organic precursors. This behavior was attributed to the effect of grain-boundary resistivity to ionic transport, which decreases with increasing sintering temperature and grain size, as commonly found for oxide solid electrolytes.

Oxygen Permeability and Ionic Conductivity of Perovskite-Related La0.3Sr0.7Fe ( Ga ) O 3 − δ

The oxygen ionic conductivity of La 0.3 Sr 0.7 Fe 1-x Ga x O 3-δ (x = 0.2-0.4), determined by the oxygen permeation, faradaic efficiency, and total conductivity measurements at 1023-1223 K, is essentially independent of oxygen chemical potential and structural changes in the wide oxygen partial pressure range from 10 -14 to 0.21 kPa. At oxygen pressures close to atmospheric air, the ion transference numbers of perovskite-like La 0.3 Sr 0.7 Fe(Ga)O 3-δ phases vary from 4 × 10 -4 to 4 × 10 -2 , increasing with gallium content and temperature. Although there is a great difference between ionic and p-type electronic conductivities in oxidizing atmospheres, the electron-hole conduction was demonstrated to affect oxygen permeation and ambipolar conductivity. In oxidizing conditions, the oxygen permeability of La 0.3 Sr 0.7 Fe 1-x Ga x O 3-δ (x = 0-0.4) membranes increase with increasing x. The bulk ionic transport process and surface exchange kinetics both influence the permeation through La 0.3 Sr 0.7 FeO 3-δ ceramics, whereas the oxygen fluxes through Ga-containing materials are predominantly determined by the bulk ambipolar conduction. Thermal expansion coefficients of La 0.3 Sr 0.7 Fe 1-x Ga x O 3-δ (x = 0-0.4) in air vary in the range (11.7-14.9) × 10 6 K -1 at 300-800 K and increase up to (19.5-26.4) × 10 6 K -1 at 800-1170 K. Substitution of iron with gallium was found to suppress both thermal expansion and chemically induced expansion of La 0.3 Sr 0.7 Fe(Ga)O 3-δ materials, originating from oxygen nonstoichiometry variations under changing the oxygen partial pressure.

Mixed electronic and ionic conductivity of LaCo(M)O3 (M=Ga, Cr, Fe or Ni).: V. Oxygen permeability of Mg-doped La(Ga, Co)O3−δ perovskites

Abstract The LaGa 1− x − y Co x Mg y O 3−δ solid solutions with rhombohedrally-distorted perovskite structure were ascertained to form in the concentration range of 0≤ y ≤0.10 at x =0.60 and 0≤ y ≤0.20 at x =0.35–0.40. Increasing cobalt content results in increasing electrical conductivity and thermal expansion of the perovskites. Thermal expansion coefficients of the LaGa 1− x − y Co x Mg y O 3−δ ceramics were calculated from the dilatometric data to vary in the range of 12.4–19.8×10 −6 K −1 at 300–1100 K. Doping La(Ga,Co)O 3−δ solid solutions with magnesium leads to increasing oxygen nonstoichiometry, electronic and oxygen ionic conductivity. Oxygen permeation fluxes through LaGa 1− x − y Co x Mg y O 3−δ membranes were found to be limited by the bulk ionic conduction and to increase with magnesium concentration, being essentially independent of cobalt content.

Faradaic efficiency and oxygen permeability of Sr0.97Ti0.60Fe0.40O3-δ perovskite

Abstract Oxygen ionic conduction in the perovskite-type Sr 0.97 Ti 0.60 Fe 0.40 O 3− δ was studied using oxygen permeability, Faradaic efficiency and total electrical conductivity measurements at 973–1223 K. The ion transference numbers of the strontium titanate–ferrite in air vary from 0.005 to 0.08, decreasing with decreasing temperature. The electron–hole conductivity of the oxide is relatively low but exceeds the ionic conductivity. The activation energy for the electronic conductivity is 35±3 kJ/mol at 470–890 K and drops at higher temperatures. Studying the oxygen permeation through dense Sr 0.97 Ti 0.60 Fe 0.40 O 3− δ ceramic membranes as a function of membrane thickness showed that at temperatures above 1170 K the permeation fluxes are limited by both bulk ionic conductivity and surface exchange rates. Depositing of porous layers of the same material or a mixture of platinum and praseodymium oxide onto the membrane feed-side surface leads to a significant increase in the oxygen permeability. Decreasing temperature results in increasing role of the bulk ionic transport in Sr 0.97 Ti 0.60 Fe 0.40 O 3− δ as the permeation-determining factor. The oxygen permeation fluxes at 1073 K are limited predominantly by the oxygen ionic conductivity of the ceramics. The thermal expansion coefficients of the ceramic material in air were calculated from dilatometric data to be 11.7×10 −6 K −1 in the temperature range 300–720 K and 16.6×10 −6 K −1 at 720–1070 K.