High-Temperature Proton Conductors Based on Strontium and Barium Cerates: The Interphase Exchange and Diffusion of Oxygen

Kinetic parameters of interphase exchange of oxygen in high-temperature proton conductors BaCe_0.95Nd_0.05O_3- and SrCe_0.95Y_0.05O_3- are determined using an isotope exchange method in the temperature range 630-820°C at oxygen pressures of 2.7-13.3 gPa and diffusion coefficients for oxygen are calculated. Effective energies of activation of oxygen exchange and diffusion coefficients for oxygen are calculated. Dependence of exchange characteristics and the diffusion coefficients on the oxygen pressure in the gas phase is investigated. The oxygen exchange on the surface of studied oxides is shown to proceed only with the participation of oxygen of the oxides, which causes high rates of interphase exchange of oxygen of studied cerates of barium and strontium. It is shown that the obtained data satisfactorily agree with those found in the relevant literature.     

Oxygen isotope exchange in the LSM-YSZ composite under the conditions of long-term tests

The stability of the properties of the composite 40 vol % La_0.6Sr_0.4MnO_{3 ? δ}-60 vol % Zr_0.83Y_0.17O_1.92 (LSM-YSZ) held for 1000 h at a temperature of 800°C and an oxygen pressure of 1.01 kPa and changes in its microstructure were studied by isotope exchange and gas-phase analysis. The oxygen exchange was found to occur by the dissociative adsorption mechanism involving the adsorption forms of oxygen on the triple-phase LSM-YSZ-O₂ boundary. The rate-determining step of the oxygen exchange of the composite did not change with time. The rate of oxygen exchange decreased by 22%, which is comparable to the decrease in the length of the LSM-YSZ-O₂ triple-phase boundary (～20%), on which oxygen exchange is dominant. The conductivity of the LSM-YSZ composite increased due to the connectivity of the material.     

Phase equilibriums, oxygen exchange kinetics and diffusion in oxides CaZr1 ? x Sc x O3 ? x/2 ? δ

Oxides CaZr_{1 ? x} Sc _x O_{3 ? x/2 - ??} (x = 0.00?C0.20) were synthesized according to the ceramic technology. The solubility boundary of scandium with formation of solid solutions on the basis of calcium zirconate CaZrO_{3 ? ??} corresponds to x = 0.07?C0.08. The second phase of CaSc₂O₄ is present in the samples with scandium content of x = 0.10, 0.15, 0.20. Its fraction grows at an increase in x. The method of full-profile Rietveld analysis was used to calculate the structure parameters for oxides CaZr_0.99Sc_0.01O_{2.995 ? ??} and CaZr_0.95Sc_0.05O_{2.975 ? ??}. The method of isotopic exchange with gas phase analysis was used to study the kinetics of gas-phase oxygen interaction with the CaZr_0.95Sc_0.05O_{2.975 ? ??} oxide in the temperature range of 700?C850°C and at oxygen pressures of 0.13?C6.67 kPa. The values of effective activation energies of the oxygen exchange and diffusion processes were 1.36 ± 0.32 and 1.92 ± 0.21 eV, accordingly. The dependence of the interphase exchange rate on the pressure of oxygen corresponds to the power law with the exponent of 0.31 ± 0.04 at the temperature of 750°C.     

Effect of oxygen nonstoichiometry on kinetics of oxygen exchange and diffusion in lanthanum-strontium cobaltites

The method of isotopic exchange was used to study the kinetics of oxygen exchange and diffusion in complex oxides of La_{1 − x} Sr _x Co_{1 − y} Fe _y O_{3 − δ} (x = 0.0, y = 0.0; x = 0.6, y = 0.2, 0.4). The rates of oxygen interfacial exchange and its diffusion coefficient were determined for LaCoO_{3 − δ} at the pressure of 5 torr in the temperature range of 600–850°C and at the temperature of 700°C in the pressure range of 1–70 torr. The contributions of the three exchange types were calculated. The order of the dependence of the interfacial exchange rate on the oxygen pressure was 0.51 ± 0.01. In the case of La_0.4Sr_0.6Co_{1 − y} Fe _y O_{3 − δ} (y = 0.2, 0.4) in the temperature range of 600–900°C at the oxygen pressure of 10 torr, the oxygen exchange rates and diffusion coefficients were determined in the material bulk and in the subsurface region; contributions of the three types of exchange were calculated. The paper considers the mechanism of oxygen exchange and diffusion as compared to nonstoichiometry in the oxygen sublattice of the La_{1 − x} Sr _x Co_{1 − y} Fe _y O_{3 − δ} oxides.     

Interphase exchange and diffusion of oxygen in lanthanum-strontium cobaltites doped with iron

Oxides La_0.4Sr_0.6Co_{1 − x} Fe _x O_{3 − δ} (x = 0.2 and 0.4) were synthesized by the ceramic technology. The kinetics of exchange and diffusion of oxygen in these oxides were studied by the isotopic exchange method over the temperature range 600–900°C at an oxygen pressure of 10 torr. The temperature dependences of interphase exchange rate and oxygen diffusion coefficient in the bulk and near-surface region were obtained, and the effective activation energies of exchange and oxygen diffusion were calculated. The concentration of iron ions was shown to have no substantial influence on the exchange rate and oxygen diffusion. Ratio between the values of exchange rate and diffusion coefficient were shown to influent on the precision of the values estimation.     

High-Temperature Proton Conductors Based on Strontium and Barium Cerates: The Content, Interphase Exchange, and Diffusion of Hydrogen

The hydrogen content and hydrogen exchange kinetics of doped cerates of strontium and barium are studied in a reducing environment by the isotope balance and isotope exchange techniques. The measurements are performed at 500-840°C and hydrogen pressures of 2.7-16 gPa. The interphase exchange of hydrogen is found to occur at high speed via dissociative mechanisms. The effective activation energy for the hydrogen heteroexchange is determined. The diffusion coefficient for hydrogen in BaCe_0.95Nd_0.05O_3- is computed. The hydrogen content per formula unit appears to equal 0.48 ± 0.05 in SrCe_0.95Y_0.05O_3- and 0.60 ± 0.05 in BaCe_0.95Nd_0.05O_3- at 550 and 720°C, respectively, and a hydrogen pressure of 6.7 gPa. This is much greater than in an oxidizing environment, where the hydrogen content in an oxide is approximately proportional to the doping level of sublattice B. A possible explanation is connected with the presence of Ce³⁺ ions in the oxides or the dissolution hydrogen in a neutral form.     

Oxygen isotope exchange and electrical conductivity of CaZr1–x Sc x O3–x/2

Journal of Solid State Electrochemistry - The effect of scandium content in CaZr1–x Sc x O3–x/2 on electrical conductivity and oxygen exchange kinetics was investigated. The electrical...     

Isotopic Exchange and Oxygen Diffusion in the La0.88Sr0.12Ga0.82Mg0.18O3 – α–Molecular Oxygen System

The oxygen isotopic exchange kinetics in La_0.88Sr_0.12Ga_0.82Mg_0.18O_{3 –} and in the same electrolyte whose surface layer is enriched with cobalt ions is studied. Rates of different types of exchange and oxygen isotope diffusion coefficients in oxide are determined at 970–1200 K and oxygen pressures of 400–1200 Pa. The activation energy for the interphase oxygen exchange and the order of heteroexchange vs. oxygen pressure relationship are determined. The presence of cobalt ions in the near-surface layer alters the surface exchange parameters, such as the oxygen heteroexchange rate (which increases), order of the heteroexchange vs. oxygen pressure relationship, and the ratio between rates of different exchange types. The oxygen isotope diffusion coefficients coincide in both electrolytes but differ from those calculated from the electroconductivity data with the Nernst–Einstein equation.     

Effect of oxygen nonstoichiometry on kinetics of oxygen exchange and diffusion in lanthanum-strontium manganites

Porous _963-1 (x = 0, 0.4, 0.6) materials are synthesized using a ceramic technique. The method of isotopic oxygen exchange with gas-phase analysis was used to study the kinetics of oxygen exchange in La_1-x Sr _x MnO_3±δ at 600–850°C and at oxygen pressure of 0.13–9.33 kPa. The values of oxygen interphase exchange rate, diffusion coefficient, and contributions of three exchange types are calculated. It is found that the rate of oxygen interphase exchange rate and diffusion coefficient grow at an increase in the concentration of strontium. The values of effective activation energies of oxygen exchange and diffusion decrease at an increase in the strontium content. Oxygen diffusion and its exchange mechanism are considered as related to the defect structure of oxides in the La_1-x Sr _x MnO_3±δ system.