Oxygen Chemical Diffusion Coefficient Research Articles

Oxygen Partial Pressure Dependence of In Situ X-Ray Absorption Spectroscopy at Co and Fe K-Edge for (La0.6Sr0.4)(Co0.2Fe0.8)O3–δ

(La0.6Sr0.4)(Co0.2Fe0.8)O3–δ (LSCF) has been promised as a cathode material of solid oxide fuel cells at intermediate temperatures. Despite the many previous studies of LSCF that have been reported, the role of Co and Fe atoms in the oxygen ion conduction is still unclear. In this work, we aimed at presenting each valence, oxygen chemical diffusion coefficient (Dchem) and activation energy (Ea) related to Co and Fe in LSCF by in situ X-ray absorption spectroscopy (XAS) at high temperatures and during reduction. For quantitative analysis of X-ray absorption near edge structure (XANES) spectroscopy, these results indicated that the Co valence decreased more easily than the Fe valence. On the other hand, from relaxation plots of the Co and Fe valence during reduction, the values of Dchem and Ea related to Co and Fe were nearly equal. Considering equations showing the oxygen ion conductivity, these results would indicate that oxygen ion conductivity was contributed by Co with more oxygen vacancies rather than Fe. According to these results, a structural model with and without oxygen vacancies and the oxygen ion conduction mechanism of LSCF was speculated, that is, we found that oxygen ion conductivity was more closely related to Co than Fe in LSCF by direct observations of in situ XAS.

Synthesis, Sintering, Transport Properties, and Surface Exchange of La2Ni0.9Cu0.1O4+δ

Dense La2Ni0.9Cu0.1O4+δ ceramics were sintered from powders synthesized through a wet‐chemical citrate nitrate route with optimized ratios of citrate to nitrate. Less citrate decreases the required sintering temperature and improves the oxygen permeativity. The oxygen permeation was measured as a function of oxygen activity gradient, membrane thickness (0.4–2.6 mm) and temperature (800°C–950°C). The oxygen self diffusion coefficient DO and the surface exchange coefficient k show Arrhenius‐type behaviors with activation energies of ~50 and ~100 kJ/mol, respectively. The oxygen chemical diffusion coefficient Dchem and surface exchange coefficient kchem, measured by conductivity relaxation, exhibit Arrhenius‐type behaviors with activation energies of 62 and 104 kJ/mol, respectively. Dchem and kchem are related to DO and k through the thermodynamic factor ωO.

Effects of surface coatings on the determination of Dchem and kchem in La 2NiO 4+δ by conductivity relaxation

In this work, we utilize surface modification to accurately determine oxygen diffusivity and surface exchange in La 2NiO 4 + δ by transient conductivity. To achieve ‘instantaneous’ gas exchange in the reaction cell, a total pressure change was adopted instead of exchange of gas mixtures at 1 atm. Conductivity relaxation measurements were performed in the temperature range of 600–900 °C and at oxygen partial pressures of 0.2, 1.0 and 1.9 atm. Due to the large uncertainties associated with deriving oxygen chemical diffusion and surface coefficients simultaneously, we limited the relaxation to pure bulk diffusion control by coating nano-grained La 2NiO 4 + δ particles on the sample surface. After determining D chem, k chem was derived by fitting the relaxation data of the uncoated sample. The transient experiments via both oxidation (a step change of pO 2 from 0.2 to 1.0 atm) and reduction (1.9 to 1.0 atm) give consistent results of D chem and k chem showing Arrhenius-type behaviour with activation energies of ~ 90 kJ/mol and ~ 150 kJ/mol, respectively.

The effect of phase composition on the transport properties of composites La0.8Sr0.2Fe0.7Ni0.3O3 − δ-Ce0.9Gd0.1O1.95

Full conductivity, diffusion and oxygen exchange processes in composites (100 − x)La0.8Sr0.2Fe0.7Ni0.3O3 − δ−xCe0.9Gd0.1O1.95 (x is the volume fraction, 0 ≤ x ≤ 71.1%) at 700°C over the oxygen partial pressure range from 0.2 to 3 × 10−3 atm are studied by the electrical conductivity relaxation method. The composites’ conductivity was shown to decrease monotonically with the increasing of Ce0.9Gd0.1O1.95 fraction, while the oxygen chemical diffusion coefficient increased. The oxygen exchange constant is higher for the composites than for the individual phases of La0.8Sr0.2Fe0.7Ni0.3O3 − δ and Ce0.9Gd0.1O1.95. Possible reason of the dependence of the parameters D chem and k chem on the temperature, oxygen pressure, and the composite composition is the effect of the interface on the oxygen transfer processes. Most effective oxygen transfer occurs in the composites whose composition approaches La0.8Sr0.2Fe0.7Ni0.3O3 − δ-Ce0.9Gd0.1O1.95 (x = 71%).

Investigation on Oxygen Chemical Diffusion Properties of Perovskite Oxides (La1−x Pr x )0.6Sr0.4Co0.8Fe0.2O3−δ

Perovskite oxide samples of (La1−xPrx)0.6Sr0.4Co0.8Fe0.2O3−δ (x = 0.2, 0.4, 0.6, 0.8) are obtained by solid-state reaction method. The oxygen chemical diffusion properties of (La1−xPrx)0.6Sr0.4Co0.8Fe0.2O3−δ are determined by electrical conductivity relaxation technique. The results show that the conductivity of (La1−xPrx)0.6Sr0.4Co0.8Fe0.2O3−δ increases with the increase of oxygen partial pressure. The (La1−xPrx)0.6Sr0.4Co0.8Fe0.2O3−δ samples have a high oxygen chemical diffusion coefficient, which decreases linearly with a decrease in temperature and an increase in Pr content. The oxygen chemical diffusion coefficient Dchem remains fairly constant at high PO2. The oxygen chemical diffusion coefficient is the highest for (La1−xPrx)0.6Sr0.4Co0.8Fe0.2O3−δ with x = 0.2, and attains a value of 9.41 × 10−5 cm2 s−1 at 600 °C. This shows the material’s promise as a cathode material for intermediate temperature solid oxide fuel cells.

Composite of Ce 0.8Gd 0.2O 2 − δ and GdBaCo 2O 5 + δ as oxygen separation membranes

A novel oxygen separation membrane using oxygen ions and a mixed oxygen ion-electron conducting CGO-GBCO (Ce 0.8Gd 0.2O 2 − δ –GdBaCo 2O 5 + δ ) dual phase composite has been proposed. The transport and surface exchange properties of the CGO-GBCO composite membrane are investigated as a function of oxygen partial pressure (OPP) over the OPP range of 10 − 4 ≤ pO 2/atm ≤ 0.21 at 1073 ≤ T/K ≤ 1323 using DC conductivity relaxation experiments. The surface exchange coefficient and the oxygen chemical diffusion coefficient of the composite are similar to those of the CGO-based composite membrane under oxidizing conditions. The characteristic thickness of the specimen, calculated from the oxygen chemical diffusion coefficient and surface exchange coefficient, L C = D̃/κ, is 0.25 mm at 1223 K. The thermally activated oxygen permeation flux monotonically increased with increasing oxygen chemical potential gradient, yielding a maximum of 0.28 cc/min·cm 2 at air/He condition for the 0.62 mm-thick CGO-GBCO specimen at 1223 K. This value is an order of magnitude higher than that of GBCO at the same condition.

Defect structure and oxide ion transport in Sr- and Cr-doped LaCoO 3 − δ

The results of oxygen nonstoichiometry, δ, measured by means of coulometric technique as a function of oxygen partial pressure, p o 2, in temperature range 1223 ≤ T, K ≤ 1323 are presented for the perovskite-type doped with chromium solely LaCo 0.7Cr 0.3O 3 − δ and simultaneously doped both with strontium and chromium La 0.7Sr 0.3Co 0.7Cr 0.3O 3 − δ cobaltites. The limit stability of the latter was found to exceed that of undoped cobaltite LaCoO 3 − δ on six orders of magnitude of p o 2 at a given temperature. The modeling of the defect structure of these perovskites was carried out and its adequate model was found. Chemical and self-diffusion coefficients of oxygen vacancies and oxygen ionic conductivity and ionic transport numbers were measured for the first time for La 0.7Sr 0.3Co 0.7Cr 0.3O 3 − δ as a function of oxygen partial pressure p o 2 and temperature in the ranges − 4 ≤ log( p o 2 , atm) ≤ 0 and 1223 ≤ T, K ≤ 1323, respectively. The additional substitution of Sr for La in LaCo 0.7Cr 0.3O 3 − δ was shown to lead to noticeable increase of ionic conductivity and oxygen chemical diffusion coefficient at given values of oxygen partial pressure and temperature as compared to lanthanum cobaltite doped with chromium solely. Self-diffusion coefficient of oxygen vacancies and their mobility in La 0.7Sr 0.3Co 0.7Cr 0.3O 3 − δ were found to be dependent on oxygen partial pressure and nonstoichiometry unlike undoped and doped with chromium lanthanum cobaltites.

Defect structure and electrical properties of LaSr 3Fe 3O 10–δ

As a mixed conductor, LaSr 3Fe 3O 10–δ with triple layer perovskite intergrowth structure can be used as an oxygen separation membrane material and cathode material in solid oxide fuell cells. LaSr 3Fe 3O 10–δ was synthesized via citrate acid route. Iodine titration method was used to determine the average valence of transition metal Fe and oxygen nonstoichiometry δ. Conductivities of LaSr 3Fe 3O 10– δ were measured in the oxygen partial pressure range from 10 −2 × 10 5 to 1 × 10 5 Pa, by Ac four probe method. Seebeck coefficient measurement together with conductivity results indicated that LaSr 3Fe 3O 10– δ is a p-type conductor. A linear relationship between the conductivity and logarithm of oxygen partial pressure was observed in the measurement. A defect model was introduced to explain the observed phenomena in the iodine titration, conductivity and Seebeck coefficient measurement. Electrical conductivity relaxation method was used to determine the oxygen chemical diffusion coefficient D chem. The apparent activation energy of oxygen chemical diffusion was calculated to be 61.4 kJ/mol.

Oxygen chemical diffusion in hypo-stoichiometric MOX

Kinetics of the oxygen-to-metal ratio change in (U 0.8Pu 0.2)O 2− x and (U 0.7Pu 0.3)O 2− x was evaluated in the temperature range of 1523–1623 K using a thermo-gravimetric technique. The oxygen chemical diffusion coefficients were decided as a function of temperature from the kinetics of the reduction process under a hypo-stoichiometric composition. The diffusion coefficient of (U 0.7Pu 0.3)O 2− x was smaller than that of (U 0.8Pu 0.2)O 2− x . No strong dependence was observed for the diffusion coefficient on the O/M variation of samples.

Oxide ion transport in undoped and Cr-doped LaCoO 3− δ

The results of chemical and self-diffusion coefficient of oxygen vacancies, oxygen ionic conductivity and ionic transport numbers measured as a function of oxygen partial pressure p O 2 and temperature in the ranges − 4 ≤ log( p O 2 , atm) ≤ 0 and 900 ≤ T, °C ≤ 1050, respectively, are presented for the perovskite-type undoped LaCoO 3− δ and doped with chromium LaCo 0.7Cr 0.3O 3− δ cobaltites. The dependencies of these properties on oxygen partial pressures were shown to have monotonic character. This can, therefore, indicate that defect clusters or associates containing oxygen vacancies do not form in the oxides over complete p O 2 range investigated. The predominant charge carriers in both undoped and Cr-doped lanthanum cobaltite are electron defects, since oxygen vacancies transport numbers do not exceed 2 · 10 −3 %. The substitution of Cr for Co was found to lead to decrease of both ionic conductivity and oxygen chemical diffusion coefficient. Activation energy of ionic conductivity of the oxides studied decreases with the oxygen nonstoichiometry increase. Self-diffusion coefficient of oxygen vacancies and their mobility were shown to be independent of oxygen partial pressure and nonstoichiometry most likely due to lack of the defects interaction in the oxide studied.

The oxidation rate of (U 0.7Pu 0.3)O 2− x with two fcc phases

The kinetics of the oxidation in hypo-stoichiometric MOX with 30%Pu content was investigated using thermal gravity and differential thermal analysis to evaluate the variation of oxygen-to-metal ratio (O/M) in the fuel fabrication process. In the isothermal oxidation examination, the O/M ratio of the MOX gradually approached 2.00 at temperatures below 200 °C. Therefore it was considered that the oxidation reaction below 200 °C proceeded by increasing the amount of fcc phase with O/M = 2.00. Finally, the oxidation rate of hypo-stoichiometric MOX was evaluated, and the effective oxygen chemical diffusion coefficient was estimated.

Oxygen nonstoichiometry and transport properties of strontium substituted lanthanum cobaltite

Oxygen nonstoichiometry, structure and transport properties of the two compositions (La 0.6Sr 0.4) 0.99CoO 3− δ (LSC40) and La 0.85Sr 0.15CoO 3− δ (LSC15) were measured. It was found that the oxygen nonstoichiometry as a function of the temperature and oxygen partial pressure could be described using the itinerant electron model. The electrical conductivity, σ, of the materials is high ( σ > 500 S cm − 1 ) in the measured temperature range (650–1000 °C) and oxygen partial pressure range (0.209–10 − 4 atm). At 900 °C the electrical conductivity is 1365 and 1491 S cm − 1 in air for LSC40 and LSC15, respectively. A linear correlation between the electrical conductivity and the oxygen vacancy concentration was found for both samples. The mobility of the electron-holes was inversely proportional with the absolute temperature indicating a metallic type conductivity for LSC40. Using electrical conductivity relaxation the chemical diffusion coefficient of oxygen was determined. It was found that accurate values of the chemical diffusion coefficient could only be obtained using a sample with a porous surface coating. The porous surface coating increased the surface exchange reaction thereby unmasking the chemical diffusion coefficient. The ionic conductivity deduced from electrical conductivity relaxation was determined to be 0.45 S cm − 1 and 0.01 S cm − 1 at 1000 and 650 °C, respectively. The activation energy for the ionic conductivity at a constant vacancy concentration ( δ = 0.125) was found to be 0.90 eV.

Modeling of Spent Fuel Oxidation at Low Temperature

AbstractDuring dry storage, the oxidation of the spent fuel in case of cladding and container failure (accidental scenario) could be detrimental for further handling of the spent fuel rod and for the safety of the facilities. Depending on whether the uranium dioxide is under the form of powder or pellet, irradiated or unirradiated, the weight gain curves do not present the same shape. To account for these different behaviours, two models have been developed. Firstly, the oxidation of unirradiated powders has been modelled based on the coexistence, during the oxidation, of two intermediate products, U4O9 and U3O7. The comparison between the calculation and the literature data is good in terms of weight gain curves and chemical diffusion coefficient of oxygen within the two phases. Secondly, the oxidation of spent fuel fragments is approached by a convolution procedure between a grain oxidation model and an empirical parameter which represents the linear oxidation speed of grain boundary or an average distance able to cover the entire spent fuel fragment. This procedure of calculation allows in one hand to account for the incubation period noticed on unirradiated pellets or spent fuel and in another hand to link the empirical parameter to physical as porosity, cracks or linear power, or operational parameters such as fission gas release (FGR) respectively. A comparison of this new modelling with experimental data will be proposed.

Oxygen transport in La0.6Sr0.4Co1−y Fe y O3−δ

The surface exchange coefficient and chemical diffusion coefficient of oxygen for the perovskites La0.6Sr0.4Co1–yFeyO3–delta (y=0.2, 0.5 and 0.8) were measured using the conductivity relaxation technique. Measurements were performed between 600 and 800 °C in an oxygen partial pressure range between 10–4 and 1 bar. Both transport coefficients decrease markedly with decreasing oxygen partial pressure below about 10–2 bar at all temperatures. This is attributed to ordering of oxygen vacancies. Implications for using La0.6Sr0.4Co1–yFeyO3–delta as an oxygen separation membrane are discussed.

Oxygen Chemical Diffusion Coefficient in Manganite Thin Films by Isothermal Electric Resistivity Measurements.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Chemical oxygen diffusion coefficient measurement by conductivity relaxation—correlation between tracer diffusion coefficient and chemical diffusion coefficient

Chemical oxygen diffusion coefficient ( D ̃ ) was measured by conductivity relaxation experiments on La 2Cu 0.5Ni 0.5O 4+δ ceramics. This method consists of abruptly changing the oxygen partial pressure in the surrounding atmosphere of the sample. The consequent evolution of the electrical conductivity is recorded as a function of time, at fixed temperature. The transient behaviour during the re-equilibration process is analysed on the basis of the second Fick's law. In parallel, the oxygen tracer self-diffusion coefficient (D*) was derived from isotopic exchange experiments monitored by SIMS analysis. Finally, the oxygen over-stoichiometry (δ) was computed from thermogravimetric experiments, under different p O2, in order to determined the thermodynamic factor (γ). A quantitative agreement between D ̃ and γ×D*, observed over a wide temperature range (500–900 °C), confirms the validity of the experimental method and of the Wagner's theory, applied to the oxygen diffusion in this compound.

Measurement of chemical and tracer diffusion coefficients of oxygen in La 2Cu 0.5Ni 0.5O 4+ δ

In mixed electronic and ionic conductors O 2−/e − (MIEC), where the electronic conductivity is dominant, the determination of the oxygen diffusion coefficient is an essential feature. In the present paper, the oxygen chemical diffusion coefficient D̃ O of a MIEC oxide has been evaluated using the conductivity relaxation method, in the temperature range 773–1173 K. Samples of La 2Cu 0.5Ni 0.5O 4+ δ were selected because of the high value of their surface exchange coefficient. In addition, the oxygen diffusion coefficient D* was determined from the 18O tracer diffusion profile. Finally, the oxygen partial pressure dependence of this compound was determined by thermogravimetric analysis (TGA). On the basis of these three independent experiments, the model of ambipolar diffusion, which describes the diffusion in mixed oxide, has been experimentally verified. For such a determination of the intrinsic diffusion characteristics, dense samples are required.

Oxygen transport in La 1− xSr xFe 1− yMn yO 3− δ perovskites

The oxygen transport in La 1− x Sr x Fe 1− y Mn y O 3− δ (LSFM) with 0≤ x≤0.5 and y=0.2 and 0.3 has been examined with a thermogravimetric method. As long as x≤ y, the oxygen transport was found to be very slow while the oxygen transport increased several orders of magnitude when x> y. For two compositions—La 0.5Sr 0.5Fe 0.7Mn 0.3O 3− δ (I) and La 0.6Sr 0.4Fe 0.8Mn 0.2O 3− δ (II)—oxygen chemical diffusion coefficients were determined and found to increase from 1.5×10 −5 to 5.6×10 −5 cm 2 s −1 for composition (I) and from 1.4×10 −6 to 1.4×10 −4 cm 2 s −1 for composition (II) in the temperature range 750–1000 °C. The activation energy for the oxygen chemical diffusion was found to be 66 kJ mol −1 for La 0.5Sr 0.5Fe 0.7Mn 0.3O 3− δ and 136 kJ mol −1 for La 0.6Sr 0.4Fe 0.8Mn 0.2O 3− δ . The oxygen surface exchange constants were found to vary between 2.5×10 −4 and 1.8×10 −2 cm s −1.

Oxygen diffusivity in melt-processed SmBCO and YBCO bulk-superconductors

We studied the diffusivity of oxygen in melt-processed Sm–Ba–Cu–O (SmBCO) and Y–Ba–Cu–O (YBCO) bulk-superconductors. The chemical diffusion coefficients of oxygen estimated from the experimental results show that the addition of silver suppresses the oxygen diffusivity in melt-processed bulk-superconductors. On the other hand, the diffusion coefficients of SmBCO/Ag and YBCO/Ag were almost identical. We discuss the effect of Ag addition on the oxygen diffusivity by examining the microstructure of the samples. It is also shown that annealing at a lower temperature than YBCO is necessary to extract the high potential of SmBCO.

Oxygen Transport Properties and Stability of Mixed-Conducting ZrO2-Promoted SrCo0.4Fe0.6O3-δ Oxides

The oxygen nonstoichiometry, transport properties, and structure stability of ZrO2-promoted SrCo0.4Fe0.6O3-δ (SCFZ) were investigated. The nonstoichiometry of SCFZ increased with increasing temperatures and decreasing oxygen partial pressures. The results of X-ray diffraction and oxygen desorption experiments showed that the addition of ZrO2 in SrCo0.4Fe0.6O3-δ (SCF) stabilized the phase structure under low oxygen partial pressure. The chemical diffusion coefficient of oxygen in SCFZ in a temperature range of 928−1178 K was obtained using the weight relaxation technique. A simple transport equation correlating oxygen flux to the oxygen diffusion coefficient was deduced. Model calculations, based on the transport equation in conjunction with data of oxygen permeation and oxygen nonstoichiometry, were performed. The rates of oxygen permeation fluxes measured at various temperatures and time showed that the long-term operation stability of the SCF membrane with the addition of ZrO2 was greatly improved. The oxygen permeation flux, however, was slightly reduced.