Oxygen Bulk Diffusion Research Articles

Systematic investigation on structure stability and oxygen permeability of Sr-doped BaCo 0.7Fe 0.2Nb 0.1O 3− δ ceramic membranes

Ba 1− x Sr x Co 0.7Fe 0.2Nb 0.1O 3− δ (BSCFN, 0.0 ≤ x ≤ 0.4) materials were synthesized by the conventional solid-state reaction process for oxygen separation application. The influences of strontium doping in BSCFN oxides on phase structure stability, oxygen nonstoichiometry, electrical conductivity and oxygen permeation behavior were systematically investigated. Improved cubic structure stability under reducing atmosphere and structure reversibility in alternative oxidizing/reducing atmospheres were obtained via strontium doping strategy. Strontium introduction effectively suppressed the phase transformation from cubic to hexagonal under high temperature and harsh atmosphere conditions. Strontium doping increased the electrical conductivity, however, slightly decreased the oxygen permeability. Structural parameters played an important role in charge compensation mechanism and oxygen permeability. Ba 0.6Sr 0.4Co 0.7Fe 0.2Nb 0.1O 3− δ membranes exhibited good structural stability and the sample with a thickness of 1 mm had a high level of oxygen permeation flux of 1.31 mL cm −2 min −1 at 900 °C under an Air/He gradient. It was demonstrated that the oxygen bulk diffusion was the rate-controlling step for oxygen permeation in BSCFN ( x = 0.4) membranes when the thickness was higher than 1 mm, while the oxygen surface exchange process should be taken into consideration when the thickness was less than 1 mm.

Effect of Dopant, Crystal Orientation, and Space Charge Layer on Oxygen Diffusion in Bi4Ti3O12 Ceramics

We studied oxygen diffusion in non- and Nb2O5-doped Bi4Ti3O12 (BIT) ceramics by two-dimensional isotope mapping. In non-doped BIT, the grain boundary acted as a barrier to oxygen diffusion, reflecting the presence of a space charge layer at the grain boundary. In Nb2O5-doped BIT, a textured morphology was found by bulk and grain-boundary diffusion of oxygen. Diffusion coefficients were determined from the individual grains and grain boundaries. The difference in diffusion paths resulted from the decrease in oxygen-defect concentration owing to the addition of Nb2O5. The bulk-diffusion coefficient along the ab-plane was about one order of magnitude larger than that in the direction of the c-axis. The grain-boundary-diffusion coefficients varied by about two orders of magnitude. The bulk and grain-boundary diffusion of oxygen depended significantly on the crystal orientation.

Effect of CuO additive on the sintering and performance of niobium-doped strontium cobaltite as oxygen separation membranes

In this work, the effects of CuO addition on sintering behavior, crystal structure and the oxygen permeation of SrCo 0.9Nb 0.1O 3− δ (SCN) membranes have been investigated. XRD characterization demonstrated that copper could incorporate into the perovskite lattices with certain solubility dependent on temperature. Small amount of CuO (5 wt.%) successfully reduced the sintering temperature of the SCN membrane by 180 °C. A relative density of 95.4% was reached for the membrane with 5 wt.% CuO additive after sintering at 1000 °C. The promoting effect on sintering is likely associated with liquid assisted sintering. The incorporation of copper into the SCN lattice has minimal effect on the membrane sintering but a significant effect on the membrane integrity. As compared to the single-phase SCN membranes, the introduction of CuO as a sintering aid does not affect the electronic conductivity of the membrane between 700 and 900 °C, but the oxygen permeability is slightly reduced. Permeation study of the membranes of 0.9 mm thickness demonstrated oxygen fluxes of 1.5, 1.4, 1.3 and 1.2 ml cm −2 min −1 [STP] at 800 °C for the membranes containing 0 (pure SCN), 1, 3 and 5 wt.% CuO, respectively. The results suggest that the introduction of CuO as a sintering aid had a more significant effect on the oxygen surface exchange kinetics than on the oxygen bulk diffusion rate.

Comparison of oxygen permeation through some perovskite membranes synthesized with EDTNAD

Three dense membranes of types SrCo0.8Fe0.2O3−δ (SCF(82)), La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF(6482)) and La0.8Sr0.2Co0.6Fe0.4O3−δ (LSCF(8264)) perovskites were prepared by complexation applying a chelating agent, ethylene diamine N,N,N′,N′-tetra-N-acetyl-diamine (EDTNAD). The oxygen permeation flux through the perovskite membranes was measured as a function of temperature within 1,073–1,223 K as well as the oxygen partial pressure of 0.1–1.0 bar. The oxygen permeation fluxes for the membranes, SCF(82), LSCF(6482), LSCF(8264) with the thickness of 0.85 mm were observed as 9.2 × 10−7 (mol/cm2 s), 1.7 × 10−7 (mol/cm2 s), and 1.0 × 10−7 (mol/cm2 s) in these cases at 1,153 K. The results indicated the oxygen permeation process was mainly controlled by the oxygen bulk diffusion through these membranes.

Oxygen Surface Exchange and Bulk Diffusion Coefficients Evaluated from Porous Mixed Ionic-Electronic Conducting Cathodes

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Effect of Thermal Aging on the Transient Kinetics of Oxygen Storage and Release of Commercial Ce x Zr1−x O2-based Solids

The use of powerful analytical methodologies for the evaluation of the performance of commercial oxygen storage materials based on oxygen transient isothermal isotopic exchange experiments is illustrated in this study. CexZr1−xO2-based oxygen storage materials were investigated. The materials were characterized for their dynamic oxygen storage and release properties through the pulse injection technique allowing the measurement of the oxygen storage capacity (OSC), and for their surface and bulk oxygen mobility through transient isothermal 18O isotopic exchange experiments. Very high OSC values were obtained after calcination in air at Tcalc. = 700 or 850 °C. However, after thermal aging at 1000 or 1100 °C of the as prepared samples, a significant deterioration of their OSC was observed. This fact was found to correlate with the significant increase in the activation energy of surface oxygen mobility. On the contrary, smaller increase in the activation energy of bulk oxygen diffusion with increasing Tcalc. was observed.

Evaluation of mixed‐conducting lanthanum‐strontium‐cobaltite ceramic membrane for oxygen separation

AbstractIn this study, La0.4Sr0.6CoO3‐δ (LSC) oxide was synthesized via an EDTA‐citrate complexing process and its application as a mixed‐conducting ceramic membrane for oxygen separation was systematically investigated. The phase structure of the powder and microstructure of the membrane were characterized by XRD and SEM, respectively. The optimum condition for membrane sintering was developed based on SEM and four‐probe DC electrical conductivity characterizations. The oxygen permeation fluxes at various temperatures and oxygen partial pressure gradients were measured by gas chromatography method. Fundamental equations of oxygen permeation and transport resistance through mixed conducting membrane were developed. The oxygen bulk diffusion coefficient (Dv) and surface exchange coefficient (Kex) for LSC membrane were derived by model regression. The importance of surface exchange kinetics at each side of the membrane on oxygen permeation flux under different oxygen partial pressure gradients and temperatures were quantitatively distinguished from the oxygen bulk diffusion. The maximum oxygen flux achieved based on 1.6‐mm‐thick La0.4Sr0.6CoO3‐δ membrane was ∼4.0 × 10−7 mol cm−2 s−1at 950°C. However, calculation results show theoretical oxygen fluxes as high as 2.98 × 10−5 mol cm−2 s−1 through a 5‐μm‐thick LSC membrane with ideal surface modification when operating at 950°C for air separation. © 2009 American Institute of Chemical Engineers AIChE J, 2009

Oxygen diffusion in SrZrO 3

Oxygen diffusion coefficients in SrZrO 3 polycrystals were determined using the isotopic exchange method with 18O as oxygen tracer. Diffusion treatments were performed at different temperatures between 1173 K and 1473 K. Oxygen diffusion profiles were established by secondary ion mass spectroscopy (SIMS). Classical diffusion equations were used to fit experimental results and to determine bulk diffusion ( D vol) and surface exchange ( k) coefficients of oxygen in SrZrO 3 polycrystalline materials. From these values, bulk diffusion and grain boundary diffusion coefficients as well as oxygen surface exchange coefficients were determined. The activation energy of oxygen diffusion in the bulk is 2.1 eV, while for the diffusion in the grain boundary, 1.8 eV was found. The surface exchange reaction has an activation enthalpy of 1.2 eV.

Diffusion of Oxygen in Thermally Grown Oxide Scales

High temperature reactivity of materials under oxidizing atmospheres is based on the formation of protective oxide scales. The protectiveness is obtained when the thermally grown oxide scales are dense, continuous and adherent to the metallic substrates (even during thermal shocks); as a matter of fact, the growth of the scale has to be governed by diffusion of species across the growing scale. The diffusing species are coming from the substrate (metallic ions) and/or from the oxidizing atmosphere (oxygen ions). The understanding of growth mechanisms can be reached by making two stage oxidation experiments, using oxygen isotopes. The experiment consists in oxidizing first the metallic substrate in 16O2, evacuating the oxygen after the desired time, and replacing it by 18O2. The distribution of oxygen isotopes given by secondary ion mass spectrometry (SIMS) across the oxide scale informs about the growth mechanisms: anionic transport, cations diffusion or mixed diffusion processes. The use of marker experiment is able to determine the oxygen diffusion coefficients within the growing scales. In this case, a thick scale is grown under 16O2, followed by a shorter diffusion experiment in 18O2. The distribution of 18O isotope across the scale by plotting ln (18O intensity) versus x (depth in oxide) allows determining the oxygen effective diffusion coefficient in the scale, according to the classical Fick’s law solution. A more accurate analysis of these profiles can provide information about bulk and grain boundary diffusion of oxygen. The results can be related to kinetics according to Wagner’s theory. The proposed work consists firstly in making a state of the art review about oxygen diffusion in thermally grown oxide scales, and secondly in connecting the so-obtained outcome (effective, bulk and grain boundary diffusion) to kinetics results. The proposed oxides are chromia, alumina and zirconia.

Study of Oxygen Diffusion in Polycrystalline ZnO by SIMS

Oxygen diffusion coefficients were measured in polycrystalline ZnO by means of the gas-solid exchange method using the isotope 18O as the oxygen tracer. The diffusion annealings were performed at 892oC and 992oC, in an Ar+18O2 atmosphere under oxygen partial pressures from 0.1 to 1atm. After the diffusion annealings, the 18O diffusion profiles were established by secondary ion mass spectrometry (SIMS). Increasing the oxygen pressure leads to an increase of the oxygen diffusion in ZnO. The bulk diffusion coefficients depends on oxygen pressure according to , at 882oC, or , at 992oC, which indicates that the oxygen bulk diffusion mechanism should preferentially take place by means of interstitial oxygen having a null effective charge. The grain boundary diffusion coefficients show little dependence on oxygen pressure at 882oC, given by , which should correspond to a diffusion mechanism by means of interstitial oxygen, with a double negative charge, but at 992oC this dependence is corresponding to a diffusion mechanism by interstitial oxygen having a null effective charge. The results also show that the grain boundary is a fast path for the oxygen diffusion in polycrystalline ZnO.

First Study of Oxygen Diffusion in a ZnO - Based Commercial Varistor

Oxygen diffusion coefficients were determined in a commercial ZnO-based varistor by means of the gas-solid exchange method using the isotope 18O as the oxygen tracer. The diffusion annealings were performed at 892, 942, 992 and 1092oC, in an Ar + 18O2 atmosphere under an oxygen partial pressure of 0.2 atm. After the diffusion annealings, the 18O diffusion profiles were established by secondary ion mass spectrometry (SIMS). The results show an increase of the oxygen diffusion in the varistor, both in bulk and in grain boundaries, when compared to the oxygen diffusion in undoped ZnO. The increase of the oxygen bulk diffusion in the varistor agrees with an interstitial mechanism for the oxygen diffusion. The results also show that the grain boundary is a fast path for the oxygen diffusion in the varistor. However, the oxygen diffusion in the grain boundaries of the varistor seems to depend on several chemical and microstructural parameters and does not allow a simple explanation.

Measurement and Modeling of the Impedance Characteristics of Porous La1 − xSrxCoO3 − δ Electrodes

Porous (LSC) electrodes having Sr composition (LSC-64) and (LSC-82) were fabricated on Sm-doped ceria electrolytes, and studied using electrochemical impedance spectroscopy between and . The faradaic portion of the impedance was found to exhibit Gerischer or Gerischer-like characteristics, indicating colimitation by kinetics and transport. The characteristic resistance and frequency response was analyzed using a transport and reaction model that considers parallel bulk and surface diffusion of oxygen, as well as three-dimensional transport effects near the electrode electrolyte interface. In the case of LSC-64, measured characteristics appear to be largely consistent with a bulk transport path, based on independent measurements of the thermodynamic, kinetic, and transport properties of LSC-64. In the case of LSC-82, which has a much lower bulk vacancy concentration under the same conditions, results were inconsistent with an entirely bulk transport path. Results for LSC-82 could be rationalized assuming a parallel surface transport path, where surface mobility is governed by some kind of interstitial or adatom diffusion mechanism.

Oxygen reduction mechanism at Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ cathode for solid oxide fuel cell

Perovskite structure Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ (BSCF) and La 0.9Sr 0.1Ga 0.8Mg 0.2O 3− δ (LSGM) powders have been successfully synthesized by glycine–nitrate combustion process. A porous and crack-free BSCF cathode is obtained by spraying the slurry of BSCF powders and terpineol onto LSGM pellet. The oxygen reduction reaction mechanism has been investigated by AC impedance spectroscopy and cyclic voltammetry method. AC impedance spectroscopy analysis shows that there are two different processes in the cathode reaction which are related to oxygen dissociation/adsorption and bulk oxygen diffusion. And the molecular oxygen is involved in the rate-determining step. The polarization resistance decreases with an increase of temperature and the oxygen partial pressure. With an increase of the applied DC bias, the logarithm of the polarization resistance decreases linearly due to additional oxygen vacancies and the lowered chemical potential of oxygen at the BSCF/LSGM interface by the applied voltage. The exchange current density reaches to 182 mA cm −2 at 700 °C, suggesting that the ORR kinetics at the BSCF/LSGM interface is high due to the excellent mixed ionic and electronic conductivity of BSCF.

Fabrication of Porous Supported MIEC Dense Thin Coating for Hydrogen Separation

Decomposition of steam under a chemical driving force at moderate temperatures offers a simple and economical way to generate hydrogen. Significant amounts of hydrogen can be separated by splitting steam and removing the oxygen using Gd0.2Ce0.8O1.9-Gd0.08Sr0.88Ti0.95Al0.05O3{plus minus}δ mixed ionic and electronic conductors. Hydrogen generation experiments for the self-standing thick membranes and porous supported thin membranes have been conducted at different oxygen pressure gradients across the membranes and steam contents on the steam side. Experimental results indicate that the hydrogen generation process is controlled by the bulk diffusion for the thick membranes, while the permeation process for the thin membranes is mixed controlled, i.e. the process is controlled by surface exchange reactions and the oxygen bulk diffusion. A mathematical model for the calculation of the area specific hydrogen generation rate is proposed based on the measured oxygen partial pressures, gas compositions of the inlet and outlet gases on the steam side.

Systematic investigation on new SrCo 1− yNb yO 3− δ ceramic membranes with high oxygen semi-permeability

SrCo 1− y Nb y O 3−δ ( y = 0.025–0.4) were synthesized for oxygen separation application. The crystal structure, phase stability, oxygen nonstoichiometry, electrical conductivity, and oxygen permeability of the oxides were systematically investigated. Cubic perovskite, with enhanced phase stability at higher Nb concentration, was obtained at y = 0.025–0.2. However, the further increase in niobium concentration led to the formation of impurity phase. The niobium doping concentration also had a significant effect on electrical conductivity and oxygen permeability of the membranes. SrCo 0.9Nb 0.1O 3− δ exhibited the highest electrical conductivity and oxygen permeability among the others. It reached a permeation flux of ∼2.80 × 10 −6 mol cm −2 s −1 at 900 °C for a 1.0-mm membrane under an air/helium oxygen gradient. The further investigation demonstrated the oxygen permeation process was mainly rate-limited by the oxygen bulk diffusion process.

Oxygen diffusion in Bi2O3-doped ZnO

In order to clarify the influence of Bi-doping on oxygen diffusion in ZnO, the bulk and grain boundary oxygen diffusion coefficients were measured in Bi2O3-doped ZnO polycrystals by means of the gas-solid exchange method using the isotope 18O as the oxygen tracer. The experiments were performed on ZnO sintered samples containing 0.1, 0.3 and 0.5 mol% Bi2O3. The diffusion annealings were performed at 942, 1000 and 1092 °C, in an Ar+18O2 atmosphere under an oxygen partial pressure of 0.2 atm. After the diffusion annealings, the 18O diffusion profiles were established by secondary ion mass spectrometry (SIMS). The results show an increase in the oxygen diffusion in the Bi2O3-doped ZnO, when compared to the oxygen diffusion in the undoped ZnO polycrystal under the same experimental conditions, both in bulk and in grain-boundaries. Moreover, it was observed that the higher the Bi2O3 concentration, the higher the oxygen diffusion. These results suggest that the incorporation of Bi2O3 increases the interstitial oxygen concentration which agrees with an interstitial diffusion mechanism both in bulk and in grain-boundaries.

High temperature oxidation of Fe–9Cr–1Mo steel in stagnant liquid lead–bismuth at several temperatures and for different lead contents in the liquid alloy

This research project deals with the feasibility studies concerning the construction of an hybrid reactor for the transmutation of long-lived radioactive wastes. In this context, the liquid lead–bismuth eutectic (LBE) is considered to be a good candidate for the spallation target material needed for the neutrons production necessary to the transmutation. In this hybrid reactor, the LBE, which is enclosed in a T91 (Fe–9%Cr) steel container, can induce corrosion concerns. If the oxygen content dissolved in Pb–Bi is higher than the needed content for magnetite formation, corrosion proceeds by oxidation of the steel. Previously, specific results were reported, obtained in stagnant liquid LBE at 470 °C. An analytical model taking into account the oxide layer structure has been carried out. It involves iron, oxygen and chromium bulk diffusion and diffusion via preferential paths such as liquid lead–bismuth nano-channels incorporated in the oxide layer structure and grain boundaries. In this paper, experimental results on corrosion kinetics, obtained at different temperatures with different percentages of lead in the lead–bismuth alloy, are presented. The model, adapted to the different experimental conditions, is compared to these kinetics and to experimental points coming from the literature at different temperatures in LBE, in pure lead and in pure bismuth.

Oxygen selective membranes based on B-site cation-deficient (Ba 0.5Sr 0.5)(Co 0.8Fe 0.2) yO 3− δ perovskite with improved operational stability

(Ba 0.5Sr 0.5)(Co 0.8Fe 0.2) y O 3− δ ((BS)(CF) y , 1.00 ≥ y ≥ 0.77) oxides were investigated for oxygen separation application with emphasis on long-term operational stability. Pure phase cubic perovskite was formed at y ≥ 0.83. The oxygen nonstoichiometry increased while electrical conductivity and permeability decreased with the decrease of y. However, the (BS)(CF) 0.97 membrane still displayed an attractive oxygen flux as high as 2.4 × 10 −6 mol cm −2 s −1 at 900 °C, as compared to 2.5 × 10 −6 mol cm −2 s −1 for a cation stoichiometric BSCF membrane. The B-site deficiency greatly restrained the A-site cation diffusion and stabilized the perovskite structure and permeation properties of the membranes. During the long-term operation of the (BS)(CF) 0.97 membrane at 850 °C for more than 300 h, a stable permeation flux of (1.8 ± 0.3) × 10 −6 mol cm −2 s −1 was achieved. Further investigation demonstrated that the improved importance of oxygen bulk-diffusion regime in the rate-determination of oxygen permeation through the membranes.

Oxygenation mechanism of TSMG YBCO bulk superconductor

For the first time isothermal thermogravimetric (TG) measurements of oxygen diffusion in to melt processed YBa2Cu3Oδ/Y2BaCuO5 (Y123/Y211 or YBCO) bulk samples were conducted in the temperature range 350 – 800°C. It is shown that the real time to a full sample oxygenation is much shorter than the calculated one assuming the bulk oxygen diffusion along the a/b-planes. It means that the diffusion distances in the oxygenated samples were much shorter at all temperatures of oxygenation then the halve sample size along the a/b-plane (0.1 cm). Estimated diffusion distances correspondent well with the spacing of the cracks formed during oxygenation. The kinetic law of virgin oxygenation at 350 and 400°C is changed during oxygenation and observed deviation from the typical parabolic dependence of the sample weight on time of isothermal oxygenation was related to oxygenation cracking during the sample oxygenation. The performed microstructural analysis of the samples studied confirmed that the change of oxygenation kinetics is accompanied with cracking. Developed oxygenation cracks along the {100} (c-cracks) and {001} (a/b-cracks) type planes play a crucial role in shortening the oxygenation time of single-grain TSMG YBCO bulk superconductors.

Investigation of the OSC performance of [formula omitted] catalysts by CO oxidation and 18O/ 16O isotopic exchange reaction

The development of a more efficient oxygen storage material is a very important approach for the optimization of automotive catalysts. To investigate the influence of the physical parameters, the Pt particle size and specific surface area (SSA) on the OSC (oxygen storage capacity) performance of the Pt / CeO 2 – ZrO 2 – Y 2 O 3 (CZY) catalysts, a series of Pt/CZY catalysts were prepared and characterized. The OSC performance was evaluated by CO oxidation from 200 to 400 ∘ C , while alternately feeding CO ( 2.0 vol % ) / N 2 and O 2 ( 1.0 vol % ) / N 2 for 60 s at a flow rate of 7.0 l/min. The CO 2 generation was analyzed continuously and used to indicate the OSC performance and oxygen storage/release behavior. The oxygen surface diffusion and bulk diffusion rates on the catalysts were further characterized by the O 18 / O 16 isotopic exchange reaction and correlated with the OSC performances. The characterization results of the Pt particle size and SSA of the Pt/CZY catalysts indicated that the Pt particle size and SSA of Pt/CZY appeared to be a hyperbolic distribution, being interdependent and difficult to control independently. OSC performance results by CO oxidation at 200 and 300 ∘ C showed that smaller Pt particle size favored OSC performance and appeared to be the optimum around 70 m 2 / g for the SSA. This corresponds with the oxygen mobility results obtained from the O 18 / O 16 isotopic exchange reaction at 400 ∘ C , that is, the oxygen surface and bulk diffusion showed a peak at the Pt particle size of 6 nm and an SSA of 40 m 2 / g .