A case study of mechanical properties of perovskite-structured Ba0.5Sr0.5Co0.8Fe0.2O3−δ oxygen transport membrane

This study has investigated mechanical properties of perovskite-structured Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) oxygen transport membrane. The Young’s modulus and fracture toughness are determined by both macroscopic-scale and microscopic-scale methods. Both three-point and ring-on-ring bending tests as macroscopic-scale methods produce broadly similar results with a Young’s modulus, which is lower than that measured from micro-indentation method under a 10 N load. Young’s modulus and fracture toughness of BSCF show strongly dependent of the porosity. However, the fracture toughness of BSCF is independent of grain size. The fracture toughness determined by macroscopic-scale method is similar with that measured by microscopic-scale method. The crack shape of BSCF under a 10 N load is determined to be a median-radial mode. The intrinsic Young’s modulus and fracture toughness are determined to be 105.6 GPa and 1.49 MPa m^0.5, respectively, according the Minimum Solid Area (MSA) model. Annealing decreases the fracture toughness of BSCF between RT and 800 °C.     

Effect of microstructure on oxygen permeation of Ba0.5Sr0.5Co0.8Fe0.2O3−δ and SrCo0.8Fe0.2O3−δ membranes

The effect of grain size on oxygen permeation properties of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) and SrCo_0.8Fe_0.2O_3?δ (SCF) membranes was investigated by variation of the dwell time. The membrane microstructure was examined by field-emission scanning microscopy (FE-SEM) and then evaluated using a statistical approach. With longer dwell times the grain growth was stimulated and leaded to grains with a narrower size distribution. The grains of SCF (average size from 11.3 to 19.9 μm) were found to be smaller than those of BSCF (average size from 13.9 to 41.3 μm). The oxygen permeation flux of BSCF membranes was found to be independent of grain size in the range from 24 to 42 μm. However, membranes with smaller grains (13.9 μm) show a decreased oxygen permeation flux. For the SCF membranes a decrease in permeation flux with larger grains was observed for average grain sizes between 11.3 and 19.9 μm. By transmission electron microscopy (TEM) formation of an oxygen ordered SrCo_0.8Fe_0.2O_2.5 brownmillerite by-phase could be observed at the oxygen-depleted sweep side of the membrane.     

Predicting grain size distributions in perovskite-structured Ba0.5Sr0.5Co0.8Fe0.2O3–δ oxygen transport membranes

ABSTRACT

This study is conducted over a 3?×?3 time–temperature matrix on Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) ceramics, and sintered bodies above 93% dense are obtained. The electron backscatter diffraction band contrast micrographs of the polished sintered samples are analysed for characterising the grain size distributions (GSDs). This study develops an algorithm for predicting the GSDs of BSCF dependence of sintering condition (time and temperature). In addition, the GSDs predicted by the algorithm agree reasonably with those experimentally observed. When individual grain size is non-dimensionalised by the median grain size, the GSDs data of all BSCF samples reduce to a single self-similar GSD curve. The median grain size is predicted by the classical kinetics equation, D^n?=?tK₀exp(?Q/RT).     

Ammonia oxidation in Ba0.5Sr0.5Co0.8Fe0.2O3−δ membrane reactor

Selective oxidation of ammonia to NO was studied in a dense mixed ion electron conducting Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ membrane reactor, which integrates the separation and catalytic reaction process in a single reactive separation unit. The influence of the temperature and feed concentration on the membrane reaction performance were investigated in detail. Under reaction conditions, the oxygen permeation flux through the dense membrane increases with increasing temperature and ammonia flow rate. The lower temperature and ammonia concentration can favor the formation of NO, in which higher catalytic performance is obtained, suggesting that the membrane reactor operation is much beneficial for selective oxidation of ammonia.     

Towards the fabrication of La0.98−xSrxCo0.2Fe0.8O3−δ perovskite-type oxygen transport membranes

La_0.98−xSr_xCo_0.2Fe_0.8O_3−δ (LSCF) is a candidate material for use as an oxygen transport membrane (OTM). In this work, fabrication-relevant properties (sintering behaviour, thermal and chemical expansion) of LSCF (x = 0.2, 0.4, 0.6, 0.8) were investigated in order to select the preferred composition for fabricating a thin-film supported membrane able to withstand the thermochemical stresses encountered during manufacturing and operation with simultaneously high oxygen permeation flux.Partial substitution of La by Sr ions in LSCF is beneficial for increasing the oxygen permeation rate, but it causes drawbacks regarding manufacturing and operation. A Sr content of x ≥ 0.6 results in a swelling of the material during sintering, which complicates the manufacturing of thin, leak-free membranes. This swelling is related to oxygen release during heating, combined with the formation of a liquid phase above 1200 °C. Furthermore, an increase in total strain with Sr content is observed. This is caused by the chemical expansion, while there is no significant change in thermal expansion with increasing Sr content.The compositions x = 0.4 and x = 0.6 showed tolerable expansion coefficients as well as adequate sintering behaviour and were therefore selected for the fabrication of thin supported membranes. These supported membranes with a thickness of 30 μm were manufactured by sequential tape casting and characterised regarding microstructure and oxygen flux.     

Yttrium doping of Ba0.5Sr0.5Co0.8Fe0.2O3-δ part II: Influence on oxygen transport and phase stability

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) in its cubic perovskite phase has attracted much interest for potential use as oxygen transport membrane (OTM) due to its very high oxygen permeability at high temperatures. However, performance degradation due to a sluggish phase decomposition occurs when BSCF is operated below 840?°C. Partial B-site substitution of the transition metal cations in BSCF by larger and redox-stable cations has emerged as a potential strategy to improve the structural stability of cubic BSCF. In this study, the influence of yttrium doping (0…10?mol-%) on oxygen transport properties and stability of the cubic BSCF phase is assessed by in situ electrical conductivity relaxation (ECR) and electrical conductivity measurements during long-term thermal annealing both at 700?°C and 800?°C. Detailed phase analysis is performed by scanning electron microscopy (SEM) after long-term annealing of the samples in air at different temperatures.     

Modeling of U-shaped Ba0.5Sr0.5Co0.8Fe0.2O3-δ hollow-fiber membrane for oxygen permeation

A mathematic model is developed for the perovskite-type mixed ionic-electronic conducting (MIEC) membrane,which makes it possible to simulate the process of oxygen separation in the U-shaped Ba0.5Sr0.5Co0.8Fe0.2O3-δ hollow-fiber membrane.The model correlates the oxygen permeation flux to the measurable variables.The trends of calculated results for purge operation coincide well with the experimental data,therefore the model is considerable for flux prediction under vacuum operation.Higher oxygen separation efficiency can be achieved with vacuum operation than purge operation.Parameter study with vacuum operation reveals that oxygen permeation flux increases with higher vacuum levels,and vacuum pressure of around 1.013 × 103 Pa is the optimal.Also,vacuum operation on the lumen side is much more efficient to achieve higher oxygen permeation flux compared with compression mode on the shell side.     

New morphological Ba0.5Sr0.5Co0.8Fe0.2O3−α hollow fibre membranes with high oxygen permeation fluxes

Perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?α (BSCF) hollow fibre membranes were fabricated by a combined phase inversion and sintering technique. The membranes were characterised by XRD, SEM and tested for air separation. The membrane possesses a novel morphology consisting of one dense layer and one porous layer. Oxygen permeation fluxes through the obtained hollow fibre membranes were measured in the temperature range 650–950 °C using helium sweep gas rates from 50 to 200 mL min^?1. Experimental results indicated the oxygen permeation flux through the BSCF hollow fibre membrane sintered at 1050 °C was approximately 11.46 mL min^?1 cm^?2 at 950 °C when the helium sweep rate was kept at 200 mL min^?1. The BSCF hollow fibre membrane showed a stable oxygen permeation flux of 8.60 mL min^?1 cm^?2 over the investigated period of 120 h at 900 °C.     

Experimental investigation and mathematical modeling of oxygen permeation through dense Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) perovskite-type ceramic membranes

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) perovskite powder was synthesized via EDTA/citrate complexation method. BSCF membranes were formed by pressing powder at 400 MPa and sintering at 1100 °C for 10 h. XRD patterns showed that a high pure powder with cubic structure was obtained. SEM micrographs revealed that the membranes are dense with large grains. Effects of temperature, feed and permeate side oxygen partial pressures, flow rates and membrane thickness on oxygen permeation flux were studied experimentally. A Nernst–Planck based mathematical model, including surface exchange kinetics and bulk diffusion, was developed to predict oxygen permeation flux. Considering non-elementary surface reactions and introducing system hydrodynamics into the model resulted in an excellent agreement (RMSD = 0.0617, AAD = 0.0487 and R² = 0.985) between predicted and measured fluxes. The results showed that oxygen permeation flux increases with temperature, feed side oxygen partial pressure and flow rates, however decreases with permeate side oxygen partial pressure and membrane thickness. Contribution of feed side surface exchange reactions, bulk diffusion and permeate side surface exchange reactions resistances in the total resistance are in the range of 8–32%, 10–81% and 11–59%, respectively. Permeation rate-limiting step was determined using the membrane dimensionless characteristic thickness.     

The effect of B-site Y substitution on cubic phase stabilization in (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ

The cubic phase mixed ionic-electronic conductor (Ba_0.5Sr_0.5)(Co_0.8Fe_0.2)O_3−δ (BSCF) is well-known for its excellent oxygen ion conductivity and high catalytic activity. However, formation of secondary phases impedes oxygen ion transport and consequentially a widespread application of BSCF as oxygen transport membrane. B-cation substitution by 1, 3 and 10 at.% Y was employed in this work for stabilization of the cubic BSCF phase. Secondary phase formation was quantified on bulk and powder samples exposed to temperatures between 640 and 1100°C with annealing time up to 44 days. The phase composition, cation valence states, and chemical composition of all samples were analyzed by high-resolution analytical electron microscopic techniques. Y doping effectively suppresses the formation of Ba_n+1Co_nO_3n+3(Co₈O₈) (n ≥ 2) and Co_xO_y phases which would otherwise act as nucleation centers for the highly undesirable hexagonal BSCF phase. This work validates for 10 at.% Y cation substitution perfect stabilization of the cubic BSCF phase at temperatures ≥800°C, while a negligible small volume fraction of the hexagonal BSCF phase was found at lower temperatures. A newly developed model describes the effect of Y doping on the formation of secondary phases and their effective suppression with increasing Y concentration.     

Oxygen permeation through dense La0.1Sr0.9Co0.8Fe0.2O3−δ perovskite membranes: Catalytic effect of porous La0.1Sr0.9Co0.8Fe0.2O3−δ layers

The oxygen permeation performance of a number of La_0.1Sr_0.9Co_0.8Fe_0.2O_3−δ (LSCF1982)-based membranes, consisting of dense LSCF1982 layer with/without porous LSCF1982 layer, was analyzed on the basis of the thickness of the dense layer and catalytic effect of the porous layer. A 0.27 mm thick dense membrane gives oxygen permeation flux ($J_{O_2}$) of 2.33 sccm min⁻¹ cm⁻² at 900 °C, which is increased to 3.55 sccm min⁻¹ cm⁻² on applying a porous layer of LSCF1982 onto the dense membrane. The membrane gives a stable flux for 300 h. The flux was further improved by reducing the thickness of the dense LSCF1982 layer and at 950 °C a flux of 4.47 sccm min⁻¹ cm⁻² is obtained with 0.012 mm thick membrane.     

Thermochemical expansion of mixed-conducting (Ba,Sr)Co0.8Fe0.2O3−δ ceramics

The thermal and chemical expansion of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and SrCo_0.8Fe_0.2O_3−δ (SCF) mixed ionic-electronic conductors were studied in combination with oxygen nonstoichiometry (δ) at 298–1223 K and p(O₂) = 10⁻⁴ to 1.00 atm. In order to minimize the effects of phase separation or oxygen-vacancy ordering processes, the data were collected in dynamic cooling mode using dense ceramic samples. The procedure was justified by a very fast equilibration at given p(O₂) in high-temperature range demonstrated for ceramics samples with different specific surface area. The difference in nonstoichiometry of BSCF and SCF at temperatures ≥973 K was found to be ≤0.03 oxygen atoms per formula unit. BSCF demonstrates favorably smaller chemical expansion compared to SCF and many other mixed conductors, originating from smaller δ variations and larger unit cell less sensitive to temperature and nonstoichiometry changes. Excessive thermochemical expansion impedes however the use of BSCF in single-phase fuel cell cathodes and planar mixed-conducting membranes.     

Microstructure coarsening in Ba0.5Sr0.5Co0.8Fe0.2O3-δ during reactive air brazing

Reactive air brazing of the ceramic oxygen transport membrane material BSCF causes microstructural modifications. The trend towards minimizing of membrane thicknesses requires a physical understanding of these modifications since they may influence both oxygen permeation and mechanical properties. To this purpose, wetting samples with variations in the Ag-xCuO braze alloy (1 < x < 25 at.-%) and the brazing time (0 < t < 120 min) were characterized by quantitative image analysis. We found that the pore size in reactive air brazed BSCF increases with increased brazing time as well as CuO-content in the braze. A local porosity minimum is always observed at the end of the reaction zone close to the unaffected bulk BSCF. Additionally, a zone with unidirectionally elongated grains at the end of which silver residues were found is observed. The observed coarsening effects are explained by liquid melt film penetration along the grain boundaries which increases the grain boundary mobility. Phase field simulations qualitatively confirmed the experimentally observed elongated grain structure.     

A novel series of Ba0.5Sr0.5Al0.2−xMgxFe0.8O3−ξ (x ≤ 0.2) membranes for oxygen permeation application

The pervoskite-type oxides have received attention due to their potential applications in catalysis, fuel cells, sensors, gas separable membranes, and electrolytes. In view of the importance of oxygen separation from air, stable Ba_0.5Sr_0.5Al_0.2−xMg_xFe_0.8O_3−ξ (x = 0–0.2) powders have been synthesized by decomposition of sol–gel derived oxalate at 950 °C for 5 h and characterized with regard to formation, nature of iron species, oxygen permeation, and electrical conductivity. It is shown that magnesium substitution leads to (i) a stable perovskite-type cubic phase with ‘a’ = 3.953–3.978 Å, (ii) weakening of metal–oxygen bond, (iii) reduction of Fe⁴⁺ ions, and (iv) enhancement of oxygen deficiency and electrical conductivity. Their compact discs act as stable oxygen permeable filters with flux density of ∼3.013–3.355 μmol cm⁻² s⁻¹ at 1000 °C. The maximum value corresponds to composition x = 0.2 and hence can be a potential membrane for oxygen separation technology.     

Enhanced sintering of Ce0.8Nd0.2O2−δ-La0.8Sr0.2Ga0.8Mg0.2O3−δ using CoO as a sintering aid

Ce_0.8Nd_0.2O_1.9 (NDC) and La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ (LSGM) electrolytes were prepared using a sol-gel method. NDC-LSGM composite electrolytes were subsequently prepared by adding 5% (w, mass fraction) precalcined LSGM powders to NDC sols. The electrolyte materials of NDC-Co and NDC-LSGM-Co were obtained by adding 1 mol% CoO to NDC sols and NDC-LSGM composite electrolytes, respectively. The microstructure and phase composition of the pellets were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and energy dispersive X-ray spectroscopy (EDS). The electrical conductivities of the pellets were measured using alternative current (AC) impedance spectroscopy. The results indicate that a single perovskite phase is observed for the LSGM ceramic, while NDC-Co, NDC-LSGM and NDC-LSGM-Co have a cubic fluorite structure similar to that of NDC. As a sintering aid, CoO can further promote grain growth and increase relative density (＞95%) of the NDC-LSGM composite electrolyte. The enhancement of the total conductivity is primarily attributed to the large increase in the conductivity of the grain boundary. However, the slight decrease of the grain boundary conductivity of the NDC-LSGM-Co electrolyte is caused by the presence of trace amounts of impurity phases in the grain boundaries.     

Evaluation of (Ba0.5Sr0.5)0.85Gd0.15Co0.8Fe0.2O3−δ cathode for intermediate temperature solid oxide fuel cell

A perovskite-type (Ba_0.5Sr_0.5)_0.85Gd_0.15Co_0.8Fe_0.2O_3?δ (BSGCF) oxide has been investigated as the cathode of intermediate temperature solid oxide fuel cells (IT-SOFCs). Coulometric titration, thermogravimetry analysis, thermal expansion and four-probe DC resistance measurements indicate that the introduction of Gd³⁺ ions into the A-site of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) leads to the increase in both oxygen nonstoichiometry at room temperature and electrical conductivity. For example, the conductivity of BSGCF is 148 S cm^?1 at 507 °C, over 4 times as large as that of BSCF. Furthermore, the electrochemical activity toward the oxygen reduction reaction is also enhanced by the Gd doping. Impedance spectra conducted on symmetrical half cells show that the interfacial polarization resistance of the BSGCF cathode is 0.171 Ω cm² at 600 °C, smaller than 0.297 Ω cm² of the BSCF cathode. A Ni/Sm_0.2Ce_0.8O_1.9 anode-supported single cell based on the BSGCF cathode exhibits a peak power density of 551 mW cm^?2 at 600 °C.     

Synthesis,CO2-tolerance and rate-determining step of Nb-doped Ce0.8Gd0.2O2−δ–Pr0.6Sr0.4Co0.5Fe0.5O3−δ ceramic membranes

In this work, CO₂-tolerant Ce_0.8Gd_0.2O_2δ–Pr_0.6Sr_0.4Co_0.5Fe_0.5−xNb_xO_3−δ (CG–PSCF_0.5−xN_x; x=0–0.125) dual-phase dense oxygen permeation membranes were successfully developed. The crystal structure, microstructure, oxygen permeability, rate-determining step and CO₂ tolerance were systematically investigated. The experimental results showed that the increase in CG content improved oxygen permeability and CO₂ tolerance. Thermogravimetry–differential-scanning-calorimetry analysis, X-ray photoelectron spectra and oxygen permeation tests indicated that the increase in Nb content caused a slight decrease in oxygen permeability, while the long-term CO₂ resistance can be improved significantly. According to the adopted permeation model, the weight ratio and thickness affect the oxygen permeability and permeation resistance distribution. By examining the distribution of three permeation resistances, we identified the rate-determining step and then optimized the weight ratio of the two phases, as well exploring the effects of thickness on oxygen permeability. All these experiments confirm that CG–PSCF_0.5−xN_x dual-phase membranes have great CO₂ tolerance and potential application in oxy-fuel combustion.     

Oxygen permeation performance of Ba0.5Sr0.5Co0.8Fe0.2O3?δ membrane after surface modification

The effect of minor surface modification on the performance of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membrane was evaluated in the temperature region from 700 to 850 °C. Oxygen permeation experiments were conducted according to membrane thickness (1.0mm and 1.6 mm) and oxygen partial pressure (0.21, 0.42, and 0.63 atm) in the absence and in the presence of carbon dioxide (300 and 500 ppm). The oxygen permeation flux of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membrane increased with increasing temperature and decreasing membrane thickness. The oxygen permeation flux through the membrane of 1.0 mm thickness with Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ -modified surface was ca. 1.23 ml/cm²·min at 850 °C under air feeding condition. It was found that the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ -modified Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membrane has better oxygen permeation flux than the pristine Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membrane. In summary, it has been demonstrated that the surface morphology is an important factor in determining the oxygen permeation fluxes through Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ membrane under mixed-control conditions.     

Electrochemical and microstructural characteristics of nanoperovskite oxides Ba0.2Sr0.8Co0.8Fe0.2O3−δ (BSCF) for solid oxide fuel cells

Nanoperovskite oxides, Ba_0.2Sr_0.8Co_0.8Fe_0.2O_3?δ (BSCF), were synthesized via the co-precipitation method using Ba, Sr, Co, and Fe nitrates as precursors. Next, half cells were fabricated by painting BSCF thin film on Sm_0.2Ce_0.8O_x (samarium doped ceria, SDC) electrolyte pellets. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS) measurements were carried out on the BSCF powders and pellets obtained after sintering at 900 °C. Investigations revealed that single-phase perovskites with cubic structure was obtained in this study. The impedance spectra for BSCF/SDC/BSCF cells were measured to obtain the interfacial area specific resistances (ASR) at several operating temperatures. The lowest values of ASR were found to be 0.19 Ω cm², 0.14 Ω cm² 0.10 cm², 0.09 Ω cm² and 0.07 Ω cm² at operating temperatures of 600 °C, 650 °C, 700 °C, 750 °C and 800 °C, respectively. The highest conductivity was found for cells sintered at 900 °C with an electrical conductivity of 153 S cm^?1 in air at operating temperature of 700 °C.     

Phase interaction and oxygen transport in La0.8Sr0.2Fe0.8Co0.2O3–(La0.9Sr0.1)0.98Ga0.8Mg0.2O3 composites

Composite ceramics made of two perovskite-type compounds, (La_0.9Sr_0.1)_0.98Ga_0.8Mg_0.2O_3−δ (LSGM) and La_0.8Sr_0.2Fe_0.8Co_0.2O_3−δ (LSFC) mixed in the ratio 60:40 wt.%, possess relatively high oxygen permeability limited by both bulk ionic conduction and surface exchange at 700−950 °C. Sintering at elevated temperatures (1320–1410 °C) necessary to obtain dense materials leads to fast interdiffusion of the components, forming almost single perovskite phase ceramics with local inhomogeneities. This phase interaction decreases the oxygen ionic transport in the composites, where the level of ionic conductivity is intermediate between those of LSGM and LSFC. The scanning electron microscopy (SEM) suggests a presence of Ga-enriched domains, probably having a high ionic conductivity. The size and concentration of these domains can be increased by decreasing sintering temperature or using preliminary coarsened LSGM powders. The maximum oxygen permeability is thus observed for the composite prepared under minimum sintering conditions sufficient to obtain gas-tight ceramics, including the use of LSGM, preliminary passivated at 1150 °C, and sintered at 1320 °C. The activation energy values for total conductivity, which is predominantly p-type electronic and slightly decreases due to component interaction, vary in the narrow range from 24.0 to 26.2 kJ/mol at 25–575 °C. The average thermal expansion coefficients (TECs) of LSGM-LSFC composites, calculated from dilatometric data in air, are (12.4–13.5)×10⁻⁶ K⁻¹ at 100–650 °C and (17.8–19.8)×10⁻⁶ K⁻¹ at 650–1000 °C.