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Sintering and oxygen permeation studies of La0.6Sr0.4CoO2Fe0.8O3-delta ceramic membranes with improved purity
Abstract
High purity raw materials are used for synthesizing La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF6428) powders to reduce the effect of impurity phases on oxygen permeability of the corresponding membranes. The as-synthesized LSCF6428 powders require a sintering temperature above 1180 degrees C to achieve membrane density over 90%. Ball milling of the powders increases the membrane sintering. It also increases oxygen permeation flux from 0.37 to 0.43 ml cm(-2) min(-1) at 950 degrees C for the membranes sintered at 1100 degrees C. A decrease in oxygen permeation fluxes with the further increase in sintering temperature is observed for the membranes with ball-milled starting powders, accompanied by an obvious increase in grain size. It suggests, at low level of impurity phases, the grain boundaries facilitate the oxygen diffusion. The combination of ball milling of the starting powders and a sintering temperature of 1100 degrees C is optimal to achieve high oxygen permeability of LSCF6428 membranes with improved purity.
... The huge demand for oxygen comes from a variety of industries, such as medical, steel, and glass. For the production of pure oxygen, a new technology based on a ceramic oxygen transport membrane (OTM) has recently emerged as an alternative method to conventional separation processes, such as cryogenic distillation and pressure swing adsorption [1][2][3][4][5][6][7][8]. OTM materials largely belonging to a class of mixed ionic electronic conductors (MIEC) that allow oxygen diffusion through vacancies in the crystal lattice [1]. ...
To fabricate a multi-layered structure for maximizing oxygen production, oxygen transport membrane (OTM) ceramics need to be joined or sealed hermetically metal supports for interfacing with the peripheral components of the system. Therefore, in this study, Ag–10 wt% CuO was evaluated as an effective filler material for the reactive air brazing of dense Ce0.9Gd0.1O2–δ–La0.7Sr0.3MnO3±δ (GDC–LSM) OTM ceramics. Thermal decomposition in air and wetting behavior of the braze filler was performed. Reactive air brazing was performed at 1050 °C for 30 min in air to join GDC–LSM with four different commercially available high temperature-resistant metal alloys, such as Crofer 22 APU, Inconel 600, Fecralloy, and AISI 310S. The microstructure and elemental distribution of the ceramic-ceramic and ceramic-metal interfaces were examined from polished cross-sections. The mechanical shear strength at room temperature for the as-brazed and isothermally aged (800 °C for 24 h) joints of all the samples was compared. The results showed that the strength of the ceramic-ceramic joints was decreased marginally by aging; however, in the case of metal-ceramic joints, different decreases in strengths were observed according to the metal alloy used, which was explained based on the formation of different oxide layers at the interfaces.
... The precursors should not be hygroscopic as a metal nitrate unless the precursors were synthesized by sol-gel or co-precipitation. The dispersant was needed to produce the homogeneity of the composite such as ethanol because it has a lower boiling point so easily evaporated and produces more uniform particles [4]. ...
Structure evolution and morphology of La 0.7 Sr 0.3 Co 0.8 Fe 0.2 O 3-δ (LSCF 7328) were investigated during two different preparation methods namely mechanochemical and combination of mechanochemical-solid state. The result shows that no characteristic peak of perovskite oxide was found on the diffractogram of the product of sole mechanochemical method at 600 rpm and up to 12 h of high energy milling process. On the other hand, the manual grinding method that was followed by solid state calcination produces irregular particle size. Due to the result, the combination of both methods was proposed to obtain the fine structure formation and particle size distribution. Rietveld refinement was used to investigate the lattice distortion. It was found that unit cell remains unchanged at increasing milling time. Moreover, the combination method produces regular particle size at milling time of 0.5 h. At longer milling time, the more regular particle size is formed which comes from highly energy transfer of milling.
... MIEC membranes are in a good position to replace conventional air distillation for pure oxygen production, because of the high purity of the generated oxygen and their low energy consumption, particularly if good thermal integration in high-temperature industrial process is realized. Amongst MIEC materials, perovskite-type materials, such as LSCF, have attracted great interest due to their high oxygen permeation fluxes and excellent stabilities345678910. Thus, in the last few years, MIEC membranes have become the focus of growing interest for implementation in oxyfuel-based power plants111213141516. ...
Abstract One of the most promising materials for oxygen separation amongst ceramic oxygen transport membranes (OTMs) is La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) due to its relatively high oxygen permeability combined with high stability. In this work, asymmetric thin-film LSCF membranes supported over a porous LSCF support were manufactured by inverse sequential tape casting. Moreover, surface activation was accomplished by depositing a porous LSCF activation layer in order to promote oxygen evolution reaction, i.e. O2− to O2 oxidation, in the permeate membrane side. In this case, the porous layer allows the surface area available for oxygen activation to be enlarged. Both the manufacturing of the asymmetric thin-film membranes and the surface activation are described in detail. A thorough study of the oxygen permeation is presented for disk-shaped 30 µm thick LSCF-supported membranes considering the following operating parameters: temperature (1000–600 °C), sweep flow rate (300–750 ml min−1 argon) and oxygen partial pressure in the feed (0.21–1 atm). High permeation fluxes were achieved, e.g., 11.87 ml min−1 cm−2 at 1000 °C and 300 ml min−1 argon sweep when using pure oxygen as feed. A change in the apparent activation energy at about 850 ºC was related to a reversible structural change in the perovskite symmetry (cubic↔rhombohedral), as revealed by XRD measurements. Furthermore, the application of an activation layer allowed the permeation process to be improved, especially at low temperatures, i.e. below 800 °C. Specifically, an improvement of up to about 300% at 600 °C is observed upon application of the activation layer. The activated membrane reached a flux of 13.3 ml min−1 cm−2 at 1000 °C under an O2/Ar gradient. Additional permeation tests using CO2-rich sweep gas demonstrated the good stability and performance of these LSCF asymmetric membranes at 900 and 1000 ºC.
Ba0.95La0.05FeO3 (BLF) was synthesized using a sol–gel method. The catalytic activity of the BLF catalyst towards the co-splitting of H2O/CO2 was investigated in a packed bed reactor and micro-channel reactor for comparison purposes. At the maximum reduction temperature of this study (700 °C), the oxygen vacancies of 3508 µmol/g were achieved. The H2O-TPSR and CO2-TPSR results revealed that H2O splitting took place at temperatures ranging from 400 to 700 °C while the CO2 splitting occurred at temperatures higher than 600 °C. The activation energy of H2O splitting and CO2 splitting was calculated at 23 and 124 kJ/mol, respectively. FTIR results suggested that the active hydroxyl group was responsible for H2O splitting, whereas the oxygen vacancy played a role in CO2 splitting. The micro-channel reactor was found to be advantageous for only the CO2 splitting. The hydroxyl groups were believed to be located on the catalyst’s surface, while the oxygen vacancies were more likely present within the catalyst’s bulk.
The development of stable mixed ionic-electronic conducting (MIEC) perovskite ceramic membranes for highly efficient oxygen separation is still a major challenge. Herein, we propose a new dense ceramic membrane of Zr-Y co-doped perovskite BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY) for stably separating oxygen from air. The BCFZY membrane demonstrated excellent thermal and chemical stability at elevated temperatures, under Ar and 10-30% CO2-containing atmospheres. A high mechanical strength of 110.9 MPa and improved thermomechanical stability can also be achieved for the BCFZY membrane, much better than classical Ba0.5Sr0.5Co0.8Fe0.2O3-δ with high cobalt content. The oxygen permeability of as-prepared BCFZY membranes was systematically investigated. The 0.5 mm-thick membrane demonstrated a very stable and high oxygen permeate flux of about 1.36 mL·min⁻¹·cm⁻² at 900 °C in the run of 100 h with Ar as the sweep gas. Furthermore, the BCFZY membranes exhibited relatively good CO2 resistance showing a flux reduction of only about 25% when 30% CO2 was introduced in the sweep gas. The obtained results demonstrate a good prospect of BCFZY ceramic membrane for highly efficient oxygen separation.
A series of one-side closed Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) hollow fiber membrane decorated with Sm/Sr co-doped ceria (SSDC) with Ce 0.8 (Sm 1-x Sr x ) 0.2 O 2-δ compositions have been studied as a function of strontium content (x = 0.0, 0.3, 0.5, and 0.7) for high temperature vacuum driven oxygen separation from air. The oxygen permeation flux of all SSDC-decorated BSCF hollow fiber membranes is higher than that decorated with Sm-doped ceria (SDC, x = 0.0). The BSCF hollow fiber membrane decorated with SSDC with Sr content x = 0.5 exhibits the highest among the series (˜8 mL/cm ² min, 950 °C), which can be mainly ascribed to the increase of oxygen vacancies for the surface exchange reaction. © 2019 The Korean Society of Industrial and Engineering Chemistry
Perovskite ceramic hollow fiber membranes (HFMs) have attracted great interest due to their potential application as the novel cost-effective method for oxygen production and favorable features for compact module preparation. However, the practical application of these perovskite HFMs is limited by the inherent brittleness of ceramic material and in cases of thick fiber wall, the insufficient oxygen flux values. In this work, highly asymmetric BaCo 0.85 Bi 0.05 Zr 0.1 O 3-δ (BCBZ) perovskite HFMs with a thin dense separation layer supported on the porous BCBZ substrate were fabricated by a modified phase inversion-sintering technique. The perovskite HFMs were integrated in a porous BCBZ perovskite matrix to form a bundle structure to simultaneously achieve high mechanical strength for membrane assembly and high O 2 permeation rate. Pure O 2 was achieved from air separation by vacuuming the permeate side of the hollow fiber bundles (HFBs). Oxygen recovery rate is a function of multi-factors including operation temperature, air feeding flow rate and the membrane area provided by the HFBs. The thermal cycle and post characterization results of the spent HFBs highlight the good stability of the fabricated HFBs. Oxygen permeation rate increased with the HFM number in the HFBs. Under static air atmosphere and the operation mode of vacuum application, the permeation rate of HFBs containing 3 HFMs (HFB_3) or 7 HFMs (HFB_7) at 1000 °C was 79.01 or 98.12 mL min ⁻¹ , respectively. Normalized by the overall membrane area, the achieved oxygen flux for HFB_3 or HFB_7 was 13.27 or 7.06 mL cm ⁻² min ⁻¹ , respectively, the difference of which is attributed to the limited air convection resulting in the insufficient oxygen supply in the feed once it is extracted.
This study investigates the suitability and effectiveness of La0.5Sr0.5Co0.8Fe0.2O3-δ (LSCF5582) membrane for integration into the reduction reactor of a Nitrogen based chemical looping air separation (CLAS) process for exclusive separation of oxygen. First, the structural and chemical characteristics as well as the oxygen separation properties of LSCF5582 membranes, prepared at sintering temperatures of 1050–1350 °C, were examined to obtain the optimum range of sintering temperatures resulting in membranes with enhanced oxygen separation from air. This was achieved by determining the oxygen permeation properties of LSCF5582 membranes under the reducing environment of the Nitrogen based CLAS process, whereby oxygen is liberated from CuO oxygen carriers on the feed side of the membrane using nitrogen as a reducing gas. Membrane characterisation results showed that a single phase dense LSCF5582 membrane was formed at the sintering temperature range of 1200–1225 °C obtaining a maximum oxygen permeation flux of 0.67 ml.min⁻¹.cm⁻² and oxygen recovery of 27% at the sintering and operating temperatures of 1200 °C and 900 °C, respectively. At the same operating temperature, under the reducing environment of nitrogen, the LSCF5582 membrane sintered at 1225 °C was found to perform best achieving an oxygen permeation flux and oxygen recovery of 0.77 ml.min⁻¹.cm⁻² and 76%, respectively.
This book addresses the application of process intensification to sustainable energy production, combining two very topical subject areas. Due to the increasing process of petroleum, sustainable energy production technologies must be developed, for example bioenergy, blue energy, chemical looping combustion, concepts for CO2 capture etc. Process intensification offers significant competitive advantages, because it provides more efficient processes, leading to outstanding cost reduction, increased productivity and more environment-friendly processes.
In this study, a dual-phase oxygen permeable membrane based on a segmented structure of yttria stabilized zirconia (YSZ) and strontium-doped lanthanum manganite (LSM) was developed using a tape casting technique. Oxygen flux was significantly increased in the segmented membrane by a significant reduction in reactions between the YSZ and LSM. The segmented membrane also exhibited stable oxygen flux in the presence of both CO2 and H2O, as well as against thermal stress. This chemically and thermo-mechanically stable YSZ based segmented membrane is proposed for real application in oxygen production.
Robust oxygen ion-conducting membranes based on doped-ceria oxides can be used as oxygen permeation membranes via the short circuit to provide the required electronic conduction. Previous studies used expensive noble metal to be coated on both sides of the ion conducting electrolyte membrane as the electronic conducting phase to fulfil the functions for electron shuttling between the two membrane surfaces required for the oxygen reduction and oxidation. During the membrane operation, the atmosphere surrounding the two membrane sides is different with feed side exposing to air and the permeate side possibly confronting CO2 or reducing gases like CH4 or H2. At high operating temperature, such different gas atmosphere poses different requirements on the material choice to prepare the membranes thus leaving space for further optimisation to reduce the material cost. In this work, a novel Ce0.9Gd0.1O2-δ (GDC) with cost-effective mixed conductive Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) layer to replace the preciously employed noble metal layer on the membrane surface facing the air atmosphere was developed to deliver a highly stable oxygen flux for possible applications in clean energy and chemical synthesis. Further on, we noted that the membranes coated with BSCF improved the oxygen fluxes compared to the membranes coated with noble metal layer. A triple phase boundary (TPB) theory has been put forward to deeply explain the observed improvement on oxygen flux values.
A microchanneled ceramic membrane has been developed for oxygen separation. Numerous parallel microchannels traverse the ceramic membrane with one open end and another end terminating with a thin dense layer. Compared with conventional dense membranes, the new membrane substantially increases oxygen permeation flux by a factor of greater than 5.
In this study, the influences of sealing materials on the oxygen permeation fluxes of three typical dense oxygen permeating membranes, including PrBaCo2O5+δ (PBC), Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF5582) and Ce0.8Sm0.2O1.9 (SDC), with high electronic conductivity, moderate electronic conductivity and dominant oxygen-ion conductivity, respectively, are systematically investigated. Silver and insulating ceramic are used as two different sealants. The effects of external short circuit for electronic conduction by applying silver sealant and potential contamination effect of ceramic sealant on membranes were taken into major consideration. PBC membranes with either silver or ceramic as sealant showed comparable oxygen permeation flues during the test. Similar conclusions were also obtained based on BSCF5582 membranes. As for SDC membrane, significant promotion of the permeation flux by applying silver sealant was clearly demonstrated. Explanations for these performance changes are proposed. The findings from this study may provide valuable insight to explain the discrepancy of oxygen permeation fluxes of some membranes as reported by different research groups in the literature.
Innovative asymmetric oxygen-transport membrane architectures were prepared by combining freeze-casting and film deposition techniques. Freeze-casting enabled the optimization of the gas transport through the support by creating a hierarchical porosity while a dense top-layer of 30 [small mu ]m was coated over this support by screen printing. The versatility of this technique was demonstrated by preparing highly porous bodies made of fast ionic conductors, e.g. perovskites and doped ceria fluorites, with a large number of applications in catalysis, electrochemistry and gas separation. Permeation tests using an all-La0.6Sr0.4Co0.2Fe0.8O3-[small delta] asymmetric membrane proved the beneficial effect of such porous supports over the O2 fluxes with a maximum value of 6.8 mL min-1 cm-2 at 1000 [degree]C, markedly above the results achieved so far with conventional preparation techniques. Gas permeance study through the porous freeze-cast support showed that the particular pore structure allows the gaseous transport resistance to be minimized. The related pressure drop is found to be very low in comparison with conventional porous supports, e.g. tape-cast supports, with for example only 0.59 bar mm-1 with argon at 800 [degree]C for an inlet flow of 400 mL min-1 cm-2. Finally, the stability of the asymmetric membrane has been evaluated under CO2 atmosphere during 48 hours and at 900 [degree]C. The membrane is found to be stable without deactivation nor decrease in the O2 permeation flux.
The objective of this work was to understand the origin of the stresses and the nature of the grinding mechanisms in a high-shear ball mill. The ball mill used was a Dynomill and the grinding media consisted of zirconium oxide beads. The ground powder was a poorly water soluble product. The particle size distribution was analyzed with laser diffraction.A population balance model enabled to calculate the breakage function from the experimental data. The breaking parameters were used to determine the grinding mechanism at different grinding conditions. The grinding mechanism depends on the orientation and intensity of the forces applied on the particles. The grinding mechanisms were determined from the grinding experiments and correlated to the movement of the grinding media in the grinding chamber and the applied mechanical stresses.The observed grinding mechanisms are cleavage and some fracture for coarse particles (15μm), cleavage and abrasion for intermediate particles (0.8μm) and cleavage for fine particles (smaller than 0.15μm).The grinding mechanisms are related to the movement of the grinding media in the grinding chamber. Cleavage and abrasion of particles are the result of compression forces and shear in the centrifuged packed bed of grinding beads. The grinding mechanism does not change when changing the operating conditions and the packed bed of media is similar in the studied ranges of operating conditions.
Generating oxygen for industrial use is big business. Now a new cog has been added to the production process in the shape of a ceramic membrane.
Gasoline reformulation will change global markets for methyl tert -butyl ether (MTBE), methanol, ethanol, and other oxygenates virtually "overnight," according to a new study done by chemical industry consultants Probe Economics Inc., Millwood, N.Y. Consequently, it also may affect prices for a host of other petrochemicals. "The combination of clean air legislation and raw material supply limitations has already begun to alter oxygenate markets, and could entirely transform them during the 1990s," says Fred Peterson, Probe's president. He predicts world oxygenate demand could increase more than 10-fold from its level today by the year 2001. Petrochemical producers and energy companies see "oxygenates as the hottest game in town and all of them want a piece of the action." Although producers enthusiastically anticipate gains, Peterson cautions that they should acknowledge the risks of getting involved in a market in which the ultimate global supply and demand balance is still uncertain. The U.S. Clean Air Act ...
La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) mixed conducting hollow fibre membranes were prepared via the combined phase inversion and sintering technique. A porous layer of silver or LSCF perovskite was coated on the outside surface of the hollow fibres to improve the surface exchange kinetics. Oxygen permeation fluxes through both the modified and the unmodified LSCF hollow fibre membranes were measured under air/He gradients at different temperatures. The oxygen permeation process through the unmodified LSCF hollow fibre membranes is mainly controlled by the oxygen dissociation on the membrane surface exposed to feed gas of air. After surface modifications, the oxygen permeation fluxes can be improved from the original values of 0.01–0.81 mL standard temperature and pressure (STP) cm−2 min−1 in the unmodified hollow fibre membrane to 0.08–1.85 mL cm−2 min−1 in the Ag-coated membrane and to 0.05–1.48 mL cm−2 min−1 in the porous LSCF-modified membrane, respectively, in the temperature range of 700–1000 °C. The maximum enhancements for the Ag-coated and the porous LSCF-modified membranes are achieved at 800 °C to be 17.8 and 9.3 times the original flux, respectively. Due to the surface modifications, the operating temperature to achieve anticipated oxygen permeation fluxes of commercial interest can be reduced noticeably.
The average linear thermal expansion coefficient (TEC) of perovskite-type Sr0.7Ce0.3MnO3−δ, calculated from dilatometric data at 300–1100 K in air, is (11.9±0.2)×10−6 K−1, compatible with that of stabilized zirconia. The high electronic conductivity, 230–270 S/cm at 773–1273 K, and the moderate thermal expansion suggest possible application of Ce-doped strontium manganite as solid oxide fuel cell (SOFC) cathode material. The oxygen permeation fluxes through dense Sr0.7Ce0.3MnO3−δ membranes at 1273–1173 K were found to linearly increase with the oxygen chemical potential gradient in the range of permeate-side oxygen partial pressures from 21 kPa down to 2×10−9 Pa, indicating a relatively high stability of this phase. However, the oxygen permeability of Sr0.7Ce0.3MnO3−δ, limited by both bulk ionic conductivity and surface exchange at the membrane/gas boundaries, is higher than that of La0.7Sr0.3MnO3−δ, but still considerably lower if compared to La(Sr)Fe(Co)O3−δ perovskites.
Oxygen transport and oxygen surface reaction measurements have been obtained for La0.6Sr0.4Co0.2Fe0.8O3−δ using the conductivity relaxation technique. These measurements are compared with tracer diffusion coefficients and surface exchange coefficients obtained for the same material. The change in oxygen stoichiometry of this material with temperature and oxygen partial pressure has been demonstrated. The values derived from these experiments have been used to calculate the theoretical oxygen fluxes and close agreement has been found with actual oxygen fluxes obtained from permeation experiments through a dense La0.6Sr0.4Co0.2Fe0.8O3−δ sample using a modified Wagner model.
Electrical conductivity relaxation (ECR) experiments were carried out on La0.6Sr0.4Fe0.8Co0.2O3−δ (LSFCO) in the temperature range of 840 to 910 °C and in the oxygen partial pressure range of 0.01 to 0.05 atm. The effects of annealing at 900 °C on the oxygen transport properties were investigated for individual samples. The values of DO2−=D̃O/ΓO and kex=kchem/ΓO agree well with the result from isotope-exchange depth profile (IEDP) experiments.
The effect of the bulk microstructure (grain size distribution, grain boundary composition) on the oxygen transport properties of La0.5Sr0.5FeO3 membranes was investigated. For this purpose, samples with different microstructures were prepared by modifying the sintering duration and/or temperature. The average grain sizes, ranging from 0.20 to 1.43μm, were determined from SEM analysis. The oxygen transport properties of these samples were characterised by permeation measurement. The fluxes presented a change in the activation energy which was attributed to a change in the rate limiting step, from bulk diffusion at lower temperature (900°C). Only the transport through the bulk was influenced by the microstructure, with the highest flux for the smallest grains. This would imply that oxygen transport occurs more rapidly along the grain boundaries than through the bulk. Grain and grain boundary compositions were analysed by TEM.
Electrical conductivity relaxation (ECR) experiments were carried out on La0.6Sr0.4Fe0.8Co0.2O3−δ (LSFCO) in the temperature range of 840 to 910 °C and in the oxygen partial pressure range of 0.01 to 0.05 atm. The effects of annealing at 900 °C on the oxygen transport properties were investigated for individual samples. The values of DO2−=D̃O/ΓO and kex=kchem/ΓO agree well with the result from isotope-exchange depth profile (IEDP) experiments.
The effect of microstructure on oxygen permeation in SrCo0.8Fe0.2O3-d membranes was investigated using disc samples fabricated under different processing conditions of applied pressure and sintering temperature. The average grain size of the samples was found to remain unchanged as a function of applied pressure, but increased considerably when the sintering temperature was increased from 950 to 1200°C. This change in grain size has a strong effect on the oxygen permeation flux, which increased considerably as the grain size was decreased. The density as well as the microhardness of these samples were also measured and found to change slightly as the processing conditions were changed.
La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) powders were synthesized respectively by an EDTA (ethylenediaminetetraacetic acid)–Citrate sol–gel process and a low-temperature auto-combustion process. The samples were characterized by XRD, SEM, BET, TGA and instant temperature analysis. The iodometric titration was used to determine the average valence of Co and Fe ions and the oxygen nonstoichiometry of the prepare powders. The catalytic properties of the synthesized powders were investigated by the hydrogen peroxide catalytic decomposition. Pure-perovskite structure was formed by both synthesis methods. The oxygen nonstoichiometry of the samples prepared by the auto-combustion process is larger than that by the sol–gel process. The catalytic activities of the powders from two synthesis processes also differed largely due to the different oxygen nonstoichiometry, surface area and crystalline sizes.
Ce0.8Sm0.2O1.9 powder for tape casting has been synthesized by oxalate coprecipitation. The rod-like particles, 1.0μm sized, were broken to 0.62μm or 0.37μm equiaxial particles by 24 or 48h wet ball milling. The initial sintering rate of tapes is linear with the reciprocal of particle size. The milled 0.37μm powder densifies to 98% of the theoretical density after sintering at 1400°C, while the original rod-like powder only densifies to 76%. The different sinterability is due to different particle size and pore texture. Forty-eight hours wet ball milling is needed to increase the sinterability of oxalate-derived doped ceria powder for tape casting.
Ceramic materials with a perovskite related structures such as non-doped and doped barium titanate ceramics are attracting much interest for their application as capacitor dielectrics, resistors, thermal sensors, etc. Since mechanical activation can be used in order to modify properties of these materials, in this study microstructure evolution and electric properties of mechanically activated BaTiO3 have been analyzed. The sintering process of high purity non-doped mechanically activated BaTiO3 was monitored using a sensitive dilatometer with a heating rate of 10°C/min. Investigation of the microstructure evolution of mechanically activated BaTiO3 was performed using scanning electron microscope (SEM) and digital pattern recognition (DPR) methods. A dielectric study of the paraelectric–ferroelectric phase transition in the barium titanate ceramics was performed by recording the temperature dependence of dielectric permittivity.
Ceramic samples of CaTi0.8Fe0.2O3−δ were obtained from mechanically activated mixtures of TiO2, Fe2O3 and CaCO3 sintered at 1150°C for 2h. The ceramics are dense with submicrometric grains with sizes in the order of 100–200nm and 400–600nm. X-ray diffraction analysis revealed a single perovskite phase structure indicating a high level of homogeneity, which was confirmed by transmission electron microscopy. The oxygen permeability through these samples, measured in the range 700–950°C, is about 50% lower than for ceramics with considerably larger grains, close to 10μm. The permeability increases linearly with the increase of the grain size in the range from ≈200nm to ≈10μm. Observations by results suggest that the grain boundaries have a negative impact on the high temperature ionic transport properties of CaTi0.8Fe0.2O3−δ ceramics.
A partially reacted mixture comprising lead zirconate titanate (PZT) phase and the starting oxides was successfully synthesized via a high-energy ball milling process by controlling the milling time within the second milling stage. The partially reacted system exhibited a sintering reaction when sintered at elevated temperature. Fully dense PZT ceramics with 99.2% of the theoretic density were achieved from the partially reacted mixture sintered at 1100 °C. The good dielectric and ferroelectric properties of the sintered PZT ceramic indicate that high-energy milling process is a promising and flexible technique for the preparation of PZT ceramics.
The rate of oxygen permeation through La1−xSrxCo1−yFeyO3−δ was found to increase with an increase in Sr or Co content, showing that the permeability was mainly controlled by the amount of oxygen vacancies. The results obtained indicate that mixed conductive perovskite-type oxides are promising materials for oxygen permeation at elevated temperatures.
SrCo1−yNbyO3−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. SrCo0.9Nb0.1O3−δ exhibited the highest electrical conductivity and oxygen permeability among the others. It reached a permeation flux of ∼2.80×10−6molcm−2s−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.
The effect of grain size distribution in perovskite-type (Ba0.5Sr0.5)(Fe0.8Zn0.2)O3−δ (BSFZ) ceramics on their oxygen permeation behaviour has been investigated by variation of calcination temperature in powder production and sintering time for the ceramics. The membranes were examined via scanning electron microscopy (SEM), transmission electron microscopy (TEM) and oxygen permeation experiments. We found that the dwell time during sintering has an important influence on the microstructure of the ceramic. The longer the dwell time, the further proceeds the grain coarsening, which affects the oxygen permeation in a positive way and leads to an enhanced permeation. Supplementary, decreasing calcination temperature in perovskite powder synthesis delivers fine powders with grain sizes less than one micrometer and thus smaller grains in the ceramic. Unfortunately, the grain size distribution in sintered membranes is not constant through membrane cross-sections since grains in the bulk are smaller compared to those at the surface which is not favorable for the oxygen permeation of the ceramics. The activation energy was determined to be in the range of 51–53kJ/mol and its variation does not exhibit a dependence of grain size changes. High-resolution transmission electron microscopy proved that grain boundaries are atomically thin without any interfacial phases. We come to the conclusion that the transport rate of the oxygen permeation is limited predominantly by bulk diffusion and due to the fact that grain boundaries in BSFZ act as barriers for bulk diffusion, this material is a high mobility material.
This paper describes a series of experiments conducted with natural gas in air and in mixtures of oxygen and recycled flue gas, termed O2/CO2 recycle combustion. The objective is to enrich the flue gas with CO2 to facilitate its capture and sequestration. Detailed measurements of gas composition, flame temperature and heat flux profiles were taken inside CANMET's 0.3MWth down-fired vertical combustor fitted with a proprietary pilot scale burner. Flue gas composition was continuously monitored. The effects of burner operation, including swirling of secondary stream and air staging, on flame characteristics and NOx emissions were also studied. The results of this work indicate that oxy–gas combustion techniques based on O2/CO2 combustion with flue gas recycle offer excellent potential for retrofit to conventional boilers for CO2 emission abatement. Other benefits of the technology include considerable reduction and even elimination of NOx emissions, improved plant efficiency due to lower gas volume and better operational flexibility.
An energy balance analysis of the effect of oxygen enrichment of combustion air in a glass melting tank (with a flue gas heat recovery system) has been carried out which takes into account the decrease in the amount of heat carried away by flue gases due to a reduction in the amount of nitrogen and in the amount of combustion products because of reduced fuel input. According to this formulation, the effects of enrichment depend only on the level of enrichment, the efficiency of the heat recovery system and on a dimensionless parameter, θ, the ratio of the useful calorific value of the fuel to the heat carried away by nitrogen when using stoichiometric amounts of normal air combustion. The value of θ does not depend significantly on the composition of the fuel. The results of the analysis are in reasonable agreement with the available experimental data.
Current theories of grain growth presume that grain boundary migration is the rate-limiting step, and either explicitly or implicitly assume that triple junctions can always move with sufficient speed to accommodate the changing positions of the grain boundaries. Following from some recent observations of triple-junction drag effects in tricrystals of zinc and in molecular dynamics models, an analytical theory is developed to explore the effects of triple-junction drag upon grain growth, for a two-dimensional solid. The theory is developed in the framework of the Von Neumann–Mullins formulation, and demonstrates that drag effects operating exclusively at the triple junctions result in a retardation of grain growth. The stability of six-sided grains in the isotropic, drag-free case of the Von Neumann–Mullins analysis is successively extended to grains of 6±N sides, where N increases with the strength of the triple-junction drag.
The high temperature phase structures of the perovskite-type oxide Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) were characterized by in situ high-temperature X-ray diffraction, which revealed that BSCF exhibits a good phase reversibility and structure stability in air from room temperature to 1273K. The XRD patterns of BSCF oxide at 1173K in different atmospheres (air, 2% O2 in Ar and pure Ar) indicated that BSCF possesses an excellent phase stability at high temperatures not only in air but also in pure Ar. From the plot of the lattice constant against the temperature, the thermal expansion coefficient of BSCF was determined to be 11.5×10−6K−1, which is smaller than that of SrCo0.8Fe0.2O3−δ (SCF) (17.9×10−6K−1). Microstructures of the membranes sintered under different conditions were characterized by scanning electron microscopy (SEM). The effect of microstructure on the oxygen permeation flux through BSCF was observed by measuring the oxygen permeation flux using samples sintered under different conditions. The oxygen permeation flux increased considerably with the increase of the grain size of the membrane.
Electrical conductivity relaxation (ECR) was utilized to determine the oxygen surface exchange coefficient, kchem, and bulk diffusion coefficient, Dchem, for La0.6Sr0.4Co0.2Fe0.8O3 − δ (LSCF 6428) solid oxide fuel cell (SOFC) cathodes. Measurements were performed at 1073 K in the 100% to 3.3% pO2 range. Both kchem and Dchem were nearly one order of magnitude larger than previously reported values. This discrepancy is attributed to the low reliability, yet commonly utilized, two-parameter fit procedure that seeks to simultaneously determine kchem and Dchem from a single measurement. The value of kchem is shown to vary by almost one order of magnitude depending on the number of terms chosen in the data fit, while the fit quality, SR, remained constant throughout. Polished LSCF 6428 samples were also individually surface-doped with La, Sr, Co, and Fe, as well as with LSCF 6428 nanoparticles (25–50 nm) to investigate trends in kchem and Dchem. Fe surface doping caused a decrease in kchem, while La, Sr, Co and nanoparticle doping did not affect the oxygen surface exchange or bulk diffusion coefficients.
Doped LaGaO3 exhibits high oxide ionic conductivity. Doping of Sr far the La site and Mg for the Ga site is the most effective method for enhancing the oxide ionic conductivity of LaGaO3. The oxide ionic conductivity of La0.9Sr0.1Ga0.8Mg0.2O3 was higher than that of Sc-doped ZrO2 and slightly lower than that of Bi2O3 oxide. Furthermore, electronic or hole conduction was negligibly small in the oxygen partial pressure region from 1 to 10(-20) atm.
Alumina compacts fabricated with different green densities and different pore size distributions were characterized and the changes of the pore characteristics during solid-state sintering were studied. A critical ratio of pore size to mean particle size for pore shrinkage was determined. Porosity in the compact could be classified into two classes: the first class contains pores smaller than the critical ratio, and the second class contains pores larger than the critical ratio. Pores belonging to a different class of porosity behaved differently during sintering. Pores larger than the critical ratio were not totally eliminated during sintering. The first class of porosity controlled the ultimate sintering shrinkage, and the second class of porosity controlled the final sintered density.
The grain growth and densification rates of Mn-Zn ferrites during sintering are closely linked to the characteristics of the calcined and milled powders used. Long milling times enlarge powder particle size distributions and tend to promote discontinuous grain growth during sintering. For fixed sintering conditions, an optimum milling time, which corresponds to minimal eddy current and hysteresis losses, exists. The electrical properties of overmilled powders deteriorate greatly because duplex structure occurs. Theoretical analysis of the probability of discontinuous grain growth occurring during sintering in relation to powder particle size distribution agrees with the experimental results.
La0.1Sr0.9Co0.9Fe0.1O3−δ (LSCF1991) dense disks with different thicknesses and surface areas were prepared to investigate the contribution of surface reactions and bulk diffusion in oxygen permeation phenomena. The bulk diffusion controlled situation became significant with increasing the membrane thickness, and the surface reaction controlled situation prevailed at smaller surface area. The increase in surface area at the low P(O2) (anode) side was more effective to increase the oxygen permeation flux than that at the high P(O2) (cathode) side. The coating of a porous catalyst layer below the optimum thickness was also effective in enhancing the oxygen permeability due to the increase in surface area, but the coating with too thick layer deteriorated the permeability probably due to the increase in the gas diffusion resistance.
The successful application of nanomaterials, with their unique mechanical, optical, magnetic and electrical properties, largely depends on the consolidation of powders into engineering components that will preserve an initial metastable microstructure. The key characteristic of the nanopowder consolidation process is to achieve densification without microstructural coarsening. This paper addresses specific densification issues related to the nanocrystalline nature of the consolidating particulates. The theoretical issues that affect the thermodynamics and kinetics of the atomic processes involved in nanopowder densification are related to higher driving force, enhanced interfacial energy and diffusion, and full density values. The experimental aspects cover powder compressibility and differential shrinkage, heating rate and pressure effects, and grain growth. Meaningful results in terms of final density values and grain growth are presented with the emphasis on processes that have demonstrated the capability to form dense specimens with retention of fine grain sizes.
Point defects are of paramount importance for electroceramics. They are key structure elements as regards materials functionality; but, in addition, they are also decisive for chemical kinetics, hence for preparation, conditioning, annealing and degradation phenomena. Concentrations and mobilities of these charge carriers are significantly changed at or near interfaces (or more generally higher dimensional defects) giving rise to depletion, accumulation, and inversion layers with respect to ionic and electronic carriers and hence to distinct electrical and chemical effects. It is discussed how these effects can be explained and how such knowledge can be used to design electroceramics purposefully. Examples refer to ionically or mixed conducting oxides and halides. Finally, in nano-structured materials the spacing of interfaces becomes relevant in that local properties can be severely affected. Such size effects do not only lead to confinement effects in the case of electronic carriers but also to anomalies with respect to ion conduction and mass transport. The potential of the nano-regime for electrical and chemical properties of electroceramics is discussed in the framework of a “soft materials science”.
Perovskite-type membranes of (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ (BSCF) and (Ba0.5Sr0.5)(Fe0.8Zn0.2)O3−δ (BSFZ) were successfully prepared via liquid-phase sintering using BN as sintering aid. The obtained membranes were examined via powder X-ray diffraction pattern (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and oxygen permeation experiments. It has emerged that the use of BN as sintering aid lowers sintering temperatures in order to obtain dense membranes with relative densities in the range of 93–96% as proven by the Archimedes method. It was further shown that the perovskite structure could be maintained after sintering with BN. Additionally, BN was completely removed from the sintered membranes. Investigation of the microstructure revealed that the average grain size of the membranes was influenced by the amount of BN added prior the sintering process. It was found that large amounts of BN effectively lower the average grain size. Oxygen permeation experiments have shown that the lower the average grain size the lower the oxygen permeation performance, particularly in the case of BSCF. Transmission electron microscopy revealed that no evidence for an amorphous layer or any other interfacial phase in the grain boundary is present.
Recent experimental data clearly show that microstructure has a significant influence on the minor contributions to the total conductivity, following similar trends observed for the major conductivity components, in oxide semiconductors and solid-electrolyte ceramics. As for microcrystalline solid-electrolyte materials, increasing grain size in ceramics with predominant electronic transport often leads to a higher ionic conductivity. Interaction of the components in oxide composite materials may play a critical role—decreasing oxygen ionic conduction. Both ionic transport in materials with dominant electronic conductivity and electronic conduction in solid electrolytes should be analyzed not only as properties of an oxide phase, determined by the overall composition, oxygen partial pressure and charge carrier mobility, but also as functions of the ceramic microstructure—the latter becoming an increasingly important tool in the design of materials performance.
Sintering trajectories were simulated for different initial grain sizes assuming grain-boundary diffusion as densification mechanism and either pore drag or intrinsic grain-boundary mobility as grain growth mechanisms. Computation of comparable sintering trajectories for initial grain sizes between 10 nm and 250 nm necessitated an adjustment of the isothermal sintering temperature. Conditions where one of the two coarsening mechanisms prevails were determined and shown to depend on density, grain size, and the ratio S of the rate constants related to the two coarsening mechanisms, but little on the dihedral angle. All computed sintering trajectories have a common shape, being very flat at low densities and exhibiting most grain growth in the last 5% of densification before reaching their end-point density. Depending on the difference between the activation energies related to the dominant densification and coarsening mechanisms, a reduction in grain size might be beneficial for densification.
Ionic-electronic mixed-conducting perovskite-type oxide La0.6Sr0.4Co0.8Fe0.2O3 was applied as a dense membrane for oxygen supply in a reactor for methane coupling. The oxygen permeation properties were studied in the pO2-range of 10−3−1 bar at 1073–1273 K, using helium as a sweeping gas at the permeate side of the membrane. The oxygen semi-permeability has a value close to 1 mmol m−2 s−1 at 1173 K with a corresponding activation energy of 130–140 kJ/mol. The oxygen flux is limited by a surface process at the permeate side of the membrane. It was found that the oxygen flux is only slightly enhanced if methane is admixed with helium. Methane is converted to ethane and ethene with selectivities up to 70%, albeit that conversions are low, typically 1–3% at 1073–1173 K. When oxygen was admixed with methane rather than supplied through the membrane, selectivities obtained were found to be in the range 30–35%. Segregation of strontium was found at both sides of the membrane, being seriously affected by the presence of an oxygen pressure gradient across it. The importance of a surface limited oxygen flux for application of perovskite membranes for methane coupling is emphasized.
Oxygen permeability of Ln1-xMxCoO3-δ (Ln = La, Pr, Nd; M = Sr, Ca, Bi, Pb; x = 0–0.9) and SrCo1-xMexO3-δ (Me = Cr, Mn, Fe, Ni, Cu; x = 0–0.5) perovskite-like oxide ceramics, which are promising materials for high-temperature electrochemical oxygen membranes where matter is transferred owing to conjugate transport of oxide ions O2− and electrons through a gas-tight ceramic material, has been investigated. Dependencies of the density of the molecular oxygen flow passing throuth the membrane on the chemical potential gradient of O2 in the gas phase and temperature have been analyzed. Physicochemical models of such dependencies are proposed. It is shown that complex oxides SrCo1-xFexO3-δ (x = 0.2–0.35) and La1-xSrxCoO3-δ (x = 0.65–0.75) having the highest oxide ionic conductivity can be used as materials for electrochemical oxygen membranes.
In this work, the effects of sintering temperature on the phase structure, oxygen nonstoichiometry, microstructure, electrical conductivity, and oxygen permeation behavior of perovskite La0.6Sr0.4Co0.2Fe0.8O3−δ membranes were systematically studied. The sintering temperature has negligible effect on the bulk properties of the phase structure and the oxygen nonstoichiometry, but has significant influence on the microstructure and electrical conductivity of the membranes, and therefore imposes large impact on its oxygen permeability. With an increase in the sintering temperature, the grain size increased steadily from 0.3 μm at 1000 °C to 3.5 μm at 1300 °C, accompanied with the substantial enhancement of electrical conductivity but the decrease of activation energy for the electrical conduction. Results indicate that the grain boundary had a much lower electrical conductivity than that of the bulk. The oxygen permeation process was mainly rate-determined by the slow oxygen diffusion over the grain boundary. Higher sintering temperature lowers the activation energy for oxygen permeation and accordingly gives rise to higher oxygen permeation flux. The oxygen permeation flux could be improved more than 10 times if the sintering temperature increased from 1000 to 1300 °C.
Antimony doped tin oxide (ATO) nanoparticles were prepared by coprecipitation reaction in methanol solution of tin(IV) chloride pentahydrate and antimony(III) chloride. The obtained ATO nanoparticles did not form any secondary particles and showed loosely agglomerated structure consisting of primary particles weakly attached with each other by van der Waals and capillary adhesive forces. In addition, ATO nanoparticles showed lower degree of agglomeration compared with those of commercial product. The reason for the formation of weakly agglomerated structure was discussed.
Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) exhibits high oxygen permeability, which is why it is being discussed for gas separation (oxygen transport membrane, OTM) in zero-emission power plants using oxyfuel technology when the membrane is operated in a clean environment, i.e. no flue gas contact. We investigate the influence of membrane processing on microstructure and oxygen permeation. Pure-phase BSCF powder is synthesized using a modified Pechini method. For comparison, commercially available powder is also used, synthesized by a solid-state reaction. Disk-shaped membranes of various microstructures, i.e. closed porosities and grain sizes, are prepared by uniaxial pressing and sintering of the powders processed in different ways. The powders and membranes are characterized by methods including BET, SEM, XRD, and DSC. The microstructures obtained by different sintering conditions are investigated by SEM and TEM. Sintering at 1150 °C leads to incongruent melting of BSCF indicated by DSC. The liquid phase appears at three-phase boundaries grain–grain–air and consists of nearly pure cobalt oxide with small impurities of barium and strontium detected by TEM/EDX analysis. Oxygen permeation of the membranes is measured in an air/Ar gradient depending on temperature and membrane microstructure. The closed porosity of different processed membranes is varied between 2 and 15% with uniform grain sizes in the range of approx. 10 μm. The average grain size is increased from 10 to 45 μm by increasing the sintering temperature. Neither porosity nor the grain size significantly influences the oxygen permeation rate of 1-mm-thick disks in the investigated parameter range.
In order to reduce overall fuel consumption, or to partially substitute a “valuable” fuel with a poor one, in industrial heating, oxygen enrichment of combustion air can be very effective. For the second option, a general criterion is stated in this paper for examining the suitability of oxygen enrichment in single cases. The topic is particularly interesting, as for the first time, it is now feasible to produce oxygen enriched air using permeable membranes on a commercial scale and with costs that are remarkably lower than those of other existing techniques. In this paper, the subject is investigated after some remarks about the definition of the “usable exergy” parameter, which was already proposed in previous papers by one of the authors and is here utilized for the above criterion.
Ammonia nitrate was applied as an oxidizer and combustion trigger to modify the normal combined EDTA-citrate complexing method into a process with autocombustion and low ignition temperature properties. Therefore, the synthesis procedure was greatly simplified. The effect of NH4NO3/metal ions to organic mole ratios and the heating temperature on the autocombustion behavior and the properties of the powders derived were investigated in detail. The critical amount of NH4NO3 for the autocombustion to occur was identified at the NO3− to citric acid to EDTA mole ratio of around 10:2:1. After the experimental optimization, well-crystallized nanostructured La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) powder with a specific surface area as high as 21 m2/g was obtained; it is comparable with that obtained from a normal complexing process. By adjusting the combustion parameters, the properties of the powders then derived can be tailored for different applications, such as nanograined dense membrane for oxygen separation membrane, and porous cathode for fuel cells and sensors.
The effect of the bulk microstructure (grain size distribution, grain boundary composition on the oxygen transport properties of La<sub>0.5</sub>Sr<sub>0.5</sub>FeO<sub>3</sub>membranes was investigated. For this purpose, samples with different microstructures were prepared by modifying the sintering duration and/or temperature. The average grain sizes, ranging from 0.20 to 1.43μ m, were determined from SEM analysis. The oxygen transport properties of the samples were characterised by permeation measurement. The fluxes presented a change in the activation energy which was attributed to a change in the rate limiting step, from bulk diffusion at lower temperature (<850°C) to surface limitations at higher temperature (>900°). Only the transport through the bulk was influenced by the microstructure, with the highest flux for the smallest grains. This would imply that oxygen transport occurs more rapidly along the grain boundaries that through the bulk. Grain and grain boundary compositions were analysed by TEM.
Oxygen diffusion and surface exchange in the mixed conducting perovskite La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ
- S J Benson
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Oxygen permeability of perovskite-type Sr 0.7 Ce 0.3 MnO 3−δ
- Vv Kharton
- Ap Viskup
- Ip Marozau
- En Naumovich
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of perovskite-type Sr 0.7 Ce 0.3 MnO 3−δ. Mater Lett 2003;57:3017–21.