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Co-Casting and Co-Sintering of Porous MgO Support Plates with Thin Dense Perovskite Layers of LaSrFeCoO3
Abstract
A tape casting co-sintering route is described in which thin dense layers of LaSrFeCoO3 (LSFC) have been formed on planar, porous MgO substrates 100- 200 micron thick. SEM analysis of the sintered structure showed that it was possible to eliminate most of the residual porosity in the LSFC layer, but maintain a porosity between 25 and 45% in the MgO support layer. The LSFC layer dit not reveal many cracks. The overall shrinkage of the co-sintered structure was about 25%. The LSFC layer topography was smooth and uniform with a metallic-like lustre. A good correlation was obtained between the observed microstructure and the gas permeability measurements made at room temperature.
... De nombreuses études ont mis en évidence la possibilité d'élaborer des membranes supportées [59,60] Les matériaux pérovskites présentent un comportement dilatométrique particulier en ...
... . Cependant, les premiers résultats nécessitaient le frittage du support, puis le dépôt de la membrane dense et son frittage et enfin l'ajout d'une couche catalytique si nécessaire[59][60][61][62]. Le nombre de traitements thermiques important, augmente le coût d'élaboration des membranes supportées. ...
... Les études s'orientent actuellement vers l'élaboration de membranes supportées par co-frittage. Les procédés d'élaboration de telles membranes décrits dans la littérature sont le co-pressage, le dépôt de suspension ou encore le coulage en bande[59][60][61][62][63]. Au cours de cette étude, les membranes supportées sont élaborées par coulage en bande, thermocompression et co-frittage des couches dense et poreuse.Ce chapitre traite donc de l'élaboration par co-frittage de membranes supportées avec LSFG 8273 comme matériau de membrane dense.Le matériau pour le support poreux doit répondre à certains critères. ...
For few years, conversion of natural gas into synthesis gas (H2+CO) is of great interest for hydrogen or fuel production via GTL process. Catalytic Membrane Reactor (CMR) constitute an economically interesting alternative for syngas production.
The design of the reactors consists of a catalyst layer, a mixed conductor membrane with La1-xSrxFe1-yGayO3-d material and an active porous support. La0,8Sr0,2FeO3-d was chosen as material for porous support in order to make co-sintering of dense and porous layers possible. By this way, chemical continuity is ensured and costs are decreased. Supported membranes LSFG8273/LSF821 and LSFN8273/LSFG8273/LSF821 were processed by tape-casting, lamination and co-firing. Reactor performances can be widely increased using the porous support and the catalyst layer. Finally, thermomechanical properties of materials were evaluated
... However, a dilemma was usually observed for the membranes based on single-phase conducting oxide, i.e., the higher the oxygen permeability the lower the chemical stability [13,14]. For the membranes with high chemical/structural stability such as iron-based perovskite, in order to obtain the oxygen permeation fluxes with practical attractiveness, surface modification, thin-film and dual-phase configurations are proposed [15][16][17][18][19][20][21][22][23][24][25][26][27][28]. The surface modification with a porous layer of oxide possessing high oxygen surface exchange kinetics was extensively studied recently [15][16][17][18]. ...
... Although significant improvement in permeation flux was sometimes observed, the absolute flux was still too low for practical application purpose due to the low oxygen bulk diffusion rate and the thick membrane configuration [18]. On the other hand, porous substrate supported thin-film oxygen separation membranes have been investigated by several researchers [19][20][21][22][23]. In order to avoid the solid-state reaction between the support and the thin-film layer, the application of the same material both for the porous substrate and the thin-film layer was proposed [23]. ...
A configuration of dense mixed ionic and electronic conducting (MIEC) membrane with layered morphological structure for oxygen separation, which combines the benefits of high oxygen permeation flux of cobalt-based membrane, high chemical stability of iron-based perovskite and high mechanical strength of thick membrane, was studied. The membrane is normally composed of two layers; each layer is a dense MIEC oxide. The substrate layer is a thick dense membrane with high oxygen permeability but relatively lower chemical stability. The feasibility of dense thick Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF5582) membrane as the substrate layer and Ba0.5Sr0.5Co0.2Fe0.8O3-delta (BSCF5528) as the thin-film layer was mainly experimentally investigated. Both the BSCF5582 and the BSCF5528 show the same cubic perovskite structure and the similar lattice constant with no detrimental reaction products formed. By optimizing fabrication parameters of a simple dry pressing process, dual-layered membrane, free of cracks, was successfully fabricated. The oxygen permeation flux of a dual-layered membrane with the thin-film BSCF5528 layer facing to the sweep gas reached 2.1 mL cm(-2) min(-1) [STP] (1.56 x 10(-6) mol cm(-2) s(-1)) at 900 degrees C, which is about 3.5 times higher than that of the BSCF5528 membrane (0.6 mL cm(-2) min(-1), [STP] (4.46 x 10(-7) mol cm(-2) s(-1)) under the same conditions. (C) 2007 Elsevier B.V. All rights reserved.
... Although using organic solvents in tape casting is a common practice in industry due to their lower latent heat and less energy-intensive drying, 10 water-based tape casting can not only significantly reduce the risks to the environment and human health but also lower the costs associated with the installation and disposal of organic waste. 11 First, Middleton et al. 12 casted very thin tapes of LSCF (5-10 µm) using water as the solvent in 2004. Fernandez-Gonzalez et al. 13 fabricated LSCF6428 tapes of 35% solid loading by waterbased tape casting with the focus on slurry stability and microstructural evolution at different sintering temperatures. ...
Tape casting is a flexible technique for manufacturing scalable ceramic sheets. This study fabricated La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF6428) tapes using water‐based tape casting. A high molecular weight plasticizer, polyethylene glycol 4000, was chosen to balance flexibility and mechanical strength. Adjusting the plasticizer‐to‐binder ratio (R‐value) and increasing relative humidity during drying led to crack‐free and flawless green tapes of 330 µm. The uniform polymer matrix improved homogeneity and consistency as well. An applicable suspension formulation was developed for the water‐based fabrication of LSCF tapes for continuous production.
... On the one hand, disk-shaped or planar configurations are the most studied in literature, presenting advantages related to manufacturing simplicity rapid evaluation of permeation values. For this design, the porous support is generally developed by uniaxial pressing and subsequent sintering of the powder, while the dense top layer is deposited using conventional coating techniques such as screen-printing or tape casting [5,9,10]. Nevertheless, from an industrial point of view such configuration presents limited membrane area for oxygen permeation and edge leakage issues [11]. ...
An original asymmetric tubular membrane for oxygen production applications was manufactured in a two-step process. A 3 mol% Y2O3 stabilized ZrO2 (3YSZ) porous tubular support was manufactured by the freeze-casting technique, offering a hierarchical and radial-oriented porosity of about 15 µm in width, separated by fully densified walls of about 2 µm thick, suggesting low pressure drop and boosted gas transport. The external surface of the support was successively dip-coated to get a Ce0.8Gd0.2O2−δ – 5mol%Co (CGO-Co) interlayer of 80 µm in thickness and an outer dense layer of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) with a thickness of 30 µm. The whole tubular membrane presents both uniform geometric characteristics and microstructure all along its length. Chemical reactivity between each layer was studied by coupling X-Ray Diffraction (XRD) analysis and Energy Dispersive X-Ray spectroscopy (EDX) mapping at each step of the manufacturing process. Cation interdiffusion between different phases was discarded, confirming the compatibility of this tri-layer asymmetric ceramic membrane for oxygen production purposes. For the first time, a freeze-cast tubular membrane has been evaluated for oxygen permeation, exhibiting a value of 0.31 ml·min−1·cm−2 at 1000ºC under air and argon as feed and sweep gases, respectively. Finally, under the same conditions and increasing the oxygen partial pressure to get pure oxygen as feed, the oxygen permeation reached 1.07 ml·min−1·cm−2.
... Due to the high price for the raw materials, an important goal is to replace the expensive LaWO substrate by a low cost structural ceramic with sufficient mechanical stability, low creep rate, good heat conductivity [10], [11] and sufficiently large thermal expansion coefficient [12]. In the present work, magnesium oxide (MgO), which is an already known support material [13][14][15], was chosen and the stability under possible application conditions was investigated, free standing MgO substrates and asymmetric systems of LaWO-MgO combinations were prepared and characterised. ...
The performance of ceramic gas separation membranes can be increased by reducing their thickness. To maintain the mechanical stability of the membrane, a supporting structure is needed. In this work magnesium oxide (MgO) is identified as support material for H2-separation membranes made of La6-xWO12-δ (LaWO). Therefore MgO is tested under application-related conditions in a water-gas-shift (WGS) catalytic membrane reactor for hydrogen production and shows a good stability. The sequential tape casting serves as the manufacturing method for the preparation of the asymmetric membrane structures. To show the compatibility of the two materials, detailed investigations of the sintering behaviour and stability tests are performed and show the well-matching properties of the two materials. Microstructural analyses reveal the inter-diffusion between LaWO and MgO.
... final shrinkage and densification kinetics) in order to obtain flat and crack-free asymmetric reactors. 21,22 In addition, to limit the thermo-mechanical stresses that can appear during the cooling stage of sintering or under working conditions (i.e. 800-1000 • C), similar thermal expansion values are necessary. ...
The decrease of the dense layer thickness can lead to increase the internal stresses in the membrane due to the chemical expansion of membrane material under a large gradient of oxygen partial pressure. This chemical expansion due to pO2pO2 gradient through the membrane leads to important mechanical stresses in the membrane and commonly to the membrane rupture under large range of pO2pO2, i.e. air/methane atmosphere. The solution suggested in this paper is the elaboration by tape casting and co-firing process of multilayer membranes with a specific design in order to decrease stresses due to the chemical expansion in working conditions.
In this work, the Mg2Al4-x (Mg0.5Ti0.5)xSi5O18 (0 = x ≤ 0.20) ceramics were successfully synthesized via solid-state reaction. The main phase was Mg2Al4Si5O18 with an orthorhombic structure, and the secondary phases (SiO2, TiO2 and Al2SiO5) were formed during the synthesis of Mg2Al4-x (Mg0.5Ti0.5)xSi5O18 ceramics. The substitution of Al³⁺ by (Mg0.5Ti0.5)³⁺ occur complex chemical reaction: Mg ions prefer to supplement to the Mg-site deficiency in Mg2Al4Si5O18; Ti ions substitute Si-site at x < 0.12, whereas Ti ions also occupy Al-site and remaining to form TiO2 phase at x ≥ 0.12. The dielectric constant (εr), quality factor (Qf) and temperature coefficient of resonance frequency (τf) are mainly dominated by the ionic polarizability, relative density, secondary phases and bond valence of Mg-site. Additionally, the bond ionicity (fi) of Mg–O, lattice energy (U) of Si–O and bond energy (E) of Al–O also influence εr, Qf and τf, respectively. The moderate (Mg0.5Ti0.5)³⁺ substitution not only effectively improve Qf while maintaining low εr, but also reduce densification temperature. The excellent microwave dielectric properties with εr = 4.45, Qf = 33,995 GHz and τf = −39 ppm/°C were obtained in Mg2Al4-x (Mg0.5Ti0.5)xSi5O18 (x = 0.08) ceramic sintered at 1375 °C, which is potential for millimeter-wave communication.
This review summarizes recent progress on dual-phase oxygen transport membranes. Existing challenges, research strategies and future application areas are discussed.
Metal oxide nanoparticles display unique properties such large bandgap, low electric constant, low refractive index, high chemical stability, and vacant oxygen presence. Magnesium oxide (MgO) nanoparticles are of particular interest because they are abundant, nontoxic, cheap, odorless, and stable. Here we review the synthesis and applications of MgO nanoparticles and their composites in various fields. MgO has antibacterial properties for medicine due to the production of superoxide anion O2−. The reactivity and stability of MgO are of interest for medicine, water purification, catalysis, and gas sensors. For membrane applications, new strategies are needed to control the pore diameter of the membranes with MgO as filler. As food packaging materials, there is a need for methods to assess the release of nanoparticles from the packing materials into food. For sensor applications, challenges include sensitivity, reproducibility, surface fouling, and poisoning. For supercapacitors and semiconductors, controlling the pore structure should improve electron transport.
Carbon encapsulated LaFeO3 perovskite composites (LaFeO3@C) were synthesized by a one-pot method and the effects of carbohydrates as carbon precursor on the activity of LaFeO3@C for catalytic transfer hydrogenation (CTH) reactions were investigated. The textural and surface properties (e.g., pore structure, surface acidity and basicity) of LaFeO3@C vary significantly with carbohydrates, which offer a facile route to tailor the sample by simply changing the carbon precursor. LaFeO3@C-M, using maltose as carbon precursor, shows the best catalyst for the CTH of furfural with isopropanol, with conversion and selectivity of 90.3% and 94.3%, respectively, owing to the surface La enrichment, rich oxygen functional groups / oxygen vacancy and matched surface acidity-basicity. This sample is also recyclable after post-treatment in N2 atmosphere and can be applied to catalytic hydrogenation of various carbonyl compounds. The current one-step synthesis method provides an efficient way to design carbon-containing composite catalysts with controllable properties for catalytic applications.
Micro-tubular solid oxide fuel cells (MT-SOFCs) have attracted much attention due to; higher volumetric output density, greater tolerance to cycling, quicker start-up capability and better mobility. Fabrication process of MT-SOFCs is one of the critical factor that ensures the success of a cell. There are three main components (i.e. anode, electrolyte and cathode) in the development of MT-SOFCs which are generally fabricated through separated and multiple laborious steps. This study aims to produce a novel anode/electrolyte/cathode (NiO-CGO/CGO/LSCF-CGO) triple-layer hollow fiber (TLHF) in a single-step using phase inversion based co-extrusion combined with co-sintering technique. The challenge lies within the diverse sintering behaviors among the anode, electrolyte and cathode layers at some point of the cell fabrication. In this study, the TLHF was found being able to survive the co-sintering at 1450 °C resulting a defect free precursor producing 0.48 Wcm⁻² maximum power densities at 525 °C, which is comparable with the cell fabricated using conventional multiple-step process.
Asymmetric membranes, with suitable mechanical properties under working conditions, lead to an enhanced oxygen permeability compared to self-supported membranes. In this work, crack-free asymmetric tubular membranes made of BaCo0.7Fe0.2Nb0.1O3-δ perovskite oxide were prepared via a dip-coating and co-sintering method. The crystal phase of the top layer of asymmetric membranes prepared was the same as that of powders, which were of the cubic perovskite phase. The thickness and quality of the dense layer can be controlled by adjusting the concentration of the suspension slurry in the range of 10–50 wt%. Moreover, dip coating repeatedly is helpful to ensure the integrity of the membrane. The microscopic structure of membrane was characterized by SEM which showed that the top layer of asymmetric membrane was dense and crack-free. Notably, when tested for oxygen separation, the oxygen flux of asymmetric membrane was significantly higher than that of self-supported membranes.
This paper delineates the fabrication of porous magnesium oxide (MgO) ceramics with high porosity and gas permeability by warm pressing using pre-calcined MgO powder and fugitive pore former (combination of graphite and polymethyl methacrylate). Effect of pore former on the microstructure development of porous MgO ceramic substrates was subjected to investigation. The resultant microstructure consisted of large spherical and elongated pores with small interconnecting pores. The total porosity (55%), mean pore neck size (0.65 μm), and the associated gas permeability (4–4.5×10−15 m2) of MgO substrates were measured and correlated. Economic analysis of the MgO substrates was performed and it was found that MgO was much cheaper compared to perovskite and fluorite materials.
An asymmetric membrane consisted of a thin functional layer and a thick support was developed for oxygen production and membrane reactors. The functional La0.8Sr0.2Cr0.5Fe0.5O3-delta(LSCrF)-Zr0.8Y0.2O2-delta(YSZ) layer was prepared by screen printing and the YSZ support by phase inversion tape casting. Performance of the membrane was evaluated under air/argon, air/CO and CO2/H-2 gradient at 750-900 degrees C. The results indicate the membrane to have good chemical stability and considerable high oxygen permeability under stringent conditions. An oxygen permeation flux of 0.058, 0.175 and 1.198 ml (STP) cm(-2) min(-1) was obtained at 900 degrees C for operation under air/argon, CO2/H-2 and air/CO gradient, respectively. The apparent activation energy for oxygen permeation was 131.22 +/- 19.74, 89.53 +/- 6.29 and 66.28 +/- 1.41 kJ/mol under air/argon, air/CO and CO2/H-2 gradient respectively. The results suggest the membrane to have potential applications in the chemical reactor.
Three different compositions of MgO compounds were investigated for use in oxygen transport membranes. Porous MgO supports were extruded using different kind (size, morphology and chemistry) of pore formers: A flaky graphite, a spherical graphite and ideal spheres of PMMA. The influence of the pore former on microstructure, gas permeation and the mechanical properties for various sintering temperatures were investigated. The gas permeation behavior of the MgO supports was highly dependent on pore neck size and total open porosity. MgO substrate, with 20% spherical graphite as a pore former, sintered at 1300 • C for 2 h, showed a total porosity of 42.5% and gas permeability of 4.7 × 10 −16 m 2 . Subsequently, the 4-point bending strengths of this substrate, scaled to an effective volume of 10 mm 3 , were 77 and 60 MPa for room and operation temperature (850 • C). Both, permeation rate and mechanical strength is sufficient for using the support for further investigations in OTM.
The perovskite mixed ionic-electronic conducting membranes (MIEC) are widely used in solid oxide fuel cells, oxygen separation and the partial oxidation of methane to syngas. The synthesis process and properties of the composite membrane including the porous support layer and the thin dense layer with the same material, Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF5582), were investigated in the present study. The powders of perovskite structure BSCF5582 for porous support layer were synthesized by solid-state reaction method (SSR) at 1233 K for 2~6 hours, while the powders of BSCF5582 for dense layer were synthesized by citrate-EDTA complexing method (CC-EDTA) after being calcined at 1173 K for 2~6 hours. The support layer with a thickness of 0.6 mm was made by tape calendering process (TCP), and then pre-sintered at 1073 K for 1 h. The dense layer with a thickness of 30 μm was formed by dip-casting process. The composite membrane of BSCF was formed by co-firing the two layers at 1413 K for 10 hours. Porous support layer and dense layer were clearly identified by scanning electron microscopy (SEM). The conductivity of the composite membrane is 35S·cm-1 at 773K. The electrical-conduction behavior of BSCF obeys the p-type small polaron-hopping mechanism.
The fabrication of mixed ionic-electronic conducting (MIEC) ceramic-based membranes for oxygen separation has extensively increased in the last three decades as a promising alternative of clean energy delivery. In recent years, the interest on supported MIEC membranes has increased due to their attractive properties such as higher mechanical strength and oxygen flux compared to self-supported membranes. This work presents a literature review on the development of supported MIEC membranes of perovskite structure. The concepts and transport mechanism of those membranes are explained and recent works on supported MIEC membranes are presented. Finally, manufacturing methods of self-supported membranes and their influence on oxygen permeation are discussed.
In this work the manufacture of commercial La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) by aqueous colloidal processing is presented. The surface behavior of LSCF as a function of pH and the effect of a polyelectrolyte (Duramax D3005) on the stability are studied using measurements of zeta potential. Concentrated suspensions were prepared to solid content as high as 35 vol.%. The best dispersing conditions were determined by means of rheological measurements for obtaining stable and fluid slurry for tape casting technique. Different relative densities of the tapes were obtained at different temperatures. The LSCF tapes are good candidates for using as gas separation membrane or cathode for SOFC.
This work reports a new design of asymmetric tubular oxygen-permeable ceramic membrane (OPCM) consisting of a porous Y2O3 stabilized ZrO2 (YSZ) tube (with ∼1 μm of pore diameters and 31% porosity) as the support and a gas-tight mixed conductive membrane. The membrane has an interlocking structure composed of a host matrix, Ag(Pd) alloy (9:1 by wt) doped perovskite-type
(LSM80, 90wt%), and the embedded constituent, pristine LSM80. The Ag(Pd) alloy component promotes not only electronic conductivity and mechanical strength but also reduction of both porosity and pore sizes in the layer (∼10-μm-thick) where it dopes. The porous structure in this layer could then be closed through a solution coating procedure by which ingress of an aqueous solution containing stoichiometric nitrate salts of La3+, Mn3+, and Sr2+ to the pore channels takes place first and the mixture of nitrate salts left after drying is subjected to pyrolysis to generate tri-metal oxides in situ. This is followed by calcinations at l,300 °C to consolidate the embedded trioxide and to cohere them with the Ag(Pd)-LSM80 host matrix. The structure formed is dubbed LSM80(S)-Ag(Pd)-LSM80, which was confirmed gas-tight by electron micrograph and N2 permeation test. Finally, we assess the chemical compatibility between LSM80 and YSZ at the sintering temperature by X-ray diffraction and electrochemical impedance analysis. The oxygen permeation of the fabricated LSM80(S)-Ag(Pd)-LSM80-YSZ membrane is within the temperature range of 600 to 900 °C. The tests reveal good compatibility between the LSM80 and YSZ and a reasonably high oxygen permeation flux in association with this OPCM assembly.
The oxygen-permeable ceramic membrane (OPCM), made of the mixed-conductive ceramic oxide Gd0.2Ce0.8O2−δ (GCO20), has been successfully fabricated on a porous CeO2 tubular support by means of the slurry-coating and co-sintering techniques. In this asymmetric membrane, the GCO20 membrane (10–20 μm thick) is intercalated in the surface layers of a porous CeO2 tube. It gives an O2 permeation flux of 0.45 sccm/cm2 from air feed at 900 °C and 1 bar with exclusive permselectivity. To promote oxygen flux at relatively lower separation temperatures, a thin layer of La0.2Sr0.8CoO3−δ (LSCO80) was deposited on top of GCO20/CeO2 to generate a dual-layer membrane. Nevertheless, raising the calcination temperature to consolidate the outer LSCO80 layer will suppress the oxygen flux. Based on SEM and XRD investigations, this phenomenon is due to removal of fractal surface features and distortion of LSCO80 crystalline phase. Furthermore, the temperature leverages on oxygen ionic conduction were assessed using impedance analysis. The electric measurement results are in agreement with the oxygen permeation testing results. Finally, the density function theory (DFT) was applied to perform simulations in order to find out the dependence of the equilibrium dimension of lattice cell on the oxygen vacancy concentration in the perovskite LSCO80 structure at the ground state of the crystal. The outcome shows that the lattice undergoes expansion upon losing oxygen, which provides a theoretical explanation for the crystalline distortion induced by high calcination temperature.
As a method for fabrication of multilayer structures, co-sintering process has attracted increasing attention due to its promising advantages. In this study, co-sintering process was applied to fabricate bi-layer ZrO2/Al2O3 membranes supported by rigid Al2O3 substrates. Following our previous work, this paper focused on the effect of membrane thickness on the co-sintering process. Experimental results showed that thinner sub-layer membrane without defects was favorable to achieve good bonding between the membrane layers and the support after co-sintering. The thickness of the top-layer membrane should be not very thin. Otherwise the compressive stress caused by fast shrinkage of the top-layer membrane would be insufficient to promote the sintering of the sub-layer membrane during the co-sintering process. The suitable thicknesses for the sub-layer and the top-layer membranes have been determined, which were about 15 and 10 μm, respectively. The bi-layer ZrO2/Al2O3 membrane co-sintered at 1200 °C had the average pore size of about 0.28 μm and the pure water flux through the membrane was about 2.82 × 10−2 L m−2 h−1 Pa−1 (2.82 × 103 L m−2 h−1 bar−1). The bi-layer ZrO2/Al2O3 membranes prepared by co-sintering process showed good performance in an oil–water separation.
Thin dense films of Ca0.8Sr0.2Ti0.7Fe0.3O3 − α (CSTF) perovskite-type oxide, which is used as an oxide ionic and electronic mixed conductor, were prepared on a CSTF porous substrate using spin coating method. For use as both thin dense film materials and porous substrates, these CSTF materials were prepared using different procedures. A porous substrate was prepared by treating carbon and CSTF composites in air at 1273 K to produce open pores by carbon combustion. The dense film thickness was controlled at 35–96 µm by varying the spin coating times. The maximum oxygen flux from air to nitrogen through the asymmetric membrane observed at 1223 K was 2.1 ml min− 1 cm− 2 for 35 µm film thickness. Oxygen permeation rates were inversely related to the film thickness. These results suggest that bulk diffusion controls the permeation of oxide ion in the CSTF thin film.
Dense mixed-conducting membranes of La0.6Sr0.4Fe0.9Ga0.1O3−δ (LSFG) with various contents of MgO as second phase particles were prepared to evaluate the influence of magnesia inclusions on LSFG stoechiometry, microstructure and oxygen permeation. XRD and EDS investigations on sintered pellets revealed that magnesia inclusions were quite inert with the LSFG matrix phase, the composition of which remained identical whatever the magnesia content. LSFG pure phase material was synthesized through a solid-state route and sintered between 1250 and 1350°C. Sintering temperature strongly affected microstructure of the LSFG membrane since rapid grain growth and decreasing density were observed when temperature increased. Small amounts of fine particles of magnesia, from 2 to 10vol%, were found to significantly reduce grain size of sintered samples and made it possible to obtain a high density on a large sintering temperature range. Average grain size experimental data of LSFG in function of the amount of second phase magnesia were also compared with numerical models from literature. Oxygen permeation rates of pure LSFG and composite LSFG/MgO dense membranes were measured in an air/argon gradient, in a temperature range from 825 to 975°C and results were discussed to explain the flux improvement of composite membranes.
High-temperature applications of perovskite-type membrane reactors require improved material performances and operational stability. The reactor microstructure and architecture controls were found to be crucial for thermo-mechanical integrity and oxygen permeation kinetics.
The use of perovskite mixed ionic–electronic conducting membranes for industrial applications, such as oxygen dissociation
from air and methane conversion into syngas, requires high oxygen permeation fluxes. In this regard, the improvement of the
permeation rates through a (LSFG) membrane was performed by modifying the surface by controlling the average grain size of the perovskite material or
by coating its surface with a thin layer of . In both cases, the permeation-limiting regime was determined and the oxygen surface coefficient k
s and diffusion coefficient D
v were evaluated. The low value of k
s
was found to be the most critical parameter for the performance of these LSFG membranes.
We report a successful paradigm of implementing the nickel catalyst for partial oxidation of methane (POM) in an oxygen-permeable ceramic membrane reactor (OPCMR). With using air feed, the OPCM (or the cathode of O2 reaction) was completely gastight but presented a desirable oxygen anion flux, which is required by POM in the anodic side. An almost 100% of CH4 conversion with high selectivity of CO and H2 was achieved in the anode, which was initially a composite of YSZ and NiO. It is also found that a mutual diffusion of Ni2+ and Zr4+ ions happens between the YSZ and NiO phases, which affected performance of POM. Besides experimental work, a new activation pathway was proposed to describe cleavage of the first C−H bond of methane. This activation pathway was simulated using the density function-theory and the computed activation energy for breaking up the first C−H bond falls in the experimental measurement range.
Thin-film La0.58Sr0.4Co0.2Fe0.8O3−δ (LSCF58428) exhibits high oxygen permeability due to its high ionic and electronic conductivity. In order to increase the oxygen flux, a thin-film membrane on a structural substrate is beneficial. Different Ni-based alloys were studied as potential substrate materials. The chemical compositions and thermal expansion coefficients of Ni-based alloys were measured in this study. LSCF58428 layers were screen printed on Ni-based alloys and cofired at a high temperature in air. Microstructural and elemental analyses of samples were conducted using a scanning electron microscope and energy-dispersive X-ray spectroscope. X-ray diffraction was used to investigate the phase compositions. The Ni-based alloy, MCrAlY (with M=Ni, Co), with a high Al content showed better chemical compatibility with perovskite material at high temperatures than other Ni-based alloys. A reaction occurred between Sr in the perovskite and the alumina-based surface layer on MCrAlY. However, the reaction zone was found to be stable in mid-term annealing at 800°C in air. Hence, it is expected that this reaction will not prevent application as an oxygen transport membrane. Three different cofiring atmospheres were investigated. Air was found to be the only possible cofiring atmosphere.
Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) shows promising oxygen transport properties for the use as oxygen separation membranes and cathode material for solid oxide fuel cells. In order to evaluate the electrical conductivity properties as well as surface exchange coefficients without the influence of grain boundaries present in polycrystalline bulk samples, high-quality epitaxial BSCF thin films on single crystal NdGaO3 (110) (NGO) substrates were prepared for the first time. This was achieved by pulsed laser deposition (PLD) using single-phase cubic targets sintered from powder prepared via the mixed oxide route. Structural and compositional properties of the thin films were verified by various X-ray diffraction techniques as well as by scanning electron microscopy (SEM). The electrical conductivity of the thin films was measured in dependence of temperature and compared to values obtained by measurements on polycrystalline bulk samples from the same powder as well as data from literature, showing different activation energies and higher general conductivity. Dependence of the conductivity on oxygen partial pressure has also been studied, revealing a rather small variation and indicating a possible phase change beginning to occur at oxygen partial pressures below 10−4 bar. Additionally, the surface exchange coefficient kChem of the purely surface exchange controlled samples was determined by a conductivity relaxation technique and yielded an activation energy of 93 kJ/mol.
In order to appraise a two-stage compaction procedure using pore-forming additives for the fabrication of asymmetric mixed-conducting membranes where the porous and dense layers are made of the same composition, the oxygen permeability of a series of (SrFeO3−δ)0.7(SrAl2O4)0.3 composite samples with varying architecture was studied at 1073–1223 K. The preparation route for the crack-free supported membranes included pressing of the starch-containing and pure dual-phase composite powders, sintering at 1623 K, and subsequent surface modification of the dense layers having the thickness of 0.12–0.15 mm. Analysis of the oxygen permeation fluxes show a significant limiting effect of oxygen diffusion through the support, where the porosity and average pore size are 20% and 2–4 μm, respectively. The overall level of oxygen transport, higher than that in the symmetric surface-activated membranes, was only achieved at 1173–1223 K for the porous layer thickness of 0.4 mm. Slow microstructural degradation due to the support sintering, evidenced by dilatometry, leads to a moderate decrease in the oxygen fluxes with time. At 1973 K, the corresponding changes were approximately 16% during 220 h. The results suggest that increased total porosity, preferential pore orientation perpendicular to the dense layer and incorporation of nano-sized catalyst particles into the pores are needed to increase the performance of asymmetric ferrite-based membranes.
An asymmetric-structured membrane, in which a dense thin La0.6Ca0.4CoO3 layer of 10 μm was deposited on porous La0.6Ca0.4CoO3 support by a slurry dropping method, were tested for its oxygen permeability at 630–930 °C. In order to increase the oxygen permeability, porous La0.6Ca0.4CoO3 and SrCo0.8Fe0.2O3 − δ oxygen evolution layers were attached on the dense layer. It was found that the asymmetric structured membranes with the porous oxygen evolution layers showed remarkably higher oxygen permeability as compared with a conventional sintered disk-type membrane (1200 μm). This suggests that oxygen permeation through membranes of 10 μm in thickness is rate-determined by both bulk O2− diffusion and oxygen evolution reaction. The maximum oxygen permeability reached 4.77 cm3 (STP) min− 1 cm− 2 (3.54 × 10− 6 mol cm− 2 s− 1) at 930 °C for the asymmetric membrane with porous La0.6Ca0.4CoO3 oxygen evolution layer of 10 μm in thickness (STP = Standard Temperature and Pressure). Further improvements in the oxygen permeability would be achieved through control of the micro-structure of the oxygen evolution layers.
The performance of a catalytic membrane reactor (CMR), for partial oxidation of methane, mainly depends on the oxygen semi-permeation properties of the membrane and on its architecture. This paper presents the elaboration, by tape-casting lamination and co-firing process, of a membrane consisting of a crack free La0.8Sr0.2Fe0.7Ga0.3O3−δ (LSFG8273) dense thin film supported by a porous layer. In the porous layer, gallium must be eliminated due to its cost.In order to avoid membrane cracking during the sintering and under working conditions, the material of the porous support was chosen and adapted to have the same shrinkage during the co-firing and a thermal expansion coefficient similar to the dense membrane material. Two materials fulfil these conditions: La0.8Sr0.2FeO3−δ (LSF821) and La0.8Ba0.2FeO3−δ (LBF821).Asymmetric membranes lead to an enhancement of the oxygen semi-permeation flux compared to self-supported membranes, with suitable mechanical properties in working conditions. The porous layer has a beneficial influence on oxygen semi-permeation of the membrane. At working temperature, the limiting mechanism of oxygen transport through the membrane is the oxygen surface exchanges.
Multilayer membranes based on La0.6Sr0.4Fe0.9Ga0.1O3−δ (LSFG) and La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF) perovskite materials were fabricated to study the impact of membrane architecture on the oxygen permeability. Thick dense membrane and asymmetric membranes were shaped by tape casting and stacked to reach the desired architecture. Asymmetric membranes composed of a thin dense LSFG layer (120 μm) and a thick porous support layer (820 μm) of the same material were co-sintered to obtain crack-free and flat membranes. The use of large corn-starch particles (14 μm) as pore forming agent to the tape-casting slurries resulted in a connected porosity in the sintered support layer with low gas diffusion resistance. Oxygen permeation measurements in an air/argon gradient between 800 and 925 °C showed that the thickness of self-supported LSFG membranes was not the determining factor in the membrane performance for our testing conditions. A catalytic layer of La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF), deposited on the membrane surfaces to catalyze the oxygen exchange reactions, leads to a significant increase of oxygen permeation rates. As the membrane thickness had no effect even if a catalyst coating was used, surface-exchange reactions were thought to be still limiting for the oxygen permeation fluxes. Thus, the improvement of surface activity of LSFG membrane was found to be a key point to reach higher oxygen permeation fluxes.
Mixed conductive perovskite materials, e.g., La1−xSrxO3−δ (LSCO), have been widely investigated to understand the leverages of doping extent and composition on the oxygen permeability with the aim of developing an oxygen-transport solid electrolyte membrane. However at the present stage fabrication of a dense thin layer of perovskite oxide on a porous tubular support possessing mechanically and chemically stability at high temperatures is still a technological challenge to the endeavor. This is because the asymmetric configuration is a desired model of the commercial oxygen-permeable ceramic membrane reactor. The present work develops a new approach that allows the formation of a complete gas-tight oxygen-permeable thin membrane on the outer surface of a porous CeO2 tube by the means of slurry coating. The oxygen-permeable membrane is a dual-phase composite containing equal volume fractions of CeO2 and LSCO-80 (x = 0.8). In the membrane CeO2 particles are uniformly embedded in the continuous LSCO phase, and this highly dispersed semi-continuous structure could successfully buffer the mechanical stress generated in the LSCO phase due to mismatch of coefficient of thermal expansion (CTE) between the membrane and the support. The oxygen permeation flux tests showed a low activation energy barrier (∼30 kJ/mol) of the whole electrochemical reaction in the temperature range from 400 to 900 °C. The surface de-sorption (or the anodic) process of the oxygen has been simulated using the extended Hückel theory (EHT). The activation energy obtained from the EHT simulation is found very close to the experiment data. In addition, according to the computer simulation, surface oxygen de-sorption activation energy relies on the surface oxygen vacancy density and thus the oxygen partial pressure.
This paper reports a new design of ceramic asymmetric tubular reactor for correlating air separation with catalytic partial oxidation of methane (POM). The tubular membrane reactor consists of three annular layers, a porous and thin La0.2Sr0.8MnO3−δ (LSM80)-Ce0.8Gd0.2O2−δ (CGO20) cathodic layer, a dense and thin YSZ⊥(Pd-TiO2) mixed conducting layer as the electrolyte layer, and a porous and thick YSZ-Ni anodic layer. For realizing mixed-conducting electrolyte layer, an electronic conductive Pd-TiO2 stripe was wedged into a dense YSZ coating layer via a specially designed two-step calcination process. The resulting membrane reactor was assessed by its POM output in a broad temperature range as well as by its capability to clean up the coke deposited on Ni(0) catalyst. It demonstrated high methane conversion (>90%), CO selectivity (>90%) and H2 selectivity (>80%) at 850 °C. Besides the experimental work, a mathematical model including the two major POM mechanisms responsible for the methane conversion over the temperature span of study was developed and employed to simulate the experimental XCH4 (conversion) ∼1/T data. The kinetic parameters obtained well accounted for the characteristics of these two reaction mechanisms.
A crack-free supported mixed-conducting membrane (composite 60 wt.%SrCo0.4Fe0.5Zr0.1O3−δ/40 wt.%MgO, SCFZ–0.4MgO) was successfully prepared by the dry-pressing technique directly on a support with the different composition (composite 60 wt.%MgO/40 wt.%SrCo0.4Fe0.5Zr0.1O3−δ, MgO–0.4SCFZ), followed by sintering. The match of thermal performances between the membrane and the support was realized by incorporating the support material (MgO) into the membrane (SCFZ) and simultaneously merging the membrane material (SCFZ) into the support (MgO). SEM and the nitrogen gas-tight test demonstrated that the surface of the supported membrane was dense and crack-free. XRD patterns showed that SCFZ was chemically compatible with MgO. The oxygen flux of the supported membrane with a dense layer of 200 μm and a support layer of 1.0 mm was about 9–11 times that of the 1.2 mm symmetric SCFZ–0.4MgO membrane.
There has been tremendous progress in membrane technology for gas separation, in particular oxygen separation from air in the last 20 years. It provides an alternative route to the existing conventional separation processes such as cryogenic distillation and pressure swing adsorption as well as cheaper production of oxygen with high purity. This review presents the recent advances of ceramic membranes for the separation of oxygen from air at high temperature. It covers the issues and problems with respect to the selectivity and separation performance. The paper also presents different approaches applied to overcome these challenges. The future directions of ceramic-based membranes for oxygen separation from air are also presented.
Strontium lanthanum manganite (LSM) and lanthanum ferrite (LSF) perovskite cathode and oxygen membrane materials were synthesized using different techniques: spray pyrolysis, a modified citrate route, oxalate and carbonate co-precipitations. The use of Ca, a cheaper substituent on the A-site, was explored along to the substitution of La by Pr. The differently sourced powders were characterized by TG/DTA, XRD, ICP, TEM, XPS, PSD and BET. The co-precipitation of La, Ca and Fe was also possible using the cyanide route. This complexation method allowed the precipitation of a crystalline phase as evidenced by XRD. Among all methods, the cyanide and carbonate co-precipitation allowed the lowest perovskite phase transformation for LSF and LCF, followed by the nitrate (i.e. 'spray pyrolysis'). These phase transformation differences affected much the particle size distribution and the surface areas of these materials, the carbonate and the cyanide routes giving rise to very fine powders in the nm range. XPS and TEM analyses indicated uneven composition distributions. These different powder characteristics are expected to affect the catalytic and electrochemical properties of these materials.
Depuis quelques années, un intérêt croissant est porté à la conversion du méthane en gaz de synthèse (H2+CO) pour la production d'hydrogène ou de carburants propres par le procédé GTL. Les réacteurs catalytiques membranaires (CMR) constituent une alternative économiquement intéressante pour cette application.
L'architecture des réacteurs intègre un catalyseur, une membrane conductrice mixte de type La1-xSrxFe1-yGayO3-d et un support poreux actif. Le choix du matériau du support s'est porté sur La0,8Sr0,2FeO3-d, en vue du co-frittage des couches denses et poreuses, ce qui permet d'assurer une continuité chimique et de diminuer les coûts. Des membranes supportées planes LSFG8273/LSF821 et LSFN8273/LSFG8273/LSF821 ont été élaborées par coulage en bande, thermocompression et co-frittage. Les performances du réacteur ont pu être largement améliorées par la présence du support poreux et de la couche catalytique. Enfin, les matériaux ont fait l'objet d'une étude thermomécanique.
A crack-free asymmetric membrane of perovskite-type oxide (La0.6Sr0.4Co0.2Fe0.8O3−δ) was successfully prepared by coating a slurry containing La0.6Sr0.4Co0.2Fe0.8O3−δ powders directly on the surface of a green support of the same composition, followed by sintering. It was found that crack-free asymmetric membranes could be obtained by controlling the powder concentration of the slurry in the range of 15–25wt.%. After sintering, the crystal phase of the top layer of asymmetric membranes prepared was the same as that of powders, which were of the cubic perovskite phase. The nitrogen permeability and SEM photograph of the support showed that the support was porous, and the gas-tight test and SEM demonstrated that the top layer of asymmetric membrane was dense and crack-free. The asymmetric membrane prepared, whose dense top layer was 200μm thick, exhibited about three to four times as high an oxygen flux as a 2mm dense sintered disc.
The partial oxidation of methane to synthesis gas (syngas, CO + H2) was performed in a mixed-conducting perovskite dense membrane reactor at 850°C, in which oxygen was separated from air and simultaneously fed into the methane stream. Steady-state oxygen permeation rates for La1-xA′xFe0.8 Co0.2O3-δ perovskite membranes in nonreacting air/helium experiments were in the order of A′x = Ba0.8 > Ba0.6 > Ca0.6 > Sr0.6. Deep oxidation products were obtained from a La0.2 Ba0.8 Fe0.8 Co0.2 O3–δ disk-shaped membrane reactor without catalyst, with a 4.6% CH4 inlet stream. These products were further reformed to syngas when a downstream catalytic bed was added. Packing the 5% Ni/Al2O3 catalyst directly on the membrane reaction-side surface resulted in a slow fivefold increase in O2 permeation, and a fourfold increase in CH4 conversion. XRD, EDS, and SEM analyses revealed structure and composition changes on the membrane surfaces. Oxygen continuously transported from the air side appeared to stabilize the membrane interior, and the reactor was operated for up to 850 h.
Ion transport membranes (ITMs) are made from ceramic materials that conduct oxygen ions at elevated temperatures. Successful application of ITM technology will allow significant improvement in the performance of several large-scale industrial processes. The ITM Oxygen process, in which ITMs are used to separate high-purity oxygen from air, has the potential for significant advantages when integrated with power generation cycles. The ITM Syngas process, by combining air separation and high-temperature syngas generation processes into a single compact ceramic membrane reactor, has the potential for substantially reducing the capital investment for gas-to-liquid (GTL) plants and for distributed hydrogen. The development efforts are major, long-term and high risk, and place severe demands on the performance and property requirements of the ITM materials. Air Products and Chemicals has joined with the U.S. Department of Energy, Ceramatec and other partners to develop, scale-up and commercialize these technologies. In addition, Air Products and Ceramatec are developing the SEOS™ Oxygen Generator, an electrically-driven, small scale, oxygen generation and removal technology using ITMs, which could have a significant impact in the global market for distributed oxygen and inert gases. This paper describes the stages of development of these three related technologies, their industrial applications, and the technical hurdles that must be overcome before successful commercialization.
The last 10 years an alternative production route for oxygen production by using mixed conductors has been investigated. Perovskite mixed electron-oxygen conducting membranes are bulk membranes with a thickness of the order of 1 mm, showing sufficient oxygen fluxes only at temperatures above 800°C. To reach commercially interesting fluxes at lower temperatures, membrane modules with a much higher surface/volume ratio or multilayer membranes with a thin dense skin need to be developed. In our laboratory, dense hollow fibres and porous multilayer substrates were synthesised. The hollow fibres were produced using a polymeric spinning technique based on phase inversion. The multilayer porous substrates were manufactured following conventional ceramic processing routes.
The prospects of using mixed ionic/electronic conducting ceramics for syngas production in a catalytic membrane reactor are analysed. Problems relating to limited thermodynamic stability and poor dimensional stability of candidate materials are addressed. The consequences for these problems, of flux improving measures like minimization of membrane thickness and minimization of the losses due to oxygen exchange over the membrane surfaces, are discussed. The analysis is conducted on two candidate materials: La0.6Sr0.4Co0.2Fe0.8O3−δ and SrFeCo0.5Ox. Finally, experimental investigations of the dimensional stability of the latter material under reducing conditions are reported.