Thin Dense Layer Research Articles

Development of Multilayer Anodes for Proton-conducting Solid Oxide Fuel Cells

Advanced cermet electrodes of Ni-BaCeO.9Y0.1O2.95, suitable for Intermediate Temperature PCFC, have been elaborated using gelling starch as pore forming agent. SEM and mercury porosimetry measurements show a strong dependence of the microstructure upon the starch content. As the starch content increase, a gradual creation of percolating starch network is clearly shown. An half cell showing a gradient of porosity and a thin electrolyte dense layer was successfully obtained by a simple co-pressing and co-sintering process. The polarization ASR (0.13 Ohm.cm2) is obtained for the cell architecture investigated in this work

PREPARATION AND CHARACTERIZATION OF LOW-PRESSURE NANOFILTRATION MEMBRANES BASEDON CROSSLINKED HYPERBRANCHED POLYESTER

Thin-film composite nanofiltration membranes were prepared by a simple dipping and crosslinking process.A polysulfone(PSF) ultrafiltration membrane was used as the support and a crosslinked hyperbranched polyester(HPE) as the active separation layer.The separation layer was immobilized onto the support via the aldol condensation of HPE with glutaraldehyde(GA).The prepared membranes were characterized by attenuated total reflection Fourier transform infrared spectroscopy(ATR-FTIR),water contact angle measurements and scanning electron microscopy(SEM).The results of FTIR examination confirmed the existence of crosslinked HPE layer on membrane surface.The results of SEM observation indicated that a thin dense layer of crosslinked HPE was deposited on the support membrane.Water contact angle measurements showed that the membrane hydrophilicity was significantly improved.The effects of HPE concentration and GA concentration during membrane preparation on the separation and permeation properties were investigated.The optimized HPE and GA concentrations were 9.8 g/L and 7.4 g/L,respectively,and the water flux for the resultant membrane reached 69.6 L/(m~2·h),and the Na_2SO_4 rejection ratio was 93.2%.The sequence of rejection ratio to various salts was Na_2SO_4NaClMgSO_4MgCl_2,which was characteristic of electronegative nanofiltration membranes.These results showed that a high performance nanofiltration membrane with low operation pressure,high permeability and high rejection could be obtained by the present process.

Fabrication of ultrathin La0.6Sr0.4Co0.2Fe0.8O3– hollow fibre membranes for oxygen permeation

An ultrathin La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) hollow fibre membrane for enhanced oxygen permeation flux was fabricated using a wet spinning/sintering method. The membrane exhibits a highly asymmetric structure comprising of a very thin dense outer layer supported by finger-like structures that are fully open on the inner surface. Oxygen permeation measurements were conducted using sweep gas as an operating mode. Effects of operating temperatures and flow rates of the sweep gas on the oxygen permeation fluxes were investigated in details. The highest oxygen permeation flux, i.e. 0.096 cm3/cm2 s (5.77 cm3/cm2 min) was obtained from the ultrathin hollow fibre membrane at 1323 K (1050 °C) and the sweep gas flow rate of 2.42 cm3/s. The results indicate that the oxygen permeation flux obtained is much higher (4.9–11.2 times) than that obtained from conventional LSCF hollow fibre membranes mainly due to the reduced thickness of the membrane as well as the porous surface on the permeate side. In addition, despite a very thin dense layer, the LSCF hollow fibre membrane possessed a reasonable mechanical strength (113.22 MPa).

Deconvolution of the Potential and Time Dependence of Electrochemical Porous Semiconductor Formation

A layer of porous InP is grown beneath a thin dense surface layer when n-InP electrodes are anodised to sufficiently high potentials in aqueous KOH solutions. The shape of the linear sweep (LSV) or the cyclic voltammogram (CV) is dependent on carrier concentration. A technique is presented to deconvolute the effects of potential and time on a CV. The results obtained from this technique are used to explain the shape of the anodic current response and it relation to porous layer formation. The accuracy of the deconvolution technique is then tested by comparison to experimental results.

Carbon dioxide removal from anaesthetic gas circuits using hollow fiber membrane contactors with amino acid salt solutions

A novel technology based on the use of hollow fiber membrane contactors with regenerable liquid absorbents is proposed for carbon dioxide removal from anaesthetic closed breathing circuits. To analyse the performance of the contactor for this specific application and the influence of the system parameters, a 2D numerical model was developed for the transport of CO 2 through the hollow fibers. The model considered potassium glycinate solutions as absorbents and a composite membrane made of a porous support layer and a dense thin layer. Both co- and counter-current operations were studied. The model simulation results were compared to those obtained with conventional mass transfer models, valid for limit conditions, and a good agreement was found. The analysis performed indicates that the use of hollow fiber membrane contactors with amino acid salt absorbent solutions is suitable for CO 2 removal from closed anaesthetic circuits. Contactor design and operating conditions are suggested.

Preparation and characterization of carbon molecular sieve membranes for gas separation—the effect of incorporated multi-wall carbon nanotubes

A new class of multi-wall carbon nanotubes (MWCNTs)/carbon nanocomposite thin films was successfully prepared by incorporating (MWCNTs) into polyimide (PI) precursor solution. The carbon films were obtained in only one coating step by spin-coating technique on a macroporous alumina substrate and were pyrolyzed at 773 K. The structure and single gas permeation properties of PI-based carbon membranes incorporated with or without MWCNTs were investigated. From the FE-SEM images, the pure carbon membranes have a thin dense permselective layer with few closed pores while the permselective layer of the MWCNTs/carbon nanocomposite thin films are porous due to the structure of nanotubes. The MWCNTs/carbon nanocomposite thin films exhibited that the ideal carbon dioxide flux was 866.6 Barrer (1 Barrer = 1×10 −10 cm 3 (STP) cm cm −2 s −1 cm Hg −1) and the separation factor of CO 2/N 2 was 4.1 at room temperature and 1 atm, which was 2–4 times of magnitude higher than that of pure carbon membrane prepared by the same procedures and conditions.

A Comparative Study of the Performance of Symmetric and Asymmetric Mixed-conducting Membranes

According to the configuration, mixed-conducting membranes are classified as symmetric membranes and asymmetric membranes consisting of a thin dense layer and a porous support. In this study, these two kinds of SrCo 0.4Fe 0.5Zr 0.1O 3-δ oxide-based membranes were systematically compared in terms of oxygen permeability and chemical stability, and their differences were elucidated by means of the theoretical calculation. For the oxygen permeability, the asymmetric membrane was greater than the symmetric membrane due to the significant decrease of bulk diffusion resistance in the thin dense layer of the asymmetric membrane. In regard to the chemical stability, the increase of oxygen partial pressure on the asymmetric membrane surface at CH 4 side produced the stable time of over 1032 h in partial oxidation of methane at 1123 K, while the symmetric membrane was only of 528 h. This study demonstrated that the asymmetric membrane was a promising geometrical configuration for the practical application.

Development of an Ionic‐Conductive Skeletal Structure for an Electrode‐Supported Solid Oxide Fuel Cell

A novel ionic‐conductive skeletal structure was developed for an electrode‐supported solid oxide fuel cell (SOFC). The skeletal structure was formed as an ionic‐conductive frame that accumulated two thick porous layers and one dense thin layer. Two types of green sheets were used to form the frame. Thin green sheets (thickness: 15–30 μm) for the dense layers were fabricated with a well‐dispersed Gd0.2Ce0.8Ox (GDC) slurry and thick sheets (thickness: 150–300 μm) were fabricated with a GDC/pore‐former slurry by the tape casting method. Sintering laminated sheets at 1450°C for 8 h in air resulted in an ionic‐conductive frame that was suitable for electrode‐supported SOFC. The relative density of the inner thin layer, which works as an electrolyte, was higher than 98%, and the outer thick layers remained porous after the sintering. The porous structure of the outer layers was controlled using agglomerated slurries instead of well‐dispersed slurries. Electrochemical measurements of the ionic‐conductive frame attached with platinum electrodes exhibited potential as high‐performance cells.

SEPARATION OF PROPANE FROM PROPANE/NITROGEN MIXTURES USING PDMS COMPOSITE MEMBRANES BY VAPOUR PERMEATION

This study deals with polydimethylsiloxane (PDMS)/polyvinylidene fluoride (PVDF) composite membranes for propane separation from propane/nitrogen mixtures, which is relevant to the recovery of propane in petroleum and chemical industry. The surface and cross-section morphology of PDMS/PVDF composite membranes was observed by scanning electron microscope (SEM). The surface morphology of PDMS/PVDF composite membranes is very dense. There are three layers, the thin dense top layer, finger-like porous middle layer and sponge-like under layer in the cross-section SEM image of PDMS/PVDF composite membranes. The effects of the types of cross-linking agents and pressure on the membrane permselectivity were investigated. The permeability of nitrogen was independent of feed pressure. However, the permeability of propane increased with the pressure increasing for all membranes. The membrane cured by a tri-functional crosslinker with attached vinyl groups had better performance than the tetra-functional one, in both ...

Oxygen Permeation of a Dense/Porous Asymmetric Membrane Using La0.6Ca0.4CoO3−δ–BaFe0.975Zr0.025O3−δ System

Abstract To achieve a high oxygen permeation rate at medium and high temperature, an asymmetrically structured membrane, in which a thin dense layer (30 μm) made of La0.6Ca0.4CoO3−δ (LCC) and BaFe0.975Zr0.025O3−δ (BFZ) was formed on a porous LCC support, was fabricated and tested for its oxygen permeability. The oxygen permeation flux significantly increased by mixing BFZ with LCC and reached a fairly high value of 1.60 cm3 min−1 cm−2 even at 780 °C.

An asymmetric coating composed of gelatin and hydroxyapatite for the delivery of water insoluble drug

An asymmetric coating composed of gelatin and hydroxyapatite on Ti6Al4V alloy implant was prepared to control the release of water-insoluble drug ibuprofen and improve the surface properties of the implant. The asymmetric coating developed into a thin dense outer layer and a thick porous inner layer using a dip-coating method and a succedent phase-inversion process. The drug loading ranged from 10 to 30% (w/w), and depended on the immersion time and drug concentration in the quenching solution. The in vitro release from this system was always at an approximately zero-order rate and at least lasted for 30 days. The in vitro studies in SBF revealed that the coating could induce the formation of apatite, and was fully covered after 14 days soaking in SBF solution. This asymmetric coating had better bioactivity of inducing the formation of apatite in vitro, compared with pure gelatin coating and bare Ti6Al4V implant.

Highly asymmetric yttria stabilized zirconia hollow fibre membranes

Highly asymmetric yttria stabilized zirconia (YSZ) hollow fibre membranes were fabricated by a modified phase inversion and sintering technique using a mixture of NMP and water as internal coagulant. The obtained YSZ hollow fibres possess a thin dense outer skin layer of thickness 3–5 μm and a thicker porous sub-layer with a surface porosity of 28.8%. The crystal phase of the YSZ hollow fibres retains the same cubic fluorite structure as the original YSZ powders after the spinning and sintering process. The thin dense skin layer is integrated perfectly with the porous layer and is especially suitable for use as an electrolyte film in micro-tubular solid oxide fuel cells (MT-SOFCs).

Shaping of advanced asymmetric structures of proton conducting ceramic materials for SOFC and membrane-based process applications

A versatile fabrication process combining three techniques easy to scale up was successfully developed to prepare advanced multilayered structures integrating proton conducting ceramic materials. This enables the production of novel solid oxide fuel cells with proton conducting electrolytes (PCFCs) and asymmetric membranes designed as thin dense layers coated on thick porous mechanical supports. The manufacturing process makes use of ceramic powders synthesized by spray-pyrolysis using water-based solutions. Porous supports of membranes and anodes of PCFCs were prepared by tape-casting and their porosity was tuned by addition of corn starch filler. Membrane and electrolyte layers were prepared by spin-coating colloidal ceramic suspensions. A single coating-sintering step was carried out to produce gas tight layers with thickness ranging from 4 to 26 μm. Cathodes of PCFCs were screen-printed using commercial Pt ink. This paper describes the successful preparation of SrCe 0.95Yb 0.05O 3− δ asymmetric membranes and novel concepts of PCFCs integrating BaZr 0.9Y 0.1O 3− δ and La 0.995Sr 0.005NbO 4+ δ electrolytes.

Preparation of electrolyte membranes for micro tubular solid oxide fuel cells

Yttria-stabilized zirconia (YSZ) micro tubular electrolyte membranes for solid oxide fuel cells (SOFCs) were prepared via the combined wet phase inversion and sintering technique. The as-derived YSZ micro tubes consist of a thin dense skin layer and a thick porous layer that can serve as the electrode of fuel cells. The dense and the porous electrolyte layers have the thickness of 3–5 μm and 70–90 μm, respectively, while the inner surface porosity of the porous layer is higher than 28.1%. The two layers are perfectly integrated together to preclude the crack or flake of electrolyte film from the electrode. The presented method possesses distinct advantages such as technological simplicity, low cost and high reliability, and thus provides a new route for the preparation of micro tubular SOFCs.

Single-step fabrication of asymmetric dual-phase composite membranes for oxygen separation

Asymmetric dual-phase composite membranes for oxygen separation were conveniently fabricated by an acid leaching technique. A thin dense layer of Ce 0.85Sm 0.15O 1.925/Sm 0.6Sr 0.4FeO 3− δ was left by controlling the degree of acid leaching, and a porous substrate of Ce 0.85Sm 0.15O 1.925 with a fluorite structure was formed after dissolution of Sm 0.6Sr 0.4FeO 3− δ with a perovskite structure in HCl. Thus, a thin dense layer and a porous substrate can be fabricated in a single step in which traditional shrinkage mismatch and chemical reaction between thin dense layers and porous substrates can be avoided. The thickness of the dense layer can be controlled by varying the acid leaching time. Hence, dual-phase composite membranes with high oxygen flux can be obtained.

Experimental and modeling study of oxygen permeation modes for asymmetric mixed-conducting membranes

An asymmetric mixed-conducting membrane consists of a thin dense layer and a porous support, and its application has drawn considerable attention, because it is expected to have a more promising potential in the practical application compared with the symmetric membrane. However, with the introduction of support in the asymmetric membrane, two possible permeation modes are produced. One mode is that oxygen permeates from the support to the thin dense layer (designated as SD mode). The other is in the direction from the thin dense layer to the support (designated as DS mode). Thus, from the viewpoint of choosing an appropriate oxygen permeation mode to make better use of the membrane, it is necessary to study the oxygen flux in these two modes. In this paper, their effects on the oxygen flux of asymmetric membranes were investigated from the experiment and the model. The modeling results showed a good agreement with the experimental data. Our study demonstrates that when the asymmetric membrane adopts the SD mode, it is beneficial for the membrane to obtain higher oxygen permeation flux.

Asymmetric porous thin film preparation by controlled solvent absorption using PDMS

We present a simple and yet effective method of producing asymmetric polymeric membranes. All it takes to prepare the membrane is to place a flat slab, which can absorb the solvent readily, on a polymer solution. The rapid absorption of the solvent by the slab leads to the formation of a thin dense skin layer and the presence of this top dense layer allows for the phase separation in the polymer solution to proceed beneath the layer. The solubility of a polymer in a solvent, as affected by solubility parameter and polymer molecular weight, determines the morphology of the membrane prepared by this controlled solvent absorption method. The preparation conditions can be tailored to produce “double-skinned” membranes and membranes with localized pore structure.

Preparation of nanocrystalline titania films with different porosity by water-based chemical solution deposition

This study describes the synthesis of nanocrystalline titania layers on silicon and glass substrates by chemical solution deposition, using a water-based citratoperoxo-Ti(IV) precursor solution. The same aqueous solution–gel precursor is used for deposition of, both, thin dense layers by spin-coating and thicker porous layers by tape-casting. In the latter, the precursor solution is modified by the addition of polyvinyl alcohol (PVA), which acts as a thickener and pore-forming agent. Phase composition, film morphology, and the hydrophilic character of the films are studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–VIS transmission measurements, variable angle spectroscopic ellipsometry (VASE), and by contact angle measurements. The thin 180 nm titania film, deposited from the unmodified precursor solution, shows a density of about 96%. Upon ultraviolet illumination, it acquires a highly hydrophilic surface. One hour of illumination is sufficient to obtain a water contact angle of almost 0°. Furthermore, the hydrophilisation process shows to be reversible. Tape-casting and thermal treatment of the modified precursor solution gives rise to the formation of a 500 nm thick, porous, pure anatase film. The nanocrystalline thick film is composed of 20–40 nm particles, and contains clearly defined pores of 20 nm, homogeneously distributed along the surface.

Electrochemical performance of LSCF based thin film cathodes prepared by spray pyrolysis

La 0.6Sr 0.4Co 0.2Fe 0.8O 3 (LSCF) based thin film cathodes were fabricated by spray pyrolysis and their performance was evaluated with area specific resistance (ASR). With a maximum processing temperature of 650 °C, these cathodes are suitable for application in micro-solid oxide fuel cells (μ-SOFC). The ∼ 500 nm thick cathodes showed performance similar to traditional thick film LSCF cathodes (10–100 μm). However, by using a new material composition Ba 0.25La 0.25Sr 0.5Co 0.8Fe 0.2O 3 − δ (BLSCF) or by modifying the microstructure, a significant improvement in performance was achieved. Reducing the grain size or introducing a thin dense cathode layer between the porous cathode and the electrolyte were beneficial modifications of the microstructure.

Topography on the D′′ region from analysis of a thin dense layer beneath a convecting cell

In this study, we examine the development of topography on a thin dense layer at the base of the lower mantle. The effect of the convecting mantle above is represented as a traction acting on the upper surface of the layer. Topography on the layer boundaries is predicted by a balance of dynamic flow stress and external traction. The nature of boundary topography depends on the magnitude of the driving tractions and the density variation within the layer. If we assume that the layer density is greatest beneath areas of mantle downwelling and decreases to a minimum beneath areas of mantle upwelling (the layer is thermally coupled to the convection in the overlying mantle) then its upper boundary develops a cusp-like peak beneath the upwelling mantle. The height of this peak is potentially much greater than the layer thickness. If, however, the layers are effectively coupled by viscous shear then internal density gradients of the opposite sign may be established. In this case, we observe solutions where the layer is completely swept away beneath areas of mantle downwelling leaving steep-sided ‘islands’ of dense material. This mechanism therefore provides a possible explanation for steep-sided anomalously slow regions at the base of the mantle observed by seismic methods (e.g. beneath south Africa) or for discrete ultralow velocity zones detected at the core–mantle boundary beneath locations of surface hotspots. The magnitude of the upper boundary driving tractions compared to the density gradient within the layer is the key parameter that determines the nature of flow in, and consequently boundary topography of, the layer. The deflection of the core–mantle boundary is small compared with that of the top of the dense layer, but a change in sign of the ratio of these deflections is observed as the magnitude of the driving tractions changes relative to the magnitude of the internal density gradient. We compare seismic measurements of core–mantle boundary topography and D′′ topography with the predictions of this model in an attempt to constrain model parameters, but no clear correlation seems to exist between D′′ thickness and CMB topography.