Thin Porous Substrates Research Articles

Thin film composite membrane with improved permeance for reverse osmosis and organic solvent reverse osmosis

Reverse osmosis (RO) is a widely used pressure-driven membrane process for desalination. Advancements in membrane materials have driven RO technologies' development and widespread application. Thin-film composite (TFC) membranes of a thin polyamide (PA) layer, porous substrate, and nonwoven fabric support are considered the gold standard in desalination technology. These membranes exhibited excellent separation performance and mechanical strength. Recently, TFC membranes with PA layers have been successfully employed in organic solvent reverse osmosis (OSRO) to differentiate organic liquids molecularly. Developing TFC membranes with improved solvent resistance and permeance is required to expand the applications of TFC membranes beyond solvent-solute systems. This study explored polyketone (PK) membranes as porous substrates in TFC membranes for general reverse osmosis processes. PK membranes offer excellent solvent resistance and are easier to prepare than other membranes. Furthermore, a surface modification approach was proposed to enhance the interfacial polymerization (IP) of the polyamide layer on both PK and polysulfone (PSf) substrates, resulting in improved TFC membranes with enhanced water permeance for RO and increased organic liquid permeance for OSRO while maintaining permselectivity. These findings highlight surface chemistry's significance in controlling crumpled PA layers' formation, ultimately improving the filtration efficiency for general RO applications. This study provides an example of the design of composite structures of TFC membranes for broader RO applications.

Thin Reinforced Ion-Exchange Membranes Containing Fluorine Moiety for All-Vanadium Redox Flow Battery.

In this work, we developed pore-filled ion-exchange membranes (PFIEMs) fabricated for the application to an all-vanadium redox flow battery (VRFB) by filling a hydrocarbon-based ionomer containing a fluorine moiety into the pores of a porous polyethylene (PE) substrate having excellent physical and chemical stabilities. The prepared PFIEMs were shown to possess superior tensile strength (i.e., 136.6 MPa for anion-exchange membrane; 129.9 MPa for cation-exchange membrane) and lower electrical resistance compared with commercial membranes by employing a thin porous PE substrate as a reinforcing material. In addition, by introducing a fluorine moiety into the filling ionomer along with the use of the porous PE substrate, the oxidation stability of the PFIEMs could be greatly improved, and the permeability of vanadium ions could also be significantly reduced. As a result of the evaluation of the charge–discharge performance in the VRFB, it was revealed that the higher the fluorine content in the PFIEMs was, the higher the current efficiency was. Moreover, the voltage efficiency of the PFIEMs was shown to be higher than those of the commercial membranes due to the lower electrical resistance. Consequently, both of the pore-filled anion- and cation-exchange membranes showed superior charge–discharge performances in the VRFB compared with those of hydrocarbon-based commercial membranes.

Pore-Filled Proton-Exchange Membranes with Fluorinated Moiety for Fuel Cell Application

Proton-exchange membrane fuel cells (PEMFCs) are the heart of promising hydrogen-fueled electric vehicles, and should lower their price and further improve durability. Therefore, it is necessary to enhance the performances of the proton-exchange membrane (PEM), which is a key component of a PEMFC. In this study, novel pore-filled proton-exchange membranes (PFPEMs) were developed, in which a partially fluorinated ionomer with high cross-linking density is combined with a porous polytetrafluoroethylene (PTFE) substrate. By using a thin and tough porous PTFE substrate film, it was possible to easily fabricate a composite membrane possessing sufficient physical strength and low mass transfer resistance. Therefore, it was expected that the manufacturing method would be simple and suitable for a continuous process, thereby significantly reducing the membrane price. In addition, by using a tri-functional cross-linker, the cross-linking density was increased. The oxidation stability was greatly enhanced by introducing a fluorine moiety into the polymer backbone, and the compatibility with the perfluorinated ionomer binder was also improved. The prepared PFPEMs showed stable PEMFC performance (as maximum power density) equivalent to 72% of Nafion 212. It is noted that the conductivity of the PFPEMs corresponds to 58–63% of that of Nafion 212. Thus, it is expected that a higher fuel cell performance could be achieved when the membrane resistance is further lowered.

Pore-Filled Ion-Exchange Membranes with Optimal Cross-Linking Degrees for Efficient Membrane Capacitive Deionization

In this work, we investigated the optimal design parameters of pore-filled ion-exchange membranes (PFIEMs) for the successful to a membrane capacitive deionization (MCDI) process. The PFIEMs were fabricated by employing a cheap and thin polyethylene-based porous substrate and filling ionomers based on the cross-linked polystyrene. In particular, the cross-linking degrees of the PFIEMs were controlled by adjusting the cross-linker (i.e., divinylbenzene) contents in the monomer mixture. The PFIEMs exhibited much lower electrical resistance compared to those of commercial membranes because of the largely reduced film thickness and the high ion-exchange capacities. The results also revealed that the cross-linking degree significantly affected the membrane characteristics by influencing the free volume and the density of fixed charge groups. In addition, the membrane permselectivity dominated by the cross-linking degree was shown to be the most significant parameter in determining the salt removal efficiency (η) of MCDI. Consequently, the PFIEMs with the optimal cross-linking conditions were able to the achieve excellent desalting efficiency (η = 88.5%) of the MCDI, which was superior to that of expensive commercial membranes (η = 78.7%).

Preparation of Anion Conducting Pore Filling Membranes for Direct Hydrazine Fuel Cells

Hydrazine fuel cells in an alkaline medium are a promising energy conversion technology due to the extraordinary properties of hydrazine as a hydrogen carrier. Alkaline electrolyte fuel cells allow the use of less expensive metals such as cobalt and nickel as an electrode catalyst. The devices consist of a pair of electrochemical reactions at electrodes and anion movement through electrolytes to complete full electrochemical cells. Anions move from cathode to anode or vice versa for electron exchanges. Pathway for ions is normally in an aqueous phase. Regardless, the introduction of any aqueous phase in energy conversion devices causes many problems during fabrication, operation, maintenance, and so on. Thus, much efforts have been devoted to develop quasi-solid electrolytes such as gel polymer, ion exchangeable polymers, impregnation of ions in porous matrix and so on. An approach to minimize areal resistance of film-type polymeric electrolytes is to make membrane thickness as thin as possible. In this study, pore filling membranes have been studied as a new type of electrolytes. Pore filling membranes are made by impregnating monomers into thin porous substrates, followed by heat or UV cured polymerization. Pore filling membranes with the thickness of 20-50 μm were prepared using porous substrates with hydrophilized surface. A technique to make the surface of pore filling membranes hydrophilic were developed so as to adsorb liquid hydrated hydrazine fuel in KOH supporting electrolyte. Characterization in terms of ion conductivity, ion exchange capacity, water uptake, dimensional stability, mechanical strength was carried out and discussed. Acknowledgment This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT ＆ Future Planning(NRF-2018M1A2A2063866).

Drying of Foam under Microgravity Conditions

Foams have recently been characterised as ideal products for pharmaceutical and topical use applications for the delivery of topical active agents. Foams are usually produced in a wet form but in a number of applications moderately dry foams are required. Drying of foam under terrestrial conditions proceeds under the action of gravity, which is impossible under microgravity condition. Below a new method of drying foams under microgravity condition is suggested. According to this method foam should be placed on a porous support, which will absorb the liquid from foam based on capillary forces only. The final liquid content inside the foam can be achieved by a proper selection of the porous support. The suggested method allows drying foams under microgravity conditions. Interaction of foams with porous support under terrestrial conditions was developed only recently and theoretically investigated (Arjmandi-Tash, O.; Kovalchuk, N.; Trybala, A.; Starov, V. Foam Drainage Placed on a Porous Substrate. Soft Matter2015, 11 (18), 3643–3652) followed by a theory of foam drainage on thin porous substrates (Koursari, N.; Arjmandi-Tash, O.; Johnson, P.; Trybala, A.; Starov, M. V. Foam Drainage Placed on Thin Porous Substrate. Soft Matter, 2019, (submitted)), where rate of drainage, radius of the wetted area inside the porous layer and other characteristics of the process were predicted. The latter model is modified below to investigate foam drying under microgravity conditions. Model predictions are compared with experimental observations for foam created using Triton X-100 at concentrations above CMC. Wetted radius inside the porous substrate was measured and results were compered to model predictions. Experimental observations for spreading area versus time show reasonable agreement with theoretical predictions for all investigated systems.

Ultra-Thin Composite Electrolytes for Energy Conversion Devices

Most of electrochemically driven energy conversion devices consist of anodic reaction, ion movement and cathodic reaction to complete full electrochemical cells. Ions move from anode to cathode or vice versa for electron exchanges. Pathway for ions is normally in an aqueous phase. Regardless, the introduction of any aqueous phase in energy conversion devices causes many problems during fabrication, operation, maintenance, and so on. Thus, much efforts have been devoted to develop quasi-solid electrolytes such as gel polymer, ion exchangeable polymers, impregnation of ions in porous matrix and so on. One of major disadvantages to use polymeric electrolytes is significant Ohmic voltage decay due to high areal resistance compared to aqueous phases. An approach to minimize areal resistance of film-type polymeric electrolytes is to make film thickness as thin as possible. Free standing polymeric electrolytes, however, show low mechanical properties. Thus, composite membranes could be used by impregnating polymeric electrolytes into thin porous substrates. In this study, three-layered (electrolyte-skeleton/electrolyte-electrolyte) composite membranes with the thickness of 12-18 μm were prepared. A technique to impregnate polymeric electrolytes into hydrophilic or hydrophobic porous substrates were intensively developed so as to prepare void-free composite membranes. Characterization in terms of ion conductivity, ion exchange capacity, water uptake, dimensional stability, mechanical strength was carried out.

Thin pore-filling membrane with highly packed-acid structure for high temperature and low humidity operating polymer electrolyte fuel cells

For the next generation polymer electrolyte fuel cells (PEFCs), membrane electrode assemblies (MEAs) should be able to operate at high temperatures and low relative humidity (RH). Under such conditions, cell performance drastically decreases owing to the high resistance of proton conductivity in the proton exchange membrane (PEM). It is thus challenging to design a PEM that exhibits high proton conductivity at high temperature and low RH. In this study we propose a pore-filling membrane, composed of a low equivalent weight (EW) perfluorosulfonic acid (PFSA) polymer and a mechanically strong thin ultra-high molecular weight polyethylene porous substrate. This pore-filling membrane suppresses the swelling of the low EW PFSA polymer and exhibits high proton conductivity. Inside the pores of the substrate, a high-density structure of sulfonic acid groups was achieved, which is advantageous for high proton conductivity at low humidity conditions. The MEA with the pore-filling membrane shows high fuel cell performance at high temperatures and low RH; the maximum power density was around 1000 mW cm−2 at 100 °C and 30% RH. Although the thickness of this membrane was around one-fourth of that with Nafion 211 (7 μm), they have comparable hydrogen crossover current density owing to the effective swelling suppression.

Imbibition with swelling: Capillary rise in thin deformable porous media

The imbibition of a liquid into a thin deformable porous substrate driven by capillary suction is considered. The substrate is initially dry and has uniform porosity and thickness. Two-phase flow theory is used to describe how the liquid flows through the pore space behind the wetting front when out-of-plane deformation of the solid matrix is considered. Neglecting gravity and evaporation, standard shallow-layer scalings are used to construct a reduced model of the dynamics. The model predicts convergence to a self-similar behavior in all regions except near the wetting front, where a boundary layer arises whose structure narrows with the advance of the front. Over time, the rise height approaches the similarity scaling of t^{1/2}, as in the classical Washburn or BCLW law. The results are compared with a series of laboratory experiments using cellulose paper sheets, which provide qualitative agreement.

Rapid evaporation-driven chemical pre-concentration and separation on paper

Airflow-enhanced evaporation is investigated as a method for rapid chemical preconcentration on a thin porous substrate. The mechanism is described by combining 1D models of capillary rise, chromatography, and pervaporation concentration. It is shown that the effective length of the column can be shorter than its actual length, allowing concentrate to be held at a stagnation point and then released for separation, and that the Péclet number, which determines the concentration performance, is determined only by the substrate properties. The differential equations are solved dynamically, and it is shown that faster concentration can be achieved during capillary filling. Experiments are carried out using chromatography paper in a ducted airflow, and concentration is quantified by optical imaging of water-soluble food dyes. Good agreement with the model is obtained, and concentration factors of ≈100 are achieved in 10 min using Brilliant Blue FCF. Partial separation of Brilliant Blue from Tartrazine is demonstrated immediately following concentration, on a single unpatterned substrate. The mechanism may provide a method for improving the sensitivity of lab-on-paper devices.

Absorption of picoliter droplets by thin porous substrates

Absorption of picoliter (pL) droplets into porous substrates is studied experimentally and numerically. In the case of pL droplets, major phenomena involved in the interaction between droplet and porous media develop at different time scales: spreading and wetting at microseconds, absorption and wicking at milliseconds, and evaporation at seconds. Therefore, one can decouple these processes to minimize the complexity of the study. A high‐speed imaging system capable of 1 million frames per second is used to visualize individual droplets impacting, spreading, and imbibing on substrates. To simulate droplet dynamics, the governing equations for flow outside and inside porous media are proposed and solved using an in‐house developed computational fluid dynamics solver. The simulation results are in good agreement with the experimental data. The effect of drop impact velocity and fluid properties on final dot shape in the porous substrates is investigated through a series of parametric numerical studies. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1690–1703, 2017

(Invited) Development of Reverse Electrodialysis System

Salinity gradient power (SGP), also called blue energy, is a newly emerging form of marine renewable energy. SGP is the energy available from the salinity difference between seawater and river water, which respectively have high and low salt concentrations. The SGP energy potential between typical seawater and river water is 231 m (~23 atm) of hydraulic water head. The global energy potential is estimated to be 2.4–2.6 TW, which is ~80% of the global electricity demand. Among membrane-based blue energy technologies, reverse electrodialysis (RED), the opposite process of electrodialysis (ED), has been applied practically. Recently, RED has been considered the attractive technology because this new process has huge potential and possibility to generate energy from abundant but largely unused resources. RED uses a stack of alternating cation (CEM) and anion (AEM) exchange membranes. For RED to find greater use, a high power density (power generated per membrane area) is essential. By increasing the power density, the membrane demand and RED stack size can be decreased. To make RED an economically attractive technology, the large scale RED stack should be developed. Recently, we have developed the kW class RED stack and evaluated the performances for the first time in Korea. And the effect of operation conditions on power density was confirmed. Furthermore, a numerical simulation has been conducted for investigating the flow distribution and pressure loss in the stack. Since the performance of RED significantly depends on the inertial fluid characteristics and the required pumping power, it is a primary importance to lower pressure drop and uniform flow distribution in the stack. With respect to the size and the height of cell, and the inlet shape of cell, the flow distribution and the pressure loss have been evaluated. The ion exchange membranes (IEMs) also strongly influences the performance, specifically, the power density, of the RED stack. Although studies have, over the last decade, improved the IEM characteristics, they do not yet match the expected characteristics for RED. Generally, IEMs for ion separation processes require high strength and long lifetime regardless of the electrical resistance and thickness; however, IEMs for RED require low electrical resistance and high permselectivity. Nevertheless, most present IEMs for RED still do not possess both the thinness and the mechanical strength required for higher power density of the RED stack. To solve this problem, we have developed pore-filling-type IEMs for RED. A pore-filling membrane consists of a very thin porous substrate to provide mechanical strength and an electrolyte polymer in the substrate pores to allow ion conductivity. Meanwhile, electrolyte-polymer-filled pores should have high ion exchange capacity (IEC) to increase the ion conductivity. The thickness of the prepared pore filling membranes was controlled between 20 and 25 micrometers to extremely lower membrane resistance and to increase ion transfer. The prepared ion exchange pore filling membranes were compared with commercial membranes. We obtained the highest power density comparing to the result reported in elsewhere.

Propagation of viscous currents on a porous substrate with finite capillary entry pressure

We study the propagation of viscous gravity currents over a thin porous substrate with finite capillary entry pressure. Near the origin, where the current is deep, propagation of the current coincides with leakage through the substrate. Near the nose of the current, where the current is thin and the fluid pressure is below the capillary entry pressure, drainage is absent. Consequently the flow can be characterised by the evolution of drainage and fluid fronts. We analyse this flow using numerical and analytical techniques combined with laboratory-scale experiments. At early times, we find that the position of both fronts evolve as $t^{1/2}$, similar to an axisymmetric gravity current on an impermeable substrate. At later times, the growing effect of drainage inhibits spreading, causing the drainage front to logarithmically approach a steady position. In contrast, the asymptotic propagation of the fluid front is quasi-self-similar, having identical structure to the solution of gravity currents on an impermeable substrate, only with slowly varying fluid flux. We benchmark these theoretical results with laboratory experiments that are consistent with our modelling assumption, but that also highlight the detailed dynamics of drainage inhibited by finite capillary pressure.

Pore-filled anion-exchange membranes for non-aqueous redox flow batteries with dual-metal-complex redox shuttles

Pore-filled anion-exchange membranes (PFAEMs) were synthesized using thin porous polymer substrates for non-aqueous redox flow battery (RFB) applications. The PFAEMs which have a thickness of about 25μm exhibited high ion conductivity and selectivity in both aqueous and non-aqueous media. In addition, they showed excellent mechanical properties as well as dimensional stability in a non-aqueous environment. The non-aqueous RFB experiments utilizing novel Fe/Ni dual-metal complex redox electrolyte resulted in the respectable energy efficiencies (i.e. 87.3% and 77.2% in non-flowing and flowing cells, respectively) much higher than those of previous papers where other non-aqueous redox electrolytes were used. The PFAEMs showed much better ion selectivity in non-aqueous medium compared to that of the commercial membrane due to the relatively low swellability. The results demonstrated that the charge–discharge performances were largely enhanced by the mitigated crossover of metal complex as well as the low mass transport resistance through the membrane.

Adjusting the Electronic and Optical Properties of Mesoporous MCM-41 Materials by Ti Doping

The objectives of this thesis are to understand the electronic structures of oxides and oxynitrides for photocatalytic water splitting, examine the Casimir interaction between oxides, and explore possible approach to bridge the Casimir force and material properties for advanced material research. The studies were performed in the framework of the density functional theory, many-body perturbation theory, i.e, the GW approximation and Bethe-Salpeter equation, as well as the Casimir-Lifshitz approach.The thesis consists of two sets of results. In the first part (papers I-VI), the electronic structures of oxynitrides, i.e., ZnO-GaN and ZnO-InN, with different compositions and local structures have been studied. The oxynitrides reduce the band-gap energies significantly compared to the binary counterparts, enabling the oxynitrides to act as visible light active photocatalysts. Formation of cluster--like structures further reduces the band-gap and delocalizes the valence bands, benefiting higher optical absorption. Furthermore, the energy levels between oxynitride and water were aligned using a surface model adapted from semiconductor heterostructure.In the second part (papers V-IX), the electronic structures of oxides as well as the Casimir interactions have been examined. In particular, we investigated the differences of optical and electronic properties between SnO2 and TiO2 polymorphs in terms of band-edge characters and electron-phonon coupling. In addition, we synthesized a mesoporous material possessing two types of pore structures (one is hexagonal ordered with pore diameter of 2.60 nm and the other is disordered with pore diameter of 3.85 nm). The pore framework contains four-coordinated titanium and oxygen vacancies, verified by both experimental measurements and density-functional theory calculations. Utilizing the predicted properties of the materials, we studied the Casimir interactions. A stable equilibrium of Casimir force is achieved in planar geometry containing a thin film and porous substrates. Both the force and equilibrium distance are tuned through modification of the material properties, for instance, optical properties and porosity. Furthermore, we adapted this concept to study the interactions between gas bubbles and porous SiO2 in water. A transition from repulsion to attraction is predicted, which highlights that the bubbles may interact differently at different surface regions.

Dynamic evolution of plasma facing surfaces in NSTX: Impact of impurities and substrate composition on fuel recycling

Abstract Understanding plasma/complex surfaces interactions in NSTX with liquid lithium on thin porous molybdenum substrate for the divertor, additional PFC surfaces such as carbon tiles, and the presence of gases and impurities require integrated implementation and the interplay of detail various physical processes. These include simultaneous and multiple ion species penetration and mixing, scattering, reflection, physical and chemical sputtering and/or compound formation, dynamic surface evolution/modification, thermal diffusion and segregation, and recombination/desorption of species in this multi-component surface composition. We enhanced our ITMC-DYN simulation package combining integrated models for studying the time-dependent dynamic surface evolution under impact of plasma particles to address all phenomena self-consistently. These phenomena include the effect of compounds formation and impurities, specific for NSTX design and conditions, the influence of Mo substrate porosity on Li erosion, mixing, and overall fuel recycling. Simulation results showed that formed lithium compounds, filling porous Mo, would reduce diffusion of deuterium into the bulk that will result in accumulation near the surface region. However, inclusion of a thin layer of impurity compounds on the surface would prevent hydrogen motion to surface and leads to a decrease in recombination and desorption rate.

Air Dehydration Membranes for Nonaqueous Lithium–Air Batteries

Robust, thin-sheet membranes were prepared and tested as an -permeating moisture barrier for the operation of nonaqueous Li–air batteries in ambient air. A membrane sheet was attached onto a carbon air electrode filled with a nonaqueous Li-ion electrolyte solution to supply dry oxygen from ambient air for the electrode reaction and to mitigate potential loss of the solvent from the electrode by evaporation. The membrane was prepared by depositing an -selective coating layer on the exterior surface of a thin porous metal substrate sheet. Silicalite zeolite and poly(tetrafluoroethylene) (PTFE) were used as inorganic and polymeric hydrophobic-type materials, respectively, for the membrane preparation in this work. A thin , dense-phase PTFE film supported on a porous metal sheet provided the best battery performance. This kind of membrane enabled the Li–air batteries with carbon black air electrodes to operate in ambient air [at 20% relative humidity (RH)] for 21 days. The specific capacity of 1022 mAh/g carbon and specific energy of 2792 Wh/kg carbon were obtained for the battery discharged to 2.0 V. The Li–air battery equipped with the selective membrane showed a much better performance in ambient air operation (20% RH) than the reference battery tested in a dry air box (1% RH).

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.

Synthesis and characterization of mixed ionic–electronic conducting Ca 0.8Sr 0.2Ti 0.7Fe 0.3O 3− α thin film

Thin dense films of Ca 0.8Sr 0.2Ti 0.7Fe 0.3O 3 − 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.

Preparation and characterization of nanostructured ZrO 2 coatings on dense and porous substrates

Nanostructured ZrO 2 coatings are prepared on both dense and porous substrates by wet-chemical deposition of non-agglomerated 5 nm precursor particle dispersions, followed by thermal processing. The precursor particle dispersions are made by modified emulsion precipitation and a purification treatment to remove reaction products and additives. The coatings are formed by depositing the precursor nanoparticle dispersion directly onto the substrate, followed by drying and heating at 600 °C. Scanning electron microscopy and cross-sectional transmission electron microscopy observations of the heat-treated coatings indicate that the ZrO 2 coating on dense Si wafer substrate has a homogeneous, dense particle packing structure with shallow meniscus-shaped depressions in the surface, and microcracks below the meniscus surface. On the other hand, coatings formed on a meso-porous γ-alumina membrane substrate are free of defects, but with a lower packing density. The mechanism of the substrate effect on the particle packing behavior and defect formation during coating deposition is discussed. It is expected that by using a thin porous substrate with reduced capillary force, a defect-free, homogenously dense-packed coating structure can be achieved.