Design and fabrication of whisker hybrid ceramic membranes with narrow pore size distribution and high permeability via co-sintering process

Ceramic microfiltration membranes (MF) with narrow pore size distribution and high permeability are widely used for the preparation of ceramic ultrafiltration membranes (UF) and in wastewater treatment. In this work, a whisker hybrid ceramic membrane (WHCM) consisting of a whisker layer and an alumina layer was designed to achieve high permeability and narrow pore size distribution based on the relative resistance obtained using the Hagen-Poiseuille and Darcy equations. The whisker layer was designed to prevent the penetration of alumina particles into the support and ensure a high porosity of the membrane, while the alumina layer provided a smooth surface and narrow pore size distribution. Mass transfer resistance is critical to reduce the effect of the membrane layers. It was found that the resistance of the WHCM depended largely on the alumina layer. The effect of the support and whisker layer on the resistance of the WHCM was negligible. This was consistent with theoretical calculations. The WHCM was co-sintered at 1000?°C, which resulted in a high permeability of ~?645?L?m^?1 h^?1 ;bar^?1 and a narrow pore size distribution of ~?100?nm. Co-sintering was carried out on a macroporous ceramic support (just needed one sintering process), which greatly reduced the preparation cost and time. The WHCM (as the sub-layer) also showed a great potential to be used for the fabrication of ceramic UF membranes with high repeatability. Hence, this study provides an efficient approach for the fabrication of advanced ceramic MF membranes on macroporous supports, allowing for rapid prototyping with scale-up capability.     

Gas permeation and thermomechanical properties for macroporous alumina focused on necking size at grain boundaries

The gas permeation and thermomechanical properties of macroporous alumina used as a support substrate for microporous ceramic permselective membranes were investigated. The porosity, pore size, and apparent necking size between grains of macroporous alumina were systematically varied, and the relationships between the porous microstructure and material properties were examined. The grain necking size at alumina grain boundaries was evaluated by microstructural observations. The nitrogen gas permeance of the porous alumina increased with increasing pore size. All the measured thermal and mechanical properties decreased with increasing porosity. The properties of porous alumina samples with extensive grain necking showed higher values even in samples with the largest pore size. The high thermal conductivity of porous alumina with extensive grain necking was due to the low interfacial thermal resistance at grain boundaries. Porous alumina with extensive grain necking had high thermal shock strength due to the higher thermal conductivity. It was demonstrated that a porous structure combining high gas permeability and excellent fracture resistance could be successfully achieved.     

致密超细球形氧化铝制备性能良好的陶瓷超滤膜

热等离子体制备的超细球形氧化铝具有表面致密光滑、分散性好等特点,本工作以超细球形氧化铝为原料,通过浸渍提拉烧结法,制备了孔径分布窄、渗透通量高的陶瓷超滤膜,研究了烧结温度对陶瓷膜微孔结构的演化、孔径分布和渗透通量的影响。随后对1250℃下烧结的陶瓷膜进行了纳米硅水分散液过滤处理,采用不同堵塞模型分析了陶瓷膜过滤纳米硅水分散液的膜污染过程。结果表明,通过调节烧结温度调控陶瓷膜的微孔结构,当烧结温度为1250℃时,陶瓷膜的孔径分布较窄,孔径大小为25?65 nm,渗透通量为986.4 L/(m2?h)。超细球形氧化铝粒径分布较窄及表面致密光滑有助于1250℃下烧结形成均匀的烧结颈,提供了陶瓷膜较窄的孔径分布。对1250℃下烧结的陶瓷膜进行了纳米硅水分散液过滤处理后其浊度下降为0.231 NTU,浊度去除率达99.96%。采用不同堵塞模型分析了陶瓷膜过滤纳米硅水分散液的膜污染过程,结果表明,纳米硅水分散液的堵塞模型是滤饼过滤,属于可逆污染。     

The membrane properties of alumina-coated alumina support layers and alumina-coated diatomite–kaolin composite support layers

ABSTRACT

Porous ceramic membranes are a current research focus because of their outstanding thermal and chemical stability. Recent research has utilised inexpensive natural materials such as diatomite to reduce the expense of these porous ceramic membranes. However, insufficient data exist for microfiltration applications using the diatomite-based membranes. The measured membrane properties of alumina-coated alumina support layers and alumina-coated diatomite–kaolin composite support layers have been compared. These experiments have been used to determine whether the average pore size could be reduced effectively by controlling the thickness of the alumina coating layer, while maintaining acceptable water permeability. The membrane properties of the alumina-coated alumina support layers and the alumina-coated diatomite–kaolin composite support layers were examined using the scanning electron microscopy, mercury porosimetry, and a dead-end microfiltration system.     

Preparation and characterisation of alumina-based composite support layers

Porous ceramic membranes are of special interest owing to their outstanding thermal and chemical stability. However, porous ceramic membranes with permeability usually suffer from low mechanical strength. Therefore, there have been a number of studies of the optimisation of membrane mechanical strength and permeability. In this paper, to avoid a trade-off between mechanical strength and permeability, we attempt to enhance these parameters by incorporating diatomite as both a pore former and a bonding phase. Because the flexural strength and air permeability of alumina support layers cannot be enhanced simultaneously by just changing the sintering temperature, we investigate whether they can be controlled by changing the amount of added diatomite. We study the effectiveness of diatomite as both a pore former and a bonding phase through a comparison of alumina–diatomite and the alumina–pyrophyllite composite support layers.     

Three-dimensional printing of high-flux ceramic membranes with an asymmetric structure via digital light processing

In this study, a novel method was proposed for preparing high-flux ceramic membranes via digital light processing (DLP) three-dimensional (3D) printing technology. Two different alumina powders were well dispersed in a photosensitive resin to form a UV-curable slurry for DLP 3D printing. The effects of the grading ratio on the viscosity of the slurry and the porosity, pore size distribution, mechanical strength, roughness, and permeability of the ceramic membranes were systematically investigated. The thermal treatment conditions were also studied and optimized. The obtained ceramic membranes exhibited a uniform pore size distribution, a high porosity, a low tortuosity factor, and an asymmetric structure. The combination of these factors led to a high flux for the 3D-printed ceramic membranes. DLP 3D printing exhibited a good potential to be a strong candidate for the next generation of ceramic membrane fabrication technology.     

Gel Casting of Free‐Shapeable Ceramic Membranes with Adjustable Pore Size for Ultra‐ and Microfiltration

The growing demand of reliable high‐performance membrane materials for separation processes requires new simple, straightforward, environmental friendly, sustainable approaches for membrane fabrication. In this study, we present an environmentally friendly gel‐casting, one‐pot process based on ionotropic‐gelation for obtaining alumina membranes. A slurry of alumina particles and the biopolymer alginate, which acts in combination with calcium iodate like a resin, was gelled in a controllable temperature dependent manner. Alumina membranes are obtained by three different shaping routes (extrusion, free‐forming, casting). The suitability of extruded capillaries in a polymer‐ceramic hybrid state (green body) and after sintering (1150°C for 2 h) for potential application in micro‐ and ultrafiltration is evaluated by monitoring the chemical and mechanical stability, permeability and separation behavior. Varying the initial alumina particle size from 200 to 900 nm, membranes with a narrow pore size distribution, predictable and tunable average pore diameters from 70 up to 480 nm and a constant open porosity of ~40%, are obtained. The permeability behavior is tested with fluorescence labeled submicron‐ and nano‐particles. Our novel colloidal processing route represents a very versatile tool for designing and manufacturing ceramic membranes with complex shapes for micro‐ (>0.1 μm) and ultrafiltration (0.1–0.01 μm).     

Fabrication of ice-templated tubes by rotational freezing: Microstructure,strength, and permeability

We demonstrate a facile and scalable technique, rotational freezing, to produce porous tubular ceramic supports with radially aligned porosity. The method is based on a conventional ice-templating process in a rotatory mold and demonstrated here with yttria-stabilized zirconia (YSZ). We investigated the effects of solid loading, freezing temperature, and volume of the slurry on the microstructure, strength (o-ring test and four-point bending), and air permeability. The results show that pore volume and pore size can be controlled by the solid loading and freezing temperature respectively, and overall tube thickness can be adjusted by the volume of slurry initially poured into the mold. Decreasing pore size and pore volume increases the mechanical properties but decreases the air permeability. These tubes could be particularly interesting as tubular membrane supports such as oxygen transport membranes.     

Effects of particle size on microstructure and mechanical strength of a fly ash based ceramic membrane

Recycling fly ash for ceramic membrane fabrication not only reduces solid waste discharge, but also decreases the membrane cost. Now, fly ash is becoming a promising substitute material for ceramic membrane preparation. A significant difference between fly ashes from different plants is the particle size, which makes performances of fly ash membranes unpredictable. The novelty of this work is to clarify the effects of the particle size of fly ash on ceramic membranes, thereby giving practical suggestions on fly ash selection for ceramic membrane preparation. Ceramic membranes were fabricated with different sizes of fly ashes. Effects of particle size on porosity, pore size, microstructure, mechanical strength and gas permeability of the membrane were investigated. Results indicate that a broader particle size distribution of fly ash leads to a denser structure of membrane with a lower porosity. Pore size and gas permeability of membrane increase while bending strength decreases with the particle size increasing. Bending strength of a fly ash membrane is largely determined by large particles in the fly ash because the large particles lead to cracks in the membrane. This work provides experimental bases for developing high performance ceramic membranes from fly ash.     

Preparation processes and characterizations of alumina-coated alumina support layers and alumina-coated natural material-based support layers for microfiltration

Recently, porous ceramic membranes have become a subject of significant interest due to their outstanding thermal and chemical stability. To reduce the high manufacturing costs of these porous ceramic membranes, recent research has focused on the utilization of inexpensive natural materials. However, there have not been any well-established direct comparisons of the membrane properties between typical alumina-based membranes and novel natural material-based membranes. Therefore, we compared alumina-coated alumina support layers (with average pore sizes ranging from 0.10 µm ~0.18 µm), alumina-coated diatomite-kaolin composite support layers (with an average pore size of 0.12 µm), and alumina-coated pyrophyllite-diatomite composite support layers (with an average pore size of 0.11 µm) via the dip-coating method and subsequent heat treatment ranging from 1200 °C–1400 °C for 1 h. The pure water permeability of the alumina-coated diatomite-kaolin composite support layer and the alumina-coated pyrophyllite-diatomite composite support layer was found to be approximately 2.0×10² L m⁻² h⁻¹ bar⁻¹, which is similar to that of an alumina-coated alumina support layer. Therefore, we suggest that the average pore size of an alumina-coated natural material-based support layer can be effectively controlled while exhibiting acceptable water permeability.     

颗粒粒径和膜孔径对陶瓷膜微滤微米级颗粒悬浮液的影响

通过测定颗粒悬浮液通过陶瓷微滤膜时的参透通量及污染阻力,确定了陶瓷膜处理微米级颗粒悬浮液时,颗粒粒径和膜孔径对微滤过程的影响和膜污染机理,获得了微米级颗粒悬浮液微滤过程中膜孔径的选择方法。     

New Inorganic Ultrafiltration Membranes: Titania and Zirconia Membranes

Destabilization of colloidal solutions permits preparation of titania and zirconia ultrafiltration membranes. In this work, layer thickness, pore diameter as a function of sintering temperature, and water permeability are developed. These membranes have a very good chemical resistance and can be used in all pH ranges in ultrafiltration processes. A comparison between alumina, titania, and zirconia membranes is given.     

Effect of sintering temperature on functional properties of alumina membranes

Supported membranes were prepared from different submicron alumina powders. The evolution of pore size, hardness and permeability were monitored after sintering the films at temperatures ranging from 1000 to 1400 °C. These functional properties and the microstructure of the films were compared with the free-standing membranes. Sintering at temperature range from 1000 to 1200 °C maintained the narrow, monomodal pore size distribution of the supported membranes. The effect of sintering temperature on the hardness of the membranes was weak. The permeability was also independent on the sintering temperature. When sintering temperature was raised up to 1300 and 1400 °C, the pore size increased significantly and distribution was changed to bimodal containing fraction of large pores. The hardness of the membranes increased while significant densification was not observed. Permeability increased due to the large pore size and the high porosity. In sintering of the free-standing membranes pore size remained almost unchanged, density increased when sintering temperature was raised, hardness was dependent on the density and permeability decreased continuously. The substrate did not have effect on the grain growth, which was dependent on the sintering temperature. Evolution of the properties of the free-standing membranes suggests local densification. The rigid substrate restricts the sintering shrinkage leading to densification of small areas. This local densification opens large flow channels between agglomerates. This increases the pore size, broadens the pore size distribution and increases the permeability. The macroscopic densification of the film is small.     

Preparation of Silica/γ-Alumina Membrane with Bimodal Porous Layer for Improved Permeation in Ions Separation

Creating secondary pores in the intermediate layer of hierarchical ceramic membranes successfully increases the permeability of bi-layered membranes by reducing the density of the separating layer. With the optimum secondary pore volume, the permeability of the silica/γ-alumina membrane with low secondary volume is enhanced with a satisfactory retention of organic ions and inorganic ions. However, the silica layer is not well formed when excessive secondary pores were generated in the intermediate layer. This is likely because the bimodal porous structure of γ-alumina with high secondary pore volume is inadequate to prevent the penetration of silica sol into the α-alumina support during dip coating. Thus, the bi-layered membrane with high secondary pore volume shows insufficient retention of Reactive Orange 16 dye and NaCl at low pH.     

The effects of diatomite addition on the pore characteristics of a pyrophyllite support layer

Recently, porous ceramic membranes have become a subject of special interest due to their outstanding thermal and chemical stability. To alleviate the manufacturing cost issues of these porous ceramic membranes, recent research has been focused on the utilization of low cost natural materials. Therefore, in this paper, we report the results of our efforts to determine whether we could utilize diatomite as a pore former and whether we could prepare a pyrophyllite–diatomite composite support layer that could effectively control the largest pore size and permeability. The pore characteristics of the specimens were studied by scanning electron micrography, mercury porosimetry, and capillary flow porosimetry.     

Preparation of alumina membranes for micro- and ultrafiltration applications

Abstract

A very thin disc type ceramic membrane (0·3-0·8 mm thickness, 25-30 mm diameter) made of pure alumina and suitable for microfiltration applications has been prepared by a tape casting process. A sol coating was applied to the disc to form an ultrafiltration membrane. T he pore size of the membrane could be varied in the range 0·1-0·7 μ m and porosity in the range 25-55% by optimising the ex perimental parameters. The most important factor for determining the pore size of the membrane was found to be the initial particle size distribution of the ceramic powder. Firing temperature and soaking time are other crucial parameters affecting volumetric porosity. Water permeability through the membrane under suction and under positive pressure is comparable with that of ceramic membranes prepared by conventional methods. In addition, a typical membrane shows a very narrow pore size distribution in the range 0·1-0·4 μm, with a median pore size of 0·28 μm. A very sharp drop in the pore size distribution pattern around 0·4 μm indicates that no pores larger than this exist. This implies that all particles with radius > 0·4 μm are trapped in the membrane, providing excellent separation efficiency. Results of microbial separation tests confirmed the possibility of micro-organism separation through these membranes.     

Properties of polyethersulfone ultrafiltration membranes modified with polyethylene glycols

Polyethersulfone ultrafiltration membranes have been prepared using polyethylene glycols (PEGs) of 400, 1000, and 10,000 gmol, as additive with dimethylacetamide as solvent. Infrared analysis proves that PEG leaves almost completely the surface of the membranes after 24 h of water immersion. Scanning electron microscopy, contact angle, and liquid–liquid displacement porometry have been used to characterize the membrane morphology, surface hydrophilicity and porous structure. The relative flux reduction factor, flux, retention—of PEG (20,000 and 35,000 g/mol) and bovine serum albumin (67,000 g/mol)—and pure water permeability have been measured for the membranes. Results show that the addition of PEG increases slightly hydrophilicity and decreases pore size and narrows the corresponding pore size distribution while thickening the skin layer, in spite of the fast disappearance of the added PEG form the membrane surface. The resulting flux and pure water permeability are higher when middle size PEGs are added but decrease again when very high molecular weight (MW) PEGs are added. Retention decreases initially for increasing MWs of PEG although for very long PEG chains (MW of 10,000 g/mol) retention increases again. After filtration, the membranes with PEG added showed a lower relative flux reduction that decreases for increasing MW of the added PEGs. © 2013 Society of Plastics Engineers. POLYM. ENG. SCI., 54:1211–1221, 2014. © 2013 Society of Plastics Engineers     

Microstructural characterization and gas permeation performance of freeze-cast alumina supports

One of the major challenges for obtaining asymmetric membranes has been the preparation of ceramic supports with tailored pore structures. The methods commonly used for processing these materials do not allow a fine control of the support structure, which may decrease their mass transport capacity. As a consequence, freeze-casting is a promising technique for obtaining supports with tailored pore structures and permeation behaviors. This work deals with the preparation of freeze-cast alumina macroporous supports. Different freezing routes and alumina powders were used in this study. It was correlated the effect of these parameters on the pore structure and permeation capacity of the obtained supports. It was shown that the isotropic layer observed at the samples bottom side plays a key role in their non-Darcian permeability. As far as we know, this is the first time that this approach is reported in the literature for freeze-cast samples.     

A Model Simulation of the Filtering Process in Multichannel Ceramic Membranes

The filtering process in ceramic membranes is analyzed by jointly solving Darcy and Laplace equations and an equation for continuous flow of an incompressible fluid to determine the pressure and velocity profiles over the cross section of a membrane cell. Computational and experimental data for multichannel ceramic membranes how that the filtering efficiency of the channels decreases from the center to the periphery depending on the pore size and on the thickness of both the selective layer and the substrate.     

新型两步法合成高质量Silicalite-1分子筛膜

通过一种新颖的两步合成方法,即先用改性剂溶液对镀有二氧化硅缓冲层的多孔氧化铝(孔径0.1～0.5μm)载体管进行浸泡处理,再水热合成,制备了silicalite-1分子筛膜,并考察了晶化温度、晶化时间对成膜的影响.采用溶胶-凝胶(sol-gel)法,在最优粘度5～25 cp内制得平整连续的介孔缓冲层,达到了修饰载体材料表面性能的目的.X射线衍射(XRD)和扫描电镜(SEM)结果表明,160℃下,晶化24 h得到表面致密、连续的6取向的silicalite-1分子筛膜.