Spheroidization of low-cost alumina powders for the preparation of high-flux flat-sheet ceramic membranes

Two kinds of low-cost alumina powders with irregular morphology were pretreated by spheroidization and the two spherical powders were used to prepare high-flux flat-sheet support and microfiltration (MF) membrane with high separation accuracy, respectively. It was found that the spheroidization pretreatment not only unified the morphology of alumina powder particles into spherical shape, but also narrowed the particle size distribution of the powders, which both were conducive to optimizing the performance of the as-prepared ceramic membranes. After sintering at 1350 °C, the open porosity, bending strength, average pore diameter and pure water permeability of alumina flat-sheet support from spheroidized alumina coarse powder were 44.3%, 36.3 MPa, 3.3 μm and 3240 L/h m² bar, respectively. The slurry derived from spheroidized alumina fine powder was dip-coated on the flat-sheet support to prepare MF membrane. The crack-free MF membrane with a thickness of 23.5 μm had a pore diameter of 0.12 μm and pure water permeability of 850 L/h m² bar. Additionally, the elaborated MF membrane was used to clarify aqueous suspension of carbon black with the maximum rejection rate of up to 99.7%, exhibiting excellent cleaning performance at the same time by completely restore the virgin permeate flux after backwash.     

Actual air separation through poly(aniline‐co‐toluidine)/ethylcellulose blend thin‐film composite membranes

Several multilayer thin‐film composite membranes were fabricated of ethylcellulose (EC) and poly(aniline‐co‐ortho‐toluidine) or poly(ortho‐toluidine) blend as selective thin films and three ultrafiltration membranes with a 10‐ to 45‐nm pore size and 100‐ to 200‐μm thickness as porous supports. The relationships between the actual air‐separation performance through the composite membranes and layer number, composition, casting solution concentration of the thin selective film are discussed. The oxygen‐enriched air (OEA) flux through the composite membranes increases steadily with increasing operational temperature and pressure. The oxygen concentration enriched by the composite membranes appears to decrease with operating temperature, but increases with operating pressure. The actual air‐separation property through the composite membranes seems to remain nearly constant for at least 320 days. The respective highest OEA flux, oxygen flux, and oxygen concentration, respectively, were found to be 4.78 × 10⁻⁵ cm³ (STP)/s · cm², 2.2 × 10⁻⁵ cm³ (STP)/s · cm², and 46% across EC/poly(o‐toluidine) (80/20) blend monolayer thin‐film composite membranes in a single step at 20°C and 650 kPa operating pressure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 458–463, 2000     

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.     

Cost‐effective porous ceramic tubes fabricated through a phase inversion/casting process using calcined bauxite

Cost‐effective ceramic tubes based on low‐price commercial calcined bauxite for economical separation were fabricated by a new phase‐inversion casting method. The thermal shrinkage and weight loss during heating of the green tubes were characterized by dilatometric analysis and TG, respectively. Three shrinkage stages appear successively, corresponding to the viscous deformation of polymeric binder at 200‐300°C, significant combustion loss of ~5.2 wt% at 500‐620°C and sintering shrinkage over 800°C, respectively. However, due to high enough viscosity of the casting suspension that can guarantee the green tube against collapse or deformation during the phase inversion/casting process, the sintered tubes display nearly uniform microstructure instead of characteristic asymmetrical structure of the phase inversion process. The influence of sintering temperature on the pore property (including pore size and porosity) and mechanical strength was investigated. As the sintering temperature increases from 1200 to 1400°C, the porosity and average pore size decrease from 46.4% to 37.0% and from 0.98 to 0.81 μm, respectively, and the flexural strength increases from 25.8 to 65.1 MPa. The cost‐effective ceramic tube sintering at the range of 1250‐1400°C can be capable of functioning as a microfiltration membrane or an ultrafiltration membrane support.     

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.     

Manufacture and characterization of ultra and microfiltration ceramic membranes by isostatic pressing

This paper describes the manufacture of tubular UF and MF porous and supported ceramic membranes to oil/water emulsions demulsification. For such a purpose, a rigorous control was realized over the distribution and size of pores. Suspensions at 30 vol.% of solids (zirconia or alumina powder and sucrose) and 70 vol.% of liquids (isopropyl alcohol and PVB) were prepared in a jar mill varying the milling time of the sucrose particles, according to the pores size expected. The membranes were prepared by isostatic pressing method and structurally characterized by SEM, porosimetry by mercury intrusion and measurements of weight by immersion. The morphological characterization of the membranes identified the formation of porous zirconia and alumina membranes and supported membranes. The results of porosimetry analysis by mercury intrusion presented an average pore size of 1.8 μm for the microfiltration porous membranes and for the ultrafiltration supported membranes, pores with average size of 0.01-0.03 μm in the top-layer and 1.8 μm in the support. By means of the manufacture method applied, it was possible to produce ultra and microfiltration membranes with high potential to be applied to the separation of oil/water emulsions.     

Near‐Net‐Shaped Porous Ceramics for Potential Sound Absorption Applications at High Temperatures

We present an interesting processing route for obtaining alumina/mullite‐based ceramics with controlled porosity and airflow resistance leading to promising microstructures for application as sound absorbers. The use of ceramic materials aims for potential applications where high temperatures or corrosive atmospheres are predominant, e.g., in combustion chambers of gas turbines. For the production of the porous ceramics we combined freeze gelation and sacrificial templating processes to produce near‐net‐shaped parts with low shrinkage (<3%) based on environmental‐friendly and low cost conditions. The obtained microstructure presents a bimodal pore size distribution, with small pores derived from the freeze gelation process (~30 μm) connecting large pores (2–5 mm diameter) originated from the expanded polystyrene template particles. These connections, called “windows” in this study, show a significant impact on the sound absorption properties, allowing the pressure diffusion effect to take place, resulting in a significant improvement of the sound absorption coefficient. By varying the template particle content and the slurry solid content, it is possible to control the sound absorption behavior at different frequencies of the open‐celled ceramics. These ceramics feature a high open porosity, from 77% to 82%, combined with sufficient compressive strength ranging from 0.27 to 0.68 MPa and sound absorption coefficients of 0.30–0.99, representing a highly promising combination of properties for noise control and reduction at corrosive environments and high temperatures.     

Solvent‐stable UV‐cured acrylic polysulfone membranes

A systematic investigation of the effect of the presence of acrylate resin on polysulfone‐based membranes was performed with the aim of obtaining chemically stable crosslinked membranes without affecting their flux performances. The membranes were prepared via UV curing of the polymer dope followed by a non‐solvent‐induced phase separation process. Two different acrylic monomers were investigated and their amount was varied in the polymer dope, to study the influence of concentration on final results. High crosslinking degrees were achieved by irradiating the solution for one minute. Morphological investigations of the active surface and of the cross‐sections of the fabricated membranes showed that the typical porosity of ultrafiltration membranes was obtained starting from solutions containing a low amount of crosslinker (10 wt%), which is consistent with the water flux values which were comparable to that of the pristine polysulfone membrane. High concentrations of crosslinker resin in the initial polymer dope produced denser membranes with lower permeability. High rejection of 27 nm particles (>90%) was measured for all samples having measurable flux. The addition of the crosslinker allowed one to obtain stability in various solvents without affecting the flux and rejection performance of the porous membranes. © 2016 Society of Chemical Industry     

Potential of DMSO as greener solvent for PES ultra‐ and nanofiltration membrane preparation

In this article, the performance of polyethersulfone (PES) ultra‐ and nanofiltration membranes, prepared with the non‐toxic solvent dimethyl sulfoxide (DMSO), was investigated. The membranes were prepared by immersion precipitation via phase inversion. Experimental results proved that DMSO is a better alternative to N‐methyl‐2‐pyrrolidone (NMP) as solvent for PES ultrafiltration membranes as the membranes had a higher permeability and rejection of bovine serum albumin (BSA). An explanation was found based on experimental cloud point data and scanning electron microscopy images showing the morphology. The rejection of BSA and rose Bengal (RB) was proportional to the polymer concentration. On the contrary, the permeability decreased with increasing polymer concentration. For a casting thickness of 250 µm, an optimal balance between permeability and rejection of macromolecules for ultrafiltration was found at 24 wt % PES. The permeability was inversely proportional to the casting thickness, but a small decrease in rejection was observed when lowering the thickness. A good balance between permeability and rejection of RB was found, using a reference nanofiltration membrane of 28.5 wt % PES with 150 µm casting thickness. This membrane achieved a RB rejection of 95.3% and a pure water flux of 2.03 L m^?2 h^?1 bar^?1. The membrane thickness and polymer concentration did not have a clear influence on the hydrophilicity of the membranes. It can be concluded that DMSO is a benign alternative as compared to traditional solvents such as NMP and also results in better PES membrane performances. DMSO is a perfectly suitable solvent for ultrafiltration applications and has potential to be used for nanofiltration applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46494.     

Multifunctional Alumina Composites with Toughening and Crack‐Healing Features Via Incorporation of NiAl Particles

In this research, NiAl particles modified Al₂O₃ composites with toughening and crack‐healing performances were prepared by spark plasma sintering. The toughening effects as a function of the NiAl particle size and content were systematically investigated. The fracture toughness was increased by 80% compared with pure alumina after incorporation of 1 μm NiAl particles at 15 wt%. A self‐healing process of damaged composites induced by indentation was found to occur, mediated by oxidation of NiAl at elevated temperatures with the size of the NiAl particles. Atomic force microscopy was adopted to characterize the crack‐healing performance and to explore the healing mechanism under each healing condition. The best crack‐healing efficiency of 99.2% was found in the composite containing 1 μm NiAl at 15 wt% that was annealed at 1300°C for 5 h. In addition, the fracture toughness of the composite was further enhanced by 40% after the crack‐healing process. These multifunctional ceramic composites may have promising and broad application in areas such as thermal barrier coatings, fuel cell membranes, etc.     

Hemodialysis membrane prepared from cellulose/N‐methylmorpholine‐N‐oxide solution. II. Comparative studies on the permeation characteristics of membranes prepared from N‐methylmorpholine‐N‐oxide and cuprammonium solutions

Two kinds of regenerated cellulose membranes for hemodialysis were prepared from casting solutions of N‐methylmorpholine‐N‐oxide (NMMO) and cuprammonium (denoted NMMO membranes and cuprammonium membranes, respectively). The concentration of cellulose in the casting solution investigated was 6–8 wt %. The permeation characteristics of both membrane series were compared in terms of the ultrafiltration rate (UFR) of pure water, the sieving coefficient (SC) of dextran, and the solute permeabilities of urea, creatinine, and vitamin B₁₂. The UFR and SC of the NMMO membranes were strongly affected by the cellulose concentration of the casting solution, and NMMO was a preferable solvent for the production of cellulose membranes with high performance; the cuprammonium solution gave low‐performance membranes. The pore structures of both types of membranes were estimated with the Hagen–Poiseuille law. The results showed that the NMMO membranes had larger pore radius and smaller pore numbers than the cuprammonium membranes. The differences in the membrane pore structures led to the differences in the performance between the two membrane series. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 333–339, 2003     

Low‐Temperature Co‐Fired Ceramic Substrates for High‐Performance Strain Gauges

Recent advances in the development of high gauge factor thin films for strain gauges prompt the research on advanced substrate materials. A glass ceramic composite has been developed in consideration of a high coefficient of thermal expansion (9.4 ppm/K) and a low modulus of elasticity (82 GPa) for the application as support material for thin‐film sensors. In the first part, constantan foil strain gauges were fabricated from this material by tape casting, pressure‐assisted sintering, and subsequent lamination of the metal foil on the planar ceramic substrates. The accuracy of the assembled load cells corresponds to accuracy class C6. That qualifies the load cells for the use in automatic packaging units and confirms the applicability of the low‐temperature co‐fired ceramic (LTCC) substrates for fabrication of accurate strain gauges. In the second part, to facilitate the deposition of thin‐film sensor structures to the LTCC substrates, pressure‐assisted sintering step is modified using smooth setters instead of release tapes, which resulted in fabrication of substrates with low average surface roughness of 50 nm. Titanium thin films deposited on these substrates as test coatings exhibited low surface resistances of 850 Ω comparable to thin films on commercial alumina thin‐film substrates with 920 Ω. The presented material design and advances in manufacturing technology are important to promote the development of high‐performance thin‐film strain gauges.     

Manufacture and optimization of tubular ceramic membrane supports

The manufacturing and optimization of centrifugally casted ceramic membrane supports is presented. For the optimization, the effect of three different powder sizes (0.25, 0.31 and 0.61 μm) and a sintering temperature range between 1050 and 1400 °C was investigated. The ceramic tubes were characterized according to tube dimensions, mercury porosimetry, water permeability, SEM and mechanical strength. It was shown that the centrifugal casting technique delivers highly reproducible support properties. A novel strength testing apparatus was developed to determine the mechanical strength of the ceramic tubes. It was found that the strength varied between 3300 MPa and 300 MPa, depending on the porosity of the supports. With increased sintering temperature, water permeability and porosity decreased, while strength and linear shrinkage increased. The pore diameter of the supports produced by the 0.31 and 0.61 μm powders decreased, while that of the 0.25 μm powders remained constant (72 nm) with increasing sintering temperature. The 0.61 μm powder sintered at the lowest sintering temperature resulted in the support with the highest porosity, pore diameter and porosity, but the lowest mechanical strength and linear shrinkage. An overall improvement concerning pure water permeability was seen when the support in this study was compared to our own previous studies and similar studies in literature.     

Direct Synthesis of Alumina Nanowires on Ceramic Substrates Using Sn Catalysts

Direct integration of nanostructures into macroscopic substrates is very important for their practical applications. In this work, we report a simple method that can be introduced for the Sn‐catalyzed growth of alumina nanowires on ceramic substrates such as porous disk, monolith, and foam. Our study focuses on the role of the Sn catalysts in the formation mechanisms governing nanowire growth. Using the proposed approach, hair‐ or grass‐like tufts of 20 nm diameter nanowires grow on the surface of the ~3 μm diameter Sn particles, in a tip growth mechanism. The nanowires of α‐phased polycrystalline structure grow and are packed via a complex process involving batch‐by‐batch, branching, and amalgamation growth. The detailed observations reveal that the Sn catalyst is key to tailoring the growth patterns of the nanowires. In addition, cathodoluminescence studies highlight the potential optical applications of the alumina nanowires.     

Separation of bovine serum albumin (BSA) using γ‐Al2O3–clay composite ultrafiltration membrane

BACKGROUND: Ceramic membranes have received more attention than polymeric membranes for the separation and purification of bio‐products owing to their superior chemical, mechanical and thermal properties. Commercially available ceramic membranes are too expensive. This could be overcome by fabricating membranes using low‐cost raw materials. The aim of this work is to fabricate a low‐cost γ‐Al₂O₃–clay composite membrane and evaluate its potential for the separation of bovine serum albumin (BSA) as a function of pH, feed concentration and applied pressure. To achieve this, the membrane support is prepared using low‐cost clay mixtures instead of very expensive alumina, zirconia and titania materials. The cost of the membrane can be further reduced by preparing a γ‐alumina surface layer on the clay support using boehmite sol synthesized from inexpensive aluminium chloride instead of expensive aluminium alkoxide using a dip‐coating technique. RESULTS: The pore size distribution of the γ‐Al₂O₃‐clay composite membrane varied from 5.4–13.6 nm. The membrane was prepared using stable boehmite sol of narrow particle size distribution and mean particle size 30.9 nm. Scanning electron microscopy confirmed that the surface of the γ‐Al₂O₃–clay composite membrane is defect‐free. The pure water permeability of the support and the composite membrane were found to be 4.838 × 10^?6 and 2.357 × 10^?7 m³ m^?2 s^?1 kPa^?1, respectively. The maximum rejection of BSA protein was found to be 95%. It was observed that the separation performance of the membrane in terms of flux and rejection strongly depends on the electrostatic interaction between the protein and charged membrane. CONCLUSION: The successively prepared γ‐Al₂O₃‐clay composite membrane proved to possess good potential for the separation of BSA with high yield and could be employed as a low cost alternate to expensive ceramic membranes. Copyright © 2009 Society of Chemical Industry     

Gas transport through ceramic membranes under super-critical conditions

In order to select and to apply a porous membrane under supercritical conditions, it is necessary to understand the transport mechanism affecting the permeation behaviour.This paper describes the investigation of gas transport through micro porous ceramic membranes consisting of several layers. The separation layer is made from TiO² with a nominal pore size diameter of 0.9 nm. Single gas permeation of helium, nitrogen, argon, methane, and carbon dioxide was measured in the temperature range of 293-443 K and in the pressure range of 1-10 MPa.Observation of the permeability of these membranes revealed that the transport of non?adsorbing gases under these conditions is governed by Knudsen diffusion and viscous flow.     

Micro‐Phase Separation within Epoxy Resin Yields Ultrathin Mesoporous Membranes with Increased Scalability by Conversion from Spin‐ to Dip‐Coating Process

Ultrathin mesoporous membranes offer highly desirable characteristics for separation tasks regarding selectivity and mass transport of proteins. They have potential applications as separation devices in microfluidics, diagnostics, sensing, and high‐precision separations for pharmaceutical formulation. Especially for large‐scale and mass production, sophisticated production processes represent a barrier for wider application. A method is developed to produce nanomembranes with a thickness of 75 nm and 40 nm pores with an epoxy resin. The novolac resin is cured with branched polyethylenimine to form a covalently crosslinked polymer membrane with perforations spanning the entire thickness. Pore formation relies on micro‐phase separation of the curing agent during casting and the selective dissolution of the emergent nanodomains which thereby serve as pore templates. The resulting membranes are hydrophilic and therefore suitable for applications with biological fluids. Mechanical testing of the flexible but robust thin films reveals an ultimate tensile strength of 15 MPa and a biaxial modulus of 0.8 GPa. Proteins with a diameter of less than 12 nm can diffuse through the pores and permeation rates are pH dependent. The entire fabrication process is transferred to a dip‐coating approach, which is more suitable for a potential large‐scale production.     

Hierarchically Structured Materials by Anodic Coagulation Casting of Fibrinogenic Alumina Suspensions

Anodic coagulation casting of fibrinogenic ceramic suspensions is a novel processing technology, which is based on the electrically induced transformation of the water soluble fibrinogen into the insoluble fibrin. Contrary to the direct coagulation casting (DCC) technology, green formation does not depend on a pH‐shift and as the fibrin coagulate forms on an anode, it can be combined with the electrophoretic deposition (EPD) technology. In this study, the conversion of fibrinogen into fibrin is activated via electron transfer processes at an electrode material and is combined with the green formation of alumina by embedding the ceramic particles in the protein matrix. The focus of this work was to establish a technology to shape thin hierarchically structured ceramic films and thick porous materials with a distinct pore structure. Film thickness and porosity were controlled by the applied voltage and the processing‐time. The range of the established green bodies included two‐dimensional and simple three‐dimensional shapes including multilayered deposition and fiber coatings. Overall the process of anodic coagulation casting can be reported to be successful for all established ceramic shapes except multilayers, where delamination was observed. The deposited alumina ceramics were characterized using light microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and synchrotron micro computed tomography (μCT), while the coagulation mechanism was studied using high‐performance liquid chromatography (HPLC).     

Microstructural,Physical, and Fluid Dynamic Assessment of Spinel‐Based and Phosphate‐Bonded Investments for Dental Applications

This work reports the experimental evaluation of physical and gas permeation parameters of four spinel‐based investments developed with or without inclusion of sacrificial fillers. Data were compared with those of three commercial formulations. Airflow tests were conducted from 27 to 546°C, and permeability coefficients were fitted from Forchheimer's equation. Skeletal densities found for spinel‐ (ρ_s = 3635 ± 165 kg/m³) and phosphate‐bonded (ρ_s = 2686 ± 11 kg/m³) samples were in agreement with the literature. The developed investments were more porous and less permeable than commercial brands, and the differences were ascribed to the different pore morphologies and hydraulic pore sizes of ceramic matrices. The inclusion of both fibers and microbeads resulted in increases of total porosity (42.6–56.6%) and of Darcian permeability coefficient k₁ (0.76 × 10^?14–7.03 × 10^?14 m²). Air permeation was hindered by increasing flow temperatures, and the effect was related to the influence of gas viscosity on ΔP, in accordance with Darcy's law. Casting quality with molten titanium (CP Ti) was directly proportional to the permeability level of the spinel‐based investments. However, the high reactivity of the silica‐based investment RP and the formation of α‐case during casting hindered the benefits of the highest permeability level of this commercial brand.     

Silane‐Crosslinked Asymmetric Polythiosemicarbazide Membranes for Organic Solvent Nanofiltration

Crosslinked polythiosemicarbazide (PTSC) membranes with a positively charged surface are fabricated via a reaction with (3‐glycidyloxypropyl)trimethoxysilane. The integrally asymmetric ultrafiltration membranes discussed here can be easily prepared by water‐induced phase separation using a PTSC solution in dimethylsulfoxide (DMSO). The crosslinked PTSC membranes are stable in DMSO, N,N‐dimethylformamide, and tetrahydrofuran and they reject molecules of molecular weights (MW) above 1300 g mol^?1. The influence of the crosslinking agent on the surface charge, membrane solvent resistance, and membrane performance is discussed. The crosslinked asymmetric PTSC membranes totally reject Direct Red dye (MW 1373 g mol^?1), while the pristine PTSC membrane does not show any rejection for this dye. This finding suggests that an inorganic‐type‐network is formed during the crosslinking reaction, which tunes the pore size of the prepared membranes.