A new aqueous sol was synthesized to prepare Ag-doped TiO2/Ti composite tight ultrafiltration (UF) membranes. This sol was prepared based on water (>95 wt%), for which the hydrodynamic size of 8 nm could be obtained by tuning the electrostatic and steric repulsive forces, breaking the limit of the polymeric sol–gel method to prepare tight UF membranes. For the preparation of the sol, high water content can significantly reduce the use of organic solvents and is more environment-benign. Moreover, Ag doping can considerably improve the toughness of the supported membranes to efficiently prevent the TiO2/Ti composite membranes from cracking and overcome the thermal expansion mismatch of ceramic membranes and their metallic supports. The prepared Ag-doped TiO2/Ti composite membranes presented a molecular weight cut-off of 3 kDa with a pure water flux of 24 L m−2 h−1 bar−1 and exhibited good dye removal capabilities. The proposed Ag doping method has considerable potential for the fabrication of integrated hybrid ceramic–metallic composite membranes.

Pinhole defects can lead to an unacceptable deterioration and sometimes loss of the usual separation ability of ceramic membranes. A modified dip-coating method was proposed in an attempt to avoid pinhole defects in ceramic membranes. This was realized by applying a tangential flow of suspension against the support during the step of support dipping in a dip-coating process, in order to achieve a dynamic particle deposition. The idea was to obtain a uniform and pinhole-free particle layer by the interaction of capillary-filtration and suspension tangential flow. One α-Al2O3 membrane with an average pore size of 0.6 μm was prepared by the modified dip-coating method. It was demonstrated that pinholes could be effectively avoided by applying a suspension flow velocity of 50 mm s−1 and a withdrawal speed of 4 mm s−1 via a single coating-sintering procedure. The largest pore size of the as-prepared membrane could be reduced to 0.76 μm, which is very close to the average pore size of the membrane, while the pure water flux of the membrane remained as high as 8.0 m3 h−1 m−2 bar−1.Research highlights▶ A modified dip-coating method is proposed to prepare pinhole-free ceramic membrane. ▶ A tangential flow of suspension assists the capillary-filtration effect. ▶ A pinhole-free membrane with high flux is prepared by applying the novel method.

The preparation of titania ultrafiltration membranes with intermediate layer of sol-coated nanofibers is briefly described. In this process, titiania nanofibers cover on the porous substrate to produce uniform layer with high porosity and flux. The use of titania nanoparticles from sol has been found to bring an improvement on the mechanical strength of the titania nanofiber membrane due to the formation of sintering neck between nanofibers with colloidal particles (sol) at lower sintering temperature. In order to reduce the pore size and achieve high separation efficiency, titania colloidal particulate sol is used to coat on the top of the titania nanofiber layer and then co-sintered at the suitable sintering temperature of the titania gel (480 °C) to prepare ultrafiltration membranes. Furthermore, the coating times of titania sol are optimized based on the measurements supplied by permeation and separation performance and a delicate control of the coating times indeed plays a key role in the preparation of the defect-free membrane with high performance. In this work it is found that the prepared membrane has homogeneous surface without obvious defects in case of three coatings of titania sol co-sintered with the fiber layer at 480 °C. Property tests display that the pure water flux reaches 1100 L m−2 h−1 bar−1 while the molecular weight cut-off is 32,000.

Cracking of ceramic films during drying is a well-known phenomenon. Literatures have shown that films cast on the nonporous substrates will crack unless polymeric additives are added to improve the critic crack thickness. This paper investigates the fabrication of crack-free zirconia membranes on porous substrate through a wet chemical method, with a focus on the influence of polymeric additives on the critical crack thickness and membrane performances. Four types of additives were used in the preparation of zirconia slurries, and polyvinyl alcohol (PVA) and methylcellulose (MC) were favorable for a stable, ready-to-dip-coating slurry suggested by viscosity, particle size distribution and sedimentation measurements. Drying experiments also showed that there was a smooth weight loss and a low risk of crack-formation during membrane drying in the case of using PVA or MC as additives. Compared with a sole polymer additive of 0.75 wt% MC, the critical crack thickness increases from 4.0 μm to 5.0 μm when 0.25 wt% PVA was further introduced into the suspension, which benefits the preparation of crack-free membranes. When the membrane thickness was controlled to be thinner than the critical crack thickness, the prepared membrane would be more likely to have a homogeneous and smooth surface, and less defects or cracks. For the membrane with a thickness of approximately 3–4 μm, the pure water permeability reached 1000 L m−2 h−1 bar−1 and the dextran rejections were up to 85%.

Knowledge of the surface properties of ceramic membranes is useful for understanding and predicting their filtration performance. In this study focus was put on investigation of the effect of membrane surface properties on filtration performance. The TiO2-doped Al2O3 composite microfiltration membrane with an average pore size of 0.2 μm was prepared by solid state sintering method. The surface wettability and surface charge properties of porous ceramic membrane were determined by measurement of the dynamic contact angle and streaming potential. The results indicate the TiO2 doping improves membrane hydrophilicity and makes the isoelectric point shift towards lower pH. The Al2O3 and Al2O3–TiO2 membrane were applied to separate oily wastewater to evaluate the filtration performance. The interactions between the membrane surface and oil droplets strongly influence the formation of membrane fouling. A higher and more stable permeate flux was observed through the Al2O3–TiO2 composite membrane, which demonstrated that in the filtration of oily wastewater enhancing membrane hydrophilicity and having the same type of charge as the oil droplets can improve the membrane filtration performance.

Pore size has been found to strongly depend on the sintering program in the preparation of porous ceramic membranes. In this paper, a model was developed to predict the variation in pore size and porosity of membranes during the sintering process. A comparison of shrinkage characteristics was made between the sintering processes of supported membranes and unsupported membranes. For supported membranes, the effect of restriction coming from a rigid substrate on the sintering behavior has been taken into account in the calculation. It is predicted that the pore size increases in supported membranes and decreases in unsupported membranes as the sintering temperature is increased. Calculations also showed that the loss of porosity in the supported membranes was less than that in the unsupported membranes. In order to verify reliability of this model, unsupported and supported membranes were prepared with α-Al2O3 powders at the sintering temperatures ranging from 1125 °C to 1325 °C. The pore size and porosity were measured by gas permeation technique and Archimedes’s method. The experimental results for the unsupported and supported α-Al2O3 membranes showed a good agreement with those calculated from the model. Therefore, this model provides an effective tool in predicting the porosity and the pore size of ceramic membranes at the different sintering temperatures.

The effects of sintering atmosphere on the properties of symmetric TiO2 membranes are studied with regard to sintering behavior, porosity, mean pore size, surface composition, and surface charge properties. The experimental results show that the symmetric TiO2 membranes display better sintering activity in the air than in argon, and the mean pore diameters and porosities of the membrane sintered in argon are higher than those of the membrane sintered in the air at the same temperature. The surface compositions of the symmetric TiO2 membrane sintered in the air and in argon at different temperatures, as studied by X-ray photoelectron spectroscopy, are discussed in terms of their chemical composition, with particular emphasis on the valence state of the titanium ions. The correlation between the valence state of the titanium ions at the surface and the surface charge properties is examined. It is found that the presence of Ti3+, introduced at the surface of the symmetric TiO2 membranes by sintering in a lower partial pressure of oxygen, is related to a significant decrease in the isoelectric point. TiO2 with Ti4+ at the interface has an isoelectric point of 5.1, but the non-stoichiometric TiO2-x with Ti3+ at the interface has a lower isoelectric point of 3.6.

Ceramic capillary membrane has received much attention due to its relatively high pack density and favorable mechanical strength. However, it is difficult to prepare capillary membrane on its thin support by a dip-coating method. In this study, alumina microfiltration membranes were prepared on the inner surface of alumina capillary support (outer diameter 4 mm, inner diameter 2.5 mm) by a dip-coating method. Scanning electron microscopy (SEM) observation, gas bubble pressure (GBP) method and membrane permeation test were carried out to evaluate membrane performance. Two major effects in preparation of crack-free membrane, capillary filtration and film-coating, upon the thin support were studied. The as-prepared crack-free membrane presents a narrow pore size distribution, a mean pore size of about 0.6 μm and a high pure water flux of 86000 L·m−2·h−1·MPa. It is proved that the membrane thickness should be sufficiently large to overcome the defects of support surface, but it is only one of the prerequisites for the formation of crack-free membrane. Furthermore, it is demonstrated that the capillary filtration effect is greatly restricted for thin capillary support with the dip-coating method and the film-coating effect plays a crucial role in the formation of crack-free membrane.