Modified dip-coating method for preparation of pinhole-free ceramic 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.