Deposition of polymer-derived ceramic layers with asymmetric porosity

Abstract

Deposition of porous polymer derived ceramic layers onto tubular macroporous silicon nitride substrates via dip coating was investigated. The aim was to establish a process that would allow the preparation of polymer-derived ceramic multilayer membranes with respect to their gas separation properties. As a first step, silicon nitride supports were prepared following a previously developed procedure. Properties of the newly produced samples were compared to previous results. Before developing a coating method for the substrates a masking procedure was developed. Two different methods - immersion and dip coating - to apply a polystyrene masking layer from highly concentrated PS solutions in toluene were investigated. After establishing a masking procedure, the substrates were dip coated with an intermediate layer and/or a top layer. One layer was applied with an organic slurry of poly(vinyl)silazane and silicon nitride powder in n-hexane, and, after pyrolysis, was supposed to fulfill the role of an intermediate layer, bridging a microporous separation layer and the macroporous substrate. The second layer was applied by dip coating with a solution of poly(vinyl)silazane in n-hexane. After pyrolysis, this should resemble an actual microporous separation layer of a ceramic membrane. The samples were either directly coated with this layer or first with an intermediate layer followed by this layer. The samples were characterized by secondary electron microscopy to determine properties like layer thickness and layer adhesion on the substrates, as well as defects in the surface or infiltration behavior of the deposition liquids. A method to mask the macroporous substrates was developed. Even though infiltration of the deposition liquids was still present in most of the samples, it could be reduced. The masking allowed for the deposition of an intermediate layer without it exhibiting a slip casting-like behavior, which in turn would lead to delamination of the layer. It was found, that layer quality depends largely on the surface and masking quality of the substrates. Intermediate layers that were deposited did not allow for a more homogenous and defect-free deposition of the separation layer, even though their surfaces were much smoother than the substrate surfaces. Direct application of separation layers led to the most promising results in terms of layer homogeneity and reproducibility.