The Slip-casting of Ceramic Membranes

A common method to slip-cast ceramic membranes is to start with a colloidal suspension or polymeric solution as described in the previous section. This is called a slip . The porous support system is dipped in the slip and the dispersion medium (in most cases water or alcohol-water mixtures) is forced into the pores of the support by a pressure drop (APJ created by capillary action of the microporous support. At the interface the solid particles are retained and concentrated at the entrance of pores to form a gel layer as in the case of sol-gel processes. It is important that formation of the gel layer starts 

The smaller and more uniform the primary particles, and the weaker the agglomerates in the sol are, the smaller the pore size and the sharper its distribution in the membrane will be. The thickness of the layer L, increases linearly with the square root of the dipping time. The process is quantitatively described by Leenaars and Burggraaf (1985). The rate of membrane deposition increases with the slip concentration or with decreasing pore size of the support as shown below. This has been experimentally confirmed for alumina and titania (Leenaars and Burggraaf 198S, Uhlhom et al. 1989). 

The capillary pressure drop, AP caused by pores with an efiective radius r for each capillary is given by 

From Equation 2.2, it can be seen that the viscosity of the slip plays an important role. It regulates the formation rate of the gel layer and helps to prevent the slip from penetrating the support pore system. In the colloidal suspension route the evolution of the viscosity during the solvent extraction is slow during the very first steps of the process and drastically increases just before gelling. With the polymeric gel route a more gradual increase of the viscosity is observed. In both cases the evolution of the viscosity can be modified by the addition of binders to the sol slip . Different kind of binders are chosen depending on the nature of the solvent, the compatibility with the precursors and the viscosity of the system. 

It is obvious that the pore diameter can be regulated by heat treatment to values as small as 3-6 nm (minimum) and up to 50-200 nm depending on the material. 

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