Fabrication of Inorganic Membranes

The fabrication starts with the preparation of porous substrates, with which the shape and configuration of the final membrane products can be determined. The porous substrate is critical for the quality of the membrane itself, because it not only provides sufficient mechanical strength but also takes effect on the permeability and selectivity of the membrane. Therefore, the commercial availability of high-quality substrates is a critical issue in the further development of membrane separation units. 

Porous substrates are mostly made from ceramics like a-Al203, but also from other materials such as metal or glass. They are formed by shaping inorganic powders and consolidation of the green body by 

Porous substrates should have a smooth surface with constant and homogeneous characteristics (wettability) and a narrow pore size distribution. Pores much larger than average and grains broken out of the surface, or irregularities in the porous substrate, may result in defects in the separation layer applied on it. Therefore, surface modification or formation of intermediate layer(s) is necessary to prepare a thin and defect-free membrane. 

The preparation of intermediate layers is actually a process to produce a porous layer with smaller pores than those in the bulk support. Since 

The separation layer, either porous or dense, can be formed using different methods such as sol-gel and template routes, hydrothermal synthesis, chemical vapor deposition (CVD), or physical sputtering, depending on the membrane material and its application. These membrane preparation methods will be described in the following chapters of this book for different membranes and membrane reactors. We note that the preparation of inorganic membranes involves a multi-step high-temperature treatment process. Therefore, inorganic membranes are much more expensive than polymeric ones. 

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Fig. 16.21. Overall scheme and stages involved for fabrication of inorganic membrane.

As inorganic membranes have a multi-layered asymmetric structure consisting of porous support, intermediate layers and a selective separation layer, the fabrication of inorganic membranes is a multi-step process, as illustrated in Figure 1.8. 

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