Dense ceramic membranes preparation

Large-scale membrane fabrication. Preparation of dense ceramic membranes is a complex multi-step process. Many factors may influence the microstructure and performance of the resulting membranes. This leads to low reproducibility and low yields to fabricate high-quality membranes on a large scale. 

Dense ceramic membranes can be fabricated in three configurations disc/ flat-sheet membrane, tubular membrane and hollow fibre membrane. Their preparation processes are described respectively in details as follows. 

As described above, dense ceramic membranes are made of composite oxides with a large number of oxygen vacancies in the crystaUine lattice. Such materials are inherently catalytic to the oxidation and dehydrogenation reactions. Therefore, dense ceramic membrane may serve as both catalyst and separator, and catalyst is not required in the membrane reactor. As shown in Fig. 7.5a, the lattice oxygen directly takes part in the chemical reactions. Since the chemical reactions take place on the membrane surface, it is required to have a very porous membrane surface so as to contain a sufficient quantity of active sites. This can be achieved in the membrane preparation process, or by coating a porous membrane material after the preparation. The main potential problems for this are that the membrane may not have sufficient catalytic activity, and the catalytic selectivity cannot be modulated with respect to the considered reactions. 

In this chapter membrane preparation techniques are organized by membrane structure isotropic membranes, anisotropic membranes, ceramic and metal membranes, and liquid membranes. Isotropic membranes have a uniform composition and structure throughout such membranes can be porous or dense. Anisotropic (or asymmetric) membranes, on the other hand, consist of a number of layers each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense, thin surface layer supported on an open, much thicker micro-porous substrate. The surface layer performs the separation and is the principal barrier to flow through the membrane. The open support layer provides mechanical strength. Ceramic and metal membranes can be either isotropic or anisotropic. 

The methods of preparing inorganic membranes with tortuous pores vary enormously. Some use rigid dense solids as the templates for creating porous structures while many others involve the deposition of one or more layers of smaller pores on a premanufactured microporous support with larger pores. Since ceramic membranes have been studied, produced and commercialized more extensively than any other inorganic membrane materials, more references will be made to the ceramic systems. 

Now considering dense membranes, attention will be focused only on ceramic membranes, since a detailed description of the preparation and properties of the interesting and promising metal membranes have been described in detail in the preceding chapter of this book. Data concerning the permeability of Ag and Pd-alloy membranes, though, are listed in Table 2 for comparison. 

Tape casting is a well known method for making thin, flat and dense ceramics [13-15]. This technique is limited by the thickness (a few millimetres) of the films obtained. The most important applications are in the electronics industry. Simon et al. [16] have described the preparation of flat ceramic membranes. This type of geometry is receiving interest because of the possibility of obtaining a high module compactness (compactness is defined by the ratio of membrane surface area to module volume). 

Mtxed-ronducting dense (nonporous) solid oxide membranes prepared out of appropriate perovskite ceramic materials are such that both ion induction and eleirtron transport can occur through the membrane at h temperatures the material is otherwise impermeable to gases. No voltage is applied across the membrane. 

The preparation of composite PCMs starts with the synthesis of porous substrates, followed by the formation of thin PCM films. Detailed procedures are illustrated in Figure 6.3. An exact procedure should be conducted to obtain thin, dense ceramic films [17].The carbon black content for the synthesis of porous substrates can be varied so as to determine conditions that match accurately the shrinkage profile of the porous substrates with that of the deposited perovskite films during the final sintering of the composite membrane. Partial sintering of the green substrates is conducted to match their subsequent shrinkage with that of coated powders so that fissures and cracks can be minimized. 

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