Porous ceramic membranes microstructure

Organic or inorganic entities as well as polymer particles can also be used as template agents in the preparation of porous ceramic membranes following either the polymeric or the colloidal sol-gel route. The strategy to control microstructure in porous material is illustrated in Fig. 7.13. The template agents are trapped during matrix formation and eliminated in a second step with the aim to define the pore size in the final material. 

Abstract This chapter discusses the research and development of porous ceramic membranes and their application as membrane reactors (MRs) for both gas and liquid phase reaction and separation. The most commonly used preparation techniques for the synthesis of porous ceramic membranes are introduced first followed by a discussion of the various techniques used to characterise the membrane microstructure, pore network, permeation and separation behaviour. To further understand the structure-property relationships involved, an overview of the relevant gas transport mechanisms is presented here. Studies involving porous ceramic MRs are then reviewed. Of importance here is that while the general mesoporous natnre of these membranes does not allow excellent separation, they are still more than capable of enhancing reaction conversion and selectivity by acting as either a product separator or reactant distributor. The chapter closes by presenting the future research directions and considerations of porous ceramic MRs. 

Dong, J., lin, Y.S., Hu, M.Z.C., Peascoe, R.A., and Payzant, E.A. (2000) Template-removal-associated microstructural development of porous-ceramic-sup-ported MFI zeolite membranes. 

The evaluation of the commercial potential of ceramic porous membranes requires improved characterization of the membrane microstructure and a better understanding of the relationship between the microstructural characteristics of the membranes and the mechanisms of separation. To this end, a combination of characterization techniques should be used to obtain the best possible assessment of the pore structure and provide an input for the development of reliable models predicting the optimum conditions for maximum permeability and selectivity. The most established methods of obtaining structural information are based on the interaction of the porous material with fluids, in the static mode (vapor sorption, mercury penetration) or the dynamic mode (fluid flow measurements through the porous membrane). 

The synthesis of nanophase ceramics is one of these concepts, it allows micro-porous ceramic materials with ceramic grains in the nanometer range to be obtained. Research in the field of nanophase materials is very active. A number of results on the control of microstructure and temperature stability of metal oxide ceramics can be applied to membrane preparation. Works carried out on non-oxide ceramics such as silicon carbide, silicon oxinitride or aluminum nitride should be regarded in order to extend the domain of available membrane materials. 

Dong JH, Lin YS, Hu MZC, Peascoe RA, Payzant FA. Template-removal-associated microstructural development of porous-ceramic-supported MFI zeolite membranes. Micropor Mesopor Mater 2000 34(3) 241-253. 

In recent years there has been tremendous interest in porous ceramics because of their applications as filters, membranes, catalytic substrates, thermal insulation, gas-burner media and refractory materials. These are due to their superior properties, such as low bulk density, high permeability, high temperature stability, erosion/corrosion resistance and excellent catalytic activity. One branch of this field is porous SiC ceramics, owing to their low thermal expansion coefficient, high thermal conductivity and excellent mechanical properties. However, it is difficult to sinter SiC ceramics at moderate temperatures due to their covalent nature. In order to realize the low temperature fabrication of porous SiC ceramics, secondary phases may be added to bond SiC. Oxidation bonded porous SiC ceramics have been found to exhibit good thermal shock resistance owing to the microstructure with connected open pores. 

Burggraaf, A.J., Bouwmeester, H.J.M., Boukamp, B.A., Uhlhom, R.J.R., and Zaspalis, V.T., Synthesis, microstructure and properties of porous and dense ceramic membranes, in Science of Ceramic Interfaces, Nowotny, J., Ed., Elsevier Science Publishers B.V., North-Holland, 1991, 525-568. 

Commercialized ceramic membranes can be of different configurations, from disc/fiat-sheet to multichannel tubes. The membrane microstructure is always very similar, that is, a thin separation layer made of finer particles supported onto a more porous multilayer substrate with a gradient pore structure, as shown in Figure 10.1. The only layer involved in separation is normally the thinnest layer with the finest pore size. The thickest layer, which consists of big particles for low resistance to permeates, provides the mechanical strength, whereas the layers in between are needed for a uniform formation of the top separation layer. Normally, different techniques are used for each individual layer, which results in soaring expenditures with the number of layers involved. 

The porous membranes consist of a porous metal or ceramic support with porous top layers which can have different morphologies and microstructures. Their essential structural features are presented in Figures 2.1 and 2.2 and are discussed later (Section 2.2). 

Schrotter I.C., Smaihi M., Guizard C. Polyimide-siloxane hybrid materials influence of coupling agent addition on microstructure and properties. J. Appl. Polym. Sci. 1996 61 2137-2149 Soler-Dlia G.I., de A.A., Crepaldi E.L., Grosso D., Sanchez C. Block copolymer-templated meso-porous oxides. Cnrr. Opin. Colloid Interface Sci. 2003 8(1) 109-126 Steele B. Ceramic Ion Conducting Membranes and Their Technological Apphcation. C.R. Acad. Sci. Paris 1998, Series lie. Title 1, pp. 533-543... 

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