Ceramic membranes by sol-gel

In this section a short introduction will be given on the synthesis of porous ceramic membranes by sol-gel techniques and anodization, carbon membranes, glass membranes and track-etch membranes. An extensive discussion will be given in Sections 2.3-2.S. 

While considerable progress has been made in the preparation of ceramic membranes by sol-gel processing, the development of membranes from hybrid polymers is in its infancy (see also Section V). This is, nevertheless, a very promising area of development, because the possibility of forming mechanically stable membranes by inorganic polycondensation is implemented by the possibility to incorporate organic functions. 

Hollow glass microbeads Porous ceramic membranes Microbeads coated by Ti02 particles Porous ceramic Ti02 and ZnO membranes prepared by sol-gel technique... 

The slip coating-sintering procedure can be used to make membranes with pore diameters down to about 100-200 A. More finely porous membranes are made by sol-gel techniques. In the sol-gel process slip coating is taken to the colloidal level. Generally the substrate to be coated with the sol-gel is a microporous ceramic tube formed by the slip coating-sintering technique. The solution coated onto this support is a colloidal or polymeric gel of an inorganic hydroxide. These solutions are prepared by controlled hydrolysis of metal salts or metal alkoxides to hydroxides. 

R.S.A. de Lange, J.H.A. Hekkink, K. Keizer and A.J. Burggraaf, Formation and characterisation of supported microporous ceramic membranes prepared by sol-gel modification techniques. /. Membr. Sci., 99 (1995) 57-75. 

Ceramic membranes, which are much stronger than polymer membranes, are conunonly made of Al, Si, Ti or Zr oxides and mixed oxides. The top layers of the membrane are prepared by sol-gel synthesis which converts a colloidal or polymeric solution of inorganic precursors (e.g. alkoxides or salts) to a gelatinous product. Viscous binders are usually added prior to layering followed by controlled calcination and finally sintering to form the ceramic membrane. The process can be controlled to provide membranes with specific MWCOs (molecular weight cut-offs) and which can sustain defined solvents, temperatures and pressures. 

Figure III - 58. Schematic drawing of the preparation of ceramic membranes by the sol-gel process 156.571...

Workers of the same group suggested using polytetrafluoroethylene (PTFE) in membranes of this type to raise the thermal resistance (Reichman et al., 2007). Nano-sized ceramic particules produced from tetraethyl orthosilicate [TEOS Si(OC2Hs)4] by sol-gel synthesis were introduced into a porous PTFE layer to give the material some degree of hydrophilicity and to secure a sufficiently... 

Sampath S., Pankratov L, Gun J., Lev O. Sol-Gel derived ceramic-carbon enzyme electrodes glncose oxidese as a test case. J. Sol-Gel Sci. Technol. 1996 7 123-128 Santos L.R.B., Belin S., Briois V., Santilli C.V., Pulcinelli S.H., Larbot A. Study of structural surface modified tin oxide membrane prepared by sol-gel route sintered at 400°C. J. Sol-Gel Sci. Technol. 2003 26 171-175... 

Okubo T., Nagamoto H. Low temperature preparation of nanostructured zirconia and YSZ by sol-gel processing. J. Mater. Sci. 1995 30 749-757 Okubo T., Takahashi T., Sadakata M., Nagamoto H. Crack-free porous YSZ membrane via controlled synthesis of zirconia sol. J. Membr. Sci. 1996 118 151-157 Palinko L, Torok B., Surya Prakash G.K., Olah G.A. Surface characterization of variously treated Nafion-H, Nafion-H supported on silica and Nafion-H silica nanocomposite catalysts by infrared microscopy. Appl. Catal. A Gen. 1998 174 147-153 Schmidt H., Wolter H. Organically modified ceramics and their applications. J. Non-Cryst. SoUds 1990 121 428 35... 

De Lange, R. S. A., Hekkink, J. H. A., Keizer, K. and Burggraaf, A. J. (1995a) Formation and characterization of supported microporous ceramic membranes prepared by sol-gel modification techniques. Journal of Membrane Science, 99,57-75. 

The sol—gel technique has been used mosdy to prepare alumina membranes. Figure 18 shows a cross section of a composite alumina membrane made by sHp coating successive sols with different particle sizes onto a porous ceramic support. SiUca or titanium membranes could also be made by the same principles. Unsupported titanium dioxide membranes with pore sizes of 5 nm or less have been made by the sol—gel process (57). 

Once the membrane was successfully produced, it was analysed for characterisation and scanning. The sol-gel technique was successfully used to obtain a crack-free unsupported membrane, which was expected to have pore size of 1-2 nm. The development of the crack-free membrane may not have the same strength without strong, solid support. The next stage of this work was to characterise the fabricated membrane. Hie objectives of this study were to develop a zirconia-coated 7-alumina membrane with inorganic porous support by the sol-gel method and to characterise the surface morphology of the membrane and ceramic support. 

Ceramic Ceramic membranes are made generally by the sol-gel process, the successive deposition of ever smmler ceramic precursor sph eres, followed by firing to form multitube monoliths. The diameter of the individual channels is commonly about 2 to 6 mm. Monoliths come in a variety of shapes and sizes. A 19-channel design is common. One manufacturer makes large monoliths with square channels. 

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... 

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