Ceramics, microporous

Most glass-ceramics have low dielectric constants, typically 6—7 at 1 MHz and 20°C. Glass-ceramics comprised primarily of network formers can have dielectric constants as low as 4, with even lower values (K < 3) possible in microporous glass-ceramics (13). On the other hand, very high dielectric constants (over 1000) can be obtained from relatively depolymerized glasses with crystals of high dielectric constant, such as lead or alkaline earth titanate (11,14). 

Ceramic, Metal, and Liquid Membranes. The discussion so far implies that membrane materials are organic polymers and, in fact, the vast majority of membranes used commercially are polymer based. However, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafHtration and microfiltration appHcations, for which solvent resistance and thermal stabHity are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported or emulsified Hquid films are being developed for coupled and facHitated transport processes. 

Because membranes appHcable to diverse separation problems are often made by the same general techniques, classification by end use appHcation or preparation method is difficult. The first part of this section is, therefore, organized by membrane stmcture preparation methods are described for symmetrical membranes, asymmetric membranes, ceramic and metal membranes, and Hquid membranes. The production of hollow-fine fiber membranes and membrane modules is then covered. Symmetrical membranes have a uniform stmcture throughout such membranes can be either dense films or microporous. 

Capillary Suction Processes. The force needed to remove water from capillaries increases proportionately with a decrease in capillary radius, exceeding 1400 kPa (200 psi) in a 1-p.m-diameter capillary. Some attempts have been made to use this force as a way to dewater sludges and cakes by providing smaller dry capillaries to suck up the water (27). Sectors of a vacuum filter have been made of microporous ceramic, which conducts the moisture from the cake into the sector and removes the water on the inside by vacuum. Pore size is sufficiently small that the difference in pressure during vacuum is insufficient to displace water from the sector material, thus allowing a smaller vacuum pump to be effective (126). 

Surface media Captures particles on the upstream surface with efficiencies in excess of depth media, sometimes close to 100% with minimal or no off-loading. Commonly rated according to the smallest particle the media can repeatedly capture. Examples of surface media include ceramic media, microporous membranes, synthetic woven screening media and in certain cases, wire cloth. The media characteristically has a narrow pore size distribution. 

The only ceramic membranes of which results are published, are tubular microporous silica membranes provided by ECN (Petten, The Netherlands).[10] The membrane consists of several support layers of a- and y-alumina, and the selective top layer at the outer wall of the tube is made of amorphous silica (Figure 4.10).[24] The pore size lies between 0.5 and 0.8 nm. The membranes were used in homogeneous catalysis in supercritical carbon dioxide (see paragraph 4.6.1). No details about solvent and temperature influences are given but it is expected that these are less important than in the case of polymeric membranes. 

Membrane materials have to withstand a pressure difference and relatively high temperatures (500 °C and up). Microporous ceramic membranes have been... 

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... 

Ohya, H., Y. Tanaka, M. Niwa, R. Hongladaromp, Y. Negismi and K. Matsumoto. 1986. Preparation of composite microporous glass membrane on ceramic tubing. Maku 11 41-44. 

Bhavc, R. R., J. Gillot and P. K. T. Liu. 1989. High temperature gas separations for coal offgas cleanup with microporous ceramic membranes. Paper 124f read at AIChE Annual Meeting, 5-10 November 1989, San Francisco. 

Uhlhom, R. J. R., M. H. B. J. Huis in t Veld, K. Keizer and A. J. Burggraaf. 1989a. Theory and experiments on transport of condensable gases in microporous ceramic membrane systems. Proc. 1st Inti Cong, Inorganic Membrane, 3-6 July, 323-328, Montpellier. 

V. Schroder, O. Behrend, and H. Schubert Effect of Dynamic Interfacial Tension on the Emulsification Process Using Microporous Ceramic Membranes. J. Colloid Interface Sci. 202, 334 (1998). 

V. Schroder and H. Schubert Emulsification Using Microporous Ceramic Membranes. In Proceedings of the First European Congress on Chemical Engineering (ECCE 1) 2491, Florence Italy (1997). 

Fe impurities may cause snow flakes , because of the nucleation and crystallisation of YAG (Y3Al50i2) [532]. The volume change during crystallisation of the grain boundary phase leads to internal stresses which can be cause micropores, or microcracks, or relax by other mechanisms [534, 535]. Such microcracks have only been detected in ceramics with crystallised / -Y2Si207 as grain boundary phase [533]. 

Finely microporous Intermediate pore-flow ceramic/carbon solution-diffusion... 

Figure 2.41 The change in nitrogen flux through a PTMSP membrane caused by the presence of a condensable vapor in the feed gas [71]. This behavior is characteristic of extremely finely porous microporous ceramic or ultrahigh-free-volume polymeric membranes such as PTMSP. The condensable vapor adsorbs in the 5- to 15-A-diameter pores of the membrane, blocking the flow of the noncondensable nitrogen gas...

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. 

Recently, attempts have been made to reduce the cost of palladium metal membranes by preparing composite membranes. In these membranes a thin selective palladium layer is deposited onto a microporous ceramic, polymer or base metal layer [19-21], The palladium layer is applied by electrolysis coating, vacuum sputtering or chemical vapor deposition. This work is still at the bench scale. 

During the last few years, ceramic- and zeolite-based membranes have begun to be used for a few commercial separations. These membranes are all multilayer composite structures formed by coating a thin selective ceramic or zeolite layer onto a microporous ceramic support. Ceramic membranes are prepared by the sol-gel technique described in Chapter 3 zeolite membranes are prepared by direct crystallization, in which the thin zeolite layer is crystallized at high pressure and temperature directly onto the microporous support [24,25],... 

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