Porous colloidal ceramic processing

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... 

FIG. 1.6 A schematic diagram of colloidal processing of ceramic specimens. The figure illustrates some of the ways in which a dispersion is densified and transformed into porous or compact films or bulk objects. (Adapted and modified from Brinker and Scherer 1990.)... 

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. 

The sol-gel process involves the transition of a system from a liquid "sol" (mostly colloidal) into a solid "gel" phase (11). By applying this methodology, it is possible to fabricate ceramic or glass materials in a wide variety of forms ultrafine or spherical-shaped powders, thin film coatings, ceramic fibers, microporous inorganic membranes, monolithic ceramics and glasses, or extremely porous aerogel materials. 

The sol-gel process is a versatile solution process for making ceramic and glass materials involving the transition of a system from a colloidal suspension (sol) into a solid phase (gel) ". The resulting porous gel can be chemically purified and consolidated at high temperatures. In the classical sol-gel process, the precursor (e.g. a metal enolate or a metal aUcoxide) is exposed to a series of hydrolysis and polymerization reactions to form... 

Monodispersely-sized submircon zirconia colloids are useful starting material for ceramics, catalysts, and chromatographic stationary phases. One such process (polymerization-induced colloid aggregation or PICA process) requires entirely reproducible aqueous zirconia sols with no surfactants [1,2]. In the PICA process developed by Her and McQueston [3], the concentrated ( 20 wt %) 100 nm zirconia colloids are aggregated by urea-formaldehyde polymerization reaction to produce the porous zirconia particles in the size range of 4 - 6 pm [1,2],... 

Macromolecules are very important in the processing of powders into porous support layers. They are used to ensure the colloidal stability of ceramic suspensions, to control the rheology of suspensions and pastes, to avoid cracking... 

The aim of ceramic membrane production is to obtain defect-free (no cracks, no pinholes) supported films with homogeneous thickness and a narrow pore size distribution. By far the most important are sol-gel based processing routes. They have in common the fact that a porous support is contacted with a colloidal precursor solution for a given time to form a film which is processed after this step. The general procedure can be broken down into the following steps ... 

In the ceramic industry the rheology of clay suspensions plays a major role. Their colloidal properties are of especial significance in the process of slip-casting, in which a clay slurry is put in contact with a porous plaster mould, which sucks up the water to leave a solid form ready for firing, Here the suction provided by the porous mould in relation to the rate of flow of water through the clay matrix is important. The structure of the clay slurry, as determined by its degree of dispersion, has also to be controlled by suitable additives. 

Cellulose nanocrystals have been used in the elaboration of ceramics [170, 171], Shin and Exarhos [170] have prepared porous Titania by using a template process with cellulose nanocrystals. A colloid suspension of cellulose nanocrystals was added into titanium (IV) bis(ammonium lactate)dihydroxide (Tyzor-LA) to form a Tyzor-LA-cellulose nanocrystals composite. After calcination at 500 °C in air, a porous structure was obtained. 

The sol-gel process, which refers to a multitude of reactions that employ a wide variety of alkoxide precursors to prepare many different inorganic oxide-based products, has been applied most often to the production of glasses and ceramic oxides [22]. Typically, the sol-gel process involves a metal alkoxide such as Si(OR>4, water, and a second solvent that acts as a miscibility agent. The particles that develop in the colloidal sol under abasic condition cross-link to form a gel, which is subsequently dried to form a porous glass. The fundamental sequence of the reactions responsible for the ultimate development of a cross-linked three-dimensional solid matrix is as... 

The main objective of the project, ie. to study the reaction bonding technique as an effective way to produce porous silicon carbide ceramics, has been successfully achieved. This includes the colloidal processing of the precursor powders, compaction through slip-casting, and finally sintering to achieve the muilitization reaction. Characterization of the sintered and polished samples using SEM and XRD have confirmed the formation of the muliite phase. 

Sasaki T., Watanabe M., Hashizume H., Yamada H., Nakazawa H. Macromolecule-like aspects for a colloidal suspension of an exfoliated titanate. Pairwise association of nanosheete and dynamic reassembling process initiated from it. J. Am. Chem. Soc. 1996 118 8329-8335 Sasaki T., Nakano S., Yamauchi S., Watanabe M. Fabrication of titanium dioxide thin flakes and their porous aggregate. Chem Mater. 1997 9 602-608 Sathiyakumar M., Gnanam F.D. Synthesis of sol-gel derived alumina powder effect of milling and calcination temperatures on sintering behaviour. Brit. Ceram. Trans. 1999 98 87-92... 

The processes where porous membranes find their main applications are pressure-driven ones, such as microfiltration, ultrafiltration and nanofiltration. These processes are also especially interesting due to their wide range of practical applications. They can be used for the processing of fine particles, colloids and biological materials such as protein precipitates and microorganisms [3]. Membranes used are commonly polymeric materials but innovative development has been made in the fields of ceramic and inorganic membranes. 

A part from EDM, a near net shaping process is very important when the final purpose is the obtainment of a complex geometry, and above all, complex porous architectures. Several techniques are available to realize ceramics with complex shape and/or tailored porous architecture and they are all based on ceramic colloidal processes (Lewis, 2000). Furthermore, water based near net-shaping is of major interest among the wet-based forming processes being more environmental friendly and low cost. 

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