Colloidal processing slip-casting

Slip-casting has been a major part of traditional ceramics processing technology for at least 250 years. It was introduced for the casting of alumina ware in the early part of the last century and since then has been extensively exploited for forming advanced ceramics. 

In relatively recent years the term colloidal processing has been coined to describe processing technologies in which the colloidal properties of ceramic powders are exploited. J.A. Lewis [6] comprehensively reviews colloidal processing and its potential. 

In the slip-casting process the ceramic powder is suspended in a fluid vehicle, usually water. The suspension, or slip , has a high solids content, typically 50vol.%, and the individual particles are fine, usually less than 10 /mi. Deflocculants, which modify the electrical environment of each particle so that they repel one another, are added to the slip. The fineness of the powder and consequent high surface area ensure that electrostatic forces dominate gravity forces so that settling does not occur. When exceptionally heavy powder particles 

The casts are usually sufficiently dense to yield low-porosity (5% or less) bodies on sintering. The relatively slow dewatering process evidently results in close-packed particles and, compared to dry powder compaction routes, with less risk of introducing defects. 

This process offers a route for the manufacture of complex shapes and, in the traditional pottery industry, is the accepted method for the production of teapots, milk jugs, figurines and large articles such as wash-hand basins. It may be necessary for the mould to be made up of a number of pieces so that the cast article can be removed. 

---

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... 

Figure 2.8. Synthesis route of alumina (a) the colloidal suspension route and (b) the slip-casting process (Larbot et al. 1987, Uhlhorn et al. 1989).

Dip coating is analogous to a slip casting process for making ceramic parts. The membrane deposition behavior by slip casting can be described by a theory of colloidal filtration for incompressible cakes [Aksay and Schilling, 1984] and compressible cakes [Tiller and Tsai, 1986). The theory predicts that the thickness of the consolidated layer, L, is given by... 

The process starts with contacting a porous support with the colloidal precursor solution in a dip-coating or a spin-coating process. Film formation starts either with a film-coating or a slip-casting mechanism as will be discussed below. 

Colloidal Processing In ceramics, a variation of slip-casting in which a stabilized colloidal dispersion of particles is poured into a mold for sintering. 

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. 

Concerning microfiltration, in which pores of less than 1 pm are needed (typically 0.1-1 pm), suspensions of submicrometer powders must be processed using slip-casting or tape-casting techniques, depending on the support shape (flat or tubular). Smaller pore sizes are required for ultrafiltration. In this case pores result from the packing of colloidal particles, which cannot... 

Alternative fabrication procedures for dispersion-reinforced glass matrix composites via wet routes have been developed such as slip and tape casting [117,118], as well as colloidal processing and sol-gel techniques [74]. 

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. 

---