Dip and Spin Coating

Porous membranes arc composed of constituent particles or polymers. A dispersion of such particles with a controlled size distribution or polymer in a solvent (in most cases, water and/or alcohol) is called a slip. The slip can be deposited onto a rigid microporous 

For microfiltration applications, pore diameters of ceramic membranes in the range of 0.1 to 10 im arc typical These membranes can be prepared by dip or spin coating of 

Prior to spin coating, both ends of a pre-fabricated porous support, either in the form of a tube or a monolithic honeycomb element, are filled with the slip and temporarily sealed with gaskets. Slips of Ni, A1 and AI2O3 have been spin coated at a rotational speed of up to about 1300 rpm to form membrane layers [Sumitomo Electric Ind., 1981 NGK Insulators, 1986]. The development of a coating layer is expected to be faster with the in coating process than the dip coating process due to the centrifugal force. 

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 

Alternatively, the coating thickness can be related to the coating or withdrawal speed by theories of dip coating  

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Dip and Spin Coating. The dip coating technique described for webs can also be used to coat discrete surfaces such as toys and automotive parts. The surface to be coated is suspended on a conveyor and the part dipped into the coating solution. The surface is then removed, the coating drains, then levels to give the desired coverage. The object is then dried or cured in an oven. 

Depending on interfacial properties, some catalyst powder films are not stable in flowing solvent. For example, SiO2-containing materials have been reported to stick badly to the IRE in flowing solvent (5P). In such a situation, the catalyst particles may be fixed to the IRE by addition of a small amount of a polymer such as polyethylene before contacting of the slurry with the IRE 59,60. Films can also be prepared by dip- and spin coating. 

Figure 7.19 Dip and spin coating are two common techniques to form polymer films on solid surfaces.

As we mentioned in the Introduction, the bottom-up approach to materials design, or building the structure one molecule/atom at a time, provides the ultimate in control over the final properties of the material. For thin-film growth, this corresponds to vapor deposition techniques, rather than the top-down approaches of dip- and spin-coating. 

The dip and spin coating methods are the most practiced techniques as indicated in the open literature. Many materials have been made into porous membranes by this route including the more commonly referenced materials such as metal oxides, silicon carbide, silicon nitride, silicon and aluminum oxynitrides and glasses. 

Typically, SEM is used to determine the continuity and the thickness of the film. Large-scale cracks and defects of the film can be determined by top view SEM images, and film thickness is easily obtained from cross-sections. It has been used extensively with powdered samples of mesoporous silica to determine morphology and thus assist in phase determination. However, thin films produced by dip and spin coating typically have a scarcity of features that may be resolved in SEM. This can be seen in Figs. 6A and 6B, SEM images of a mesoporous films templated with P123. 

Obviously, as in the case of washcoating, the solid content of the sol impacts directly on the film thickness, but viscosity might also play a direct role for both dip and spin coating. 

In general, the sol-gel process is used as a phenomenological term to describe the conversion of colloid suspensions from a liquid (sols) into a solid (gel). The gel consists then of two phases a solid network penetrated by a second phase, either a liquid (wet gel) or a gas (xerogel). The most important technologies used to deposit sol-gel films technically are dip and spin coating [28]. 

We present an introduction to sol-gel processing with an emphasis on sih-cate formation. Dip- and spin-coatings, the two most nsefnl thin-tilm processing techniques for electrochemical applications, are briefly described. The section on thin films also includes an expanded account of recent developments in sol-gel electrodeposition of functional silicates, a field that has recently attracted considerable scientific attention. A classification of ways to electrodeposit thin films is provided. 

Polymer-modified electrodes can be prepared either by direct deposition of polymer onto the surface (via drop-, dip- or spin-coating methods) or by polymerization onto the electrode surface (via chemical, electrochemical or photochemical routes). The simplest method to prepare a polymer-based sensor is by drop-coating a small volume of polymer dissolved in a solvent. With time, the solvent evaporates leaving the polymer adsorbed onto the electrode surface. Dip- and spin-coating methods have also been used to obtain more uniform films. These methods are used when polymers are aheady synthesized and need to be immobilized as they are. In situ polymerization is another effective method to prepare polymer-modified electrodes. For electropolymerization, the electrode is immersed in a monomer solution (e.g., pyrrole, thiophene, phenol, aniline...) and a suitable potential (either cathodic or anodic) is applied to allow the formation of the polymer film on the electrode surface. Photopolymerization is rarer in the case of electrochemical sensors. Nevertheless, poly(vinyl alcohol) functionalized with styrylpyridinium and acrylated polyurethane have been used for the development of electrochemical sensors. 

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