Silica coating process

The economic impact of Iler s silica coating process will continue to play a major role in the coatings industry, from the perspective of the pigment manufacturers as well as the coatings producer. A conservative estimate of 300 mUlion is placed on the value of dense silica coated titanium dioxide pigments that were sold in 1990 in the world pigment marketplace. The total value of manufactured coatings products is estimated to be in excess of 4 billion. 

In bulk coating processes, bulk materials are joined to the substrate either by a surface melt process or by attachment of the solid material. An example of the latter is the application of heat-resistant tiles of silica-type material to the aluminum alloy skin of a space shutde vehicle, enabling the vehicle to withstand the reentry heat. 

A remaining question is the morphology of the interface between the silica top-layer and the intermediate Y-AI2O3 support layer. In the dip-coating process the silica layer may either form an almost flat layer on top of the polished support surface or partly infiltrate the pores in the y-layer. With conventional techniques (SEM-EDS, or TEM) the microscopic geometry of this interface cannot be elucidated. With RBS analysis a more clear indication can be obtained on the depth distribution of the silicon atoms with respect to the y-alumina surface. 

A dielectric oxide layer (eg. silica) is useful as a shell material because of the stability it lends to the core and its optical transparency. The classic method of Stober for solution deposition of silica are adaptable for coating of nanocrystals with silica shells.111 This method relies on the pH and the concentration of the solution to control the rate of deposition. The natural affinity of silica to oxidic layers has been exploited to obtain silica coating on a family of iron oxide nanoparticlcs including hematite and magnetite111 Such a deposition process is not readily extendable to grow shell layers on metals. The most successful method for silica encapsulation of metal nanoparlides is that due to Mulvaney and co-workers.114 In this method, the surface of the nanoparticles is functionalized with aminopropyltrimethylsilane. a bifunctional molecule with a pendant silane group which is available for condensation of silica. The next step involves the slow deposition of silica in water followed by the fast deposition of silica in ethanol. Fig. 13 shows the TEM images... 

Polyethylene oxide). This ether-rich polymer (PEO) apparently forms hydrogen bonds with silanol groups, with concomitant adsorption of a PEO layer at silica surfaces. In the case of CZE, the fused silica capillaries are pretreated with 0.1 M NaOH and 0.1 M SDS at the beginning of each day. The typical coating protocol is to flush the capillary with 1.0 M HC1, followed by a solution of 0.2% PEO, then washing with an electrophoretic buffer. The coating process has to be repeated before each run. The EOF is reduced by 60-70%, and the columns thus treated work well for basic proteins. 

With very smooth, high quality y-alumina supporting layers on a a-alumina support ultra-thin (100 nm) silica membranes with a pore diameter of 0.45-0.5 nm could be obtained. High quedity membranes with low defect levels are obtained in a two-step coating process. The thermal stability of these systems is limited to about 500°C and relatively low water partial pressures. Reseeirch to improve this is necessary. 

Y. Huang, K. Chou, Studies on the spin coating process of silica films, Ceram. Int. 29, 485-493 (2003). 

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