Carbon silica coatings

For electrical insulation china clay is commonly employed whilst various calcium carbonates (whiting, ground limestone, precipitated calcium carbonate, and coated calcium carbonate) are used for general purpose work. Also occasionally employed are talc, light magnesium carbonate, barytes (barium sulphate) and the silicas and silicates. For flooring applications asbestos has been an important filler. The effect of fillers on some properties of plasticised PVC are shown in Figure 12.21 (a-d). 

Whitsitt, E.A., et al., LPD silica coating of individual single walled carbon nanotubes. Journal of Materials Chemistry, 2005.15(44) p. 4678-4687. 

Whitsitt, E.A. and A.R. Barron, Silica Coated Single Walled Carbon Nanotubes. Nano Letters, 2003. 3(6) p. 775-778. 

Chen, R. and Li, Z. (1993). A study of silica coatings on the surface of carbon or graphite fiber and the interface in a carbon/magnesium composite. Composites Sci. Technol. 49, 357-362. 

Hybrid bonded-phase columns are being produced with carbon chains cross-linked chemically to the silica surface to reduce the amounts of free silanols and to increase the stability of the surface in the presence of high pH media. Hybrid silica column with a bridged organo-silica coating (Fig. 5.2)... 

Takenaka, S., Orita, Y., Umebayashi, H., Matsune, H., and Kishida, M. High resistance to carbon deposition of silica-coated Ni catalysts in propane stream reforming. Applied Catalysis. A, General, 2008, 351 (2), 189. 

Hydrated silica—coated activated carbon fiber... 

FIG. 15 (A) Optical micrograph of a silica-coated 10 /xm carbon fiber at a mag-... 

In addition to the above, preparation in w/o microemulsions of nanoparticles of various other types of compounds, viz. silica-coated iron oxide, Fe203-Ag nanocomposite, oxides of ytrium, erbium, neodymium, vanadium and cobalt, titanates of barium and lead, ferrites of barium, strontium, manganese, cobalt and zinc, oxide superconductors, aluminates, zirconium silicate, barium tungstate, phosphates of calcium, aluminium and zinc, carbonates of calcium and barium, sulphides of molybdenum and sodium, selenides of cadmium and silver etc. have been reported. Preparative sources and related elaboration can be found in [24]. 

Thus, silica coating appears to retard both the formation of the hard carbon skin and the growth of the secondary (less dense) coke deposit at cracks in the skin, with the overall result that in cracking runs up to three days in length in the laboratory ESC reactor the coke formation is inhibited by a factor of 3-4 compared to uncoated tubes. 

Coatings can be deposited on carbon fibers either from liquid precursors, e.g., organometallic species or sols, or from gaseous precursors, e.g., by chemical vapor deposition (CVD) or infiltration (CVI). For example, a silica coating deposited from tetraethylorthosilicate lowers the oxidation rate of carbon fiber at 650°C by a factor of 5 and at 430°C by a factor of 30 [68]. Likewise, a 50 nm thick B4C or SiC coating deposited by reactive CVD significantly lowers the oxidation rate of carbon fibers [69]. 

The major deficiency of carbon fibers is their sensitivity to oxidation even at relatively low temperatures. Although silicon carbide (SiC) fibers are also sensitive to oxidation, their oxidation starts at higher temperatures and yields a protective silica coating. In an oxidative environment, SiC and Si-C-0 fibers are generally more useful than carbon fibers [1-3]. Large diameter silicon carbide fibers are obtained by chemical vapor deposition (Chapter 4). Small diameter silicon carbide and oxycarbide fibers are derived from solid polydimethylsilazane precursor fibers (this chapter). 

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