Aqueous sol-gel route

According to Pierre [9], during powder formation via both alkoxide and aqueous sol-gel routes, the precursors undergo the following sequence of transformations in the presence of water ... 

Figure 26.2 (a) Aligned hollow barium ferrite nanoparticles prepared by nonhydrolytic sol-fibers prepared by aqueous sol-gel route. gel reaction coated with oleic acid, (c) Mag-... 

The ability to form hybrid silica glasses under aqueous, room-temperature conditions (at which proteins and cells are active) opens up the possibility to extend sol-gel processing to the encapsulation of biologicals. However, as mentioned earlier, the traditional sol-gel route using silicon alkoxides has to be adapted to avoid excess alcohol as well as low pH conditions (see section 3.2.1). Moreover, because biospecies could be denatured by covalent bonding, most bioorganic-inorganic composites are Class 1 hybrids. 

The preparation of the required microporous ceramic layers is possible by the sol-gel route from stable colloidal dispersions with individual nanoparticles of less than 10 nm. Different types of ceramic nanofilters have been prepared from such aqueous or organic sols of the following oxides y-alumina, zirconia, ° hafnia," and titania. ... 

To a suspension of tin(IV) oxide gel in triply distilled water was added a solution of chromium(VI) oxide and the solution stirred for 24h. The resulting yellow mixture was filtered and the yellow solid air dried at 60°C for 24h. If desired, tlie powdery material was then washed with distilled water imtil the washings were colourless. The resulting material was again air dried. Copper(II)-doped tin(lV) oxide was either prepared in a similar manner using a solution of copper(II) nitrate, or by coprecipitation from an aqueous solution containing tin(IV) chloride and copper(II) nitrate in the required ratio. Both chromiun(VI)- and copper(II)-doped catalysts were also prepared by treatment of tin(VI) oxide sol-gel using aqueous solutions of cliromium(VI) oxide or copper(II) nitrate. Mixed Sn-Cu-Cr-0 catalysts were obtained by the sol-gel route. 

Kozhukharov V., Machkova M., Brashkova N. Sol-gel route and characterization of supported perovskites for membrane apphcation. J. Sol-Gel Sci. Technol. 2003 26 753-757 Kragten J. Atlas of Metal-Ligand Equilibria in Aqueous Solution. New York Ellis Horwood limited publisher, John Whey and Sons, 1978. 

Besides the sol-gel processing, there are many aqueous routes to synthesize ceramic powders, fibers, films and bulks. Niesen and DeGuire reviewed these low temperature and non-electrochemical processes (Niesen and DeGuire, 2001). According to them, the processes include a chemical-bath deposition (CBD), successive ionic-layer adsorption and reaction (SILAR), liquid-phase deposition (LPD), electroless deposition (ED), and film deposition on organic self-assembled monolayers (SAMs). Of course, an electrochemical route is an important process. Another non-sol-gel route is a spray pyrolysis of solution or sol, and is applied to the direct preparation of oxide powders and films. Since these processes do not form the gel phase, they are not described here. 

There are other many aqueous, non-sol-gel routes including electro-deposition (Izaki and Omi, 1997 Peulon and Lincot, 1998), deposition on SAMs and tepmplates (Boyle et al., 2002 Saito et al., 2002), and spray pyrolysis (Caillaud et al., 1993 Maldonado et al., 1999 Loldiande and Uplane, 2000). 

Bhattacharya A.K., Hartridge A., MaUick K.K., Taylor D. A novel aqueous route for the synthesis of mullite fibres. J. Mater. Sci. Lett. 1996a 15 1654—1656 Bhattacharya A.K., Hartridge A., MaUick K.K., Taylor D. Inorganic sol-gel synthesis of zirconium titanate fibres. J. Mater. Sci. 1996b 31 5583-5590 Boulton J.M., Jones K., Enblem H.G., Preparation of cordierite fibers by a sol-gel route. J. Mater. Sci. Lett. 1989 8 1087-1088... 

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