Hydrolytic Sol-Gel Processes

Both stages are controlled by condensation chemistry that can include, as a first step, hydrolysis of hydrated metal ions or metal alkoxide molecules [6, 7] (hydrolytic sol-gel processing). The condensation chemistry in this case is based on olation/oxolation reactions between hydroxylated species. The hydroxylated species for further condensation can be formed also by a non-hydrolytic route, that is, by reactions between metal chlorides and alcohols with electron-donor substituents [8]. The nonhydrolytic sol-gel processing may... 

PMMA-Si02-Zr02 nanohybrid films were prepared successfully using a novel non-hydrolytic sol-gel process and, owing to their excellent optical transparency combined with high thermal stability, were stated to have high potential in applications for optical devices at elevated temperatures [307]. 

Acosta S., Corriu R.J.P., Leclercq D., Lefevre P., Mutin P.H., Vioux A. Preparation of alumina gels by a non-hydrolytic sol gel processing method. J. Non-Cryst. Solids 1994 170 234-242 Acosta S., Corriu R., Leclercq D., Mutin P.H., Vioux A. Novel non-hydrolytic sol-gel route to metal oxides. J. Sol-Gel Sci. Tech. 1994 2 25-28... 

Crouzet L., Leclercq D. Polyphosphazene-metal oxide hybrids by non-hydrolytic sol-gel processes. J. Mater. Chem. 2000 10 1195-1201... 

Hay J.N., Porter D., Raval H.M. A versatile route to organically-modified silicas and porous sihcas via the non-hydrolytic sol-gel process. J. Mater. Chem. 2000 10 1811-1818 Hay J.N., Raval H.M. Synthesis of organic-inorganic hybrids via the non-hydrolytic sol-gel process. Chem. Mater. 2001 13 3396-3403... 

Janackovic D., Orlovic A., Skala D., Drmanic S., Kostic-Gvozdenovic L.K., Jokanovic V., Uskokovic D. Synthesis of nanostructured mullite from xerogel and aerogel obtained by the non-hydrolytic sol-gel method. Nanostructured Mater. 1999 12 147-150 Jansen M., Guenther E. Oxide gels and ceramics prepared by a non-hydrolytic sol-gel process. Chem. Mater. 1995 7 2110-2114... 

Starting from solution as in sol—gel processes, the key to design the desired dispersion and mixing of the different metal centers lies in an understanding of the chemical reactivity and structural chemistry of the precursors. Typical precursors for sol-gel processes are metal alkoxides, for example, Ti(OR)4 for titania or Si(OR)4 for silica. A challenge in hydrolytic sol-gel processing of these different alkoxides is the different reaction rate for the hydrolysis reaction. This reaction typically proceeds via a nucleophilic substitution mechanism and therefore depends on the partial charge, 5, on the metal atom (Si or Ti). The... 

Depending on the desired materials properties and structure, these approaches have been explored for sol-gel processing of alkoxide-based precursors and further developed in combination with many experimental parameters that can also be adjusted to control the sol composition and particle size, including alkoxide/ water ratio, concentration of the precursors, reaction temperature, reaction time, drying methods, choice and concentration of the catalyst, and the solvent used [42]. Besides the well-known hydrolytic sol-gel processes, even nonhydro-lytic approaches have been reported as a powerful method in the synthesis of mixed oxide materials and will be discussed in more detail subsequently [43]. Another processing option to tailor the material properties is the application of soft or hard templates [44]. In addition to the chemical parameters, drying and thermal treatments or firing processes determine the final architecture, chemical... 

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