Metal Alkoxide Precursor Types

In monolith febrication it is required to take into account the influence of the precursor gradient chemical composition on the sol-gel process. Parameters that affect the process are metal alkoxide precursor types, pH effect - type of catalyst used, H2O Si molar ratio (/ ), steric effect of precursor ligand groups, and functionality of organically modified silanes (ORMOSIL). 

The sol-gel process can be performed with a variety of metal alkoxides. The most common metal alkoxides used in the sol-gel process are silicon-based alkoxides such as tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS). Silicon-based sol-gel process results in the formation of an inorganic polymer of silica, Si02, with tetrahedral imit cell geometry. A dried gel xerogel after firing at 700 °C will transform completely to fused sflica. Additional frequently used metal alkoxides for the sol-gel process are transition metal alkoxides such as Ti, 

As is well known, acidic and basic catalysts differently affect hydrolysis and condensation reaction rates in the sol-gel process. Acidic catalysts exert more influence on the hydrolysis reaction due to an electrophilic reaction [9]. On the other 

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Besides pH, other preparative variables that can affect the microstructure of a gel, and consequendy, the properties of the dried and heat-treated product iaclude water content, solvent, precursor type and concentration, and temperature (9). Of these, water content has been studied most extensively because of its large effect on gelation and its relative ease of use as a preparative variable. In general, too Httie water (less than one mole per mole of metal alkoxide) prevents gelation and too much (more than the stoichiometric amount) leads to precipitation (3,9). Other than the amount of water used, the rate at which it is added offers another level of control over gel characteristics. 

The volatile metal-containing precursors which satisfy the ALD criteria fall into four main categories (i) halides, (ii) y0-diketonate complexes, (iii) alkoxides, and (iv) true organometallics, viz. metal alkyls and cyclopentadienyl-type compounds (Fig. 3). Also amido complexes have recently gained attention as possible ALD precursors. Occasionally other compounds have been used as ALD precursors for thin films, for instance, metal nitrates, carboxy-lates and isocyanates [17,18]. 

Metal alkoxides are of interest as precursors to oxide materials with ceramic or electronic applications.1 Barium incorporation in such a precursor is particularly interesting because of the known high-Tc superconductor, YBa2Cu307. The title compound has vast possibilities for reactions of the type... 

Note that the equilibrium of the following type of reaction was postulated by Dislich (13) in 1971 to explain the ultrahomogeneity of the final ceramic product obtained in the sol-gel process that employed a mixture of alkoxides with different metals as precursors ... 

Sol-gel processing can be roughly divided into two areas based on the type of precursor used the alkoxide route, i.e., the controlled hydrolysis and condensation of a hydrolytically unstable precursor (usually based upon a metal alkoxide species) soluble in non-aqueous solvents and the colloidal route, the peptidization of a water-soluble precursor (also known as colloidal processing). Hydrolysis of an aqueous metal precursor occurs on changing the pH of the precursor solution. These types are usually discussed separately. 

There are two types of precursors inorganic precursors (metal salts) or metal-organic (alkoxide) precursors. It is not the aim of this chapter to give an overview on all materials made by sol-gel processing, but instead to discuss the chemistry of the precursors, including ways to influence their reactivity, and the basic chemistry of sol-gel processes. 

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