Sol—gel process

A sol is basically a stable dispersion of nanometer-sized particles and most of the voluminous research that has been conducted in this field is related to the formation of metal oxide sols. Metal oxide sols can be prepared by the following various techniques as summarized by Livage [1986]  

In the first route where typically a metal salt or a hydrated oxide is added to a non-stoichiometrically excess amount of water, a particulate sol is precipitated which consists of gelatinous hydroxide particles. The hydrolysis rate is fast. The primary colloidal particles so obtained are usually in the range of 5 to 15 nm. 

The hydrolysis can be acid- or base-catalyzed. In the case of silica, acid-catalyzed hydrolysis results in sols with weakly-branched polymers that lead to final products having smaller pores while base-catalyzed hydrolysis produces particulate sols and final porous bodies showing larger pores [Brinker ct al.. 1982 Shafer ct al.. 1987]. 

The membrane pore diameter appears to depend on the acid concentration (Table 3.1). In the case of alumina membranes, as the ratio of acid to alkoxide increases, the pore diameter of the final membrane decreases [Leenaars et al., 1984]. It should be mentioned that the mean pore diameter may not always decrease as the acid to sol concentration ratio increases. In fact, Yoldas [1975b] demonstrated a maximum pore diameter at a value of the acid to sol ratio near 0.05-0.06. Below that ratio, the pore diameter actually increases slightly as the ratio increases. 

Effect of acid concentration on diameters of precusor particles and membrane pores 

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C. J. Brinker and G. W. Scherer, Sol-Gel-Science The Physics and Chemistry of Sol-Gel Processing Academic Press, San Diego, Calif., 1990. 

Sol—Gel Sintered Aluminum Oxide. A new and much more versatile sintered alumina abrasive is now produced from aluminum monohydrate, with or without small additions of modifiers such as magnesia, by the sol—gel process (see Sol-gel technology). The first modified sol—gel abrasive on the market, Cubitron, was patented (27) and produced by the 3M Corporation for products such as coated belts and disks. The success of this material promoted intensive research into sol—gel abrasives. 

The precursor glass powders may be produced by various methods, the simplest being the milling of quenched glass to an average particle size of 3—15 p.m. Sol gel processes, in which highly uniform, ultrafine amorphous particles are grown in a chemical solution, may be preferable for certain apphcations. 

Sodium Poly(4-styrene sulfonate). The sol—gel processing of TMOS in the presence of sodium poly-4-styrene sulfonate (NaPSS) has been used to synthesize inorganic—organic amorphous complexes (61). These sodium siUcate materials were then isotherm ally crystallized. The processing pH, with respect to the isoelectric point of amorphous siUca, was shown to influence the morphology of the initial gel stmctures. Using x-ray diffraction, the crystallization temperatures were monitored and were found to depend on these initial microstmctures. This was explained in terms of the electrostatic interaction between the evolving siUcate stmctures and the NaPSS prior to heat treatment at elevated temperatures. 

Figure 17 summarizes the avadable sol—gel processes (56). The process on the right of the figure involves the hydrolysis of metal alkoxides in a water—alcohol solution. The hydrolyzed alkoxides are polymerized to form a chemical gel, which is dried and heat treated to form a rigid oxide network held together by chemical bonds. This process is difficult to carry out, because the hydrolysis and polymerization must be carefully controlled. If the hydrolysis reaction proceeds too far, precipitation of hydrous metal oxides from the solution starts to occur, causing agglomerations of particulates in the sol. 

The sol—gel technique has been used mosdy to prepare alumina membranes. Figure 18 shows a cross section of a composite alumina membrane made by sHp coating successive sols with different particle sizes onto a porous ceramic support. SiUca or titanium membranes could also be made by the same principles. Unsupported titanium dioxide membranes with pore sizes of 5 nm or less have been made by the sol—gel process (57). 

Titanium alkoxides are used for the hardening and cross-linking of epoxy, siUcon, urea, melamine, and terephthalate resins in the manufacture of noncorrodable, high temperature lacquers in the sol-gel process as water repellents and adhesive agents (especially with foils) to improve glass surfaces as catalyst in olefin polymeri2ation, and for condensation and esterification. 

W. Lacourse andS. Kim, Use of Mixed Titanium Alkoxides for Sol-Gel Process, Wiiey-ln.tetscien.ee,New York, 1986, pp. 285—303. 

Transparent Vitreous Silica. Clear, transparent, bubble-free vitreous sihca may be obtained by melting natural quart2 minerals, ie, fused quart2, by flame or plasma vapor deposition (synthetic fused siUcas), and by sol—gel processing. 

Production of net-shape siUca (qv) components serves as an example of sol—gel processing methods. A siUca gel may be formed by network growth from an array of discrete coUoidal particles (method 1) or by formation of an intercoimected three-dimensional network by the simultaneous hydrolysis and polycondensation of a chemical precursor (methods 2 and 3). When the pore Hquid is removed as a gas phase from the intercoimected soHd gel network under supercritical conditions (critical-point drying, method 2), the soHd network does not coUapse and a low density aerogel is produced. Aerogels can have pore volumes as large as 98% and densities as low as 80 kg/m (12,19). 

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