Sol-gel processing

Sol-gel syntheses are typically carried out in the presence of polar solvents such as alcohol or water media, which facilitate the two primary reactions of hydrolysis and condensation (Eqs. 15 and 16, respectively). During the sol-gel process, the molecular weight of the oxide product continuously increases, eventually forming a highly viscous three-dimensional network (step-growth polymerization - Chapter 5). 

The most widely used metal alkoxides are Si(OR)4 compounds, such as tetra-methoxysilane (TMOS) and tetraethoxysilane (TEOS). However, other alkoxides of Al, Ti, and B are also commonly used in the sol-gel process, often mixed with TEOS. For instance, aluminum silicates may be generated through hydrolysis/condensation of siloxides (Eq. 17), which proceed through an intermediate Al-O-Al network known as alumoxanes. Alumoxanes are important for applications in antiperspirants, 

As one would expect from the similar electronegativities of Si and O, the hydrolysis of silicon alkoxides are significantly slower than other metal analogues. For identical metal coordination spheres and reaction conditions, the general order or reactivity 

It should be noted that a sol-gel process may also take place through nonhydrolytic pathways. In these systems, a metal halide reacts with an oxygen donor such as ethers, alkoxides, etc. to yield a crosslinked metal oxide product (Eq. 18). 

In stark contrast to other metal alkoxides, the kinetics for the hydrolysis of Si(OR)4 compounds often require several days for completion. As a result, acid e.g., HCl, HE) or base (e.g., KOH, amines, NH3) catalysts are generally added to the mixture, which also greatly affects the physical properties of the final product. Under most conditions, condensation reactions begin while the hydrolytic processes are underway. However, altering the pH, [H20/M(0R) ] molar ratio, and catalyst may force the completion of hydrolysis prior to condensation. 

As condensation reactions progress, the sol will set into a rigid gel. Since the reactions occur within a hquid alcoholic solvent, condensation reactions result in a three-dimensional oxide network [M-0-M] that contains solvent molecules within 

Sol-gel processing is a chemical method. It is a more expensive process compared to the earlier two processes. The advantage of this process is that it produces powder with more surface area. This makes the powder sintered at a lower temperature, thereby compensating for some of the costs involved in the production of the powder. 

The reactants in most sol-gel processes are the corresponding metal alkoxy compounds. Alkoxides are produced by reacting the metal in the 

R is an organic group. For example, if the alcohol is propanol, R is C3H7. Reaction rates are increased by the use of catalysts. In order to prepare aluminum isopropoxide, mercuric chloride will be needed. This is because there will always be an oxide layer on aluminum. This aluminum oxide layer prevents any reaction with the environment. The chloride ions in mercuric chloride attack and remove the oxide layer on the aluminum. The reaction with alcohol now becomes possible. The reaction is further enhanced by increasing the temperature to 80°C. 

After forming the alkoxide, it is dissolved in an alcohol. This solution is called the sol. To this sol, water is added. Then the following reactions 

Solubility versus temperature for several ionic compounds. 

Many elements of glass processing are similar regardless of the product, so that an understanding of the basics of glass formation, melting 

Many methods could be applied to prepare polymer-inorganic nanocomposites, such as sol-gel processing, in situ polymerization, particle in situ formation, blending, radiation synthesis, and so on. 

The wet chemistry methods have attracted increasing attention because homogeneous BST nanoparticles are necessary for the development of uniform microstructures with desired properties. Nanometer-scaled and well-dispersed BST powders have been successfully synthesized by wet chemistry methods. 

Oxolation is a condensation reaction in which an oxo bridge is formed. Oxola-tion is favored when the metal center is coordinatively unsaturated. Oxolation is a nucleophilic addition that results in edge- or face-shared polyhedra. Oxolation is 

There are seven steps to making a sol-gel material mixing, casting, gelat-ing, aging, drying, dehydration or stabilization, and finally, densification. Mixing 

Aging of the gel is called syneresis, which is the process of separating the liquid from the gel to allow further solidification. As the sols become interconnected, the solvent and water from the condensation are pushed outside the pores. The condensation reactions continue as the colloidal sols are brought closer together. Aging determines the average pore size and the resulting density of the 

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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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