The sol-gel process

The development of the sol-gel process in the beginning of the eighties can be considered as the breakthrough in inorganic membranes comparable to the Loeb-Sourirajan process 

From the above mentioned silica preparation paths, the sol-gel route is the most studied and consequently most documented. Sol-gel processing is used not only for the preparation of silica gels, but also for the synthesis of ceramic products, ranging from thin films and coatings over porous membranes to composite bodies.4 

This broad success of the method is due to its ability to form pure and homogeneous products at very low temperatures.5 Thus, the sol-gel technology is replacing the millennia old ceramic fabrication processes in which powders are shaped into objects and subsequently densified at temperatures close to their liquidus. This allowed a transformation of ceramics and glasses from stone age materials to space age materials . 

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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. 

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. 

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. 

The sol—gel process can be utilized to yield products within a wide range of appHcations. Some of these appHcations include production of nanocomposites, films, fibers, porous and dense monoliths, and biomaterials. 

Sol—Gel Technology. The sol—gel process involves conversion of a metal alkoxide or mixture of metal alkoxides, dissolved in an organic solvent (generally the patent alcohol) into a hydroxooxyalkoxide sol, followed by gelation and sintering to give the desired ceramic material. 

Ceramic Orarnic membranes are made generallvbv the sol-gel process, the siiccessiv e deposition of ev er smaller ceramic precursor... 

In the present report the opportunity of use of the sol-gel process for preparation of supports for chromatography is considered by three ways ... 

Despite the progress outlined in this chapter, much work remains to be done in the metal surface preparation arena. For example, there is still no ideal surface preparation method that does for steel what anodization processes do for aluminum and titanium. The plasma spray process looks encouraging but because it is slow for large areas and requires rather expensive robot controlled plasma spray equipment, its use will probably be limited to some rather special applications. For more general use, the sol-gel process has potential if future studies confirm recently reported results. 

An inorganic membrane can be prepared by various methods such as sol-gel, phase separation and leaching.2,3 The sol-gel process is considered the most practical method among those used to prepare inorganic membrane. Sol-gel processing is a simple technology in principle but requires considerable effort to become of practical use. The advantage of this... 

A recent competitor to CVD in the planarization of silicon dioxide is the sol-gel process, where tetraethylorthosilicate is used to form spin-on-glass (SOG) films (see Appendix). This technique produces films with good dielectric properties and resistance to cracking. Gas-phase precipitation, which sometimes is a problem with CVD, is eliminated. 

Mehrotra RC (1991) Present Status and Future Potential of the Sol-Gel Process. 77 1-36... 

Sol-gel chemistry offers a unique advantage in the creation of novel organic-inorganic hybrids. The sol-gel process begins with a solution of metal alkoxide precursors [M(0/f) ] and water, where M is a network-forming element, and R is typically an alkyl group. Hydrolysis and... 

Sol-gel techniques have been successfidly applied to form fuel cell components with enhanced microstructures for high-temperature fuel cells. The apphcations were recently extended to synthesis of hybrid electrolyte for PEMFC. Although die results look promising, the sol-gel processing needs further development to deposit micro-structured materials in a selective area such as the triple-phase boundary of a fuel cell. That is, in the case of PEMFC, the sol-gel techniques need to be expanded to form membrane-electrode-assembly with improved microstructures in addition to the synthesis of hybrid membranes to get higher fuel cell performance. 

Ceramic Ceramic membranes are made generally by the sol-gel process, the successive deposition of ever smmler ceramic precursor sph eres, followed by firing to form multitube monoliths. The diameter of the individual channels is commonly about 2 to 6 mm. Monoliths come in a variety of shapes and sizes. A 19-channel design is common. One manufacturer makes large monoliths with square channels. 

The fabrication of colloidal silica and optical glasses by the sol-gel process has attracted a great deal of attention (8). The process relies on the hydrolytic polycondensation reactions of alkoxysilanes, usually (EtO)4Si, in which the reactive silanols (EtO)4 Si(OH)n (n = 1-4) are formed. These then undergo acid- or base-catalyzed condensation with both water and alcohol formation, as shown in Scheme 2. 

The formation of polymeric silica or a gel from this process is clearly complicated (9) and will not be discussed further in this article for reviews of the sol-gel process, see Ref. 10. The important point for this general discussion is that silanol species play a vital role in the formation of numerous industrially important materials. 

Figure 1. Schematic of the sol-gel process for preparing glasses and ceramics.

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