Particulate sol-gel processes

FIGURE 26.2 (a) The capacity versus cycle number showing the first 30 cycles for LiNiOj synthesized using the particulate sol-gel process, and (b) the first cycle voltage vs. lithium content for the low defect concentration of LiNi02-... 

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. 

Immiscible wetting in mixers Sol-gel processes Pellet flocculation <0.3 Low Up to 10 ton/hr Wet processing based on flocculation properties of particulate feed Coal fines, soot and oil removal from water Metal dicarhide, silica hydrogels Waste sludges and slurries... 

Either particulate sol or polymeric sol has been used for thin film coatings. The polymeric sol was fabricated by partial hydrolysis of corresponding metal alkoxide. If the rate of hydrolysis or condensation is very fast, then some kinds of organic acids, beta-dicarbonyls, and alkanolamines have been used as chelating agent in sol-gel processes to control the extent and direction of the hydrolysis-condensation reaction by forming a strong complex with alkoxide. [2]. 

SOL-GEL TECHNOLOGY. The goal of sol-gel technology is lo use low temperature chemical processes to produce net-shape, net-surfacc objects, films, fibers, particulates, or composites that can be used commercially after a minimum of additional processing steps. See also Thin Films. Sol-gel processing can provide control of microstructures in the nanometer size range, i.e 1-100 nm (0.001-0.1 pm), which approaches the molecular level These materials often have unique physical and chemical characteristics. See also Nanotechnology (Molecular). 

In the particulate-sol method a metal alkoxide dissolved in alcohol is hydrolyzed by addition of excess water or acid. The precipitate that results is maintained as a hot solution for an extended period during which the precipitate forms a stable colloidal solution. This process is called peptization from the Greek pep—to cook (not a misnomer many descriptions of the sol-gel process have a strong culinary flavor). The colloidal solution is then cooled and coated onto the microporous support membrane. The layer formed must be dried carefully to avoid cracking the coating. In the final step the film is sintered at 500-800 °C. The overall process can be represented as ... 

In sol-gel processing, particulate sols of hydrous metal oxides can be formed using a peptization reaction to prevent aggregation of the primary particles. This is the case for y-alumina obtained from boehmite. Usually mesoporous membranes with a pore diameter down to 5 nm are easily elaborated from commercial boehmite using nitric acid as the peptization agent. Larbot et al. [53]... 

In immiscible liquid agglomeration, particles with a small amount of adsorbed binding liquid on their surfaces collide and coalesce to form larger entities. In the sol-gel process (see also Section 5.3.2, Fig. 5.52), another agglomeration technique that occurs in liquid phase, fine particles are initially suspended in a binder liquid. The suspension is then formed into spherical droplets and the excess binder is removed to solidify the droplets into a particulate product. 

In the nuclear industry one of the main applications of colloid technology is in the handling and reprocessing of radioactive waste, where the problems are mainly concerned with the flocculation and separation of particulate radioactive materials. A second application is that of the sol-gel process to the preparation of nuclear fuel in the form of spherical particles of uniform size. 

Sol-gel processes properties of particulate feed Metal dicarbide, silica hydrogels... 

The term sol-gel is used broadly to describe the preparation of ceramic materials by a process that involves the preparation of a sol, the gelation of the sol, and the removal of the liquid. A sol is a suspension of colloidal particles in a liquid or a solution of polymer molecules. The term gel refers to the semirigid mass formed when the colloidal particles are linked to form a network or when the polymer molecules are cross-linked or interlinked. Two different sol-gel processing routes are commonly distinguished the particulate (or colloidal) gel route in which the sol consists of dense colloidal particles (1 to 1000 nm) and the polymeric gel route in which the sol consists of polyma- chains but has no dense particles >1 nm. In many cases, particularly when the particle size approaches the lower limit of the colloidal size range, the distinction between a particulate and a polymeric system may not be very clear. 

TABLE 2-1. THE MAJOR PROCESSING STEPS FOR A PARTICULATE-BASED SOL-GEL PROCESS... 

Rabinovich E. Particulate silica geUs and glasses from the sol-gel process. In Sol-Gel Technology for Thin Films, Fibers, Preforms, Electronics, and Speciality Shapes, Klein L.C., ed. Park Ridge, NJ Noyes, 1988, chap 13, pp. 479 82... 

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