Sol-gel process routes

Segal, D. L., Sol-gel processing Routes to oxide ceramics using colloidal dispersions of hydrogen oxides and alkoxide intermediates, J. Non-Cryst. Solids, 63, 183 (1984). 

There are also a great number of studies on sol-gel processing routes to modify the surface of CNTs. The surface modified CNTs are targeted to produce final composites with increased wettability and hence optimised load transfer from the matrix to the CNTs. Ceramics that have been considered to coat carbon nanotubes by sol-gel methods include titania [60], silica [57] and alumina... 

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. 

In order to prepare sulfide glass planar waveguides, germanium sulfide glass films were recently prepared in our group. GeS based films could be prepared by three different sol-gel processing routes, namely B, C and D. 

Titanium dioxide is commonly found in three crystalline forms rutile, anatase and brookite. For bulk Ti02, mtile is the thermodynamically stable form, while for small spherical particles (<14 nm) the anatase form is most stable (Gribb, 1997 Zhang, 1998, 2000). Therefore, most sol-gel processing routes 0fXiO2 nanoparticles yield anatase and ratile is far less common (Matijevic, 1977 Park, 2000 Wang, 1999). 

The downstreaming of sol-gel processes routes to the production of different nanostructures with distinct and unique surface properties. For example "xerogels" and "aerogels" are produced... 

S.3.2 Sol-Gel Encapsulation of Reactive Species Another new and attractive route for tailoring electrode surfaces involves the low-temperature encapsulation of recognition species within sol-gel films (41,42). Such ceramic films are prepared by the hydrolysis of an alkoxide precursor such as, Si(OCH3)4 under acidic or basic condensation, followed by polycondensation of the hydroxylated monomer to form a three-dimensional interconnected porous network. The resulting porous glass-like material can physically retain the desired modifier but permits its interaction with the analyte that diffuses into the matrix. Besides their ability to entrap the modifier, sol-gel processes offer tunability of the physical characteristics... 

Mechanistic studies are particularly needed for the hydrolysis and polymerization reactions that occur in sol-gel processing. Currently, little is known about these reactions, even in simple systems. A short list of needs includes such rudimentary data as the kinetics of hydrolysis and polymerization of single alkoxide sol-gel systems and identification of the species present at various stages of gel polymerization. A study of the kinetics of hydrolysis and polymerization of double alkoxide sol-gel systems might lead to the production of more homogeneous ceramics by sol-gel routes. Another major area for exploration is the chemistry of sol-gel systems that might lead to nonoxide ceramics. 

There are various routes to synthesize rubber-based organic-inorganic hybrid composites through sol-gel process. These are mentioned in brief below. 

There are a variety of routes currently utilized to fabricate a wide range of hollow capsules of various compositions. Among the more traditional methods are nozzle reactor processes, emnlsion/phase-separation procednres (often combined with sol-gel processing), and sacrificial core techniques [78], Self-assembly is an elegant and attractive approach for the preparation of hollow capsules. Vesicles [79,80], dendrimers [81,82], and block hollow copolymer spheres [83,84] are all examples of self-assembled hollow containers that are promising for the encapsnlation of various materials. 

Similarly to the above-mentioned entrapment of proteins by biomimetic routes, the sol-gel procedure is a useful method for the encapsulation of enzymes and other biological material due to the mild conditions required for the preparation of the silica networks [54,55]. The confinement of the enzyme in the pores of the silica matrix preserves its catalytic activity, since it prevents irreversible structural deformations in the biomolecule. The silica matrix may exert a protective effect against enzyme denaturation even under harsh conditions, as recently reported by Frenkel-Mullerad and Avnir [56] for physically trapped phosphatase enzymes within silica matrices (Figure 1.3). A wide number of organoalkoxy- and alkoxy-silanes have been employed for this purpose, as extensively reviewed by Gill and Ballesteros [57], and the resulting materials have been applied in the construction of optical and electrochemical biosensor devices. Optimization of the sol-gel process is required to prevent denaturation of encapsulated enzymes. Alcohol released during the... 

Sol-gel processing forms the basis for various routes employed for the fabrication of a wide diversity of functional materials. To impart a structural organization at various length scales, the syntheses are performed using templates. Most consist of a self-organized ensemble of surfactants and co-polymers [1-10]. They have been successfully applied to control the geometry and dimensions of pores that are periodically arranged as in the initial structures. Mesoporous silica materials of the MCM family, which were first synthesized by a team from the Mobil oil company [11,12], are a well-known example. 

An alternative strategy to obtain silica immobilised catalysts, pioneered by Panster [23], is via the polycondensation or co-condensation of ligand functionalised alkoxysilanes. This co-condensation, later also referred to as the sol-gel process [24], appeared to be a very mild technique to immobilise catalysts and is also used for enzyme immobilisation. Several novel functional polymeric materials have been reported that enable transition metal complexation. 3-Chloropropyltrialkoxysilanes were converted into functionalised propyltrialkoxysilanes such as diphenylphosphine propyltrialkoxysilane. These compounds can be used to prepare surface modified inorganic materials. Two different routes towards these functional polymers can be envisioned (Figure 3.4). One can first prepare the metal complex and then proceed with the co-condensation reaction (route I), or one can prepare the metal complex after the... 

Figure 1.14 In a novel synthetic route made possible by the versatility of the sol-gel process, using spray-drying of the precursor sol affords spheres of mesoporous ORMOSIL whose surface consists of highly ordered hexagonal domains. (Reproduced from ref. 30, with permission.)... 

Over the past few years, a large number of experimental approaches have been successfully used as routes to synthesize nanorods or nanowires based on titania, such as combining sol-gel processing with electrophoretic deposition,152 spin-on process,153 sol-gel template method,154-157 metalorganic chemical vapor deposition,158-159 anodic oxidative hydrolysis,160 sonochemical synthesis,161 inverse microemulsion method,162 molten salt-assisted and pyrolysis routes163 and hydrothermal synthesis.163-171 We will discuss more in detail the latter preparation, because the advantage of this technique is that nanorods can be obtained in relatively large amounts. 

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