Principles of sol-gel process

The explosion of literature in the field of sol-gel science in the past decade made it difficult for newcomers to catch up to the state of the art and overwhelmed researchers already involved in the subject. The book by Brinker and Scherer (8) and the chapter by Brinker (Chapter 18) fill the need for a coherent account of the principles of sol-gel processing. Additional detailed discussions on this subject are included in the section on sol-gel technology in this book, in the collection of articles edited by Klein (72), and in the Proceedings of the Materials Research Society (79-81) and the Ultrastructure Processing meetings (82-84). 

This chapter focuses only on the syntheses of oxide ceramic powders. The precursors of these powders are metal-organic compounds, mainly metal alkox-ides. The different varieties of ceramic powders synthesized by mixed metal alkoxide precursors are the focal points of this chapter. Numerous references on the fundamental principles of sol-gel processing are available in the literature and within other chapters of this book [10-12]. 

The principle of sol-gel processing is summarized in Sec. 2.6. Sol-gel is used on a large scale in the production of alumina-based fibers Nextel by the 3M Co.) and more recently of silicon-carbide and silicon-nitride fibers. 

Microemulsion-based synthesis of nanodimensional materials provides a physical means to control particle size and shape while still exploiting the advantages of sol-gel processing. The principle limitation of emulsion methods is that... 

The basic chemical principle behind sol-gel processing of metal alkoxides is the transformation of M—OR groups to M—O—M units via M—OH species. The direct formation of M—O—M units from metal alkoxides by ether cleavage is possible, but is only rarely observed under sol-gel conditions (see Section 7.10.3.3.1). Thus, the alkoxo ligands first have to be converted into hydroxo ligands (hydrolysis reaction), which can then undergo condensation reactions. 

One merit of special attention in this chapter is the fact that sol-gel synthesis offers a convenient method for hosting chemical reactions, a process which is not possible using other synthesis techniques. Typically, in sol-gel encapsulation, silica nanoparticles surround the captive molecules during gel formation. In principle, the sol-gel process can be considered as a phase separation by sol-reactions, sol-gelation and finally, removal of the solvent resulting in a ceramic material Depending on the preparation, dense oxide particles or polymeric clusters will be obtained [16]. 

In the following, the chemical and physical principles behind the individual steps of sol-gel processing will be discussed in more detail. 

The basic chemical principle behind sol-gel processing of silica-based materials is the transformation of Si—OR- and Si—OH-containing species to siloxane compounds by condensation reactions. From a structural point of view, this corresponds to connecting Si04 tetrahedra (or RSiOs tetrahedra in hybrid materials) by corner sharing. To obtain a stable gel, the number of siloxane bonds (Si—O—Si) has to be maximized and consequently the number of silanol (Si—OH) and alkoxo (Si—OR) groups has to be minimized. 

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). 

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... 

This chapter focuses on silica synthesis via the microemulsion-mediated alkoxide sol-gel process. The discussion begins with a brief introduction to the general principles underlying microemulsion-mediated silica synthesis. This is followed by a consideration of the main microemulsion characteristics believed to control particle formation. Included here is the influence of reactants and reaction products on the stability of the single-phase water-in-oil microemulsion region. This is an important issue since microemulsion-mediated synthesis relies on the availability of surfactant/ oil/water formulations that give stable microemulsions. Next is presented a survey of the available experimental results, with emphasis on synthesis protocols and particle characteristics. The kinetics of alkoxide hydrolysis in the microemulsion environment is then examined and its relationship to silica-particle formation mechanisms is discussed. Finally, some brief comments are offered concerning future directions of the microemulsion-based alkoxide sol-gel process for silica. 

The principle of the sol-gel process is as follows. A sol is prepared by peptizing a well suited alumina powder with an acid. The powder particles or agglomerates are stabilized by a positively charged layer on the surface of these particles. An increase in pH allows the hydroxyl ions to neutralize the positive ions on the surface of the particles. These particles will grow together to larger particles and gelation will occur. The sol-gel process can be carried out in several ways ... 

The dynamic self-assembly processes of such supramolecular systems undergoing continous reversible exchange between different self-organized entities in solution may in principle be connected to kinetically controled sol-gel process in order to extract and select an amplified supramolecular device under a specific set of experimental conditions. Such dynamic marriage between supramolecular self-assembly and in sol-gel polymerization processes which synergistically might communicate leads to constitutionnal hybrid materials. ... 

Dynamic self-assembly of supramolecular systems prepared under thermodynamic control may in principle be connected to a kinetically controlled sol-gel process in order to extract and select the interpenetrated hybrid networks. Such dynamic convergence between supramolecular self-assembly and inorganic sol— gel processes, which synergistically communicate, leads to higher self-organized hybrid materials with increased micrometric scales. 

For the synthesis of the nanocomposites, additionally to the reaction principles described above, nanoscaie particles have to be generated within or added to the hetero polysiloxane composite matrix. As described elsewhere [26], the sol-gel process, is a suitable means. It can be considered as a growth process from solution where a controlled growth reaction takes place, that leads to a stabilized colloidal system and thus avoiding precipitation. 

Instead of ground powders, spherical fuel particles can be used as a starting material. This has advantages with respect to fabrication, reactor utilization, and fuel reprocessing. These oxide or carbide particles are very small, < 1 mm in diameter. The particles are produced by the sol—gel process, which in principle consists of the following steps ... 

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