Metal oxide gels preparation

The various steps in the sol-gel technique described above may or may not be strictly followed in practice. Thus, many complex metal oxides are prepared by a modified sol-gel route without actually preparing metal alkoxides. For example, a transition metal salt solution is converted into a gel by the addition of an appropriate organic reagent. In the case of cuprate superconductors, an equimolar proportion of citric acid is added to the solution of metal nitrates, followed by ethylene-diamine until the solution attains a pH of 6-6.5. The blue sol is concentrated to obtain the gel. The xerogel is obtained by heating at approximately 420 K. The xerogel is decomposed at an appropriate temperature to obtain the cuprate. 

Examples of common metal oxide membranes prepared by sol-gel process... 

The sol-gel technique uses mild conditions to prepare a cross-linked, robust metal oxide gel. Gelation of silanes, such as tetraethoxysilane (TEOS) or sodium silicate, involves the use of aqueous solutions, sometimes containing alcohol, with a slight adjustment of pH to catalyse the condensation polymerisation. The cross-link density, material porosity and homogeneity of functionalised sites, are dependent on the solution pH, extent of ageing, method of solvent evaporation, water content in solution and type of catalyst, all of which provide a number of handles to modify the material of interest. 

Example. The Pechini method for fuel cell electrode preparation. La, Ba, Mn niU ates - - CgHgO — citrate complex - - C2FI6O2 — gel. Metal nitrates are complexed with citric acid, and then heated with ethylene glycol to form a transparent gel. This is then heated to 600 K to decompose the organic content and then to temperatures between 1000 and 1300K to produce tire oxide powder. The oxide materials prepared from the liquid metal-organic procedures usually have a more uniform particle size, and under the best circumstances, this can be less than one micron. Hence these particles are much more easily sintered at lower temperatures than for the powders produced by tire other methods. 

These are generally reserved for specialist applications, and are in the main more costly than conventional soap-based greases. The most common substances used as nonsoap thickeners are silica and clays prepared in such a way that they form gels with mineral and synthetic oils. Other materials that have been used are carbon black, metal oxides and various organic compounds. 

Transition-metal mixed oxides active in combustion catalysis have been prepared by two main procedures i) classical coprecipitation / calcination procedures starting from metal nitrates and/ or alkoxides ii) preparation based on the supercritical drying of gels prepared from organic complexes (alkoxides, acetylacetonates or acetates), producing aerogels . Details on the second preparation can be found in Ref. 13. 

According to Ref. [12], template for synthesis of nanomaterials is defined as a central structure within which a network forms in such a way that removal of this template creates a filled cavity with morphological or stereochemical features related to those of the template. The template synthesis was applied for preparation of various nanostructures inside different three-dimensional nanoporous structures. Chemically, these materials are presented by polymers, metals, oxides, carbides and other substances. Synthetic methods include electrochemical deposition, electroless deposition, chemical polymerization, sol-gel deposition and chemical vapor deposition. These works were reviewed in Refs. [12,20]. An essential feature of this... 

---