Various techniques of sol-gel processing for the preparation of electronic and related materials are described and reviewed. Typical examples are chosen from thin films and coatings of gels, crystalline materials and glasses as also bulk glasses to illustrate the variations in processing parameters and material properties.

This paper is a preliminary report on the preparation of silica glass containing very low amount of hydroxyl by the sol-gel processing technique. Gels were prepared from optimized amounts of tetraethyl orthosilicate, fumed silica and water. Acids and bases in small quantities were added for catalysing hydrolysis and adjusting the pH. Dried gels were heated up to 1400°C in various atmospheres to obtain transparent silica glass of the required density and very low (<5 ppm) hydroxyl content.

An indigenous sol-gel derived yttria-partially stabilized zirconia (Y-PSZ) powder has been characterized and its suitability for plasma spraying applications evaluated. The powder, determined to have about 5·1% yttria content, predominantly consisted of spherical particles with an average equivalent particle diameter close to 25µm. Furthermore, it was found that the powder did not contain any particles >50µm, which is considered the ideal upper size limit for spray-grade ceramic powders in order to ensure complete melting during spraying. The sol-gel produced powder exhibited good flow characteristics and the plasma sprayed coatings developed using this powder were also found to have excellent thermal shock resistance. The corresponding results obtained using an imported Y-PSZ powder are also presented for the purpose of comparison.

The various parameters related to sol-gel processing are discussed with special reference to those which usually attract less attention but depending on the final product in mind, can play important roles. The versatility of the sol-gel technique in materials preparation is demonstrated by discussing the various products developed at the author’s laboratory by using this processing method.

Gelation times of tetraethyl orthosilicate-derived sols containing selected di- to tetravalent cations as dopants were noted at different temperatures (15°–50°C). An analysis of these data, in conjunction with relevant published information, led to the tentative conclusions that (i) unhydrolyzed cations in bare form linked negatively charged silicate polymers in sols, thereby accelerating gelation and (ii) hydrolyzed cations retarded the process by offering molecular size-related hindrance and competition for the available water for hydrolysis.

The basic experimental steps in the preparation of sol-gel glass as developed in recent times, and their relevance have been discussed taking high purity silica glass as an example. Current developments in sol-gel derived ultra-low expansion glasses, rare earth doped laser glasses, semiconductor-doped non-linear glasses, gradient index lenses, microoptics and organic molecule-doped (mainly dyes) glasses for sensor and other applications have been discussed in brief.

The basic characteristics of the sol-emulsion-gel (SEG) process are described as derived from water-in-oil type emulsions when ‘water’ is replaced by an aqueous sol and ‘oil’ indicates water-immiscible organic liquids. The main roles of a surfactant in emulsion formation are discussed. Steps in the generation of ceramic particles from the SEG process through a variety of experimental options are explained. Relevant examples are described where control of surfactant contents (below and above the working range of critical micelle concentration) in a sol-emulsion can lead to oxide particles of different sizes and shapes. Attempts are made to correlate the products of high-surfactant emulsions with micelles and mesophase structures known to form by surfactant molecules in large concentrations.