Sol–Gel Process and Products

As alternatives to traditional melt-quenching, sol–gel methods are wet chemical routes based on colloidal chemistry techniques that are used to synthesize glassy, hybrid (organic–inorganic), and crystalline materials at relatively low temperatures. They are reviewed in terms of the main steps of solution preparation, aging, forming, drying and sintering into dense nanoparticles, glassy/ceramic films, or bulk materials, with particular emphasis on dip-coating and spin-coating deposition of films. This review is followed by a systematic consideration of the advantages and drawbacks of these methods. The former include the low-temperature nature of the process, the preparation of homogeneous noncrystalline solids in unusual composition ranges, and the fabrication of special products like nanoparticles, powders, films, and fibers with improved purity and homogeneity. As for the drawbacks, they are mainly the cost of raw materials, shrinkage during processing, and the difficulties of eliminating residual hydroxyl species or obtaining large monoliths. Then, the most representative products and applications at present are reviewed in the automotive, architectural, ophthalmic, electronic, household, and optical domains. The final section focuses on new perspectives in the field of sol–gel science and technology; for example, new glasses, more reproducible preparation conditions, increasing use of spray- and roller-coating methods for film deposition, ink-jet and 3-d printing, and biomedical applications.