Birth of the sol–gel method: early history

Congratulations on “United Nations International Year of Glass 2022.” I would like to acknowledge the efforts made by the International Commission on Glass with the Community of Glass Associations and the Glass Art Society to promote the status of glass. I am very much pleased to contribute an article on the birth of the sol–gel method and its growth in early times for a Special Issue of Journal of Sol–Gel Science and Technology, which is published to celebrate the Year of Glass. At present, the sol–gel method is employed not only for manufacturing optical glass fibers for communication, but also for processing a wide range of functional, high technology materials, such as photonic, electronic, micromechanical, chemical, and bionic materials. The sol–gel method typically involves the following steps: (1) starting from a solution containing pure precursors, (2) conversion of the solution to a sol and then to a gel by hydrolysis and polycondensation, (3) forming into desired shapes such as bulk, film, and fiber during gelation of the sol, and (4) thermal treatment of the gel into a solid material at low temperatures. It should be noted, however, that there are many variants. For instance, the method of fabrication of a solid material by heating the shaped compact of gel powders prepared from a solution or sol is also called the “sol–gel method.” In this case, “starting from a solution” is important.