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for Advanced Inorganic Materials

Masahiro Yoshimura and Jacques Livage, Guest Editors The development of human society has been brought about by the advance of artificial materials and has led to increased consumption of energy and resources. Even though this consumption brings about global environmental problems, we cannot avoid the production, use, and disposal of these advanced materials if we wish to sustain our civilized society with its everincreasing population. However, in recent years, we have recognized the limited capacity of the Earth, not only in energy and resources, but also in space for wastes (materials, heat, and entropy). Particularly, even though expended heat and exhausted gases like CO2 appear not to be harmful in and of themselves, they can contribute to a serious problem such as global warming. This has pressured scientists to search for alternatives to such high-energy production methods as high-temperature firing, gaseous and/or vacuum technologies, ion beam/plasma processing, and so on. In this regard, chemical processing using solutions—including (1) soft chemistry, or “wet” chemical processing; (2) biomimetic processing; and (3) soft solution processing—have been attracting increased interest. Here, we briefly describe their concepts and practices.

Soft Chemistry, or “Wet” Chemical Processing Much of the development of materials has been based on high-temperature processing. New materials were possible once we became able to produce higher temperatures for processing. After copper came the age of bronze and then iron; metals such as aluminum or silicon were not produced until the end of the 19th century. For thousands of years, glasses and ceramics have been made by heating powders at high temperature; the most widely used method for the synthesis of solid materials is still

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the direct reaction, in the solid state, of a mixture of powders. For the reaction to proceed at a reasonable rate, high mobility of the reactants and maximum contact surface between the reacting particles are desirable. The rates of these processes are enhanced by higher temperatures and finer particle sizes. In the so-called “shake and bake” processes, powders are crushed together and heated to high temperatures (1000C), where diffusion of the atoms in the solid can occur. Plants and animals have developed strategies, very different from those used by materials engineers, to produce biomaterials. Diatoms, for instance, build their siliceous cell walls at room temperature, directly from the silicic acid [Si(OH)4] dissolved in water. Using such biological models, new synthesis methods, based on solution chemistry, have been developed during the last few decades. They are known as chimie douce (“soft chemistry”), a term introduced by French scientists in the 1970s.1 In soft, or “wet” chemistry methods, solute precursors are mixed in solution and then transformed into a solid via precipitation or gelation. Heating at moderate temperatures yields a mixture of finely divided and intimately mixed metal oxides. L