Hydrothermal synthesis
Hydrothermal synthesis includes the various techniques of crystallizing substances from high-temperature aqueous solutions at high vapor pressures; also termed "hydrothermal method". The term "hydrothermal" is of geologic origin.Geochemists and mineralogists have studied hydrothermal phase equilibria since the beginning of the twentieth century. George W. Morey at the Carnegie Institution and later, Percy W. Bridgman at Harvard University did much of the work to lay the foundations necessary to containment of reactive media in the temperature and pressure range where most of the hydrothermal work is conducted.
Hydrothermal synthesis can be defined as a method of synthesis of single crystals that depends on the solubility of minerals in hot water under high pressure. The crystal growth is performed in an apparatus consisting of a steel pressure vessel called an , in which a nutrient is supplied along with water. A temperature gradient is maintained between the opposite ends of the growth chamber. At the hotter end the nutrient solute dissolves, while at the cooler end it is deposited on a seed crystal, growing the desired crystal.
Advantages of the hydrothermal method over other types of crystal growth include the ability to create crystalline phases which are not stable at the melting point. Also, materials which have a high vapour pressure near their melting points can be grown by the hydrothermal method. The method is also particularly suitable for the growth of large good-quality crystals while maintaining control over their composition. Disadvantages of the method include the need of expensive autoclaves, and the impossibility of observing the crystal as it grows.
The first report of the hydrothermal growth of crystals was by German geologist Karl Emil von Schafhäutl (1803-1890) in 1845: he grew microscopic quartz crystals in a pressure cooker. In 1848, Robert Bunsen reported growing crystals of barium and strontium carbonate at 200 °C and at pressures of 15 atmospheres, using sealed glass tubes and aqueous ammonium chloride ("Salmiak") as a solvent. In 1849 and 1851, French crystallographer Henri Hureau de Sénarmont (1808-1862) produced crystals of various minerals via hydrothermal synthesis. Later (1905) Giorgio Spezia (1842-1911) published reports on the growth of macroscopic crystals. He used solutions of sodium silicate, natural crystals as seeds and supply, and a silver-lined vessel. By heating the supply end of his vessel to 320-350 °C, and the other end to 165-180 °C, he obtained about 15 mm of new growth over a 200-day period. Unlike modern practice, the hotter part of the vessel was at the top. A shortage in the electronics industry of natural quartz crystals from Brazil during World War 2 led to postwar development of a commercial-scale hydrothermal process for culturing quartz crystals, by A. C. Walker and Ernie Buehler in 1950 at Bell Laboratories. Other notable contributions have been made by Nacken (1946), Hale (1948), Brown (1951), and Kohman (1955).
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