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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. more...

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Geochemists and mineralogists have studied hydrothermal phase equilibria since the turn of the 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 which 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 autoclave, in which a nutrient is supplied along with water. A gradient of temperature is maintained at the opposite ends of the growth chamber so that the hotter end dissolves the nutrient and the cooler end causes seeds to take additional growth.

Possible 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 also be grown by the hydrothermal method. The method is also particularly suitable for the growth of large good-quality crystals while maintaining good control over their composition. Disadvantages of the method include the need of expensive autoclaves, good quality seeds of a fair size and the impossibility of observing the crystal as it grows.

History

In 1839, the German chemist Robert Whilhelm Bunsen contained aqueous solutions in thick-walled glass tubes at temperatures above 200°C and at pressures above 100 bars. The crystals of barium carbonate and strontium carbonate that he grew under these conditions mark the first use of hydrothermal aqueous solvents as media. Other early reports of the hydrothermal growth of crystals were by Schafhäult in 1845 and by de Sénarmont in 1851 who produced only microscopic crystals. Later G. Spezzia (1905) 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. Other notable contributions have been made by Nacken (1946), Hale (1948), Brown (1951), Walker (1950) and Kohman (1955).

Read more at Wikipedia.org