Hydrothermal synthesis

Hydrothermal synthesis includes the various techniques of <a href="/facts/Chemical_synthesis/3KXsVVwC">synthesizing</a> substances from high-temperature <a href="/facts/Aqueous_solution/eRNr3MH2">aqueous solutions</a> at high <a href="/facts/Pressure/eyc37GRG">pressures</a>; also termed "hydrothermal method". The term "<a href="/facts/Hydrothermal/hqwwksSm">hydrothermal</a>" is of <a href="/facts/Geology/WGvVRIYF">geologic</a> origin. <a href="/facts/Geochemistry/05094gDu">Geochemists</a> and <a href="/facts/Mineralogist/MeIK9lx9">mineralogists</a> have studied hydrothermal <a href="/facts/Phase_equilibria/nDXuS8PT">phase equilibria</a> since the beginning of the twentieth century. George W. Morey at the <a href="/facts/Carnegie_Institution/zS5MqNUP">Carnegie Institution</a> and later, <a href="/facts/Percy_W._Bridgman/h72BUYPt">Percy W. Bridgman</a> at <a href="/facts/Harvard_University/Lhy9L1cx">Harvard University</a> 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. In the broadest definition, a process is considered hydrothermal if it involves water temperatures above 100 °C (212 °F) and pressures above 1 atm.
In the context of <a href="/facts/Material_science/N5HrXRYW">material science</a>, hydrothermal synthesis focuses on the production of <a href="/facts/Single_crystal/UMG5pFp9">single crystal</a>. Under high temperature > (300 °C) and pressure (> 100 atm), ordinarily insoluble minerals become soluble in water. The <a href="/facts/Crystal_growth/QwhSDA5y">crystal growth</a> is performed in an apparatus consisting of a steel pressure vessel called an <a href="/facts/Autoclave/bg1J3cXJ">autoclave</a>, in which the reactant ("nutrient") is supplied along with <a href="/facts/Water/0lkXnJPK">water</a>. 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 vapor 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 if a steel tube is used. There are autoclaves made out of thick walled glass, which can be used up to 300 °C and 10 bar.

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Hydrothermal synthesis