Hydrothermal Synthesis of Transition Metal Oxides



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1.2 History

The hydrothermal technique has been most popular, garnering interest from scientists and technologists of different disciplines, particularly in the last fifteen years. The hydrothermal term is primarily of geological origin, pertaining to the natural processes characterized by joint action of heat and water under pressure. It was first used by the British Geologist, Sir Roderick Murchison (1792-1871), to describe the action of water at elevated temperature and pressure in bringing about changes in the earth’s crust leading to the formation of various rocks and minerals [8]

A majority of the minerals formed in the presence of water at elevated temperature and pressure conditions are called “of hydrothermal origin”. This contains a huge number of mineral species including ore deposits.

It is well known that the largest single crystals formed in nature and some of the largest quantities of single crystals made by man in one experimental process (quartz crystals of 1000 kg) are both of hydrothermal origin. A number of minerals have formed under hydrothermal conditions over 100oC and 1 bar [9]. With the understanding of the mineral formation in nature under elevated pressure and temperature conditions in the presence of water, the hydrothermal technique has developed.

The first hydrothermal synthesis was carried out in 1845 by Schafhault to obtain quartz crystals upon transformation of freshly precipitated silicic acid in Papin’s digestor [10]. Thus, Geologists and mineralogists have determined the conditions needed for the mineral formation in the laboratory during simulations [11].

The first purposeful man-made hydrothermal chemical reaction was probably performed by the German chemist Robert Wilhelm Bunsen in 1839. He contained aqueous solutions in thick walled glass tubes at temperatures above 200oC and at pressures above 100 bars. The crystals of barium carbonate and strontium carbonate that he formed under these conditions marked the first use of hydrothermal aqueous or other solvents as reaction media [12]. This work was followed by that of deSenarmont, He investigated the synthesis of various crystalline solids in superheated water sealed in glass ampoules and counter-pressured in welded gun barrels [13]. The early work was so productive, and more than 80 types of known minerals were prepared from various recipes in water above its boiling point. Most of this early work was located in the realm of geochemistry, and workers were able to grow many various known minerals, including oxides, silicates, phosphates and sulfides under hydrothermal conditions [14].

During this period, two important achievements appeared. One was the development of the Morey vessel. Morey’s development allows hydrothermal fluids to be contained in autoclaves lined with an inert metal, such as silver, gold or platinum, at pressures up to 800 bar [15]. The other was invention of the Bridgeman seal by the great Percy Bridgeman. This invention allowed solutions to be contained at much higher pressures up to 7 kbar [16]. Both of these inventions are used in much the same form today.

After World War II, the hydrothermal method became important for industrial growth of extremely pure crystals with interesting physical properties [17]. Today there are several dozen companies throughout the world that produce -quartz commercially more than 500 000kg each year [18]. Improvements in electronic industry required new materials, especially as a single crystal, and this necessity made hydrothermal method important.

The first successful commercial application of hydrothermal technology began with mineral extraction or ore beneficiation in the 19th century [19]. The commercial importance of the hydrothermal technique for the synthesis of inorganic compounds was realized with the beginning of the synthesis of large single crystals of quartz [20] and zeolites [21].

The hydrothermal technique has captured the attention of scientists and technologists from different branches of science. Today it is a highly interdisciplinary subject and the technique is popularly used by geologists, biologists, physicists, chemists, ceramists, hydrometallurgists, material scientists, engineers, and so on [7].

Figure 1.1 shows the number of publications year-wise and this number is increasing sharply with the entry of scientists from other branches of science.

As evident from Figure 1.2, the preparative chemists and ceramists have dominated this field, particularly with the recent advances in the advanced materials and electronic ceramics. It is interesting to note that the hydrothermal technique of material synthesis has gained its momentum during the postwar period, although began in the 19th century. Prior to that, majority of the compounds synthesized under hydrothermal conditions were essentially the natural analogues, as the main thrust was on the study of the origin of rocks, minerals and ores through laboratory simulations.



Figure 1.1 Number of publications year-wise.
With the availability of the improved equipment and also with the knowledge on the preparative chemistry routes to synthesize many inorganic compounds, with or without natural analogues under hydrothermal conditions, the popularity of the technique grew fast. Today, the number of compounds without any natural analogue synthesized under hydrothermal conditions is more than the number of compounds with natural analogues synthesized. With the advent of new mineralizers, a wide variety of organic and inorganic compounds until now unknown are being prepared by hydrothermal technique. The technique is being popularly used for crystallization of materials, crystal growth, and materials processing, and so on.

Figure 1.2 Number of papers on hydrothermal research in materials.


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