HYDROTHERMAL METHOD

HYDROTHERMAL METHOD 

This equipment has a steel tube closed at one end. 

Another and has a screw cap with a gasket at soft copper to provide a seal. 

This creates reaction bom to achieve high desired water vapour pressure. 

Role of water in hydrothermal method 

1. It serves as a pressure transmitting medium. 

2. Some or all reactants are partially soluble in water because soluble at high pressure. 

3. The reaction that uses water hydrolysis is speeded up under high pressure. 

4. Volatile components can be prevented to escape from reaction bombs. 

5. The reaction can be carried out at a critical temperature of water i.e. at 374 degrees Celsius 

Critical temperature: - above the temperature where the water remains at an aqueous phase. 

Application of hydrothermal method: 

1. Synthesis of new phases: Ca-silicates hydrates that an important for cement and concrete is prepared by the hydrothermal method. 

CaCo3 SiO2 (Quartz) 150oC to 500oC pressure 0.1 to 2.00 k bar. 

100 to 2000 bar Ca6 Si6 O17 (OH)2 (The product obtained) 

2. Single crystals can be grown by the use of a mineralizer (seed crystal) 

Hydrothermal synthesis includes the various techniques of crystallizing substances from high-temperature aqueous solutions at high vapor pressures; also termed the "hydrothermal method". 

The term "hydrothermal" is of geologic origin. 

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 autoclave, 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 Ability to create crystalline phases which are not stable at the melting point. 

Also, materials that 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 hydrothermal method 

There is a need for 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. 

Uses 

A large number of compounds belonging to practically all classes have been synthesized under hydrothermal conditions: elements, simple and complex oxides, tungstates, molybdates, carbonates, silicates, germinate, etc.

Hydrothermal synthesis is commonly used to grow synthetic quartz, gems, and other single crystals with commercial value. 

Some of the crystals that have been efficiently grown are emeralds, rubies, quartz, alexandrite, and others. 

The method has proved to be extremely efficient both in the search for new compounds with specific physical properties and in the systematic physicochemical investigation of intricate multicomponent systems at elevated temperatures and pressures. 

Equipment for hydrothermal crystal growth 

The crystallization vessels used are autoclaves. 

These are usually thick-walled steel cylinders with a hermetic seal that must withstand high temperatures and pressures for prolonged periods of time. Furthermore, the autoclave material must be inert with respect to the solvent. 

The closure is the most important element of the autoclave. 

Methods Temperature-difference method 

This is the most extensively used method in hydrothermal synthesis and crystal growing. 

Supersaturation is achieved by reducing the temperature in the crystal growth zone. The autoclave is heated in order to create a temperature gradient. 

The nutrient dissolves in the hotter zone and the saturated aqueous solution in the lower part is transported to the upper part by convective motion of the solution. Temperature-reduction technique In this technique, crystallization takes place without a temperature gradient between the growth and dissolution zones. 

The disadvantage of this technique is the difficulty in controlling the growth process and introducing seed crystals. For these reasons, this technique is very seldom used. 

Metastable-phase technique 

This technique is based on the difference in solubility between the phase to be grown and that serving as the starting material. 

The nutrient consists of compounds that are thermodynamically unstable under growth conditions. 

The solubility of the metastable phase exceeds that of the stable phase, and the latter crystallize due to the dissolution of the metastable phase. 

This technique is usually combined with one of the other two techniques above.