Flux method

Crystallization
Fundamentals
Crystal Crystal structure Nucleation
Concepts
Crystallization Crystal growth Recrystallization Seed crystal Protocrystalline Single crystal
Methods and technology
Boules Bridgman–Stockbarger method Van Arkel–de Boer process Czochralski method Epitaxy Flux method Fractional crystallization Fractional freezing Hydrothermal synthesis Kyropoulos method Laser-heated pedestal growth Lely method Micro-pulling-down Shaping processes in crystal growth Skull crucible Verneuil method Zone melting

The flux method of crystal growth is a method where the components of the desired substance are dissolved in a solvent (flux). this is also called "Molten Flux Synthesis". The method is particularly suitable for crystals needing to be free from thermal strain. The flux, which serves as the solvent for crystallization and is often composed of a single, straightforward inorganic compound which melts at conveniently low temperatures, is a high-temperature solution. To get an even lower melting flux, various substances can be combined to create eutectic compositions. The ability to dissolve a sizable amount of the reagents, a significant change in solubility with temperature, a low melting point, low volatility, low cost, and finally, ease of removal after crystal growth via dissolution in typical solvents, ideally water, are all characteristics of a "good" flux. Although numerous fluxes can be used interchangeably and most crystals can be formed using more than one solvent system, there is no "universal flux"—each of them has some benefits and drawbacks. Since many fluxes are very reactive and will dissolve or chemically react with different containers, the reaction vessel utilized for crystal development is also a crucial factor. Given that silver is largely inert to both hydroxide and fluoride fluxes, it is a preferable choice. As long as a silica compatible flux is selected, fused silica is an excellent option for crystal growth investigations requiring a sealed reaction environment, such as for producing crystals containing elements in reduced oxidation states.There is no one type of container that is perfect for all flux growth investigations; instead, a variety of various refractories and metal containers have been employed as suited for the flux used.

As a result, the compatibility of the reaction vessel and the flux must be taken into account. Platinum and gold are examples of "inert" materials, although they are also highly expensive and, in the case of gold, have a temperature limit on crystal formation below 1064 °C. Alumina crucibles are frequently employed as a less expensive option since they are thought to be chemically resistant to halide melts but not to fluoride melts. However, hydroxide fluxes can dissolve them, which will include aluminum into the final product. Fluoride-based fluxes cannot be used with alumina because alumina crucibles are easily reacted with or dissolved by them. It takes place in a crucible made of highly stable, non-reactive material. For production of oxide crystals, metals such as platinum, tantalum, and niobium, silver are common. Production of metallic crystals generally uses crucibles made from ceramics such as alumina, zirconia, and boron nitride. The crucibles and their contents are often isolated from the air for reaction, either by sealing them in a quartz ampoule or by using a furnace with atmosphere control. A saturated solution is prepared by keeping the constituents of the desired crystal and the flux at a temperature slightly above the saturation temperature long enough to form a complete solution. Normally as common approach is implementing a temperature that is 100 °C higher than the melting point of the flux. Then the crucible is cooled in order to allow the desired material to precipitate. Crystal formation can begin by spontaneous nucleation or may be encouraged by the use of a seed. As material precipitates out of the solution, the amount of solute in the flux decreases and the temperature at which the solution is saturated lowers. This process repeats itself as the furnace continues to cool until the solution reaches its melting point or the reaction is stopped artificially. In flux method synthesis, divergent crystal growth kinetics may emerge, with a small number of crystallites growing at the expense of neighboring ones, resulting in abnormal grain growth.

Advantages

1. Flux method or molten flux synthesis is very efficient way to synthesize the crystal of oxides, sulfides and many other lanthanides and actinides.
2. Unlike solid state synthesis, it is possible to run the reaction at a comparatively lower temperature.
3. A good number of fluxes exist i.e. alkali halides, alkaline earth metal oxides etc.
4. This method does not require any sophisticated equipments.

Disadvantages

1. Sometimes the flux itself reacts with the starting material which then turns out to be an undesired product.
2. Often it is difficult to separate the product from the flux when the reaction is done. It is needed to purify the target product after the reaction. So it is important to find a solvent that dissolves only the flux but not the product.

See also

• Chemical vapor deposition
• Crystal growth
• Crystallography
• Czochralski process
• Epitaxy
• Hydrothermal synthesis
• Micro-pulling-down
• Verneuil process

External links

• Flux Method for Preparing Crystals
• Growth of single crystals from metallic fluxes
• Flux Technique