Flux method
 | This article needs additional citations for verification. (January 2021) |
The flux method of crystal growth is a method where crystal starting materials are dissolved in a solvent (flux), typically a solid at room temperature, and heated until molten.[1] Crystals of the desired compound are then precipitated out. The method is particularly suitable for crystals needing to be free from thermal strain. The flux is typically molten in a crucible made of highly stable, non-reactive material. For production of oxide crystals, metals such as platinum, tantalum, and niobium are common. Production of metallic crystals generally uses crucibles made from ceramics such as alumina, zirconia, and boron nitride.[2] For air sensitive growths, contents are often isolated from the air, either by sealing them in a quartz ampoule or by using atmosphere controlled furnace.
The starting materials, flux and crucible are heated to form a complete solution, then cooled to a temperature where the solution is fully saturated. Further cooling allows the desired material to precipitate, decreasing the concentration of starting materials in solution. 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 neighbouring ones, resulting in abnormal grain growth.
One advantage of this method is that the crystals grown often display natural facets, which often makes preparing crystals for measurement significantly easier. A disadvantage is that most flux method syntheses produce relatively small crystals. However, some materials such as the "115" heavy fermion superconductors (CeXIn5, X=Co,Ir,Rh) may grow up to a few centimeters. Flux growth is preferable to a pure melt in that it avoids the situation of incongruent melting at high temperatures required to melt the substance by replacing it with a lower melting temperature flux.[3]
Flux separation
Following crystallization, a solid crystal-flux matrix may develop, which may cause defects in the crystals due to thermal stresses caused by thermal expansivity differences. A solvent can be used to dissolve the matrix around the crystals, but it may be difficult to find a suitable solvent capable of only dissolving the matrix, especially if the matrix and crystals possess similar chemical properties. One alternative to separate the flux is to do so in the liquid form through the use of either a centrifuge or by attaching a seed to a pulling rod. This method is best used when thermal stresses are an issue. In the centrifuge technique, silica wool acts as a sieve, separating the mixture. In the seed method, such as the Czochralski method, a seed crystal is grown in the liquid solution, allowed to grow and cool, and pulled out, leaving only the crystal. Flux separation can also be done at room temperature through removal of the solid matrix through a drill or evaporation.[4]
See also
- Chemical vapor deposition
- Crystal growth
- Crystallography
- Czochralski process
- Epitaxy
- Hydrothermal synthesis
- Micro-pulling-down
- Verneuil process
External links
References
- ^ Byrappa, K.; Ohachi, Tadashi (Eds.) (2003). "17.2.4 Flux method". Crystal Growth Technology. Norwich, N.Y.: William Andrew Pub. p. 567. ISBN 3-540-00367-3.
Components of the gem materials desired in a single crystal form are dissolved in a flux (solvent).
- ^ Tachibana, Makoto (2017). Beginner's Guide to Flux Crystal Growth. Tsukuba, Ibaraki Japan: Springer. pp. 82–87. ISBN 978-4-431-56586-4.
- ^ Fisk, Z.; Remeika, J. P. (1989-01-01), "Chapter 81 Growth of single crystals from molten metal fluxes", Handbook on the Physics and Chemistry of Rare Earths, vol. 12, Elsevier, p. 54, retrieved 2023-11-08
- ^ Wolf, Thomas (July 2012). "Flux separation methods for flux-grown single crystals". Philosophical Magazine. 92 (19–21): 2458–2465. doi:10.1080/14786435.2012.685193. ISSN 1478-6435.