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The flux method is a crystal growth method where starting materials are dissolved in a solvent (flux), and are precipitated out to form crystals of a desired compound. The flux lowers the melting point of the desired compound, analogous to a wet chemistry recrystallization.[1] The flux is molten in a highly stable crucible that does not react with the flux. Metal crucibles, such as platinum, titanium, and niobium are used for the growth of oxide crystals. Ceramic crucibles, such as alumina, zirconia, and boron nitride are used for the growth of metallic crystals.[2] For air-sensitive growths, contents are sealed in ampoules or placed in atmosphere controlled furnaces.
Fluxes edit
Oxide fluxes are often combined to reduce volatility, viscosity, and reactivity towards the crucibles. Metallic fluxes aren't typically combined, as they do not suffer from the same volatility, viscosity, and reactivity issues. An ideal flux should have the following properties[2]:
- Good solubility for desired compound at growth temperatures.
- Low melting point.
- Large gap between melting and boiling point.
- Easily removed from crystals.
- Unreactive with crucible and starting materials at growth temperatures.
| Metallic Flux | Oxide Flux | ||||
|---|---|---|---|---|---|
| Flux | Melting Point (°C) | Boiling Point (°C) | Flux | Melting Point (°C) | Boiling Point (°C) |
| Aluminum | 660 | 2470 | Lead(II) Oxide | 888 | 1477 |
| Bismuth | 271 | 1564 | Lead(II) Fluoride | 824 | 1293 |
| Gallium | 30 | 2400 | Bismuth (III) Oxide | 817 | 1890 |
| Indium | 157 | 2072 | Lithium Oxide | 1438 | 2600 |
| Tin | 232 | 2602 | Molybdenum Trioxide | 802 | 1152 |
| Lead | 328 | 1749 | Potassium Fluoride | 858 | 1502 |
Furnace Procedure edit
The growth (starting materials, flux, and crucible) are heated to form a complete liquid solution. The growth is cooled to a temperature where the solution is fully saturated. Further cooling causes crystals to precipitate from the solution, lowering the concentration of starting materials in solution, and lowering the temperature where the solution is fully saturated. The process is repeated, decreasing temperature and precipitating more crystals. The process is then stopped at a desired temperature, and the growth is removed from the furnace. Practically, the flux method is done by placing the growth into a programmable furnace:
- Ramp - The furnace is heated from an initial temperature to a maximum temperature, where the growth forms a complete liquid solution.
- Dwell - The furnace is maintained at the maximum temperature to homogenize the solution.
- Cool - The furnace is cooled to a desired temperature over a specified rate or time.
- Removal - The growth is removed from the furnace. The growth can be quenched, centrifuged, or simply removed if already at room temperature.
Additional steps may be added to this basic temperature profile, such as additional dwells or different cooling rates over different points of the cool. Crystallization can occur through spontaneous nucleation, encouragement with a seed, or through mechanical stress.
Air-Sensitive Growths edit
The growth of intermetallic compounds are air-sensitive. Oxygen in air reacts with heated metals to form metal oxides. Less commonly, nitrogen in the air can react with heated metals to form metal nitrides. Intermetallic growths are sealed under vacuum or inert-atmosphere ampoules to prevent oxidation. These ampoules are made out of quartz glass. The ampoules limits the maximum temperature of the growth to the melting point of the specific quartz glass, since it's typically lower than the melting point of the crucible or starting materials. One method of ampoule preparation is as follows:
- A quartz glass tube is filled with: quartz glass wool; a crucible containing starting material (grey) and flux (green); a filter; an upside-down crucible; and more quartz glass wool.
- A thin neck is formed above the tube's contents using a blowtorch. The tube is attached to a closed system containing a vacuum pump and an inert gas canister. The air in the tube is removed using the vacuum pump, and then the tube is filled with inert gas such as argon. This process is repeated to remove as much oxygen and water vapor from the tube as possible.
- The ampoule is filled with inert gas, and the neck is sealed using a blowtorch. The ampoule is often placed inside a larger crucible to keep it upright. The ampoule and crucible are placed into the furnace, and heated to the maximum temperature
- The furnace is maintained at the maximum temperature and the growth forms a homogenous liquid mixture.
- The furnace is cooled, and crystals precipitate out of solution. The solution becomes more flux-rich (less grey).
- At a specific temperature, the ampoule is removed from the furnace, and immediately flipped and spun in a centrifuge. The flux-rich (even less grey) solution passes through the filter, while the crystals cannot. The solution solidifies during the centrifuging, and the ampoule is allowed to cool to room temperature. The ampoule is broken, and the crystals are removed.
Flux separation edit
After crystallization, often some solidified flux remains on the surface or inside the desired crystal. This flux may cause defects in the crystal due to the different thermal expansivities of the flux and crystal.[3] A solvent (often an acid) can dissolve the flux, but it's difficult to find a solvent that doesn't also dissolve the crystal. If the flux is clearly visible, potentially with a microscope, it can be removed mechanically using a blade or drill. If the crystal and flux have significantly different boiling points, the flux may be removed with evaporation.
The removal of excess flux is important to assess a crystals properties, as the flux can affect measurements. For example, tin and lead super conduct at low temperatures, if a sample has tin or lead flux superconductivity can be observed even if the desired crystal is not a superconductor.
See also edit
External links edit
References edit
- ^ 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).
- ^ a b Tachibana, Makoto (2017). Beginner's Guide to Flux Crystal Growth. Tsukuba, Ibaraki Japan: Springer. pp. 61–87. ISBN 978-4-431-56586-4.
- ^ Wolf, Thomas (July 2012). "Flux separation methods for flux-grown single crystals". Philosophical Magazine. 92 (19–21): 2458–2465. Bibcode:2012PMag...92.2458W. doi:10.1080/14786435.2012.685193. ISSN 1478-6435. S2CID 137541564.