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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. The flux is molten in a highly stable
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. A solvent (typically an acid or a base) can dissolve the flux, but it's difficult to find a solvent that doesn't also dissolve the crystal. The flux 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. Flux can also be removed through recrystallization through use of a seed in the liquid phase, leaving the flux behind as the crystals accumulate.
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.
Flux Method for Preparing CrystalsGrowth of single crystals from metallic fluxesFlux Technique
crucible
A crucible is a container in which metals or other substances may be melted or subjected to very high temperatures. Although crucibles have historically tended to be made out of clay, they can be made from any material that withstands temperat ...
that does not react with the flux. Metal crucibles, such as platinum
Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...
, titanium
Titanium is a chemical element; it has symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
, and niobium
Niobium is a chemical element; it has chemical symbol, symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and Ductility, ductile transition metal. Pure niobium has a Mohs scale of mineral hardness, Mohs h ...
are used for the growth of oxide crystals. Ceramic crucibles, such as alumina
Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly ...
, zirconia
Zirconium dioxide (), sometimes known as zirconia (not to be confused with zirconium silicate or zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral ba ...
, and boron nitride
Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula B N. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexago ...
are used for the growth of metallic crystals. For air-sensitive growths, contents are sealed in ampoules or placed in atmosphere controlled furnaces.
Choice of flux
Oxide fluxes are often combined to reduce volatility,viscosity
Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...
, 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:
* Good solubility for desired compound at growth temperatures.
* Low melting point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...
.
* Large gap between melting and boiling point
The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.
The boiling point of a liquid varies depending upon the surrounding envi ...
.
* Easily removed from crystals.
* Unreactive with crucible and starting materials at growth temperatures.
Furnace procedure
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 theconcentration
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', '' number 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
In thermodynamics, nucleation is the first step in the formation of either a new Phase (matter), thermodynamic phase or Crystal structure, structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically def ...
, encouragement with a seed
In botany, a seed is a plant structure containing an embryo and stored nutrients in a protective coat called a ''testa''. More generally, the term "seed" means anything that can be Sowing, sown, which may include seed and husk or tuber. Seeds ...
, or through mechanical stress.
Flux separation
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. A solvent (typically an acid or a base) can dissolve the flux, but it's difficult to find a solvent that doesn't also dissolve the crystal. The flux 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. Flux can also be removed through recrystallization through use of a seed in the liquid phase, leaving the flux behind as the crystals accumulate.
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
*Chemical vapor deposition
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high-quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (electro ...
* Crystal growth
Crystal growth is a major stage of a crystallization, crystallization process, and consists of the addition of new atoms, ions, or polymer strings into the characteristic arrangement of the crystalline lattice. The growth typically follows an ini ...
* Crystallography
Crystallography is the branch of science devoted to the study of molecular and crystalline structure and properties. The word ''crystallography'' is derived from the Ancient Greek word (; "clear ice, rock-crystal"), and (; "to write"). In J ...
* Czochralski process
The Czochralski method, also Czochralski technique or Czochralski process, is a method of crystal growth used to obtain single crystals (monocrystals) of semiconductors (e.g. silicon, germanium and gallium arsenide), metals (e.g. palladium, plati ...
* Epitaxy
Epitaxy (prefix ''epi-'' means "on top of”) is a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited cry ...
* Hydrothermal synthesis
Hydrothermal synthesis includes the various techniques of synthesizing substances from high-temperature aqueous solutions at high pressures; also termed "hydrothermal method". The term "hydrothermal" is of geologic origin. Geochemists and mine ...
* Micro-pulling-down
* Verneuil process
External links
Flux Method for Preparing Crystals
References
{{DEFAULTSORT:Flux Method Crystallography Methods of crystal growth fr:Cristallogénèse pt:Cristalografia sk:Kryštalizácia