Crystal growth

In Crystal Growth ( rarely also crystal growth ) means the artificial production of crystals. Consequently, both terms describe the technical process that provides the crystal. This is to be distinguished in the crystal growth of the German ( a chemical or physical process that leads by attachment of atoms or molecules to form crystals ). In English, all of these terms are described with crystal growth. Crystallization (english crystal lization ), however, means the formation of one or many crystals in an unspecified process under consideration.

Since crystals constitute the essential material basis for many modern technological applications, the growth of crystals of some materials is operated ( first silicon ) today in the order of several thousand tons per year worldwide industry. In these cases, therefore, the crystal as such, or the component produced therefrom, the goal of these efforts represent a different objective, however, is often pursued in preparative chemistry of new substances. This small but well -formed crystals of the test substance are routinely examined by X-ray structure analysis ( crystallography ), and determined the crystal structure. Was already in the 1950s by such studies, the structure determination of deoxyribonucleic acid ( DNA) as the carrier of the genetic information of living organisms.

Breeding methods

The classification is carried out expediently on the nature of the phase transition, resulting in the formation of the crystal:

Growth from the melt

The crystal growth from a melt is initiated in all these processes is that an originally above the melting temperature Tf befindliches volume ( the melt) is slowly cooled to a temperature below Tf and thereby crystallized.

• Czochralski method: The melt (temperature only slightly above Tf) is located in a crucible. From top plunges a seed crystal is pulled under and upward rotation.
• Bridgman method or Bridgman -Stockbarger method: The melt is contained in a vial. It is lowered by a vertical tube furnace, which generates a temperature at the top of T1 > T2, Tf and at the lower part < TF. This initial version is also called a vertical Bridgman method; but it is also a variant operated with horizontal temperature gradients (horizontal Bridgman method )
• Zone melting process: When operated in two versions: Zone melting in the crucible: Similar to the Bridgman method, a horizontal or vertical; However, the furnace is designed so that only a narrow zone located above Tf. This zone "wanders" through movement of the furnace or crucible by at T < Tf contained other material.
• Free crucible zone melting (floating zone ): Works only for electrically conductive materials and is used in particular for silicon crystals of the highest quality. In this case, a polycrystalline supply rod is inserted vertically through a coil, produced by the applied AC voltage an eddy current in the material. This eddy current leads for the electrical resistance of the material to heat up to above Tf in a narrow melting zone. These moves in turn through movement crystal and stock rod relative to the coil through the material.

Growth from the gas phase

In the growth of the gas phase, there are basically two methods: the sublimation or physical vapor deposition and chemical vapor deposition.

The sublimation or physical vapor deposition ( PVD) is carried out by the to-grow substance is first vaporized by physical means, such as by heating them to the point that it passes into the gas phase. The substance must not necessarily be previously melted ( sublimation). Such behavior shows, for example, elemental iodine. The gas moves to a seed crystal and allowing under suitable conditions where the growth of a crystal.

The chemical vapor deposition ( CVD) works technically similar; but here the junction of the substance S to be grown in the gas phase by a first auxiliary substance ( Transport, H) is possible because otherwise the vapor pressure and thus also the transfer rate would be too low. So there will be a reaction S H → SH at the source. The gaseous SH then moves to the seed crystal, where the back reaction SH → S H is the original substance is again formed and deposited as crystal. The auxiliary substance H is thus again available and is not consumed in the process. Such processes are often carried out in the epitaxial growth of semiconductors.

Breeding from the solution

In the simplest case, the substance ( for salts often water ) is dissolved in a suitable solvent to saturation. The crystal growth from the solution is then triggered either by evaporation of the solvent or by temperature change. ( Usually due to cooling because the solubility usually decreases with decreasing temperature. ) Alternatively, the solubility can be (eg ethanol to aqueous solutions ) also reduced by the addition of other substances. Crystal growth of salts from aqueous solutions can be carried out simply and partially funded by the chemistry teaching in schools or at home. Corresponding experimental kits are commercially available. Suitable substances are, for example, copper sulphate, alums ( KAl (SO4) 2:12 H2O or other ) or yellow prussiate of potash ( Kaliumhexacyanidoferrat (II ) trihydrate).

Some substances, such as silica (SiO2) dissolve under atmospheric pressure much better very poorly in water, but under hydrothermal conditions. This is exploited in hydrothermal growth method, for example by SiO2 ( ie sand) is dissolved in supercritical basic solvents, which are in autoclave and crystallized at colder places in the reactor again. α - quartz ( quartz) as the most important piezoelectric crystal is produced. For special purposes, and usually only in research melting solutions are used. Herein is for a molten substance other than a solvent for the material to be grown.

History

The ( large) industrial crystal growth has only existed since the early 20th century with the invention of the Verneuil process for the commercial cultivation of ruby.

Crystals are, however, already grown considerably longer, the Chinese developed allegedly already 2700 years BC process for the recovery of salt crystals. Other records of crystals and their extraction can be found in Pliny the Elder ( 23-79 AD) and Georgius Agricola over vitriols ( 1494-1555 ), who wrote inter alia on the purification of saltpeter by crystallization.

One of the first solvothermal synthesis goes back to Robert Bunsen, who thus bred in 1839 barium and strontium carbonate.

First time in 1852 herapathite was prepared which was later bred commercially from a solution and is the predecessor of Polaroid film.

The first successful attempts Rubin artificially before it came to a breakthrough by the Verneuil process, H. Gaudin reach 1837-1840, however, believed this to have only made ​​of glass, because the density apparently did not agree with that of natural ruby, but this was probably due to trapped gas bubbles.

, Jan Czochralski developed in 1912 was named after him Czochralski method in Berlin. This process is now computer and solar industries mainly used for pulling large silicon single crystals for.

In the following years, the crystal growth grew by the need for suitable materials for progressive technologisation rapidly and is today one of the foundations for many technological achievements, such as lasers and semiconductor technology.

Rating

Crystals are indispensable key materials for many scientific and engineering applications and corresponding crystal growth at a high qualitative and quantitative level is now established both in industry and in universities and research institutes worldwide. Scheel specifies the following data for the total global production of all kinds of crystals: 1979: 5,000 tons, 1986: 11,000 tons, 1999: 20,000 tons. These totals are distributed approximately as follows: