Crystal growing

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Under crystal growth (more rarely crystal growth ) is the artificial production of crystals understood. Both terms describe the technical process that delivers the crystal. This is to be distinguished in German from crystal growth , the chemical or physical natural process that leads to the formation of crystals through the addition of atoms or molecules . In English, both of these terms are described as crystal growth . Crystallization (English crystallization ) generally means the formation of one or many crystals, regardless of the specific process.

Since crystals represent the indispensable material basis for many modern technical applications, the growing of crystals of some materials ( silicon in the first place ) is nowadays carried out on an industrial scale in the order of several thousand tons annually worldwide. In these cases, the crystal as such, or the component made from it, represents the goal of the endeavors. A different goal, however, is often pursued in the preparative chemistry of new substances. Here, small but well-formed crystals of the substance to be examined are routinely examined by X-ray structure analysis (single crystal diffractometry), and the crystal structure is thereby determined. As early as the 1950s, such studies were used to determine the structure of deoxyribonucleic acid (DNA) as the carrier of genetic information in living beings.

Breeding methods

The classification is expediently based on the type of phase transition that leads to the formation of the crystal:

Growing from the melt

In all of these processes, crystal growth from a melt is triggered by the fact that a volume (the melt) originally located above the melting temperature T f is slowly cooled to a temperature below T f and crystallizes in the process.

  • Verneuil process : The powdery starting material is liquefied by means of a burner and drips onto a seed crystal ( seed crystal ).
  • Bridgman process or Bridgman-Stockbarger process : The melt is in an ampoule. This is lowered by a vertical tube furnace, which generates a temperature T 1  >  T f in the upper part and T 2  <  T f in the lower part . This original variant is also known as the vertical Bridgman method ; However, a variant with a horizontal temperature gradient is also operated ( horizontal Bridgman method )
  • Neck Kyropoulus procedure : named after Richard Nacken and Spyro Kyropoulos . The melt with a temperature only slightly above T f is in a crucible and a somewhat cooler seed crystal dips into it from above . The crystal grows into the crucible.
  • Czochralski process : further development of the above process. Greater lengths are possible by slowly pulling the crystal upwards. The crystallization front remains at the same height so that the temperature can be better controlled. In addition, the crystal is rotated slowly to compensate for horizontal temperature gradients. This enables larger diameters. The rotation also makes the boundary layer in front of the crystallization front thinner, so that impurities that are not supposed to build into the crystal are transported away more quickly.
  • Zone melting process : Is operated in two variants:
    • Zone melting in the crucible: Similar to the Bridgman process, horizontal or vertical; however, the furnace is designed in such a way that there is only a narrow zone above T f . This zone "migrates" through the movement of the furnace or crucible through the remaining material at T  <  T f .
    • Crucible-free zone melting ( floating zone ): only works for electrically conductive materials and is used in particular for silicon crystals of the highest quality. A polycrystalline supply rod is pushed vertically through a coil, which generates an eddy current in the material through the AC voltage applied . Because of the electrical resistance of the material, this eddy current leads to heating to above T f in a narrow melting zone. This in turn moves through the material as the crystal and supply rod move relative to the coil.

Growth from the gas phase

There are essentially two processes involved in growing from the gas phase : sublimation or physical gas phase deposition and chemical gas phase deposition .

The sublimation or physical vapor deposition (PVD) is carried out in that the substance to be grown is first physically vaporized, for example by heating it so far that it changes into the gas phase. The substance does not necessarily have to be melted beforehand (sublimation). Such behavior shows e.g. B. elemental iodine . The gas moves to a seed crystal and, under suitable conditions, enables a crystal to grow there.

Chemical vapor deposition (CVD) works in a technically similar way; but here the transition of the substance S to be grown into the gas phase is only made possible by an auxiliary substance ( means of transport , H ), because otherwise its vapor pressure and thus also the transport rate would be too low. A reaction S  +  H  →  SH takes place at the source . The gaseous SH then moves to the seed crystal , where in the reverse reaction SH  →  S  +  H the original substance is formed again and deposited as a crystal. The auxiliary substance H is thus available again and is not used up in the process. Such processes are often carried out in the epitaxy of semiconductors .

Culture from solution

In the simplest case, the substance is dissolved in a suitable solvent ( often water in the case of salts ) until it is saturated . Crystal growth from the solution is then triggered either by evaporation of the solvent or by a change in temperature. (Usually by cooling, because the solubility usually decreases with decreasing temperature.) Alternatively, the solubility can also be reduced by adding other substances (e.g. ethanol to aqueous solutions). Crystal growth of salts from aqueous solutions is partly easy and can be carried out with the help of chemistry lessons in schools or at home. Appropriate experiment kits are available in stores. Suitable substances are e.g. B. vitriol , alums (KAl (SO 4 ) 2 · 12H 2 O or others) or yellow blood liquor salt ( potassium hexacyanidoferrate (II) trihydrate).

Some substances such as silicon dioxide (SiO 2 ) dissolve only very poorly in water under normal pressure , but much better under hydrothermal conditions. This is exploited in hydrothermal cultivation processes by e.g. B. SiO 2 (ie sand) in supercritical basic solvents, which are located in autoclaves , dissolved and recrystallized at colder points in the reactor. α-quartz ( quartz ) as the most important piezoelectric crystal is produced in this way. Melting solutions are used for special purposes and mostly only in research. Here a melted other substance serves as a solvent for the material to be grown.

history

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

Crystals have been grown for much longer, however, as the Chinese allegedly developed as early as 2700 BC. Chr. Process for the extraction of salt crystals. Further records of crystals and their extraction can be found in Pliny the Elder (23–79 AD) on vitriols and in Georgius Agricola (1494–1555), who u. a. wrote about the purification of saltpetre using crystallization.

One of the first solvothermal syntheses goes back to Robert Bunsen , who used it in 1839 to grow barium and strontium carbonate .

Herapathite was first produced in 1852 , which shows strong polarization and was later grown commercially from a solution; later these crystals were detached by polarizing foils.

The first successful attempts to artificially produce ruby ​​before the breakthrough with the Verneuil process came from H. Gaudin between 1837 and 1840, although Gaudin believed he had only produced glass because the density did not seem to match that of natural ruby. but this was probably due to trapped gas bubbles.

Jan Czochralski developed the Czochralski process named after him in Berlin in 1916 . This process is now mainly for towing large silicon - single crystals used for the computer and solar industries.

In the years that followed, crystal growth grew rapidly due to the need for suitable materials for the advancing technology and is now one of the foundations for many technical achievements, for example in lasers and semiconductor technology .

Rating

Crystals are indispensable key materials for many scientific and technical applications and accordingly crystal growing at a high qualitative and quantitative level is now established worldwide in industry as well as in universities and research institutes. Scheel gives the following data for the global total production of all types of crystal: 1979: 5,000 tons, 1986: 11,000 tons, 1999: 20,000 tons. These total amounts are distributed roughly as follows:

Material group proportion of Examples
semiconductor 60% Silicon , gallium arsenide
Scintillators 12% Tl: CsI, BGO
optics 10%
Acousto-optics 10% Lithium niobate
Laser , NLO 05% Cr: Al 2 O 3 ( ruby ), Nd: YAG
Watches , jewelry 03% Al 2 O 3 crystal as a watch glass

literature

  • Klaus-Thomas Wilke, Joachim Bohm (Eds.): Crystal growth . 2nd edition. Verlag Deutsch, Thun 1988 (2 volumes).
  • Hans J. Scheel: Historical aspects of crystal growth technology . In: Journal of Crystal Growth . tape 211 , no. 1-4 , 2000, pp. 1-12 , doi : 10.1016 / S0022-0248 (99) 00780-0 .

Web links

Individual evidence

  1. ↑ mineralienatlas.de .
  2. ↑ Klaus-Thomas Wilke, Joachim Bohm (Eds.): Kristallzüchtung , 2nd edition. Verlag Deutsch, Thun 1988 (2 volumes).
  3. ↑ RA Laudise: Hydrothermal Synthesis of Crystals . In: C&EN . 28, 1986, pp. 30-43.
  4. ^ D. Harris: A Century of Sapphire Crystal Growth . In: Proceedings of the 10th DoD Electromagnetic Windows Symposium Norfolk, Virginia. 2004.
  5. ^ Hans J. Scheel: Historical aspects of crystal growth technology . In: Journal of Crystal Growth . tape 211 , no. 1-4 , 2000, pp. 1-12 , doi : 10.1016 / S0022-0248 (99) 00780-0 .