How do crystals grow?

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PANEL_05.inddMonopotassium phosphate (KDP) crystals are used in industry as frequency doublers or piezoelectric materials. The National Ignition Facility is an ambitious project that uses 192 lasers to create a controlled fusion reaction – the same reaction that feeds the Sun – here on Earth. In order to achieve it, 600 metric size panels of KDP crystals of more than 300 kilograms are needed, like the one in the photo, grown in the Lawrence Livermore National Laboratory (USA). They are obtained carefully controlling the cooling velocity of a solution and the orientation of the initial crystalline seed.

This same technique of growth using solutions and a similar product (monoammonium phosphate) is used in the Crystallization Competition organized in Spanish schools (http://www.lec.csic.es/concurso/).

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Video showing how giant KDP crystals, shown top left, grow.

Until the nineteenth century the only crystals used were those found in nature – minerals. But as new properties were discovered with applications in the industries of electronics, communications, pharmacology and food, different techniques were developed to grow crystals better and more efficiently in university and industrial laboratories. Today, young students learn to make crystals at school (http://www.lec.csic.es/concurso/).

AND HOW DO CRYSTALS GROW?

Crystals grow by accretion, that is, by the stacking of molecules and atoms on the surface of their faces. You probably played at stacking blocks as a child, putting one on top of another and another alongside, well ordered. That is more or less how crystals grow. The molecules that are in the solution come close to the surface of the crystal and arrange themselves upon it. They pass over the face until arriving at a step, where they connect with more force. Then they move along the step until they reach a corner position, in which they settle permanently.

image005It is just like the famous game Tetris. In fact, there is a variety of the game, “CrystalTetris”, which is used to teach how crystals grow. You can play Crystal Tetris online on the Laboratorio de Estudios Cristalográficos website.

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image010 Silicon for semiconductors is usually obtained using the melt crystallization technique. The silicon is heated in a crucible to a temperature somewhat higher than melting temperature. A small, cooled seed is placed on top, which, when it touches the surface of the melt, crystallizes the liquid on contact. Pulling slowly on the growing seed, cylinders of up to two metres long are achieved.

Research into the techniques of crystal growth is fundamental today to move industry forward.

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Oxide crystals for lasers or high-temperature superconductors Gallium arsenide crystals for high-frequency electronic devices.
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Diamond crystals for use as abrasives and cutting tools have been obtained habitually for fifty years. Today gem-like diamond crystals of up to 4 carats are also obtained. Many new technology applications need thin crystalline films few microns thick, like these of silicon oxide. To grow them, techniques of “Chemical Vapour Deposition” are used.

PROTEIN CRYSTALS

Biomedicine and structural biology projects need to obtain protein and nucleic acid crystals of perfection. By subjecting them to powerful beams of X-rays or neutrons, it is possible to reveal the atomic structure of these compounds. Although only crystals of less than a millimetre are needed for X-rays, and of a few millimetres for neutrons, obtaining these crystals is one of the greatest challenges of crystallization today.

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Composition of microphotographs of crystals of biological macromolecules published in the journal Acta Crystallografica F of the International Union of Crystallography.

CRYSTALLIZATION IN SPACE

The Granada Crystallization Facility (GCF) is a device for the crystallization of proteins in space, designed and constructed by the Laboratorio de Estudios Cristalográficos [LEC – Crystallographic Studies Laboratory] of CSIC and the company NTE. It is the equipment that is most used by different international laboratories to carry out crystallization experiments in microgravity conditions, as well by the agencies NASA, ESA and JAXA. The GCF and other LEC devices have performed crystal growth experiments on the Discovery (Mission STS-95) and Columbia (Mission STS-107) shuttles, on the International Space Station and on the crewless spacecraft Foton, using the Progress/Soyuz rockets and currently the SpaceX-3 launcher.

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Pedro Duque on the International Space Station receiving the Granada Crystallization Facility.

CRISTALES

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