Crystallography: A Very Short Introduction by A. M. Glazer

Author:A. M. Glazer
Language: eng
Format: epub
ISBN: 9780198717591
Publisher: OUP Oxford
Published: 2016-02-19T05:00:00+00:00

Crystals of quartz are vitally important for their piezoelectric properties, especially for their use in the electronics industries as oscillators and timing devices. This is a huge crystal industry. These crystals are grown by the hydrothermal method, which uses the fact that the solubility of quartz crystal in an alkaline liquid varies with high temperatures and pressures. The vessel for growth is a pressure vessel called an autoclave, and in industrial processes may be several metres in height. Raw material, called lasca (small pieces of natural quartz crystals), is placed in the lower part of the autoclave and in the upper part, many seed crystals of quartz in the form of thin plates are suspended in racks above a permeable baffle. The vessel is filled with water and a mineralizer such as Na2CO3 or NaOH, and after encapsulating, it is heated up to very high pressure (this is a dangerous method of crystal growth!). Finally, it reaches a supercritical state, and the growth begins. By keeping the temperature of the upper part of the autoclave below that of the lower part, convection occurs and the solution is moved from the lower part to the upper part, where quartz crystals precipitate on to the seed crystals. Since the synthetic quartz crystals are grown under strict control, they have precise shapes, dimensions, and properties. Almost perfect crystals of quartz, several centimetres in length, can be grown like this over a period of weeks or months.

Controlled vapour deposition (CVD) is a popular industrial method to grow crystals by depositing the solid material from a vapour on to a cold surface. There are many variants, which have been used to make semiconductor thin films. It is also used to make artificial diamonds. While diamond growth can be done by applying high pressures and temperatures to carbon in a press, CVD has increasingly been employed instead. In this technique a hydrocarbon gas is passed into a chamber on to a substrate, and diamond crystals form by chemical reactions that are still a matter of research. CVD is useful because it enables diamond crystal to be deposited over large areas, so that one could, for instance, coat electronic components. This is of interest because diamond has the unusual property of being a good electrical insulator but at the same time an excellent thermal conductor, thus enabling heat to be carried away from the electronic components. Today, gem-quality diamonds can be grown artificially, and in order to protect the diamond gem market, spectroscopic and other methods have been developed in order to distinguish between synthetic and natural diamonds.

None of these methods are suitable for the growth of protein crystals, for which special techniques have been developed. The most commonly used are hanging and sitting drop vapour diffusion methods. The hanging-drop method involves a drop of protein solution being placed on an inverted cover slip, which is then suspended above a reservoir containing a liquid reagent. The sitting drop crystallization apparatus places the drop on a pedestal that is separated from the reservoir.

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