single crystal
A single crystal, also called monocrystal, is a crystalline solid in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries. The opposite of a single crystal sample is an amorphous structure where the atomic position is limited to short range order only. In between the two extremes exist polycrystalline and paracrystalline phases, which are made up of a number of smaller crystals known as crystallites. Because of a variety of entropic effects on the microstructure of solids, including the distorting effects of impurities and the mobility of crystallographic defects and dislocations, single crystals of meaningful size are exceedingly rare in nature, and can also be difficult to produce in the laboratory under controlled conditions (see also recrystallisation).
Because grain boundaries can have significant effects on the physical and electrical properties of a material, single crystals are of interest to industry, and have important industrial applications. The most notable of these is the use of single crystal silicon in the fabrication of semiconductors. On the quantum scale that microprocessors operate on, the presence of grain boundaries would have a significant impact on the functionality of field effect transistors by altering local electrical properties. Therefore, microprocessor fabricators have invested heavily in facilities to produce large single crystals of silicon.
Fabrication of single crystals an example single crystalline silicon usually involves the building of a crystal layer by layer of atoms. Techniques to produce large single crystals (boules) include slowly drawing a rotating "seed crystal" in a molten bath of feeder material silicon(as in the Czochralski process and the Bridgeman technique). The feeder polycrystalline silicon is the purest material in the world. It is grown in rods and then melted for the formation of the single crystalline Czochralski and Bridgeman crystals.
An entirely different technology to create single crystalline materials is called epitaxy. This proces deposits micron layers of the same or different materials on the surface of an existing single crystal.
Uses
 Monocrystals of silicon and other semiconductors are important for manufacture of integrated circuits.
Monocrystalline silicon photovoltaic cells have superior performance characteristics and lower price to chemically identical polycrystalline solar cells.
Monocrystals of sapphire and other materials are used for lasers and nonlinear optics.
Monocrystals of fluorite are sometimes used in the objective lenses of apochromatic refracting telescopes.
Monocrystals of metals, especially superalloys, are used for their special mechanical properties. Turbine blades of some gas turbines are made of single crystal cast superalloy.
Monocrystals of copper (crystalline copper) are used for fine crystalline powders and hi tech wires.
Monocrystaline superconductors have superior performance characteristics to chemically identical polycrystaline superconductors.
The detailed study of the Crystal structure of a material by Bragg diffraction techniques is much easier with monocrystals. They may be grown for this purpose, even when the material is otherwise only needed in polycrystalline form.
See also
Crystal
Crystal structure
Crystallite
Crystallization and engineering aspects
Fractional crystallization (chemistry)
Polycrystalline silicon
Recrystallization
Seed crystal
Wafer (electronics)
References
"Small Molecule Crystallization" (PDF) at Illinois Institute of Technology website
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