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Crystal structure · Solid
A single crystal or monocrystalline solid is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries. The absence of the defects associated with grain boundaries can give monocrystals unique properties, particularly mechanical, optical and electrical, which can also be anisotropic, depending on the type of crystallographic structure. These properties, in addition to making them precious in some gems, are industrially used in technological applications, especially in optics and electronics.
Because entropic effects favour the presence of some imperfections in the microstructure of solids, such as impurities, inhomogeneous strain and crystallographic defects such as dislocations, perfect single crystals of meaningful size are exceedingly rare in nature, and are also difficult to produce in the laboratory, though they can be made under controlled conditions. On the other hand, imperfect single crystals can reach enormous sizes in nature: several mineral species such as beryl, gypsum and feldspars are known to have produced crystals several metres across.
The opposite of a single crystal is an amorphous structure where the atomic position is limited to short range order only. In between the two extremes exist polycrystalline, which is made up of a number of smaller crystals known as crystallites, and paracrystalline phases.
Single crystal silicon is used 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.
Another application of single crystal solids is in materials science in the production of high strength materials with low thermal creep, such as turbine blades. Here, the absence of grain boundaries actually gives a decrease in yield strength, but more importantly decreases the amount of creep which is critical for high temperature, close tolerance part applications.
Single crystals provide a means to understand, and perhaps realize, the ultimate performance of metallic conductors.
Of all the metallic elements, silver and copper have the best conductivity at room temperature, so set the bar for performance. The size of the market, and vagaries in supply and cost, have provided strong incentives to seek alternatives or find ways to use less of them by improving performance.
The conductivity of commercial conductors is often expressed relative to the International Annealed Copper Standard, according to which the purest copper wire available in 1914 measured around 100%. The purest modern copper wire is a better conductor, measuring over 103% on this scale. The gains are from two sources. First, modern copper is more pure. However, this avenue for improvement seems at an end. Making the copper purer still makes no significant improvement. Second, annealing and other processes have been improved. Annealing reduces the dislocations and other crystal defects which are sources of resistance. But the resulting wires are still polycrystalline. The grain boundaries and remaining crystal defects are responsible for some residual resistance. This can be quantified and better understood by examining single crystals.
As anticipated, single-crystal copper did prove to have better conductivity than polycrystalline copper.
|Material||ρ (μΩ∙cm)||IACS |
|Single-crystal Ag, doped with 3 mol% Cu||1.35||127%|
|Single-crystal Cu, further processed||1.472||117.1%|
|High purity Ag wire (polycrystalline)||1.59||108%|
|High purity Cu wire (polycrystalline)||˃103%|
But there were surprises in store (see table). The single-crystal copper not only became a better conductor than high purity polycrystalline silver, but with prescribed heat and pressure treatment could surpass even single-crystal silver. And although impurities are usually bad for conductivity, a silver single-crystal with a small amount of copper substitutions was a better conductor than them all.
As of 2009, no single-crystal copper is manufactured on a large scale industrially, but methods of producing very large individual crystal sizes for copper conductors are exploited for high performance electrical applications. These can be considered meta-single crystals with only a few crystals per metre of length.
Single crystals are essential in research especially condensed-matter physics, materials science, surface science etc. The detailed study of the crystal structure of a material by techniques such as Bragg diffraction and helium atom scattering is much easier with monocrystals. Only in single crystals it is possible to study directional dependence of various properties. Furthermore, techniques such as scanning tunneling microscopy are only possible on surfaces of single crystals. In superconductivity there have been cases of materials where superconductivity is only seen in single crystalline specimen. They may be grown for this purpose, even when the material is otherwise only needed in polycrystalline form.
In the case of silicon and metal single crystal fabrication the techniques used involve highly controlled and therefore relatively slow crystallization.
Specific techniques to produce large single crystals (aka boules) include the Czochralski process and the Bridgman technique. Other less exotic methods of crystallization may be used, depending on the physical properties of the substance, including hydrothermal synthesis, sublimation, or simply solvent-based crystallization.
A different technology to create single crystalline materials is called epitaxy. As of 2009, this process is used to deposit very thin (micrometre to nanometer scale) layers of the same or different materials on the surface of an existing single crystal. Applications of this technique lie in the areas of semiconductor production, with potential uses in other nanotechnological fields and catalysis.