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A polycrystalline solid.



an aggregate of small crystals of a substance, which, because of their irregular form, are sometimes called crystallites or grains. Many materials of natural or artificial origin—such as minerals, metals, alloys, and ceramics—are po-lycrystals.

The properties of a polycrystal are due to the properties of the constituent crystalline grains; the average size of the grains, which ranges from 1–2 × 10-6 μ to a few millimeters, the crystallographic orientation of the grains; and the structure of the grain boundaries. If the grains are randomly oriented and their dimensions are small in comparison with the size of the polycrystal, the polycrystal does not exhibit anisotropy of physical properties, which is characteristic of single crystals. If the polycrystal has a texture, or preferred crystallographic orientation of its grains, it exhibits anisotropic properties. The presence of grain boundaries has a considerable effect on the physical, particularly the mechanical, properties of polycrystals, since such phenomena as the scattering of conduction electrons, the scattering of phonons, and the inhibition of dislocations occur at the boundaries.

Polycrystals are formed during crystallization, during polymorphic transformations, and as a result of the sintering of crystalline powders. A polycrystal is less stable than a single crystal. For this reason, during prolonged annealing of a polycrystal there occurs recrystallization, or the preferential growth of certain grains at the expense of others, which results in the formation of large crystalline blocks.


References in periodicals archive ?
The behavior of a polycrystal under a uniaxial compressive load offers a striking resemblance to the behavior of plasma which was the subject of study of Bohm [11].
The texturing of the polycrystal is tracked by computing the change in orientation [DELTA]g = v[DELTA]t of crystals at each time step.
For implementation of this approach the finite element model of the polycrystal was used in the form of a thin plate consisting of one layer of grains in the shape of hexagonal prisms (39 hexahedrons and fragments forming a rectangular plate).
Nonetheless the accurate analytical description of these boundaries and lines in a real polycrystal would be unfeasible.
Our elastic constants estimated for a full-dense polycrystal depart strongly from those given in Munro's review (2), especially in the uncertainties.
Compared to conventional solar technology utilizing single crystal or polycrystal silicon, United Solar Ovonic's thin-film solar cells are 100 times thinner, do not use crystalline silicon material and, therefore, are unaffected by the scarcity of silicon worldwide.
They will produce new low-temperature polycrystal silicon LCDs for smaller and lighter equipment such as compact cellular phones and DVD players, the reports said, with the new LCDs featuring glass substrates incorporating an integrated circuit for displaying images.
Among the topics are enhancing simulation in complex systems, a numerical investigation of deployable drag surfaces used for recovery systems, liquid crystal thermography for determining the heat transfer coefficient in rectangular ducts, evaluating crack growth rate and the growth model of ultrafine-grained copper, the compression property of a waste polyurethane rubber/unsaturated polyester composite cube, a non-contact breath sensor based on a Doppler detector, new solid-state organic membrane based lead-selective micro-electrode, simulating intergranular crack nucleation and evolution in polycrystal metals on mesoscale, a coupled numerical-experimental study of armor perforation by armor piercing projectiles, and lessons from airlines for railway disruption recovery.
The relation between polycrystal deformation and single crystal deformation, Metallurgical and Material Transactions 1: 1121-1144.
The yield and flow stress dependence on polycrystal grain size, Yield, Flow and Fracture of Polycrystals.
Key words: exchange coupling; magnetocrystalline anisotropy; micromagnetics; polycrystal.