polycrystalline

Polycrystalline describes a solid composed of many crystallites, or grains, each with its own crystal orientation. The grains are separated by grain boundaries, regions where the lattice is misaligned and defects are concentrated. In contrast to a single crystal, a polycrystalline material has no single continuous crystallographic direction across its entire volume, and its properties often reflect the collective behavior of many grains.

Polycrystalline structures form during solidification from a melt, by deposition of atoms in multiple islands that coalesce, or by consolidation of ceramic or metal powders followed by sintering. Grain size typically ranges from nanometers to millimeters and can be altered by heat treatment or plastic deformation, which drives grain growth or recrystallization.

The presence of grain boundaries influences mechanical, electrical, optical, and thermal properties. Boundary regions can impede dislocation motion, strengthening metals at smaller grain sizes (the Hall-Petch effect), but they can also be preferential sites for corrosion, creep, and diffusion. Electrical conductivity and carrier mobility can be reduced by boundary scattering in polycrystalline semiconductors or metals. In optics, grain boundaries affect scattering and translucency in ceramics and thin films.

Common examples include polycrystalline silicon used in solar cells and integrated circuits, polycrystalline ceramics for structural components and insulators, and polycrystalline metals in many alloys. Production methods include casting and solidification, powder metallurgy with sintering, and thin-film deposition techniques such as chemical vapor deposition or sputtering, which can produce polycrystalline films with controlled grain structure.