policristallino

Policristallino refers to a solid material composed of many small crystalline regions, called grains, each with its own lattice orientation. The grains are separated by grain boundaries. This contrasts with a monocrystal, where the material has a single continuous crystal lattice. Policrystalline materials are common among metals, ceramics, and some semiconductors.

The microstructure arises when the material solidifies from a melt, or when powder particles are joined by sintering, or when films are deposited and grains grow but do not coalesce into a single crystal. The resulting average grain size can range from a few nanometers to millimeters, and the processing conditions largely determine it. The grain boundaries act as barriers to dislocation movement, which often increases strength and hardness, a relation described in the Hall-Petch effect. However, very small grains can lead to changes in deformation mechanisms and, at the extreme, to a softening behavior known as inverse Hall-Petch.

Grain boundaries also influence transport properties: they can scatter electrons and phonons, reducing electrical and thermal conductivity, and can be diffusion paths that affect corrosion and high-temperature stability. In optics and electronics, polycrystalline materials may be less uniform than single crystals, affecting performance in certain applications.

Applications: polycrystalline silicon is widely used in solar cells and electronics; many structural metals (steel, aluminum alloys) are polycrystalline; polycrystalline ceramics and diamond films are common in cutting tools and coatings.