polykrystalline

Polycrystalline refers to a material composed of many small crystals, or grains, each with a distinct lattice orientation. The internal interfaces between grains, called grain boundaries, separate the crystallites and often host defects and segregated species. Polycrystalline materials contrast with single-crystal materials, in which the lattice is continuous across the sample, and with amorphous materials, which lack long-range order.

In practice, most structural metals, most ceramics, and many semiconductors are polycrystalline. The random orientation of grains typically leads to different properties in different directions at the microscale, but macroscopic properties can be more isotropic if grain orientations are well distributed. However, grain boundaries act as barriers to dislocation motion and diffusion, influencing strength, hardness, creep, conductivity, and corrosion resistance. Grain size is a key parameter; smaller grains generally increase strength through the Hall-Petch effect but can reduce ductility.

Formation and processing: polycrystalline materials form during solidification of a melt, powder metallurgy, sintering, or chemical vapor deposition. Techniques such as annealing and controlled grain growth influence grain size and texture. In thin films, polycrystalline growth can produce textures or grain boundaries that affect electronic and optical properties.

Applications and examples: polycrystalline metals like steel and aluminum alloys, polycrystalline ceramics such as silicon nitride or zirconia, and polycrystalline semiconductors such as polycrystalline silicon used in photovoltaics and electronics. Photovoltaic devices often use large-area polycrystalline silicon or thin-film polycrystalline layers (CdTe, CIGS) where grain boundaries play a crucial role in performance and stability.

Characterization: X-ray diffraction and electron microscopy are used to determine crystal structure, grain size, and grain boundary characteristics. Electron backscatter diffraction (EBSD) provides maps of grain orientation; the Scherrer equation estimates average grain size from diffraction peak broadening.