Piezoelectric Working Principle, Perovskite and Polarization
Piezoelectricity is based on the ability of certain crystals to generate an electrical charge when mechanically loaded with pressure or tension (direct piezo effect). Conversely, these crystals undergo a controlled deformation when exposed to an electric field – a behavior referred to as the inverse piezo effect. The polarity of the charge depends on the orientation of the crystal relative to the direction of the pressure.
Ceramics exhibiting piezoelectric properties belong to the group of ferroelectric materials. Today’s systems are almost exclusively based on lead zirconate titanate (PZT); i.e., they consist of mixed crystals of lead zirconate (PbZrO3) and lead titanate (PbTiO3). Piezoceramic components have a polycrystalline structure comprising numerous crystallites (domains) each of which consists of a plurality of elementary cells. The elementary cells of these ferroelectric ceramics exhibit the perovskite crystal structure, which can generally be described by the structural formula A2+B4+O32-.
Schematic diagram of an ideal perovskite structure, neglecting distortions due to spontaneous polarization below Curie temperature. The bivalent cation is located in the center of the cube, while the tetravalent cations form the cube corners. The bivalent anions are located in the center of each cube edge in this illustration. For the PZT (lead zirconate titanate) mixed crystal, the formula is: A: Pb2+, B: Ti4+ / Zr4+
Image: Before, during and after polarisation
Immediately after sintering, the domains of a ceramic body (i.e., the areas consisting of elementary cells of uniform dipole direction) will show an arbitrary (statistically distributed) orientation, i.e., the macroscopic body is isotropic and shows no piezoelectric properties.
These piezoelectric properties have to be originated by “polarization“. In this process, the ceramic body is exposed to a strong electric DC field that causes the electric dipoles to become aligned in the direction of field. They will maintain this orientation even when the DC field is no longer applied (remanent polarization) – a necessary condition for the piezoelectric behavior of ferroelectric ceramics.