Fundamentals of Piezoelectricity and Piezo Actuators
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Since the piezo effect exhibited by natural materials such as quartz, tourmaline, Rochelle salt, etc. is very small, polycrystalline ferroelectric ceramic materials such as barium titanate and lead (plumbum) zirconate titanate (PZT) with improved properties have been developed. These ferroelectric ceramics become piezoelectric when poled.
PZT ceramics are available in many variations and are still the most widely used materials for actuator applications today. At temperatures below the Curie temperature, PZT crystallites exhibit tetragonal or rhombohedric structure. Due to their permanent electrical and mechanical asymmetry, these types of unit cells exhibit spontaneous polarization and deformation. Groups of unit cells with the same polarization and deformation orientation are called domains. Because of the random distribution of the domain orientations in the ceramic material, a ferroelectric poling process is required to obtain any macroscopic anisotropy and the associated piezoelectric properties (see Fig. 5.).
If heated above the Curie temperature, however, the PZT crystallite unit cells take on cubic centrosymmetric (isotropic) structure. When cooled, the domains reform, but with randomized orientations, and the material does not regain its macroscopic piezoelectric properties.
The asymmetric arrangement of the positive and negative ions imparts permanent electric dipole behavior to the crystals. Before the poling treatment, the domains are randomly oriented in the raw PZT material. During poling, an intense electric field (> 2000 V/mm) is applied to the piezo ceramic. With the field applied, the material expands along the axis of the field and contracts perpendicular to that axis as the domains line up. When the field is removed, the electric dipoles stay roughly, but not completely, in alignment. The material now has a remanent polarization (which can be degraded by exceeding the mechanical, thermal and electrical limits of the material).
Subsequently, when a voltage is applied to the poled piezoelectric material, the ions in the unit cells are shifted and, additionally, the domains change their degree of alignment. (see Fig. 6.). The result is a corresponding change of the dimensions (expansion, contraction) of the PZT material.
Fig. 5. PZT unit cell:
1) Perovskite-type lead zirconate titanate (PZT) unit cell in the symmetric cubic state above the Curie temperature.
2) Tetragonally distorted unit cell below the Curie temperature
Fig. 6. Electric dipoles in domains; (1) unpoled ferroelectric ceramic, (2) during and (3) after poling (piezoelectric ceramic)