Piezo Materials Tutorial: The Piezoelectric Effect
Piezoelectric ceramics belong to the group of ferroelectric materials. Ferroelectric materials are crystals which are polar without an electric field being applied. This state is also termed spontaneous polarization. Characteristic of this state is the thermodynamically stable reversibility of the axis of polarization under the influence of an electric field, described graphically by a hysteresis loop. The reversibility of the polarization, and the coupling between mechanical and electrical effects are of crucial significance for the wide technological utilization of piezoceramics. From a crystallographic point of view, these piezoelectric materials exhibit what is called Perovskite crystalline structure. This applies to a series of compounds with three types of atoms with the general formula ABC.
The piezoelectric effect in common piezoceramics: PbTiO3 - PbZrO3 are sythesized from the oxides of lead, titanium and zirconium. BaTiO3 is also used. Special dopings of these leadzirconate- titanate ceramics (PZT) with, for example, Ni, Bi, Sb, Nb ions etc., make it possible to adjust individual piezoelectric and dielectric parameters as required. These materials are not ferroelectric above a characteristic temperature, known as the Curie temperature. They are in a paraelectric state, i.e. no dipoles are present. The relative dielectric constant has a distinct maximum in the vicinity of the Curie temperature. Below the Curie point of the material, the cubic, electrically neutral crystalline form gives way to lattice distortions, resulting in the formation of dipoles and rhombohedral and tetragonal crystallite phases, which are of interest for piezo technology.
Ferroelectric domains and polarization of piezo ceramic
A ferroelectric crystal can be divided into spatial regions having different directions of polarization, called ferroelectric domains. What is generally meant by a domain in a solid body is a physically bounded spatial region in which a vector quantity characterizing the state at a point in the solid body has the same direction everywhere. For a ferroelectric domain this characteristic quantity consists in the same alignment and the same absolute value of the spontaneous polarization. Depending on the particle size of the polycrystalline ceramic material, the individual crystallites contain only a few domains, bounded by domain walls.
In the event of large changes of the electric field or mechanical stress, shifting occurs and the polarity of whole regions can be reversed as a result of domain reforming. These processes, and the irreversible displacement of domain walls, are some of the reasons for the familiar phenomenon of ferro-electric hysteresis.
During manufacture, after the sintering process the polycrystalline piezoelectric ceramics are in a thermally depolarized state after the sintering process. From a statistical point of view, there is an almost uniform distribution of spontaneous polarization directions among the domains, and the material is isotropic, i.e. not piezoelectric. By applying a strong electric field E, the spontaneous polarization is ferroelectrically reoriented to the saturation polarizationPs This produces a residual polarization parallel to the direction of the field, and the material is anisotropic, i.e. piezoelectric.
Direct Piezo Effect
Mechanical stresses arising as the result of an external force acting on the piezoelectric body induce displacements in the positive and negative lattice elements which manifest themselves in dipole moments. The resulting formation of an electric field puts an electric potential on the insulated electrodes. This direct piezoelectric effect is frequently referred to as the generator effect in the literature.
Inverse Piezo Effect
The application of an electric voltage to an unrestrained piezoceramic body results in ist deformation. The amount of movement is a function of the polarity of the voltage applied and the direction of the polarization vector. Applying an AC voltage generates a cyclical change in the geometry (e. g. increase or reduction in the diameter of a disk). If the body is clamped, i.e. if free deformation is constrained, a mechanical stress or force is generated. This inverse piezoelectric effect is frequently also called the motor effect.
Cubic (paraelectric) and tetragonal (ferroelectric) structure of PZT and BaTiO3 before and after an electric field has been applied or a mechanical stress taken effect.
Symbolic representation of the electrical reorientation processes in piezoelectric ceramic crystallite and domain structure.
An opposing electric field will only depolarize the material if it exceeds the coercivity strength. A further increase in the opposing field leads to repolarization, but in the opposite direction.
The direct piezo effect: Force on a piezoelectric cylinder results in a charge
The inverse piezoelectric effect: Deformation of a piezoceramic body when a voltage is applied