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Ferroelectric hysteresisAn 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 effect of a force on a piezoelectric cylinder
Deformation of a piezoceramic body when a voltage is applied
The Piezoelectric Effect
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 piezo 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 effect is frequently also called the motor effect.
