Also, piezoelectric technologies are better suited than electromagnetic ones for MEMS implementation, because of the limitations in magnets miniaturization with current state-of-the-art microfabrication processes [21].2.?Transduction PrincipleThe piezoelectric effect converts mechanical strain into electric current or voltage. It is based on the fundamental structure of a crystal lattice. Certain crystalline structures have a charge balance with negative and positive polarization, which neutralize along the imaginary polar axis. When this charge balance is perturbed with external stress onto the crystal mesh, the energy is transferred by electric charge carriers creating a current in the crystal. Conversely, with the piezoelectric effect an external charge input will create an unbalance in the neutral charge state causing mechanical stress.

The connection between piezoelectricity and crystal symmetry are closely established. The piezoelectric effect is observed in crystals without center of symmetry, and the relationship can be explained with monocrystal and polycrystalline structures.In a monocrystal (Figure 3) the polar axes of all of the charge carriers exhibit one-way directional characteristics. These crystals demonstrate symmetry, where the polar axes throughout the crystal would lie unidirectional even if it was split into pieces.Figure 3.Monocrystal.Instead, a polycrystal (Figure 4) is characterized by different regions within the material with different polar axes. It is asymmetrical because there is no point at which the crystal could be cut that would leave the two remaining pieces with the same resultant polar axes.

Figure 4.Polycrystal.In order to attain the piezoelectric effect, the polycrystal is heated to the Curie point along with strong electric field. The heat allows the molecules to move more freely and the electric field forces the dipoles to rearrange in accordance with the external field (Figure 5).Figure 5.(a) Polarizations; (b) Surviving Polarity.As a result, the material possesses piezoelectric effect: a voltage of the same polarity as of the poling voltage appears between electrodes when the material is compressed; and opposite polarity appears when stretched. Material deformation takes place when a voltage Dacomitinib difference is applied, and if an AC signal is applied the material will vibrate at the same frequency as the signal [23�C25].Piezoelectricity is governed by the following constitutive equations, which link the stress T, the strain S, the electric field E and the electrical induction D:{Tp=cpqESq?ekpEkDi=eiqSq+?ikSEk(1)where cpqE is the Young’s modulus, ekp is the piezoelectric coefficient and ?ikS is the clamped permittivity.