Origin of piezoelectricity

Fig. 1 and 2 explain the origin of piezoelectricity from both the intrinsic and extrinsic effects of the piezoelectrics. The intrinsic effect corresponds to the relative anion/cation shift that preserves the ferroelectric crystal structure. As shown in Fig. 1, when the external electric field is applied, the cations are drawn in the direction of the electric field and the anions in the opposite direction, leading to a deformation of crystal lattice. The accumulation of lattice deformations becomes the strain of the ceramics. This is the so-called inverse piezoelectric effect. If the ceramic is stressed, the ions are also displaced from its original positions, resulting in the distortion of lattice and appearance of electrical charges on the surface, i.e. the direct piezoelectric effect occurs.

Fig. 1. Intrinsic effect of piezoelectricity

Depending on the crystal structure, the centers of the positive and negative charges do not coincide even without the application of external electric field. Such crystals are said to possess spontaneous polarization (Denoted as Ps in Fig. 1 and 2). Especially when the spontaneous polarization of the dielectric can be altered by an electric field, it is called ferroelectric. Piezoelectrics in general consist of many domains to reduce the electrostatic energy of the system. In each domain, the spontaneous polarization has the same direction as shown in Fig. 2. The domains may reorient under electric field through domain wall motions and formation of new domains where the direction of spontaneous polarization is close to that of the electric field as shown in Fig. 2. The contribution of the domain wall motions to the piezoelectric response is the extrinsic properties of the ceramics. From the published results, the extrinsic contributions to dielectric constant e, piezoelectric constant d and compliance s of the soft PZT ceramics at room temperature and low field levels are significant and may account for 50-80% of the overall response. From the viewpoint of lattice as shown in Fig. 2, the extrinsic contribution is the rotation of lattice caused by the domain wall motions. The a- and c-axis is exchanged for a tetragonal structure ceramic. The rotation results in the contraction of the overall crystal in one direction and elongation in the perpendicular direction.

Fig. 2. Extrinsic effect of piezoelectricity