Vibrations & Displacements

Piezoelectric Ceramic Vibrations & Displacements

When the crystalline structure of a material has no centre of symmetry, it is noncentrosymmetric. A single crystal with this structure has anisotropic characteristics: the properties of the material differ according to the direction of measurement. Piezoelectricity is an anisotropic characteristic. The non-centrosymmetric crystalline structure provides a net electric dipole moment within the crystal unit cell. Any dipoles aligned in the same direction will arrange themselves into regions called domains.

Piezoelectric ceramics are ferroelectric materials. These materials have noncentrosymmetric unit cells below a certain temperature and a centrosymmetric structure above that temperature. That temperature is the Curie temperature. Above the Curie temperature, these ceramics have a centrosymmetric structure and have no piezoelectric characteristics. Below the Curie temperature, these ceramics have a noncentrosymmetric structure.

A ceramic material is composed of many randomly oriented crystals or grains, each having one or a few domains. With the dipoles randomly oriented, the material is isotropic and does not exhibit the piezoelectric effect. By applying electrodes and a strong d.c. electric field, the dipoles will tend to align themselves parallel to the field, so that the material will have a permanent (or remanent) polarization. Refer to Figure 5. Not as many domains can align their dipoles in ceramic materials as in single crystals, but enough do so that the material will become piezoelectric. After poling, the material has a remanent polarization (Pr) and remanent stress (Sr) as shown in Figures 6A and B.

Fig 5 Poling
Unpoled	Poled

Fig 6 Hysteresis
Fig 6A	Fig 6B

As this "poled" ceramic is subjected to stress, the crystal lattice or grains distort causing some of the domains to grow at the expense of others. This changes the total dipole moment of the material. Within a certain range of stress, this change of dipole moment with stress gives rise to piezoelectric property (and its converse) can be used practically, because the material's response is nearly linear.