0.16 Section 4.5. piezo-electricity and ferro-electricity.  (Page 5/2)

Section 4.5. Piezo-electricity and Ferro-electricity.

Piezo-electric effect was discovered by Jacques and Pierri Curie in 1880. They discovered that certain materials like quartz, Rochelle Salt, tourmaline and Sodium Potassium Tartarate exhibited polarization on the application of mechanical stress as shown in Figure 4.6.

Application of electrical field causes mechanical deformation. This is called Inverse Piezo-electric Effect. Using Inverse Piezo-electric Effect, Ultra-sonic transducers can be built. Alternating Electric field applied in the frequency range 20kHz to 100MHz will set the piezo-crystal in mechanical vibration at the same frequency as long as the frequency happens to be the natural frequency of resonance as determined by the physical dimensions. If the applied alternating electric field is off-resonance then very weak mechanical vibrations will be set.

As seen in Figure 4.6, there is no polarization under zero strain. When tensile or compressive strain is applied to such a crystal, it alters the separation between (+)ve and (-)ve charges in each elementary cell. This leads to a net polarization in each unit cell at the crystal surface. The Polarization is proportional to the applied Stress and its polarity is direction dependent. In the Figure 4.6, Elongation or tensile strain causes positive polarization and Compressive strain causes negative polarization.

Stress creates Electric Field and Electric Field creates Elastic Strain causing the physical dimension to alter in accordance with the electric field.

Besides Quartz, Rochelle Salts, Tourmaline, Sodium Potassium Tartarate we have piezoelectric ceramics. PZT is an example of piezo-electric ceramics. PZT is polycrystalline ferroelectric material with PEROVSKITE crystal structure.

Perovskite has Tetragonal/Rhombohedral structure very close to Cubic Structure. They have a general formula as follows:

Here A is trivalent metal ion such as La.

B is trivalent metal ion such as Al.

Rhombohedral Perovskite is LaAlO ₃ .

PZT is a mass of minute crystallites. Above Curie Temperature it exhibits simple cubic structure with (+)ve charge center and (-)ve charge center being coincident as shown in Figure 4.7.a. Hence the crystal is centro-symmetric with no permanent dipoles. It is found to exhibit paraelectric behavior. Ability for undergoing electronic polarization as discussed in Physics of Dielectric Chapter.

Paraelectricity is the ability of many materials (specifically ceramic crystals) to become polarized under an applied electric field. Unlike Ferroelectricity; this can happen even if there is no permanent electric dipole that exists in the material, and removal of the fields results in the polarization in the material returning to zero. The mechanisms which give rise to paraelectric behaviour are the distortion of individual ions (displacement of the electron cloud from the nucleus) and the polarization of molecules or combinations of ions or defects.