Piezo and Ferroelectric Ceramics

PIEZO AND FERROELECTRIC CERAMICS

We know that the piezoelectric crystals undergo a change in polarisation when they are subjected to a stress. The application of compressive stress results in the displacement of charge in one direction while tensile stress leads to the displacement of charge in opposite direction.

Pyroelectric crystal is one which produces spontaneous polarisation due to application of electric field. Further in these crystals, a change in temperature also produces a change in spontaneous polarisation.

 On the other hand, if one changes the direction of electric field, the direction of spontaneous polarisation also changes. The crystals exhibiting this additional property are known as ferroelectrics.

Therefore, ferroelectric crystal exhibit spontaneous and reversible polarisation. Most of the properties of ferroelectricity and ferromagnetism are common.

The ferroelectric properties disappear at certain temperature T_C (curie temperature). The crystal will be in paraelectric state above T_C, which is analogous to paramagnetism.

The crystal in the paraelectric state obeys Curie-Weiss law. Therefore, the spontaneous polarisation takes place, below T_C

Examples for the ferroelectric ceramics are Rochelle salt, BaTiO₃, SITO₃ PbTiO₃ LiNbO₃, NaNbO₃, KNbO₃, PbTa₂O_3, etc. The structure, Curie temperature and Curie constants of ferroelectric ceramics are shown in Table.

Table: 4.1

Structure, Curie Temperature and Curie Constant of Ferroelectric Ceramics

Hysteresis

Ferroelectric ceramic materials exhibit the hysteresis behaviour. The hysteresis behaviour exhibited by a single crystal and polycrystalline material is shown in fig. 4.24 and fig. 4.25 respectively.