The word ‘piezo’ means to press or squeeze (from Greek word ‘piezein’). Piezoelectricity is the electricity (generation of electrical potential) which is produced as a result of piezoelectric effect of some materials when subjected to mechanical stress or compression or expansion. This was first revealed by Pierre Curie and Jacques Curie in 1880. This effect is reversible i.e. its converse effect also exist. That means when some material (normally crystals and ceramics) with the application of electrical potential may result in change in mechanical character (produce compression or expansion or strain). The reason for the piezoelectric effect is mainly the uneven structure of the molecules in the crystal. When electrical potential is given, the unevenness or asymmetry increases result in the production of piezoelectricity.
Generation of piezoelectric effect on material
Now, we can discuss how this effect is produced inside the molecule of the crystal OR ceramics. A molecule is said to be symmetrical when the polar axes (an unreal line passing through the middle of the positive and negative charges in a molecule) of whole dipoles remain in same orientation. This crystal is called mono crystal which is represented below.
However, in Poly crystal, the orientation of polar axes will be dissimilar which can produce piezoelectric effect as represented in Fig 2.
The piezoelectric effect is produced in this poly crystals by heating it subjected to heavy electric field. As a result of heating, molecules will shift freely and electric field causes the whole dipole to form unidirectional as shown in Figure 3(B) and it will retain in polarised state after the removal of the heavy electric field as shown in Figure 3(C).
Now, we can move on to the illustration of piezoelectric effect with the help of the Figure 4. The Figure 4(A) shows the piezoelectric crystal without any application of mechanical energy. When the material is pressed or compressed, a voltage is developed in the middle of the two electrodes. The polarity will be the same as that of voltage of polarisation as shown in Figure 4(B). Next, the material is extended and as a result, a voltage having opposite polarity is formed as shown in Figure 4(C).
For the reverse or backward effect, application of a heavy potential with diverse polarity is required to the crystal and the result in enlargement of the crystal is shown in Figure 4(D). If the voltage having same polarity is applied, piezoelectric material will undergo contraction as shown in Figure 4(E). In Figure 4(F), the response of the material when AC voltage is applied is shown. The material will swing back and forth with the same frequency of the applied AC.
Materials Exhibiting Piezoelectric Effect
The natural crystals such as Quartz, cane sugar, Rochelle salt, bone, Topaz, Berlinite, Tourmaline group minerals are piezoelectric crystals. The other man made materials are GaPO4, La3Ga5SiO14, BaTiO3, KNbO3, LiTaO3 etc are piezoelectric materials. Polyvinylidene fluoride is a polymer which can produce piezoelectric effect.
Application of Piezoelectricity
- Used as sources of voltage and power in electric cigarette lighter, in equipments used in battle field, in piezoelectric transformer etc.
- As sensors in microphones, pickups in guitar and other musical instruments, power monitoring in medical treatment etc.
- As actuators in loudspeakers, inkjet printers, piezoelectric motors etc.
- As frequency standard in quartz clock.