Piezo - Electric Oscillator Method

Piezo – electro effect

When mechanical pressure is applied on the opposite faces of the quartz crystals, electric charges appear on the other pair of opposite faces of the crystal. This effect is called piezoelectric effect.

Fig. 1.1 Piezo – electric effect

The piezo- electric effect was discovered by P $aul$ Jean curie and Pierre curie in 1880.

Inverse piezo – electric effect

The piezo – electric effect is reversible. When electric field is applied on the opposite faces of the quartz crystal, alternative mechanical deformation (expansion or contraction) is produced across the other pair of opposite faces of the crystal. This effect is called inverse piezo – electric effect. This is also called as electrostriction.

This effect is used for generating ultrasonic waves using piezo – electric oscillator. Ultrasonic waves which are produced by the piezo – electric method is more efficient than magnetostriction oscillator method.

The materials which exhibit this effect are called piezoelectric transducers. Eg. Quartz, and Barium titanate.

Fig. 1.2 Inverse piezo – electric effect

Piezo – electric crystals

The crystals which produce piezo-electric effect are termed as piezo – electric crystals.

Example: Quartz, tourmaline, Rochelle salt and Zinc blend.

Fig. 1.3 Piezo-electric crystals

Natural quartz crystal has a hexagonal structure with a pyramid attached to each end.
The line joining the end points of the pyramid is called z-axis (or) optic axis.
The line passing through the opposite corners of the hexagon is called $x$ -axis or electrical axis.
The lines passing through the opposite sides of the hexagon is called y-axis or mechanical axis.
In quartz crystal, all the three axis (x, y, z) are mutually perpendicular to each other.

X-cut crystal

When the crystal is cut perpendicular to the

x

– axis is called

X

– cut crystal. Generally X – cut crystals are used to produce longitudinal ultrasonic waves.(Fig 1.4).

Fig. 1.4 X – cut crystals

Y- cut crystal

When the crystal is cut perpendicular to the y-axis is called Y-cut crystal. Generally Y-cut crystals are used to produce transverse ultrasonic waves as shown in (Fig 1.5)