Piezo-Electric Effect and Piezo-Electric Method

PIEZO‒ELECTRIC EFFECT AND PIEZO‒ELECTRIC METHOD

Piezo‒Electric Crystals

The crystals which produces piezo‒electric effect and converse piezo‒electric effect are termed as piezo‒electric crystals.

Examples: Quartz, Tourmaline, Rochelle salts etc.

A typical example for a piezo‒electric crystal (Quartz), is as shown in Fig.5.1. It has an hexagonal shape with pyramids attached at both ends. It consists of 3 axes, viz., (i) optic axis (Z‒ axis), which joins the edges of the pyramid, (ii) Electrical axis (X‒axis), which joins the corners of the hexagon and (iii) mechanical axis (Y‒axis), which joins the centre (or) sides of the hexagon, as shown in Fig.5.1.

NOTE: All the 3 axes are mutually perpendicular to each other.

X‒cut and Y‒cut crystals

X‒cut crystal: When the crystal is cut perpendicular to the X‒axis, as shown in Fig.5.2, then it is called X‒cut crystal.

Generally X‒cut crystals are used to produce longitudinal ultrasonic waves.

Y‒cut crystals: When the crystal is cut perpendicular to the Y axis, as shown in Fig.5.3, then it is called Y‒cut crystal.

Generally Y‒cut crystals produces transverse ultrasonic waves.

Piezo electric effect

When pressure (or) mechanical force is applied along certain axis (mechanical axis) with respect to optic axis of the crystals like quartz, tourmaline, rochelle salts etc., then equal and opposite charges are produced along the perpendicular axis (electrical axis) with respect to optic axis of the crystal as shown in Fig 5.4. This effect is called piezo electric effect.

Inverse piezo electric effect

When potential difference (or) e.m.f. is applied along certain axis (electrical axis) with respect to optic axis of the piezoelectric crystals then the crystal starts vibrating along the perpendicular axis (Mechanical axis) with respect to the crystal as shown in Fig 5.5. This effect is called as inverse piezo electric effect.

Principle

Inverse piezo electric effect is the principle behind the production of ultrasonics using piezo electric oscillator circuit. Here ultrasonics are produced at resonance (i.e) when the frequency of the oscillatory circuit is equal to the frequency of the vibrating crystal.

Construction

The piezo‒electric generator consists of primary and secondary circuits. The primary circuit is arranged with coils L₁ and L₂. The coil L₁ is connected to the base of the transistor and coil L₂ is connected to the collector of the transistor. The capacitor C₁ is used to vary the frequency of the oscillatory circuit [L₁C₁]. The coil L₂ is inductively coupled to the secondary circuit, which comprises of the coil L₃ and two metal plates A and B as shown in Fig 5.6.

The crystal is kept inbetween the plates A and B for the production of ultrasonics. Necessary biasing, i.e., Emitter is forward biased ('n' is connected to negative of the battery) and the collector is reverse biased ('n' is connected to positive of the battery) is given with the help of the battery.

Working

The battery is switched ON and hence current is produced by the transistor, in the circuit. The current is passed through the coil L₁ and L₂ of the primary circuit. This current is transfered to the coil L₃ in the secondary circuit due to transformer action and is fed to the plates A and B. Due to the principle of inverse piezo‒electric effect the crystal starts vibrating along the mechanical axis of the crystal.

The frequency of the oscillatory circuit is adjusted by the capacitor C₁ and when this frequency is equal to the frequency of the vibrating crystal, resonance occurs. At resonance the crystal vibrates vigorously and ultrasonic waves are produced along both the ends of the crystal.

Condition for Resonance

Frequency of the oscillatory circuit = Frequency of the vibrating crystal

(i.e.)

Where

 l is the Length of the crystal

E is the Youngs modulus of the crystal

ρ is the Density of the crystal

P = 1,2,3.... etc. for fundamental, first over tone, second overtone etc. respectively.

Limitations

(i) It can produce frequency upto 500 MHz.

(ii) It can produce longitudinal as well as transverse ultrasonic waves by properly cutting and shaping the crystal with respect to the optic axis.

(iii) The production of ultrasonics is independent of temperature and hence produces high power ultrasonics at constant frequency.