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Piezoelectric | Ultrasonic

Piezoelectric Effect

When pressure is applied to one pair of opposite faces of crystal like quartz, tourmaline, Rochelle salt, etc., cut with their faces perpendicular to its optic axis, equal and opposite charges appear across its other faces as shown in figure. This phenomenon is known as piezoelectric effect. The frequency of the developed emf is equal to the frequency of dynamic pressure.


The sign of the charge gets reversed if the crystal is subjected to tension instead of pressure.

The electricity produced by means of piezoelectric effect is called piezoelectricity. The material which can undergo piezoelectric effect is called piezoelectric material or crystal.

Note:
  1. The type of crystal used for ultrasonic production is generally the X-cut crystal.
  2. An X-cut crystal is piezoelectric crystal cut in a direction perpendicular to its X-axes(Electrical axes).
The cross-section of a natural quartz crystal is hexagonal. The lines joining the midpoint of the opposite faces of the hexagonal are called the mechanical axes or Y-axes.



Piezoelectric Method

Principle : Inverse piezoelectric effect

If an alternating voltage is applied to one pair of opposite faces of the crystal, alternatively mechanical contractions and expansions are produced in the crystal and the crystal starts vibrating. This phenomenon is known as inverse piezoelectric effect or electrostriction effect.
If the frequency of the alternating voltage is equal to the vibrating frequency of the crystal, then the crystal will be thrown into resonant vibration producing ultrasonic waves.

Construction

The circuit diagram is shown in figure.

The quartz Q is placed between two metal plates A and B. The plates A and B are connected to the coil L3. The coils L1, L2 and L3 are inductively coupled to the oscillatory circuit of a transistor. The coil L2 is connected to the collector circuit, while the coil L1 with a variable capacitor C1 forming the tank circuit is connected between the base and the emitter. The battery is connected between free end of L2 and the emitter of transistor.

Working

When the battery is switched on, the oscillator produces high frequency alternating voltage given by,
The frequency of oscillation can be controlled by the variable capacitor C2. Due to the transformer action an emf is induced in the secondary coil L3. This emf is impressed on plates A and B and thus excite the quartz crystal into vibrations. By adjusting the variable capacitor C1, the crystal is set into one of the modes of resonant condition. Thus, the vibrating crystal produced longitudinal ultrasonic waves in the surrounding air. The frequency of vibration of the crystal is,
where E is the Young's modulus, ρ is the density of the material and p=1,2,3... for fundamental, first overtone, second overtone,... respectively.
At resonance condition,
Frequency of the oscillatory circuit = Frequency of the vibrating crystal
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