Engineering Physics 1 - Ultrasonics- Piezo-Electric Effect- Piezo-Electric Generator

Introduction 

Can all the crystals exhibit piezoelectric effect? What is special about the piezoelectric crystal?
Is the piezoelectric effect direction dependent? 

Learning Objectives
On completion of this chapter you will be able to:
1. define piezoelectric effect
2. define inverse piezoelectric effect
3. know what type of crystals will exhibit piezoelectric effect
4. Understand the working of piezoelectric generator 

Piezoelectric effect: 

When crystals like quartz or tourmaline are stressed along any pair of opposite faces, electric charges of opposite polarity are induced in the opposite faces perpendicular to the stress. This is known as Piezoelectric effect.

Piezoelectric effect- Mechanism:

Piezoelectric and inverse piezoelectric effects are only exhibited by certain crystals which lack centre of symmetry. In a piezoelectric crystal, the positive and negative electrical charges are separated, but symmetrically distributed, so that the crystal overall is electrically neutral. Each of these sides forms an electric dipole and dipoles near each other tend to be aligned in regions called Weiss domains. The domains are usually randomly oriented, but can be aligned during poling , a process by which a strong electric field is applied across the material, usually at elevated temperatures.

When a mechanical stress is applied, this symmetry is disturbed, and the charge asymmetry generates a voltage across the material. For example, a 1 cm cube of quartz with 2 kN (500 lbf) of correctly applied force can produce a voltage of 12,500 V.

Piezoelectric materials also show the opposite effect, called converse (inverse) piezoelectric effect, where the application of an electrical field creates mechanical deformation in the crystal.

Inverse piezoelectric effect: 

When an alternating e.m.f is applied to the opposite faces of a quartz or tourmaline crystal it undergoes contraction and expansion alternatively in the perpendicular direction. This is known as inverse piezoelectric effect. This is made use of in the piezoelectric generator.

Piezoelectric generator:

A slab of piezoelectric crystal is taken and using this a parallel plate capacitor is made. Then with other electronic components an electronic oscillator is designed to produce electrical oscillations >

20 kHz. Generally one can generate ultrasonic waves of the order of MHz using piezoelectric generators. Quartz slabs are preferred because it possesses rare physical and chemical properties. A typical circuit diagram is given below.

The tank circuit has a variable capacitor 'C' and an inductor 'L' which decides the frequency of the electrical oscillations. When the circuit is closed current rushes through the tank circuit and the capacitor is charged, after fully charged no current passes through the same. Then the capacitor starts discharging through the inductor and hence the electric energy is in the form of electric and magnetic fields associated with the capacitor and the inductor respectively. Thus we get electrical oscillations in the tank circuit and with the help of the other electronic components including a transistor, electrical oscillations are produced continuously. This is fed to the secondary circuit and the piezoelectric crystal (in our case a slab of suitably cut quartz crystal) vibrates, as it is continuously subjected to varying (alternating) electric field, and produces sound waves. When the frequency of electrical oscillations is in the ultrasonic range then ultrasonic waves are generated. When the frequency of oscillation is matched with the natural frequency of the piezoelectric slab then it will vibrate with maximum amplitude. The frequency generated is given as follows:

E- the Young's modulus of the piezoelectric material
and ρ- the density of the piezoelectric material

Check your understanding

1. Choose the right answer from the options given below: The following crystals show piezoelectric effect:
a) NaCl
b) Barium Titanate

c) Diamond
d) Quartz

Answer : b,d

2. State if the following statement is true or false?
One can generate ultrasonic waves of frequency f, 2f, ..., where f is the fundamental freqency 

a) True

b) False

Answer :a

3. State if the following statement is true or false?
The magnitude of the Piezoelectric effect of a crystalline material does not depend on direction
a) True

b) False

Answer :b

4. State if the following statement is true or false?
The Piezoelectric effect of a material is something to do with the crystal symmetry

a) True
b) False

Answer :a

5. State if the following statement is true or false?
The Piezoelectric effect of a material depends on its crystal structure

a) True
b) False 

Answer :a

Summary
On completion of this chapter you have learned :
1. piezoelectric effect and inverse (converse) piezoelectric effect.
2. the type of crystals showing piezoelectric effect.
3. principle of piezoelectric oscillator.
4. Working of piezoelectric oscillator.

Activity
Students can read standard texts or browse the net and read the relevant material before coming to the class. They can discuss extra points not covered during the lectures. They can find the actual working circuit for the piezoelectric oscillator (generator) to generate ultrasonic waves.

Suggested Reading
1. Engineering Physics by P.K.Palanisamy (Scitech publications)
2. http://en.wikipedia.org/wiki/piezoelectric