Piezo-electric effect.pptx Piezoelectric effect

Complete description of piezoelectric effect

1. https://www.youtube.com/watch?v=YEJ2qryXcIQ Piezoelectric effect When a mechanical stress / pressure is applied on opposite faces of piezo-crystals , then equal and opposite charges are produced on other opposite faces of crystal generating an electric voltage . Inverse Piezoelectric effect When an electric field is applied on opposite faces of piezo -crystals , then change in dimensions (contraction/expansion) are produced on other opposite faces of crystal.

2. Piezoelectric materials Materials which exhibit piezo -electric effect are called piezoelectric materials The effect is exhibited, when the crystal is cut with reference to optic axis Eg . Quarzt , Tourmaline, Rochelle Salt PZT – Lead Zirconate Titanate

3. Piezo -electric generator / oscillator It is a device consisting of piezo -electric crystal and an electronic oscillator . It is used to produce ultrasonic wave . Principle Inverse Piezo -electric effect When an alternating voltage is applied on opposite faces of piezo -crystals , then alternative mechanical compressions and expansions are produced on other opposite faces of crystal. At resonance , mechanical vibrations produces ultrasonic waves of high frequency

4. construction L 3 B C E L 1 C 1 S + - Battery T A B L 2

5. A quartz crystal is placed between two metal plates A and B forming a parallel plate capacitor with crystal as dielectric L 1 and variable capacitor C 1 are connected to the base of NPN transistor and they form an electronic oscillator Coil L 2 is connected to the collector of transistor and forms a closed circuit through a switch S and battery Coils L 1 and L 2 are from the primary transformer T

6. Working When battery is turned ON, the oscillator L 1 C 1 produces an alternating current of frequency, Current flows through the coil L 2 and e.m.f . is induced in coil L 3 by means of transformer When alternating magnetic field is applied to the plates A and B, inverse piezoelectric effect is produced and the quartz crystal mechanically contracts and expands By varying the capacitor C 1 , the frequency of alternating current is made equal to natural frequency of quartz crystal and R esonance is attained

7. n  natural frequency p  fundamental and overtones (1, 2, 3,……) l  length of the crystal Y  Young;s modulus of crystal ρ  density of crystal At resonance, the crystal vibrates with high amplitude and produce ultrasonic waves of high frequency

8. merits Cost of piezo -electric quartz is very high Cutting and shaping of crystals are very complex Mishandling the crystal might damage the oscillator demerits Oscillator can produce 500 MHz of frequencies Natural frequency is independent of temperature and humidity. More efficient than magnetostriction method.

9. Magnetostriction method Ultrasonic waves of frequency – 3000 kHz can be produced Less efficient method Ultrasonic waves are produced when alternating magnetic field are applied to ferro -magnets. High power applications - mechanical drilling, grinding and welding processes Piezoelectric method Ultrasonic waves of frequency as high as 500 MHz can be produced More efficient method Ultrasonic waves are produced when mechanical stress applied on opposite faces of piezo -crystals Low power applications – flaw detection and medical application