July 4, 2022

Understanding of piezoelectric effect

What is the Piezoelectric Effect?

Piezoelectric effect is the effect produced by certain materials to generate voltage after applying mechanical pressure. For example Electronic stethoscopes. It can convert sound waves to electrical signals which enable us to listen,record and playback sound waves coming from the body. I will explain the working of stethoscopes in detail at last. So in this article I am going to explain you how these materials are able to exhibit this unique property.

How does piezoelectric material work?

There are so many piezoelectric materials available that are natural and manmade. Some naturally occurring piezoelectric materials are cane sugar, quartz, Rochelle salt, topaz, bone etc. For understanding the piezoelectric effect I have considered one quartz crystal. The quartz structure from a specific angle looks like a hexagonal shape as shown in Fig.1.

Why does the piezoelectric material produce an EMF across it when you apply a force?

To understand this first, I will explain a concept called electronegativity. The Quartz molecular structure is SiO4. The oxygen and silicon atoms share one electron. As you can see in Fig.2 Due to the smaller size of oxygen atoms, the shared electron is closer to the oxygen's nucleus, so oxygen will exert more force on the electron than silicon .

As a result, oxygen is more electronegative than silicon and will have a slight negative charge. This also means the silicon atom will have a slight positive charge. So the oxygen and silicon atom together constitute a dipole. In the quartz crystal these dipoles are arranged such that the centre of charges of the negative and positive atoms coincide for better understanding refer Fig.3.

Now what will happen to the centre of charge if I apply a compressive force across the molecules? The centre of charges do not coincide anymore and there will be a charge separation. This is called polarisation as I shown in Fig.4. This also means an electric field will be created between the net positive and negative charges. This leads to generation of a small voltage across it.

The quartz structure consists of multiple similar hexagons connected in series which lack a centre of symmetry. So if I apply a compressive force across the entire material, the individual charge separations of these hexagons will add up to generate a voltage across the material. If I replace these charge pairs with small cells more will be the net voltage developed just because the thickness of the material is more.

Piezoelectricity is reversible:

Now what do you think will happen if I applied voltage across the piezoelectric material? If a voltage is applied across the material the positive and negative charges of the material will experience forces in the opposite directions and the hexagon will get stretched.

Similarly if i reverse the polarity of the voltage the hexagon will get compressed. This stress or strain is transferred across the material resulting in a net compression or expansion of the material. It means that Piezoelectricity is reversible.

Electronic stethoscope

The electronic stethoscope has a very thin diaphragm connected to the piezoelectric material. When the sound waves hit the diaphragm it oscillates, making compressions or expansions in the piezoelectric material. This produces voltage across the material proportional to the amount of deflection. The voltage is then transferred to an amplifier and filter circuit to get the desired output. A foam spacer is connected to the piezo material and a steel plate to enhance the vibrations which in turn results in generating more voltage.

That’s it, it's all about the piezoelectric effect. I hope you guys understand this concept. Thanks for reading!

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