Piezoelectric Effect
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22-01-2011, 10:53 AM




Introduction

Some materials have the ability to produce electricity when subjected to mechanical stress. This is called the piezoelectric effect. This stress can be caused by hitting or twisting the material just enough to deform its crystal lattice without fracturing it. The effect also works in the opposite way, with the material deforming slightly when a small electric current is applied. Piezoelectricity was discovered more than one hundred years ago and has many applications today. It is used in electronic clocks, gas ovens, inkjet printers, and many other appliances. It also used in scientific instruments which require extremely precise movements, like microscopes.

Applications

Small piezoelectric crystals can produce enough voltage to create a spark large enough to ignite gas. These igniters are used in many gas-powered appliances like ovens, grillers, room heaters, and hot water heaters. They are even small enough to fit inside lighters, although most lighters still use flint because it costs less, and only the more expensive lighters use piezo igniters. While there have been many attempts at generating electricity from the effect, it has proven impractical on a large scale.

Piezoelectric crystals are used in electronic clocks and watches to maintain the time and provide the alarm noise. They are also called quartz clocks because the crystal they use is often made from quartz. It has a natural frequency that is ideal for creating the oscillations needed to maintain exact time. Quartz clocks are also used to organize the flow of data in computers. Discs of piezoelectric material are also used to create thin speakers that fit inside wristwatches.

Sonar transducers apply an electrical pulse to a piezoelectric crystal to create a pressure wave, and then produce a current when the reflected wave deforms the crystal. The time gap between the two currents is used to work out how far away an object it. Industrial inkjet printers use the converse piezoelectric effect to move ink through the hundreds of nozzles in their print heads. An electric current makes a tiny crystal in each nozzle bend, creating a pressure pulse that forces the ink out. Ink is drawn into the nozzle when the current stops and the crystal relaxes.


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28-06-2012, 01:41 PM

Piezoelectric Effect



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Piezoelectric Effect Basics

A piezoelectric substance is one that produces an electric charge when a mechanical stress is applied (the substance is
squeezed or stretched). Conversely, a mechanical deformation (the substance shrinks or expands) is produced when an
electric field is applied. This effect is formed in crystals that have no center of symmetry. To explain this, we have to look at
the individual molecules that make up the crystal. Each molecule has a polarization, one end is more negatively charged
and the other end is positively charged, and is called a dipole. This is a result of the atoms that make up the molecule and
the way the molecules are shaped. The polar axis is an imaginary line that runs through the center of both charges on the
molecule. In a monocrystal the polar axes of all of the dipoles lie in one direction. The crystal is said to be symmetrical
because if you were to cut the crystal at any point, the resultant polar axes of the two pieces would lie in the same direction
as the original. In a polycrystal, there are different regions within the material that have a different polar axis. It is asymmetrical
because there is no point at which the crystal could be cut that would leave the two remaining pieces with the
same resultant polar axis. Figure 1 illustrates this concept.


Using the Piezoelectric Effect

The piezoelectric crystal bends in different ways at different frequencies. This bending is called the vibration mode. The
crystal can be made into various shapes to achieve different vibration modes. To realize small, cost effective, and high performance
products, several modes have been developed to operate over several frequency ranges. These modes allow us
to make products working in the low kHz range up to the MHz range. Figure 4 shows the vibration modes and the frequencies
over which they can work.
An important group of piezoelectric materials are ceramics. Murata utilizes these various vibration modes and ceramics to
make many useful products, such as ceramic resonators, ceramic bandpass filters, ceramic discriminators, ceramic traps,
SAW filters, and buzzers.
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15-10-2012, 12:27 PM

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