Piezoelectric Sensors and Self Monitoring Planes

Box 1 | More about Piezoelectricity

The word 'piezoelectricity' comes from two Greek words; one for ‘pressing’ or ‘squeezing’ and the other for amber, since rubbing that hardened tree sap was the first way static electricity was generated. The phenomenon was first found in the late 19th century by the Curie brothers of France, Pierre and Jacques (Pierre was later the husband of Marie Curie and shared with her the Nobel Prize for discovering radium).

The brothers found that the effect went both ways between mechanical stress and the build-up of electric charge. Other researchers found that some 20 naturally occurring minerals including quartz, topaz and cane sugar showed the effect, but it did not seem initially to have any practical use. That soon changed. Within a few decades piezoelectric devices were being used in phonograph pick-ups, to help detect submarines underwater by echolocation, and in many scientific instruments.

By the mid-20th-century man-made piezo-ceramics could outperform any naturally occurring piezoelectric substance. There was also a new word to describe the two-way behaviour of the devices.  We started calling them ‘transducers’, able to couple together mechanical stresses and electrical behaviour in either direction. For example in sonar, which can find objects underwater by bouncing sound waves off them, the same sort of devices are used to transmit and to receive. Electrical currents cause the vibrations that make the sound waves; when those waves are received back, the same devices turn the vibrations back into currents that are able to be measured.

These days one of the best-known applications of piezoelectricity is in gas lighters. Flicking the lighter used to make steel strike flint and generate a spark to ignite the flammable gas released by the lighter fluid. Now the hammer falls on a piezoelectric crystal to make an electric spark for the same purpose. Running in the other direction, piezoelectric devices generate the high-frequency sound waves that are produced by digital watch alarms, and that are needed in medical imaging.

In a piezoelectric material, natural or man-made, mechanical and electrical properties are closely linked. The particles that make up a crystal carry electric charges which are unevenly spread, so forming ‘electric dipoles’. Squeezing the crystal to make it change shape ever so slightly shifts the charges relative to each other and so makes an electric current. Alternatively, pushing a current through the crystal moves the charges together or apart and the shape of the crystal changes. If the current changes direction many times a second (that is, an alternating current), so does the shape of the crystal, so that it vibrates.

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Box 2. Other piezoelectric devices

Related sites
Introduction to Piezoelectric materials (Piezocryst)
Piezoelectricity (PCB Piezotronics)
Piezoelectricity (Kids.net.au)

External sites are not endorsed by the Australian Academy of Science.
Posted July 2011, Edited August 2012