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Piezoelectric Materials and Applications

The History

Piezoelectrics are materials that can create electricity when subjected to a mechanical stress. They will also work in reverse, generating a strain by the application of an electric field.

The phenomenon was first discovered in 1880 when Pierre and Jacques Curie demonstrated that when specially prepared crystals (such as quartz, topaz and Rochelle salt) were subjected to a mechanical stress they could measure a surface charge. A year later, Gabriel Lippmann deduced from thermodynamics that they would also exhibit a strain in an applied electric field. The Curies later experimentally confirmed this effect and provided proof of the linear and reversible nature of piezoelectricity.

One of the first applications of the piezoelectric effect was an ultrasonic submarine detector developed during the First World War. A mosaic of thin quartz crystals glued between two steel plates acted as a transducer that resonated at 50MHz. By submerging the device and applying a voltage they succeeded in emitting a high frequency 'chirp' underwater, which enabled them to measure the depth by timing the return echo. This was the basis for sonar and the development encouraged other applications using piezoelectric devices both resonating and non-resonating such as microphones, signal filters and ultrasonic transducers. However many devices were not commercially viable due to the limited performance of the materials at the time.

The Applications

The continued development of piezoelectric materials has led to a huge market of products ranging from those for everyday use to more specialised devices. Some typical applications can be seen below:

Automotive Air bag sensor, air flow sensor, audible alarms, fuel atomiser, keyless door entry, seat belt buzzers, knock sensors.
Computer Disc drives, inkjet printers.
Consumer Cigarette lighters, depth finders, fish finders, humidifiers, jewellery cleaners, musical instruments, speakers, telephones.
Medical Disposable patient monitors, foetal heart monitors, ultrasonic imaging.
Military Depth sounders, guidance systems, hydrophones, sonar.

The Science

Piezoelectric ceramic materials are ionically bonded and consist of atoms with positive and negative charges, called ions. These ions occupy positions in specific repeating units (called unit cells). If a unit cell is non-centro symmetric, i.e. lacking a centre of symmetry, then the application of a stress produces a net movement of the positive and negative ions with respect to each other and results in an electric dipole or polarisation.

piezoelectric fibres
The degree of polarisation is dependent upon the stress and whether tensile or compressive stresses are applied affects the charge produced. The dipoles, which are present due to the non-centro symmetric structure, form domains that are regions where neighbouring dipoles have the same alignment.

Initially the domains are randomly oriented (see figure on the left) and there is no overall polarisation of the ceramic and therefore it exhibits I no piezoelectric effect. By applying heat and a strong DC field the domains are subjected to 'poling', causing the domains that are nearly aligned to the field to grow at the expense of those at differing alignments. After cooling to room temperature and removing the DC field, the domains are 'locked' resulting in an overall alignment and the material is now piezoelectric.

Tennis Racquet Case Study

A more recent innovation using piezoelectric technology is in the sports industry. Tennis manufacturers, Head, were requested by players to design racquets with comfort as well as power. Previously, racquets had been designed to be stiff so that they return maximum energy to the ball when it is hit but this means that the racquet transmits shock vibration to the players arm.

Tennis racquet
In an attempt to reduce vibration, piezoelectric fibres have been embedded around the racquet throat and a computer chip embedded inside the handle. The frame deflects slightly when the ball is hit so that the piezoelectric fibres bend and generate a charge (by the direct effect) which is collected by the patterned electrode surrounding the fibres. The charge and associated current is carried to an embedded silicon chip via a flexible circuit containing inductors capacitors and resistors, which boost the current and send it back to the fibres out of phase in an attempt to reduce the vibration by destructive interference.

The fibres then bend (by the converse effect) to counter the motion of the racket and reduce vibration. The current generated is said to be only a couple of hundred micro amps generating 600 to 800 volts in only 2 to 3 milliseconds.

The manufacturers claim 50% reduction in vibration compared with conventional rackets and the International Tennis Federation have approved them for tournament play.

This article is based on a case study developed under the supervision of Dr. Irene Turner of the University of Bath.