Ac

source

Metal contact

- Crystal

Fig. 8.1 An oscillator based on the piezoelectric effect

When the process is reversed, i.e. by applying force to the crystal, mechanical strain is caused, the electrical polarity is changed, and a voltage is produced between the metal plates. This is known as the 'generator effect' and it is this that is the basis of the knock sensor that is used in many engine management systems (Fig. 8.2).

Natural quartz contains impurities and to overcome this problem quartz is 'grown' in controlled conditions. In addition to quartz and a few other materials, certain ceramics that are oxide alloys of platinum, zirconium and titanium (PZT), also possess piezoelectric properties.

Force

Piezo crystal

Piezo crystal

Electric field

Part of the crystal

Force

Part of the crystal

Piezo crystal

Piezo crystal

Electric field

Gives out electrical energy (generator effect)

Changed shape of crystal structure

Strains the crystal and elasticity causes vibration (motor effect)

Fig. 8.2 The piezoelectric generator and motor effects

Gives out electrical energy (generator effect)

Changed shape of crystal structure

Strains the crystal and elasticity causes vibration (motor effect)

Fig. 8.2 The piezoelectric generator and motor effects

An important property of a piezoelectric material is its coupling coefficient k.

This expresses the relation between electrical and mechanical energy.

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