piezoelectricity and pyroelectricity
Piezoelectricity is a property manifested by various crystalline materials, especially quartz and Rochelle salt, whereby mechanical pressure exerted upon the crystal results in the appearance of electric charge along its surfaces. The reverse effect, the distortion of the crystal, can be achieved by the application of electric charges. Piezoelectricity is due to electrical polarization resulting from mechanical stress. The effect was discovered by Pierre Curie (1859–1906), the French physicist and chemist, who is better known for his work in radioactivity, in collaboration with his wife.
Both the piezoelectric effect and its converse are of immense importance. Because of the effect, a quartz crystal that is mechanically vibrating gives rise to an AC voltage across its face. As the frequency of mechanical vibration of the crystal depends upon the dimensions and other parameters of the crystal, the AC voltage thus generated possesses a vary constant frequency This is utilized in stabilizing the frequencies of electronic signal generators.
The reverse effect provides a convenient means of producing ultrasonic waves (see ultrasonics). If an AC voltage is applied to the faces of a crystal, and the frequency of the voltage is identical to the natural frequency of the crystal's mechanical vibration, the crystal can be made to vibrate violently and thus serve as a source of ultrasonic radiation. The crystal in this instance behaves as a transducer: it transforms electrical into mechanical energy.
It is observed also that when a crystal with piezoelectric properties is heated or cooled it develops charge at its surfaces – an effect known as pyroelectricity. These are piezoelectric charges resulting from the strain associated with thermal expansion and contraction of the crystal.