GITAM, Department of Engineering Physics
Piezoelectric Effect
Crystals which acquire a charge when compressed, twisted or distorted are said to be piezoelectric. This provides a convenient transducer effect between electrical and mechanical oscillations. Quartz demonstrates this property and is extremely stable. Quartz crystals are used for watch crystals and for precise frequency reference crystals for radio transmitters. Rochelle salt produces a comparatively large voltage upon compression and was used in early crystal microphones. Barium titanate, lead zirconate, and lead titanate are ceramic materials which exhibit piezoelectricity and are used in ultrasonic transducers as well as microphones. If and electrical oscillation is applied to such ceramic wafers, they will respond with mechanical vibrations which provide the ultrasonic sound source. The standard piezoelectric material for medical imaging processes has been lead zirconate titanate (PZT). Piezoelectric ceramic materials have found use in producing motions on the order of nanometers in the control of scanning tunneling microscopes.
The word piezo is Greek for "push". The effect known as piezoelectricity was discovered by brothers Pierre and Jacques Curie when they were 21 and 24 years old in 1880.
Piezoelectric Transducers
The
conversion of electrical pulses to mechanical vibrations and the conversion of
returned mechanical vibrations back into electrical energy is the basis for
ultrasonic testing. The active element is the heart of the transducer as it
converts the electrical energy to acoustic energy, and vice versa. The active
element is basically a piece polarized material (i.e. some parts of the
molecule are positively charged, while other parts of the molecule are
negatively charged) with electrodes attached to two of its opposite faces.
When an electric field is applied across the material, the polarized molecules
will align themselves with the electric field, resulting in induced dipoles
within the molecular or crystal structure of the material. This alignment of
molecules will cause the material to change dimensions. This phenomenon is
known as electrostriction. In addition, a permanently-polarized material such
as quartz (SiO2) or barium titanate (BaTiO3) will produce an electric field
when the material changes dimensions as a result of an imposed mechanical
force. This phenomenon is known as the piezoelectric effect.