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 ultrasonic transducer as it converts the electrical energy to acoustic energy, and vice versa. The active element is basically a piece of 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.
Originally, crystals made from quartz were used as a material for piezoelectric transducers. In the early 1950s, quartz crystals began to give way to piezoelectric ceramic as the primary transducer material. The advantages offered by a ceramic transducer when compared to other materials include ceramic’s ability to be manufactured in a wide variety of shapes and sizes, its capability of operating efficiently at low voltage, and its ability to function at temperatures up to 300oC.
Piezoelectric transducers are a type of electroacoustic transducer that converts the electrical charges produced by some forms of solid materials into energy. The word “piezoelectric” literally means electricity caused by pressure.