Ceramics, Transducers

Abstract The various effects that relate the input—electric field, magnetic field, stress, heat and light—to the output—charge/current, magnetization, strain, temperature, and light are listed. Conducting and elastic materials that generate current and strain outputs, respectively, from input voltage, or stress are sometimes called “trivial” materials. Conversely, pyroelectric and piezoelectric materials that generate an electric field from the input of heat or stress are called “smart” materials. These off‐diagonal couplings have a corresponding converse effect such as electrocaloric and converse piezoelectric effects, and both “sensing” and “actuating” functions can be realized in the same materials. “Intelligent” materials should possess a “drive/control” or “processing” function which is adaptive to the change in environmental conditions, in addition to the actuation and sensing functions. Note that ferroelectric materials exhibit most of these effects, except magnetic‐related phenomena. Thus, the ferroelectrics are said to be very “smart” materials. The “actuator” in a narrow meaning stands for materials or devices that generate mechanical strain (or stress) output. Solid state actuators use converse piezoelectric, magnetostriction, elasticity, thermal expansion, or photostriction phenomena. A shape‐memory alloy is a kind of thermally expanding material. On the other hand, a “sensor” requires charge/current output in most cases. Thus, conducting/semiconducting, magnetoelectric, piezoelectric, pyroelectric, and photovoltaic materials are used for detecting electric fields, magnetic fields, stress, heat, and light, respectively. In this sense, piezoelectric materials are most popularly used in smart structures and systems because the same material is applicable to both sensors and actuators, in principle. In this article, we treat mainly piezoelectric transducers, sensors, and actuators. Although, transducers, in general, are devices that convert input energy to a different energy type of output, the piezoelectric “transducer” is often used to denote a device that possesses both sensing and actuating functions, exemplified by underwater sonar.