ABSTRACT
During World War I, the quartz (SiO2) transducer was rst used in the sonar system to detect submarines. Among the 20 crystal classes with piezoelectricity, 10 are pyroelectric, which show a spontaneous polarization that changes with temperature. If the dipole moment of a pyroelectric medium can be redirected by an externally applied electric eld, the material is called ferroelectric. Around the end of World War II, piezoelectric ceramics were developed by making a suitably balanced mixture that included an appropriate binder, followed by pressing and then ring at a high temperature. Domains of the same polarization direction are formed within the polycrystallites, and these are bound by other domains with different polarization directions. Overall, because the domains are randomly oriented, the macroscopic behavior of the polycrystalline ceramic is approximately isotropic. By applying a high DC electric eld at a temperature close to the Curie point and with the eld present when the temperature is lowered, many of the domains can be made to align with the applied eld and some grow in volume, causing the piezoelectric properties to be greatly enhanced, a process called poling. Piezoceramics have the important advantage of easy fabrication into a variety of shapes with controlled directions of polarization. Relaxor-based ferroelectric materials can be grown in single-crystalline form to a sufciently large size for fabricating arrays and have a higher piezoelectric coupling factor than lead zirconate titanate (PZT) ceramics. The performance of ultrasound transducers can also be improved through the use of a composite arrangement of piezoelectric and non-piezoelectric (i.e., polymer and epoxy) materials. A piezocomposite combines the superior piezoelectric properties of the ceramic with the much lower acoustic impedance of the polymer, resulting in an effective acoustic propagation. Relaxor-based single crystals can also be used in building piezocomposites. Polyvinylidene uoride (PVDF), a semicrystalline polymer with long molecular chains and structure, exhibits ferroelectricity and,
therefore, piezoelectric properties. It is used in the fabrication of broadband hydrophones, high-frequency imaging (i.e., 50−200 MHz), and the acoustic microscope.
