Piezoelectric ceramics based on transition metals

Ferroelectric ceramic materials form an important class of piezoelectric materials. They feature spontaneous electrical polarization of the domains, which generates mechanical deformations. The coupling between polarization modulation and lattice deformation due to piezoelectricity in ferroelectric materials is characterized by significant variations in polarization (or deformation) when mechanical stress (or electric field) is applied, and ferroelectrics have the highest piezoelectric coefficients.

Since the 1960s-70s, the most technologically important piezoelectric materials have been ferroelectric ceramics with lead-based perovskite structures such as lead zirconate titanate (PZT), with a solid solution range extending from lead titanate to lead zirconate. In these perovskite structures, ferroelectric properties vary continuously with the rate of cationic substitution, and a large number of chemical modifications are possible to modulate the piezoelectric properties.

In addition, there is a specific behavior, present in a whole series of lead-based perovskites, characterized by ferroelectric phase transitions and manifested by the presence, within a narrow range of composition, of a morphotropic phase boundary called FPM, and for which piezoelectric properties are maximized. These features highlight the fact that these perovskites combine the desired properties for a wide range of applications.

In 2002, European regulation EU-Directive 2002/95/EC recommended the replacement of lead-based compounds with lead-free materials. This has prompted extensive research into the development of lead-free ceramics with properties at least equivalent to those of PZT. Oxide compounds of varying complexity (inorganic oxide compounds with several cationic sublattices) with very good piezoelectric properties in the single-crystal state are used to develop textured lead-free ceramics. The results are very encouraging, but further study of these materials is needed before large-scale industrialization and effective replacement of PZT can be envisaged.

The high electromechanical coupling coefficients of ferroelectric piezoelectrics are widely used for transduction applications (sensors and actuators) and for broadband filtering. Piezoelectric materials that can operate at high temperatures are being sought for specific sensors or actuators, and are currently under development. However, their use at high temperatures requires avoiding the phase transitions that cause instability of properties with temperature, and in the case of ferroelectric materials, having a Curie temperature well above that of the application.