Lead-Free Piezoelectric Materials. Jing-Feng Li

Figure 1.3 Polarization vs. electric field hysteresis loop in ferroelectric materials.

Once a ferroelectric crystal is cooled across the Curie temperature, a polarization develops. The ferroelectric phase transition is a structural phase transition, during which the displacements of ions produce lattice distortions and change the symmetry of the crystal. The magnitude of the ion displacements along certain crystallographic directions in the materials is specific to a given crystal structure and composition. If the polarization develops uniformly throughout the whole crystal, a depolarizing electric field will be produced. To minimize the electrostatic energy associated with this field, the crystal often splits into regions, called domains; a region in which the polarization is uniform is called a domain. The regions between two adjacent domains are called domain walls. Their thickness is typically of the order of 10–100 Å. Domain represents a region within a ferroelectric material in which the direction of polarization is uniform. The saturation polarization, P_s, corresponds to the total polarization at an extreme state where (almost) all domains are aligned along the direction of applied electric field. Some of these domains stay at the same direction even after the removal of electric field, resulting in the remnant polarization. It can be readily envisaged that a ferroelectric material at a state with remnant polarization can be used a piezoelectric material, since it can generate electric charges when subjected to mechanical stress. In other words, if a ferroelectric material, at least polycrystalline bulk materials should show no piezoelectric response if it has not been subjected to an electric field. This is because the charges will be canceled collectively if the domains are randomly distributed along different directions, resulting in zero change when the whole material receives mechanical deformation. As such, piezoelectricity can be regarded as one of the functionalities of ferroelectric materials, and in general, ferroelectric materials need to be poled before they can be used as piezoelectric materials. Therefore, electrical poling is an indispensable process for ferroelectric piezoelectric materials. During poling, a strong electric field is applied across ferroelectric materials and consequently, a majority of the domains switch their pristine polarization and become aligned along the electric field direction. Figure 1.4 schematically shows the poling process. The virgin materials are subjected to an electric field, which should be sufficiently higher than the coercive field (E_C), so that the domains can be re‐orientated almost along the same direction. As shown in Figure 1.4b, the poling process is accompanied with an expansion of the poled materials or tensile strains, which is basically consistent with the converse piezoelectric phenomenon. As shown in Figure 1.4c, although most domains are kept along the poling direction, part of them revert back or change their orientations after the removal of the poling electrical field in order to reduce the mechanical strains. After the poling treatment, the material possesses a macroscopic polarization, which is equal to the remnant polarization (P_r) in the P–E loop shown earlier. Therefore, the poling process is very important for piezoelectric materials. Even for the same materials, if not completely poled, the resultant piezoelectric properties, especially piezoelectric charge coefficient (d₃₃), will be very low. Also, it is clear that the poling process is not applicable to non‐ferroelectric materials. That is why high‐performance piezoelectric materials must be ferroelectric in the first place.