In response to the current environmental regulations against the use of lead in daily electronic devices, a number of investigations have been performed worldwide in search for alternative piezoelectric ceramics that can replace the market-dominating lead-based ones, representatively Pb(Zr
 x
 Ti1-x
 )O3 (PZT)-based solid solutions. Selected systems of potential importance such as chemically modified and/or crystallographically textured (K, Na)NbO3 and (Bi1/2Na1/2)TiO3-based solid solutions have been developed. Nevertheless, only few achievements have so far been introduced to the marketplace. A recent discovery has greatly extended our tool box for material design by furnishing (Bi1/2Na1/2)TiO3-based ceramics with a reversible phase transition between an ergodic relaxor state and a ferroelectric with the application of electric field. This paired the piezoelectric effect with a strain-generating phase transition and extended opportunities for actuator applications in a completely new manner. In this contribution, we will present the status and perspectives of this new class of actuator ceramics, aiming at covering a wide spectrum of topics, i.e., from fundamentals to practice.

Giant electric-field-induced strains in lead-free ceramics for actuator applications – status and perspective

Thermodynamical formulation for coupled electromechanical hysteresis effects—I. Basic equations

Electromechanical properties of a number of ferroelectric films including PbZrxTi1-xO3(PZT), 0.9PbMg(1/3)Nb(2/3)O(3)-0.1PbTiO(3)(PMN-PT), and SrBi2Ta2O9(SBT) are investigated using laser interferometry combined with conventional dielectric measurements. Effective electrostriction coefficients of the films, Q(eff), are determined using a linearized electrostriction equation that couples longitudinal piezoelectric coefficient, d(33), with the polarization and dielectric constant. It is shown that, in PZT films, electrostriction coefficients slightly increase with applied electric field, reflecting the weak contribution of non-180 degrees domains to piezoelectric properties. In contrast, in PMN-PT and SBT films electrostriction coefficients are field independent, indicating the intrinsic nature of the piezoelectric response. The experimental values of Q(eff) are significantly smaller than those of corresponding bulk materials due to substrate clamping and possible size effects. Electrostriction coefficients of PZT layers are shown to depend strongly on the composition and preferred orientation of the grains. In particular, Q(eff) of (100) textured rhombohedral films (x = 0.7) is significantly greater than that of (111) layers. Thus large anisotropy of the electrostrictive coefficients is responsible for recently observed large piezoelectric coefficients of (100) textured PZT films. Effective electrostriction coefficients obtained by laser interferometry allow evaluation of the electromechanical properties of ferroelectric films based solely on the dielectric parameters and thus are very useful in the design and fabrication of microsensors and microactuators. (C) 2001 American Institute of Physics.

Characterization of the effective electrostriction coefficients in ferroelectric thin films

The mechanisms involved in the polarization switching process in soft and hard Pb(Zr53,Ti47)O3 (PZT) bulk ceramics were investigated through the dependency of the hysteresis loop on the frequency. In order to determine the influence of the defects on the domain switching dynamics, the samples were characterized in the virgin state and after a fatigue or a depinning process. The frequency dependence of the polarization revealed a strong relaxation of the 90° domain walls at ∼100 Hz. The results also revealed a strong influence of the kind of defect and their distribution in the ferroelectric matrix on the domain switching dynamics, which were reflected in the frequency dependence of the coercive field and the percentage of the backswitching. Initially, it was observed that the frequency dependence of the coercive field for the soft and the hard PZT in the virgin state had just one rate of change per decade in the entire frequency range investigated, which is the standard behavior found in the literature. However, after the fatigue or the depinning process, two rates of changes were noticed. Consequently, evidence of an upper-frequency limit for the coercive field changes was found. The percentage of the backswitching and its behavior for the soft PZT was almost independent of the fatigue state in the entire frequency range investigated. Nevertheless, for the hard PZT, an opposite behavior was verified. The reorientation of the domains was modeled as occurring in a viscous medium where several forces, such as viscous and restoring forces, act on them.

90° domain wall relaxation and frequency dependence of the coercive field in the ferroelectric switching process

Thermodynamical formulation for coupled electromechanical hysteresis effects. III: Parameter identification

The polarization reversal of some tetragonal-phase PLZT ceramics is studied. The results are explained in terms of 90° domain wall motions. A reported theory originally derived for rhombohedral-phase PLZT 6/65/35 ceramics is shown to be applicable to tetragonal-phase PLZT ceramics also by introducing the remanent degree of 90° domain alignment γ90. As a result, the calculation of coercive force is possible for various PLZT compositions considering the interaction of 90° domain wall motions and reacting forces established by the internal stress state of ferroelectric ceramics. The frequency dependence of coercive force and measurements of switching strain, including depolarization data obtained from either mechanically or electrically produced external forces, confirm the interpretation of predominantly occuring 90° reorientations and are consistent with the extended theory.

N. A. Schmidt

Papers

Coercive force and 90° domain wall motion in ferroelectric PLZT ceramics with square hysteresis loops