The formation and effect of defect dipoles in lead-free piezoelectric ceramics: A review

Abstract

As one of the most crucial intrinsic contributions, defect dipoles show unique and prominent influence on the responsive behavior of piezoelectric ceramics. Here, we present the formation of the defect dipoles and the state-of-the-art effect on lead-free piezoelectric ceramics in detail. First, the formation of defect dipoles in ceramics are demonstrated. The coupling effect between defect dipole polarization and normal ferroelectric spontaneous polarization, which results in the unusual ferroelectric properties (e.g., “pinched” or asymmetric polarization hysteresis loops) and high electric field induced-strain performance in piezoelectric ceramics, is elaborated. Thereafter, we systematically introduce the switching and decoupling process of defect dipoles under different external conditions. Finally, the researches about the effect of defect dipoles on various lead-free piezoelectric ceramics are presented herein, with special attention on the three systems: the barium titanate, sodium-bismuth titanate and potassium sodium niobate. The remaining challenges, however, such as weak temperature/cycling stability, still need further effort for the extensive applications of these materials.

Introduction

Lead-based piezoelectric ceramics, e.g., Pb(Zr,Ti)O₃-based ceramics (PZT), were utilized as the core component of actuators, sensors, and transducers [[1], [2], [3], [4], [5]]. However, due to the toxicity of lead which brings serious environment and health problems, it is urgent to find lead-free alternatives for the lead-based piezoelectric ceramics [[6], [7], [8], [9], [10], [11]]. In recent years, lead-free piezoelectric ceramics (e.g., Na_1/2Bi_1/2TiO₃- (BNT-) [[12], [13], [14], [15], [16], [17]], BaTiO₃- (BT-) [[18], [19], [20], [21]] and K_0.5Na_0.5NbO₃-based (KNN-based) [[22], [23], [24], [25], [26], [27], [28], [29], [30], [31]] ceramics) have attracted people's attention as potantial alternatives for PZT-based ceramics.

For high-power applications, the piezoelectric ceramics are required to obtain high mechanical quality factors and low dielectric loss, which can avoid excessive heat loss during delivering high acoustic power process. To achieve these performances, the piezoelectric ceramics are usually acceptor-doped, bringing in the “hardening” effect [32,33]. Acceptor ion has a lower valence than the ion that will be substituted, e.g., Ti⁴⁺ in BaTiO₃ substituted by Mn²⁺ or Nb⁵⁺ in K_0.5Na_0.5NbO₃ substituted by Cu²⁺. Consequently, the oxygen vacancies are created due to the charge compensation, and form defect dipoles with acceptor dopant ions during aging process for reducing the elastic and electrostatic field [1,34]. Although the concentration of defect dipoles may be very low, they usually play an important role in the responsive behavior of piezoelectric materials. For example, the defect dipoles can act as the pinning centers in domains, which blocks the switching of the ferroelectric domains under the externally applied electric field (AEF), leading to the “harden” behavior of piezoelectric ceramics.

Here, we present the recent researches on defect dipoles in lead-free piezoelectric ceramic field. Firstly, the formation of defect dipoles in ceramics are demonstrated. Some unusual phenomena (e.g., “pinched” or asymmetric polarization hysteresis loops) caused by the coupling effect between defect dipoles and ferroelectric spontaneous polarizations in piezoelectric ceramics are demonstrated in detail. Subsequently, electric field induced-strain (electro-strain) performance of acceptor-doped piezoelectric ceramics under the effect of defect dipoles is compiled. Furthermore, the switching/decoupling processes of defect dipoles under external conditions are introduced. Finally, we summarize the researches on the defect dipoles modified lead-free piezoelectric ceramic systems, e.g., Na_1/2Bi_1/2TiO₃-, BaTiO₃- and K_0.5Na_0.5NbO₃-based ceramics etc.