Shifting the phase boundary: Potassium sodium niobate derivates
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ntroduction Matthias et al. conducted an early investigation of potassium and sodium niobate in 19491 and reported evidence of ferroelectricity in single crystals of the binary end-members constituting (K,Na)NbO3 (KNN) solid solution. Egerton et al. pioneered research on ceramics in this system in 1959.2 In 2004, Saito et al. revealed the tremendous potential of textured polycrystals as a substitute for lead zirconate titanate (PZT) ceramics, which in turn stimulated widespread scientific interest in developing lead-free alternatives.3 The past decade has witnessed a surge in publications on lead-free piezoelectrics,4–9 as well as continual enhancement of their electromechanical properties; this was further prompted by worldwide regulations restricting the use of lead and leadcontaining materials. The historical evolution of the performance of KNN ceramics from the perspective of the piezoelectric constant d33 is shown in Figure 1,10–57 where state-of-the-art KNN piezoceramics have been tailored to demonstrate properties comparable to those of PZT in certain scenarios. Strategies in terms of enhancing electromechanical properties within the KNN system mainly focus on phase-boundary engineering,13,15,52–56 domain engineering,57,58 and optimization of processing.59,60 Wu et al. observed a large d33 of 570 pC/N in polycrystalline KNN ceramics by simultaneously shifting dual-phase transition temperatures and constructing a distinct phase structure.20
Li et al. achieved ultrahigh electromechanical properties with d33 of 700 pC/N and a planar electromechanical coupling factor of 76% (measure of the conversion efficiency between electrical and mechanical energy) in highly textured KNN ceramics.61 In terms of d33, KNN materials are indeed comparable to soft commercial PZT (350–700 pC/N). For practical applications, d33 and its thermal stability are considered to be the most important criteria, suggesting KNN could be used at least for certain operating regimes. KNN-based materials have certain inherent, often neglected, advantages over PZT. For example, the density of KNN is 4.5–4.8 g/cm3, much lower than that of PZT (7.5–8.0 g/cm3), resulting in superior performance for transferring ultrasound into low-density media.5 Also, recent research has revealed the feasibility and reliability of KNN-based multilayer ceramics with nickel inner electrodes.62 For PZT devices, the thermodynamic incompatibility between the metallic nickel and lead oxide during co-firing precludes the possibility of using nickel as inner electrodes for multilayer structures. Thus, there are considerable cost benefits in using KNN. Researchers from industry recently demonstrated the mass production of granulated KNN powders, a technical achievement, and were able to reproducibly produce 100 kg per batch.63 However, it should be noted that there remain many challenges, such as poor sinterability and weak reproducibility, which still hinder extensive application of these materials.
Ke Wang, School of Materials Science and Engineering, Tsinghua University,
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