A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Perspective on the Development of Lead-free Piezoceramics

Potassium-sodium niobate lead-free piezoelectric materials: past, present, and future of phase boundaries.

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

Environment-friendly lead-free piezoelectric ceramics have been studied extensively in the past decade with great progress particularly in systems based on a niobate perovskite compound formulated as (K, Na)NbO3 (abbreviated as KNN). A comprehensive review on the latest development of KNN-based piezoelectric ceramics is presented in this article, including the phase structure, property enhancement approaches, and sintering processes as well as the status of some promising applications. The phase structure of KNN was reexamined and associated with the effect of chemical modification on its tetragonal-to-orthorhombic transition. Then, a special focus is placed on the temperature dependence of piezoelectric properties of KNN-based ceramics, followed by reviewing the recent approaches devoted to the temperature-stability enhancement. The processing fundamentals related to the sintering of KNN-based ceramics are also presented with an emphasis on compositional and microstructural control. Finally, this review introduces several industrial attempts of traditional piezoceramic products using KNN-based ceramics and the studies on some promising application in authors' laboratory.

(K, Na)NbO3-Based Lead-Free Piezoceramics: Fundamental Aspects, Processing Technologies, and Remaining Challenges

Recent development in lead-free perovskite piezoelectric bulk materials

Highly dense (1−x)(Na0.5K0.5)NbO3–x(Bi0.5Na0.5)TiO3 (NKN-BST) solid solution piezoelectric ceramics have been fabricated by ordinary sintering. All compositions show pure perovskite structures, showing room-temperature symmetries of orthorhombic at x⩽0.02, of tetragonal at 0.03⩽x⩽0.09, of cubic at 0.09 0.20. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal ferroelectric phases was identified in the composition range of 0.02<x<0.03. The materials near the MPB exhibit a strong compositional dependence, owning peak values of the planar electromechanical coupling factor kp∼43%, the piezoelectric constant d33∼195pC∕N, and the Curie temperature of 375°C comparable to that of commercial lead zinconate titanate ceramics. The results indicate that NKN-BST ceramic is a promising lead-free piezoelectric candidate material.

Phase structures and electrical properties of new lead-free (Na0.5K0.5)NbO3–(Bi0.5Na0.5)TiO3 ceramics

0.94(Bi 0.5 Na 0.5 )TiO 3 –0.06BaTiO 3 (BNT6BT) ceramics doped with 0–4 mol.% tantalum were investigated in terms of the sintering, microstructure, phase transition, dielectric and piezoelectric properties. Tantalum doping has no remarkable effect on the microstructure and densification within the studied doping content. Up to 4 mol.% tantalum can dissolve into the lattice of BNT6BT ceramics, and the structure symmetry is not changed. However, a significant change in the dielectric behavior and piezoelectric properties took place. With increasing tantalum content, the antiferroelectric phase zone gets broader. Simultaneously, the temperature for a ferroelectric to antiferroelectric phase transition is clearly reduced, and the temperature for a transition from antiferroelectric phases to paraelectric phases is only a bit increased. The results indicate that tantalum occupies B site of a perovskite and behaves as a donor, generating A-site cation vacancies. The properties of the material thus become softer, concerning a reduced coercive field and an improved piezoelectric constant. More than 2 mol.% tantalum doping induces antiferroelectric properties, showing a typical double hysteresis loops, accompanied by a large maximum strain.

Tantalum doped 0.94Bi0.5Na0.5TiO3–0.06BaTiO3 piezoelectric ceramics

Na0.5K0.5NbO3–BiFeO3 lead-free piezoelectric ceramics

(1−x)(Na0.5K0.5)NbO3–(Bi0.5K0.5)TiO3 solid solution ceramics were successfully fabricated, exhibiting a continuous phase transition with changing x at room temperature from orthorhombic, to tetragonal, to cubic, and finally to tetragonal symmetries. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal ferroelectric phases was found at 2–3 mol% (Bi0.5K0.5)TiO3 (BKT), which brings about enhanced piezoelectric and electromechanical properties of piezoelectric constant d33=192 pC/N and planar electromechanical coupling coefficient kp=45%. The MPB composition has a Curie temperature of 370°–380°C, comparable with that of the widely used PZT materials. These results demonstrate that this system is a promising lead-free piezoelectric candidate material.

Phase Transitional Behavior and Piezoelectric Properties of Lead-Free (Na0.5K0.5)NbO3–(Bi0.5K0.5)TiO3 Ceramics

A well-known Bi-containing perovskite ferroelectric, BiScO3 (BS), was successfully utilized to develop Na0.5K0.5NbO3 (NKN)-based lead-free piezoelectric ceramics, in which there is a morphotropic phase boundary (MPB) between ferroelectric orthorhombic and rhombohedral phases. The addition of a small amount of BS tends to promote the sinterability of NKN ceramics, and simultaneously, to retard the grain growth, and change the crystalline structures, resulting in a dense (1-x)NKN–xBS solid-solution ceramic upon conventional sintering. Enhanced piezoelectric and electromechanical properties appear near the MPB composition with x=0.02, the piezoelectric constant being 210 pC/N and the planar electromechanical coupling factor being 45%. This material also has a comparable Curie temperature (340 °C) to that of commercial Pb(Zr,Ti)O3 as well. All these results reveal that BS-substituted NKN ceramics may become promising candidates for lead-free piezoelectric materials.

https://iopscience.iop.org/article/10.1143/JJAP.46.6733/pdf

Chun Ye

Papers

Phase structures and electrical properties of new lead-free (Na0.5K0.5)NbO3–(Bi0.5Na0.5)TiO3 ceramics

Tantalum doped 0.94Bi0.5Na0.5TiO3–0.06BaTiO3 piezoelectric ceramics

Na0.5K0.5NbO3–BiFeO3 lead-free piezoelectric ceramics

Phase Transitional Behavior and Piezoelectric Properties of Lead-Free (Na0.5K0.5)NbO3–(Bi0.5K0.5)TiO3 Ceramics

Dielectric and Piezoelectric Properties of Lead Free Na0.5K0.5NbO3–BiScO3 Ceramics