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

Investigations in the development of lead-free piezoelectric ceramics have recently claimed comparable properties to the lead-based ferroelectric perovskites, represented by Pb(Zr,Ti)O3, or PZT In this work, the scientific and technical impact of these materials is contrasted with the various families of “soft” and “hard” PZTs On the scientific front, the intrinsic nature of the dielectric and piezoelectric properties are presented in relation to their respective Curie temperatures (T
 C) and the existence of a morphotropic phase boundary (MPB) Analogous to PZT, enhanced properties are noted for MPB compositions in the (Na,Bi)TiO3-BaTiO3 and ternary system with (K,Bi)TiO3, but offer properties significantly lower The consequences of a ferroelectric to antiferroelectric transition well below T
 C further limits their usefulness Though comparable with respect to T
 C, the high levels of piezoelectricity reported in the (K,Na)NbO3 family are the result of enhanced polarizability associated with the orthorhombic-tetragonal polymorphic phase transition being compositionally shifted downward As expected, the properties are strongly temperature dependent, while degradation occurs through the thermal cycling between the two distinct ferroelectric domain states Extrinsic contributions arising from domains and domain wall mobility were determined using high field strain and polarization measurements The concept of “soft” and “hard” lead-free piezoelectrics were discussed in relation to donor and acceptor modified PZTs, respectively Technologically, the lead-free materials are discussed in relation to general applications, including sensors, actuators and ultrasound transducers

Lead-free piezoelectric ceramics: Alternatives for PZT?

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

After twenty years of partly quiet and ten years of partly enthusiastic research into lead-free piezoceramics there are now clear prospects for transfer into applications in some areas. This mimics prior research into eliminating lead from other technologies that resulted in restricted lead use in batteries and dwindling use in other applications. A figure of merit analysis for key devices is presented and used to contrast lead-containing and lead-free piezoceramics. A number of existing applications emerge, where the usage of lead-free piezoceramics may be envisaged in the near future. A sufficient transition period to ensure reliability, however, is required. The use of lead-free piezoceramics for demanding applications with high reliability, displacements and frequency as well as a wide temperature range appears to remain in the distant future. New devices are outlined, where the figure of merit suggests skipping lead-containing piezoceramics altogether. Suggestions for the next pertinent research requirements are provided.

Transferring lead-free piezoelectric ceramics into application

Perovskite lead-free dielectrics for energy storage applications

Piezoelectric Properties of Bismuth Sodium Titanate Ceramics

The phase transition temperatures of divalent and trivalent ions substituted (Bi1/2Na1/2)TiO3 ceramics were investigated in detail by electrical measurement. BNT ceramics substituted by divalent (Ca2+, Sr2+ and Ba2+) and trivalent (La3+, Nd3+, Ho3+ and Yb3+) ions were prepared by conventional ceramic fabrication process. For these ceramics, the depolarization temperature, Td, the rhombohedral-tetragonal phase transition temperature, TR-T, and the temperature of the maximum dielectric constant, Tm, were determined on the basis of the temperature dependences of dielectric and piezoelectric properties. In this study, we demonstrated the relationship between phase transition temperature and ionic radius of ions substituted in the A-site of BNT ceramics.

Phase Transition Temperatures of Divalent and Trivalent Ions Substituted (Bi1/2Na1/2)TiO3 Ceramics

The piezoelectric properties of lead-free x(Bi1/2Na1/2)TiO3-y(Bi1/2K1/2)TiO3-zBaTiO3 [BNBKy:z(x); y:z = 4:1 and 2:1, x+y+z=1] with tetragonal compositions were investigated, focusing on the secondary phase transition T2 from the ferroelectric to antiferroelectric phases. Although the T2 temperatures of BNBK near the MPB composition remained at about 115°C, T2 rapidly shifted to a higher temperature of 200°C or higher for the off-MPB composition [x≤0.75 for BNBK4:1(x) and x≤0.78 for BNBK2:1(x)]. The piezoelectric activities of off-MPB compositions decreased with decreasing the amount (x) of BNT. The larger piezoelectric constant d33=135 pC/N, the higher secondary phase transition temperature T2=197°C, and the coupling factor k33=0.45 with a good temperature dependence were obtained for BNBK2:1(0.78).

https://iopscience.iop.org/article/10.1143/JJAP.44.4350/pdf

(Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3-BaTiO3-Based Lead-Free Piezoelectric Ceramics

Phase transition temperatures of (1-x)(Bi1/2Na1/2)TiO3—x (Bi1/2K1/2)TiO3 (BNKTx) and (1-x)(Bi1/2Na1/2) TiO3—xBaTiO3 (BNBTx) ceramics were investigated by using electrical measurements such as dielectric and piezoelectric properties. We discussed how to determine the depolarization temperature, Td, and rhombohedral-tetragonal phase transition, TR−T, for (Bi1/2Na1/2) TiO3 (BNT)-based solid solutions, and the Td TR−T and the temperature that dielectric constant become maximum Tm, for BNKTx and BNBTx were clarified in this study.

Investigation of Phase Transition Temperatures on (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3 and (Bi1/2Na1/2)TiO3-BaTiO3 Lead-free Piezoelectric Ceramics by Electrical Measurements

In this study, Bi2O3-doped bismuth potassium titanate, (Bi1/2K1/2)TiO3, (abbreviated to BKT-Bix; x=0, 0.1, 0.2, 0.4, and 0.6 wt %) ceramics were fabricated by hot-pressing method, and its electrical properties were investigated in detail. Moreover, BKT-Bi0.6 was annealed at 1040 °C for 4–20 h to develop grains [abbreviated to BKT-Bi0.6(y h); y=4, 10, and 20]. The ferroelectric properties were improved by grain growth, and the remanent polarization, Pr, of BKT-Bi0.6(10 h) was 27.6 µC/cm2. The piezoelectric properties were improved by field cooling method, and the electromechanical coupling factor, k33, and piezoelectric constant, d33, of BKT-Bi0.6(10 h) improved up to 0.40 and 101 pC/N, respectively. Moreover, the temperature dependences of piezoelectric properties were measured, and piezoelectricity disappeared at 280 °C for BKT-Bi0.6(10 h).

https://iopscience.iop.org/article/10.1143/JJAP.46.1081/pdf

Yuji Hiruma

Papers

Piezoelectric Properties of Bismuth Sodium Titanate Ceramics

Phase Transition Temperatures of Divalent and Trivalent Ions Substituted (Bi1/2Na1/2)TiO3 Ceramics

(Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3-BaTiO3-Based Lead-Free Piezoelectric Ceramics

Investigation of Phase Transition Temperatures on (Bi1/2Na1/2)TiO3-(Bi1/2K1/2)TiO3 and (Bi1/2Na1/2)TiO3-BaTiO3 Lead-free Piezoelectric Ceramics by Electrical Measurements

Grain-Size Effect on Electrical Properties of (Bi1/2K1/2)TiO3 Ceramics