Lead-Free Piezoelectric Materials. Jing-Feng Li

et al. in 1961 [33], BNT had been considered as a promising lead‐free piezoelectric material system [34, 35]. BNT shows strong ferroelectric properties at room temperature, including a large remnant polarization P_r = 38 μC/cm² and a coercive field E_c = 73 kV/cm [36]. Due to the poor sintering property of the pure BNT ceramics and the relatively low piezoelectric constant (d₃₃ ~ 100 pC/N), many studies have thus focused on improving the piezoelectric properties through cation substitution at the A and/or B sites of BNT [36–38].

Figure 2.5 (a) Phase diagram of of (Bi_1/2Na_1/2)TiO₃–BaTiO₃ proposed by Takenaka and (b) compositional dependence of dielectric constant.

      Source: Reprinted with permission from Takenaka et al. [35]. Copyright 1991, The Physical Society of Japan.

Takenaka and his research group had done extensive investigations about BNT‐based lead‐free piezoceramics. In 1991, they obtained the phase diagram of the BNT and BaTiO₃ solid solution based on the combined measurement results of X‐ray diffraction patterns and dielectric and ferroelectric properties [35]. Most importantly, as shown in Figure 2.5, there exists an MPB between BNT and BaTiO₃ at the composition close to 6 mol% BaTiO₃. Separated by this phase boundary are the rhombohedral and tetragonal phases, similar to that in PZT. Therefore, it is also possible to enhance the piezoelectric properties of BNT‐based ceramics by utilizing the MPB effect via compositional modification. However, the BNT–BaTiO₃ phase diagram has obtained several newer versions due to the incorporation of other characterization methods [39, 40]. A similar system is the solid solution between (Bi_1/2Na_1/2)TiO₃ and (Bi_1/2K_1/2)TiO₃ compounds, first reported by Sasaki et al. [41]. The MPB between BNT (rhombohedral) and (Bi_1/2K_1/2)TiO₃ (tetragonal) exists in the range of 16–20 mol% (Bi_1/2K_1/2)TiO₃ [41].

      (Bi_1/2Na_1/2)TiO₃ is an interesting system. The structural description of BNT is quite complicated, and some details of the structure are still under debate [36]. Although it is generally agreed that BNT at room temperature is rhombohedral as indicated in the phase diagram, the high‐resolution X‐ray measurements of single crystals also revealed the hints of a monoclinic phase [42]. Some studies also indicate that BNT has a composite structure at nanoscale, namely, the dispersion of short‐range‐ordered orthorhombic phase domains within a long‐range‐ordered rhombohedral or monoclinic phase matrix. The complexity may stem from the fact that there exist two kinds of ions with different valances at the A‐site of the perovskite structure. In addition, unmodified NBT and BNT‐rich BNT–BaTiO₃ ceramics often show relaxor ferroelectric features, although the early phase diagram proposed by Takenaka et al. indicated an antiferroelectric region above a certain temperature as shown in Figure 2.5.

Compared with other lead‐free piezoelectric systems, the BNT system exhibits an exceptionally high electric‐field‐induced strain, which makes it particularly attractive for applications in piezoelectric actuators such as camera focusing, printing nozzle, and impact driving. It has been reported that the poling strain of BNT–BaTiO₃ ceramics could reach a high value range of 0.3–0.7% by composition modification [43, 44]. However, one of the drawback of BNT‐based ceramics is the existence of a depolarization temperature (approximately 150 °C) below the Curie point, which poses a limit on the application temperature, and its underlying mechanism is still not completely understood.

2.5 BiFeO₃

Bismuth ferrite (BiFeO₃) possesses a rhombohedrally distorted perovskite structure as shown in Figure 2.6, which can be denoted as a pseudocubic structure with lattice parameter a = 0.396 nm and α = 89.6° [45]. BiFeO₃ is the only room‐temperature multiferroic material with coexisting ferroelectric and magnetic properties in a single compound [46]. Extensive studies have been conducted on this compound in the form of thin films, which are expected to open up new applications [47]. As reported in the epitaxial thin films, BiFeO₃ has a high Curie point (T_c ~ 830 °C) and large remnant polarization (P_r ~ 100 μC/cm²) at the same time [47]. In addition, the cost of resource is low because of the earth abundance of the constituent elements. Most importantly, BiFeO₃ is a lead‐free ferroelectric compound where Bi has a similar electronic structure with Pb. Therefore, recently, increasing attention has been paid to the development of BiFeO₃‐based lead‐free piezoelectrics.

However, BiFeO₃ and BiFeO₃‐based ceramics have high leakage current issues, mainly induced by defects and secondary phases, which hinder the poling process required for piezoelectric ceramics [48]. To resolve this problem, extensive investigations have been conducted based on chemical modifications to BiFeO₃ [49]. In 2008, Takeuchi and coworkers introduced another rhombohedral–orthorhombic (R–O) MPB that was formed by A‐site doping with 14% Sm in BiFeO₃ films grown on SrTiO₃. The piezoelectric coefficient d₃₃ near this MPB could reach 110 pm/V [50]. This report stimulated many studies about Sm‐doping on BiFeO₃, and the addition of other rare earth elements were also investigated to promote the piezoelectricity and ferroelectricity. For example, a large piezoelectricity (d₃₃ ≈ 50 pC/N) was reported in Sm and La co‐doped BiFeO₃ ceramics [51]. Higher properties are obtained in BiFeO₃ ceramics with the addition of ABO₃. In a study by Lee et al., excellent piezoelectric properties with a Curie temperature above 400 °C were reported in BiFeO₃–BaTiO₃ ceramics with the addition of either Bi_1.05GaO₃ or Bi_1.05(Zn_0.5Ti_0.5)O₃ fabricated in a sintering‐and‐quenching process [52]. Such a special process has proved to effectively suppress the formation of point defects and secondary phase and thereby reduce the high current leakage [53].

Figure 2.6 Crystalline structure of BiFeO₃.

      The main applications of BiFeO₃‐based ceramics are considered to be high‐temperature piezoelectric sensors, but their lower usage temperatures should be lower as compared with bismuth layer‐structured ferroelectric ceramics (BLSFs). Nevertheless, BiFeO₃‐based ceramics have the potentials to achieve much higher piezoelectric coefficients, so their applications can be extended to broader areas including piezoelectric actuators and transducers at middle and high temperatures.

2.6 Summary

      For decades, Pb(Zr,Ti)O₃ (PZT)‐based ceramics have been market‐dominating due to its excellent properties and flexibility in terms of compositional modifications. On the other hand, developing high‐performance lead‐free piezoelectric ceramics has been one of the most active materials research topics in the last decades. At present, a single replacement for PZT may not be available, but the abovementioned lead‐free systems are already applicable for some applications. Among them, KNN appears to be the most promising one for its well‐balanced combination of high piezoelectricity and Curie temperature. In addition, as shown in Chapter 3, KNN‐based ceramics can be co‐fired with base metals like nickel, which is a critical factor for the cost‐effective