Lead-Free Piezoelectric Materials. Jing-Feng Li

can be seen between the cubic phases of PbTiO₃ and BaTiO₃, but between their tetragonal phases. Within the latter phases, the distributions in the Pb–O and Ti–O planes of PbTiO₃ are significantly different from those in the Ba–O and Ti–O planes of BaTiO₃. The electron‐density distribution around the Pb and O2 sites in tetragonal PbTiO₃ is quite anisotropic as shown in Figure 2.3c, i.e. extending toward the O2 and Pb, respectively. The electron densities of Pb and O2 are overlapped along the Pb–O2 direction in the tetragonal PbTiO₃. However, the electron densities around Ba and O are isotropic for tetragonal BaTiO₃, as shown in Figure 2.3a. These results indicate that the PbO bonds of tetragonal PbTiO₃ are covalent, although they are ionic in the cubic phase. By comparing Figure 2.3b,d, it is clear that the Ti in PbTiO₃ has a larger displacement than that in BaTiO₃, consistent with the fact that the spontaneous polarization of PbTiO₃ is larger than that of BaTiO₃. This provides clear evidence for the Pb–O hybridization in tetragonal PbTiO₃, which has been theoretically predicted as a key factor of much larger ferroelectricity of PbTiO₃ as a representative Pb‐containing ferroelectric compound than that of BaTiO₃ as a lead‐free counterpart.

Figure 2.2 Change in relative dielectric constants of BaTiO₃ ceramics as a function of temperature. Inserted are the crystal structures at the corresponding temperatures.

      Source: Modified from Merz [12].

The aforementioned comparison between BaTiO₃ and PbTiO₃ indicates that partial covalent bonding due to the Pb–O hybridization should be responsible for the high performance of Pb‐containing piezoceramics. In lead‐free counterparts, Saito et al. also found that substituting Nb with Sb, which has higher electronegativity than the former, leads to further piezoelectric property enhancement in (K,Na)NbO₃‐based ceramics [10]; this is also because of the hybridization of covalency onto ionic bonding. In addition, Bi as a neighbor of Pb has a similar electronic structure with the latter, and can be used a modifier to enhance the contribution of covalent bonding.

Figure 2.3 Charge‐density distributions on (a) the (010) plane (Ba–O plane), (b) the (020) plane (Ti–O plane) of tetragonal BaTiO₃ at 300 K, compared to that of tetragonal PbTiO₃ at 300 K on (c) the (010) plane (Pb–O plane), and (d) the (020) plane (Ti–O plane).

      Source: Reprinted with permission from Kuroiwa et al. [20]. Copyright 2001, American Physical Society.

2.3 (K,Na)NbO₃

(K,Na)NbO₃ (shortened as KNN) is a solid solution of KNbO₃ and NaNbO₃. KNbO₃ is a ferroelectric material with T_c = 435 °C, while NaNbO₃ is expected to an antiferroelectric material with T_c = 640 °C according to the early studies; evidences for the latter's antiferroelectricity are still being investigated. Nevertheless, both compounds can form solid solutions in the full compositional range, analogous to the PZT system that is a solid solution of ferroelectric PbTiO₃ and antiferroelectric PbZrO₃ [21].

      (K,Na)NbO₃ ceramics have been investigated quite early, which was found as a piezoelectric material in 1950s. Jaeger and Egerton [22] fabricated highly dense KNN ceramics using hot pressing and reported that the hot‐pressed KNN ceramics possessed high piezoelectric constants and Curie temperature. However, since then there had been only a few reports about piezoelectricity of NaNbO₃, KNbO₃, or KNN ceramics, probably because of their poor sinterability and high volatility, in addition to their low piezoelectricity compared with the PZT ceramics that have been widely used so far. KNN ceramics have begun to attract special attention again because of a breakthrough made by Saito et al., who obtained high d₃₃ (~416 pC/N) in textured KNN‐based ceramics [10]. Their work not only revealed the potentials of KNN ceramics as a promising candidate for lead‐free piezoelectric ceramics, but also stimulated widespread scientific interest in developing lead‐free piezoceramics.

Figure 2.4 shows the phase diagram of the KNN system [23]. It is quite complicated when compared with the PZT system. Similar to KNbO₃, the KNN shows the following polymorphisms within a wide composition range: low temperature (less than −10 °C) rhombohedra (R) phase, room‐temperature orthorhombic (O) phase, high temperature tetragonal (T, 225~435 °C), and cubic (C, >435 °C) phase. However, the phase transitions of NaNbO₃ are even more complicated, exhibiting seven polymorphs across the whole temperature range. The temperature‐dependent phase transition of KNN is similar to the case of BaTiO₃, but the cubic to non‐cubic phase transition temperature in the former, corresponding to the Curie temperature, is much higher than that of BaTiO₃, which is a major advantage of KNN. Therefore, KNN‐based ceramics are considered as the most promising lead‐free piezoceramics with good comprehensive performance (large d₃₃ and high T_C).

Figure 2.4 Phase diagram of KNbO₃–NaNbO₃ system.

      Source: Reprinted with permission from Li et al. [23]. Copyright 2013, John Wiley & Sons, Inc.

      Significant research progress on KNN‐based ceramics has been achieved in the past years [23–26]. The record d₃₃ of the textured KNN‐based ceramics has been boosted to 704 pC/N in 2018 [27] from the original report of 412 pC/N by Saito et al. in 2004 [10]. Phase boundary engineering is an effective approach to the piezoelectricity enhancement in KNN. Pristine KNN has an orthorhombic phase (O) at room temperature, which transfers to rhombohedral (R) and tetragonal (T) phases at approximately −200 and 200 °C, respectively. Shifting these phase transition temperatures to room temperature by chemical modification has proved to be an effective way to enhance the piezoelectric performance of KNN materials significantly. That is, these two or three phases can be rendered to co‐exist near at room temperature, thereby leading to enhanced piezoelectricity. Such an enhancement is based on the polymorphic phase transition (PPT) effect, which is different from the classic MPB effect. The high piezoelectricity of KNN‐based ceramics deriving from the PPT effect, however, is sensitive to temperature changes [23, 28]; small variations from room temperature can lead to significant drops in the piezoelectricity. Therefore, a simultaneous enhancement of the piezoelectricity and its thermal stability has been one of the focused topics, which is crucial for extending the large‐scale applications of KNN‐based piezoceramics. An effective strategy to this end is making the phase transition boundary diffused [29–31], which will be described later in the corresponding chapter. In addition to the outstanding overall piezoelectric performance, another advantage of the KNN‐based ceramics is their compatibility of co‐firing with Ni electrodes that are much cheaper than Ag/Pd electrodes currently used for PZT multilayer piezoelectric ceramics [32].

2.4 (Bi_1/2Na_1/2)TiO₃

      (Bi_1/2Na_1/2)TiO₃ (abbreviated as BNT