Lead-Free Piezoelectric Materials. Jing-Feng Li

motors, and sonar systems. In the future, the applications will be extended to energy harvesting systems for distributed low‐power systems. The global market for piezoelectric devices is estimated to be valued at US$ 25.1 billion in 2019 and projected to expand at a compounded annual growth rate of 6.2% through the next five years [1]. One of the most widely used piezoelectric ceramic materials is the solid solution of lead zirconate (PbZrO₃) and lead titanate (PbTiO₃), commonly known as PZT, with general formula PbZr_1−xTi_xO₃ [2]. As mentioned in Chapter 1, although BaTiO₃ was discovered before lead zirconate titanate (PZT), the latter soon dominated the market for piezoelectric applications because of its comprehensively excellent piezoelectric properties and high Curie temperature as well as the flexibility for compositional modification to adjust to specific applications.

The outstanding properties of PZT originate from the nature of end‐number compounds and the phase structure evolution in their solid solutions. Both PbZrO₃ and PbTiO₃ have perovskite structure with Pb ions at the same A‐site. PbZrO₃ is antiferroelectric, but PbTiO₃ is ferroelectric [3]. Solid solutions are formed between them in a full range, but there exists a steep boundary where a transition occurs between the rhombohedral Zr‐rich phase and the tetragonal Ti‐rich phase, as shown in Figure 2.1. It should be noted that the tetragonal PbTiO₃ has a high transition temperature to cubic phase, which endow high Curie temperature to PZT. The phase boundary separating two phases in a solid solution system is called morphotropic phase boundary (MPB), where piezoelectric coefficients, dielectric constant, and electromechanical coupling factor are greatly enhanced [4]. The “phase boundary” is also considered as a composition where the two phases are believed to coexist, which renders the materials more responsive to extra stimuli. It should be noticed that the MPB is nearly temperature independent, which is why the properties of PZT ceramics have excellent temperature stability. More in‐depth studies had been conducted regarding the nature of the MPB using advanced structural characterization techniques, leading to several modified versions of the PZT phase diagram. One representative work suggested that the MPB is a narrow region where monoclinic phases exist in between the tetragonal and rhombohedral phases [5, 6]. It seems that the hierarchical structures ubiquitously present in ferroelectric materials makes the phase diagram studies more complicated than non‐ferroelectric materials [7]. Nevertheless, a similar concept of MPB is useful and highly desirable for the development of lead‐free piezoelectric materials.

Figure 2.1 Phase diagram of the PbZrO₃–PbTiO₃ system. Note that there are revised phase diagrams showing an area instead of a line in the middle composition.

In addition to the existence of MPB, the properties of PZT ceramics can be adjusted relatively easily by chemical doping or substitution, which made PZT like “all‐around player” to meet a wide range of applications. In fact, piezoelectric ceramic devices can be classified as actuators, sensors, and transducers, which have specific property requirements. For example, “hard” PZT ceramics for high‐frequency transducers actuators need high mechanical quality factor (Q_m), but sensors usually use “soft” PZT with enhanced d₃₃. All kinds of PZT ceramics are commercially available. However, the large amount of lead contained in PZT materials has drawn much attention during the past decade, due to the environmental concern as well as governmental regulations against hazardous substances. In 2006, the European Union has already adopted the well‐known directives: Waste Electrical and Electronic Equipment (WEEE) and Restriction of the use of certain Hazardous Substances (RoHS) in electrical and electronic equipment, with the purpose of protecting human health as well as environment by exclusion or substitution of hazardous substances used in electrical and electronic devices [8]. Similar regulations have also been planned or established in North America, Japan, Korea, and China. Lead ranks foremost among the hazardous substances list due to its notoriety, with maximum allowed concentration of merely 0.1 wt%. Bearing the goal to reduce the production and waste disposal of toxic lead, the research on lead‐free piezoceramics strives to make PZT and similar perovskite materials redundant. Substantial effort has been devoted to the development of high‐performance lead‐free piezoelectric ceramics based on the several perovskite compounds [9], including BaTiO₃, (K,Na)NbO₃, (Bi_0.5Na_0.5)TiO₃, and BiFeO₃. In particular, great progress has been made since 2004 when Saito et al. reported (K,Na)NbO₃ based lead‐free piezoelectrics with properties comparable with those found in PZT [10].

      This chapter is set up to enable the readers quickly learn about the lead‐free piezoelectrics by introducing their brief history, crystal structure, and physical natures for BaTiO₃, (K,Na)NbO₃, (Bi_0.5Na_0.5)TiO₃, and BiFeO₃ systems. We first introduce BaTiO₃ not only because it is the first lead‐free ferroelectric compound, but also for the purpose to show the intrinsic role of Pb by comparing BaTiO₃ with PbTiO₃, which is a representative lead‐containing ferroelectric. The detailed research progress about each system will be given in Chapters3–7.

2.2 BaTiO₃

BaTiO₃ is a well‐known ferroelectric material that was discovered before PZT in the 1940s during the World War II [11]. Before the emergence of PZT, BaTiO₃ had been the most important piezoelectric material, which has a higher piezoelectric coefficient d₃₃ = 120–190 pC/N than that of preceding piezoelectric materials such as triglycine sulfate (TGS), potassium dihydrogen phosphate (KDP), and quartz. BaTiO₃ undergoes a series of phase transitions, from cubic to tetragonal approximately at 130 °C, then to orthorhombic at 0 °C, and further to a rhombohedral crystal structure at −90 °C, as shown in Figure 2.2 [12].

      Although BaTiO₃ is the first lead‐free piezoelectric compound, its Curie point (approximately 130 °C) is moderate, which limits its piezoelectric applications. Nevertheless, there are also many studies on lead‐free piezoelectrics based on BaTiO₃ [13, 14]. High piezoelectricity has been reported in pure BaTiO₃ single crystals [15, 16] and fine‐grained ceramics [17, 18]. In 2009, Liu and Ren reported a large piezoelectric response for the BaTiO₃‐based ceramic system with a d₃₃ coefficient >600 pC/N [19], which is comparable with soft PZT ceramics. The details will be given in Chapter 5.

BaTiO₃ is often used as a model ferroelectric material for fundamental studies because of its simple composition and rich phase structures. For example, early studies have been conducted to reveal the role of Pb from a comparison between BaTiO₃ and PbTiO₃ [20]. Both compounds have similar cohesive properties such as unit‐cell volume (0.0632 and 0.0642 nm³, respectively), yet with distinct ferroelectric behavior. PbTiO₃ has a high Curie temperature of 490 °C, and a large value of spontaneous polarization P_s (57 μC/cm²), whereas those of BaTiO₃ are much lower (P_s ~ 26 μC/cm²). PbTiO₃ has no low‐temperature phase transitions, whereas BaTiO₃ is known to have successive phase transitions, from the cubic structure to tetragonal, orthorhombic, and rhombohedral structure at low temperature. To explain why BaTiO₃ and PbTiO₃ show different ferroelectric behaviors, the studies have been focused on the difference in the chemical bonds of the two perovskite compounds. Kuroiwa et al. demonstrated through calculations that the Pb and O electronic states hybridize in tetragonal PbTiO₃, whereas the interaction between Ba and O is almost purely ionic in the tetragonal BaTiO₃, as shown by the charge‐density distributions of cubic and tetragonal PbTiO₃ and BaTiO₃ in Figure

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