Advances in Lead-Free Piezoelectric Materials

Piezoelectric materials are currently used in many electronic devices because of excellent properties. Here, we briefly introduce the historical evolution of piezoelectric effect and also emphasize the importance of some factors (e.g., phase transition, microstructure, poling behavior) on the piezoelectricity of a material. Due to the toxicity of Pb in lead-based piezoelectrics, lots of attention has been given to lead-free piezoelectric materials, especially the use of phase boundaries. Importantly, we summarize the development of lead-free piezoelectrics, and some great advances have been demonstrated. We believe that the advances in lead-free piezoelectric materials will promote the practical applications.

Preparation technique and characterization method are critical aspects to a functional material. The preparation techniques strongly affect the electrical properties of a material. In this chapter, we pay much attention to several preparation techniques of lead-free piezoelectric materials including ceramic, nanostructure, thin film and single crystal. The influences of different preparation techniques on electrical properties and microstructure of a material are also addressed. In addition, various characterization methods on crystal structure, domain structure and electrical properties are also introduced.

As one of the most promising lead-free candidates, alkali niobate-based piezoelectric ceramics have been investigated for more than fifty years due to their moderate piezoelectricity and high Curie temperature. Several advances are speeding up the research of alkali niobate-based piezoelectric ceramics, and considerable efforts are given to the study of phase boundaries, generating a serial of high d₃₃ values of 400–700 pC/N. This chapter reviews the researches on structure and property of alkali niobate-based ceramics, with a focus on KNN-based ceramics. The phase boundaries, piezoelectric properties, and temperature stability of KNN-based ceramics are systematically discussed with the supporting of advanced physical mechanisms involving phase structure and domain configuration. Finally, the future direction of KNN-based ceramics is outlined, focusing on the balanced development of piezoelectricity and temperature stability.

In this chapter, we overview the development of electrical properties (mainly piezoelectric and strain properties) in BNT-based materials through chemical modifications, and two kinds of new physical effects (energy storage and electrocaloric behavior) are briefly introduced. More importantly, the relationships between phase boundaries and piezoelectric properties are established, and strain mechanisms are discussed in detail to better understand the ultrahigh strain responses. It is accepted that chemical modification can be generally employed to promote the piezoelectric and strain properties of BNT-based materials by forming different phase boundaries, such as the enhanced piezoelectricity (d₃₃ ~ 271 pC/N) and giant strain (S_max ~ 0.70%). Finally, we also address the tough issues including the controversial microscopic origins and somewhat unclear in this complex relaxor ferroelectric system. Further studies are still required to transfer this material into the real applications.

In this chapter, we present a comprehensive review of barium titanate (BaTiO₃,BT)-based piezoelectric materials, mainly consisting of the design of new material systems, the construction of phase boundaries, the origin of high piezoelectricity, and the exploration of low-temperature sintering technique. Firstly, effects of synthesis methods, microstructure and sintering aids on piezoelectricity are discussed. Thereafter, we introduce the approaches to modulate electrical properties of BaTiO₃ by focusing on phase boundary construction and oxides additives, the typical candidate materials including (Ba,Ca)(Zr,Ti)O₃, (Ba,Ca)(Sn,Ti)O₃ and (Ba,Ca)(Hf,Ti)O₃. In addition, we summarize the recent advances of (1 − x)Ba(Zr_0.2Ti_0.8)O₃–x(Ba_0.7Ca_0.3)TiO₃ thin films from the views of piezoelectric, ferroelectric and dielectric properties. Particularly, the electrocaloric effects of BaTiO₃-based materials are also reviewed with theory, property modification, and typical material systems. Finally, the related physical origin for high piezoelectricity is addressed by focusing on the role of intermediate phase in phase boundary and domain structure.

Bismuth ferrite materials including ceramics and thin films have attracted lots of attention due to their multi-functional properties. This chapter reviews the relationship between crystal structure and electrical properties of BFO-based ceramics through composition engineering. In addition, several crucial issues of BFO thin films are also pointed out, such as orientation, multilayer structure, buffer layer, thickness dependence, and so on. The detailed review of BFO-based materials gives a clear direction on the further researches about piezo/ferroelectric properties.

The high-temperature piezoelectric materials have been given to considerable attention due to the requirement of some electronic devices. However, relative low Curie temperature of K_0.5Na_0.5NbO₃ (T_C ~ 415 °C) and BaTiO₃ (T_C ~ 120 °C) as well as the depolarization temperature (T_d ~ 100 °C) of (Bi_0.5Na_0.5)TiO₃ limit the high-temperature applications. In this chapter, bismuth layer-structured ferroelectrics (BLSFs) with high T_C (generally higher than 500 °C) are referred. However, some shortcomings are also addressed, such as high coercive field, poor ferro/piezoelectricity. Due to the lack of phase boundaries, lots of attention mainly focus on the composition modification to improve ferro/piezoelectricity. In addition, some advanced preparation technologies are also introduced.

Lead-free perovskite piezoelectric ceramics are important to both scientific and industrial communities due to the promising candidates to replace the toxic lead-based ones. After the great efforts by researchers for several decades, some remarkable progresses are obtained, covering the breakthroughs in electrical properties, temperature stability and physical mechanisms. This chapter briefly reviews the recent development of lead-free piezoelectric ceramics, including temperature stability, electrical properties (e.g., piezoelectricity, electrocaloric effect, energy storage and electrostrictive effect) and physical mechanisms with a focus on crystallographic structure and domain configuration. Finally, the competition and challenge for each lead-free piezoelectric are listed, which could guide the development of lead-free piezoelectrics towards the practical applications.

After twenty years of enthusiastic researches into lead-free piezoelectric materials, the most eager prospects are transforming into the real applications. This chapter reviews the recent application progresses for lead-free piezoelectric materials, including piezoelectric energy harvesting devices, ultrasonic transducers, piezoelectric actuators, pyroelectric IR detectors, piezoelectric transformers and ultrasonic motors. The electrical parameters of active elements and devices performance are systematically discussed, which are almost compared with lead-based ones. Additionally, those challenges in lead-free piezoelectric materials and suggestions for the next research requirements for practical applications are also proposed.