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Due to ever-increasing environmental concerns, lead-free piezoceramics have been studied for more than half a century with the purpose of replacing toxic lead-based counterparts. A series of notable breakthroughs have been reported in perovskite-structured lead-free piezoceramics, such as ultra-high piezoelectric and strain properties. By contrast, the development of the temperature stability of lead-free piezoceramics has left far behind and has always been the one of the biggest hindrances for practical applications. In this context, we have summarized the most cutting-edge advances in the temperature stability of perovskite-structured lead-free piezoceramics. We first emphasized the measurement methods of evaluating temperature stability, then summarized the regulating strategies (including phase boundary engineering, texturing, composite ceramics, defect engineering, quenching, and others) used for improving the temperature stability of these lead-free piezoceramics, and addressed the physical mechanisms from a multi-scale view. Finally, we concluded advantages and disadvantages of these strategies and provided our perspective on the challenges and future research of the temperature stability. We hope that this timely review could help the development of the temperature stability of lead-free piezoceramics towards practical applications. [ABSTRACT FROM AUTHOR]

Developing eco-friendly high-performance piezoceramics without lead has become one of the most advanced frontiers in interdisciplinary research. Although potassium sodium-niobate {(K,Na)NbO3, KNN} based ceramics are believed to be one of the most promising lead-free candidates, the relatively inferior piezoelectric properties and strong temperature dependency have hindered their development for more than 50 years since being discovered in the 1950s. It was not until 2014 that our group initially proposed a new phase boundary (NPB) that simultaneously improved the piezoelectric properties and temperature stability of non-textured KNN-based ceramics to the level of partly lead-based ceramics. The NPB has been then proved by some researchers and believed to pave the way for "lead-free at last" proposed by E. Cross (Nature, 2004, 432, 24). However, the understanding of the NPB is still in its infancy, leaving many controversies, including the phase structure and physical mechanisms at the NPB as well as the essential difference when compared with other phase boundaries. In this context, we systematically summarized the origin and development of the NPB, focusing on the construction, structure and intrinsic trait of the NPB, the effects of the NPB on the performance, and the validity and related incipient devices of the NPB. Particularly, we concluded the phase structure and domain structure locating at the NPB, analyzed the physical mechanisms in depth, proposed the possible methods to further improve the performance at the NPB, and demonstrated the validity and scope of the NPB as well as the device application. Finally, we gave out our perspective on the challenges and future research of KNN-based ceramics with NPB. Therefore, we believe that this review could promote the understanding of the NPB and guide the future work of KNN-based ceramics. [ABSTRACT FROM AUTHOR]

Abstract: In this work, we report a lead‐free piezoelectric ceramic of (0.9‐x)NaNbO3‐0.1BaTiO3‐xBaZrO3, and the effects of BaZrO3 on the phase structure, microstructure, electrical properties and temperature stability are investigated. A morphotropic phase boundary‐like region consisting of rhombohedral (R) and tetragonal (T) phases is constructed in the compositions with x = 0.035‐0.04. More importantly, in situ temperature independence of the piezoelectric effect {piezoelectric constant (d33) and strain} can be achieved below the Curie temperature (Tc). Intriguingly, the electric field‐induced strain is still observed at T ≥ Tc due to the combined actions of the electrostrictive effect and the electric field‐induced phase transition. We believe that NaNbO3‐based ceramics of this type have potential for applications in actuators and sensors. [ABSTRACT FROM AUTHOR]

Abstract: To improve the temperature stability and electrical properties of KNN‐based ceramics, we simultaneously consider the phase boundary and the addition of rare earth element (La), 0.96K0.5Na0.5Nb0.96Sb0.04O3‐0.04(Bi1‐xLax)0.5Na0.5ZrO3 (0 ≤ x ≤ 1.0) ceramics. More specifically, we investigate how the phase boundary and the addition of La3+ affect the phase structure, electrical properties, and temperature stability of the ceramic. We show that increasing the La3+ content leads to a change in phase structure, from a rhombohedral‐tetragonal (R‐T) phase coexistence to a cubic phase. More importantly, we show that the appropriate addition of La3+ (x = 0.2) can simultaneously improve the unipolar strain (from 0.127% to 0.147%) and the temperature stability (i.e., the unipolar strain of 0.147% remains unchanged when T is increased from 25 to 80°C). In addition, we find that the ceramics with x = 0.2 exhibit a large piezoelectric constant (d33) of ~430 pC/N, a high Curie temperature (TC) of ~240°C and a fatigue‐free behavior (after 106 electric cycles). The enhanced electrical properties mostly originate from the easy domain switching, whereas the improved temperature stability can be attributed to the R‐T phase boundary and the appropriate addition of La3+. [ABSTRACT FROM AUTHOR]

Although a rhombohedral-tetragonal (R-T) phase boundary is known to substantially enhance the piezoelectric properties of potassium-sodium niobate ceramics, the structural evolution of the R-T phase boundary itself is still unclear. In this work, the structural evolution of R-T phase boundary from −150°C to 200°C is investigated in (0.99− x)K0.5Na0.5Nb1− ySb yO3-0.01CaSnO3- xBi0.5K0.5HfO3 (where x = 0-0.05 with y = 0.035, and y = 0-0.07 with x = 0.03) ceramics. Through temperature-dependent powder X-ray diffraction ( XRD) patterns and Raman spectra, the structural evolution was determined to be Rhombohedral (R, <−125°C)→Rhombohedral + Orthorhombic (R + O, −125°C to 0°C)→Rhombohedral + Tetragonal (R + T, 0 °C to 150°C)→dominating Tetragonal (T, 200°C to Curie temperature ( TC)) → Cubic (C, > TC). In addition, the enhanced electrical properties (e.g., a direct piezoelectric coefficient ( d33) of ~450 ± 5 pC/N, a conversion piezoelectric coefficient ( [ABSTRACT FROM AUTHOR]

Phase boundary engineering (PBE) has remarkably enhanced the piezoelectric properties of potassium sodium niobate {(K, Na)NbO 3 , KNN} piezoceramics, yet the physical mechanisms need to be further understood. Here we outline a new physical phenomenon to describe piezoelectricity enhancement in KNN-based ceramics with PBE. We propose that the enhancement is due to the multi-phase coexistence featured with strong relaxation behavior. The strong relaxation behavior was unambiguously revealed by cryogenic experiments and originated from the polar nanoregions (PNRs) exhibiting a scale of 2.1 nm and a weak tetragonality (c/a = 1.0040). in situ temperature-dependent experiments uncovered the thermal evolution of the ferroelectric matrix and PNRs in both unpoled and poled samples, the first report in KNN-based ceramics. Our experiments combined with phenomenological theory revealed that ultra-fine nanodomains, PNRs, and easy polarization rotation together promote macro dielectric and piezoelectric properties in the relaxation-featured multi-phase coexistence. This work reveals the physical mechanism from different levels (e.g., local-mesoscopic-macroscopic), thus providing a new systematic understanding of the observed enhancement of piezoelectricity. Proposing a new physical mechanism, relaxor-featured multi-phase coexistence, to explain piezoelectricity enhancement in potassium sodium niobate-based ceramics. [Display omitted] [ABSTRACT FROM AUTHOR]

In this work, (1 − x)(KNa)NbO- x(BiAg)ZrO [(1 − x)KNN- xBAZ] lead-free piezoceramics was prepared by the conventional solid-state method, and a new phase boundary consisting of three phases [e.g., rhombohedral, orthorhombic, and tetragonal (R-O-T) phases] has been constructed by adding both (BiAg) and Zr. The ceramic with x = 0.05 possesses an R-O-T phase coexistence. A large d of ~347 pC/N and a high T of ~318 °C have been shown in the ceramic with x = 0.05. In addition, such a ceramic also possesses enhanced thermal and temperature stability of piezoelectricity and ferroelectricity. Both the phase boundary and the grain size play a critical role in large piezoelectricity and good stability. We think that this material belongs to be one of the promising candidates for the high-temperature piezoelectric devices. [ABSTRACT FROM AUTHOR]

Bi 0.5 Na 0.5 TiO 3 is regarded a potential alternative to lead-based piezoelectric materials due to its large electro-strain. Generally, the improved electro-strain is considered to arise from a relaxor-ferroelectric transition, and the random fields play a critical role. In this study, to investigate the effect of random electric field or strain field on the strain value, the dopant Sb5+, Ta5+, Zr4+, and Hf4+ are introduced into the Bi 0.5 (Na 0.8 K 0.2) 0.5 Ti 1- x B x O 3 matrix, respectively. The 1 % Ta5+ doped sample has the largest unipolar electro-strain, up to 0.49 %, followed by the 3 % Zr4+ doped sample (∼0.448 %). The electro-strain of 1 % Sb5+ and 3 % Hf4+ samples are 0.404 % and 0.377 %, respectively. The results show the random electric field or strain field can effectively regulate the relaxor-ferroelectric transition temperature, which enhances strain, but the strain value cannot be determined directly. The strain value is determined by the lattice structure before and after the electric field is applied. [ABSTRACT FROM AUTHOR]

In this work, the multiphase coexistence of rhombohedral-orthorhombic and orthorhombic-tetragonal (R-O/O-T) was constructed in (1- x - y )BaTiO 3 - x CaTiO 3 - y BaZrO 3 ceramics with 0.14 ≤ x ≤ 0.16 and 0.10 ≤ y ≤ 0.12, and thus a large piezoelectric constant ( d 33 ) of 600 pC/N was attained in the ternary (1- x - y )BaTiO 3 - x CaTiO 3 - y BaZrO 3 ceramics using the optimization of x and y . The R-O/O-T multiphase coexistence as well as the enhancement of dielectric and ferroelectric properties can be responsible for the high d 33 of this work. In addition, a high strain of 0.15% was observed in the multiphase coexistence. As a result, electrical properties of BaTiO 3 can be optimized by the construction of multiphase coexistence through the co-doping of CaTiO 3 and BaZrO 3 . [ABSTRACT FROM AUTHOR]

Inducing local structural heterogeneity is one of the most attractive strategies to enhance the piezoelectricity in lead-free piezoceramics, while the underlying mechanisms have not been well clarified, such as phase coexistence and domain boundary. Here, we mainly focus on analyzing the structure evolution mechanism of 0.96(K 0. 48 Na 0. 52)(Nb 1 − x Sb x)O 3 -0.04(Bi 0. 5 Ag 0. 5)ZrO 3 (0 ≤ x ≤ 0.1) (KNN) lead-free piezoelectric ceramics to study the intrinsic structural contributions from chemical modifying and/or alloy replacement. We explore the relation between structural characteristics and physical functionality by combining macro-spectral and micro-heterogeneity characterizations. It was found that Raman- and infrared-active phonon evolutions with respect to the motion of oxygen octahedron reveal the enhanced lattice symmetry when x increases to 0.06, along with the upswing optical electronic transitions. Moreover, we clarify the local heterogeneous structure of ceramics in terms of the spatial distribution of phonon traits on the micron scale accompanied with the atom-resolution polar vector state. The detected nanoscale atomic displacement clusters are directly related to the excellent polarization properties. Additionally, the phonon thermodynamics evolution was investigated in a wide temperature range of 80-720 K with unpolarized and polarized geometries. The detailed phase transition evolution and rhombohedral–orthorhombic–tetragonal multi-phase coexistence have been confirmed with Sb incorporation. The present work boosts the understanding of the compound-structure-functionality relation of KNN-based heterophase coexistent system, which could promote the development of the promising high-performance piezoelectric materials. [Display omitted] [ABSTRACT FROM AUTHOR]

A feasible replacement for lead-based piezoelectric ceramics is desperately required for increasing environmental concerns. Benefiting from the construction of a rhombohedral-tetragonal (R-T) phase boundary, a potential lead-free candidate (Na 0.5 K 0.5 )NbO 3 -(Bi 0.5 Li 0.5 )TiO 3 -BaZrO 3 is presented in this study, with a large piezoelectric coefficient d 33 of 330 pC/N and a unipolar strain of 0.16% at 4 kV/mm. More importantly, the piezoresponse is extremely thermally stable that the strain can maintain a considerable value of 0.12% at the temperature as high as 190 °C, rendering the current system very promising for piezoelectric actuator applications. The temperature dependent phase structure evolution as well as piezoelectric response is investigated systematically for interpretation of underlying mechanisms. [ABSTRACT FROM AUTHOR]