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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]

By studying the alone and synergetic effects of Zr4+ and Bi3+ on potassium sodium niobate ((K, Na)NbO 3 , KNN) ceramics, we revealed how Bi 0.5 A 0.5 ZrO 3 (A = K, Na, Ag, and (Na 0.82 K 0.18)) reduces the orthorhombic-tetragonal phase transition temperature (T O–T) of KNN ceramics. Investigations into the alone effects reveal that aliovalent substitutions on K+/Na+ (Nb5+) with Bi3+ (Zr4+) inevitably destroy long-range ordering (LRO) and thus worsen piezo/ferroelectric properties. Despite this, Zr4+ can replace Nb5+ within a high content, remain an orthorhombic (O) phase, and slightly increase T O–T. Although substituting on K+/Na+ with Bi3+ decreases T O–T , it already significantly destroyed LRO before shifting T O–T to room temperature. Then, investigations into the synergetic effects show that Zr4+ acts as a buffer, Bi3+ is an accelerator, and A+ is a stabilizer. The buffer can exist in KNN ceramics within a high content and neutralizes the charges caused by the accelerator that concentrates on decreasing T O–T, and the stabilizer compensates for the stability of the perovskite phase. Their synergetic effects explain why Bi 0.5 A 0.5 ZrO 3 can gradually reduce the T O–T of KNN ceramics without significantly destroying LRO. Therefore, this work helps understand how Bi 0.5 A 0.5 ZrO 3 decreases T O–T and further design the phase boundary for KNN ceramics. Image, graphical abstract [ABSTRACT FROM AUTHOR]

This work reveals the role of Bi3+ in the controlling ability of (Bi 0.5 Ag 0.5)ZrO 3 to the phase structure of potassium sodium niobate based ceramics by introducing Ba2+. The substitution on (Bi 0.5 Ag 0.5)2+ with Ba2+ results in the reduced degree of phase coexistence, accompanying the phase transition from coexisting orthorhombic-tetragonal (O–T) phases to an orthorhombic (O) phase, which is demonstrated by the systematic phase structure analysis and variations of electrical properties. The analysis reveals that the high electronegativity of Bi3+ results in the increased covalent bond character and then increases the stabilization of the T phase, leading to a decrease in orthorhombic-tetragonal phase transition temperature (T O-T). Therefore, this work not only compares the controlling ability of Ba2+ and (Bi 0.5 Ag 0.5)2+ to T O-T but also reveals which and how elements cause the decreased T O-T , promoting the understanding of the effect of additives on the phase structure. [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 Phase boundaries and composition design were explored to achieve both high piezoelectricity and favorable temperature stability in potassium-sodium niobate ceramics, using (1- x)(K,Na)(Nb,Sb)O 3 - x Bi(Na,K)(Zr,Sn,Hf)O 3 ceramics. A rhombohedral-tetragonal (R-T) phase boundary was constructed at x =0.035–0.04 by co-doping with Sb5+ and Bi(Na,K)(Zr,Sn,Hf)O 3. More importantly, a superior temperature stability was observed in the ceramics with x =0.035, accompanying with a stable unipolar strain at room temperature to 100 °C. The ceramics with x =0.035 also exhibited improved piezoelectric properties (e.g., piezoelectric coefficient d 33 ∼465 pC/N and electromechanical coupling factor k p =0.47) and Curie temperature (T c ∼240 °C). The Rietveld refinement and in-situ temperature-dependent piezoresponse force microscopy (PFM) results indicated that the enhancement of the piezoelectric properties was caused by the easy domain switching, high tetragonal fraction, and tetragonality, while the improved temperature stability mainly originated from the stable domain structures. [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]

As one of the most important lead-free piezoelectric candidates, potassium sodium niobate [(K, Na)NbO 3 , KNN] has gained popularity due to its attractive functional properties, which are much improved by chemical modifications. However, there still remains an insufficient understanding of how these dopants influence the structure and performance of KNN ceramics. Herein, we comparatively studied a series of changes triggered by introducing (Bi 0.5 Na 0.5)ZrO 3 , BaZrO 3 , and PbZrO 3 into the KNN matrix. The analysis highlights their different roles in modulating the orthorhombic-tetragonal phase transition temperature (T O-T) by considering discrepancies in the electronegativity and radius among Bi3+, Ba2+, and Pb2+ ions. The synergistic effects of the large electronegativity and the small radius of the Bi3+ ion are the key to regulating T O-T , which is relatively unaffected by Pb2+ and Ba2+ ions. Based on experimental results and first-principles calculations, a multi-scale model is proposed to summary the general law of how the electronegativity and radius of the A-site ion in AZrO 3 -type dopants synergistically affect the phase structure, ferroelectric domains, and electrical properties of KNN ceramics. Therefore, this work helps understand the role of chemical dopants in the structure and performance of KNN ceramics and promote the future composition design for high performance. [Display omitted] Proving the large off-center displacement of Bi3+ is the root cause to effectively reduce T O-T and enhance d 33 , which is absent in Pb2+ and Ba2+. [ABSTRACT FROM AUTHOR]

The mechanisms behind the high piezoelectricity of (K,Na)NbO 3 -based lead-free ceramics were investigated, including electric field-induced phase transitions and composition-driven nanodomains. The construction of a rhombohedral-tetragonal (R-T) phase boundary, confirmed using several advanced techniques, allowed a large piezoelectric constant ( d 33 ) of 450 ± 5 pC/N to be obtained in (1- x )K 0.4 Na 0.6 Nb 0.945 Sb 0.055 O 3 - x Bi 0.5 Na 0.5 (Hf 1- y Sn y )O 3 (0 ≤ x ≤ 0.06 and 0 ≤ y ≤ 0.5) ceramics possessing an ultralow Δ U T-R of 7.4 meV. More importantly, the existence of an intermediate phase, i.e., the electric-induced phase (EIP), bridging the rhombohedral R [ P s //(111)] and tetragonal T [ P s //(001)] phases during the polarization rotation was demonstrated. Striped nanodomains (∼40 nm) that easily responded to external stimulation were also observed in the ceramics with an R-T phase. Thus, the enhanced piezoelectric properties originated from EIP and the striped nanodomains. [ABSTRACT FROM AUTHOR]

Energy storage properties of {Bi 0.5 [(Na 0.8 K 0.2 ) 1- z Li z ] 0.5 } 0.96 Sr 0.04 (Ti 1- x - y Ta x Nb y )O 3 (BNKLSTTN- x / y / z ) lead-free ceramics are investigated. It is found that Ta performs better than Nb in the case of their energy storage density values, and the addition of optimum Li contents can enhance the energy storage properties by enhancing the dielectric breakdown strength (DBS). Enhanced energy storage density of 1.60 J/cm 3 under a low electric field of 90 kV/cm is achieved in BNKLSTTN-0.025/0/0.10 samples, and the fatigue-free properties are also observed. In addition, the BNKLSTTN-0.025/0/0.10 samples show the enhanced temperature dependence of energy storage density. These results indicate that the BNKLSTTN- x / y / z ceramics are one of the most promising lead-free materials for energy storage applications. [ABSTRACT FROM AUTHOR]

(1 − x )K 0.5 Na 0.5 Nb 1 − y Sb y O 3 - z SrZrO 3 - x Bi 0.5 Na 0.5 HfO 3 (KNNS-SZ-BNH) lead-free ceramics were developed by the conventional solid-state reaction method. Effects of the additives (Bi 0.5 Na 0.5 HfO 3 , SrZrO 3 and Sb 5 + ) on their phase structure, microstructure, and electrical properties were investigated. The rhombohedral-orthorhombic-tetragonal (R-O-T) phase boundary can be established in the ceramics with 0.03 ≤ x ≤ 0.05, 0.04 ≤ y ≤ 0.06, and 0.01 ≤ z ≤ 0.025, and then their piezoelectric properties were improved. The ceramics with x = 0.03, y = 0.04 and z = 0.01 possess the optimum piezoelectric properties ( d 33 = 470 ± 5 pC/N, k p = 0.51 ± 0.02, and T C = 244 °C). We believe that R-O-T multiphase coexistence is mainly responsible for the enhancement of piezoelectric properties. [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]

• A new concept of tuning the trade-off between LRO and PNRs is proposed. • The concept relieves the sensitivity of d 33 and enhances temperature stability. • d 33 shows higher values at x = 0.9–0.95 and a high retention of > 83% at x = 0.8–1.1. • S uni of x = 1.1 is enhanced and has a high retention of ≥ 79% at T = 20–180 °C. To relieve the sensitivity of piezoelectric coefficient (d 33) to composition and strengthen temperature stability of strain in potassium sodium niobate {(K, Na)NbO 3 , KNN} ceramics, we proposed a new concept, tuning the trade-off between long-range ordering (LRO) and polar nanoregions (PNRs), and realized it by tailoring the content of bismuth (Bi) in an already-constructed multiphase coexistence, namely, 0.96(K 0.48 Na 0.52)(Nb 0.955 Sb 0.045)O 3 -0.04(Bi x Na 4-3 x) 0.5 ZrO 3 -0.3 mol%Fe 2 O 3 ceramics. We obtained not only the high retention of > 83% at x = 0.80–1.10 in d 33 but also higher d 33 at x = 0.90–0.95, relieving the sensitivity of d 33 to composition. We also obtained not only the enhanced strain but also the high retention of ≥ 79% over a wide temperature range of 20–180 °C at x = 1.10, irrespective of the electric field, strengthening the temperature stability. We demonstrated that high d 33 values hinge on the trade-off between LRO and PNRs, and the enhanced temperature stability of strain originates from the diffused multiphase coexistence and the reduced contribution of domain switching. Therefore, the new concept helps further design high-performance KNN-based ceramics for practical application. [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]