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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]

High strain and good temperature stability are contradictory properties in (K, Na)NbO3 (KNN)‐based materials. Herein, good temperature stability with high strain is obtained in a multiphase coexistent [ie, orthorhombic‐tetragonal (O‐T) and rhombohedral‐orthorhombic‐tetragonal (R‐O‐T)] KNN. A second‐order transition‐like characteristic should contribute to the temperature stability, in which an intrinsic lattice structure forms a bridge between them. The observed second‐order transition‐like characteristic is due to the reduced discrepancy among different lattice symmetries and a broadened temperature region for the phase transition. These integrated factors can slow the latent heat in a first‐order transition and extend it over a wide temperature region, thereby exhibiting second‐order transition‐like behavior. Correspondingly, the abrupt increase in strain near the phase transition temperature significantly slows. In addition, the appearance of pure tetragonal symmetry (P4mm) is deferred to a much higher temperature than TO‐T, in which the strain will inevitably decrease. As a result, good temperature stability with a high strain response can be realized in multiphase coexistent KNN materials, including d33*=448 pm/V, ‐27.5%≤fluctuation≤4.2% for O‐T, and d33*=446 pm/V, ‐17.5%≤fluctuation≤7.6% for R‐O‐T, over the whole temperature range 25 °C‐190 °C. [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]

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]

Abstract: (0.99‐x)(K0.5Na0.5)(Nb0.965Sb0.035)O3‐0.01SrZrO3‐x(Bi0.5Na0.5)ZrO3 ceramics are chosen as an example to illustrate the effects of phase boundaries on strain. Temperature‐dependent dielectric permittivity and convergent beam electron diffraction patterns confirm the multiphase coexistence of rhombohedral, orthorhombic, and tetragonal phases in the ceramics with x = 0.03–0.04. Intriguingly, the ceramics with x = 0.03 display improved temperature stability in strain, e.g., a high retention (≈95%) of unipolar strain at 180 °C and low variation of 24% at 20–180 °C. A mode is proposed to show how to effectively tailor the strain and consolidate its temperature stability in potassium sodium niobate‐based ceramics. [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]

Different types of defect dipoles form, F e A.. − V A ′ for A-site and F e N b ″ − V O.. for B-site according to the dopant sites [Fig. 1(a)]. Both of the temperature stable miniatured domain [Fig. 1(b)] and active defect dipole F e N b ″ − V O.. [Fig. 1(c)] in the B-site 0.05 leads to the preferable temperature stability, that is, the remnant polarization (P r) and unipolar strain (S) vary less than 10% and 15% in the temperature range of 20-140 °C, which is superior to other systems [Fig. 1(d)]. We believe that the strategy of designed defect dipoles is helpful for improving electrical properties and further promoting the development of KNN-based ceramics. [Display omitted] The designed defect-engineering has been proved to be an effective way to promote properties in potassium sodium niobate (KNN)-based ceramics. Here, we focus on the introduction of Fe element into different sites of KNN-based ceramics to form defects and optimize properties. Two types of defect dipoles can be formed, F e A.. − V A ′ for A-site and F e N b ″ − V O.. for B-site. Compared with the defect dipoles of A-site, the defect dipoles of B-site are sensitive to both electric field and temperature. So that, the B-site defect dipoles switch easily under the thermo-electric treatment, and share the same behavior with spontaneous polarization after removing the electric field, which compensates the remnant polarization. In addition, the domain miniaturizes as the Fe ions doping. Both of the defect dipole behavior and nanodomain lead to the preferable temperature stability: remnant polarization (P r) and unipolar strain (S) of the B-site engineered ceramics vary less than 10% and 15% in the temperature range of 20-140 °C. We hope that the strategy of designed defect dipoles is helpful for improving electrical properties and further promoting the development of KNN-based ceramics. [ABSTRACT FROM AUTHOR]

Although the composition-driven multi-phase coexistence strategy has been widely used to modify the piezoelectric properties of potassium sodium niobate ((K, Na)NbO 3 , KNN) based ceramics, the trade-off between long-range ferroelectric ordering and chemical additives-induced disorder hinders the further improvement of piezoelectricity and makes the electric field-induced strain temperature-dependent. Herein, to resolve two issues, we proposed a new concept, that is, manipulating defect dipoles and local stress of a pre-constructed multi-phase coexistence through controlling zirconium (Zr) content. We validated the new concept by designing 0.96(K 0.4 Na 0.6)Nb 0.955 Sb 0.045 O 3 -0.04(Bi 0.5 Na 0.5)Zr (1+ x) O 3 ceramics. In samples with Zr deficiency (i.e., x =-0.1), we obtained high retention of 91% in normalized unipolar strain (S uni) over the temperature range of 30-160 °C, even under low electric fields of 10-20 kV/cm, superior to those of other representative KNN-based ceramics. In samples with slight Zr excess (i.e., x =0.07), we achieved an increase in direct piezoelectric coefficient (d 33) and converse piezoelectric coefficient (d 33 *) by 12% and 25%, respectively, in comparison with that at x =0. The enhanced temperature stability stems from the released domain walls that are pinned by defect dipoles, and the increased d 33 (and d 33 *) originates from the synergetic contributions of the multi-phase coexistence, increased grain size, and stabilization of the ZrO 2 secondary phase. Therefore, our new concept would benefit the composition design and performance improvement of KNN-based ceramics in the future. [Display omitted] [ABSTRACT FROM AUTHOR]