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Lead-free potassium sodium niobate [(K, Na)NbO3, KNN]-based ceramics have aroused great interest due to their excellent piezoelectricity and high Curie temperature. However, the deteriorative microstructure of KNN-based ceramics restricts their further developments. In this work, various kinds of oxide additives (Al2O3, Co2O3, Nb2O5, SnO2 and HfO2) are introduced into the KNN-based ceramics to increase the grain size and wipe the microcracks to improve their microstructures. Herein, large piezoelectric coefficient values (d33 ≈ 450–500 pC/N) and high Curie temperature values (TC ≈ 255–263 °C) together with high electromechanical coupling coefficient values (kp ≈ 0.50–0.56) are obtained in constructed R-O-T multiphase coexisted KNN-based ceramics. Moreover, the KNN ceramic with the addition of Al2O3 exhibits an optimal unipolar strain value (S = 0.124%) and a high inverse piezoelectric constant (d33* = 494 pm/V). We believe that our findings provide a simple, effective, and applicable approach to optimize the electrical performances of KNN-based system materials. [ABSTRACT FROM AUTHOR]

Defect engineering plays an important role in property modification for piezoelectric materials. In this work, we pay much attention to the effect of Nb nonstoichiometry on structure and properties of typical 0.95(K0.45Na0.55)Nb1+xO3–0.05Bi0.5Na0.5HfO3 ceramics. Large piezoelectric constant (d33 ~ 425 pC/N and d33∗ ~482 pm/V) together with high Curie temperature (TC ~ 315°C) have been achieved in the ceramics with excess Nb content (x = 0.01). However, the ceramics with deficient Nb element have seriously suppressed cryogenic εr‐T curves and deteriorated electrical properties. Multi‐scale characterizations including phase structure, microstructure, defect structure and domain structure have been adopted to explain the corresponding phenomenon. Defect complex VNb″″′-VO·· caused by deficient Nb induces clamped domain wall motion, leading to blocked polarization vector and poor electrical properties. On the contrary, the enhanced properties for the ceramics with excess Nb are attributed to easier domain switching due to the suppressed vacancies. We believe that defect engineering, for example nonstoichiometry, cannot only modulate electrical properties but also help us to understand some fundamental and critical problems about 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]