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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]

The strategy of modulating polarization rotation of potassium sodium-niobate ((K 0.5 Na 0.5)NbO 3 , KNN) lead-free piezoelectric materials was reported to boost piezoelectric properties and resultant piezocatalytic activities. The effectiveness of modulating polarization rotation was proved by degrading Rhodamine B (RhB) using three KNN-based samples with different phase structures (i.e., orthorhombic (O) phase, orthorhombic-tetragonal (O-T) coexistence phase, and rhombohedral-orthorhombic-tetragonal (R-O-T) coexistence phase). Poled samples with the R-O-T coexistence phase show a reaction rate constant of 0.091 min−1 owing to the easiest polarization rotation, 2.12 times more than that of poled O-phase featured samples with the most difficult polarization rotation. Enhanced piezocatalytic activities primarily originate from the easier polarization rotation and improved carrier concentration, accompanied by the trace of the mechano-charge generation. Therefore, modulating polarization rotation effectively boosts piezocatalysis of KNN-based materials, promising for harnessing natural energy and disease treatment. [Display omitted] • lead-free (K, Na)NbO 3 materials are eco-friendly piezocatalyst. • Proposing a new strategy to enhance there piezocatalysis. • Obtaining a high k value of 0.091 min−1, much superior to those of other materials. • Unveiling the physical mechanisms of the superior piezocatalytic activities. [ABSTRACT FROM AUTHOR]

Ultrasonic energy harvesting has been developed as a promising power solution for implantable medical devices (IMDs) to perform tasks such as diagnosis, therapy, and monitoring. However, most current ultrasonic electronics use toxic lead-based piezoelectric materials (e.g., lead zirconate titanate) as core components and feature bulk or flat structures, representing common barriers to the development of future IMDs that should be eco-friendly and long-term fixation in the implanted position. Here, we present a KNN composite-based lead-free piezoelectric helix for ultrasonic transcutaneous energy harvesting. The device contains multilayer components (a matching layer, lead-free composites, a copper foil, and a Parylene-C film) to form a final self-supporting helix harvester, which can adapt to cylindrical objects (e.g., blood vessels) to maintain fixation. Nanodomain-induced high-performance (d33 ∼ 465 pC N−1) KNN ceramic was fabricated and further processed into 1–3 composites to enhance electromechanical coupling properties and mechanical flexibility. The developed prototype can be excited ultrasonically to generate adjustable power output up to ∼22.97 mW cm−2. The generated electricity was stored in capacitors and used to operate light-emitting diodes. Further ex vivo tests showed that the device is still sufficient to support wireless energy transfer through porcine tissues, demonstrating the great potential of the lead-free piezoelectric helix for IMDs. [ABSTRACT FROM AUTHOR]