Grain Boundary Diffusion Hardening in Potassium Sodium Niobate‐Based Ceramics with Full Gradient Composition and High Piezoelectricity

Abstract

Reducing mechanical losses and suppressing self‐heating are critical characteristics for high‐power piezoelectric applications. For environmentally friendly Pb‐free piezoelectric ceramics, traditional acceptor doping or annealing treatments have successfully improved the mechanical quality factor (Qm) based on a ceramic matrix with a poor piezoelectric coefficient (d33<100 pC/N). Nevertheless, a ceramic with high Qm and d33 values has not been reported owing to the inverse relationship between Qm and d33. Herein, a novel hardening method called grain boundary diffusion is used to develop Pb‐free potassium sodium niobate ceramics, where Qm increased by more than two‐fold (from 51 to 132) and a high d33 value (d33 = 360 pC/N) is maintained. Significantly, d33 retained 98% of its initial value after 180 days, exhibiting improved aging stability. The established properties are associated with the formation of the core‐shell microstructure and the full gradient composition distribution using structural characterizations and phase‐field simulations, where the core maintains a high d33 and the shell provides a hardening effect. The novel hardening effect in piezoelectric materials, known as grain boundary diffusion hardening, highlights the enhancement of the mechanical quality factor with high piezoelectricity, providing a new paradigm for the design of functional materials.

Composition Regulation of Potassium Sodium Niobate Thin Films through Post-Annealing under Alkali Element Atmospheres

“…As heightened concerns regarding environmental issues associated with lead (Pb) continue to grow, the substitution of lead-based piezoelectric materials with lead-free alternatives has garnered significant attention [1,2]. Among various materials, potassium sodium niobate (KNN) has been extensively studied owing to its promising piezoelectric-related characteristics, such as large piezoelectric coefficients comparable to that of lead-based ceramics, high Curie temperature, and high modifiability through composition regulation and texture design [3][4][5][6][7]. Currently, notable progress has been achieved in employing KNN ceramic bulk across various applications such as transducers, sensors, etc.…”

Large Piezoelectricity Induced by Internal Stress in (K,Na)NbO3 Single Crystals

“…Piezoelectric materials, with the superior function of energy conversion from mechanical form to electrical form or vice versa, have found extensive applications in the fields of sensing, medical ultrasound imaging, underwater communication, and other fields. − Among all of the piezoelectric materials, the ferroelectric family shows the most superior piezoelectric response, thereby attracting numerous research interests. Although ferroelectric materials vary, the core functions have always been to enable devices with excellent performance outputs within a wide operational range in numerous electromechanical applications; therefore ferroelectric materials with high piezoelectric response and excellent stability are highly required, , flourishing research of exploring the origin of ultrahigh piezoelectricity and improving its piezoelectric performance as well as stability. − Numerous strategies have been proposed to acquire large piezoelectricity in ferroelectric materials, including the constructions of multiphase coexistence, polar nanoregions (PNRs), and electric-field-induced phase transition, etc. ,− Among them, Pb-based relaxor ferroelectric crystals have made remarkable progress in the design, research, and application of ultrahigh piezoelectricity. ,,, However, lead-free ferroelectrics with high performance and excellent stability are preferred over Pb-based materials due to environmental issues. Among all the lead-free ferroelectrics that are widely considered, the potassium sodium niobate ((K, Na)NbO 3 , KNN) family has drawn extensive attention owing to their high Curie temperature and excellent piezoelectric properties. ,− …”