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

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 (Q_m) based on a ceramic matrix with a poor piezoelectric coefficient (d₃₃<100 pC/N). Nevertheless, a ceramic with high Q_m and d₃₃ values has not been reported owing to the inverse relationship between Q_m and d₃₃. Herein, a novel hardening method called grain boundary diffusion is used to develop Pb-free potassium sodium niobate ceramics, where Q_m 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.