Enhancing piezoelectric response and electric-field-induced strain in KNN-based ceramics for ultrasonic transducers

High-performance lead-free piezoelectric ceramics are critical components in electromechanical devices including actuators, sensors, and transducers. In this study, we present Ta₂O₅-doped 0.96K_0.48Na_0.52NbO₃-0.04Bi_0.5Li_0.5HfO₃ [KNN_(1–x)T_x-BLH] ceramics synthesized via a conventional solid-state reaction route. The structural and piezoelectric characteristics of the KNN-based matrix are significantly modified by the introduction of Ta. A modest Ta content (x = 0.02) yields a pronounced enhancement in piezoelectric response, reaching a considerable piezoelectric coefficient (d₃₃) of 368 pC/N and retaining a Curie temperature of 324 °C. Further Ta incorporation (x = 0.08) optimizes the balance between structural stability and field-induced polarization, resulting in a significant electrostatic strain of 0.86%, as well as a d₃₃ value of 300 pC/N. X-ray diffraction and piezoelectric force microscopy analyses reveal that Ta substitution promotes phase evolution and domain reorientation, which collectively underpin the observed performance improvements. These findings demonstrate an effective compositional design approach that can enhance both the piezoelectric activity and the electrostrain of lead-free ceramics simultaneously, thus promoting their application in practical electromechanical systems.