Comprehensive Performance Optimization in KNN-Based Piezoelectric Ceramics for an Ultrasonic Transducer

Potassium sodium niobate (KNN) ceramics, characterized by their high Curie temperature (<i>T</i>_C), favorable piezoelectric coefficients, and relatively low density, represent an environmentally friendly alternative to lead-based piezoelectrics. The fundamental challenge lies in simultaneous optimization of their piezoelectric performance─particularly piezoelectric coefficient <i>d</i>₃₃ and electromechanical coupling coefficient <i>k</i>_p─while preserving both thermal stability and <i>T</i>_C. Herein, a targeted doping strategy is proposed to optimize the composition of the KNN-based ceramics. The controlled acceptor doping in the multiphase coexistence system enables the regulation of grain size, and the disturbance of the long-range ordered structure leads to the formation of nanodomains, which further enhance the piezoelectric response. Excellent densification and uniform microstructure contribute to both high <i>T</i>_C and enhanced piezoelectric performance while ensuring stability across varying temperatures and frequencies. The optimized composition (<i>x</i> = 0.4) achieves exceptional piezoelectric properties: <i>d</i>₃₃ = 358 pC/N and <i>k</i>_p = 56.6%. In addition, it demonstrates a <i>T</i>_C of 315 °C and good thermal stability. The optimized KNN-based ultrasonic transducer exhibits a 3.19 MHz center frequency with 60% bandwidth (-6 dB) and achieves high-resolution imaging, demonstrating excellent signal transduction capabilities suitable for mid-to-high frequency ultrasonic devices and energy harvesting applications.