Fe-doped BNKBT lead-free piezoceramics with enhanced electromechanical performance for 10 MHz ultrasonic imaging

Ultrasonic transducers have long relied on lead-based piezoelectric materials. However, the potential harm of lead to the environment and human health has spurred an urgent demand for environmentally friendly lead-free alternative materials. Bismuth sodium titanate (BNT) is regarded as a promising piezoelectric material for lead-free transducers due to its low dielectric constant and acoustic impedance. To address the problem that the previously developed 0.84BNT–0.133BKT–0.027BT composition still struggles to meet the requirements of high-quality biological tissue imaging, this study adopted an acceptor doping strategy and prepared a B-site Fe³⁺-doped lead-free piezoceramic system. The results show that appropriate Fe³⁺ doping produces a favorable balance between R3c/P4bm phase-structure modification and defect-related pinning, thereby enhancing the electromechanical response. At x = 0.01, the piezoelectric coefficient reaches the maximum value of 180 pC/N, while the thickness-mode electromechanical coupling factor (k_t) reaches 0.42, indicating enhanced thickness-mode electromechanical coupling. The planar 10 MHz ultrasonic transducer fabricated with this composition exhibits high sensitivity, with an insertion loss (|IL|) of 16 dB. After introducing an acoustic focusing lens, the focused transducer achieves axial and lateral resolutions of 182 and 264 μm, respectively. In addition, clear imaging of the internal structure of a porcine eyeball and the surface of a commemorative coin was successfully achieved. These results confirm that the B-site Fe³⁺-doped BNT–BKT–BT ternary lead-free ceramic has great application potential in high-performance environmentally friendly medical ultrasonic transducers.