Fabrication and Characterization of Single-Aperture 3.5-MHz BNT-Based Ultrasonic Transducer for Therapeutic Application

This paper discusses the fabrication and characterization of 3.5-MHz single-element transducersfor therapeutic applications in which the active elements are made of hard lead-free BNT-based and hard commercial PZT (PZT-841) piezoceramics. Composition of (BiNa 0.88 K 0.08 Li 0.04 ) 0.5 (Ti 0.985 Mn 0.015 )O 3 (BNKLT88-1.5Mn) was used to develop lead-free piezoelectric ceramic. Mn-doped samples exhibited high mechanical quality factor ( $Q_{m}$ ) of 970, thickness coupling coefficient ( $k_{t}$ ) of 0.48, a dielectric constant ( $\varepsilon ^{T}_{33}$ ) of 310 (at 1 kHz), depolarization temperature ( $T_{d}$ ) of 200 °C, and coercive field ( $E_{c}$ ) of 52.5 kV/cm. Two different unfocused single-element transducersusing BNKLT88-1.5Mn and PZT-841 with the same center frequency of 3.5 MHz and similar aperture size of 10.7 and 10.5 mm were fabricated. Pulse-echo response, acoustic frequency spectrum, acoustic pressure field, and acoustic intensity field of transducerswere characterized. The BNT-based transducershows linear response up to the peak-to-peak voltage of 105 V in which the maximum rarefactionalacoustic pressure of 1.1 MPa, and acoustic intensity of 43 W/cm 2 were achieved. Natural focal point of this transducerwas at 60 mm from the surface of the transducer.