Fabrication and evaluation of a single element lead free-based ultrasonic transducer

This paper discusses the fabrication and characterization of a single element ultrasonic transducer with a lead-free piezoelectric active element. A piezoelectric ceramic with composition of 0.88Bi_0.5Na_0.5TiO₃-0.08Bi_0.5K_0.5TiO₃-0.04Bi_0.5Li_0.5TiO₃ was chosen as the active element of the transducer. This composition exhibited a thickness coupling coefficient (k_t) of 0.45, a dielectric constant of 440 (at 1 kHz), and a longitudinal piezoelectric coefficient (d₃₃) of 84 pC.N^-1. To make the transducer, the ceramic was sandwiched between an epoxy-tungsten backing layer and a silver epoxy matching layer. An epoxy lens was also incorporated into the transducer's design to focus the ultrasound beam. The focused transducer with a center frequency of about 23 MHz demonstrated a -6 dB bandwidth of 55% and an insertion loss of -32 dB, while the -20dB pulsed length was measured to be 150 ns. A phantom made of copper wires (30 μm in diameter) was utilized to investigate the imaging capability of the transducer. The results indicated that the fabricated transducer with a lateral resolution of 260 μm and a relatively high depolarization temperature could be considered as a candidate for replacement of lead-based ultrasonic transducers.