Fabrication and Evaluation of a Single-Element Bi0.5Na0.5TiO3-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(0.5)Na(0.5)TiO(3)-0.08Bi(0.5)K(0.5)TiO(3)-0.04Bi(0.5)Li(0.5)TiO(3) 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 (d(33)) 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; the -20-dB pulsed length was measured to be 150 ns. A phantom made of copper wires (30 mu m 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 mu m and a relatively high depolarization temperature, could be considered as a candidate for replacement of lead-based ultrasonic transducers.