A dual frequency ultrasonic transducer based on BNBT-6/epoxy 1-3 composite

A lead-free piezoelectric ceramic, 0.94(Bi0.5Na0.5) TiO3–0.06BaTiO3 (BNBT-6), was used to fabricate 1-3 composite ultrasonic transducers. BNBT-6 ceramic has a high thickness electromechanical coupling coefficient kt of 0.46 and a low planar coupling coefficient kp (∼0.26). A 1-3 composite with 0.575 mm ceramic width and 0.225 mm epoxy gap was fabricated by a dice-and-fill technique. Resonance characteristics of the composite were measured as a function of sample thickness. Mode-coupling theory was applied to calculate the resonance frequencies of the ceramic rods inside the 1-3 composites. The changes in resonance frequencies with composite thickness showed reasonable agreement with the theoretical predictions. With the width to thickness ratio (L/H) of the ceramic rods close to 1.45, the thickness mode coupled to the lateral mode and the composite showed two strong resonance modes in the impedance spectrum. At this aspect ratio, the composite was constructed into a dual frequency ultrasonic transducer. The pulse-echo response, bandwidth and insertion loss of the transducer were measured. The good transducer performance indicated that the BNBT-6/epoxy 1-3 composite would be a promising lead-free material for medical imaging applications.