Development of lead-free single-element ultrahigh frequency (170–320 MHz) ultrasonic transducers

This paper presents the design, fabrication and characterization of single-element ultrahigh frequency (UHF) ultrasonic transducers in which the center frequency ranged from 170 to 320 MHz. The center frequency of >300 MHz is the highest value of lead-free ceramic ultrasonic transducers ever reported. With concern in the environmental pollution of lead-based materials, the transducer elements presented in this work were lead-free K0.5Na0.5NbO3/Bi0.5Na0.5TiO3 (KNN/BNT) composite thick films. All transducers were evaluated in a pulse-echo arrangement. The measured −6 dB bandwidth of the transducers ranged from 35% to 64%. With the optimized piezoelectric properties of the composite film, the insertion loss of the UHF transducers was measured and determined to range from −50 to −60 dB. In addition to the pulse-echo measurement, a 6 μm tungsten wire phantom was also imaged with a 205 MHz transducer to demonstrate the imaging capability. The measured −6 dB axial and lateral resolutions were found to be 12 μm and 50 μm, respectively. The transducer performance presented in this work is shown to be better or comparable to previously reported results even though the frequency is much higher.

► We fabricate lead-free KNN/BNT single-element UHF ultrasonic transducers.
► We develop transducers with an ultrahigh frequency range of 170–320 MHz.
► 324 MHz is the highest center frequency of ceramic transducers ever reported.
► UHF transducers exhibit adequate bandwidth and insertion loss.
► Measured transducer characteristics are comparable to the theoretical ones.