Investigation of potassium niobate as an ultrasonic transducer material

This paper investigates the use of single crystal potassium niobate (KNbO₃) as an ultrasonic transducer material. Crystallographic engineering allows the samples to be optimized for high coupling coefficients for the vibration modes of interest. The piezoelectric properties of KNbO₃ in both thickness and length extensional mode have been measured for two different crystallographic orientations. The measured high coupling coefficients (k_t = 68% and k₃₃ = 75%) mid-range Q_m(mechanical) of 40 and high longitudinal velocities of 8000 m/sec enable the use of KNbO₃ for high frequency transducers. A potential drawback is high measured values of electrical loss. In order to investigate losses at the frequencies of interest a curve-fitting program was used. The results show low losses above 1 MHz. A 10 Mhz transducer was fabricated using two optimally designed matching layers to match the high acoustic impedance with tissue. Modeling was performed both with the measured losses at 1kHz and the low loss values obtained through curve fitting. The experimental pulse echo response indicates that the effect of losses on the transducer performance is minimal, indicating that losses are not a major factor. A broad bandwidth of 64% and insertion loss of 17 dB was obtained.