Lead-free piezoelectric ceramic transducer in the donor-doped K<sub>1/2</sub>Na<sub>1/2</sub>NbO<sub>3</sub> solid solution system

Abstract

Lead-based piezoelectric ceramics are suitable materials for noninvasive applications of ultrasound in medicine. However, the embedded therapeutic and diagnostic procedures require the use of lead-free piezoelectric materials as active elements in transducers. With this goal in mind, we investigated the substitution of Ba(2+) cations in a lead-free piezoelectric system of K(1/2)Na(1/2)NbO(3)-LiTaO(3)-LiSbO(3) (KNN-LT-LS) with perovskite structure. The Ba(2+) was added to the system as an A-site dopant in the range of 0-2 mol% in increments of 0.5 mol%. The addition of Ba(2+) improved the piezoelectric charge coefficient, d(33), and longitudinal coupling coefficient, k(33). The composition with 1 mol% Ba2+ had 36% and 58% higher d(33) and k(33), respectively, than the undoped composition. It appeared that the addition of Ba(2+) induced "soft" characteristics in this lead-free piezoelectric system. This was verified by the increase of remnant polarization along with the decline of coercive field. The Ba(2+) behaved as a grain growth inhibitor and caused a drastic reduction in polarization level (approximately 60%) when the grain size became smaller than approximately 1.5 microm. Incorporation of Ba(2+) up to 1.5 mol% increased the bulk resistivity of the KNN-LT-LS system and then reduced it drastically at higher dopant concentrations. The electron-hole compensation model fit well with the results obtained in this study and verified the A-site substitution of donor-doped barium. KNN-LT-LS ceramics with 0 mol% and 1 mol% Ba(2+) were used to fabricate single-element ultrasonic transducers resonating at 5.5 MHz. The -6 dB fractional bandwidth and -20 dB pulse length of the probe made of doped ceramic were 50% and 1.68 micros, respectively. This indicated that this system could be considered as a candidate for invasive and/or embedded medical ultrasound applications.