Lead-free piezoelectric ceramic transducer in the donor-doped K1/2Na1/2NbO3 solid solution system

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 (~60%) when the grain size became smaller than ~1.5 mum. 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 mus, respectively. This indicated that this system could be considered as a candidate for invasive and/or embedded medical ultrasound applications.