Piezoelectric ceramics with high dielectric constants for ultrasonic medical transducers

Complex system ceramics Pb(Sc/sub 1/2/Nb/sub 1/2/)O/sub 3/-Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-Pb(Ni/sub 1/2/Nb$ d1/sub /2/)O/sub 3/-(Pb/sub 0.965/,Sr/sub 0.035/)(Zr,Ti)O/sub 3/ (PSMNZT) have been synthesized by the conventional technique and the dielectric and piezoelectric properties of the ceramics have been investigated for ultrasonic medical transducers. High capacitances of the transducers are desired in order to match the electrical impedance between the transducers and the coaxial cable in array probes with composite transducers. Although high-K ceramics that have high dielectric constants (/spl epsiv//sub 33//sup T///spl epsiv//sub 0/>5000, k33'<70%) are produced in many foundries, the dielectric constants are insufficient. On the other hand, we have reported that low molecular mass B-site ions in the lead-perovskite structures are important in realizing better dielectric and piezoelectric properties. We focused on the complex system ceramics PSN-PMN-PNN-PSZT which consist of light B-site elements. The maximum dielectric constant, /spl epsiv//sub 33//sup T///spl epsiv//sub 0/=7,200, was confirmed in the ceramics, where k/sub 33/'=69%, d/sub 33/=940pC/N and T/sub c/=135/spl deg/C were obtained. Pulse echo characteristics were simulated using the Mason model. The PSMNZT ceramic probe showed echo amplitude about 5.5dB higher than that of the conventional PZT ceramic probe (PZT-5H type). In this paper, the electrical properties of the PSMNZT ceramics and the simulation results for pulse echo characteristics of the phased array probes are introduced.