The characteristics of ultrasonic therapeutic transducers and used lead-free non-stoichiometric NKN-based piezoelectric ceramics

The lead-free ceramics with composition of (Na0.5K0.5)0.998Ca0.001Nb1+x/5O3 + 1 mol% CuO (NKCN-x-C1) were synthesized by the mixed-oxide method with the sintering temperature at 1080 °C. The effects of excess Nb2O5 doping on the phase structure, microstructures, dielectric, piezoelectric and ferroelectric properties with the x mole variation were systematically investigated. Experimental results showed that the grains in the microstructures slightly decreased with increasing the excess Nb contents, and the corresponding bulk densities were enhanced for NKCN-x-C1 ceramics. Furthermore, they showed that the samples presented the optimum values of density ρ = 4.45 g/cm3, dielectric constant ɛ33T/ɛ0  = 255, dielectric loss tan δ = 0.003, electromechanical coupling coefficient, kt = 0.46 (thickness mode), Kp = 0.38 (planar mode), the high mechanical quality factor Qm = 2800, and internal dipolar field (Ei) 5.4 kV/cm. The high Ei value induced the high Qm value due to the defect dipole increase. Then, we successfully construct 3-MHz ultrasonic transducers utilizing our developed piezoceramics and examine their acoustic power characteristics driven by a power driver with an open-loop control. Thus, the non-stoichiometric ceramics presents the good electrical properties, which is suitable for power transducers applications.