Temperature stability of the piezoelectric properties of Li-modified KNN ceramics

Presently, the most promising family of lead-free piezoelectric ceramics is based on K0.5Na0.5NbO3 (KNN). Lithium doped KNN ceramics show high piezoelectric properties at room temperature. An issue of particular interest for medical applications is the temperature stability of the properties because of the need for repeated sterilization process at around 140 °C. A concern with Li-doped KNN is that the properties may not be thermally stable because the orthorhombic–tetragonal phase transition temperature is close to the room temperature. We report on electromechanical properties of KNN-modified with 3–6 atomic % Li, measured by resonance technique in function of temperature from 20 to 140 °C, during two heating and cooling cycles. The piezoelectric coefficients d31 and radial coupling coefficients kp are decreased up to 30% of their initial value after the first heating cycle. However, after the second cycle, the properties stabilize. The depoling is smaller in compositions that do not undergo a phase transition in the examined temperature interval.