Lead-free KNNLT piezoceramic multilayer actuators with Ni electrodes cofired under low oxygen partial pressure

The fabrication of piezoelectric multilayer actuators (MLA) with lead-free piezoceramic materials and excellent performance represents a great challenge in the process of replacing the current family of lead-based actuators. Sintering under low pO2 allows co-firing with base metal electrodes, which is an important step towards competitive multilayer actuator fabrication technologies. We prepared sodium potassium niobate (KNNLT) piezoelectric multilayer actuators with Ni electrodes by sintering at low pO2 of 10−11 atm at 950 °C, followed by reoxidation at 850 °C in 10−6 atm. We demonstrate the impact of the sintering protocol, i.e., sintering temperature Ts and pO2, on the phase composition and continuity of Ni electrodes, and on the actuator performance. We discuss the subtle interplay between sintering conditions, microstructure, reoxidation kinetics and piezoelectric performance. The Ni-MLA exhibits a unipolar strain of 0.74 ‰ at 3 kV/mm, a normalized strain coefficient d33* = 247 p.m./V and a loss of tanδ = 0.041.