Effect of heterovalent-ion doping and oxygen-vacancy regulation on piezoelectric properties of KNN based lead-free ceramics

Next generation advanced piezoelectric transducers significantly rely on lead-free piezoelectric ceramics. Previous investigations have indicated that defect engineering by acceptor doping is a preferable approach to customize piezoelectric materials, but this method leads to a trade-off between d33 and the electromechanical qualities (Qm, kp, etc.), which make the collaborate modulation to be a great challenge. In this work, a novel synergistic modulation combing heterovalention donor doping and oxygen-vacancy regulation is proposed to achieving preferably balanced piezoelectric properties. Herein, (K0.5Na0.5)1-xCaxNbO3 ceramics (abbreviated as 100xKNCN) are prepared based on a solid-state reaction method, and the effects of electromechanical properties of potassium sodium niobate (KNN) ceramics are regulated by defect dipoles and oxygen vacancies. The results elucidate that an appropriate quantity of Ca ion doping could construct a rhombohedral-tetragonal (R-O) phase boundary, which is beneficial to the improvement of d33 and Qm. Furthermore, a preferably balance of electromechanical qualities and piezoelectric coefficient is realized by annealing the sample in N2 atmosphere (2KNCN/N2). Compared with 2KNCN, the d33 and Qm of 2KNCN/N2 increase by approximately 30% and 41%, respectively. This work is expected to provide an effective paradigm to develop defect engineering and make it a promising candidate for industrial piezoelectric application.