Dielectric and piezoelectric characteristics of the non-stoichiometric (Na,K)NbO3 ceramics doped with CuO

In this study, the non-stoichiometric composition [(Na0.5K0.5)0.97(Nb0.96 Sb0.04)]O3 + x mol% CuO + 0.2 wt% Ag2O (where x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, respectively) ceramics were fabricated as a function of the amount of CuO addition. Then, their dielectric and piezoelectric properties were investigated. With a 0.8 mol% CuO content, these ceramics (abbreviated as NKNS) showed excellent values of density = 4.459 g/cm3, electromechanical coupling coefficient (kp) = 0.469, mechanical quality factor (Qm) = 540, dielectric constant (ɛr) = 410 and piezoelectric constant (d33) = 70 pC/N at the sintering temperature of 1080 °C, respectively.

Research highlights▶ The (KxNa1−x)(NbySb1−y)O3 ceramics which have an ABO3 perovskite structure, if an A site mol component of (KxNa1−x) is smaller than a B site one of (NbySb1−), can be made as a non-stoichiometric composition. ▶ The addition of CuO is performed with raw materials such as K2CO3, Na2CO3, Nb2O5, and Sb2O5 in the mixing process. ▶ Then, after calcination, excess B site components can be reacted with CuO. ▶ As the results, secondary products such as KaCubNbcOd and KaCubSbcOd are formed. ▶ These secondary products, that are capable of acting as sintering aids, can be sintered in lower sintering temperature and improve both dielectric and piezoelectric properties. ▶ Also, it is well known that the CuO is good sintering aid for low temperature sintering of PZT-based ceramics as well as shifting the tetragonal–orthorhombic phase transition (To-t) toward higher temperature and proved to be an effective acceptor dopant to induce hardening features in NKN compositions. ▶ Furthermore, from the previous result, because Ag2O addition might enhance piezoelectric and dielectric properties and achieve the low sintering temperature with minimized property degradation, Ag2O addition to (K0.5Na0.5)NbO3 system was performed. ▶ Accordingly, in this study, in order to develop the lead-free piezoelectric ceramics with high mechanical quality factor (Qm) with high electromechanical coupling factor (kp) and to increase the tetragonal–orthorhombic phase transition (To-t) for piezoelectric transformers and ultrasonic motors, non-stoichiometric [(K0.5Na0.5)0.97(Nb0.96Sb0.04)]O3 + 0.2 wt% Ag2O composition ceramics as a function of the amount of CuO (x = 0–0.1 mol%) addition were fabricated using a conventional sintering process and their piezoelectric and dielectric characteristics were investigated. ▶ With a 0.8 mol% CuO content, these materials (abbreviated as KNNS) sintered at 1080 °C showed excellent values of density, electromechanical coupling coefficient, mechanical quality factor, dielectric constant (ɛr) and piezoelectric constant (d33) of 4.459 g/cm3, 0.469 pC/N, 540 pC/N, 410 pC/N and 70 pC/N, respectively.