Structural And Properties Of Doped K_0.5Na_0.5NbO₃-based Lead-free Piezoelectric Ceramics

In the past decades, the (K,Na)NbO3 ceramics, as an environmental friendly lead-free piezoelectric ceramics, has been drawing much attention to the researchers across the world. However, compared with the commercial Pb(Zr,Ti)O3-based ceramics, the KNN ceramics still have some drawbacks such as poor sintering behavior, high volatility of alkaline elements and low electric properties. Therefore, numerous works have been carried out to enhance the performance of the KNN-based ceramics by chemical modification and optimization preparation techniques.In this work, the 0.5Ba(Ti0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3 (abbreviated as BCTZ) and Li0.8Ni0.2Nb0.96O3 (abbreviated as LNN) have been selected as doping components adding into KNN ceramics to improve the properties of KNN-based ceramics. The KNN-based powders were synthesized by liquid-coating method. The systems of K0.5Na0.5NbO3-x[0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3] (abbreviated as KNN-BCTZx, x= 0-0.06)、K0.48Na0.52NbO3-xLi0.8Ni0.2Nb0.96O3 (abbreviated as KNN-LNNx, x= 0-0.040) and [(K0.49Na0.51)1-xLix(Nb0.90Ta0.04Sb0.06)O3 (abbreviated as KNLxNTS, x= 0.04,0.07) were prepared by conventional solid-reaction method. The crystal structure, microstructure, electric properties and the relationships between the structure and properties of KNN-BCTZx ceramics and KNN-LNNx ceramics have been studied, respectively. Meanwhile we investigated the electric-field-driven phase transition process in KNLxNTS (x=0.04,0.07) lead-free piezoceramics.With the addition of BCTZ, the ceramics formed a single phase perovskite solid solutions with the symmetry of orthorhombic at x<0.03. The crystal phase of the ceramics changed from orthorhombic phase to pseudocubic phase when x>0.04. The coexistence of orthorhombic and pseudocubic (tetragonal) phases was observed near room temperature when 0.03≤x≤0.04. The grains grew up obviously ε33T/ε0 when 2 mol% BCTZ was added, but the grain size was found to reduce gradually with further increasing BCTZ content. The TC and TO-T decreased with the increasing BCTZ content. The ferroelectric and piezoelectric properties were abruptly degraded as x≥0.05. Optimum properties (d33= 136 pC/N, kp=27%, kt=26.5%, Qm=25, Pr= 14.67μC/cm2, Ec=11.23 kV/cm, Tc=347℃,= 861.5, tan δ=0.04) were obtained for the ceramic with x=0.03.For the KNN-LNNx system, all the samples exhibited the main orthorhombic perovskite phase structure with adding LNN, and the secondary phase K5.N75Nb10.85O30 was detected when x ≥0.010. Our work revealed that the Ni2+ ions entered into the B-site and lead to the formation of small amounts of the secondary phase K5.75Nb10.85O30. Moreover, the addition of LNN increased the density of the KNN ceramics. With increasing addition of LNN, the Tc increased while the TO-T decreased, respectively. The ceramic doped with 2 mol% LNN exhibited optimum electrical properties with Pr of 23.92μC/cm2, d33 of 110±3 pC/N, kp of 27.15% and k1 of 21.75%. The Qm increased significantly with increasing LNN content, and the maximum value of 195 is obtained at x=0.035.The electric-field-driven phase transition in KNLxNTS (x=0.04,0.07) lead-free piezoceramics has been investigated. After poling under different electric fields, the degrees of orthorhombic phase have been increased, indicating the crystal structures of the ceramics are strong sensitivity to external stimuli. A secondary phase K3Li2Nb5O15 induced by electric field was detected in the ceramic with Li content of 7 mol%, which is close to the Li solubility, and this field-induced secondary phase is resulted from the movement of Li ions and the structural deformation induced by electric field. Moreover, piezoelectric constant d33 increases with the increasing poling field and the enhancement can be attributed to the field-triggered domain switching. The present study implies that in addition to temperature or composition which has been reported in previous researches, electric-field might be an effective way for inducing phase transition in lead-free piezoelectric ceramics and improving the electrical performances.