Preparation And Modification Of K_0.5 Na_0.5 NbO₃-based Lead-free Piezoelectric Ceramics

Lead-free materials are of interests as new candidates to replace the widely used lead-based ceramics because of their pollution free environmental friendly character during the preparation process. Among different lead-free materials, the K0.5Na0.5NbO3 (KNN)-based ceramics have attracted significant attention due to their outstanding dielectric and electromechanical properties and high Curie temperature (Tc). However, compared with the commercial Pb(Zr,Ti)O3-based ceramics, the piezoelectric performance of the KNN-based ceramics need to be further improved. In recent years, numerous studies have been devoted to KNN-based ceramics, as a result, the piezoelectric performance of the ceramics have been enhanced significantly. However, there are a few problems for the KNN-based ceramics need to be solved. The mechanical quality factor and the temperature stability of the electrical properties of KNN-based ceramics need to be improved.In this work, the KNN based powders were synthesized by liquid-coating method. The ceramics of (1-x)(0.96K0.5Na0.5NbO3-0.04LiSbO3)-xBi0.5Na0.5TiO3 (KNN-LS-BNTx,0≤x≤0.02), (1-x)(0.99K0.5Na0.5NbO3-0.01Bi0.5Na0.5TiO3)-xLiSbO3 (KNN-BNT-LSx,0≤x≤0.10), (1-x)(KMN-BNT-LS0.05)-xNaTaO3 (KNN-BNT-LS-NTx,0≤x≤0.10), and KNN-BNT-LS-NT-CuQx(0≤x≤2.0) have been prepared with conventional ceramic technique. The effects of BNT, LS, NT and CuO on the crystal structure, phase transition, ferroelectric, dielectric and piezoelectric properties of KNN-based ceramics were investigated, and the related physical mechanisms were discussed.For the ceramics of KNN-LS-BNTx and KNN-BNT-LSx, Tc and TO-T of the ceramics were shifted to lower temperature with addition of BNT or LS. A polymorphic phase transition (PPT) between orthorhombic and tetragonal phases near room temperature was identified in the composition range of 0.03≤x≤0.04 for KNN-BNT-LSx ceramics. A trace amount of secondary phases K3Li2Nb5O15 and LiSbO3 can be detected when x≥0.06 for KNN-BNT-LSx ceramics. The ceramics with a PPT near room temperature exhibited enhanced piezoelectric properties. The piezoelectric coefficient, d33, of KNN-BNT-LS0.04 reached the maximun value of 250 pC/N, where the values of other electrical properties were as follows:the relative permittivity,ε33T/ε0=640, loss tangent, tanδ=0.031, mechanical quality factor, Qm=33.3, electromechanical coupling factors,κp=0.294 andκt=0.352.In order to improve the electromechanical coupling coefficient and the temperature stability of the electrical properties of KNN-based ceramics, NT was added into the system of KNN-BNT-LS to form the KNN-BNT-LS-NTx ceramics. The effects of NT on the structure and electrical properties of the ceramics were investigated. It was found that the lattice parameter c, tetragonality cla and the unit cell volumes of the KNN-BNT-LS-NTx ceramics declined with addition of NT. The grain size of the ceramics increased and the microstructure of the ceramics became homogeneous with addition of NT, as a result, the density of the ceramics was enhanced. Tc and TO-T have been shifted to lower temperature with addition of NT, the TO-T of the ceramics was below room temperature at x=0.06. It was found that kp and kt of the ceramics increased with increasing x, while remanent polarization, Pr, and coercive field, Ec, decreased, which suggested that the addition of NT weakened the ferroelectric properties of the ceramics. The dielectric and piezoelectric properties of ceramics were enhanced by addition of NT. The outstanding electrical properties ofκp=0.442,κt=0.426, d33=249 pC/N, Qm=51,ε33T/ε0= 1604, tanδ=0.023, Pr=17,3μC/cm2 and Ec=16.1 kV/cm were obtained at x=0.06.The KNN-based ceramics doped with CuO were prepared and studied with the purpose of enhancing the mechanical quality factor of the ceramics. The phase structure of KNN-BNT-LS-NT-CuOx ceramics changed from tetragonal to orthorhombic with addition of CuO. A trace amount of secondary phases K4CuNb8O23 can be detected when x≥1.5. The microstructure of the ceramics became more homogeneous and the grain of the ceramics shrinked a little with addition of CuO. With increasing x, TO-T and Qm of the ceramics raised while Pr, Ec,ε33T/ε0, tanδ, d33,κp andκt decreased. The maximum Qm value of 166 was obtained at x=1.5, where the values of PT=11.2μC/cm2, Ec=11.2 kV/cm,ε33T/ε0=1300, tanδ=0.0135, d33=183 pC/N,κp=0.404 andκt,=0.388.