Preparation And Characterization Of The NKN-Based Lead-Free Piezoelectric Ceramics

In this thesis, three series of NKN-based perovskite lead-free piezoelectric ceramics, (Na_0.5K_0.5)NbO₃(NKN), x(Na_0.5K_0.5)NbO₃-(l-x)LiNbO₃(NKN-LN),and y(Na_0.5K_0.5) NbO₃-(1-y)LiNbO₃(NKN-LT), were prepared by conventional ceramics processing. Furthermore, the samples modified by doping CuO, Fe₂O₃, Mg, and Mn were also prepared by the same process. X-ray diffraction, scanning electronic microscope, impedance analyzer and piezoelectric workstation were used to determine the crystal structure, micrographic topography, dielectric, piezoelectric and ferroelectric properties of NKN—based ceramics. Our work includes:1. NKN-based lead-free piezoelectric ceramics with pure perovskite structure were obtained by the conventional mixed oxide method. The sintering temperatures, depending on the variation of compositions, are in the range between 1065℃and 1120℃. The sintering temperatures of the ceramics are sensitive to the variation of compositions. In addition, a small amount of modifiers could improve the sinterability of the ceramics effectively.2. The crystal structure of NKN-LN and NKN-LT ceramics was investigated by XRD analysis, and the morphotropic phase boundary (MPB) of the ceramics was also determined. It is revealed that the perovskite structure of NKN-LN ceramics transforms from orthorhombic phase to tetragonal phase when the LN content is 6%. And the perovskite structure of NKN-LT ceramics transforms from orthorhombic phase to tetragonal phase when the LT content is in the range between 5% and 7%, where orthorhombic and tetragonal phases coexist.3. Both the piezoelectric constant (d₃₃) and the planar electromechanical coupling factor (k_p) reach maximum near the MPB. The d₃₃ of NKN-6LN and NKN-6.5LT ceramics is 210 pC/N and 232 pC/N, respectively. The Curie temperture (T_c) of NKN-LN and NKN-LT ceramics increases with increasing of LN and LT content, respectively. The ceramics possess Curie tempertures (up to 470℃).4. The sinterability of NKN-based ceramics could be improved by adding CuO and K₄CuNb₈O₂₃. The bulk density of the ceramics could reach more than 96% of the theoretical values when 5% CuO and K₄CuNb₈O₂₃ were doped, respectively.5. The mechanical quality factor (Q_m) could be improved from about 100 of pure NKN cermics to about 1000 of K₄CuNb₈O₂₃-doped NKN ceramics. Nevertheless, if Na⁺ or K⁺ were replaced partly by Li⁺, the Q_m does not show significant improvement by doping K₄CuNb₈O₂₃.6. The piezoelectric properties of most of the NKN-based ceramics decreased after 4-6 months. The decreasing of piezoelectric properties of NKN-LN ceramics is more evident. However, the time aging didn't affect samples modified by Fe₂O₃ and K₄CuNb₈O₂₃, which suggested that the doping of Fe₂O₃,K₄CuNb₈O₂₃, etc. can improve the stability of NKN based ceramics.7. The remanent polarization (P_r) of NKN-LN and NKN-LT ceramics increases with increasing of LN and LT content, respectively. The maximum value of P_r reaches up to 51μC/cm². For pure NKN ceramis, the coercive filed (E_c) is 1050V/cm. The E_c of NKN ceramics increases to 1300 V/cm and 1500 V/cm by adding CuO and 6% LN, respectively. Doping of Fe₂O₃ and high content of solid solution of LN or LN lead to leakage current's increasing.