Structure, Dielectric, And Piezoelectric Properties Of Rare Earth Doped Sodium-potassium Niobate Based Lead-free Piezoelectrics

KNN based ceramics with good piezoelectric properties and high Curie temperature are attracting more attention and emerging as a promising lead free piezoelectrics candidate. Nevertheless, the dielectric and piezoelectric properties still need further significant improvement. In addition to the optimum of processing parameters, A-site or B-site doping of KNN based lead free piezoelectrics are being extensively studied to improve the above-mentioned performance. Rare earth elements (Re=La, Ce, Pr, Nd, Sm, Eu, Ga, Dy, Er, and Lu) doped PbTiO3, (Pb0.5Zr0.5)TiO3, BaTiO3, (Bi0.5Na0.5)TiO3, SrTiO3 ceramics have been widely studied and reported. The RE elements doping was found to have a strong effect on the MPB (Morphortropic Phase Boundary) by the phase transition among the tetragonal, cubic, and orthorhombic phases in ceramics of perovskite structure.In this thesis, the structure, phase transition, dielectric, and piezoelectric properties of rare earth Nd, Dy and Ho doped in 0.942(Na0.53K0.47)NbO3-0.058LiNbO3 lead-free piezoelectric ceramics with x=0.1%, 0.5%, 1%, 2%, and 4% synthesized by conventional solid state synthesis method were studied. XRD patterns show the single phase KNN is dominant with a small amount of secondary phase for ceramics calcinated at 750oC and sintered at 1080oC. Infrared spectra and Raman spectra results showed that the bond of NbO6 regularly changed with doping content. XRD combining with the IR and Raman show that the crystal structure and phase transition can be influenced by Re doping into the A-site ions in KNN-LN. With the increasing Dy, Nd and Ho doping, successive phase transition took place from coexistence of tetragonal and orthorhombic to tetragonal and cubic. MPB consisting of the tetragonal and cubic phases is identified to form around 1%~2% Re doping content. These results show that the MPB region can be adjusted by small amount of Re doping. Although it is still relatively low, the density of rare earth doped KNN-LN is higher Re doping content than that of undoped KNN. It increases with the increasing content. It is interesting we observe that the morphology, grain size, and microstructure observed by SEM significantly change for Re-doped KNN-LN ceramics. In general, the grain size of Re-doped KNN ceramics is more homogeneous and much smaller than those of undoped KNN ceramics. Curie temperature Tc and loss tangent are characterized by impedance analyzer meter, the results show that Curie point is shifted to higher temperature above 400oC and loss tangent of doped KNN-LN at room temperature is reduced to a certain extent. Piezoelectric constant d33 and dielectric constant are significantly improved by rare earth doping. This is believed to be related to the shift of MPB. The highest d33 with 128pC/N appears in ceramics with 2% Nd doping and its dielectric constant is 694. In contrast, the d33 and Curie temperature of undoped KNN are 87pC/N and 544.7, respectively. We do not observe the significant change of Curie temperature in Re-doped KNN-LN lead-free piezoelectrics. These findings indicate that Re doping can be used to effectively improve the dielectric and piezoelectric properties of KNN–LN ceramics by tailoring the MPB region and microstructure.