Phase Boundary Structure And Electrical Properties Of KNNLT-BNT Based Lead Free Piezoelectric Ceramics

Sodium potassium niobate-based ceramics have become one of the hot spots for research on lead-free piezoelectric materials due to their high electrical constants and Curie temperatures.One of the main ways to improve piezoelectric materials is by constructing component phase boundaries.Compared with morphotropic phase boundaries（MPB）constructed from lead-based materials,the piezoelectric properties and temperature stability of polycrystalline phase transition（PPT）components constructed from sodium potassium niobate-based materials are yet to be improved.In this paper,the phase structure of KNN-based ceramics will be tuned by solid solution and elemental doping with different systems of ceramics to investigate the mechanism of influence on electrical properties.Firstly,ceramics of（1-x）KNNLT-x BNT（x=0.01,0.02,0.03,0.04）systems were prepared by the conventional solid-phase reaction method.The phase structures of the KNNLT-BNT ceramics were characterized and analyzed,and both were orthorhombic-quadratic polycrystalline phase transition structures at room temperature.The optical band gap of the ceramics was investigated,and it was found that Bi³⁺and Ti⁴⁺entered the lattice to produce distortion to widen the band gap,and the optical band gap of0.97KNNLT-0.03BNT ceramics widened to 3.15 e V.The dielectric constant of0.98KNNLT-0.02BNT ceramics can reach 1229,while the increased relaxation coefficient of the ceramics allows the ceramic properties to maintain a high level within a certain temperature range.By analyzing the piezoelectric properties of ceramics,it is found that the high dielectric response is also beneficial to the piezoelectric properties,but the improvement of the piezoelectric properties of ceramics is limited by the influence of the electrostriction coefficient Q₁₁.By analyzing the dielectric intrinsic properties of the ceramics,it was found that the dielectric intrinsic values of the ceramics improved significantly,indicating that the main reason for the improvement of the dielectric properties was the change of the crystal structure.Secondly,The electrical properties of the KNNLT-BNT ceramics were further improved by doping with La₂O₃ or Mn O₂.After doping with La₂O₃,the 0.97KNNLT-0.03BNT ceramics transformed into a tetragonal-cubic mixed phase at room temperature,and the optical band gap dropped to a minimum of 3.01 e V.The dielectric properties are substantially improved,with the dielectric constant at room temperature increasing to 1972,but the piezoelectric properties decrease,indicating that the phase structure seriously affects the electrical properties of the ceramics.After doping with Mn O₂,the ceramic remains as an orthorhombic-quadratic polycrystalline phase transition structure at room temperature,and the optical band gap decreases significantly,down to 2.66 e V,while the dielectric properties are greatly improved,with a dielectric constant of 1924 at room temperature when the doping amount is0.004,but the piezoelectric properties decrease significantly.Therefore,KNN-based ceramic materials possessing high dielectric properties were prepared by solid solution of BNT system,La₂O₃ or Mn O₂ doping of KNNLT ceramics and modulation of phase structure,which confirmed the development prospect of KNN-based piezoelectric materials possessing high electrical properties.