Study On Phase Boundary Construction And Microstructure Modification Of Potassium Sodium Niobate-based Lead-free Piezoelectric Ceramics

Piezoelectric ceramics which achieve the transformation between electrical and mechanical energy, are used in military industries and civil applications for transducers, filters, etc. Potassium sodium niobate (KNN) lead-free piezoceramics are environmental-friendly materials, and have attracted many researches so far. The properties of KNN ceramics can be controlled and adjusted, and in combination with high Curie temperature (-420℃), the families of KNN piezoceramics are considered as promising candidates for lead zirconate titanate (PZT) piezoceramics. Doping modification is an efficient and feasible way to obtain comparable piezoelectric properties to PZT. The dopants enter into the lattice to induce lattice deformation, resulting in polymorphic phase boundary with rhombohedral (R) and tetragonal (T) phase as the boundaries. Mixed phase coexistence generates much more polarization states and is in favor of domain rotation, leading to dramatic property enhancement. Besides, another effective way to optimize sintering behavior and enhance comprehensive properties is to improve the synthesis of KNN particles. So, the work mainly includes two aspects as fellow:1) the modification of the dopants with appropriate ionic radius and electronegativity to construct R-T phase boundary so as to significantly enhance piezoelectric properties; 2) the synthesis of nano or submicro KNN particles by easy hydrothermal method in order to acquire improved piezoelectric properties. The main work and achievements are as follows:(1) KNN piezoceramics modified by Bi(Zn0.5Zr0.5)O3 and Ba0.6Ca0.4ZrO3 were prepared by conventional solid-state reaction method, and the role of the dopants in the formation of R phase as well as the construction of polymorphic phase boundary, and effects of R-O phase boundary on the micro structure and comprehensive properties, were studied in this work. The study on (1-x)K0.48Na0.52NbO3-xBi(Zn0.5Zr0.5)O3 reveals that the substitution of larger ions in A site and the substitution of smaller ions in B site at the same time, helps to form R phase in KNN. Moreover, the results obtained in the research on (1-x)(K0.48Na0.52)(Nb0.96Sb0.04)O3-xBa0.6Ca0.4ZrO3 indicate that weak molecular filed induced by the dopants in A site is bad for the lattice to stretch along c axis and no T phase forms. Therefore, the two materials only achieve R-O phase boundary, and piezoelectric properties are strengthened in some extent. Detailed properties are listed:(1-x)K0.48Na0.52NbO3-xBi(Zn0.5Zr0.5)O3 with x=0.0075, d33～156 pC/N, kp～0.44, Pr～27.5 μC/cm2, Ec～11.2 kV/cm; (1-x)(K0.48Na0.52) (Nb0.96Sb0.04)O3-xBa0.6Ca0.4ZrO with x=0.035, d32～214 pC/N,kp～0.44, Pr～10.4 μC/cm2,Ec～9.18 kV/cm.(2) (1-x)(K0.48Na0.52)(Nb0.96Sb0.04)O3-x[Bi0.5(Na0.7Ag0.3)0.5]0.90Zn0.10ZrO3 ceramics were prepared by conventional solid-state reaction method, and effects of the dopants on the microstructure, the construction of polymorphic phase boundary and piezoelectric properties, were systematically studied. Monoclinic phase with space group of Pmas a transition from T phase to R phase is demonstrated by XRD patterns, Rietveld refinement, temperature-dependent dielectric constants and Raman measurement. When 0.04≤x≤0.05, R-M-T phase boundary is constructed at room temperature and piezoelectric properties are greatly enhanced (x=0.0425时,d33～425 pC/N,d*33～608 pm/V kp～0.43,Pr～14 μC/cm2). Additionally, domain rotation is verified to make main contribution to piezoelectric properties through calculating the proportion of clamping and initial piezoelectric constants (dlatt/dinit), and thus the properties are enhanced in mixed phase coexistence regime with lowered anisotropy energy.(3) (1-x)(K0.40Na0.60)(Nb0.95Sb0.05)O3-x(Bi0.5K0.5)HfO3 piezoceramics were prepared by conventional solid-state reaction method, R phase is investigated in the composition range of 0.03<x<0.05 by simulated XRD patterns, indexed HR-TEM, temperature-dependent dielectric constants, and in the region of R-O-T phase coexistence, dramatically improved piezoelectric properties are obtained (when x=0.035,d33～400pC/N,d*33～614 pm/V, and the decrease of piezoelectric properties is limited in 10% when the temperature is below 200 ℃, suggesting the increase of temperature stability.) Besides, the mechanism of R phase formation and the transition of R-O-T phase is explained in the view of lattice deformation and lowered anisotropy energy induced by the dopants, and the physical model is established.(4) KNN superfine particles are synthesized by hydrothermal method, effects of different K/Na in the initial solution and the reaction time on the products and their microstructure were studied, and piezoelectric properties of as-sintered ceramics were investigated. Phase structure of the products is identified by XRD patterns, TEM analysis and Raman measurements. Piezoelectric properties of as-sintered ceramics increase up 50%(d33-120 pC/N) compared to the ceramics prepared by conventional solid-state reaction method. When K/Na=6,240 ℃/4h, KNN nanorods with the diameter of 100-200nm and excellent orientation of [010] direction are observed. Moreover, growth procedure and the mechanism is reasonably explained.(5) NaNbO3 nanowires are synthesized by one-step method under the hydrothermal condition of 240 ℃ for 4h, and effects of the surfactant and hydrothermal condition on phase structure and the microstructure of the products were studied. Prolonging reaction time results in the formation of submicron NaNbO3 with hexagon shape. Besides, superlattice consisting of rhombohedral and orthorhombic NaNbO3 periodic lattice is detected in the products of Na2Nb2O6·H2O calcinated at 400 ℃ for 3h, which can be applied to nonlinear optical fields.