Study Of The Dielectric Relaxation Modulation,Electromechanical Properties And Mechanisms In NaNbO₃-based Morphotropic Lead-free Piezoelectric Ceramics

Environmentally-friendly lead-free piezoelectric and ferroelectric ceramic materials have become an imperative requirement due to the social sustainable development strategy in recent years.Alkaline niobate-based lead-free materials have attracted extensive attention due to their relatively superior piezoelectric properties.However,the drawback of the temperature instability of the piezoelectric and electromechanical coupling properties for most of the reported potassium sodium niobate-based multicomponent lead-free ceramics severely restricts the practical usage of these materials in the piezoelectric devices.Besides,the processing sensitivity seemed to exist in（Na,K）NbO₃（NKN）-based lead-free compositions owing to the existence of highly hygroscopic potassium carbonates and the high volatility of K2I at high temperatures.In view of the above-mentioned problems,NaNbI3（NN）was chosen as the matrix in this thesis.The macroscopic relaxor behavior of NN-based materials was modulated by optimizing the compositional design,and then morphotropic phase boundary（MPB）between rhombohedral（R）and tetragonal（T）phases was designed in current lead-and potassium-free ferroelectric ceramics.Moreover,the structure mechanism of dielectric relaxation behavior and electrostrictive properties were also systematically studied together with a further discussion on the piezoelectric,electromechanical coupling performance,high-field strain behavior and their related structure mechanism.The main contents are outlined as follows:（1）In chapter 2,the ferroelectricity of NN was stabilized by the addition of BaTiO₃（BT）.With the substitution of BT for NN,the room-temperature phase structure gradually changed from antiferroelectric orthorhombic（OAFE）to ferroelectric orthorhombic（OFE）phase to ferroelectric T（TFE）phase and then to relaxor pseudocubic（PC）phase.Excellent electrical properties were achieved at the OAFE-OFE,OFE-TFE and nonergodic-ergodic relaxor phase boundaries.The weak relaxor behavior of NN-BT binary lead-free ceramics was analyzed at multiple length scales by means of in-situ synchrotron x-ray diffraction（XRD）,Raman spectra and x-ray absorption fine structure（XAFS）.With increasing BT content,the average symmetry seemingly evolved from a T to a PC phase according to the conventional XRD results.However,the in-situ high resolution synchrotron XRD indicated that the local structure of the polar nanoregions（PNRs）in NN-BT relaxors was still tetragonal symmetry.Afterwards,XAFS analysis further proved that the local spontaneous polarization of PNRs was still along[001]direction for Nb in NbO6 octahedron.With the increase of dielectric relaxation,an obvious softening of the B-O vibration would be found from Raman results.Because of the difference of Nb（[001]）and Ti（[111]）displacement vectors,the long-range ferroelectric ordered domains would be disrupted into PNRs.（2）In chapter 3,the NN-based relaxor ferroelectric ceramics were found to have excellent electrostrictive properties.A hysteresis-free large electrostrictive strain was generated in（1-x）NN-xBT binary lead-free relaxor ferroelectric ceramics owing to both high Q33 and high dielectric permittivity.Theoretical analysis indicated that Q33 should be strongly correlated with chemical species of cations in a perovskite structure in accordance with the Q33～Σq2/z33-*2.In this thesis,the high Q33 and high dielectric permittivity of NN-based ceramics were ascribed to the strong A-O ionic bond and B-site ionic ferroelectric displacement,respectively.Afterwards,tunable electrostrictive properties were achieved in（1-x）NN-x（Bi0.5Na0.5）TiO₃（（1-x）NN-xBNT）lead-free binary system.Interestingly,temperature and frequency insensitive large electrostrictive strains（～0.196%@ 20 kV/mm）were generated in the range of 25～250 ℃ and 0.01～100 Hz for the 0.24NN-0.76BNT ceramic.The study suggests that the thermally stable electrostrictive strain was mainly related to the temperature insensitive high dielectric response,suggesting the great potential of these NN-based electrostrictive materials for the application of high-precision ceramic actuators.（3）In chapter 4,by introducing the R phase stabilizer ABO₃（CaZrO₃（CZ）,BaZrO₃,SrZrO₃,BiFeO₃ and NaSbO₃）into T-phase 0.9NN-0.1BT ceramics,a novel R-T MPB perpendicular to the composition axis was successfully achieved in the NN-BT-ABO₃ ternary system.According to the TEM observation and Rietveld refinement results,the MPB compositions were coexisted with P4mm and R3c phases.Combining with the temperature dependence of in-situ synchrotron XRD,dielectric and ferroelectric properties,it could be further determined that the coexistence of R and T phases was temperature stable,indicating that the phase boundary was morphotropic in nature,which is similar to the phase boundary properties of the traditional Pb（Zr,Ti）O₃ system but different from that of the lead-free systems such as NKN and BT etc.Excellent piezoelectric and electromechanical properties could be achieved in the wide temperature range（d33～208-305 pC/N,kp～31%-36%,ε33T/ε0～1598-2815,Qm～114-295 and Tc～80-149 ℃）.This study provides new opportunities and ideas for the design of environmentally friendly piezoelectric materials.（4）In chapter 5,the evolution of the phase structure and domain structure of NN-BT-xCZ lead-free ceramics during poling process at room temperature was studied in terms of the ferroelectric,piezoelectric,in-situ synchrotron XRD and TEM,revealing the structure mechanism of the enhanced electromechanical properties in the compositions close to the MPB.Nanodomains could be found in TEM images for the MPB compositions owing to the existence of mismatch stress caused by hetero-phase coexistence.The reduction of the domain size suggested the decrease of the domain wall energy,causing the reconstruction of the domain（growth,orientation and symmetry change）more easily under the application of the electric field.As a result,extrinsic piezoelectric contribution was enhanced.In-situ synchrotron XRD measurements suggested that the initial R-T phase coexitence would be destroyed and reconstructed into low symmetry MA-T coexisted phase after poling for the compositions close to the MPB,resulting in the enhancement of the intrinsic piezoelectric contribution.（5）In chapter 6,the high-field electrostrain behavior as well as its temperature stability of the MPB composition of 0.875NaNbO₃-0.1BaTiO₃-0.025CaZrO₃ ceramic was studied.An electric field induced low-hysteresis（<13%）strain（-0.15%@ 6 kV/mm）varying within less than ±10%of its room temperature value in the temperature range of 25-180 ℃ was found in this ceramic.The mechanisms for generating strain on heating were quantitative analyzed by means of in-situ synchrotron XRD.The contributions for the high-field strains could be discussed from three stages on heating.Temperature insensitive piezoelectric strains were obtained from room temperature up to 140℃ owing to the thermally stable R-T phase coexistence.The enhancement of strains in the proximity of the Curie temperature（from 140 to 175 ℃）was fundamentally attributed to the growth of PNRs into R microdomains and subsequent field-induced R-T phase transition as well as the further alignment of T domains to the direction of electric field.Above 175 ℃,giant electrostrictive strains were generated.In addition,the low strain hysteresis is mainly attributed to the low strain contribution from domain switching.