Electron Microscopy Study Of The Microstructure Of Perovskite Lead-Free Piezoelectric Ceramics

Bismuth sodium titanate(Bi0.5Na0.5)TiO3(BNT)ceramics,potassium sodium niobate(K0.5Na0.5)NbO3(KNN)ceramics and barium titanate BaTiO3(BT)ceramic materials are the three widely studied perovskites lead-free piezoelectric ceramic system.Researchers obtain BNT-based,KNN-based,and BT-based ceramic materials through doping,modification,substitution and changing preparation methods to improve the electrical properties of lead-free piezoelectric ceramics.This article is based on transmission electron microscopy,using the spherical aberration correction transmission electron microscopy with the sub-angstrom resolution to characterize the microstructure of(Bi0.5Na0.5)0.94Ba0.06TiO3(BNBT)ceramics,hot pressing sintered KNN ceramics and 0.9BaTiO3-0.1Bi(Mg0.5Zr0.5)O3(0.9BT-0.1BMZ)ceramics quantitatively on the subangstrom scale.The microstructure of BNBT ceramics prepared by cold sintering process(CSP)and high-temperature annealing was studied to determine the effect of hightemperature annealing on the microstructure of BNBT ceramics;the microstructure of vacuum hot pressing sintered KNN ceramics were explored as well.The crystal structure and interface features of the B-doped KNN and intergrowth phase K3Nb3B2O12 were characterized on a sub-angstrom scale;the domain structure of 0.9BT-0.1BMZ ceramics was quantitatively calculated with picometer accuracy using digital image numerical processing methods.The domain structure and electrical properties of BT ceramics after BMZ doping were analyzed.This work mainly includes the following aspects:(1)The crystal structure,secondary phase,domain structure,grain boundary,and precipitate of cold sintered BNBT ceramics,900? annealed and unannealed samples,were characterized by scanning transmission electron microscopy(STEM)and X-ray energy dispersive spectrum(EDS)to determine the effect of high-temperature annealing on the microstructure of BNBT ceramics.The results show that the 900? annealed cold sintered BNBT ceramics have a R3c trigonal structure,and there exists a secondary Bi2O3 phase at the grain boundary.The secondary phase and the BNBT matrix satisfy the crystallographic relationship of[010]Bi2O3//[011]BNBT and[200]Bi2O3//[100]BNBT.The secondary phase and the BNBT matrix grain boundary grow coherently and exhibits a stepwise like interface.In 900? annealed cold sintered BNBT ceramics,a large number of closured domains were observed,most of which inherent A-site antiphase structure and the domain wall is Na-rich.There are many cubic vapor pore regions with about 5 nm length and nano-scale precipitates in the BNBT parent crystal grains,and the precipitates have Bi element-enriched perovskite structure.Cold sintering with not high-temperature annealing BNBT ceramics have a high density of small crystal grains,and some grain growth at the grain boundaries.There are also anti-phase structure and nano-scale pore regions in the grains.The atom arrangement of the antiphase domains of unannealed samples is similar to the annealed sample,the domain wall is rich of Na elements,exhibits an A-site anti-phase structure.(2)Using scanning transmission electron microscopy(STEM),which is based on high-angle annular dark-field image,selected area electron diffraction(SEAD)and bright-field image(BF)technology,the crystal structure and domain structure of KNN ceramics were studied.The domain orientation of KNN ceramics is along the[110]c,which is the self-polarized direction of orthorhombic phase.The crystal structure and interface of the B-doped KNN intergrowth phase K3Nb3B2O12 were characterized at subangstrom scale.The results show that there is a stepped coherent interface between the K3Nb3B2O12 phase and the matrix in the B-doped KNN ceramic,and the two satisfy the crystallographic relationship of[100]KNN//[001]K3Nb3B2O12,and[010]KNN//[010]K3Nb3B2O12,forming a coherent interface.The interface exhibits a"212121…" step structure of the Nb atoms,and the step region of the interface has a"3111" structure.The included angle between the reconstructed surface of K3Nb3B2O12 and the external exposed surface of KNN is 120°.There are multiple 180°self-symbiotic structures of K3Nb3B2O12 at the interface,and the crystallographic orientations of the multiple 180° self-symbiotic structures and their included angles are illustrated.These self-symbiotic interfaces are consistent with the specific crystallographic angles of 30°and 120° between them.The multiple 180° self-symbiotic interfaces regions have linear growth and Zigzag structure growth.The surface of the K3Nb3B2O12 grains undergoes significant surface atomic reconstruction to achieve a stable state where the surface energy is minimized.(3)The electrical domain structures of 0.9BT-0.1BMZ ceramics and undoped BT ceramics are numerically calculated and analyzed with picometer accuracy and characterized by scanning transmission electron microscopy(STEM)based on high-angle annular dark-field images and high-resolution image numerical algorithms.The results show that when BT ceramics are doped with 0.1BMZ,the residual polarization Pr will decrease,the coercive field Ec will increase,the saturation of the hysteresis loop will decrease,and the area of the hysteresis loop will become smaller.The atomic polarization domain structure in BT ceramics is illustrated,and the average offset of Ti atoms is 22.5 pm.In 0.9BT-0.1BMZ ceramics,the displacement of Ti atoms is suppressed due to elemental doping,and the average polarization of displacement is 0.5 pm.This is consistent with the results that the BT ceramic is transformed from ferroelectric materials to relaxing ferroelectric materials by doping.