| Piezoelectric ceramics are a class of multifunctional materials that are widely used.However,lead-based piezoelectric ceramics firmly occupy the market at present,which is not conducive to the sustainable development of the environment.Therefore,the study of environmentally friendly high-performance lead-free piezoelectric ceramics has become a research focus.In many lead-free systems,(Bi0.5Na0.5)TiO3(BNT)-based lead-free piezoelectric materials have attractive application prospects in the fields of high-power ultrasonic transducers and highprecision actuators due to good comprehensive piezoelectric properties.This thesis takes BNT-based ceramics as the research object,and improves its piezoelectric properties such as piezoelectric response,temperature stability and electrostrain.Importantly,in situ synchrotron X-ray diffraction(SXRD)and total scattering method are used to analyze the evolution of phase structure,domain texture and lattice strain as a function of the electric field,and clarify the structural mechanism of high piezoelectric properties.The representative(1-x)(Bi0.5Na0.5)TiO3-xBaTiO3(BNT-BT,x=0.06,0.07)ceramics around morphotropic phase boundary(MPB)were investigated via in situ SXRD,tran smission electron microscopy(TEM).The dynamic and static structures of the components were thoroughly investigated.It is found that both components present the coexistence of P4bm and R3c phases.The structural distortion of x=0.07 is significantly smaller than x=0.06,showing a significant improvement in the electric field-induced phase transition and structural flexibility.Thermodynamic calculations demonstrate that x=0.07 possess a flatter Landau free energy,which is responsible for the enhancement of piezoelectricity.Based on the results of prototype BNT-BT,BiAlO3 with a larger tolerance factor is introduced into BNT-BT to form BNT-BT-BiAlO3,which achieves the reduced structural distortion and the oxygen octahedron tilting angle,and activated phase transition.Enhanced piezoelectric response(d33)and piezoelectric strain are obtained.By quantifying the lattice strain and domain switching,it is found that the calculated total lattice strain is consistent with the measured piezoelectric strain.The calculated total lattice strain includes the contribution of domain switching and lattice strain,indicating the macroscopic strain is mainly from the synergistic effect of lattice strain and domain switching.The d33 of BNT-BT is usually enhanced by chemical substitution,while the depolarization temperature(Td)is increased via introducing point defects(A-site defects or oxygen vacancies).Combining two factors,a new method of introducing oxygen vacancy perovskite(ABO2.5)is proposed,and BNT-BT-BaAlO2.5 ternary systems are prepared to improve the comprehensive performance.Meanwhile,insitu SXRD techniques,TEM,and first-principles calculations are used to reveal the relationship between dynamic structural evolution and piezoelectric properties.It demonstrates that the high d33 originates from the enhanced lattice strain and domain switching,and the improved Td originates from the enhanced tetragonality induced by oxygen vacancies.Through the method of introducing ABO2.5,oxygen vacancies are ingeniously introduced,which provides the possibility to achieve a giant piezoelectric strain.BNT-BaAlO2.5 with a simple composition is prepared,and the largest electrostrain(1.12%)and relatively high d33*(1330 pm/V)in the lead-free system are obtained.By performing reverse Monte Carlo(RMC)simulation of pair distribution function(PDF),the polarization information of the defect and electric dipoles are extracted,and the local structure mechanism of the giant piezoelectric strain is elucidated.Based on the in-depth study of the refine structure under the electric field,the relationship between the evolution of the microstructure and macroscopic performance is clarified.A new method is proposed to design high-performance lead-free piezoelectric system,which provides a basis for the design of highperformance materials. |
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