Phase Transition And Electrical Properties Of (Bi_0.5Na_0.5)TiO₃-Based Lead-free Ferroelectric Ceramics

Ferroelectric materials are widely used in military and civil industries due to their rich phase transition behavior and complex coupling effects.At present,the dominant applied materials are based on lead containing materials,such as Pb（Zr,Ti）O₃ systems.However,the use of lead gives rise to environmental concerns,which is the driving force for the development of alternative lead-free ferroelectric materials.(Bi_0.5Na_0.5)TiO₃-based lead-free ferroelectrics have attracted much attention in recent years due to their excellent ferroelectric properties,relaxation characteristics,and depolarization behavior.Among them,the(Bi_0.5Na_0.5)TiO₃-BiAlO₃（BNT-BA）possess high remnant polarization P_r and low dielectric loss,which makes it very valuable for scientific research and practical application.However,the phase transition under different external fields still need to be further studied.In addition,the electrical properties could be further optimized.This thesis aims to optimize the electrical properties,study the phase transition under the external field of BNT-BA lead-free ferroelectric ceramics,and to explore the application in pulsed power supplies.The effect of Mn doping and NaNbO₃ adding on the structure and electric properties was investigated.Moreover,the phase transition and crystal structure of BNT-BA based ceramics under electric field,temperature and stress were systematically studied.Particularly,the depolarization behavior of BNT-BA-based ceramics under shock wave compression was studied.Firstly,the thermal and electric field induced structure evolution and phase transition of（1-x）BNT-x BA ceramics were investigated.It was found that the long-range ferroelectric phase（FE）with R3c structure was induced and the rhombohedral distortion was obviously increased after poling.With increasing E_o,the variation of the hysteresis area<A>and P_r clearly suggested three stages of polarization reversal,which were the domain wall motion,then the formation of microdomains within nanodomains or polar nanoregions,followed by the induced macrodomains aligning parallel to the applied electric field.As temperature increased,the induced FE phase irreversibly transformed to relaxor phase（RE）,which corresponded to a gradual structural change from rhombohedral to pseudocubic phase.Therefore,the thermal depolarization behavior of BNT-BA ceramics was proposed to be directly related to the rhombohedral-pseudocubic transition.Secondly,the microstructure,ferroelectric properties and depolarization behavior of 0.96(Bi_0.5Na_0.5)(Ti_1-xMn_x)O₃-0.04BiAlO₃ ceramics were investigated.It was found that the average grain size were enlarged and P_r were enhanced with small Mn addition.The P_r reached an optimal value（⁴1μC/cm²）as Mn content increased up to 0.7 mol%.The enhancement of ferroelectric performance can be attributed to the improvement of sintering performance,the increase of domain size and the reduction of tangent loss.Moreover,appropriate Mn addition（x=0.7%）can improve the depolarization temperature from 140 ^oC to 161^oC,which can be attributed to that Mn doping stabilized the rhombohedral-pseudocubic structure transition to a higher temperature.Thirdly,a new ternary（1-x）(0.98(Bi_0.5Na_0.5)(Ti_0.995Mn_0.005)O₃-0.02BiAlO₃)-xNaNbO₃（0.98BNT-0.02BA-xNN）ceramics was fabricated.The NN solution significantly affect the microstructure,phase transition and pyroelectric properties of0.98BNT-0.02BA-xNN ceramics.It was found that NN addition tends to reduce the rhombohedral phase while favoring the formation of the tetragonal phase.The compositions exhibit excellent pyroelectric performance.The highest pyroelectric coefficient p of 8.45?10^-88 C/cm²K at room temperature was obtained at x=0.03.The optimal figure of merit（FOMs）at room temperature were obtained at x=0.02 with F_i=2.66?10^-1010 m/V,F_v=8.07?10^-22 m²/C,and F_d=4.22?10^-55 Pa^-1/2.Furthermore,the compositions with x?0.02 possess relatively high depolarization temperature（?155^oC）.Those results unveil the potential of 0.98BNT-0.02BA-xNN ceramics for infrared detector applications.Furthermore,the effects of composition,temperature and poling on the phase transition behavior under hydrostatic pressure of 0.98BNT-0.02BA-xNN ceramics were investigated.It was found that hydrostatic pressure can induce FE-RE phase transition.Meanwhile,the transition pressure(P_FR)tends to decrease as the NN addition increases and the temperature increases.The reduced P_FR were considered to result from the reduced FE stability and increasing proportion of RE phase.However,P_FR was obviously enhanced after poling,which can be attributed to the induced FE phase and the formation of macrodomains with application of an electric field.Finally,a current profile with a maximum value of²5 A,an initial rise of less than 0.1?s and a FWHM of².3?s was obtained under shock wave compression for0.98BNT-0.02BA-xNN ceramics.As the external resistance increases,the waveform changes and the oscillations weaken.The maximum breakdown electric field is as high as 4.0 kV/mm.Particularly,the poled x=0.01 ceramics are almost completely depolarized under high strain rate loading,releasing a high charge density J of 38?C/cm²,which was 18%higher than that of PZT95/5 ceramics.The shock-induced depolarization mechanism can be attributed to the FE-RE phase transition.These results reveal the BNT-based ceramics as promising candidates for pulsed power application.