Vacancy Defects Regulation And Dielectric Performance Of Bismuth Sodium Titanate-Based Relaxor

With the rapid development of economy and technology,the requirements for integrated and miniaturized energy storage appliances are increasing drastically.Therefore,searching for dielectric materials with large energy storage and strong dielectric stability has become one of the current research hotspots.Sodium bismuth titanate（BNT）has a large dielectric constant,but it possesses dielectric loss in high temperature environment.Therefore,in this thesis,the multicomponent compounding of BNT-based relaxor is prepared,and the dop ing of A/B site to tailor the stoichiometric ratio,adjust the defect mechanism and phase,therefore achieve the purpose of improving dielectric stability and energy storage performance.Here the intrinsic defect and polarization mechanism are investigated in A-site-deficient 0.66(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃-0.28（BixSr1-3x/2□x/2）TiO₃（BNT-BT-BST）relaxors where strontium vacancies（VSr"）are designed to compensate the negative charge shortage when Sr²⁺ is substituted by Bi³⁺（BiSr·）.Incremental Bi³⁺ is conducive to the polarization in nano-regions（PNRs）at high temperatures resulted in an enhancement of the temperature stability of dielectric constant.It reveals a strong defect induced disorder accompanied with slightly increased dielectric loss due to the partial reduction of Ti⁴⁺.Furthermore,the existing oxygen vacancies confirmed by X-ray photoelectron and atmosphere dependent AC impedance spectroscopies are suppressed effectively when sintering at O2 for BNT-BT-BST to the benefit of maintaining stable dielectric properties.Remarkably high energy storage density and efficiency are achieved simultaneously under relatively low electric fields.This work not only provides promising candidates in BNT-based relaxors but also demonstrates an effective route considering the importance of oxygen vacancies in the optimization for dielectric applications.For enhancing the relaxation polarization,refining the P-E loop and improving the dielectric properties,different mass ratios of Nb₂O₅ were doped on 0.66(Bi_0.5Na_0.5)TiO₃0.06BaTiO₃-0.28（Bi2Sr0.7）TiO₃.0.66(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃-0.28（Bi2Sr0.7）TiO₃-x wt%Nb₂O₅（BS-xNb）were prepared by solid-state method,and the structure and electrical properties were characterized.With the increase of the content,the substance in the second phase gradually increases.A small amount Nb₂O₅ doping promotes the dielectric temperature moves to the lower temperature region,which is beneficial to the transition of ergodic relaxation phase to nonergodic relaxation phase and suppress the concentration of VO··.Meanwhile,the ferroelectric long-range order is broken beneficial to the disorder of PNRs at higher temperature and the stable dielectric temperature range is broadened.Furthermore,a small amount of Nb doping can improve the resistivity and reduce the dielectric loss.Moreover,BS-2Nb still has great polarization under medium and low field intensity,and its energy storage density is 1.89 J/cm³ at 170 kV/cm.Under variable temperature and frequency,the energy storage parameters have almost negligible change（ΔPmax=2.36 μC/cm2）indicating a good dielectric energy storage stability.Perovskite solid solutions of SrTi(Cu_1/3Nb_2/3)O₃（STCN）modified 0.8Bi_0.5Na_0.5TiO₃0.2Bi_0.5K_0.5TiO₃（BNT-BKT）in the vicinity of morphotropic phase boundary composition were prepared.The dielectric and impedance properties associated with their structural evolution are investigated.Strong unsteadiness and large hysteresis（dielectric loss）under elevated temperature and variational frequency restrict the development of BNT-BKT based ceramics.Here we show a distinct phase transition from ferroelectric to ergodic relaxor phase that occurs with increasing STCN.Incremental chemical defects resulted from the substitution of the A/B site facilitate the distributions of polar nanoregions（PNRs）.The flatter and smoother dielectric curves between Ts and Tm illustrate good temperature stability.The complex impedance spectrums demonstrate the compositions possess a mixed electronic and ionic conduction mechanism.More oxygen vacancies can be introduced into the lattice contributing to a decremental impedance as the increased dopant.Meanwhile,Nb^·_Ti will suppress p-type conduction and reduce dielectric loss.Favorable stabilities of dielectric permittivity,energy storage（2.57 J/cm³ under 220 kV/cm）and electrostrictive strain are achieved attributed to the existence of nanodomains.