Optimization Of Energy Storage Characteristics Of A-site Rare-earth Ion-doped BNBST Ceramics Under Low Electric Field Condition

Energy storage capacitors,as the most critical electronic components in pulsed power system circuits,are widely used in aerospace,military,medical,geological exploration and other fields,which have been of great interest to researchers.Energy storage capacitors have very high requirements on energy storage density,charge/discharge rate,discharge density,working stability and other properties.Among them,ceramic capacitors have high dielectric constant,high electric breakdown strength and better temperature stability compared with other capacitors,which have become a research hotspot in recent years.However,most ceramic capacitors have low energy storage density and low energy storage efficiency,which are not conducive to practical applications.In our previous work,an optimized composition of Bi_0.3Na_0.3Ba_0.12Sr_0.28Ti O₃（BNBST）ceramics was obtained using(Bi_0.5Na_0.5)Ti O₃ and(Ba_0.3Sr_0.7)Ti O₃ as end members,which effectively reduced the remnant polarization intensity（P_r from 38μC/cm² to 10μC/cm²）and improved the energy storage characteristics.However,compared with other energy storage materials,the energy storage performance of BNBST ceramics needs to be further optimized.In this work,based on BNBST ceramic system,by introducing trivalent rare earth La³⁺,Nd³⁺and Sm³⁺ions non-equivalently replacing Sr²⁺at A-site,using defect engineering to design Sr²⁺vacancies for charge compensation,the influence of different kinds of rare earth ion doping on phase structure,microstructure,domain structure,dielectric and ferroelectric properties of BNBST ceramics was investigated.The specific research contents are as follows.Firstly,the effect of A-site La³⁺doping amount on the microscopic morphology,phase structure and energy storage characteristics of BNBSL_xT（x=0.00,0.01,0.03,0.05,0.07）ceramics was investigated.The results show that all ceramic samples have a single pseudo-cubic phase perovskite structure.With the increase of La³⁺doping,the dielectric constant of the ceramics first increases and then decreases,and the temperature T_m corresponding to the dielectric peak gradually moves to the lower temperature,leading to the improvement of the temperature stability of BNBSL_xT ceramics.Meanwhile,the remnant polarization P_r decreases significantly,and the“premature saturation”phenomenon of the P-E hysteresis loop disappears,resulting in the effective increase of energy storage efficiencyη.At the optimized composition with x=0.05,the effective energy storage density W_rec（1.56 J/cm³）and energy storage efficiencyη（85.88%）are obtained at a low electric field of 95 k V/cm.In addition,the properties of the ceramic sample have a good temperature,frequency and cycling stability.Secondly,the effect of A-site Nd³⁺doping amount on the phase structure,microstructure,electrical properties and energy storage characteristics of BNBSN_xT（x=0.00,0.01,0.03,0.05,0.07）ceramics was investigated.The results show that the Nd³⁺doping can effectively inhibit grain growth and increase the electric breakdown strength E_b.From the XRD patterns and Rietveld refinement data,it is clear that all ceramic samples have a single-phase perovskite structure.With the increase of Nd³⁺doping content,the amount of the pseudo-cubic Pm-3m phase and tetragonal P4bm phase has little change.Ferroelectric tests show that the P-E hysteresis loop gradually become slender and P_rdecreases significantly.Both high W_rec（1.73 J/cm³）and highη（79.42%）are obtained in optimized BNBSN_0.03T ceramics with good temperature,frequency and cycling stability only at a low electric field of 110 k V/cm.Finally,the effect of Sm³⁺doping amount on the properties of BNBSS_xT（x=0.00,0.01,0.03,0.05,0.07,0.08）ceramics in terms of phase structure,microstructure,electric domain structure,dielectric and ferroelectricity was investigated.Similar to other rare earth ions,Sm³⁺doped ceramics within a certain doping content（x≤0.07）remain as a single-perovskite structure,but when x=0.08,a second phase Bi₄Ti₃O₁₂ is generated.It can be seen from the Rietveld refinement data that although the content of the Pm-3m and P4bm phases has little change,the cell volume and cell parameters（a and b axes）of the P4bm phase do not follow the same trend,the symmetry of the cell decreases,the cation disorder increases,and the dipole orientation distribution in the electric domain is more scattered.The PFM results show that the ferroelectric domains of BNBSS_0.07T ceramics transform into smaller polar nanodomains（PNRs）as compared to BNBST ceramics,which reduces the“premature saturation polarization”phenomenon.The phenomenon of“premature saturation polarization”slows down,and the P_r significantly reduces due to the characteristics of PNRs.In addition,since the polarization rate of Sm³⁺is higher than that of Sr²⁺（:1.30?³,:2.37?³）,the increment of Sm³⁺content delays the decrement of P_max and overcomes the problem of excessive decline of P_max due to dopants.As a result,BNBSS_0.07T ceramics gain both high W_rec（2.2 J/cm³）and highη（85.22%）at a low electric field of 115 k V/cm.BNBSS_0.07T ceramic also has excellent temperature,frequency,and charge/discharge cycle fatigue stability.In addition,the BNBSS_0.07T ceramic sample exhibits a fast charge/discharge rate of t_0.9=119 ns and an ultra-high actual discharge density of W_d（1.52 J/cm³）at room temperature and 80 k V/cm electric field strength.