Preparation And Energy Storage Performance Of Barium Titanate Based Lead-free Ferroelectric Ceramics

Eco-friendly dielectric ceramics capacitors play an important role in renewable energy storage,pulse weapons,and medical fields owing to their remarkable characteristics of high power density,rapid charge-discharge rates,and thermal stability.In recent years,due to dielectric/energy storage stability and high energy storage efficiency（η≥80%）,a series of“relaxor end-member”Bi（M）O₃ doped BaTiO₃ lead-free relaxation ferroelectric（RFE）ceramics are considered to be the best energy storage materials for military and special environments.Unfortunately,the relatively small energy-storage density which severely restrict by the low breakdown strength（E_b<500k V/cm）hinders their further applications.In order to meet the increasing integration and miniaturization requirements,how to break through the limitation of low E_b and prepare high energy storage ceramics is a major challenge.In summary,novel Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃（BCZT）-based RFE ceramics are designed and optimized through the adjustment of domain structure,grain size,band gap width,point defects and other internal factors which affect the polarization intensity and/or E_b by doping modification and optimization of sintering process.In addition,the interaction mechanism of phase structure,microstructure,defects and E_b as well as energy storage performance（ESP）is studied in detail.The main research contents are as follows:（1）The introduction of Bi(Mg_0.5Sn_0.5)O₃（BMS）makes the ferroelectric domains of BCZT to transform into high dynamic polarity nano-regions（PNRs）,and thus triggers the ferroelectric（FE）-RFE phase transition.BCZT-BMSx ceramics exhibit obvious dielectric relaxation behavior and approximately linear polarization-electric field hysteresis loop（P-E loop）,which can significantly reduce the residual polarization intensity（P_r）.BCZT-BMSx with x=0.08 ceramic simultaneously achieves an ultrahigh energy storage efficiency（η）～97.6%with a high recoverable energy storage density(W_rec)～1.703 J/cm³ under Eb～300 k V/cm.Ultrahighηis of great significance for further applications.（2）We have selected Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃（BCZT）as a host matrix and prepared novel lead-free RFE ceramics using the"relaxor end-member"of stoichiometric Bi(Mg_2/3Ta_1/3)O₃（BMT）and A-site deficient Bi_2/3(Mg_1/3Ta_2/3)O₃(B_2/3MT),respectively.The interaction mechanism of oxygen vacancies and breakdown strength is investigated by electron paramagnetic resonance（EPR）.The design of A-site deficient combined with B-site donor Ta ion can effectively reduce grain growth（～1.72μm）and leakage current density by inhibiting the formation of oxygen vacancies,thus enhances E_b.As a result,high Eb～570 k V/cm and Wrec～5.22 J/cm³are obtained in BCZT-B2/3MT ceramic.Oxygen vacancies are the key factor affecting breakdown strength of barium titanate based ceramics and these findings provide some reference for the preparation of high energy storage RFE ceramics by defect engineering.（3）New BCZT-based RFE ceramics are successfully designed and fabricated using the"relaxor end-member"of Bi(Zn_2/3Ta_1/3)O₃（BZT）which contain wider band gap Ta⁵⁺donor ion and highly insulated ZnO.Breakdown strength（E_b）and polarization intensity difference(ΔP=P_max-P_r)are significantly improved through the synergistic effect of defect chemistry and energy band engineering.After doping small tolerance factor BZT,an obvious relaxor behavior can be induced in BCZT-BZTx ceramics because of the formation of highly dynamic PNRs,and the positive effect of Bi³⁺on the intrinsic polarization of BCZT ceramic,thereby improvingΔP.Meanwhile,B-site donor Ta⁵⁺ion can reduce grain size and dielectric loss by inhibiting the formation of oxygen vacancies,and thus E_b is further enhanced to～640 k V/cm.BCZT-BZTx with x=0.08ceramic exhibits ultrahigh W_rec of～7.11 J/cm³,η～80.5%,and excellent charge-discharge performance.（4）BCZT-BMSx ceramics with relatively high energy efficiency are taken as the research object,aiming to prepare dense and fine-grained BCZT-BMSx-TSS ceramics by two-step sintering method（TSS）.It is found that BCZT-BMSx-TSS with x=0.08ceramic has small grain size（～1.85μm）and high chemical composition uniformity,and thus obtain high resistivity and grain boundary relaxation activation energy(E_GB).A high E_b～830 k V/cm accompanied by a large maximum polarization contribute to the ultrahigh W_rec～9.153 J/cm³ andη～85.71%.Furthermore,the ceramic exhibits excellent thermal/frequency stability and charge-discharge performances.In summary,we completed the design and preparation of a series of high energy storage lead-free barium titanate based ceramics.Structure-performance relationship of these ceramics have been systematically analyzed and summarized.We believe that this work will be provide a new effective strategy for developing the novel lead-free relaxor ferroelectric ceramics.