Dielectric Energy Storage Properties Of （1-x）BaTiO₃-xBiMeO₃-based Bulk Ceramics

Dielectric materials have the advantages of high charge-discharge rate,low energy loss and outstanding thermal and frequency stabilities,which make them increasingly widely applied in electric vehicles,portable power electronic equipments and power distribution equipments.However,the energy storage density of traditional dielectric ceramics is relatively low,it is urgent to develop dielectric ceramic materials with high energy storage density（W）,especially with high recoverable energy storage density(W_rec)and large energy storage efficiency（η）.Compared with traditional ferroelectrics,relaxor ferroelectrics exhibit higher energy density and energy efficiency due to their advantages of high linearity of hysteresis loop（P-E）,large maximum polarization(P_max)and small remnant polarization（P_r）.Therefore,the design and synthesis of relaxor ferroelectric ceramics is considered to be one of the effective ways to prepare novel energy storage materials.As a classical electronic ceramic,Barium Titanate（BaTiO₃,BT）has been one of the research hotspots of energy storage materials.However,a series of defects such as narrow band gap,reducibility of Ti⁴⁺,relatively low breakdown field strength and high residual polarization strength lead to low energy storage density of traditional BaTiO₃ ferroelectric ceramics.In order to improve the energy storage density of BaTiO₃-based ceramics,（1-x）BaTiO₃-x Bi Me O₃ ceramics have been prepared by doping Bismuth based perovskite Bi Me O₃ into BaTiO₃in this work.By adjusting the chemical composition,BaTiO₃-based relaxor ferroelectric ceramics were successfully synthesized.And their structure and properties have been systematic studied.The specific research results are as follows:（1）（1-x）BaTiO₃-x Bi(Zn_3/4W_1/4)O₃[（1-x）BT-x BZW]（0.00≤x≤0.20）ceramics have been prepared via a traditional solid-state reaction using Bi₂O₃,Zn O,WO₃ and BaTiO₃.X-ray diffraction result indicated that Bi(Zn_3/4W_1/4)O₃ was completely dissolved into BaTiO₃ lattice within the range of 0.05≤x≤0.20.The ceramics formed A single ABO₃ structure was formed in the ceramic,and the formation of a second phase was not observed.With the introduction of BZW,Bi³⁺and(Zn_3/4W_1/4)³⁺cations replaced the A and B sites of BaTiO₃,respectively.When x≥0.05,the crystal structure of ceramics underwent tetragonal to pseudo-cubic.As the increase of BZW content,average grain size of the ceramics gradually increased from 2.36μm at x=0.05 to 5.93μm at x=0.20.In addition,it was observed from hysteresis loops（P-E）that modification with appropriate amount of BZW was an effective approach to modulate relaxation behavior of BT,this approach could reduce the residual polarization as well as improve the breakdown field strength（BDS）,and then the energy-storage properties of ceramics could also be effectively improved.Finally,0.85BT-0.15BZW ceramics prepared in this work exhibited the optimum comprehensive energy-storage performance,its W_rec andηcould reach 1.14 J cm^-3 and～92.60%,respectively,at 190 k V cm^-1.（2）The introduction of MgO can effectively improve the resistivity of ceramics and increase the insulation of ceramics.Compared with Zn²⁺,introduction of Mg²⁺can further reduce the average grain size of ceramics,thereby enhancing the BDS of ceramics and improving energy storage density.（1-x）BaTiO₃-x Bi(Mg_3/4W_1/4)O₃[（1-x）BT-x BMW]（0.00≤x≤0.20）ceramics have been prepared via a solid-state reaction using Bi₂O₃,MgO,WO₃ and BaTiO₃.XRD characterization indicated that all samples showed perovskite structure without the formation of a second phase,the tetragonal phase of original BT transformed into a pseudo-cubic phase by introduction of BMW.With the introduction of BMW,Bi³⁺and(Mg_3/4W_1/4)³⁺ions replaced the A and B sites of BaTiO₃ respectively,which transformed（1-x）BT-x BMW ceramics into relaxor ferroelectrics.The average grain size of the ceramics gradually increased from～1.64μm at x=0.01 to～3.75μm at x=0.05,but it decreased from～3.75μm at x=0.05to～2.54μm at x=0.15,then it increased again to～2.83μm at x=20.It was also observed from P-E curves that the addition of appropriate amount of BMW was beneficial to enhance the relaxation behavior of BT.This approach could reduce P_r as well as improve BDS,and then the energy-storage properties of ceramics could be effectively improved.Then,the optimum comprehensive energy-storage performance of W_rec～1.71 J cm^-3 andη～91.97%at 260 k V cm^-1could be achieved in the 0.85BT-0.15BMW ceramic,accompanied with outstanding thermal and frequency stabilities.Additionally,0.85BT-0.15BMW ceramic also showed quick charge/discharge performance and excellent pulse stability.（3）It is widely reported that doping with trace amounts of La³⁺ could improve the ceramics BDS of dielectric ceramics,and then effectively enhance ceramics comprehensive properties.Moreover,introduction of La³⁺could increase the disorder of A-site cation,further disrupting the long-range order of ferroelectric domains and improving the dielectric relaxation properties of ceramics.Therefore,La₂O₃ has been doped into 0.85BaTiO₃-15Bi(Mg_3/4W_1/4)O₃ ceramics in this chapter,and0.85Ba_（1-1.5x）La_xTi O₃-0.15Bi(Mg_3/4W_1/4)O₃（0.00≤x≤0.05）ceramics have been prepared via solid-state reaction method.XRD characterization indicated that all samples show pseudo-cubic phase（paraelectric phase）,and diffraction peaks of samples could be attributed to BaTiO₃ crystal structure,which indicated that La³⁺was successfully dissolved into BT lattice.The dielectric thermal stability and energy-storage properties of ceramics have been significantly improved with the addition of La³⁺.When x=0.02,the optimum energy-storage performance of W_rec～2.70 J cm^-3(at300 k V cm^-1)could be achieved.Although theηhas declined slightly,but it still remained above 79%.In addition,0.85Ba_0.97La_0.02Ti O₃-0.15BMW ceramic exhibited excellent thermal and frequency stability at 160 k V cm^-1.