| Energy storage ceramic capacitors have been widely used in pulse-power technology owing to their high power density,fast charge/discharge speed,good mechanical properties,corrosion resistance,good thermal stability and long life cycle.In recent years,the leading development trend of modern electronic products is lead-free,miniaturization,integration and lightweight.Thus development the lead-free ceramic capacitors with high energy density has significant practical and strategic significance.There are two bottlenecks for their development.First,high energy storage density(Wrec)and high energy storage efficiency(?)are difficult to obtain simultaneously in one ceramic capacitor.Second,obtaining high Wrec need ultrahigh electric field usually,which inhibits its practical application.In order to solve the problems,this article selects Bi0.5Na0.5TiO3(BNT)which has high energy storage potential as the research object.Intends to through the component design to prepare the BNT-based ceramics with high Wrec,high 77 and high energy storage potential(?)at the same time.The effects of phase structure,breakdown electric field,tolerance factor,ionic radius and ionic valence on energy storage properties of BNT-based ceramics is studied in detail.Deeply analyze the physical mechanism of high energy storage properties in ceramics.For preparing high energy storage properties ceramics provides new ideas.The main innovative results in this work are as follows:1.The(1-x)BNT-xSr0.7Bi0.2TiO3(abbreviated as(1-x)BNT-xSBT)ceramics with decent energy storage density and good thermal stability are prepared successfully.The physical mechanism of phase structure regulation in(1-x)BNT-xSBT ceramics is clarified.The microstructure,phase structure,relaxation characteristics,energy storage properties of ceramics are systematically studied.With an increase in the SBT content,the perovskite structure gradually changes from a polar rhombohedral phase to a weak polar pseudocubic phase,and the cell volume is inflation.Besides,the mean grain size is decreased from 3.79?m for BNT ceramic to 1.27?m for 0.55BNT-0.45SBT ceramic.Pure BNT is a typical ferroelectric at room temperature,along with large saturated polarization and remanent polarization,the Wrec is only 0.05 J/cm3.With increasing of SBT content,the relaxation ferroelectric phase gradually moving towards room temperature from high temperature range(200??320?),because of the lower Curie temperature(-75?)of SBT,which is far below 320?.The introduction of Sr2+ causing inhomogeneity of the component distribution in ceramics,which results in the widened dielectric peak,then the temperature stability of dielectric constant is improved.Besides,the Sr2+ has different ionic radii and valence from the Bi3+and Na+ in the A site,which can disrupt the long-range ferroelectric order,leading to an increment of relaxation behaviour,resulting P-E loops become thinner.The ceramic of 0.6BNT-0.4SBT possesses a decent Wrec(2.2 J/cm3),a high ?(75%)and a wider temperature stability range(28?326?)of dielectric constant.2.The rare earth ions of La3+and Sm3+are introduced into the A-site of SrTiO3(ST)ceramics,the ceramics of(1-x)BNT-xSr0.7Sm0.2TiO3(abbreviated as(1-x)BNT-xSST)and(1-x)BNT-xSr0.7La0.2TiO3(abbreviated as(1-x)BNT-xSLT)are prepared successfully.The physical mechanism for high energy storage properties is illustrated.It is found that the Sm3+can improve the electric field of phase transition(EF)and the La3+can enhance the breakdown electric field(Eb)significantly.The effects of Sm3+and La3+doping in BNT-ST ceramic on structure,electric properties and energy storage properties are systematically studied.Due to the ionic radius of Sr2+(1.44 A)is larger than the host ions,the perovskite structure gradually changes from a rhombohedral phase to a pseudocubic phase,and the cell volume is inflation after doped SST and SLT.The grain size of BNT is 3.79 ?m,the cersmics of 0.5BNT-0.5SST and 0.5BNT-0.5SLT show smaller grain size and are 0.56?mand 0.62?m,respectively.The dielectric constant temperature stability range of 0.6BNT-0.4SST and 0.5BNT-SLT are-46?331? and-22?230?,respectively.Compared to La3+,Sm3+is more helpful in optimizing the dielectric constant temperature stability range of ceramics.Besides,the SST can also effectively reduce the t of BNT ceramic,As x increases from 0.2 to 0.5,the Er improves from 70 kV/cm to 180 kV/cm.Due to the high EF exists in 0.6BNT-0.4SST ceramics,it exhibits good energy storage properties:Wrec=3.52 J/cm3,?=84%.At room temperature and 1 kHz,the dielectric constant of 0.6BNT-0.4SST and 0.55BNT-0.45SLT ceramic are 1100 and 1536,respectively,suggesting the La3+is more helpful in improving the dielectric constant of BNT-ST ceramics,which is contributed to boost the polarization intensity.Besides,the band gap and resistivity of BNT are also enhanced after doped SLT,which identified the insulation performance is optimized.The Eb of 0.55BNT-0.45SLT ceramic is significantly improve to 315 kV/cm,and the value is far high 260 kV/cm of 0.6BNT-0.4SST ceramic.Ultimately,the 0.55BNT-0.45SLT ceramic displays very good energy storage properties:Wrec=4.14 J/cm3,?=92.2%.3.The different valence ions of Sn4+and Ta5+are introduced into the B-site of ST ceramics,the ceramics of(1-x)BNT-xSrTi0.8Sn0.2O3(abbreviated as(1-x)BNT-xSTS)and(1-x)BNT-xSrTi0.8Ta0.16O3(abbreviated as(1-x)BNT-xSTT)are fabricated successfully.The relationship between point defects and energy storage properties are discussed.The effect of Sn4+and Ta5+doping in BNT-ST ceramic on crystal structure,microstructure,relaxation behavior,defects and energy storage properties are systematically studied.In the ceramics of(1-x)BNT-xSTS,on the one hand,the defect of Ti'Ti can be inhibited duo to Sn4+replaces Ti4+.On the other hand,STS also can decreases the average grain size.As x increases from 0 to 0.5,the average grain size reduces from 3.79?m to 1.1?m,and the Eb increases from 120 kV/cm to 215 kV/cm.Eventually,the 0.6BNT-0.4STS ceramic exhibits a decent Wrec of 2.47 J/cm3 and ? of 83.2%.In the ceramics of(1-x)BNT-xSTT,in order to meet the requirements of material is electrically neutral,STT not only inhibits the defect of Ti'Ti,but also reduces oxygen vacancy concentration.Compared with Sn4+,Ta5+ is more heple in reducing defects concentration.The average grain size is 0.47?m and 1.18?m,while the Eb is 245 kV/cm and 205 kV/cm for 0.6BNT-0.4STT and 0.6BNT-0.4STS ceramics,respectively.Due to the lower defects concentration and higher Eb,0.65BNT-0.35STT displays a larger Wrec of 3.30 J/cm3 and a higher ? of 90.4%.4.The antiferroelectric of AgNbO3(AN)is introduced into 0.6BNT-0.4SBT ceramic,the(1-x)(0.6BNT-0.4SBT)-xAN ceramics with high energy storage density and high energy storage potential are fabricated successfully.The effect of oxygen vacancy concentration,grain size,relaxation behavior and tolerance factor on energy storage properties are systematically studied.The impedance analysis results show that the AN can improve resistivity and activation energy,then improve the insulation performance of 0.6BNT-0.4SBT ceramic.The results of SEM prove that the mean grain size of 0.6BNT-0.4SBT ceramic is reduced after doped AN,as x increases from 0 to 0.07,the grain size reduce from 1.38?m to 0.72?m.Besides,Ag+and Nb5+ions have different valence states,ion radii and polarization with the matrix cations,their introduction in BNT-SBT can increase the inhomogeneity of the component distribution,thus the relaxation behavior is enhanced,and the polarization difference ?P is improved.AN has a smaller t of 0.93 and BNT-SBT has a larger t of 0.97,then the t is reduced and the EF is improved.Due to the low oxygen vacancy concentration,small grain size,large ?P and high EF,the ceramic of 0.95(BNT-SBT)-0.05AN exhibits good energy storage properties:Wrec=3.62 J/cm3,?=89%,Eb=246 kV/cm,?=1.47%.According to relationship between the factors of phase structure,phase transition electric field,breakdown electric field and point defects with the energy storage properties of ceramics,SMT(M=Bi3+?La3+?Sm3+),STN(N=Sn4+?Ta5+)or AN doped BNT ceramics are fabricated successfully.The key physical mechanism for high energy storage properties is clarified in different system ceramics.The target for achieving a ceramic with high energy storage density,high energy storage efficiency and high energy storage potential is implemented.The ceramics of 0.55BNT-0.45SLT,0.6BNT-0.4SST and 0.95(BNT-SBT)-0.05AN provide alternative materials for high performance ceramic capacitors. |
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