| Recently,dielectric capacitors were widely used in pulse power systems such as electric vehicles,high frequency inverters,electromagnetic pulse bombs,and medical devices etc.,thanks to their ultrahigh power density,high temperature stability and ultrafast charge-discharge capability.However,compared to other energy storage techniques like supercapacitors and batteries,dielectric capacitors possess relatively lower energy storage density thus require larger volume and weight to provide equivalent output power;this drawback goes against the development trend of lightweight,integration and miniaturization of electronic components nowadays.Therefore,dielectric capacitive materials with high energy storage density need to be developed urgently.Relaxor ferroelectrics are deemed as promising candidates for such energy storage application due to their high permittivity,large Pmax,small Pr,and medium dielectric breakdown strength.In this thesis,Bi0.5Na0.5TiO3 based relaxor ceramics were then selected as the studying subject.Through adding SrTiO3(ST)and CuO separately,we realized lowered Curie temperature,reduced remanent polarization and coercive electric field,enhanced polarization characteristics and dielectric breakdown strength,and eventually improved energy storage properties.The interrelation of microstructure,phase structure,dielectric properties,ferroelectric properties and charge-discharge performances of these dielectric materials were then analyzed.Firstly,(1-x)BNT-x ST lead-free ceramics were prepared by the citrate sol-gel method.The effect of ST doping on phase structure,microstructure,dielectric properties and energy storage performances were carefully characterized and discussed.XRD showed that all samples are of cubic phase structure,and that the lattice parameter first decreases then increases with increasing ST content.Strong frequency dispersion behavior and dielectric broad peak were detected in all samples,indicating an obvious relaxor character.This could be attributed to the polar nano regions(PNRs)since Sr substitution at A site disrupts the long-range dipolar interaction due to the size and charge difference.As the content of ST increases from 0.2 to 0.7,Tm decreases from 200? to-50?;in addition,corresponding DBS of BNT ceramics increases from 70 kV/cm at x=0.2 to 180 kV/cm at x=0.7.Overall,the maximum recoverable energy density Wrec?1.22 J cm3 and energy efficiency ??80%were simultaneously realized in the ceramic 0.4BNT-0.6ST ceramic.Next,0.5(Bi0.5Na0.5)TiO3-0.5SrTiO3-x mol%CuO ceramics were synthesized via facile citrate sol-gel method.The dielectric properties,ferroelectric properties,breakdown electrical fields,energy storage properties and discharge behaviors of synthesized ceramics were systematically studied.The motivation of CuO addition is to reduce the sintering temperature of BNT-based materials as sintering aids,and the effect is very significant.The optimal sintering temperature of 0.5BNT-0.5ST upon CuO addition(x mol%)got reduced from 1130? for x=0 to 820? for x=2.Meanwhile,the dielectric permittivity peak first increases and then decreases with increasing CuO content.In addition,the dielectric loss decreases gradually from 0.07 for x=0 to 0.0018 for x=1.5 at the room temperature,corresponding to the maximum DBS of 250 kV/cm.At x=1.5,we obtained outstanding energy storage performance at an ultralow sintering temperature of 860?,i.e.,Wrec of 2.20 J/cm3 and ??72.39%under the applied electric field of 230 kV/cm.Remarkably,we also achieved a current density CD?1740.97 A/cm2 and power density PD?139.28 MW/cm3 under an electric field of 160 kV/cm in sample 0.5(Bi0.5Na0.5)TiO3-0.5SrTiO3-1.5 mol%CuO;moreover,a relatively high discharge energy density Wd of 1.23 J/cm3 at 150 kV/cm and an ultrafast discharge time t0.9 of 82 ns were also observed in the same sample.All the above results imply that 0.5BNT-0.5ST-1.5mol%CuO ceramics via sol-gel method could become a practical choice for fabricating pulsed power ceramic capacitors. |
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