| (Bi0.5Na0.5)TiO3(BNT)is a ferroelectric material with ABO3 type perovskite structure and complicated phase transition behavior.BNT possesses good piezoelectric,dielectric and ferroelectric properties,as well as a very high Pmax(～40μC/cm2),which has been a hot topic material for theory and application of ferroelectric in a long time.However,pure BNT ceramics have some drawbacks such as high remnant polarization Pr(～38 μC/cm2)and high coercive electric field Ec(～73 kV/cm),which severely limits its application as energy storage dielectrics.Therefore,how to optimize the polarization behavior through A/B site chemical modification and effectively utilize its high Pmax improving energy storage characteristics has become the aim of this research.The research work includes as follows:(1)Paraelectric Ba0.3Sr0.7TiO3(BST),which is more suitable for pulse power systems,is selected to modify BNT,and(Ba0.3Sr0.7)x(Bi0.5Na0.5)i-xTiO3 ceramics(BSxBNT,x=0.3,0.35,0.4,0.45,0.5,0.6,0.7 and 0.8)are fabricated.The effect of BST contents on phase compositions,microstructures,dielectric and energy storage performances of the ceramics are investigated systemically.As BST content increases,the crystal structure transforms from tetragonal phase(x=0.3)to pseudocubic phase(x≥0.35),and grain sizes slightly decrease with all samples show dense microstructure.The maximum dielectric constant εm at dielectric peak and its corresponding Tm gradually decreases,and the room temperature εm=5920 for x=0.5 composition(Tm=36℃)can be obtained.The hysteresis loop initially shows an obvious clamp characteristic for x=0.35 composition and gradually change toward"slim" shape with further increasing x value.The maximum difference of Pmax-Pr=26.86 μC/cm2 under 80 kV/cm is achieved for BSxBNT ceramics with x=0.5,at which optimized energy storage performances are obtained:the maximum recoverable energy density Wrec=1.04J/cm3 with an energy storage efficiency η=77%under 100 kV/cm.(2)BS0.35BNT ceramics suitable for high dielectric and medium voltage type pulse power capacitors are selected as the new matrix,and BS0.35BN[Ti1-x(Nb2/3Mg1/3)x]O3(BS0.35BNT-NM100x,x=0,0.01,0.02,0.03,0.04 and 0.06)ceramics are designed and fabricated.The effect of(Nb2/3Mg1/3)4+complex contents on phase compositions,microstructures,dielectric properties and energy storage performances are investigated systemically.As(Nb2/3Mg1/3)4+content increases,the dielectric peaks are gradually suppressed and broadened,which enhances dielectric temperature stability.An optimized energy storage performances of the maximum recoverable energy density Wrec=0.92 J/cm3 with energy storage efficiency η=70%are obtained for x=0.02 composition under 96 kV/cm.In addition,the energy storage properties of the sample with x=0.02 shows good temperature stability and fatigue endurance under the condition of 20-180℃ and 104 cycles,respectively.(3)Similarly,BS0.35BNT ceramics are still selected as the new matrix due to suitable for preparation of high dielectric and medium voltage type pulse power capacitors,and(1-x)BS0.35BNT-xNaNbO3[(1-x)BS0.35BNT-xNN,x=0,0.02,0.04,0.06,0.08 and 0.10]ceramics are designed and fabricated.The effect of NN contents on phase compositions,microstructures,dielectric properties and energy storage performances are investigated systemically.As NN content increases,the dielectric peaks are gradually suppressed and broadened,which enhances the dielectric temperature stability.An optimized energy storage performances of the maximum recoverable energy density Wrec=1.25 J/cm3 with energy storage efficiency η=76%are obtained for x=0.06 composition under 116 kV/cm.The energy storage properties of the sample with x=0.06 shows a good temperature stability and fatigue endurance under the condition of 20-160℃ and 105 cycles,respectively.Moreover,the power density PD and discharge density Wd of 0.94BSBNT-0.06NN ceramics both possess good temperature stability at 20-100℃,which should be the candidate material for compact pulsed power capacitors. |
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