Composition Design And Low-field Energy Storage Properties Of NBT-based Antiferroelectric-like Ceramics

For efficient energy storage and conversion,the dielectric ceramics should possess high energy storage density(Wd)and power density(PD)under low field(<220 kV/cm).Usually linear dielectric capacitors have high PD but low Wd.For batteries,the opposite is vice versa.Antiferroelectrics' field-induced phase transition provides potential of high energy strorage under low field.However,most of the antiferroelectrics possess toxic elements(such as Pb)and some precious metal elements(such as Ag),which is not beneficial to commercial applications.So we raise a lead-free antiferroelectric-like stretagy.Compared with BT,due to the evolution of two PNRs(R3c and P4bm),NBT has more excellent high temperature stability and higher Pm=43 ?C/cm2,but higher coercive field strength(?73 kV/cm)and leakage current limiting the application of pure NBT,and the pinchend P-E loop behavior in NBT(antiferroelectric-like behavior)is doubtful.In this paper,aiming at the current key issues of NBT and dielectric energy storage,we have adopted equivalent solid solution,heterovalent solid solution and enhanced B-site disorder,to develop a series of antiferroelectric-like NBT-based ceramics.And we have investigatied the relationship among phase structure,polarization structure,microstructure,leakage current,etc.and their low-field energy storage performance.The main conclusions obtained are as follows:BH Modification can effectively increase the transmittance of NBT ceramics.XRD results show that the solid solubility of BH in NBT is?10%,and BH can reduce its tolerance factor and promote the anti ferroelectric orthorhombic symmetry.Raman results show that BH can enter the lattice of NBT,improve the symmetry of NBT,enhance its local disorder,and further improve its relaxation characteristics.The addition of BH can effectively reduce the volume density of high-temperature PNRs of NBT.As the BH content increases,the Tm of NBT ceramics decreases from 338? to 204?.SEM and electrical modulus show that there is a core-shell structure in the BH-modified NBT ceramics and improved the temperature stability of the dielectric ceramics.The sample with 20%BH contents achieved a superior dielectric constant(>1000)and a tan?(<0.02)at the temperature range of 75-400 ?.The ferroelectric hysteresis loop shows that the BH-added NBT ceramic exhibits an obvious antiferroelectric-like hysteresis loop.Landau-Devonshire model and I-E curve can be used to explain antiferroelectric-like phenomenon.With the increase of BH content,Wrec firstly increased from 0.3 J/cm3 to 2.12 J/cm3.This is because the sample with 15%BH content has a large Pm-Pr(29?C/cm2)and High Eb=175kV/cm,mainly due to its uniform and dense microstructure and small leakage current.Compared with the work reported in the literature,the ceramics of this system can obtain an energy storage density higher than 2 J/cm3 under an electric field of less than 200 kV/cm,and it is a candidate matrix for low electric field and high energy storage dielectric ceramics.BSN can effectively reduce the phase transition temperature of NBT ceramics.XRD results show that the crystal structure of the sample changes to an orthorhombic phase with low symmetry after adding BSN.Combined with Raman analysis,it can be known that the addition of BSN can increase the structural complexity and phase diffusibility of NBT ceramics and thus improve its relaxation characteristics.The SEM test showed that the grain size of the sample was reduced to 1.27 ?m.With the increase of BSN content,Tm decreases from 256 ? to 46 ?,which provides an effective method to reduce the phase transition temperature of NBT.Due to the dual effect of grain size and BSN composition,its breakdown strength increased from 140 kV/cm to 240 kV/cm.The ferroelectric hysteresis loop shows that the addition of BSN can effectively reduce the ferroelectric loss of NBT ceramics and improve the stability of its dielectric constant with electric field.Due to the high maximum polarization difference(23?C/cm2)and high breakdown field strength(190 kV/cm),a high energy storage density(1.9 J/cm3)and efficiency(86.4%)were obtained in NBT with 25%BSN content.Furthermore,MgO and ZnO modified 0.775NBT-0.225BSN ceramics were prepared by traditional solid state method.The MgO modified 0.775NBT-0.225BSN ceramic exhibits typical antiferroelectric double hysteresis loop,while the ZnO modified does not.SEM and EDS prove that 0.775NBT-0.225BSN ceramic modified by MgO has local compositional heterogeneity(MgO tends to accumulate at grain boundaries).XRD and Raman results show that after the addition of MgO,a slight substitution of Mg for the B-site ions occurred.By changing the content of BSN and MgO,it is found that the double ferroelectric hysteresis loop still exists.The dielectric and complex impedance spectrum show that the high-temperature electrical conductivity of the ceramic modified by MgO has changed from intrinsic conductivity to oxygen ion conductivity,which further proves the possibility of Ti/Sn substituted by Mg at the B position.A model of the coexistence of PNRs and defect dipoles is proposed to explain the reason for the generation of double ferroelectric hysteresis loops.Compared with other compounds,the NBT-BSN-based ceramic modified by MgO has an energy density greater than 2 J/cm3 at 210 kV/cm,which is beneficial for its application in low electric fields.The modification differences between BMN and BZN on NBT ceramics were studied.Compared with BZN,BMN shows more obvious antiferroelectric-like P-E loops,which is beneficial to obtain high energy storage density under lower electric field and light addition.When addition content is 10%,the energy storage density of BMN(1.11 J/cm3)modified is greater than that of BZN(0.88 J/cm3)modified,when addition content is 15%,BZN(0.95 J/cm3)modified is larger than BMN(0.9 J/cm3).XRD and Raman results show that BMN has greater solid solubility in NBT than BZN,and the distortion of oxygen octahedron is also more obvious.PFM test results show that the polar structure size of BZN modified NBT ceramics is smaller than that of BMN.Dielectric test shows that BZN-modified NBT has more significant dispersion phase change behavior at high temperature than BMN.Furthermore,NBT-BMZNT ceramics were designed and prepared.XRD test shows that the solid solubility of BMNZT in NBT is less than 18%,and it has a pseudo cubic crystal structure after modification of BMZNT.Raman results show that the addition of BMNZT significantly improves the lattice disorder of NBT ceramics and breaks the ferroelectric long-range order.Combining XRD,backscattered electron image and dielectric temperature spectrum,it can be proved that the second phase appearing is Na0.5Bi4.5Ti4O15.The PFM results show that the addition of BMZNT significantly reduces polar structure size of NBT.When addition content is greater than 18%,the polarization structure signal cannot even be detected.The SEM test shows that the grain size increases to 5 ?m from 2 ?m.The dielectric test shows that the high temperature peak of Tm is continuously depressed,and the dielectric peak shifts from 274? to 113?.The low-temperature peak T1 slowly moves to the lower temperatures.These changes in Tm and T1 have led to a broadening of the dielectric peak,and a higher dielectric constant than most current substance can be obtained in a wide temperature range.Since the polar structure size decreases,the Wrec of this system increases from 0.24 J/cm3 to 1.32 J/cm3,and the ? increases from 3.8%to 99%.LT can promote the sintering of NBT ceramics.The microscopic morphology shows that LT can effectively reduce the sintering temperature and increase density of NBT.The average grain size drops to 2 ?m after 10%LT addition.XRD and Raman analysis show that LT addition can promote the lattice distortion and oxygen octahedral distortion of NBT,break the long-range order of ferroelectricity,and improve relaxor behavior.The dielectric test shows that the dispersion behavior occurs at both low and high temperature dielectric peaks.With the increase of LT content,the low temperature peaks continue to decrease,while the high temperature peaks slightly increase.This leads to a broadening of the dielectric peak.When LT content is 7%,a high dielectric constant of 1568 ± 15%was obtained in the range of 60-600?.The Weibull breakdown distribution shows that the breakdown strength increases from 150 kV/cm to 200 kV/cm,reaching the electric field for antiferroelectric dielectric materials.With the increase of LT addition,the leakage current reaches the minimum when LT content is 10%,which proves that proper LT addition can reduce the leakage current of NBT ceramics.The P-E and I-E test results show that LT addition reduces the size of the NBT polar structure and improves the speed of the polar structure responding to the electric field.The NBT-LT system obtains a higher energy storage density(2.3 J/cm3)and efficiency(74.2%)at 200 kV/cm,which is due to the combination of the dense microstructure,small and uniform grain size,low leakage current and high Pm=28 ?C/cm2.Furthermore,pulse discharge tests show that this system has high discharge energy density Wd=0.51±5 J/cm3,high current density(430 A/cm2(±10%))and PD(22 MW/cm3(±10%))in the temperature range of 60-150?.The LA modification can reduce the leakage current of NBT ceramics.The grain morphology of NBT continuously changed into tetragonal with increasing LA content,and the average grain size decreased from 3.64 ?m to about 1.32 ?m.XRD and Raman analysis showed that the addition of LA can increase the lattice distortion and oxygen octahedral distortion of NBT,break the long-range order of ferroelectrics,and improve relaxor behavior.The dielectric test shows that after LA is modified,the high-temperature dielectric peak continues to broaden,while the low-temperature dielectric peak continues to move to lower temperatures.Compared with the LT modified ceramics,with the increase of LA content,the low temperature dielectric loss continuously decreases,indicating that LA is one kinds of additives to reduce low temperature loss.The analysis of the relaxor behavior shows that the samples with LA contents(5-7%)have better dielectric dispersion behavior.Due to the decrease in grain size and the significant reduction in leakage current,the breakdown strength increases from 120 kV/cm to 210 kV/cm.The P-E and/-E test results prove the reduced polar structure size and increased transition-field after LA modification.When LA content is 7%,the NBT-LA system ceramics obtained a higher Wrec(3.18 J/cm3)and ?(60%)at 210 kV/cm.Then,pulse discharge tests show that the 0.93NBT-0.07LA ceramic obtains high power density(30 MW/cm3)and current density(300 A/cm2)under an electric field of 100 kV/cm.At the same time,achieved a high energy storage density(0.45 J/cm3)and a power density(20 MW/cm3)at 50?.