Study On Preparation And Electric Properties Of BNT-BT Based Lead-free Anti-ferroelectric Ceramics

Lead-free ferroelectric BNT-BT perovskite materials have excellent electrical performance.They are now considered as possible alternative material candidates for lead zirconate titanate(PZT).The BNT-BT perovskite materials can be formed by doping some active elements or groups to achieve the solid solution with Morphotropic Phase Boundary(MPB).The doped material does not only have good dielectric properties but also have higher energy storage performance.In this thesis,BNT-BT is used as the research object,and is modified using A/B site elements(La,Zr,Sn,Mg,Nb,Sr)or groups(KNN,Mg4Nb2O9,etc.).The composition of the ceramics is controlled,and the sintering parameters are adjusted to obtain optimum ceramics with the desired properties.The prepared ceramics are analyzed by XRD,SEM,precision impedance analyzer,thermogravimetric analyzer and ferroelectric analyzer to investigate the phase structure,microstructure,bulk density,dielectric properties,energy storage performance,piezoelectric performance,wear performance and phase transition behavior.The forms of doped elements or groups present in the crystal structure and their effects on the material properties are revealed.The main conclusions obtained in this thesis are as follows:1.Ball grinding time,molding pressure,calcination temperature,sintering temperature and other operational parameters have significant effects on the density and grain size of BNT-BT based ceramics.Ball grinding achieves the purpose of sintering and refining the grains.The increase in molding pressure increases the density and linear shrinkage.Ideal ceramic samples is obtained with high density and small grain size when the ball milling time is controlled to around 15 h,with 300 MPa pressure molding,calcination at 800℃ and final sintering at around 1150℃.2.In the as-prepared ceramics[(Bi1/2Na1/2)0.94Ba0.06](1-1.5x)LaxTiO3(x=0,0.02,0.04,0.06),the splitting of the(200)diffraction peak of perovskite phase is caused by the addition of La element,indicating that doping with La can create quasi-homogeneous phase boundary in the ceramics at room temperature.When T<Tc,the dielectric loss(tan δ)increases with the increase in electric field frequency,while it decreases with the increase in electric field frequency when T>Tc.When x=0,the maximum polarization strength Pmax is 15 μC/cm2.When x increased to 0.06,the maximum polarization strength decreases to 7.2μC/cm2,and the remaining polarization strength(Pr)decreases from 2.5 μC/cm2 to 0.65μC/cm2.Although the doping of La element reduces the dielectric constant of ceramics to a certain extent,the effect of temperature on the dielectric constant is weakened due to the expanding effect of MPB region.Thus,the application range of ceramic materials can be expanded.When the content of La element is 0.02,the maximum energy storage density is 0.34 J/cm3.3.In the[(Bi1/2Nai/2)0.92 Ba0.05La0.02]ZrxTi(1-x)O3(x=0,0.02,0.04,0.06)ceramics,it is found that the MPB region of the material expands further as the content of Zr increases.With the increase in content of Zr,the grain size of samples is gradually reduced.At the same time,the dielectric constant is also gradually reduced,and the dielectric relaxation dispersion degree increases.The low-temperature peak shows obvious frequency dispersion characteristics of dielectric constant.However,at high temperature,the frequency dispersion characteristics of the dielectric constant are not obvious.In other words,the frequency dispersion is not obvious in the MPB region.Both the maximum wear resistance strength and the energy storage density of the material first increases and then decreases.When the content of Zr is 0.04,the maximum wear resistance strength reaches 90kV/cm,and the energy storage density reaches 1.58 J/cm3.4.In the[(Bi1/2Nal/2)o.92Ba0.05La0.02][SnxZr(0.95-x)Ti0.05]O3(x=0,0.03,0.06,0.09)ceramics,there is no obvious change in morphology with the increase in Sn content.However,the grain size shows an obvious change trend.The splitting of the(200)diffraction peaks of the perovskite structure gradually disappears and the(111)peak gradually sharpens.The crystal structure shows four phases,which decreases gradually with rising rates of three phase structure.This result indicates that Sn element doping causes the ceramic material to gradually move away from the coexistence of tripartite and tetragonal phases.As a result,the doped series changes from anti-ferroelectric ceramics to ferroelectric ceramics,thus significantly worsening the electrical properties of the material.5.Three-element(BNT-B ST-KNN)ceramic systems are used which are doped simultaneously with KNaNbO3 and Sr element.With the increase in Sr element doping,the grain size of ceramic samples becomes smaller.The addition of Sr element lead to the splitting of the(200)peak,which indicates that Sr element doping also promotes the transformation of tripartite phase-square phase.Moreover,it also effectively expands the MPB region and improves the Curie temperature of the material.Although the maximum value of the dielectric constant decreases from 5000 to about 3500,the dielectric loss of the material decreases significantly and the dielectric performance becomes significantly more stable.6.In the[(Bi1/2Na1/2)0.92Ba0.05La0.02](Mg1/3Nb2/3)xTi(1-0.5x)O3(x=0.01,0.03,0.05,0.07)ceramics,the grain size of ceramic materials gradually decreases as the content of the b-doped group Mg4Nb2O9 increases.Although the energy storage performance is lower than that of the ceramic doped with single element,the remaining polarization strength decreases significantly,from 38.9μC/cm2 at x=0.01 to 10.4 μC/cm2 at x=0.05.Moreover,the difference between the maximum polarization strength(Pmax)and the residual polarization strength(Pr)increases gradually,indicating that the addition of Mg and Nb elements increases the transformation trend of anti-ferroelectric phase.7.The energy storage performance of BNT-BT based anti-ferroelectric ceramics is closely related to the range of MPB region,which can be adjusted by A-bit,B-bit and group doping.The doping elements causes changes in the value of material tolerance factor.When the tolerance factor is between 0.7530-0.7803,single element doped ceramic material has a larger MPB area.When Ti or Zr element is dominant in the ceramic composition with content of 0.92 mol or above,there are MPB area,with the lead only when Zr PZT ceramics research report Zr:Ti =52:48 MPB zone,only means that the lead against and lead-free ferroelectric materials against the ferroelectric area of ferroelectric materials and phase structure has the very big difference.