Preparation And Properties Of BiFeO₃ Based Materials

With the popularization of new lifestyles,such as mobile payment,online car-hailing services and so on,electronic devices are more and more frequently applied in our daily life.Realizing multi-functional devices is a more reliable way to solve the miniaturization and lightweight of devices,which is bring more convenience to people.So,the materials with multifunction properties has attracted more and more attention.Bismuth ferrite(BiFeO3)is just this kind of materials,which exhibits excellent ferroelectric and ferromagnetic properties.However,the application of bismuth ferrite is serviously limited due to the large current leakage and impurity phase.Therefore,BiFeO3(BF)ceramics is chosen as the research target in this thesis.The evolutions of leakage conductance behavior,electromagnetic and energy storage properties of BF ceramics are investigated through equivalent/non equivalent doping,binary composite.In addition,composite materials have been prepared through the combination of Ti-doped BiFeO3 ceramics and cyanate ester resin(CE).Dielectric properties and energy storage properties of composites have been investigated.Equivalent/non-equivalent Ti-doped BF ceramics were prepared by solid state reaction.The research results show that Ti-doping makes BF ceramics grain refinement.The leakage conductance behaviors of BF ceramics are well suppressed,the resistivity increases by 4-6 orders of magnitude compared with undoped BF ceramics.The frequency stability of dielectric constant is improved.Obvious hysteresis loops are observed for Ti-doped BF ceramics,but the ferroelectric performances are weak,however,the ferromagnetic properties are obviously enhanced.The XRD show that Ti-doping has a good suppressing effect on the impure phase with Bi25FeO40 and Bi2Fe4O9,which are common phases in BF ceramics.Single-phase can be obtained in non-equivalent/equivalent Ti-doped BF ceramics with doping amount of 10mol%and 7.5mol%,respectively.The high temperature dielectric spectrum shows the less internal defects of equivalent Ti-doped BF ceramics than the non equivalent doped.So it exhibits the stronger ferroelectric and ferromagnetic properties.The maximum magnetization of 0.53emu/g and the maximum energy storage density of 0.205J/cm3 have been obtained in 7.5mol%equivalent Ti-doped BF ceramics.The(1-x)BiFe0.95Ti0.05O3-xCa0.4La0.4TiO3 dense ceramics were prepared by solid phase sintering.XRD results show that the composite ceramics with BiFeO3 phase and Ca(Bi,La)4(Ti,Fe)4O15 phase are synthesized.Dielectric spectra show that all of(1-x)BiFe0.95Ti0.05O3-xCa0.4La0.4TiO3 ceramics have higher dielectric constant and lower dielectric loss(tan8<0.03)at low frequency than those of BiFe0.95Ti0.05O3 ceramics.The resistivity is further increased by 1-2 orders of magnitude,the ferromagnetic properties are obviously enhanced.The ceramics of x=0.1 displays the residual magnetization Mr=0.1emu/g and coercive magnetic field Hc>60000e.And ferroelectric properties are weak,so the energy storage efficiency is higher than 90%.And ceramics with x=0.2 and x=0.3 reach the maximum storage density 0.115J/cm3.After low temperature heat treatment,(1-x)BiFe0.95Ti0.05O3-xCa0.4La0.4TiO3 ceramics pellets combined with cyanate ester to obtain the composites.The dielectric constant of composites is lower than that of ceramics,but the breakdown electric field increases by 2-3 times,the maximum is up to 321 kV/cm,the maximum energy density reaches 0.143J/cm3.However,the interface of composites is not formed well enough,which causes the leakage conductance.