Regulation And Mechanism Of Magnetoelectric Coupling And Energy Storage In Bismuth Based Perovskite Films

Multiferroic magnetoelectric?ME?materials have both ferroelectric and ferromagnetic order,and the coupling between them is called“magnetoelectric effect”,which can realize magnetic field controlling of electric polarization or electric field controlling of magnetization.It has significant potential applications in spintronic devices and new-type ME memory devices,and has attracted wide attention.However,most single-phase multiferroic materials have ME coupling effect at low temperature and that are very weak,which is difficult to be used in practical industrial applications.The ME composite multiferroic heterojunction has incomparable flexibility in material selection and coupling strength regulation at interface,especially 2-2?ferromagnetic/ferroelectric?multiferroic heterojunction.In this thesis,we mainly study the influence of the selection of ferromagnetic and ferroelectric materials and the interface regulation on the magnetoelectric coupling effect in 2-2 type multiferroic heterojunction.In addition,ferroelectric ordered materials are famous for their hysteresis behavior after electric polarization.The ferroelectric domains are switched directionally and arranged orderly under an external electric field to realize energy storage,while the energy is released after the electric field is removed and this is called dielectric energy storage.When the electric field is removed,the electrical polarization does not disappear immediately,and this kind of hysteresis will generate remanent polarization,that is to say,the ferroelectric domain does not immediately return to the original state,resulting in the hysteresis behavior with the loss of energy.Therefore,for dielectric energy storage,a small or almost no hysteresis behavior is needed to reduce energy loss and improve energy storage density and efficiency.This dielectric capacitor stands out for its ultrahigh power densities?ultrafast charging-discharging rates?,high voltage endurance,and good reliability?wide working temperature range and long working life?.However,the energy storage capability of dielectric capacitors is generally low,and the general way to improve the energy storage density is to build complex solid solutions to achieve.In this thesis,a simple method is adopted to regulate the energy storage characteristics and achieve high energy storage density and efficiency.In view of the above research hotspot,this thesis mainly includes the following research contents:1.The excellent magnetoelectric coupling effect was obtained in the 2-2 type composite film formed by the single-layer perovskite Bi_0.5(Na_0.85K_0.15)_0.5TiO₃ and BiFeO₃films.As one of the few materials with weak magnetoelectric effect at room temperature,BiFeO₃ is limited in practical application due to its large leakage current.For the ferroelectric material Na_0.5Bi_0.5TiO₃,by introducing K⁺into the position of Na⁺to induce the morphotropic phase boundary?MPB?,the polarization and piezoelectric coefficient of Bi_0.5(Na_0.85K_0.15)_0.5TiO₃ at the MPB are improved.In addition,the similar single-layer perovskite structure of BiFeO₃ and Bi_0.5(Na_0.85K_0.15)_0.5TiO₃ make the interface have a good coupling effect while effectively suppressing the leakage current generated by BiFeO₃.Finally,BiFeO₃/Bi_0.5(Na_0.85K_0.15)_0.5TiO₃ composite films obtained a good magnetoelectric effect,and its magnetoelectric coupling coefficient reached?_E=31 mV/cm·Oe.2.The Aurivillius phase multi-layer perovskite ferroelectric Na_0.5Bi_4.5Ti₄O₁₅ was introduced as piezoelectric phase and BiFeO₃ as piezomagnetic phase to form 2-2type composite films.As a natural superlattice structure,Na_0.5Bi_4.5Ti₄O₁₅ is composed of perovskite-like layer?polarization layer?and?Bi₂O₂?²⁺layer.As a high resistance layers?Bi₂O₂?²⁺has space charge effect,which can prevent the accumulation of oxygen vacancy in the domain wall and reduce the pinning effect of the domain,so it has good ferroelectric and piezoelectric properties.Therefore,an improved magnetoelectric effect was obtained in BiFeO₃/Na_0.5Bi_4.5Ti₄O₁₅5 composite films with the magnetoelectric coupling coefficient reached?_E=136 mV/cm·Oe.However,Na_0.5Bi_4.5Ti₄O₁₅5 and BiFeO₃ did not achieve a good lattice match.The multi-layer perovskite Bi₅Ti₃FeO₁₅ was introduced as the piezomagnetic phase again,which resulted in a good interface coupling between Na_0.5Bi_4.5Ti₄O₁₅5 and Bi₅Ti₃FeO₁₅ due to the similar lattice structure,which not only increased the stress transfer between the interfaces,but also improved the leakage current characteristics of the composite films.Finally,a huge magnetoelectric effect is obtained,and the magnetoelectric coupling coefficient?_E is as high as 410 mV/cm·Oe.3.The energy storage behavior of lead-free multil-ayer perovskite BaBi_4-xLa_x Ti₄O₁₅ relaxor ferroelectric films is regulated by layer selection engineering for La³⁺doping.As x?0.2,Bi³⁺is substituted by La³⁺at perovskite-like layers,which increases the disorder degree of A-site cations and depresses the defects.While continuing to increase the concentration,La³⁺?x?0.2?tends to enter?Bi₂O₂?²⁺high resistance layers,which decreases the insulating properties and results in the reduction of the energy storage density.Therefore,a high energy storage density of44.7 J/cm³ and an energy storage efficiency of 60.1%achieved in BaLa_0.2Bi_3.8Ti₄O₁₅films under an applied electric field of 1667 kV/cm.This layer selection engineering for rare-earth doping provides an approach to regulate the energy storage performances in multi-layered perovskite relaxor ferroelectric films.4.The energy storage characteristics of bismuth based multi-layered perovskite(Na_0.85K_0.15)_0.5Bi_4.5-xLa_xTi₄O₁₅5 films were regulated by introducing bidirectional effect of La-O orbital hybridization.The weak orbital hybridization between La-O replaces the strong orbital hybridization between Bi-O:on the one hand,the introduction of La-O orbital hybridization weakens the dipole-dipole interaction and enhanced the relaxation characteristics.On the other hand,the increase of band gap and the decrease of defect concentration lead to the increase of breakdown strength.Therefore,we achieved minimized hysteresis while maintaining high polarization and achieved a giant energy density U_re?111 J/cm³ with high effciency??83%in(Na_0.85K_0.15)_0.5Bi_3.2La_1.3Ti₄O₁₅ relaxor ferroelectric films.Compared with other dielectric capacitors,this work is to obtain huge energy storage density and efficiency after introducing ions of the same radius and valence state,which will provide a concise approach for the successful design of next-generation pulsed power capacitors.