Structure And Microwave Electromagnetic Properties Of Bismuth Ferrite Based Nanoparticles And Ceramics

The booming microwave communication technology puts forward higher demands on the multi-functionalization of electromagnetic materials.Electromagnetic materials need to meet the necessary conditions such as adjustable performance,energy saving,environmental protection,high reliability,etc.Multiferroics with various properties are typical representatives among them.The coexistence and synergetic loss effect of ferroelectricity and ferromagnetism in multiferroics are cornerstones for exploring strong absorption and broadband microwave materials,which have great significance to explore novel microwave electromagnetic materials and enrich the interaction theory between electromagnetic wave and multiferroics.Therefore,this thesis takes bismuth ferrite based multiferroics as the research object to study structural evolution,magnetic mechanism and microwave electromagnetic properties.Pure BiFeO₃ nanoparticles were synthesized by the modified sol-gel method.The structure,micro-morphology,Raman characteristics,ferroelectricity,ferromagnetism and microwave electromagnetic properties of BiFeO₃ nanoparticles were studied systematically.BiFeO₃ nanoparticles have remarkable ferroelectric response,and the ferroelectric domains can flip 180° under the external electric field.The ferromagnetism of BiFeO₃ nanoparticles was analyzed,the grain size of partial grains destroyed the 62nm periodic spiral spin structure of iron ions.The finite size effect plays an important role in constructing the relationship between magnetism and crystal structure.The improvement of the ferromagnetism of BiFeO₃ nanoparticles also has a positive impact on microwave absorption performance.BiFeO₃ has two absorption regions in 2-18 GHz with well performance,and the maximum reflection loss can reach-18 d B,which provides ideas for further understanding the ferromagnetic mechanism of BiFeO₃.The effects of A-site rare earth ions doping(Sm³⁺,Gd³⁺,Y³⁺)on the crystal structure,micro-morphology,ferromagnetism and microwave electromagnetic properties of BiFeO₃ nanoparticles were studied.The stable crystal structure of BiFeO₃ changes from rhombohedral to orthogonal perovskite as the doping content increases or the doping ion radius decreases.Nanoparticle size tends to decrease under the synergy of cell volume reducing and inhibited grain growth.The reduction in the size of nanoparticles promotes the destruction of the 62 nm spatially modulated spiral spin structure and the distortion of FeO₆,leading to an enhanced ferromagnetic superexchange between iron ions in doped BiFeO₃.The electromagnetic parameters of nanoparticles of each component in the range of 2-18 GHz were studied.The ion polarization enhances with the increase of doping,which has more contribution to the??of nanoparticles.The high frequency permeability??of doped nanoparticles are around one due to the weak ferromagnetic nature of BiFeO₃.The negative??of nanoparticles is closely related to the eddy current loss when the frequency exceeds 9 GHz,which is a special phenomenon of antiferromagnetic materials.BiFeO₃ and Bi_0.95M_0.05FeO₃(M=Sm,Gd,Y)ceramics doped with A-site rare earth ions were prepared by rapid liquid-phase sintering method,and the crystal structure,morphology,Raman properties and microwave electromagnetic properties were studied.Raman analysis shows that the doping of rare earth ions at the A-site has a greater effect on the E mode of the crystal than the A₁ mode,and the structure of FeO₆ octahedron affected obviously by the ion doping.Compared with pure BiFeO₃ ceramics,doped ceramics show better magnetic properties.However,the magnetic moment inside the ceramics are not completely released due to the conflict between the spatially modulated spiral spin structure and the grain size,and the ferromagnetism of ceramics is weaker than that of nanoparticles.The electromagnetic characteristics of ceramics in 2-18 GHz were studied.Due to the dielectric loss changes with the tilt of FeO₆ octahedron,doped ceramics exhibit natural ferromagnetic resonance around 16-17 GHz,and the input of A-site rare earth ions can adjust the resonance loss of BiFeO₃ ceramics.The crystal structure,micro-morphology,ferroelectricity,ferromagnetism,light absorption and microwave electromagnetic parameters of(K_0.5Na_0.5)NbO₃-xFe₂O₃(x=0,0.01 and 0.05)ceramics prepared by solid state method were studied.Fe₂O₃ doping has a positive effect on improving the ferroelectric properties of KNN-based ceramics,and it can also improve the ferromagnetic properties of ceramics.(K_0.5Na_0.5)NbO₃-xFe₂O₃(x>0)ceramics not only exhibit intrinsic ferroelectric hysteresis loops,but also have typical magnetic hysteresis loops derived from the exchange interaction between Fe³⁺ions and spin polarized electrons at oxygen vacancies.In addition,it is found that proper Fe₂O₃doping can effectively adjust the band gap of KNN-based ceramics through first-principles calculations and experiment.The composite layers can accurately control the electromagnetic properties of composite materials by studying of the electromagnetic parameters of the BFO-KNN multilayer structure.The results of this thesis are beneficial to establishing the physical connection among the structure,ferroelectricity,ferromagnetism and microwave electromagnetic properties of multiferroics,as well as the exploring the application of perovskite oxide materials with novel properties in the field of microwave electromagnetics.