Study On Preparation And Properties Of Bismuth Ferrate Doped Multiferroic Ceramics

Multiferroics refers to the materials which simultaneously exhibit at least two of the ferroic properties,such as ferroelectric,ferromagnetic and ferroelastic orders,in a single phase.It is a special multifunctional material.Moreover,various kinds of iron properties can be adjusted among order parameters of more dimensions by coupling and synergistic action.For example,the coexistence of ferroelectric order and ferromagnetic order and their mutual coupling?magnetoelectric coupling effect?,i.e.the manipulation of polarization and magnetization in the magnetic fields and electric fields,respectively.Thus it offers additional degrees of freedom for designing multifunctional devices and has been thought to have widespread prospects in the area of information storage,sensor,integrated circuit,etc.BiFeO₃ with a simple rhombohedral perovskite structure?belonging to the R3c space group?is one of the very few single-phase multiferroic materials that has both ferroelectricity and antiferromagnetism at room temperature.The theoretical spontaneous polarization intensity is as high as 90-100?C/cm².Due to the high ferroelectric Curie temperature?T_C=830??and antiferromagnetic Néel temperature?T_N=370??at room temperature and the coupling effect between ferroelectric and magnetic,it has become the current research of multiferroic materials one of the hotspots.However,on the one hand,it is difficult to prepare pure phase BiFeO₃ due to the reasons of Bi volatility and Fe variability,which makes the sample leakage current large and low resistivity,which makes it difficult to measure a hysteresis loop with good shape.On the other hand,BiFeO₃ has an inherent special space-modulated spiral G-type antiferromagnetic structure with a period of 62-64 nm.This structure is easy to cause the weakening or disappearance of the overall macroscopic magnetic moment in the bulk sample,that is,the weakening or loss of ferromagnetism.These shortcomings hinder the practical application of BiFeO₃ ceramic materials.In order to solve these problems,people have tried various methods to optimize the preparation method,improve the doping of A and B elements,and form solid solutions with other perovskites to improve the ferroelectricity and ferromagnetism of BiFeO₃ ceramics.In this study,we focused on BiFeO₃ perovskite material.Research work is carried out from the aspects of element doping modification and the introduction of other ABO₃ type perovskite structures to form solid solutions.Ceramic samples were prepared by solid phase synthesis.The crystal structure of the ceramics was analyzed by X-ray diffraction?XRD?analysis.The microstructure of the ceramics was observed by scanning electron microscopy?SEM?.X-ray photoelectron spectroscopy?XPS?was used to analyze the Fe element valence of the ceramic sample.The dielectric properties were measured by an LCR meter.The leakage current and ferroelectric properties of the ceramics were measured using a ferroelectric analyzer?TF?at room temperature.The magnetic properties of the ceramics were studied using the Physical Property Measurement System?PPMS?.The main results of this thesis are as follows:?1?Bi_0.9La_0.1Fe_1-xTe_xO₃?BLFTO-x,x=0,0.01,0.02,0.03?ceramics were prepared by the conventional solid-state sintering method.The influences of Te doping on crystal structure,microstructure,Fe element valence,dielectric,ferroelectric and magnetic properties were investigated.The results of the XRD patterns suggest that BLFTO-x samples have a rhombohedral structure?space group R3c?.Scanning electron microscopy analysis shows that the average grain size restrains with a small amount of Te doping.X-ray photoelectron spectroscopy shows that the concentration of Fe³⁺ions in the ceramic increases,reducing oxygen vacancies.Performance test research shows that Te doping improves the insulativity,ferroelectricity and ferromagnetic properties of ceramics.?2?The 0.7 Bi_1-x-x Gd_xFe_0.95Ga_0.05O₃-0.3 BaTiO₃?BG_x FG-BT,x=0,0.05,0.1,0.15,0.2?ceramics were successfully synthesized via the conventional solid-state reaction method.The effects of Gd doping on crystal structure,microstructure,dielectric,ferroelectric and magnetic properties were systematically investigated.X-ray diffraction analysis indicates that Gd doping induce a structural transition from rhombohedral?R3c?to pseudo-cubic?P4mm?in BG_xFG-BT ceramics.Scanning electron microscopy results shows a decrease of grain size with doping Gd in BFG-BT.The average grain sizes of the ceramics range from 3.2?m to6.2?m.The dielectric constant and loss tangent are drastically increased and reduced respectively with introducing Gd into the ceramics.Temperature dependent dielectric constant presents a broad peak in the vicinity of Néel temperature?T_N?for all the samples,signifying strong magnetoelectric coupling.An increment in T_N is also observed as a result of Gd-doping in the temperature regions of 230 to 340°C.The leakage current density is reduced by about two orders of magnitude under the electric field of 20 kV/cm.This can be ascribed to the reduction of the oxygen vacancy concentration,which is confirmed by the X-ray photoelectron spectroscopy result.The ferroelectricity and ferromagnetism are also improved after the addition of Gd seen from the polarization hysteresis?P-E?loops and the magnetization hysteresis?M-H?loops.The greatly enhanced magnetism with Mr=0.0186emu/g and M_s=1.084 emu/g is obtained in the ceramic with x=0.2,almost three point six times larger than that of an undoped ceramic.