| Multiferroics refers to the materials which simultaneously exhibit at least two of the ferroicproperties, such as ferroelectric, ferromagnetic and ferroelastic orders, in a single phase. Multiferroicmaterials possess the ferromagnetic (FM)/antiferromagnetic (AFM) and ferroelectric (FE) propertiessimultaneously with magnetoelectric (ME) coupling, i.e. the manipulation of magnetization by meansof an electric field or manipulation of polarization by means of a magnetic field. Thus it offersadditional degrees of freedom for designing multifunctional devices and has been thought to havepromising prospects in the area of information storage，sensor，integrated circuit, etc.BiFeO3with ferroelectric and antiferromagnetic orderings above room-temperature (TC~1103K,TN~643K), large value of polarization and simple perovskite structure, has been extensively studiedin recently years. However, the shortcomings of BiFeO3hamper its practical application, such asweek macroscopic magnetism due to canted G-type antiferromagnetic spin structure, large leakagecurrent caused by volatilization of Bi3+ion and the valence fluctuation of Fe3+ion becoming a majorobstacle to get good ferroelectrical hysteresis loops, hindering the practical application of bismuthferrite materials. Up to now, many methods have been proposed and carried out to improve the roomtemperature ferroelectricity and ferromagnetism, aiming at therealizaion of the mutual regulation ofelectricity and magnetism. Among them, ion substitution with large difference in ionic radius isthought to be one of the most effective and simplest methods.In this study, we focused on the property of BiFeO3with different ions doping at A or B sites orco-doping, which was synthesized by a tartaric acid modified sol-gel method. And measurements ofX-Ray diffraction (XRD) and Raman were carried out to characterize structural transformation. Theferroelectric test and physical property measurement were used to characterize the change offerroelectric and magnetic property with different ion doping. And we discussed the relation betweenthe crystal structure and the change of property. In addition, we researched the potential application ofBiFeO3-based ceramics in the field of dye absorption. The main results and conclusions aresummarized as following:1. The Bi1.04xYxFeO3(0≤x≤0.30) ceramics were prepared by tartaric acid modified sol-gelmethod. With the increase of concentration of Y3+doped at A site, the XRD and Raman resultsdemonstrated that the crystal structure transformed from rhombohedral (R3c) to orthorhombic (Pn21a).The ferroelectric test results showed that the leakage current can be effectively suppressed by Ysubstitution. When the concentration of Y3+was increased up to30%, the room temperature ferromagnetism was strongly enhanced, the saturation magnization reaching0.31emu/g. Under theextra electric field of120kV/cm, the saturation polarization of Bi0.74Y0.30FeO3was about25μC/cm2,much better than the undoped sample.2. In order to explore the origin of enhanced room temperature ferromagnetism in Ba2+dopedBiFeO3, which is thought to be tetragonal when the doping concentration up to30%, we synthesizedBi0.74Ba0.30FeO3and Bi0.74Ba0.30Fe0.95Ti0.05O3ceramics by sol-gel method. The XRD and Rietveldrefinement results show that Bi0.74Ba0.30FeO3has a pseudo-tetragonal structure, while triclinicdistortion was induced by5%Ti4+substitution. From the I-V curves and PPMS, we can conclude thatthe addition of Ti4+significantly reduced the leakage current but had little influence on the magneticproperties. Compared with BiFeO3(Mr=0.001emu/g; Hc=80Oe), the remanent magnetization (Mr=0.16emu/g) and coercivity (Hc=4.2KOe) of Bi0.74Ba0.30FeO3are strongly enhanced. For further study, wecarried out the temperature-dependent magnetization and found out that the magnetic transitiontemperature is around720K, which is much different from pure BiFeO3(643K). However it was veryclose to Curie temperature of permanent magnet of BaFe12O19. The magnetic hysteresis loop of bothsamples, after annealing in vacuum, showed two FM phase. Combining the result of DSC analysis, weconcluded that the enhanced ferromagnetism on Ba2+doped BiFeO3might be attributed to the traceimpurity of BaFe12O19.3. The magnetic property of BiFeO3(especially doped BiFeO3) makes it one of the promisingmaterial of magnetic separation in the field of water purification. Therefore, the initial attempt wasstudying the interface chemical adsorption properties and discussing the sorption mechanism ofBiFeO3and BiFe0.95Co0.05O3in adsorption of a typical dye pollutant-Rhodamine B (RhB) fromaqueous solution. From the results of the XRD, Raman and PPMS, we found out that5%Cosubstitution at Fe sites could enhance magnetic dramatically at room temperature without inducingany impurity phase. Under the condition of dark state, we investigated the adsorption kinetic curves,isotherm, saturated adsorption capacity under different pH conditions, zeta potential and the magneticseparation effect at room temperature by batch adsorption experiments. The calculation results ofadsorption kinetic combined with thermodynamic theory, showed that the sorption process of BiFeO3and BiFe0.95Co0.05O3on removal of Rhodanmine B is an irreversible exothermic process and mainlyattributing to physical adsorption. Isothermal adsorption curves fitted by Langmuir model showed thatthe equilibrium adsorption amount of both samples are0.185mg/g and0.216mg/g. This illustratedthat Co substitution had no significant effect on the saturated adsorption capacity. The saturatedadsorption capacity increased with acidic enhancement. Based on this, we concluded the possible sorption mechanism in the acidic solution or alkaline solution. Compared with BiFeO3,BiFe0.95Co0.05O3with improved magnetism exhibits comparable sorption performance in terms ofsorption rate as well as saturated sorption capacity, suggesting possible application in efficientmagnetic separation of powder adsorption. |
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