Preparation And Magnetic Properties Of High-coercivity Iron Oxide Nanomaterials

Iron oxide nanomaterials have been widely used in many fields because of their special physical and chemical properties,such as high-density magnetic storage,magneto-electric machines,magnetic fluids,sensors,biomedicine,solar cells and other fields.Therefore,iron oxide nanomaterials have been widely studied.ε-Fe₂O₃ is an iron oxide nanomaterial that has attracted much attention in recent years,and it has extremely high coercivity at room temperature.ε-Fe₂O₃ has important application value in the fields of high-density magnetic storage and microwave absorption.But the synthesis of high-qualityε-Fe₂O₃ nanomaterials is still a difficult task.It is of great significance to explore high-efficiency and low-cost methods forε-Fe₂O₃nanomaterials.This paper focuses on the preparation of pureε-Fe₂O₃ and Al-dopedε-Fe₂O₃nanomaterials using fumed silica compact as porous carrier.This method has the advantages of simplicity and convenience,less time-consuming,etc.,and theε-Fe₂O₃nanomaterials have large coercivity and few impurity phases.This paper also investigates the preparation of high-coercivity iron oxide nanowires using porous anodic alumina templates.The main research contents of this paper are as follows:1.The fumed silica compact were used as the carrier,which is immersed in a mixed solution of ferric nitrate nonahydrate and aluminum chloride with different iron-to-aluminum molar ratios,and thenε-Al_xFe_2-xO₃（x=0,0.125,0.167,0.222 and0.4）nanoparticles were prepared by annealing.The effect of Al doping onε-Al_xFe_2-xO₃ nanoparticles was explored.It is found that doping Al³⁺can inhibit the nucleation and growth ofε-Fe₂O₃,reduce the particle size ofε-Al_xFe_2-xO₃ nanoparticles,and effectively reduce the formation ofα-Fe₂O₃impurities in the sample.It found that the low temperature magnetic phase transition temperature ofε-Al_xFe_2-xO₃ samples decreased with the increase of Al³⁺.And the low-temperature magnetic phase transition finally disappears in the sample with x=0.222.Theε-Al_xFe_2-xO₃ sample has a large coercivity.With the increase of Al³⁺,the coercivity ofε-Al_xFe_2-xO₃ at room temperature first increased to 16.81 k Oe,and then decreased gradually.The effect of the pressure for preparing fumed silica compacts onε-Al_xFe_2-xO₃nanoparticles was studied.It was found that the pores of the fumed silica support decreased as the pressure increased,and the size of theε-Al_xFe_2-xO₃ nanoparticles in the samples decreased.Moreover,theα-Fe₂O₃ impurities in the samples were also reduced.2.Alumina was used as a template,which was immersed in a mixed solution of ferric nitrate nonahydrate and aluminum chloride,and the iron oxide nanowire arrays containingε-Fe₂O₃ were prepared after annealing..The average diameter of the nanowires is about 39 nm,which is basically consistent with the pore size of the alumina template.It is found that in addition toε-Fe₂O₃,the synthesized iron oxide nanowires also contain most ofα-Fe₂O₃ grains,and the coercivity of the iron oxide nanowire array is 4833 Oe.The coercivity is smaller than that of the general pure phaseε-Fe₂O₃,which should be the fact that the iron oxide nanowires contain most of theα-Fe₂O₃ phase.But the coercivity is larger than the usualα-Fe₂O₃,which caused by the magnetic coupling effect between theε-Fe₂O₃ phase and theα-Fe₂O₃ phase in the iron oxide nanowires.3.Co Fe₂O₄ nanowire arrays were prepared with alumina as template.Its structure and magnetic properties were also studied.The Co Fe₂O₄ nanowires are uniformly arranged with a diameter of 38 nm.The coercivity at room temperature is776 Oe,and the remanence ratio of Mr/Ms is about 0.3.The coercivity and remanence of Co Fe₂O₄ nanowires are relatively low,because Co Fe₂O₄ nanowires are polycrystalline nanowires composed of nanoparticles with smaller diameters.The research provides a method for large-scale preparation of Co Fe₂O₄ nanowire arrays,and has a high application prospect in high-density digital recording media.