Studies On Preparation, Structure And Properties Of Magnetic Particles

Magnetic nanoparticles show unique physical and chemical properties different from traditional magnetic materials.Since many applications of magnetic nanomaterials have been found as magneticfluids,magnetic recorder,catalysis,giant magnetoresistive materials, biomedicine materials,radar absorbing materials,etc.,much attention has been paid to develop the magnetic nanoparticles with unique structure and composition as well as function thereof. However,magnetic nanoparticles tend to self-aggregate as agglomerates and are lack of functional groups on the surface,which limits their application.As a result,the coating of magnetic nanoparticles becomes the focus.Silver is a favorable coating because of its good chemical stability,nontoxicity,good biocompatibility and conductibitily,from which the silver-coated magnetic nanomaterials can have a better magentic and electronic properties,and hence extend the applications of magnetic matericals.This thesis investigated the preparation of FeNi nanoparticles,Fe₃O₄ nanoparticles and the silver coated core-shell nanoparticles,such as FeNi@Ag and Fe₃O₄@Ag.Furthermore,the structures,compositions,morphologies and magnetic properties of these magenitic nanomatericals were characterized.In addiiton,we also firstly prepared a novel microrod-structured Co-Mn compound and its structure and magenitic properties were studied.The creative points in this thesis are as follows:(1) The structure and properties of FeNi nanoparticles was found to be dependent upon the external conditions,such as microwave, ultrasonics and mechanical stirring.As a result,the regulation of these external conditions could become a new concept to control control and optmize the size and magnetic property of FeNi nanoparticles.(2) Different methods had been attempted to coating Ag shell onto FeNi and Fe₃O₄ nanoparticles.The resultant core-shell FeNi@Ag and Fe₃O₄@Ag nanoparticles involved magentic and electronic properties,and hence extend the applications of magnetic matericals.(3) It is the first report that the magnetic Co-Mn microrods with about 50～150μm in length and 1～3μm in diameter were prepared through a convenient hydrothermal method.By studying the effects of preparation conditions on the structure and magnetism of Co-Mn compound,the size and morphology of the microds could be controlled to some extent.The main results in this thesis were summarized as follows:(1) The FeNi nanoparticles were reduction-prepared from the Fe and Ni salts by hydrazine hydrate,under the conditions of ultrasonic,mechanically stirring and microwave,respectively. The FeNi nanoparticles of differernt Fe/Ni molar prepared under ultrasonic were spherical and their diameters located in the range of 50～120 nm,whereas the condition of mechanical stirring produced the FeNi nanospheres of 100 nm covered with a papillary structure while the microwave radiation resulted in monodisperse FeNi nanocubes.Consequently,the morphologies and sizes of the FeNi nanoparticles might be controlled by controlling the reaction conditions and the feeding molar ratio.At last,the magnetic properties of all the FeNi nanoparticles were measured,and all the FeNi nanoparticles showed a good character of soft-magnetic materials.The FeNi nanoparticles prepared from equivalent Fe/Ni feeding molar ratio under mechanical stirring showed the maximum magnetization,and reached 92.68 emu/g.On the basis of the FeNi nanoparticles metioned above,we prepared the Ag-coated nanoparticles.Firstly,the FeNi nanoparticles were modified by 3-mercaptopropyltrimethoxysilane (MPTS) or aminopropyltriethoxysilane(APTES),respectively,and then coated with an Ag shell. The FeNi@Ag core-shell nanoparticles via the MPTS pre-modification were monodisperse spheres with a papillary structure of several nanometer dispersed onto the surface,and the average diameter were about 100 nm with a narrow distribution.Furthermore,the EDS and XRD verified that the crystalline structure of FeNi alloy and Ag co-existed in such core-shell nanoparticles while the maximum magnetization Of FeNi@Ag was 41.14 emu/g.(2) Monodisperse Fe₃O₄ nanoparticles with ca.30 nm were prepared through coprecipitation method.The Fe₃O₄ nanoparticles were modified by APTES,MPTES and tetraethyl orthosilicate (TEOS),and then coated with an Ag shell.Based on the modified Si-layer,the Fe₃O₄@Ag nanoparticles via the MPTES pre-modification were ca.50 nm with a good dispersion.XRD verified that the the cubic-phase crystalline of Fe₃O₄ and the crystalline of Ag co-existed in the Fe₃O₄@Ag nanoparticles.Both Fe₃O₄ and Fe₃O₄@Ag nanoparticles showed a character of superparamagnetic property,and the maximum magnetization of Fe₃O₄@Ag nanoparticles was 16.81 emu/g.(3) The Co-Mn microrods with about 50～150μm in length and 1～3μm in diameter were firstly prepared through a convenient hydrothennal method.The composition,structure and morphology of the as-prepared products were characterized by electron microscopy, energy-dispersive X-ray spectrum(EDS) and X-ray diffraction(XRD).The formula of the resultant microrods could be assigned to Co₉MnP₄O₁₁.The morphology of the products have a great important with the feeding molar ratio of Co:Mn,temperature and reducing agent.Magnetic hystersis loop showed that this Co₉MnP₄O₁₁ microrod-structured Co-Mn compound was a kind of weak magnetism material.However,its ability of keeping magnetization was relatively strong.This thesis presented some new concepts for the controlled preparation and modification of magnetic particles.The stufdies on the structure-properties relationships had the academic value. In addition,the magnetic particles mentioned in this thesis showed a great potential for the biomedical application,such as NMR imaging,targeted drug delivery,cell separation, electron-magnetic shielding and so on.