Study On Preparation And Properties Of Iron And Its Oxides Peculiar Micro/Nano Structures

Recently, the micro/nano materials with a peculiar morphology and structure have attracted scientists'interest. Because tof their peculiar morphology, structure and surface state, they exhibit a lot of special properties. At the same time, these properties depend on the morphology and structure. Therefore, synthesizing and developing micro/nano materials with peculiar morphology, and then studying their special properties of them have been one of the research focus of scienists. On the other hand, as an important traditional material, iron and its oxides have always been the most attractive materials. It is found that when the size of iron and its oxides particles decreases to the micro/nano level, they will show peculiar properties different with the bulk counterparts. They show special electrical, magnetic, microwave and catalytic properties, which depen on their morphology, structure and size. Therefore, synthesizing iron and its oxides micro/nano particles with peculiar morphology, and studying their properties would be of great significance. In this dissertation, hematite, magnetite and iron micro/nano particles with several peculiar morphologies are prepared by chemical strategy, including monodispersed hollow microspheres, dendritic structure, nanoplatelets, etc. Their magnetic, microwave and catalytic properties are also studied systemically.A simple method was developed to prepare the uniform hematite and magnetite hollow submicro-spheres with controllable structure and different diameter based on monodisperse poly(styrene-co-acrylic acid) [P(St-co-AA)] particles. The structure and formation mechanism of the hollow spheres were investigated in detail. The control mechanism of shell thickness was also discussed. The results indicated that the shell thickness and coarseness of the synthesized core-shell spheres could be tuned simply by the surface carboxyl content of the P(St-co-AA) particles. Then, by simply changing the atmosphere in the process of heat treatment, hematite or magnetite hollow spheres with controlled dameter and shell thickness are obtained. The magnetic properties of the as-obtained hematite hollow spheres were investigated. The results show that the coercivity and saturated magnetization could be tuned by the diameter and shell thickness of the hollow spheres.Single crystal magnetite hollow submicro-spheres with a narrow diameter distribution are synthesized by a one-pot solvothermal strategy, using ethylene glycol as solvent, ferric chloride as iron source and urea as precipitant without any surfactant or template. The determined role of water is studied in the solvothermal process. It is found that a small amount of water is crucial for the formation of the magnetite hollow spheres. A novel formation mechanism of the magnetite hollow spheres is proposed based on the bubble-assisted Ostwald ripening. It is supposed that the appropriate amount of CO2 gas bubbles in-situ produced by urea hydrolysis is crucial for the formation of hollow spheres. As the existence of gas microbubbles, magnetite solid spheres with a loose core and compact shell form, which is the key factor for the following inside-out Ostwald ripening and the formation of the hollow spheres. So, by simply changed the water dosage, magnetite hollow spheres with different diameter and shell thickness or sold magnetite nanoparticles with different morphology were obtained controllably. The saturation magnetization of the magnetite hollow submicro-spheres was larger than the solid submicro/nano particles and increased with the decrease of the shell thickness. However, the coercivity and remanent magnetization were smaller than the solid submicro/nano particles and increased with the increase of the shell thickness or shell defection, which related to their surface anisotropy.Based on the above synthesis, superparamagnetic magnetite sub-microspheres with a narrow diameter distributon are obtained by the assistance of trisodium citrate in the solvothermal process. Because of the strong coordination of carbonxylate group with iron cations, the surface of the initial produced magnetite nanoparticles adsorbe a large amount of citrate. So the growth of the maguetite crystal and the Ostwald ripening process after the formation of sub-microspheres are inhibited. In the formation process of the microspheres, the initially formed magnetite nanoparticles are negatively charged since there are citrate existed on their surface, so there is repulsion between different nanoparticles. On another hand, because of the large surface energy, there is also a driving force for the aggregation of the initial magnetite. When the two inverse forces reach balance, the magnetite sub-microspheres with a certain diameter obtained. So the diameter of the magnetite nanoparticles and sub-microspheres could be tuned by simply changing the concentration of trisodium citrate or ferric chloride. Because of the citrate group adsorbed on the magnetite surface and the small diameter of the magnetite nanoparticles, the obtained magnetite sub-microspheres have a relatively small saturation magnetization and coercivity, and exhibit a superparamagnetic property.Magnetite and iron dendritic micro-pines are synthesized in a fluidized bed reducing furnace by a hydrogen reduction, where the hematite dendritic micropines are used as starting materials. The influences of reduction conditions to the final products were studied systematically. It is found that the reduction temperature and time have an important effect to the morphology and composition of the finally obtained products. At the same time, because the reduction was a slow and multi-step process, so the dendritic morphologies of the obtained magnetite and iron were maintained well after reduction. However, since there were crystal growth and recrystallization in the reduction process, when the reduction time prolonged, the dendritic morphology was damaged gradually and the fine structure was disappeared. The as-obtained dendritic magnetite and iron exhibit enhanced coercivity and remanent magnetizations at room temperature. The dendritic iron has a high complex permittivity at 2-18 GHz due to the peculiar shape anisotropy and good crystallinity, which could result in large charge polarization and the conductivity. The negative imaginary permeability is observed at 14.5-18.0 GHz. The paraffin-based composites containing 30 wt% dendritic irons have a minimal reflection loss (RL) of -37.4 dB at 7.4 GHz when the thickness (d) is 2.0 mm. The RL values less than -20 dB are obtained in the frequency range of 5.5-12.9 GHz when d increases from 0.9 to 3.0 mm.Iron nanoplatelets with a large edge-length to thickness aspect ratio have been prepared by a facile chemical reduction process under a magnetic field and a shear field. Since the nanoplatelets formation requires both a magnetic field and a shear field, as well as a high reaction temperature and a reactant concentration are needed, a growth mechanism is proposed based on a magnetic and shear field induced self-assembly of concentrated iron nuclei. The magnetic and catalytic properties are studied. Compared with isotropic iron nanoparticles; the as-synthesized amorphous iron nanoplatelets exhibit a decreased saturation magnetization of 120-130 emu-g"1 and an enhanced coercivity of>220 Oe, which are attributed to the poor crystalline nature and shape anisotropy of the nanoplatelets. Moreover, the nanoplates have a much higher conversion rate and a higher selectivity for oxidation of cyclohexane to cyclohexanol. The high conversion rate is attributed to the high specific surface area and the high activity of amorphous irons on the surface sites, and the good selectivity may be explained by the different activities of the iron atoms located on the faces and the edges of the nanoplates.