Preparation of Monodisperse Magnetic Nanoparticles for Bionic Assembly and Properties Characterization

• Main Authors: Chih-JungChen, 陳志榮
• Other Authors: Hsin-Yi Lai
• Format: Others
• Language: zh-TW
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/41232724289052646150

博士 === 國立成功大學 === 機械工程學系碩博士班 === 98 === Magnetic nanoparticles (NPs) or magnetic fluids have already been extensively applied to various engineering fields in recent years. However, the efficiency is still difficult to improve all the time, mainly because the present technology can not simulate the organism, such as magnetotactic bacteria (MTB) to produce magnetic NPs with mono-size, shape peculiar one, well disparity, order assembly, and also not very clear about their properties of the isolated NPs and assembled nanostructures. In view of this, this research proposes an experimental and theoretical approach for the preparation, self-assembly, and properties characterization of the monodisperse magnetic NPs from a bionic viewpoint. Moreover, the method is also used to find out the system parameters and to improve application efficiency on bionic magnetic NPs, in addition to confirm the feasibility. In order to prepare the magnetic NPs, and study their basic magnetic properties, this research adopted a thermally-decomposed method to synthesize mono-disperse magnetic iron oxide nano-particles (MMIONPs) with controllable structural compositions with various sizes and shapes to character their basic magnetic properties. The results indicate that (1) the relatively inexpensive natural hematite (α-Fe2O3) powders can be used as a starting material to prepare MMIONPs by following dissolution-recrystallization mechanism. The compositions of the MMIONPs were determined first by M?ssbauer spectroscopy. It appears that the compositions are dependent on the NP sizes. As the size is smaller than 10.6 nm, it is in pure γ-Fe2O3 phase. As the size goes beyond 12.1 nm, it becomes the mixture of both γ-Fe2O3 and Fe3O4 phases; (2) by using porous hematite to prepare iron oleate complexes (IOCs), and then using IOCs to thermally decompose into MMIONPs, the manufacture time can save 2~3 hours; (3) the mono-disperse FeO NPs is then synthesized by decomposition of IOCs in hot oleic acid as the solvent at 380 ℃ for 2 hours, and the composition, such as FeO/Fe3O4, Fe3O4, γ-Fe2O3 can be controlled by various degree of oxidation; (4) by adding CTAB (cetyl trimethyl ammonium bromide) ligand for thermally-decomposed reaction can yield octahedral MMIONPs. The particle size can be controlled by adjusting the reaction temperature and the ratio of CTAB to precursors; (5) the saturated magnetization and moment of the NP is clearly found decreased as the NP size decreased due to surface spin canting; (6) FeO/Fe3O4 Core-Shell NPs can produce exchange anisotropic phenomenon, and the TN of FeO core is about 206 K, which is close to bulk; (7) the super-paramagnetic limited size of octahedral MMIONPs is about 31 nm, and the best magnetic recording sizes are in the range of 66 nm. In order to self-assemble the magnetic NPs, the present work employs a dry-mediated method and a magnetic-induced method to fabricate nano-membranes and directional super-lattice nanostructures. The theory is set out by the inter-particle potential model for finding the mechanism of structural formation by probing into the relation of assembled structures and energy difference. The results indicate that (1) the van der Waals force is a short range interaction. Its energy is a major contributor to attract neighboring NPs to form the NP core. On the other hand, the NPs are not perfectly sphere, it consists of many crystal facets, and the crystal facets will influence of NP arrangement; (2) When the formation of the nano-strucutes is due to magnetic dipole force, it shows that the ＜110＞ direction structure of magnetic dipole is more stable than that of the ＜100＞ directional arrangement. Moreover, the system energy calculated near the NP core does not have too much difference, except those which are far away from the core center. Thus, it can be concluded that the mechanism for the nanostructure formation is mainly of the long-range interaction. In order to characterize the nanostructure properties, we used the SQUID (superconducting quantum interference device) technique to analyze magnetic properties with different structural forms, and also used nano-indenter technique to analyze the mechanical properties to understand the quality of nano-membrane. The results show (1) the coercive force of self-assembled nano-menbrane can be increased by interaction of magnetic dipole; (2) the long axis of the nano-chains can be easily magnetization than that of the short axis of nano-chains due to the effect of easy-axis magnetization; (3) The distribution of the hardness and elastic modulus of magnetic membrane is varied approximately 1~3 times. The deformation of the membrane is mainly contributed by softer surface-protected ligands of the NPs. In order to chain-assemble bionic MTB NPs, and to character the properties of the nano-chain, this research used Monte Carlo simulation to find the relation of manufacturing parameters and assembled micro-structures first, and then analyze the effect of chain-geometric factors on the mechanical and magnetic properties for coming up a set of design rules for the MTB nano-chains. The results indicate that (1) if the assembly of nano-chains uses magnetic NPs, the adopted NP size must be larger than 16 nm, and the adopted strength of magnetic filed must be larger than 0.005 T, that ensures the formation of NP chain-assembly; (2) The coercive force is increased as the chain length increased, and coercive force reaches saturation as the chain length becomes larger than 10 NPs; (3) effective chain length and sensitivity of magnetic recording can be improved either by using a larger NP size or by using a higher elastically modulus of surface-protected ligands of NPs. When compared our research results with those of referenced data, it is found that the experimental and theoretical approach proposed by this study agrees favorably with MTB by using a sort of cheaper manufacture mono-disperse NPs with tunable size, shape, interface, and good colloidal stability. Moreover, by applying the proposed theoretical and experimental approach to design MTB nanochain with geomagnetic recording efficiency of 90 %, the ideal NP size is found to be around 48 ~ 70 nm, and the ideal chain length is found to be around 10 ~31. When the NP size of 70 nm and the chain length of 10 are used to design MTB nanochain, it presents higher efficiency of geomagnetic recording and orientation time. The results indicate that the proposed experimental and theoretical approach presented in this study is accurate and effective.