Study On Preparation And Properties Of Nickel Nanoparticles And Ni/PANI Nanocomposites

The control of magnetic particle size, size distribution, shape, crystal structure, surface chemistry and the overcoming of its agglomeration have still been the key issues for the theory study and application of magnetic nanocomposite materials. As one of the metal magnetic materials, the nickel nanoparticles often exhibit fantastic electrical, magnetic and chemical properties which make them have potential applications in many areas such as catalysts, magnetic materials, sintered activators, hard alloy adhesives and so on. The PANI/Ni nanocomposites with both conducting and ferromagnetic properties, which are adjustable with changing their chemical compositions and microstructures, can be obtained by dispersing nickel magnetic nanoparticles in conducting polymer matrices polyaniline and have the important prospect in the field of microwave absorbers, and electromagnetorheological suspensions.The nickel nanoparticles were prepared via polyol process under homogeneous nucleation in this dissertation. The synthetic condition, particle size control, surface modification of the nickel nanoparticles were systematically investigated as well as self-assembly properties and magnetic properties. Based on these studies, PANI/Ni nanocomposites were prepared by emulsion polymerization and in-situ synthesis and the relations between their structures and properties were discussed.The nickel nanoparticles with pure with fcc crystalline structure were available in the polyol solution. The results showed that the propanediol as the reductant could speed the reaction and benefit to obtain pure uniform nickel micrometer particles. Polymer-protected monodispersed nickel nanoparticles were synthesized by an improved polyol reduction method in the presence of poly(N-vinyl-2-pyrrolidone) (PVP), and the chemical interaction between the PVP and nickel nanoparticles is tested by FTIR. PVP was considered to be important for preventing the nickel nanoparticles from agglomeration. The concentration of NaOH strongly influenced on the size, the degree of agglomeration and uniformity of nanoparticles. The average diameters of nickel nanoparticles were discreased with the increasing of the molar ratios between NaOH and Ni²⁺.The linear chains or rings self-assembled morphologies of nickel nanoparticle in magnetic fluid are found by changing the solution concentration. The magnetic interactions among nanoparticles play a crucial role in the formation of these closed loops. The calculations of magnetostatic interaction energy per particle of straight chains and ring indicate that the self-assembled rings have lower energy and better stability. The formation conditions and mechanism of the self-assembly structures were elucidated.The nickel nanoparticles were also modified by an improved polyol reduction method using the oleic acid as a protective agent. The results indicated that the molecules of oleic acid coating on the nanoparticles held back the growth rate, decreased the crystal-size and improved the degree of agglomeration. Oleic acid gets chemisorbed as a carboxylate on the Ni nanoparticle surfaces, and the two oxygen atoms in the carboxylate are coordinated symmetrically to the Ni atoms, which led to the formation of the covalent Ni-O bond. The hydrophobic groups formed by the organic layer on the surface of Ni particles responded for the good stability and oil-solubility of the nanoparticles.The core-shell nanocomposites particles, consisting of nickel core and polyaniline shell, are prepared by emulsion polymerization of polyaniline. The formation of the nanostructures are proposed by four steps such as the disperse of nickel nanoparticles in the oil phase, the formatiom of O/W emulsion due to anionic surfactant SDS, the adsorption of anilinium cations on the micellar surface and the oxidative polymerization of aniline on the interface of the oil phase and the aqueous phase. These magnetic polyaniline microspheres are polydispersed. The particles size is ranged from 60 nm to 200 nm in diameter and the shell thickness is about 10 nm. In order to obtain these nanocomposites that are composed of metal magnetic nanoparticles and polyaniline, it is very important to control the initial pH values by adjusting the molar ratios of HVaniline and the pH values during the polymerization by controlling the concentration of anilinium cations. The results of FTIR, XPS and UV-Vis show that the polyaniline is doped by H+. The nanocomposites via emulsion method have a higher conductivity and lower magnetic properties than those via in-situ synthesis. The former, which can be diluted and undoped like normalpolyaniline, show better stability in aquous solution. The UV-Vis results indicate that the composite emulsion exhibits different undoping properties from the polyaniline prepared by solution polymerization method. The composites with both conductivity and magnetic susceptibility can solve the processable problem of polyaniline and a prospective application is guaranteed.The polyaniline/nickel (PANI/Ni) nanocomposites with the nickel content up to 19.22 wt% can be obtained by high temperature polyol reduce process in the presence of polyaniline. The concentration of dodecylamine significantly influences on both the sizes of nickel nanoparticles and the reducing reaction. The magnetic Ni nanoparticles can improve the thermal stability of polyaniline/nickel nanocomposites due to the interaction between Ni particles and polyaniline chain. The magnetic properties and conductivity of nanocomposites are dependent on the Ni content and the sizes of nanoparticles.