Graduate Thesis Or Dissertation
 

Synthesis and characterization of zero valent iron nanoparticles

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  • Synthetic methods for the preparation of magnetic nanocomposites with improved high frequency (into the GHz range) performance over conventional magnetic materials were examined. The most successful work involved synthesis of nanocrystalline zero valent iron particles (nc-Fe) through a reverse micelle method. Here, aqueous Fe(II) solution micelles formed using cetyl trimethylammonium bromide (CTAB) in heptane were reacted with aqueous sodium borohydride micelles also using CTAB and heptane. This work was carried out both in a batch reactor and in a T-mixer setup. Particle post-synthesis processing involved vacuum drying followed by high temperature (500°C) annealing of the resultant nanoparticles. This was done to improve the crystallinity of the nanoparticles. Given the highly reactive nature of zero valent Fe, all materials made consisted of mixtures of nc-Fe and Fe oxides (FeO, Fe₂O₃, Fe₃O₄). Additionally, precipitation of NaBr was observed in some samples, which proved difficult to remove without fully oxidizing the desired nc-Fe particles. Since the magnetic properties of the nanoparticles are dependent on the size of coherent crystalline domains in the material, which will be smaller than the size of agglomerated particles in suspension, size characterization was carried out by X-ray diffraction analysis using the Scherrer formula. Particle size was controlled by varying the concentration of Fe(II) in the solution used to prepare the reverse micelles, and the concentration of Fe(II) in aqueous solution was varied from 0.5M to 0.05M. XRD results confirmed average particle sizes in the desired sub 25nm range, and indicated size dependence based on the amount of iron used in solution in the case of the T-mixer setup. In the batch reaction process, magnetic filtering of the materials resulted in samples with sizes consistently in the 10-20 nm range. The nanocomposite was then prepared by mixing the resulting powder with either epoxy or KBr matrices, with the use of KBr as a matrix proving to be the simpler approach to making small toroid shaped materials due to the ease of higher pressure casting. Both epoxy and KBr composites yielded similar magnetic results, with typical quality factors of about 100. By comparison, a commercial nanocrystalline nickel ferrite (nc-NiFe₂O₄) standard had a quality factor of about 10. Likewise, materials based on fully oxidized nc-Fe (nc-Fe₃O₄) produced magnetic materials with low quality factors. This leads to the conclusion that the nc-Fe materials, even with heavy oxidation, represent a potential avenue for improvement over air core inductors.
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