Silica-encapsulated magnetic nanoparticles formed by a combined arc evaporation/chemical vapor deposition technique
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A multistep technique has been developed for the generation of metallic/alloy nanoparticles coated with amorphous silica. As a proof of concept, an inert-gas blown-arc geometry was used to create nanoparticles from a bulk nickel source, and silica coating formation was accomplished via tetraethyloxysilane (TEOS) decomposition over the nanoparticles in an adjacent chemical vapor deposition chamber. The composite particles exhibit resistance to hydrochloric acid attack over extended times, thereby confirming the protective nature of the silica coating, and magnetic measurements indicate a superparamagnetic transition temperature of 41 K. TEOS flow rate was found to have a profound effect on particle morphology, and individually coated dispersed particles were observed for the intermediate flow rate studied. These results, combined with the well-established field of silica functionalization, offer the possibility that a variety of industrially significant coated magnetic nanostructures may be synthesized with this versatile approach.
I. INTRODUCTION
Nanoparticles have received considerable attention in the past two decades due to the interesting properties afforded by their size. Their magnetic properties are of particular interest for the work at hand. Depending upon factors such as composition, average size, size distribution, and temperature, nanoparticles can exhibit magnetic behavior ranging from highly coercive to superparamagnetic. As a result of this wide spectrum of available coercivities, these materials may find use in such diverse applications as biomedical agents,1–4 ferrofluids,5 and magnetic data storage.6–8 Despite their appeal, metallic nanoparticles are hampered by issues of chemical stability, dispersion, and surface functionalization. Due to their large surface area to volume ratio, such materials are susceptible to attack by oxidative or corrosive environments that can alter their chemistry and diminish their properties. One strategy that has been developed to overcome this obstacle is to generate a protective coating (e.g., graphite or silica) around the particles. Graphite coatings have been promoted
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J. Mater. Res., Vol. 18, No. 4, Apr 2003 Downloaded: 20 Mar 2015
through the use of modified Kratschmer–Huffman arc evaporation geometries9–18 and hydrocarbon decomposition.19 However, it has proven difficult to synthesize separable, individually coated particles due to the encapsulating carbon’s tendency to form crystalline networks around groups of particles. As revealed in this paper, amorphous silica offers an attractive alternative to the use of carbon coatings. Chemical techniques have been used to form silica coatings on nanoparticles of various mate
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