Magnetic Nanocomposite Aerogels

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Magnetic Nanocomposite Aerogels Anna Corrias, Danilo Loche, Maria F. Casula Dipartimento di Scienze Chimiche and INSTM, Università di Cagliari, S.S. 554 bivio per Sestu, 09042 Monserrato (Cagliari), Italy ABSTRACT Aerogels are regarded as ideal candidates for the design of functional nanocomposites containing supported metal or metal oxide nanoparticles. The large specific surface area together with the open pore structure enables aerogels to effectively host finely dispersed nanoparticles up to the desired loading, to provide nanoparticle accessibility and/or to prevent nanoparticle agglomeration, as required to supply their specific functionalities. The preparation of highly porous nanocomposite aerogels containing magnetic metal, alloy or metal oxide nanoparticles dispersed into amorphous silica, with high purity and homogeneity, was successfully achieved by a novel sol-gel procedure involving urea-assisted cogelation of the precursor phases. This method allows fast gelation, giving rise to aerogels with 97% porosity, and it is very versatile allowing to vary composition, loading and average size of the nanoparticles. The characterization of the morphological and structural features of the nanocomposite aerogels is carried out using different techniques, such as X-ray diffraction, Transmission Electron Microscopy and X-ray Absorption Spectroscopy. The characterization of the magnetic properties is carried out by SQUID magnetometry. INTRODUCTION Silica aerogels provide unique features such as chemical inertness, lightness, transparency and low dielectric constant, related to the porous, fractal-like silica matrix[1-2]. Innovative magnetic nanocomposites such as high coercivity NdFeB-SiO2 aerogels have been prepared by exploiting the large and accessible porosity of aerogels [3]. Nanocomposites containing ferrite (general formula MFe2O4, where M is a bivalent cation) nanoparticles are of particular interest [4-5], due to the broad range of applications of ferrites, including magnetic fluids [6], drug delivery [7], and magnetic high density information storage [8]. Pure iron oxide-silica aerogel nanocomposites have been obtained either by diffusion from the vapour phase [9], or by impregnation from solution into a preformed porous silica aerogel matrix [10], or by co-gelation of silica and iron oxide precursors [11]. Nanocomposites containing FeCo alloy nanoparticles are also very interesting, iron cobalt alloys, FexCo1-x, in bulk form being soft magnetic materials with high Curie temperatures and saturation inductions [12-13] used in the technology of transformer cores and electrical generators. Nanocomposite materials constituted by FexCo1-x nanoparticles embedded in SiO2 or Al2O3 xerogel and aerogels have been successfully prepared [14-16]. The collective magnetic properties of such nanocomposites are related not only to the individual features of the magnetic nanoparticles such as shape, size distribution and composition, but also to their location in the matrix. In particular, by tuning the nanoparticles

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