Iron-cobalt alloy nanoparticles embedded in an alumina xerogel matrix

ENNAS, GUIDO;FALQUI, ANDREA;Paschina G;Marongiu G.

2005-01-01

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Abstract

Nanocrystalline gamma-Al2O3 and FeCo-Al2O3 nanocomposite xerogels with high surface areas and pore volumes were prepared from alcogels obtained by a fast sol-gel procedure. The formation of gamma-Al2O3 occurs via a sequence of stages starting from a disordered pseudo-bohemite phase which around 700 degrees C gives rise to amorphous allumina; this progressively cristallizes as gamma-Al2O3, which is stable up to 1100 degrees C, when microcrystalline alpha-Al2O3 becomes the dominant phase; in the range 1000-1200 degrees C minor traces of delta-Al2O3 and theta-Al2O3 are present. Xerogels containing iron and cobalt are amorphous up to 700 degrees C; calcination at 800 degrees C gives rise to a spinel phase similar to gamma-Al2O3 where metal ions partially fill the vacancies; at 1000 degrees C gamma-Al2O3 progressively disappears to form alpha-Al2O3 and Co(Fe)Al2O4, which are the only phases present at 1200 degrees C. Reduction in hydrogen flow of the xerogel, previously calcined at 450 degrees C, leads to a nanocomposite constituted of FeCo alloy nanoparticles around 10 nm dispersed into alpha-Al2O3 nanocrystalline matrix. Zero-field-cooled (ZFC) and FC magnetic curves are typical of superparamagnetic materials and indicate the occurrence of high-strength particle interactions.