Iron-cobalt alloy nanoparticles embedded in an alumina xerogel matrix

Nanocrystalline γ-Al₂O₃ and FeCo-Al ₂O₃ nanocomposite xerogels with high surface areas and pore volumes were prepared from alcogels obtained by a fast sol-gel procedure. The formation of γ-Al₂O₃ occurs via a sequence of stages starting from a disordered pseudo-bohemite phase which around 700°C gives rise to amorphous allumina; this progressively cristallizes as γ-Al₂O₃, which is stable up to 1100° C, when microcrystalline α-Al₂O₃ becomes the dominant phase; in the range 1000-1200°C minor traces of δ-Al₂O ₃ and θ-Al₂O₃ are present. Xerogels containing iron and cobalt are amorphous up to 700° C; calcination at 800°C gives rise to a spinel phase similar to γ-Al₂O ₃ where metal ions partially fill the vacancies; at 1000°C γ-Al₂O₃ progressively disappears to form α-Al₂O₃ and Co(Fe)Al₂O₄, which are the only phases present at 1200°C. Reduction in hydrogen flow of the xerogel, previously calcined at 450°C, leads to a nanocomposite constituted of FeCo alloy nanoparticles around 10 nm dispersed into α-Al ₂O₃ nanocrystalline matrix. Zero-field-cooled (ZFC) and FC magnetic curves are typical of superparamagnetic materials and indicate the occurrence of high-strength particle interactions.