Iron oxide nanoparticles hosted in silica aerogels

The investigated hybrid materials consist of non-agglomerated iron oxide particles hosted in silica aerogels. The composite material can be produced as a monolith, in any shape, and with different dilutions of the iron oxide phase. Two sol–gel chemistry routes have been followed: a solution of Fe(NO₃)₃·9H₂O has been added either to the silica gel or to the initial sol; in the latter, the iron salt provided the water required for the gel polymerisation. To obtain monolithic aerogels, the gels were dried by hypercritical solvent evacuation. On the other hand, some gels were dried by slow and controlled evaporation of the solvent, resulting in xerogels. Several heat treatments have been performed and the iron oxide particle phase, growth mechanism and crystallinity have been analysed. The composite materials were characterised by means of X-ray diffraction, M?ssbauer spectrometry and superconducting quantum interference device magnetometry. It was observed that the particle sizes (in the nanometre range) and the thermal stability of the iron oxide phases strongly depend on the preparation method that determines the microstructure of the host material. Consequently, the magnetic properties of the nanoparticles can be controlled via synthesis conditions, matrix properties and thermal treatments. Received: 5 March 2001 / Accepted: 16 June 2001 / Published online: 30 August 2001