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Characterization of Nanocrystalline Al2O3 Aerogels Containing Fe and Co

Maria F. Casula 1Anna Corrias Andrea Falqui Giorgio Paschina 

1. Universita di Cagliari, Cittadella Universitaria di Monserrato, Monserrato (Cagliari) 09042, Italy

Abstract

The sol-gel technique has been succesfully adopted for the preparation of composites in that it allows a homogeneous distribution of the dispersed phase thanks to the mixing of the precursors at a molecular level. Moreover, composites with tailored morphology and texture can be obtained by using the appropriate gel drying procedure.
In this work, we used the sol-gel method followed by high temperature supercritical drying to prepare Al2O3 aerogels containing Fe and Co. Pure Al2O3 aerogels were also prepared as a reference. The crystal structure of the aerogel samples was characterized by X-Ray powder diffraction (XRD), selected area electron diffraction and X-Rays absorption spectroscopy (XAFS). The texture was studied by N2 physisorption and TEM. Thermogravimetric analysis (TGA and DTA) and calcination treatments were performed in order to study the evolution of the porous texture, the particle size and the crystal phase as a function of the heat treatment.
The aerogel sample after the supercritical drying are mesoporous and have surface areas around 500 m2•g-1. Bright field TEM observations show the presence of needle-like particles. Dark field TEM observations indicate that the needles are not single crystals, but are made out of spherical nanocrystals arranged into chains.
After calcination, the aerogels retain a mesoporous texture and exhibit a significant surface area even after heat treatment at 900C. Upon calcination, the formation of a cubic nanocrystalline phase which has a structure similar to γ -Al2O3 with substitutional Fe and interstitial Co is observed. Structural details are obtained by XAFS and Mossbauer spectroscopy.
On the other hand, thermal treatments in reducing atmosphere give rise to phase separation resulting in FeCo alloy-Al2O3nanocomposite formation.

 

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Presentation: oral at E-MRS Fall Meeting 2004, Symposium I, by Maria F. Casula
See On-line Journal of E-MRS Fall Meeting 2004

Submitted: 2004-05-20 10:06
Revised:   2009-06-08 12:55