Synthesis of alumina-metal nanocomposites by mechanical alloying

The synthesis of nanometer-sized [MATH]-Al2O3-metal composites can be performed by room temperature ball-milling of mixtures of metal-oxides and aluminium as shown by Matteazzi and Le Caër. The average crystallite size of the alumina-metal composite so obtained is in general about 10nm. Such composites may also be prepared by direct grinding of a mixture of [MATH]-Al2O3 and of a metal or an alloy. The present work is devoted to the study of the reaction mechanisms by X-Ray diffraction and 57Fe Mössbauer spectroscopy for the [MATH]-Al2O3-Cr, [MATH]-Al2O3-Fe, [MATH]-Al2O3-(Fe-Cr) and [MATH]-Al2O3-Ti systems. Mössbauer spectroscopy shows that non-completely reduced Fe, partly in the form of Fe2+, still exists at the end of the synthesis and belongs to a ternary Al-Fe-O oxide in [MATH]-Al2O3-Fe composites although it cannot be indexed by X-Ray diffraction. The Fe2+ component is mainly associated with iron environments in or similar to those of hercynite. Species like Fe3+ in Al2O3 or in grain boundaries are observed, Fe4+ and Feo cannot be excluded. Differences are obtained by reactive milling and by direct milling the reaction products. Reactive milling of Al-TiO2 (anatase) yields [MATH]-Al2O3-Ti nanocomposites. The transient formation of a high-pressure orthorhombic form of TiO2 of the [MATH]-PbO2 type is observed