Synthesis of alumina-metal nanocomposites by mechanical alloying

Abstract
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 Fe_o 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.