Synthesis and characterization of Fe/Co/Ni alloys-MgO nanocomposite powders

Abstract

A mixed oxalate β-Mg_0.896Fe_0.047Co_0.034Ni_0.023C₂O₄·2H₂O of well controlled size and morphology was prepared by coprecipitation. The corresponding quaternary solid solution between MgO, FeO, CoO and NiO was prepared by thermal decomposition and calcination in a H₂/H₂O/N₂ atmosphere. The selective reduction of the solid solution in an H₂ atmosphere was studied by a combination of X-ray diffraction, Mössbauer spectroscopy, transmission electron microscopy and associated analysis. This work has brought to light the very high stability of Fe²⁺, Co²⁺ and notably Ni²⁺ when substituted for Mg²⁺ in the MgO rocksalt lattice. It is necessary to perform the reduction at 1300 °C fully to reduce the transition metal ions. The alloy particles are either distributed as relatively large particles (tens to hundreds of nanometers) at the surface of the MgO grains or as much smaller particles (≤20 nm) probably located inside the matrix grains and epitaxial with it. The composition distribution of the large surface particles is fairly broad when the reduction was performed at 1100 and 1300 °C. In contrast, it is much narrower in the powder prepared by reduction at 1200 °C, although the particles are still low in Ni compared to the target composition.

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