B. Hallouche

Synthesis and characterization of Fe-Al2O3 composites

Abstract We have synthesized composites of the type Fex(Al2O3)100−x by arc-melting compacted pellets of iron and alumina powders with starting compositions of x=0.4, 2, 3, 5, 10, 20, 40 and 60. Aluminas of two different nominal grades (99.7% and 99.99%) were used in the preparation of the composites. The samples were characterized by X-ray diffractometry, magnetization and Mossbauer spectroscopy. The results revealed the formation of the spinel FeAl2O3+z (hercynite phase) and of metallic iron partially nanostructured by this melting process. The Mossbauer subspectral areas of the formed phases were plotted against x, showing that increasing the starting iron content of the mixed powders favors the hercynite reaction. It was also observed that when the more pure alumina is used for cast, more hercynite is formed or, alternatively, less iron nanocrystallites precipitate, when compared to the less pure oxide. An arc-melted sample (x=2) was annealed at 1200°C in a hydrogen atmosphere, revealing hercynite reduction and phase separation of metallic iron.

Phase evolution and magnetic properties of a high-energy ball-milled hematite–alumina system

The system (α-Fe2O3)x(α-Al2O3)1−x was subjected to 24 h of high-energy ball-milling varying its nominal concentration, x. The milled samples were structurally and magnetically characterized at room temperature by x-ray diffraction, Mossbauer spectroscopy, and magnetic measurements. Mossbauer studies were also performed in the temperature range 250–6 K. As a result of the earlier analyses, it was observed that the milling products were extremely dependent on the hematite starting concentration. In samples with low α-Fe2O3 initial concentration (i.e., x⩽0.12), the paramagnetic solid solution α-(FeΔYAl1−ΔY)2O3, the α-Fe and the FeAl2O4 phases were identified, along with alumina, which was always residual. The presence of spinel and metallic iron was attributed to the stainless-steel vial and balls abrasion. For x>0.12, the iron component was no longer present but another magnetic component, corresponding to an aluminum-substituted hematite phase, α-(Fe1−ΔWAlΔW)2O3, could be seen to increase with increasing x...

Structural and Mössbauer characterization of the ball-milledFex(Al2O3)100−x system

Metal-oxide composites were synthesized by high-energy ball milling of metallic iron (α-Fe) and alumina (α-Al2O3) powders, varying the starting relative concentration and the milling time. The samples were characterized by scanning electron microscopy, x-ray diffraction, and Mossbauer spectroscopy. The results revealed the formation of a FeAl2O3+W spinel phase (hercynite) and of iron (super)paramagnetic nanoprecipitates, in addition to residual magnetic iron and alumina. We also observed that the relative amounts of nanoprecipitates and hercynite for isochronally milled samples were correlated with the sample nominal concentration x, with the precursor iron being relatively more converted in those phases for low x values. Particularly for x=10 milled sample, the relative amounts of the (super)paramagnetic and spinel phases were observed to increase linearly with the milling time. An x=20∕24h milled sample was annealed in H2 atmosphere and revealed the reduction of hercynite, with iron phase separation.