Synthesis of Fe–FeAl2O4–Al2O3 by high-energy ball milling of Al–Fe3O4 mixtures
Introduction
Intensive mechanical treatments on crystalline solids produce development of new surfaces, plastic deformation and accumulation of structural defects, which may induce reactions with a kinetic and thermodynamic behavior very different from that of thermally initiated reactions [1], [2].
Mechanochemical processes involving reactions between metals and crystalline oxides have become of interest in the last decade as a means of developing metastable and non-crystalline materials with controlled properties [3], [4]. This is mainly due to their potential technological applications in structural, magnetic or electric materials.
Mössbauer spectroscopy has been extensively applied to studies of spinel structures containing iron [5], [6], most of which have technological applications because of their magnetic properties. Through this technique it is possible to know the oxidation state of iron cations (Fe2+ or Fe3+) and its occupancy in the different interstitial sites of the crystalline lattice. This knowledge can be very important because the magnetic properties of these materials depend critically on the site distribution of the different cations and on their interactions.
Hercynite (FeAl2O4) is a mixed oxide with a normal spinel structure, where one eighth of the tetrahedral sites are occupied by Fe2+ cations and one half of the octahedral sites are occupied by Al3+ cations. In addition, as a result of the formation processes [7], it commonly contains Fe3+ cations in octahedral positions. Crystalline hercynite exhibits a high saturation magnetization (123 emu/g), higher than other magnetic ceramics like magnetite (92 emu/g) [8]. Hence, it is interesting to study this material both from the point of view of its structural modification and its potential applications as a semi-hard magnetic ceramic.
It is a well-known fact that the mixture Al–Fe3O4 undergoes the self-sustained reaction (1), which releases a great amount of heat:Previous studies have been devoted to the mechanochemical activation of this system [9], [10]. In these reports, the investigations were focused on the structural details, performing exhaustive compositional analyses of a complex and changing system. However, a complete description of the reactions that occur during the mechanochemical process has not been yet provided.
In this work, using information yielded by complementary analytical techniques, like Mössbauer spectroscopy (MS), X-ray diffraction (XRD) and differential thermal analysis (DTA), we have investigated the physicochemical behavior of a mechanochemically activated mixture Al/Fe3O4, contributing to elucidate the sequence of reactions in the formation of Fe–FeAl2O4–Al2O3 composite powder.
Section snippets
Preparation of samples
Reactive mixtures were prepared from metallic Al (commercial reagent, 99.9 wt.%) and a concentrate of magnetite ore, from Sierra Grande (Chubut, Argentina), with ⩾97.5 wt.% Fe3O4. The major impurities in magnetite were a clay mineral, most probably illite and quartz (both in similar weight concentrations). The average particle size of both reactants was below 44 μm. An Al:Fe3O4 molar ratio of 2.67:1 was used. A planetary laboratory ball-mill (Fritsch Pulverisette 7) with vials and balls of
X-ray diffraction
Fig. 1 shows the diffraction patterns of the as-milled samples. From 10 up to 30 min of treatment, a gradual reduction of the crystallinity is evidenced by a notable decrease in the diffracted intensity (reaching 25% in the case of Fe3O4). Moreover, a broadening of the peaks with increasing activation times can be observed, due to the reduction of the crystallite size and microstrains generated by the milling. The XRD diagram of sample S37 reveals that the powder has undergone an almost
Discussion
According to the above results, the reactive mixtures experience a self-sustained reaction between 30 and 37 min of activation, resulting in a powder composed of Fe, Al2O3 and FeAl2O4.
If only the XRD diagrams (Fig. 1) were analyzed, it might be concluded that for the first 30 min of mechanochemical activation, amorphization of the crystalline phases and particle agglomeration are the only changes produced. However, the Mössbauer spectra (Fig. 2, Fig. 3) already reveal the presence of a small
Conclusions
The mechanochemical activation of Al–Fe3O4 mixtures triggers a self-sustained reaction, which results in the formation of an Fe–FeAl2O4–Al2O3 metal–ceramic complex powder of variable composition. The evolution of the system can be described as a three-step reaction. Towards this formulation, the sensitivity of Mössbauer spectroscopy was essential because the emergence of the reaction products, like hercynite and iron, could be detected ahead of XRD.
DTA analyses indicate that the resultant α-Fe
Acknowledgements
This work has received partial economic support by CONICET, ANPCyT, CICPBA and UNMdP, Argentina.
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