The ball milling induced transformation of α-Fe2O3 powder in air and oxygen atmosphere

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Abstract

The mechanochemical treatment of α-Fe2O3 powder was done concurrently in air and oxygen atmospheres using a conventional planetary ball mill. The influence of the duration of milling and of the balls-to-powder mass ratio on the transformation of α-Fe2O3 was investigated. Under appropriate milling conditions, α-Fe2O3 completely transforms to Fe3O4, and for prolonged milling to the Fe1−xO phase, either in air or oxygen atmosphere. Owing to the higher oxygen pressure, the start of the reaction in oxygen is delayed by ∼1 h in comparison with the reaction in air. The reverse mechanochemical reaction Fe1−xO→Fe3O4α-Fe2O3 takes place under proper oxygen atmosphere. The oxygen partial pressure is the critical parameter responsible for the mechanochemical reactions. The balls-to-powder mass ratio has a considerable influence on the kinetics of mechanochemical reactions. Below the threshold value the reaction does not proceed or proceeds very slowly. Plausibly, three phenomena govern mechanochemical reactions: (i) the generation of highly energetic and localized sites of a short lifetime at the moment of impact; (ii) the adsorption of oxygen at atomically clean surfaces created by particle fracture; and (iii) the change of activities of the constituent phases arising from a very distorted (nanocrystalline) structure.

Introduction

A number of experimental results regarding the mechanochemical treatment of iron oxides, i.e. α-Fe2O3 (hematite), Fe3O4 (magnetite), γ-Fe2O3 (maghemite) and FeO (wüstite) have been published [1], [2], [3], [4], [5], [6], [7], [8], [9]. The transformation of α-Fe2O3 to Fe3O4 and, subsequently, to the FeO phase either in argon or air atmosphere was observed when the starting α-Fe2O3 powder was milled in a shaker mill [3]. Opening of the vial, thus replenishing the vial atmosphere with air suppresses the formation of FeO. Similar results were obtained by milling a SiO–25 at.% Fe2O3 powder mixture in a planetary ball mill: under ‘closed’ milling conditions the α-Fe2O3 phase transforms into an iron-rich spinel phase while under ‘open’ milling the α-Fe2O3 phase remains untransformed [4]. The α-Fe2O3 powder was milled in a so-called ‘low-energy’ mill in various milling atmospheres, i.e. vacuum, argon or air under dry or wet (water, glycerine or benzene) conditions [5]. The complete transformation of α-Fe2O3 to Fe3O4 was accomplished by milling in vacuum and argon atmosphere, while a transformation in air was not observed. On the other hand, metastable γ-Fe2O3 easily transforms to stable Fe3O4 [6], [7]. The transformation kinetics of γ-Fe2O3 and Fe3O4 powders were recently compared by milling in a planetary ball mill [8]. In both cases, mechanochemical treatment leads to the formation of α-Fe2O3, whereas the kinetics of γ-Fe2O3 transformation are faster than the Fe3O4 phase.

Recently, we reported the results of the mechanochemical treatment of α-Fe2O3 powder in air atmosphere using a planetary ball mill [10]. We demonstrated that the transformation of α-Fe2O3 to Fe3O4 greatly depends on the manner in which the milling was performed. Frequent opening of the vial suppresses the transformation, otherwise when keeping the vial closed the complete transformation of α-Fe2O3 to Fe3O4 occurs. We observed that the reverse reaction of the already formed Fe3O4 to α-Fe2O3 may also take place during milling. Therefore, from those experimental results, we concluded that the oxygen partial pressure is a dominant parameter controlling the transformation of α-Fe2O3 to Fe3O4.

Considering all the above reports it becomes clear that the mechanochemical treatment of iron oxides is very sensitive to the milling conditions, i.e. type of mill, the mill speed, number and mass of balls used, etc. as well as the vial atmosphere, above all the oxygen partial pressure. Therefore, we decided to undertake a new series of experiments with the goal to further understand the phenomena occurring during the mechanochemical treatment of iron oxides. Our prime interest was to examine the behaviour of α-Fe2O3 powder under milling action in oxygen atmosphere since such investigations have not been done up to now. Milling was concurrently done in air atmosphere. We investigated the influence of the milling time and the balls-to-powder mass ration (BPMR) on the transformation of α-Fe2O3 either in air or oxygen atmosphere. Previously [10], in an attempt to explain the experimental results we elaborated a hypothesis that mechanochemical reactions proceed through excited short lifetime sites. Here, new hypotheses induced from the obtained experimental results have been proposed. In addition to localized highly energetic states, we discuss two other phenomena which plausibly also have an important effect on mechanochemical reactions. One is the adsorption of oxygen on atomically clean surfaces and the other is the change of activities of the constituent phases arising from a very distorted (nanocrystalline) structure.

Section snippets

Experimental procedure and methods

Commercial α-Fe2O3 powder of a purity higher than 99% was used as the starting material. Mechanochemical treatment was performed in a Fritsch Pulverisette 5 planetary ball mill. Two hardened-steel vials of 500 cm3 volume, charged with 40 hardened-steel balls of a nominal diameter of 13.4 mm were used as the milling medium in all the experiments. The angular velocity of the supporting disc and vials was 33.2 (317) and 41.5 rad s−1 (396 rpm), respectively. The powder was milled concurrently in

Influence of milling time

The magnetization of the powders prepared by milling the starting α-Fe2O3 powder (mass of 10 g) for various milling times in air and oxygen atmosphere is shown in Fig. 1. For powders milled in air atmosphere, magnetization remarkably increases after 1 h of milling, reaches a maximum for 3 h and for powders milled for 4, 5 and 7 h (without any interruption or opening of the vials) decreases. For powders milled in oxygen, the curve is similar except that it is shifted by ∼1 h toward longer

Discussion

In our previous work [10], in order to explain the transformation of α-Fe2O3 to Fe3O4 in air atmosphere we proposed a simple model: the reaction 6Fe2O3⇔4Fe3O4+O2 occurs at the moment of impact by a process of energization and the freezing of highly localized sites of a short lifetime (sometimes called ‘hot spots’ or ‘thermal spikes’). By local modelling of the collision event we derived a plausible temperature of ∼1500 K generated at the moment of impact. For that temperature, from a

Conclusion

Significant structural changes occur during the mechanochemical treatment of α-Fe2O3 powder either in air or oxygen atmosphere. The experimental results show that the oxygen partial pressure is a critical parameter influencing the transformation α-Fe2O3⇔Fe3O4⇔Fe1−xO in both, the forward and reverse directions. Under proper milling conditions, i.e. power injected to the powder material, the mechanochemical treatment of iron oxides may be controlled by oxygen pressure.

Mechanochemical treatment

Acknowledgements

The authors thank Dr A. Dekanski (ICTM, University of Belgrade) for the XP spectroscopy (RIBER OPX 150 spectrometer) examination. Helpful discussions with Professor D. Poleti are gratefully acknowledged. This work was supported by the Ministry of Science and Technology of the Republic of Serbia.

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