Effects of mechanical treatment on phase transformation and sintering of nano-sized γ-Fe2O3 powder

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Abstract

The effects of mechanical treatment on the phase transformation and sintering of nano-sized γ-Fe2O3 powder were studied. γ-Fe2O3 powder was obtained by calcining iron tartrates at 300 °C. The mechanical treatment increased the contact areas in theγ-Fe2O3 powder, which acted as nucleation sites for the γ→α-Fe2O3 phase transformation, and resulted in lowering the transformation temperature. The greater surface area and fine equiaxed particle of the milled powder thus obtained were due to the vermicular microstructure development being inhibited. Consequently, the samples with mechanical treatment after sintering developed a uniform fine-grained microstructure.

Introduction

Iron oxide is widely used as a catalyst, pigment and gas sensitive material [1]. In many cases nanocrystalline iron oxide can enhance materials performance or improve industrial processing. For instance, synthesis of ferrites can be achieved at a much lower temperature by using nano-sized iron oxide as a raw material [2]. Additionally, the gas sensitivity of α-Fe2O3 also can be improved remarkably by using the ultrafine α-Fe2O3 powders [3]. Therefore it is of importance to control the particle size, morphology, and texture of the iron oxide system.

Recently, many different methods have been used in preparing nanosized α-Fe2O3. Among the various methods, the thermal decomposition of precursors, such as tartrates [4] and ferrous oxalate dihydrate [5] have been widely investigated due to the benefits of easily controlled process and lower cost. There are two well-known crystalline types of Fe2O3.: maghemite (the γ phase) and hematite (the α phase). The phase transition of γ→α-Fe2O3 takes place during calcination at about 400 °C [6]. The phase transformation which occurs during calcination gives rise to transformed α-Fe2O3 powder which has undergone considerable aggregation and grain growth [7]. The above characteristics are detrimental to the formation of nano-sized α-Fe2O3 powder.

Mechanical treatment (mechanochemical reactions) of inorganic solids has become extremely important in many processes in chemical technology, and materials science [8], [9]. Many solid state reactions, which normally occur at elevated temperatures, can be facilitated by mechanical treatment [10], [11]. In mechanical treatment induced polymorphic transformation, the transition is from a metastable to a stable phase due to the strain energy and shear energy induced by the mechanical treatment.

Mechanochemical effects on the γ→α-Fe2O3 phase transformation have been widely discussed in literature. [12], [13] The activation energy for the transformation is generally reduced by mechanical treatment applied to γ-Fe2O3. However, the effects of mechanical treatment on the morphology of the intermediate and final products and the microstructure development after sintering have not been reported.

The purposes of this study are to use the mechanical activation of γ-Fe2O3 to prepare the nano-sized α-Fe2O3. and to investigate the mechanical treatment dependence of the phase transformation and sintering of γ-Fe2O3 in terms of strain energy using transmission electron microscopy (TEM), X-ray diffractometer (XRD) and scanning electron microscopy (SEM) techniques.

Section snippets

Sample preparation

Iron nitrate hydrate (reagent grade) was dissolved in ethanol to prepare Fe+3 ethanol solution with concentrations of 0.2 mol/l. A mixture of 0.4 mol/l tartaric acid ethanol solution was prepared. The Fe+3 ethanol solution changed abruptly into a viscous sol as soon as tartaric acid solution was added. By heating the sol at 50 °C, ethanol was evaporated off, and a dry iron tartrate powder was obtained. γ-Fe2O3 powder was obtained by calcining iron tartrate powder at 300 °C for 2 h. The

Phase identification

Fig. 1 shows the X-ray diffraction patterns of the gel before and after heat treatment. It can be seen that the phase of the powder calcined at 300 °C was γ-Fe2O3. A γ→α-Fe2O3 phase transformation took place during calcination between 300 and 400 °C. An abrupt increase in the amount of α phase occurred when the calcination temperature rose above 400 °C. α- Fe2O3 was the only phase present for the powder calcined above 500 °C.

The X-ray diffraction patterns of γ-Fe2O3 (obtained by calcination at

Effects of mechanical treatment on γ→α-Fe2O3 phase transformation

Fig. 4 illustrates that the temperature of γ→α-Fe2O3 phase transformation for milled powder is lower than that of the powder without mechanical treatment. Zieliński et al. [14] also obtained similar results for Al2O3 powder during γ→α-Al2O3 phase transformation. The temperature of γ→α-Al2O3 phase transformation decreased with increasing milling time. It was proposed that the decrease in temperature resulted from the presence of internal stress induced by milling in the Al2O3 particles. In the

Conclusion

  • 1.

    Mechanical treatment increased the number of particle contacts, which acted as nucleation sites for the γ→α-Fe2O3 phase transformation and resulted in lowering the transformation temperature.

  • 2.

    Mechanical treatment of γ-Fe2O3 powders could be an attractive method for the production of nano-sized and equiaxed α-Fe2O3 powders.

  • 3.

    The mechanical treatment of γ-Fe2O3 powder enhanced microstructure development by preventing the development of vermicular pore structure during transformation, thus obtaining

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