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To investigate the microwave (MW) processing of Fe3O4, for which occurrence of decrystallization has been reported, the micro/nanostructures of MW-heated Fe3O4 powder were observed in this study. The specimens were irradiated by 2.45 GHz MW at the position of magnetic (H)-field maximum in a TE10 single mode applicator. The specimen was heated well above the Curie temperature in H-field. The heated specimen above 1000  C revealed the glass-like surface with the diminished x-ray diffraction (XRD) peak intensities. They resemble the reported features of decrystallization in an earlier work performed at Penn State University. According to the XRD profiles of the MW-heated specimens, formation of FeO and shift of Fe3O4 peaks to the lower angle with the broadened width were observed. To account for the findings, a model is presented that phase separation occurred into FeO and Fe3O4 resulting in an increased lattice parameter due to the increased oxygen content. This activity is caused by local transport of oxygen in nanoscale. Considering the shape of the main XRD Fe3O4 peak with a shoulder and the existence of halo in nanobeam diffraction (NBD), amorphous phase areas exist. As a result of transmission electron microscopy observation, it was shown that they were in nanoscaled localized regions, and it was not confirmed that the glass-like morphologies (or decrystallized morphologies) are totally amorphous. The observed micro/nanostructures and mechanism of the amorphous phase formation were discussed considering the Fe-O phase diagram. I. INTRODUCTION

Microwave (MW) processing has attracted worldwide attention for causing enhanced reaction kinetics and sintering rates1 and for enabling formation of different phases and microstructures in materials processing.2 Although a thorough understanding of the origins of this characteristic mechanism has not been accomplished yet, some phenomena providing clues for interpretation of the heating mechanisms and some other effects have been reported.3,4 First, MW provides selective heating5 in a mixture of materials having different absorbance of MW. Local temperature difference is brought about, leading to formation of the different phase constitutions from those expected from thermodynamics. The direct observation of the local temperature gradients has been made within a scale of several hundred microns.6 Second, by heating materials in a separated electric (E)-field and magnetic (H)-field, it was possible to discuss the MW heating mechanism in terms of either field contribution.7,8 In addition, this information provides clues to further develop the possible mechanisms of the selective heating in MW. a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0192 J. Mater. Res., Vol. 24, No. 5, May 2009

http://journals.cambridge.org

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Meanwhile, reports have been published that indicate occurrence of “de-crystallization.” This has been discovered first by irradiation of 28 GHz millimeter wave to Fe3O4 and Y2O