Transmission electron microscopy investigation on the electron-stimulated oxidation of iron nitrides by 2-MeV electron i
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H. Hashimoto Department of Mechanical Engineering, Okayama University of Science, Okayama 700-0005, Japan
T. Sakata and H. Mori Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Osaka 565-0871, Japan
M. Song, K. Mitsuishi, and K. Furuya High Voltage Electron Microscopy Station, National Institute for Materials Science, Tsukuba 305-0003, Japan (Received 18 December 2004; accepted 25 April 2005)
An iron nitride sample was irradiated by 2-MeV electrons intermittently for 2100 s with a dose rate of 6.3 × 1024 e.m.−2 s−1 inside a 3-MV high-voltage transmission electron microscope. The electron-stimulated oxidation of Fe4N and Fe2–3N was investigated in situ and ex situ using conventional transmission electron microscopy and high-resolution electron microscopy. It was found that both Fe4N and Fe2–3N nitrides were oxidized by the residual gas in the vacuum chamber to form Fe3O4 oxides. The orientation relationship between Fe4N (␥⬘) and Fe3O4 (o) was (110)␥⬘//(220)o, [001]␥⬘//[001]o, and that between Fe2–3N (⑀) and Fe3O4 (o) was (110)⑀//(−220)o, [1–11]⑀//[001]o. Crystal lattice deformation from iron nitride to iron oxide took place during the dynamic oxidation process. Structural models were proposed to understand the oxide formation, and the models were confirmed by experimental observations. The irradiation effects of Fe4N and Fe2–3N crystals were compared. The results show that Fe4N is more sensitive than Fe2–3N to electron irradiation. These results are important not only for the fabrication of insulating iron oxide film, but also in the field of the surface modification of iron nitride to improve its mechanical properties. I. INTRODUCTION
Electron irradiation effects on different materials have been investigated for a long time. Studies have shown that electron irradiation can stimulate many surface and bulk processes like adsorption,1 desorption,2 decomposition,3 migrations,4 and oxidation. Electron-stimulated oxidation, which is an important aspect of electron irradiation-induced processes, offers significant advantages for studying the oxidization process over thermal oxidation. For example, the excited states of the reactant molecules achieved by electron impact lead to new surface reaction pathways that differ from thermally stimulated pathways, hence improving the otherwise low rate of oxidation in some materials. Electron-stimulated oxidation
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0240 1918
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 7, Jul 2005 Downloaded: 18 Mar 2015
can also be used as a fast, accurate, and controllable method to grow insulating oxide films because of the flexibility and spatial resolution of electron beam, which is useful for the fabrication of materials. In contrast to extensive reports devoted to the electron-stimulated oxidation of metal5–7 and semiconductor8–10 surfaces, few works have been carried out on iron nitride materials. It is known that the formation of an iron nitrid
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