Fabrication of Fe-Al nanoparticles by selective oxidation of Fe-Al thin films
© Jang et al.; licensee Springer. 2013
Received: 15 July 2012
Accepted: 8 February 2013
Published: 2 April 2013
The possibility of a new technique for fabricating nanoparticles from thin films using selective oxidation in an atmosphere mixture of water vapor and hydrogen was investigated. Fe-5wt.%Al films were RF-sputtered and annealed in the atmosphere mixture at 900°C for up to 200 min, in order to oxidize aluminum selectively. Thermodynamics simulation showed that temperatures exceeding 800°C are necessary to prevent iron from being oxidized, as confirmed by the depth profile of XPS. As the annealing time increased, the morphology of the 200-nm Fe-Al films changed from the continuous to the discontinuous type; thus, particulate Fe-Al films formed after 100 min. The particulate 10- to 100-nm Fe-Al films showed super-paramagnetic behavior after the oxidation. Thus, a new technique for fabricating nanoparticles was successfully introduced using selective oxidation.
Aluminum oxide, Al2O3, formed on the surface can be used as a mechanically protective, oxidation-resistive, electricity-insulating film. For example, it was reported that in Fe-Al-X bulk alloys, the aluminum elements out-diffused along the α-Al2O3 grain boundary formed in an alumina network on the boundary by the selective oxidation of aluminum when the alloys were annealed in the atmosphere . The Al2O3 layer could also be formed exclusively by selective oxidation when Fe-Al alloys were annealed in a mixed atmosphere of water vapor and hydrogen at elevated temperatures [2, 3]. It was also reported that the Fe depletion zone appeared in the annealed Fe films on an Al2O3 substrate . This indicates that continuous Fe films changed to discontinuous films, i.e., particulate films. However, they did not focus on the morphological change of the Fe/Al2O3 films, nor the reasons for it. It is interesting to investigate the morphological changes and related properties of Al2O3/Fe-Al films, in which oxide is formed on the surface of Fe-Al films by the selective oxidation of aluminum in Fe-Al films in a mixed atmosphere.
In this study, morphological change, as well as analyses of the chemical, structural, and magnetic properties of selectively oxidized Fe-Al films formed on SiO2 substrates are investigated by X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM), with a special emphasis on the possibility that nanoparticles in the shape of a sphere can be formed by the selective oxidation of aluminum in Fe-Al films.
Results and discussion
Therefore, silicon dioxide in contact with the Fe-Al film is reduced to silicon while the metallic aluminum in the Fe-Al films is oxidized into Al2O3. The silicon created diffuses out while the Al2O3 diffuses into SiO2 and forms a mullite (3Al2O3·2SiO2). Thus, the SiO2 layer transforms into a mixture of mullite and SiO2. The out-diffused silicon can be dissolved into small Fe-Al particles, which are formed in an early stage of oxidation. The reason for non-detection of Si in large particles is not clear yet.
The 10- to 200-nm-thick RF-sputtered Fe-Al films were oxidized in the atmosphere mixture at 900°C for up to 200 min. Small particles formed in the early stage of oxidation while large particles formed by contraction of the film or the growth of black holes. Formation of Al2O3 on the surface of the film was confirmed by both the depth profile and chemical shift of the Al2p state upon XPS analysis. The 10- to 100-nm-thick films after oxidation showed superparamagnetic behavior that was due to Fe-Al nanoparticles. Thus, a new technique for fabricating nanoparticles by selective oxidation has been successfully introduced.
This work was supported in part by the 2011 WATC program of Korea Ministry of Knowledge Economy and in part by the 2011 R&D program of Korea Ministry of Education Science and Technology.
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