Phase transition of iron inside carbon nanotubes under electron irradiation
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Selective encapsulation of different materials or phases of a material inside a carbon nanotube leads to controlling local properties of the nanotube. We report a method of synthesizing stable ␥-Fe selectively inside a carbon nanotube by transforming ␣-Fe through electron irradiation in situ inside a transmission electron microscope. Therefore, this method enables a single nanotube to encase both high (␥-Fe) and low (␣-Fe) temperature phases of iron simultaneously. ␥-Fe produced by this method may be used as a novel catalyst, and its presence inside a carbon nanotube may affect the physical properties of the nanotube, which therefore can be used to modify the nanotube. I. INTRODUCTION
Iron has a body-centered-cubic (bcc) crystal structure (␣-Fe, space group: Im3m) in the ambient atmosphere. ␣-Fe transforms to ␥-Fe (space group: Fm3m) with a face-centered-cubic (fcc) crystal structure at 912 °C. The volume of iron is reduced by about 9% upon this ␣-to-␥ transformation. At 1394 °C, ␥-Fe transforms back to the bcc structure (␦-Fe) before it melts at 1538 °C. This rather abnormal transformation sequence of iron is induced by its transition in magnetic properties; the Curie transition from the ferromagnetic (spin ordered) state to the paramagnetic (spin disordered) state causes a substantial change in the heat capacity and entropy, bending the ␣-␥ boundary from a negative slope to a positive slope in the pressure–temperature phase diagram of iron.1–4 It has been known that ␥-Fe is antiferromagnetic at low temperatures (the Neel temperature is around 67 K).5,6 However, it has also been reported that ␥-Fe may be ferromagnetic when its crystal structure is deviated from the bulk fcc structure of iron as is the case for a very thin ␥-Fe film on a copper substrate.7 Also, ␥-Fe has an electronic structure different from ␣-Fe.8,9 ␥-Fe has a higher density and a higher carbon solubility than ␣-Fe. Carbon nanotubes (CNTs), which are one of the carbon polymorphs, are graphitic sheets rolled up into a seamless tubular shape. They have a lot of unique mechanical, optical, and electrical properties.10–14 Iron is one of the most important catalysts used for synthesis of single-walled or multiwalled CNTs (SWNTs and MWNTs, respectively) by chemical vapor deposition, arc discharge, or laser ablation.15 Thereby synthesized CNTs a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0242 J. Mater. Res., Vol. 19, No. 6, Jun 2004
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encase nanosized catalytic particles. Because ␥-Fe is different in many respects from ␣-Fe as mentioned above, synthesis of ␥-Fe inside CNTs can lead not only to the exploitation of the high-temperature phase of iron, but to the novel alteration of the electronic structure of the nanotube. According to a recent report,16 some iron particles in CNTs synthesized below the ␣-␥ transition temperature can form and retain ␥-Fe (fcc) instead of the more stable ␣-Fe (bcc) at room temperature, which suggests a way of using ␥-Fe
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