| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 92 === In the past decade, copious researches in carbon nanotubes (CNTs) have been preformed after their discovery in 1991 due to their symmetric structure, unique properties and potential applications. In recent years, many studies of the special properties of carbon nanotubes, such as mechanical properties, electron transport properties, and field emission at nanoscale dimensions have been attempted.
A new approach to prepare novel CNT bundles has been developed in this study. This approach entails chemical vapor deposition of carbon within the pores of an alumina template membrane using acetylene pyrolysis with ferrocene as catalyst. Many attempts have so far been made to fabricate one-dimensional nanostructures using template synthesis; however, little attention has been paid to such a bundle structure.
The bundles with 200nm in diameter and 60μm in length are composed of amorphous carbon and tangled multi-walled carbon nanotubes with an average outer diameter of 12.5nm. The CNTs, with a narrow distribution of diameters, are formed by highly crystalline graphite layers.
In addition, some carbon nanotubes in the bundles are filled with Fe or Fe3C inside the tube in the form of nanorod (in a diameter of 5–15nm and length of <1μm) or nanoparticle (in a diameter of 5–10nm). These Fe or Fe3C-filled nanotubes exhibit high coercivities of 392Oe at 350K and 1633Oe at 5K, characterized by a superconducting quantum interference device. The coercivity measured at room temperature is 510Oe which is far greater than that of bulk iron (~1Oe), possibly due to the tube’s geometry and orientation. The bundles exhibit magnetic susceptibilities of 4.37×10-3 emu/g•Oe at 5K and 17×10-3 emu/g•Oe at 300K.
The mechanical properties of the bundles were studied by nanoindentation. The elastic modulus of the bundles was calculated to be 166 GPa. After graphitization, the bundles became CNTs and graphite which exhibited a superior elastic modulus.
The I-V characteristics of the bundles measured by a two-probe method show that the resistance increases with decreasing temperature, showing a semiconducting behavior. The resistivity of the bundle at 300K is about 5.77��-cm. Furthermore, an unusual voltage jump has been observed, suggesting that it is due to high contact resistance between the electrodes and the bundle.
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