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The nanotubes were produced with the dc arc-discharge method (20 V, 70 to 90 A) with graphite electrodes of 6 mm in diameter in a 350-mbar He atmosphere. As usual, the nanotubes were found in the soft sooty fibrous core material that was encapsulated in a hard-shelled cylindrical deposit on the cathode. The core material was ultrasonically dispersed in ethanol, dried, and then oxidized in air, first for 10 min at 550°C and subsequently for 10 min at 650°C, whereby about 60% of the tubes were opened. This process usually leaves some amorphous carbonaceous material at the tip (13). This material was removed by annealing the sample for 30 min at 2000°C. The high-temperature treatment graphitizes the amorphous material (which may otherwise plug the tube) and also removes dangling bonds left by the oxidation procedure. The sample was thoroughly mixed with silver nitrate, and the resulting powder was then heated to 230°C for 30 min (that is, slightly above the melting point of silver nitrate at 212°C). The purity and evolution of the sample were analyzed after each step of the procedure with a transmission electron microscope (Philips EM 430 ST).
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12644276360
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electron microscopy studies were performed on samples prepared by crushing the solidified silver nitrate deposit (containing nanotubes) after the heat treatment; in consequence, the tubes were often partially embedded in large salt particles (see Fig. 2). Our attempts to remove the excess silver nitrate enrobing the tubes by dissolving it in an ultrasound bath of distilled water were unsuccessful. This treatment caused the material inside the tubes to be removed as well.
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It is possible that the narrow tubes are dosed. Even though the high-temperature annealing of nanotubes opened by oxidation removes the amorphous carbon material at the opened end (13), the annealing could conceivably close the tubes again in a process that could be more efficient for narrower tubes. This closure could remain undetected by electron microscopy. However, in experiments with molten metals or wet chemical techniques, the typical diameters of the material filling the tubes were also ∼5 nm (11, 14, 17). This result is relevant because in these cases the narrower tubes were certainly open.
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We assume that the polarizability of the filling material is similar to that of the bulk. This approximation is reasonable for the metal and the salt, because for metallic particles of these sizes deviations are <1% [W. A. de Heer, Rev. Mod. Phys. 65, 611 (1993)], and for the salt we expect even smaller deviations because of the very short range of the electrostatic forces. In contrast, polarizability deviations for the graphitic systems are much more important because of the highly directional nature of covalent bonds. We have, however, ignored nanotribological and nano-viscous effects, which could affect the filling rates. These properties are now under further investigation.
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De Heer, W.A.1
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33
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12644288279
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note
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We acknowledge simulating discussions and valuable suggestions from Dr. P. M. Ajayan. The electron microscopy studies were performed at the Centre lnterdépartamental de Microscaple Electronique (CIME), Ecole Polytechnique Fédérale de Lausanne. We are indebted to the Brazilian Council for Scientific and Technologic Research (CNPq) and the Swiss National Science Foundation for financial support
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