Filling of Chrysotile Nanotubes with Metals
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P. Ulmer Eidgenössishe Technische Hochschule Zu¨rich, Institute of Mineralogy and Petrography, 8052 Zu¨rich, Switzerland (Received 5 September 2001; accepted 19 February 2002)
Nanowires were produced by injection of molten Hg and Pb into chrysotile nanotubes. The breakdown of chrysotile and the surface tension of the molten metals are the limiting factors for the filling procedure. The thermal stability of chrysotile nanotubes was investigated by infrared spectrometry, thermogravimetry, differential thermal analysis, and x-ray diffraction analyses. For short-term thermal annealing (30 min) the tube morphology remains stable up to 700 °C. The high surface tension of both molten Pb and Hg (␥LV > 200 mN/m) requires external pressure for the melts to penetrate into the tubes. Filling of the tubes was achieved under high pressure and high temperature conditions compatible with the stability range for chrysotile determined in the annealing experiments. Transmission electron microscopy observations confirmed high filling yields for both metals. Almost all nanotubes were partially filled with lead. The length of continuous wires ranged from tens to hundreds of nanometers. Additional experiments with tin were not successful.
I. INTRODUCTION
The potential applications for nano-sized, tube-shaped components includes wires, wire-templates, microreactors, hydraulic tubes, and gas storage devices among others. The most promising contender for such applications are synthetic carbon nanotubes. Other layered materials known to form nanotubes are boron nitride,1 molybdenum disulfide,2 and tungsten disulfide.2 A naturally occurring, nano-sized, tube-shaped phase is the silicate mineral chrysotile, Mg3Si2O5(OH)4, which may represent an alternative to carbon nanotubes for certain applications such as the manufacture of nanowires. The structure of chrysotile3 consists of wrapped sheets composed of a layer of tetrahedrally coordinated silicon cations and a layer of octahedrally coordinated magnesium cations, which can be substituted by aluminum and iron. Aluminum can also be present in the tetrahedral sheet instead of silicon (Tschermak’s substitution). Chrysotile is common in hydrothermally altered ultramafic rocks and is easy to extract in large quantities. The tubes have an outer diameter ranging from 10 to 50 nm and an inner diameter between 1 and 10 nm. The tube walls are always multilayered.4 Just as for carbon nanotubes, different wrapping schemes are known.5 The most frequent polytypes are normal chrysotile with a fiber axis perpendicular to the [010] direction and parachrysotile with a fiber axis in the [010] direction.5 Cylindrical, spiral and multilayer wrapping is found for both chrysotile polytypes.5 The chrysotile nanotubes differ from the carbon J. Mater. Res., Vol. 17, No. 5, May 2002
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nanotubes in some important physical parameters, e.g., they are nonconducting, they have lower mechanical strength, their length can reach the millimeter range, and they are always un
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