Abstract
(Englisch)
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The partners of CARBEN are Montena, a manufacturer of capacitors, Innogy (formerly National Power), a electricity utility interested in the electrochemical storage of energy, Microcoat, a manufacturer of vacuum deposition systems, and three university groups at Cambridge, Milan and Fribourg with extensive expertise on carbon.
The objectives of CARBEN are to control the surface area of nano-structured and nanotube carbon films to give large, useful surface areas for electrochemical and field emission applications. Montena use nano-structured carbon films from Milan University and nanotube mats from Fribourg University to make electrochemical double layer capacitors or 'super-capacitors', with higher energy densities and power densities. Montena look towards these new forms of carbon to replace the large surface area graphite particles presently used. Innogy are testing mats of aligned carbon nanotubes from Cambridge University, random CVD grown nanotubes from Fribourg University and nanostructured carbon films from Milan, to develop electrode surfaces with low over-potentials for electrochemical reactions. The particular interest is for regenerative fuel cells for bulk electricity storage. Fribourg and Cambridge Universities are evaluating the field emission properties of nanotubes and nanostructured carbon. The project involves Microcoat developing a larger, industrial scale version of the cluster-beam deposition system for the deposition nano-clustered carbon (ns-C).
We have synthesized supported nanotub films by a CVD process directly on an aluminum foil. A special thermal treatment just below the melting temperature of aluminum leads to an excellent sticking of the nanotubes on the surface of the aluminum. Therefore, these electrodes are directly used in a small electrochemical cell and the properties of the electrodes are investigated by means of impedance spectroscopy. The nanotube films have an interesting specific capacities (10 F·g-1) and a low serial resistance. Detailed investigation of the carbon nanotubes filmes have shown that the gravimetric density of the films is as low as 0.3 g·cm-3. This value indicates that about 90% of the nanotube films are empty. Therefore, we will modify the groth process in order to increase the density of the carbon films.
By means of field emission spectroscopy we determined the emitter work function and the local electric field present at the emission site independently. From this data we could relate the electron emission at low applied fields, observed for various carbon thin films (e.g. CVD (chemical vapor deposition) diamond and DLC (diamondlike carbon)), to sharp protruding features present of the surface of these films. These sharp tip-like features enhance the applied electric field of around 5 Vmm-1 locally to 2500 Vmm-1 which is sufficient for Fowler-Nordheim tunneling. The factor by which the applied field is enhanced locally depends on the aspect ratio (height/radius) of the tip-like feature. In this context carbon nanotubes are ideally suited to give rise to large field enhancement, as with a length larger than 1 mm and diameters down to 1.4 nm their aspect ratio can easily exceed 1000.
Therefore we are concentrating now our efforts on the field emission properties of carbon nanotubes. The success of carbon nanotube field emitters in real world applications will depend mainly on three parameters:
- Site selective nanotube growth or deposition with micrometer precision.
- Homogenous field emission properties in large area, multiple emitter arrays.
- Stable emission current over long periods of time.
In collaboration with the EPFL Lausanne (J.M. Bonard and K. Kern) we have demonstrated the feasibility of growing micro-patterned nanotube thin films using micro-contact printing. We succeeded in growing well-defined micrometer sized patterns of multiwalled carbon nanotubes on silicon substrates. The field emission properties of these micro-patterned nanotube films were investigated using scanned anode field emission microscopy. Using this technique we evidenced the influence of electrostatic shielding on the field emission properties. Which means that for very dense nanotube films a decrease in the field enhancement could be observed. From experiment and computer simulations we could identify the relevance of electrostatic shielding to the inhomogenity in emission properties.
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