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转，已经索索，无重复。 1 December 2005 Nanotube mattress doesn't sag A film of vertically aligned carbon nanotubes provides a strong, compressible foam that could lead to superior shock absorbers. Philip Ball Scanning electron micrograph of the crimping at the bottom of a film of vertically aligned carbon nanotubes after 1,000 cycles of compression. Reprinted (in part) with permission from ref. 1. Copyright (2005) American Association for the Advancement of Science. Films of aligned carbon nanotubes act as stiff, resilient and highly compressible foams, researchers in the USA have found1. They say that such nanotube foams could make much better energy-absorbing and cushioning materials than the existing foams, made from polymers or metals, that are typically used for this purpose. What's more, the electrical conductivity of carbon nanotubes could provide a useful combination of properties for these nanotube foams, enabling them, for example, to serve as compliant interconnect structures between microelectronic chips. Anyuan Cao of the University of Hawaii at Manoa and his colleagues suspected that carbon nanotubes might provide good struts for a strong, lightweight foam because they knew from previous experiments that nanotubes are extremely flexible and resilient. They can be bent through large angles without breaking, and will spring back when released2,3. And being made of carbon, nanotubes are very lightweight. The researchers made films of vertically aligned nanotubes about a millimetre thick by using standard chemical-vapour-deposition techniques with a carbon-rich gas and a metal (ferrocene) catalyst. They peeled these films from the substrate on which they were deposited, and subjected them to repeated squeezing. The films look like a densely packed forest of parallel nanotubes, although in fact they have a porosity of about 87 per cent — just 13 per cent of the film's volume is occupied by nanotubes. The films could be compressed to only about 15 per cent of their original thickness, and yet they would spring back completely when released. This recovery was very fast — as fast, in fact, as the compression head was retracted, which was at a rate of about 2 mm per second. Conventional low-density foams made from polymers such as polyurethane generally recover much more slowly, which may severely limit their effectiveness for packaging or cushioning in environments subjected to repeated stresses. The nanotube films are also much stronger than polymer foams: the compressive stress that brings about an 85 per cent strain (reduction in film thickness) is typically several hundred times greater. Metal foams, made from aluminium for example, also have high compressive strength, but aren't nearly so flexible. And the nanotube foams have a good 'sag factor': a measure of how much the film pushes back against continued compression. This is determined by measuring the stress ratio for strains of 25 and 65 per cent, and it is an important characteristic for cushioning foams. Although the nanotube films at first recover more or less their full thickness after being compressed, their thickness is slightly reduced after several hundred cycles of squeezing. Eventually this shortfall stabilizes, remaining at about a fifth of the original thickness after thousands of cycles. To understand this behaviour, Cao and colleagues took a closer look at how the nanotubes were deforming. They saw under the electron microscope that the array of tubes buckles in a strikingly coordinated way. Towards the bottom of the film, the tubes fold in a concertina-like pattern, producing zigzag folds in the film edges with wavelengths of around 12 µm. These ripples get progressively less pronounced further up the nanotubes, until they are all but indiscernible near the middle of the film. The researchers figure that the buckling and folding of each nanotube is coherent with that of all the others because they are so closely packed. This buckling becomes permanent and clearly visible after release once the film has been squeezed a few hundred times, which makes the film both softer and slightly thinner. After a few thousand cycles, the buckling is pronounced at the foot of the film. All the same, the nanotubes continue to act like springs that bounce back after compression. References 1. Cao A., Dickrell P. L., Sawyer W. G., Ghasemi-Nejhad M. N. & Ajayan P. M. Super-compressible foamlike carbon nanotube films. Science 310, 1307–1310 (2005) Article 2. Qian D., Wagner G. J., Liu W. K., Yu M. F. & Ruoff R. S. Mechanics of carbon nanotubes. Appl. Mech. Rev. 55, 495–533 (2002) Article 3. Falvo M. R. et al. Bending and buckling of carbon nanotubes under large strain. Nature 389, 582–584 (1997) Article |
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