Feb 15, 2012 |
A rapid, uniform dispersion method for carbon nanotubes in solutions and solids
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(Nanowerk News) Harnessing the power of carbon nanotubes could get considerably easier, thanks to an advance by engineers from the University of South Carolina and the University of Georgia.
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A team led by Xiaodong Li, a professor in the College of Engineering and Computing at USC, reported a breakthrough in the handling of nanotubes in the Feb. 14 issue of Advanced Materials ("Hydrogen Passivation Induced Dispersion of Multi-Walled Carbon Nanotubes"). They combined two methods, hydrogen passivation and ultrasonication, to generate remarkably uniform dispersions of multi-walled carbon nanotubes.
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A method combining hydrogen passivation and ultrasonication was developed for the first time to disperse multi-walled carbon nanotubes (MWCNTs) in ethanol solution and epoxy resin. Excellent dispersion of MWCNTs was achieved in both media. Three-point bending tests of the MWCNT/epoxy nanocomposites revealed a remarkable increase in elastic modulus with increasing MWCNT content.
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"In applications such as lightweight and energy-efficient composites, electronic and optoelectronic devices, energy harvesting, energy conversion, and energy storage systems, carbon nanotubes have demonstrated superior performance," said Li, "but unfortunately dispersing them was always a major barrier in applications. This new technique is low cost, easy to use, and environmentally friendly--it should be quickly adapted in a wide range of areas."
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Carbon nanotubes have many desirable properties, ranging from outstanding mechanical strength to unusual electrical behavior. By incorporating them into materials, even in small 'doses,' researchers can dramatically improve a material's utility.
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But working with carbon nanotubes, which are strongly hydrophobic, can be difficult. In many solvents and polymers, their insolubility and tendency to clump together is a major obstacle to getting uniform coatings on surfaces or distributions within solids or gels.
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Ultrasonication has long been used to try to disperse carbon nanotubes in solvents, but its success is slow, middling, and all too often reversed when the sonication stops.
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Li's team combined ultrasonication with a simultaneous flow of hydrogen gas, producing fully dispersed multi-walled carbon nanotubes in ethanol in just 2 hours. The uniform dispersion, which is evident even to the naked eye, was characterized by scanning and transmission electron microscopy.
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They then fabricated a nanotube-epoxy composite with the method and examined its mechanical properties. The elastic modulus of the nanocomposite (with 1% nanotube by weight) prepared with hydrogen passivation increased nearly 100% compared to that of the pure epoxy, whereas in the absence of hydrogen passivation an increase of less than 40% was earlier reported.
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The engineers reason that energy from ultrasonication drives the breaking of C-C bonds in the nanotubes, which then react with the hydrogen to create C-H bonds. X-ray photoelectron spectroscopy confirms the addition of C-H bonds.
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This kind of modification is particularly useful, added Li, because the absorbed hydrogen is readily removed from the multi-walled carbon nanotubes by heating. "The conventional techniques--fluorine, alkane, or ionic modification, for example--introduce impurities into matrix materials," he said.
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