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Forest Bundles


“Structural model for dry-drawing of sheets and yarns from Carbon Nanotube Forests”

The strength of multiwall carbon nanotube (MWNT) forests can be increased by dry-drawing them together into yarn or string. Studies using SEM pictures show that carbon nanotubes (CNT’S) with strong adjacent forces that stand up at a 90 degree angle are the best for dry-drawing and classified as drawable CNT forests. Van der waals forces seem to be the prominent force that forms the bundles of CNT forests. If this secondary force is strong, it will result in a sufficient vertical alignment within the CNT necessary for drawability. However, van der waals alone do not hold the bundles together. After closely observing the drawing of CNT it is evident that the first bundle completely peels off from the forest yet somehow is still able to remain attached and pull out the second bundle. Scientists observed in SEM images that in the pulling-out process there is a phenomenon called the “unzipping process” in which the bending of bundles attaches it to a neighboring bundle in the forest and these interconnections move in the direction that it is being pulled. The unzipping of CNT forests increases its overall density and can be concentrated up to a point until an external force clings to the next bundle and starts pulling it to a point as well. If the pulling-out of the forest is not kept at a constant distance to maintain interconnections then the bundles will detach and get separated from the forest.

Camouflaging 3D objects


A group of researchers from the University of Michigan are using the carbon nanotube forest’s low refractive index of low density aligned nanotubes to camouflage 3-d objects or simply make it a black sheet. The refractive index of a given material is how much it can slow down light. Carbon nanotubes have a very low index almost as close to air so it doesn’t reflect any light when it hits the coating. Jay Guo, a professor in the Department of Electrical Engineering and Computer Science says "It's not cloaking, as the object can still cast a shadow. But if you put an object on a black background, then with this coating, it could really become invisible." They demonstrate this by making a microscopic, silicon based tank shape and then growing the nanotube carpet on top of it. Although the tank’s contour’s was seen under white light, it was not seen when the nanotube coating was applied. As shown in the image on the right, it is not perceivable. Carbon nanotubes are also known for absorbing light, but researchers were able to push it to a great percentage so it doesn’t absorb the light. This has also inspired other applications such as using this “black material” as camouflage paint for stealth aircrafts to improve their detection so that when they go into combat, enemy lines won’t be able to sense the aircraft. Overall, the university is also looking to put this this new technology eventually into the market.

Connecting Computer Chips onto Heat sinks


Researchers at Brigham Young University (BYU) have created stronger microstructures that can form taller and narrower 3-d shapes for microelectromechanical systems (MEMS). MEMS are extremely small, normally measured at the scale of microns, and are usually made from silicon-based materials. The process called carbon nanotube templated microfabrication (CNT-M) is the idea to make a carbon nanotube forest as a sort of scaffolding or riser. This scaffold is extremely fragile however until filled with almost any type of filler material. Once fortified, one can form tall and fine featured microstructures. This new technique is already being used to make chemical detection devices can detect chemical reactions during pharmaceutical production. It also is being looked at for being used in long-lasting batteries.

To help thermal interface materials cool off computer chips, researchers in Purdue formed carbon nanotube forests on pieces of indium to give it greater contact for optimum heat transfer. First, forming dendrimer templates on a silicon surface, to have a structure for which the forest to grow. Then adding metal catalyst particles between the dendrimers, needed in growing said nanotube forest. Finally by adding heat, the polymer will burn away leaving only the metal particles. The silicon chip is then placed in a chamber and exposed to methane gas and microwave energy. The microwave energy breaks down the methane leaving carbon atoms. The catalyst then promotes the carbon to align itself into tube like figures and they grow vertically up. "The dendrimer is a vehicle to deliver the cargo of catalyst particles, making it possible for us to seed the carbon nanotube growth right on the substrate," Amama said. "We are able to control the particle size. what ultimately determines the diameters of the tubes and we also have control over the density, or the thickness of this forest of nanotubes. The density, quality and diameter are key parameters in controlling the heat-transfer properties."