Twisting spires, concentric rings and gracefully bending petals are a few of the new three-dimensional shapes that University of Michigan engineers can make from carbon nanotubes using a new manufacturing process.
Called "capillary forming," the process takes advantage of capillary action, the phenomenon at work when liquids seem to defy gravity and spontaneously travel up a drinking straw.
The new miniature shapes have the potential to harness the exceptional thermal, mechanical, electrical and chemical properties of carbon nanotubes in a scalable fashion, said A. John Hart, an assistant professor in the department of mechanical engineering and in the School of Art & Design at the Ann Arbor, Mich.-based university. In fact, the 3-D nanotube structures could enable new materials and microdevices, including probes that can interface with individual cells, microfluidic devices, and lightweight materials for aircraft and spacecraft.
"It's easy to make carbon nanotubes straight and vertical like buildings," Hart says. "It hasn't been possible to make them into more complex shapes. Assembling nanostructures into three-dimensional shapes is one of the major goals of nanotechnology and nanomanufacturing. The method of capillary forming could be applied to many types of nanotubes and nanowires, and its scalability is very attractive for manufacturing."
Hart's method starts by patterning a thin metal film on a silicon wafer. This film is the iron catalyst that facilitates the growth of vertical carbon nanotube "forests" in patterned shapes that serves as sort of a template. Rather than pattern the catalyst into uniform shapes such as circles and squares, Hart's team patterns shapes such as hollow circles, half circles and circles with smaller ones cut from their centers. The shapes are arranged in different orientations and groupings, creating different templates for later forming the 3-D structures using capillary action.
He uses a chemical vapor deposition process to grow the nanotubes in the prescribed patterns. Chemical vapor deposition involves heating the substrate with the catalyst pattern in a high temperature furnace containing a hydrocarbon gas mixture. The gas reacts over the catalyst, and the carbon from the gas is converted into nanotubes, which grow upward like grass.
Then he suspends the silicon wafer with its nanotubes over a beaker of a boiling acetone. He lets the acetone condense on the nanotubes and then evaporate. As the liquid condenses, it travels upward into the spaces among the vertical nanotubes. Capillary action kicks in and transforms the vertical nanotubes into the intricate three-dimensional structures. For example, tall half-cylinders of nanotubes bend back to form a shape resembling a three-dimensional flower.
"We program the formation of 3-D shapes with these 2-D patterns," Hart says. "We've discovered that the starting shape influences how the capillary forces manipulate the nanotubes in a very specific way. Some bend, others twist, and we can combine them any way we want."
The capillary-forming process allows the researchers to create large batches of 3-D microstructures, all much smaller than a cubic millimeter, Hart says. In addition, the researchers show that their 3-D structures are up to 10 times stiffer than typical polymers used in microfabrication. Thus, they can be used as molds for manufacturing of the same 3-D shapes in other materials.
A paper, "Diverse 3-D Microarchitectures Made by Capillary Forming of Carbon Nanotubes," by postdoctoral researcher Michael De Volder, and Sameh Tawfick, a doctoral candidate in mechanical engineering, was published in a recent issue of Advanced Materials.
The research is funded by the University of Michigan College of Engineering and the U-M Department of Mechanical Engineering, the Belgium Fund for Scientific Research, and the National Science Foundation. The university is pursuing patent protection for the intellectual property, and is seeking commercialization partners to help bring the technology to market.