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Univ of Michigan Scientists Use Capillary Action To Build 3D CNTs

by Editor1 last modified October 25, 2010 - 15:51

University of Michigan engineers are devising 3D carbon nanotubes that may deliver more mechanical, thermal, electrical and chemical properties. Scientists say their new CNT manufacturing process uses capillary forming techniques.

Univ of Michigan Scientists Use Capillary Action To Build 3D CNTs

Capillary forming, as it name implies, takes advantage of capillary action, the phenomenon at work when liquids seem to defy gravity and spontaneously travel up a drinking straw.

The scalable 3D nanotube structures, which include twisting spires, concentric rings, bending petals and other shapes, could enable new materials and microdevices for biomedicine, microfluidics and industrial-strength lightweight materials for aircraft and spacecraft, according to A. John Hart, an assistant professor in the school’s Department of Mechanical Engineering.

“It’s easy to make carbon nanotubes straight and vertical like buildings, [but] it hasn’t been possible to make them into more complex shapes,” Hart said. “The method of capillary forming could be applied to many types of nanotubes and nanowires, and its scalability is very attractive for manufacturing.”

U-M’s 3D method starts by patterning a thin metal film on a silicon wafer. This film is the iron catalyst or template, which facilitates the growth of CNT vertical forests in patterned shapes. Rather than patterning uniform shapes such as circles and squares, Hart's team patterned a range of unique shapes and then arranged them in different orientations and groupings.

These novel shapes, when assembled together, then became templates for building 3D nanostructures that take advantage of capillary action. “We program the formation of 3D shapes with these 2D patterns,” Hart said. “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.”

U-M's Cookbook for 3D Nanotubes
Growing these CNTs, Hart uses a chemical vapor deposition, which 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. The silicon wafer with its nanotubes is then suspended over a beaker of boiling acetone. That acetone condenses on the nanotubes, and then evaporates.

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, CNTs can be formed into tall half-cylinders and bent backwards to form a shape resembling a three-dimensional flower.

The capillary forming process allows the researchers to create large batches of 3D microstructures—all much smaller than a cubic millimeter, Hart said. In addition, the researchers show that their 3D structures are up to 10 times stiffer than typical polymers used in microfabrication.
“We think this opens up the possibility to create custom nanostructured surfaces and materials with locally varying geometries and properties,” Hart said. “Before, we thought of materials as having the same properties everywhere, but with this new technique we can dream of designing the structure and properties of a material together.”

The research appears in the October edition of Advanced Materials in a paper called “Diverse 3D Microarchitectures Made by Capillary Forming of Carbon Nanotubes,” The lead authors are postdoctoral researcher Michael De Volder, and Sameh Tawfick, a doctoral candidate in Mechanical Engineering.

The research is funded by U-M’s College of Engineering and Department of Mechanical Engineering, the Belgium Fund for Scientific Research, and the National Science Foundation.