University of Michigan engineers are using a new manufacturing process called “capillary forming” to make carbon nanotubes in shapes that span the scope of the imagination.
According to a press release, capillary forming takes advantage of capillary action, the phenomenon at work when liquids seem to defy gravity and travel up a drinking straw of their own accord.
The miniature shapes of twisting spires, concentric rings and gracefully bending petals are difficult if not impossible to build using any material. Engineers say they have the potential to harness the exceptional mechanical, thermal, electrical and chemical properties of carbon nanotubes in a scalable fashion.
“It’s easy to make carbon nanotubes straight and vertical like buildings,” says A. John Hart, an assistant professor in the Department of Mechanical Engineering and in the School of Art & Design. “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. The method of capillary forming could be applied to many types of nanotubes and nanowires, and its scalability is very attractive for manufacturing.”
According to the press release:
Hart’s method starts by stamping patterns on a silicon wafer. His ink in this case is the iron catalyst that facilitates the vertical growth of the carbon nanotubes in the patterned shapes. Rather than stamp a traditional, uniform grid of circles, Hart stamp hollow circles, half circles and circles with smaller ones cut from their centers. The shapes are arranged in different orientations and groupings. One such grouping is a pentagon of half circles with their flat sides facing outward.
He uses the traditional “chemical vapor deposition” process to grow the nanotubes in the prescribed patterns. Then he suspends the silicon wafer with its nanotube forest over a beaker of a boiling solvent, such as acetone. He lets the acetone condense on the nanotubes, and then lets the acetone evaporate.
As the liquid condenses, capillary action forces kick in and transform the vertical nanotubes into the intricate three-dimensional structures. For example, tall half-cylinders of nanotubes bend backwards to form a shape resembling a three-dimensional flower.
A paper on the research is published in the October edition of Advanced Materials.