Nanotubes get loopy in 3-D
U. MICHIGAN (US) — Twisting spires, concentric rings, and bending petals are a few of the new 3-D shapes engineers can make from carbon nanotubes using a new manufacturing process.
The process is called “capillary forming,” and it 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 mechanical, thermal, electrical, and chemical properties of carbon nanotubes in a scalable fashion, says A. John Hart, an assistant professor at the University of Michigan.
The 3-D nanotube structures could enable countless new materials and microdevices, including probes that can interface with individual cells, novel microfluidic devices, and lightweight materials for aircraft and spacecraft.
A paper on the research is published in the October edition of Advanced Materials, and is featured on the cover.
“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. It’s a sort of template. Rather than pattern the catalyst into uniform shapes such as circles and squares, Hart’s team patterns a variety of unique 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 backwards 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. Which means, they can be used as molds for manufacturing of the same 3-D shapes in other materials.
“We think this opens up the possibility to create custom nanostructured surfaces and materials with locally varying geometries and properties,” Hart says. “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 work is funded by the University of Michigan, 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.
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