Carbon nanotubes, or CNTs, are cylinders of pure carbon with nearly mythical properties. Depending on the angle at which the carbon layer is rolled into cylinders, a carbon nanotube can have perfect electrical conductivity, be a semiconductor or be non-conducting. According to Nanowerk, CNTs can be 400 times stronger than steel at one-sixth the weight, and have better thermal conductivity than diamond. They are extremely resistant to corrosion, and the hollow interior can be filled with pharmaceuticals, water molecules or other nanomaterials to shield them from the surrounding environment.
A Solution for Tangled Tubes
Ever since Sumio Iijima discovered carbon nanotubes in 1991, researchers have been working to literally untangle their potential. CNTs can be 1,000 to 1,000,000 times longer than they are wide, which makes them prone to tangling during synthesis. The diameter of a carbon nanotube can be less than one nanometer (10-9 meters) or more than 100 nanometers, and the length can range from micrometers (10-6 meters) to millimeters (10-3). Carbon nanotubes are manufactured at a large scale, with highly tangled forms being used in composite materials for sporting goods, bullet-proof vests, boats, cars and aircraft.
In order to unlock the full potential of carbon nanotubes, they need to be untangled and assembled into precise structures. A new solvent reported in Science Advances now allows high quality CNTs to be used in advanced manufacturing processes such as 3D printing.
Challenges of the Nanoscale
The challenge of untangling CNTs is explained by Eric Barnes, a Northrop Grumman Fellow specializing in additive manufacturing and 3D printing who was not involved in the study: “Carbon nanotubes exist in the realm that starts to be governed by nanophysics. In this realm, mass-driven properties like gravity and inertia slowly become less dominating, and elements of quantum physics begin to exert influence,” he explained. “We find that things like Van der Waals attractive forces (attraction between molecules that leads to effects like surface tension) can become an asset or a liability. If you are a gecko lizard, the nanoscale structures on the pads of your feet allow you to take advantage of the Van der Waals attractive forces to walk up a sheet of glass. If you are wanting to disperse a nanomaterial into a fluid — such as a resin — the Van der Waals forces may cause your nanotubes to clump together.”
CNTs are often dissolved in “superacids,” such as oleum or chlorosulfonic acid (CSA), which give the sidewalls of the CNT a partial positive charge without permanently changing their structure. This positive charge causes adjacent CNTs to repel each other and move into solution, which allows the carbon nanotubes to be detangled further while remaining at high enough concentrations to be extruded through a needle to produce high performance materials. Unfortunately, these superacids are incompatible with many manufacturing processes and commonly used plastics.
A Gentle Touch Unlocks Possibilities
Researchers found that two weaker acids, p-toluenesulfonic acid (pToS) or methanesulfonic acid (MSA), had the same effect on CNTs when mixed with a small amount of oleum, which is also known as fuming sulfuric acid. These acids can be used in die coating, screen printing and injection molding with common substrates, including polyethylene terephthalate (PET) and polycarbonate plastics, according to Barnes. PToS is a solid that melts at 40 degrees Celsius (104 degrees Fahrenheit), which is particularly useful for 3D printing because the materials can be processed at moderate temperature and then solidified by cooling. The researchers found that MSA is best for fiber spinning and roll-to-roll production (for example, printing copies of an electrical device on a roll of flexible plastic).
After printing, the acid solvent can easily be removed by submersing it in water. It can then be recycled, reducing the cost and environmental impact of producing CNTs. The oleum still requires special handling, but when it’s diluted with the milder acids it can be used on unventilated bench tops with common plastics.
According to Barnes “This novel approach allows for direct write of CNT structures by transforming the disarranged CNTs into precise, functional morphologies.” These 3D printed structures “could potentially be used to carry electrical currents within a multifunctional aerospace structure. The ability to apply the CNTs to polycarbonate and polyester fabric indicates the potential for textiles containing CNT sensors for human health monitoring.”
In short, the possibilities are endless.
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