Although considerable advancements have been made for processing carbon nanotube (CNT) reinforced composites during the past two decades, there are still several technical barriers to overcome to integrate their nanoscale diameters and microscale lengths for use on a macroscopic scale.
Universities around the world are conducting research in CNT fiber processing. Here’s an update on the latest CNT fiber, fabric and composite research at two of them – North Carolina State University (NCSU) and Rice University.
Stretch Winding CNT Composites
Professor Yuntian Zhu’s group in the Materials Science and Engineering Department at NCSU is developing a new stretch winding method to fabricate CNT-based yarn, sheets and paper composites.
Xin Wang, a postdoctoral researcher who works with Zhu, says the group’s research focuses on the most important features of CNTs in composites formed with this new method: the effect of long nanotubes, high-volume fractions, straightness and alignment, and how strongly they are integrated with the polymer.
She currently works on improving CNT alignment and volume fraction to improve the load bearing efficiency. “The industry expects CNTs to increase mechanical strength, stiffness and electrical conductivity in composites,” says Wang. “Our approach is to create structures with a large amount of CNTs and small amounts of polymers.”
The CNTs that NCSU adds to composite structures comprise 50 to 60 percent of their total weight. “Unlike other groups who are adding CNTs to polymers, we’re adding polymers to nanotubes,” says Wang. The group synthesizes its own multi-walled CNTs by chemical vapor deposition (CVD) and combines them with many different thermoplastic polymers and polymeric carbon composites, such as PVA, nylon, epoxy and polyimide.
NCSU’s unique stretch winding and rewinding process is a dry process that forms continuous sheets, like a fabric. The process starts with aligned CNT arrays. “We drop them into a horizontal sheet, wind them onto a mandrel and spray a polymer matrix for molecular integration,” says Wang. “The stretching and rewinding are both done in the same machine using different parts for stretching and rotating to straighten the CNTs. The advantage of this technique is that we start with aligned CNT arrays, so the sheets maintain the alignment.”
Wang says that by straightening the nanotubes in the array, the stretch winding process improved the strength of the composite by 90 percent and the stiffness by more than 100 percent. It almost tripled the thermal conductivity and the electrical conductivity also increased by 50 percent. “These results are far higher than the strongest composites commercially available,” Wang says.
The stretch winding process requires long spinnable nanotube arrays, which Wang says only a handful of research groups in the world can currently produce. The majority use dispersion methods and produce shorter arrays. Her group believes that if they can scale this new technique and make larger samples, the industry will be interested.