Tikalon Header

Strong Carbon Nanotube Composites

November 12, 2012

When contemplating materials and their properties, every engineer has a silent wish that one property or another had a higher or lower value. His, or her, head swims with the possibilities that this property enhancement would bring. One example is superconductivity.

Superconductors were first limited to operation at very low temperatures, which are expensive to attain and maintain. Once superconductors were available at higher temperatures, above the boiling point of inexpensive liquid nitrogen, they became useful in many more applications. Of course, a room temperature superconductor would be nicer still.

There's a long path from ideal material properties, as measured in a laboratory, to a useful material you can design into novel components. Carbon nanotubes (CNTs), when measured individually, have exceptional strength because of their regular arrangement of carbon atoms and the 346 kJ/mole bond energy of the carbon-carbon bond.

The experimentally determined Young's modulus of single-walled CNTs is about one 1000 GPa, with tensile strengths measured up to about fifty GPa.[1] CNTs also have low density, high thermal conductivity, and high electrical conductivity. Their strength, coupled with these other desirable properties, makes carbon nanotubes an ideal structural material.

Atomically resolved chiral carbon nanotubeAtomically resolved chiral carbon nanotube.
(NIST image by Taner Yildirim, via Wikimedia Commons.)

The practical problem in utilizing this strength is that CNTs are individually very small, so you need to assemble many of them together to get a usable fiber or ribbon from which composites can be made. The usual route for this, mixing loose nanotubes into a matrix material, doesn't do the trick. The volume fraction of CNTs is typically limited to about 5%, and the CNTs tend to agglomerate. Making composites from buckypaper, which is an aggregate of CNTs formed into a thin sheet, is a promising way to form composites, and a buckypaper/bismaleimide composite was produced with a 3 GPa tensile strength.

A major advance in fabricating CNTs into a usable form is the dry-drawing method in which it's possible to pull highly-aligned nanotubes into fibers or sheets.[2] One published process allows spooling of CNT sheets, impregnated with a polymer binder, onto a rotating spool, which allowed the production of composite sheets with a tensile strength of 1.8 GPa, and an electrical conductivity of 780 S-cm−1.[3]

Once the alignment problem is solved, there's still the problem that the individual CNTs are not perfectly straight, but wavy. This wavy texture limits strength, since the force loading is not uniform along the length, and there are fewer contacts between the individual CNTs. Now, the team that pioneered this spooling approach has taken it one step further by stretching the CNT sheets before spooling to straighten the CNTs.[4-5]

The team, led by Yuntian Zhu, a professor of materials science and engineering at North Carolina State University, included scientists and engineers from North Carolina State University, the Suzhou Institute of Nano-Tech and Nano-Bionics, the Marshall Space Flight Center, and Oak Ridge National Laboratory.[4-5] Their stretch-winding process produced polymer-CNT composites with a 46% volume fraction of CNTs.[5] At a 12% stretch, their material had a Young's modulus as high as 293 GPa, a tensile strength of 3.8 GPa, a thermal conductivity of 41 W-m−1-K−1, and an electrical conductivity of 1230 S-cm−1 .[5]
Mechanical properties of CNT fibersTensile strength and Young's modulus of CNT matrix composites.

Prior work is shown in blue, and the properties of the stretch process material are shown in red.

Graphed using Gnumeric from the data included in ref. 5.[5]

As shown in the figure, CNTs that grow as hairlike structures are pulled away as a sheet from a flat substrate. Two fixed rods stretch the sheet as it passes over them. A contact angle between the rods and the sheet of 150-165° was found to give the best results. The stretched sheet was wound on a polytetrafluoroethylene spool, and a spray of bismaleimide resin forms a composite sheet with a 50-55% volume fraction of CNTs.[5] Says Yuntian Zhu,
"The new technique begins with a CNT array... which looks like a forest of CNTs growing up out of a flat substrate. By grabbing the CNTs at one end of the array, we are able to pull them over onto their sides – and all of the other CNTs in the array topple in the same direction."[4]

Nanotube stretching apparatus
The carbon nanotube stretching apparatus. The CNT sheet is stretched by passage along two stationary rods, wound onto a rotating mandrel, and fixed with a matrix spray. (Image by the author, rendered using Inkscape).

This research was supported, in part, by the Air Force Office of Scientific Research.[4]

References:

  1. S. Bellucci, "Carbon nanotubes: physics and applications," Physica Status Solidi (c), vol. 2, no. 1 (January 2005), pp. 34-47.
  2. K.L. Jiang, Q.Q. Li and S.S. Fan, "Nanotechnology: Spinning continuous carbon nanotube yarns – Carbon nanotubes weave their way into a range of imaginative macroscopic applications," Nature, vol. 419, no. 6909 (October 24, 2002), pp. 801.
  3. Wei Liu, Xiaohua Zhang, Geng Xu, Philip D. Bradford, Xin Wang, Haibo Zhao, Yingying Zhang, Quanxi Jia, Fuh-Gwo Yuan, Qingwen Li, Yiping Qiu and Yuntian Zhu, "Producing superior composites by winding carbon nanotubes onto a mandrel under a poly(vinyl alcohol) spray," Carbon, vol. 49, no. 14 (November 2011), pp. 4786-4791.
  4. Matt Shipman, "New Techniques Stretch Carbon Nanotubes, Make Stronger Composites," North Carolina University Press Release, October 15, 2012.
  5. X. Wang, Z. Z. Yong, Q. W. Li, P. D. Bradford, W. Liu, D. S. Tucker, W. Cai, H. Wang, F. G. Yuan and Y. T. Zhu, "Ultrastrong, Stiff and Multifunctional Carbon Nanotube Composites," Materials Research Letters, 2012; Open Access PDF File available here.