Key words :
Challenges in Making Superstrong Materials Based on Carbon Nanotubes
28 Jan, 2008 10:07 pm
Thinking of carbon nanotube (CNT) composites, one might expect superstrong materials. But a closer look at the steep challenges to improve the mechanical properties of CNT composites shows, that other applications such as electromagnetic shielding, preventing plastic to charge and dampening vibrations in sporting goods or airplanes, are more likely applications to be seen at a wider scale in the near future.
Challenge number one: CNTs tend to agglomerate and they are difficult to disperse. Strong mixing can damage the tubes. Polymer nanotubes mixtures are highly viscous due to the large CNT surface making them difficult to flow and to mould. Successful mixing is obtained by dry mixing and melting, polymerization of monomers on nanotube surfaces, and the use of surfactants. The larger surface of CNTs compared to fibers results in larger interaction with the matrix influencing the structure of the matrix. One would expect tougher and stronger composites when extrapolating the properties of individual tubes. High aspect ratio of the tubes has the effect that the tubes form a connecting network (percolation) at very small volume fractions of less than 1w% and can in theory be as low as 0.01w%. CNTs are semiconducting or metallic which makes CNT polymer composite electrical conductive (1 S/m). A conducting composite absorbs efficiently microwaves and can be used for shielding applications and to evacuate static charges.
Challenge number two: CNTs are atomically smooth making it difficult to transfer the load from the matrix. Cracks of CNT composites show the CNTs get pulled out of the matrix. But using covalent bonding of functional groups with the tubes can destroy or strongly modify the intrinsic properties of the tubes. For multi wall carbon nanotube, the external wall can be used to interact strongly with matrix and the internal walls can be used for their intrinsic properties. Double wall CNTs have the advantage that the high aspect ratio is maintained while being able to interact strongly with matrix. Non-covalent interacting molecules such as surfactants or aromatic functional groups are promising in defining the interface between the matrix and CNTs.
Current situation: Up to now only moderate improvement of the Young modulus and stiffness has been reported (1). Clearly there is a lot of room for improvements or simply we need to know more how to disperse and functionalize carbon CNTs efficiently. It has been recently observed that vibrations can be efficiently damped by incorporating CNTs into polymers. Clearly this opens applications for sporting goods and airplanes. Spinning of single wall CNTs into fibers and growth of CNT forests gives the possibility to increase the fiber length, to fabricate sheets and to control CNT structure in the two and three dimensions. Self assembly techniques can be used to control the distribution and orientation of CNTs in composites. Layer by layer assembly techniques are promising in controlling CNT distribution and adherence to the matrix. Sequential deposition of oppositely charged polyelectrolytes and CNTs into multi layer assemblies has the advantage that layers can be introduced to improve cross linking (2) or density of CNTs. While the mechanical properties of CNTs are most promising, this potential has not been fully realized yet due to challenges in dispersing CNTs and defining the interface between matrix and CNTs (4). However, making plastic conducting by incorporating a tiny amount of CNTs has found applications in situations where evacuating static charges is essential (3). Many of the applications of CNT composites are limited by the current cost of CNTs, typically a multiple of the cost of gold. While production costs are expected to fall in the future with increasing demand, CNT processing such as purification, dispersing and fuctionalization will remain a substantial if not major part of the end cost of CNTs. Given the fact that CNTs have been available for more than a decade and CNTs do have an enormous potential to be used in composites it is rather surprising to see not more research groups focusing on the key challenges in this field.
1. P. A. Ajayan, J. Tour, Nature 447 (2007)1066
2. M. Olek,J. Ostrander, S. Jurga, H. Mohwald,| N. Kotov, K. Kempa, M. Giersig, Nano Letters 4 (2004) 1889
3. Hyperion Catalysis, Cambridge, MA, U.S.A.
4. P. J. F. Harris, International Materials Reviews 49 (2004) 31