Conventional Techniques

Metals: Metals like copper, silver and aluminium have always been the forerunner for conductivity applications. Subsequently, when otherwise insulating polymer matrices were attempted to be tuned for similar applications by addition of suitable filler materials, the obvious choices were once again similar metals, in the forms of powders, flakes and fibers. Stainless steel powder filled acrylonitrile-butadiene-styrene (ABS) was tried in power measurement recorders¹. Metallic silver filled elastomeric gaskets / adhesives^{2, 3} and metal mesh embedded fiber composites are also used in electronic applications especially for EMI shielding requirements⁴. It was observed that a minimum volume fraction (percolation threshold) of the conducting reinforcement was required to establish a continuous conductive network within the polymer matrix to affect a sudden drop in its bulk resistivity. It has been reported that addition of 19 volume % of silver powder makes polymer composites electrically conductive⁵. However, this comes with a heavy penalty on the weight, mechanical behaviour and cost.

Carbon: As a result, the search for better alternatives continued. The next obvious choice was carbon. Carbon had been long used as conductive fillers in polymer matrices, both in crystalline (graphite) and amorphous forms (carbon black). Electrically/ thermally conducting carbon fiber, either in the form of chopped strands or as fabrics has been used to enhance the structural as well as the conductivity properties of the composite materials. These fibers are generally developed from Pitch or PAN based precursors, the choice of which plays an important role in the final properties of the fibers. This apart, other factors like the volume fraction and inherent conductivity of the fibers, the aspect ratio and their orientation are among the ones that play major roles in determining the conductivity properties of such composites⁶.

Aspect Ratio: Other factors remaining constant, the composite conductivity for a fiber-reinforced matrix was reported to be directly proportional to the aspect ratio of the fibers6. The aspect ratio of the common carbon/ graphite fibers is in the range of 10 to 50. However, for carbonnanotubes (CNT), which are essentially rolled up graphene sheet(s) having diameters in the range of 10 nm and length up to a few microns, the aspect ratio is often in the order of 500 and above.

Fiber Conductivity: Another important factor, which determines the composite conductivity, is the inherent conductivity of the fibers. In this regard, the CNTs offer some unique advantages over the conventional metallic or graphitic fibers. To appreciate that, one needs to have a basic understanding about the formation and structure of the CNTs.

CNT Structure: Single wall nanotubes (SWNT) are formed by rolling individual graphene sheets (one layer of crystalline hexagonal graphite) as shown in Fig 1.

Fig.1: Formation of SWNT

Likewise, multiwall nanotubes are formed if two or more graphene sheets are rolled together, such that it results in a tubular structure with hollow concentric shells with spacing of 0.34 nm.

Now, if we consider any two arbitrary points O and A on the graphene sheet such that they coincide once the sheet is rolled to form a SWNT, then the vector joining the two points is defined as the chiral vector of the CNT, as explained in Figure 2. The line OA thus becomes the circumference of the SWNT cross section.

Figure 2: Chiral Vector                         Ù       Ù        Ù     Chiral Vector (Ch)= na1 + ma2 Ù      Ù a1 & a2 = unit vectors in 2-d hexagonal lattice n & m = integers q = chiral angle

The CNT structures are often categorized on the basis of their chiral vector and chiral angle q. If n = m and q = 30°, the resultant CNT is known as an armchair nanotube. If either n or m = 0 and q = 0°, it is referred as a zigzag nanotube. In all other cases, the CNTs are called chiral nanotubes.

In other words, CNT is a tube rolled along the chiral vector, the values of n and m determining the chirality or twist (q). The chirality in turn affects the conductance of the nanotube, its density, lattice structure and other properties.

As mentioned above, not only structures, the electrical properties of the CNTs are also a function of their chirality. Depending on their chirality, CNTs are either semi-conducting or metallic7. The differences in conducting properties are caused by the different extent of p electron mismatch that results in a different band structure and band gap8. It was shown that a (n, m) nanotube behaves as a metallic conductor if (n-m) = 3i, where i is an integer. Otherwise, the CNTs behave as a semiconductor. All armchair CNTs are metallic. In theory, metallic CNTs can have electrical current density more than 1000 times stronger than metals like silver or copper. On the other hand, based on the exact values of n, m and q, the semi conducting CNT can have a band gap of either 0.01 ev or 1.0 ev (same order of conventional germanium / silicon semiconductors). This allows the designer to tune the materials for almost all levels of electrical conductivity. Generally, CNTs have very high electrical conductivity, much higher than those of conducting polymers.

Current State of Research:

Reports on the effects of the length and aggregate size of multi wall nanotubes (MWNT) on the AC conductance of epoxy based composite reveals the percolation threshold for the fillers to be as low as 0.5 weight %⁹.  The volume resistivity values of carbon nanotube – polyimide composites for different frequencies (0 to 1 MHz) also indicate similar trends¹⁰. The electromagnetic shielding capabilities of MWNT filled poly methyl methacrylate (PMMA) nanocomposites are also being explored¹¹. These, along with the reported mechanical reinforcing/ stiffening capabilities of the CNTs have together made them extremely attractive candidates for polymer reinforcement^9,12.

A Comparison:

As shown in Figure 3, it is clearly evident that compared to other conductive reinforcements like copper and aluminium fibers and vapour grown carbon fibers, single wall nanotubes (SWNT) possess a much lower percolation threshold (1-2 wt.%) and saturation surface resistivity value (102 ohm/sq).

Figure 3: Percolation phenomena in different conductive polymer composites13 PE: Polyethylene Cu: Copper fiber PP: Polypropylene VGCF: Vapour grown carbon fiber PVC: Polyvinyl chloride Al: Aluminium fiber ABS: Acrylonitrile-butadiene-styrene SWNT: Single wall nanotube

Applications

The tendency of the CNTs to form ropes provides inherently very long conductive pathways even at ultra-low loadings, which may be exploited in EMI/RFI shielding composites, gaskets, and other uses such as electrostatic dissipation (ESD), antistatic materials, transparent conductive coatings and radar absorbing materials (RAM) for stealth applications. The same properties also make these composites attractive for electronics packaging and interconnection applications like adhesives, potting compounds, coaxial cables and other types of connectors. In the long run, such multifunctional composite materials hold sufficient promise for applications in futuristic smart structures for aerospace and other critical applications.

References

• “Applications of Thermoplastics in Electronics”, Electronic Packaging & Prodn., July 1992, pp 44-49
• “Suppressing EMI by Proper Gasketing”, Electronic Packaging & Production, July 1992, pp 52-54
• Product Literature on “Conductive Adhesives” of Chomerics Co., http://www.chomerics.com
• Sankaran S., Dasgupta S., Ravi Sekhar K., Jagdish Kumar M. N., “Thermosetting Polymer Composites for EMI Shielding Applications”, Proceedings of International Conference on Electro-Magnetic Interference and Compatibility - INCEMIC 2006,  (23-24 Feb 2006) pp 01-06
• Olivero, A., Radford, D. W., “Integrating EMI shielding into composite structure”, SAMPE Journal, Vol.33, No.1, January/February 1997, pp 51-57
• Tsotra, P., Friedrich, K., “Electrical and mechanical properties of functionally graded epoxy-resin/carbon fibre composites”, Composites Part A: applied science and manufacturing, 34 (2003) pp 75-82
• Website: http://students.chem.tue.nl/ifp03/synthesis.html
• P. Avouris, “Carbon nanotube electronics”, Chemical Physics 281, (2002) pp 429-445
• Toshio, O., Yuichi, I., Takashi. I., Rikio. Y., “Characterization of multi-walled carbon nanotubes/phenylethynyl terminated polyimide composite”, Composites Part A: applied science and manufacturing, 35 (2004) pp 67-74
• Bai, J.B., Allaoui, A., “Effect of the length and the aggregate size of MWNTs on the improvement efficiency of the mechanical and electrical properties of nanocomposites-experimental investigation”, Composites Part A: applied science and manufacturing, 34 (2003) pp 689-694
• Kim. H. M., Kim. K., Lee. C. Y., Joo. J., Cho. S. J., Yoon. H. S., Pejacovic. D. A., Yoo. J., Epstein. A. J., “Electrical conductivity and electromagnetic interference shielding of multiwalled carbon nanotubes composites containing Fe catalyst”, Applied Physics Letter, Vol. 84, No.4 (26 January 2004) pp 589-591
• Sankaran. S., Dasgupta. S., Ravi Sekhar. K., Ghosh. C., “Carbon nanotubes reinforced syntactic composite foams”, Proceedings of ISAMPE National Conference on Composites - INCCOM IV,  (09-10 Dec 2005) pp 82-82XII
• Website: http://www.owlnet.rice.edu/~chem597/BarreraNotes/Introduction.pdf