NANO LETTERS
Novel Carbon Nanotube−Polystyrene Foam Composites for Electromagnetic Interference Shielding
2005 Vol. 5, No. 11 2131-2134
Yonglai Yang and Mool C. Gupta* Department of Electrical and Computer Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22904
Kenneth L. Dudley and Roland W. Lawrence Electromagnetics Research Branch, NASA Langley Research Center, Hampton, Virginia 23681 Received July 18, 2005; Revised Manuscript Received September 5, 2005
ABSTRACT A novel carbon nanotube−polystyrene foam composite has been fabricated successfully. The electromagnetic interference (EMI) shielding effectiveness measurements indicated that such foam composites can be used as very effective, lightweight shielding materials. The correlation between the shielding effectiveness and electrical conductivity and the EMI shielding mechanism of such foam composites are also discussed.
Electrically conducting polymer-matrix composites have gained popularity for electromagnetic interference shielding applications because they are lightweight, resistant to corrosion, flexibile, and cost less than metals.1-10 These kinds of flexible conductive composites may be used for typical antenna systems, lightning-protected aircraft composite panels, avionics line replaceable unit (LRU) enclosures, connector gaskets, electrostatic and space charge dissipation materials, different types of electronic pressure sensitive switches or sensors, and many other uses. These polymeric composites must be made conductive by compounding them with an electrically conducting filler, such as metal particles and filaments,11,12 carbon particles (e.g., graphite and carbon black),13-15 and carbon fibers,16 in order to achieve the required shielding effectiveness. For any filler, the EMI shielding effectiveness increases with increasing filler concentration in the composites, but the maximum filler loading is limited by the poor composite mechanical properties at high filler loadings resulting from the poor filler-matrix bonding. For materials and process cost saving and good mechanical properties, the attainment of a high shielding effectiveness at a low filler loading is desirable. Carbon nanofibers and nanotubes, as compared to conventional metals and carbon fillers, have remarkable structural, mechanical, and electrical properties, such as smaller diameters, larger aspect ratios, and much higher conductivities and strengths. The use of these carbon nanostructures as fillers * Corresponding author. E-mail:
[email protected]. 10.1021/nl051375r CCC: $30.25 Published on Web 09/27/2005
© 2005 American Chemical Society
in polymeric composites allows systems with low filler loadings to provide the desired electrical and EMI shielding properties.17-22 For the effective and practical EMI shielding systems, being lightweight is an important technological requirement. In our recent report,23 we creatively fabricated a unique conductive carbon nanofiber-polymer foam structure and preliminarily demonstrated its potential application for EMI shielding in the frequency range of 8.2-12.4 GHz (X band). According to the electromagnetic wave percolation theory, if the conductive filler in the polymer possesses a high aspect ratio, then the filler forms a conductive network easily and the critical concentration of the conductive filler required to achieve the EMI shielding effect is low. Additionally, electromagnetic radiation at high frequencies penetrates only the near surface region of an electrical conductor. This is known as the skin effect. The depth at which the field drops to 1/e of the incident value is called the skin depth (δ), which is given by δ ) (2/f µσ)1/2, where f ) frequency, µ ) permeability, and σ ) electrical conductivity. Hence, the skin depth decreases with increasing frequency, permeability, or conductivity.24 As a result, it is expected that the use of carbon nanotubes with larger aspect ratios and higher conductivities would allow a higher shielding effectiveness to be attained at the same filler loading. In this paper, we report the fabrication of carbon nanotube-polystyrene foam composites and their potential applications for EMI shielding in the X-band frequency region. The aim of this study is to demonstrate the EMI shielding capability of the foam
Figure 2. EMI shielding effectiveness as a function of frequency measured in the 8.2-12.4 GHz range of CNT-PS foam composites with various CNT concentrations.
Figure 1. SEM images of the cross section of the CNT-PS foam composite containing 5 wt % carbon nanotubes. (a) Overview of the foam structure. (b) High-magnification image showing the carbon nanotube networks within the PS matrix.
composites containing carbon nanotubes at a lower filler loading compared to the carbon nanofiber filler. The purified multiwalled carbon nanotubes were supplied by NanoLab, Inc. (Newton, MA), and were 10-20 nm in diameter and 5-20 µm in length. The polymer matrix used in this work was polystyrene (PS) with a molecular weight of 100 000. The carbon nanotube-polystyrene foam composites were synthesized according to our previous technique for preparing carbon nanofiber-polystyrene foam composites.23 Here, we describe the preparation procedure briefly. Initially, carbon nanotubes were dispersed in the PS/toluene solution containing 5 wt % foaming agent (2,2′-azoisobutyronitrile, AIBN) through ultrasonication to give a blackcolored stable nanotube suspension. Then, the resulting mixture was sprayed onto a flat plate via a microsprayer and dried at room temperature overnight. Afterward, the dried film was cured thermally in an air-circulating oven at 80 °C. Finally, the carbon nanotube-PS thin film containing foaming agent was put into a mold and hot-pressed to form a thicker structure. In the hot-pressing process, the foaming agent, AIBN, decomposed and gave off nitrogen gas within the composite system to eventually generate the carbon nanotube-PS foam composite. The morphology of carbon nanotube-PS foam composites was observed by scanning electron microscope (SEM, Hitachi-4700). Figure 1 shows the representative SEM images of the cross section of the 5 wt % carbon nanotubePS foam composite. It is obvious that the foam structure was formed throughout the carbon nanotube-polymer system as 2132
shown in Figure 1a. We can also observe from Figure 1a that the gas bubbles are almost spherical and uniform and the cell sizes range from 40 to 170 µm. The formation of this unique foam structure is ascribed to the use of a gasproducing solid foaming agent, AIBN, which is stable at room temperature but decomposes readily at a higher temperature to give off a large volume of nitrogen gas. Herein, in the hot-compression-molding process, the melted PS matrix filled with carbon nanotubes was expanded uniformly by the nitrogen gas originated from the decomposition of AIBN to generate a carbon nanotube-PS foam structure with controlled pore size distribution. Conventional polymer foams generally consist of a minimum of two phases, a solid polymer phase and a gaseous phase derived from a foaming agent.25 In this study, our CNT-PS foam composite is composed of two solid phases, the PS matrix and the carbon nanotube fillers, and a gaseous phase. At a relatively high magnification (Figure 1b), we can see clearly that the carbon nanotubes are dispersed and embedded throughout the PS matrix and an interconnected carbon nanotube network has formed. This conductive nanotube network may establish electrical conduction pathways throughout the whole system, which is responsible for the conductivity and EMI shielding characteristics. The EMI shielding effectiveness (SE) of a material is defined as the ratio between the incoming power (Pi) and outgoing power (Po) of an electromagnetic wave. In general, SE is expressed in decibels (dB). In this study, the EMI shielding effectiveness of CNT-PS foam composites was measured using an HP 8510 vector network analyzer. An electromagnetic wave was injected directly into the foam using a waveguide setup. Details on the EMI shielding effectiveness measurement of the foam composites are provided in the Supporting Information. The frequency was scanned from 8.2 to 12.4 GHz (X band), and 201 data points were taken within this frequency range. Figure 2 shows the variation of the EMI shielding effectiveness over the frequency range of 8.2-12.4 GHz for CNT-PS foam composites with various CNT loadings. It is observed that the shielding effectiveness of each foam composite is almost Nano Lett., Vol. 5, No. 11, 2005
Table 1. Comparison of the Average EMI Shielding Effectiveness of CNT-PS and CNF-PS Foam Composites in X band filler content (wt %) 0.5 1 3 7 20
Figure 3. EMI shielding effectiveness at 10 GHz as a function of electrical conductivity of CNT-PS foam composites.
independent of the frequency in the measured frequency region. The results also show that the EMI shielding effectiveness increases with increasing content of carbon nanotubes in the foam composite. The shielding effectiveness of the foam composite containing 7 wt % carbon nanotubes is measured to be 18.2-19.3 dB over a frequency range of 8.2-12.4 GHz. This increment of the EMI shielding effectiveness is attributed mainly to the formation of conducting interconnected nanotube networks in the insulating PS matrix. The increase in filler loading increases the number of interconnected nanotubes in the foam composite that will interact with the incident radiation and lead to the higher shielding effectiveness. Because of aerospace applications related to EMI shielding, the specific EMI shielding effectiveness (EMI shielding effectiveness divided by the density) is more appropriate for use in comparing the shielding performance between typical metals and the CNT foam composites. In this work, the specific EMI shielding effectiveness of the foam composite containing 7 wt % carbon nanotubes is calculated to be 33.1 dB‚cm3/g, which is much higher than that of typical metals (compared to 10 dB‚cm3/g for solid copper26). It is well established that the shielding effectiveness of a conductive composite is related to its conductivity. Figure 3 describes the correlation between the shielding effectiveness and electrical conductivity of CNT-PS foam composites. We can observe that a minor increase in shielding effectiveness occurred with the initial variation of conductivity from -14 to -5.4, but at higher conductivities (log σ > -3) the system became more efficient in shielding and the shielding effectiveness increases dramatically. This variation in the shielding effectiveness against conductivity can be correlated with the change in conductivity of CNT-PS composites against CNT loading. Initially with the addition of carbon nanotubes in the PS matrix, the change in conductivity remained marginal up to the critical concentration (percolation threshold). Then, the increase in conductivity was abrupt, that is, the insulating PS matrix became conductive at and above this critical concentration because of the formation of conductive nanotube networks. This is a typical percolation threshold for carbon nanostructure-filled polymer comNano Lett., Vol. 5, No. 11, 2005
EMI shielding effectiveness (dB) CNT-PS foams
CNF-PS foams
2.84 5.73 10.30 18.56
0.41 0.73 3.09 8.53 20.51
posites as reported in the literature.18,21,27-29 Conductivity is a consequence of conductive network formation in the insulating PS matrix. This conductive network is distributed uniformly in the polymer matrix (as shown in Figure 1b), which interacts with electromagnetic waves and contributes to the EMI shielding effectiveness. Table 1 gives the average EMI shielding effectiveness (averaged over 201 data points from 8.2 to 12.4 GHz) of the foam composites. It is found that the target value of the EMI shielding effectiveness needed for commercial applications, around 20 dB (less than 1% transmission of the electromagnetic wave), could be obtained for CNT-PS foam composites of only 7 wt % addition of carbon nanotubes into the PS matrix. Table 1 also provides the comparable EMI shielding effectiveness of the carbon nanotube-PS foam composites of this study and the carbon nanofiberPS foam composites at the same filler content. As shown in Table 1, there is a significant difference in the efficiency of carbon nanotubes and carbon nanofibers in imparting shielding effectiveness to the foam composites. For example, the CNT-PS foam composite exhibited a higher shielding effectiveness (above 10 dB) compared to 3 dB for the CNFPS foam composite at the same filler loading (3 wt %). Even the foam composite with 7 wt % carbon nanotubes gave an EMI shielding effectiveness comparable to that of the foam composite with 20 wt % carbon nanofibers. These differences result from the fact that carbon nanotubes possess much smaller diameters and larger aspect ratios with higher electrical conductivities as compared to carbon nanofibers. The smaller size of the carbon nanotubes may provide a larger interfacial area; therefore, the number of conductive interconnected nanotubes increases. And the larger aspect ratio of carbon nanotubes helps to create extensively continuous networks that facilitate electron transport in the foam composite with very low nanotube loading. As discussed above, we may conclude that the carbon nanotube-PS foam composites are more effective in providing EMI shielding compared to the carbon nanofiber-PS foam composites. When electromagnetic radiation is incident on a shielding material, phenomena such as reflection, transmission, and absorption are observed. The corresponding reflectivity (R), transmissivity (T), and absorptivity (A) are described as R + T + A ) 1. In this study, for the polymer foam composite containing 7 wt % carbon nanotubes, the reflectivity (R), transmissivity (T), and absorptivity (A) were 0.81, 0.01, and 0.18, respectively. On the basis of this result, we infer that 2133
such carbon nanotube-PS foam composites are more reflective and less absorptive to electromagnetic radiation, that is, the primary EMI shielding mechanism of such foam composites is reflection rather than absorption in the X-band frequency region. This conclusion is consistent with the investigation of the shielding mechanism of carbon fiber and nanotube-filled polymer solid composites.21,30 In summary, a novel carbon nanotube-polystyrene foam composite was developed successfully for EMI shielding applications. The EMI shielding effectiveness provided by the polymer foam composite at just 7 wt % CNT loading was around 20 dB, which implies that such a CNT-PS foam composite can be used commercially as a shielding material against electromagnetic radiation. Carbon nanotube-PS foam composites were found to be more effective in providing EMI shielding compared to carbon nanofiber-PS foam composites at the same filler loading. The primary EMI shielding mechanism of such CNT-PS foam composites was demonstrated and ascribed to the reflection of electromagnetic radiation. Acknowledgment. This work was supported by the National Science Foundation Industry-University Cooperative Research Center and NASA Langley Research Center. We also thank Dr. Jianjun Wang, of the College of William & Mary, for his assistance in SEM observation. Supporting Information Available: Details on the EMI shielding effectiveness measurement of the carbon nanotube-polystyrene foam composites. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Kaynak, A. Mater. Res. Bull. 1996, 31, 845. (2) Laight, A.; Court, R.; Treen, A. Polym. Compos. 1997, 18, 418. (3) Cheng, K. B.; Ramakrishna, S.; Lee, K. C. Composites, Part A 2000, 31, 1039. (4) Lee, B. O.; Woo, W. J.; Kim, M. S. Macromol. Mater. Eng. 2001, 286, 114.
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NL051375R
Nano Lett., Vol. 5, No. 11, 2005
