Charge transport properties of composites of multiwalled carbon nanotube with metal catalyst and polymer: application to electromagnetic interference shielding☆
Introduction
Carbon nanotubes have been intensively studied in both fundamental and applied research fields for over one decade [1]. Multiwalled carbon nanotubes (MWCNTs) have been promising nanomaterials for the commercial applications because of mass production. It has been considered to make composite of MWCNTs as filler with conventional polymer matrix such as poly (methyl methacrylate) (PMMA) for low cost, flexibility, and high quality of electrical properties [2]. One of important application fields using MWCNTs is electromagnetic interference (EMI) shielding. Comparing typical EMI shielding materials such as typical metals, the composites of MWCNT-polymer have many advantages such as ease of synthesis, flexibility, and low cost [3]. Although MWCNTs have many advantages in applications, metal catalyst should be considered in raw MWCNTs. It is important to know about the properties of MWCNTs with metal catalyst.
In this study, MWCNT–PMMA composites have been prepared in free-standing film form for the investigation of charge transport properties and application to EMI shielding materials.
Section snippets
Experimental
MWCNTs containing Fe [MWCNTs (Fe)] or Co catalyst [MWCNTs (Co)] were synthesized by chemical vapor deposition method. For raw MWCNTs, amorphous carbons and ferromagnetic Fe or Co catalysts were included. For the formation of free-standing film, MWCNTs were dispersed in toluene with PMMA by stirring and high power sonication for 24 h. Composites of MWCNTs in PMMA matrix were synthesized with various MWCNT mass concentrations. The homogenous distribution of MWCNTs in PMMA matrix was confirmed by
Results and Discussion
Fig. 1(a) shows the log–log plot of σDC(RT) of MWCNTs (Fe)–PMMA composites as a function of mass concentration (p) of MWCNTs (Fe). The increase of σDC in the order of ∼1010 S/cm was observed across pc(≅0.3 wt.%). The σDC of Fig. 1(a) shows that σDC of MWCNTs (Fe)–PMMA composites was explained with scaling law of percolation theory, σ(p)∝|p−pc|t [5]. The critical exponent t was ∼2.15, and pc≅0.003 was estimated from the plot of Fig. 1. Considering the percolation threshold (pc) is ∼0.16 for
Conclusion
We observed that σDC and EMI SE of MWCNTs (Fe or Co)–PMMA composites increased with increasing MWCNT mass concentration as expected in the percolation theory. We observed the relatively low percolation threshold as pc≅0.003 for MWCNTs (Fe)–PMMA composites, which originated from the 1D large aspect ratio of MWCNT. From the measured ρ(T), the charge transport of MWCNT–PMMA composites was followed by Sheng's tunneling conduction model, which was applied to granular metal or metal–insulator
Acknowledgements
This work was supported in part by the IMT-2000 Project, Ministry of Commerce, Industry, and Energy, Korea and the BK-21.
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Original version presented at QTSM&QFS 2003 (Quantum Transport Synthetic Metals & Quantum Functional Semiconductors), Seoul National University, Seoul, Korea, 20–22 November 2003.