Impedance of Aligned Carbon Nanotube Arrays and Polymeric Nanocomposites

Benefits of employing carbon nanotubes (CNTs) as a component of large-scale composite structures include mechanical (strength, toughness, etc.) as well as multifunctional property tailoring. In this experimental work, aligned-CNT forests and polymeric composites of aligned-CNTs are tested for both DC and AC electrical properties. Understanding the aligned-CNT forest and nanocomposite behavior is critical for understanding the improvements that can be expected in large-scale aligned-CNT hybrid nano-engineered composites under development. Numerous experimental and theoretical issues surround the measurement and interpretation of electrical properties of aligned-CNT structures, e.g., separation of contact vs. bulk resistance in the simple DC resistance case. These issues are discussed and highlighted in the interpretation of the new results presented here. Electrical properties measured are compared with existing models from the literature. Theoretical and practical considerations of electrode materials for use with CNT arrays are described, and four materials and methods (e-beam deposited Al and Pd, solder, and conductive epoxy) are compared. The aligned-CNT forests exhibit a linear (Ohmic) increase in resistance with length suggesting primarily metallic conduction. Significant increases in conductivity due to the aligned-CNTs are noted in the polymeric nanocomposites. AC impedance measurements suggest the CNT forests can be considered as an RC-circuit in series with the contact resistances for conduction along the CNT axis. Dependence on the amount of applied pressure on the forest during electrical measurements, and the lack of a mechanistic model and understanding of CNT-CNT interactions inside the forest, motivate future work in this area for microelectronics, MEMS, and structural (nanocomposite and nano-engineered composite) applications. Nomenclature C = Capacitance RQ = Quantum resistance RNT = Nanotube resistance RC = Contact resistance Z = Impedance Z’ = Real part of impedance Z’’ = Imaginary part of impedance 1 Visiting Graduate Student (Electronics Dept., Politecnico di Torino, Italy; INP Grenoble, France; EPF Lausanne, Switzerland), Dept. of Aeronautics & Astronautics, MIT. 2 Visiting Student (Instituto Tecnologico de Aeronautica, Brazil), Dept. of Aeronautics & Astronautics, MIT. 3 Postdoctoral Associate, Dept. of Aeronautics & Astronautics. Currently Chief Engineer, Gamesa Innovation and Technology, Sarriguren (Spain). 4 Postdoctoral Associate, Dept. of Aeronautics & Astronautics. Currently Assistant Prof., Dept. of Mech. Eng., University of Michigan. 5 Charles Stark Draper Assistant Professor, Dept. of Aeronautics & Astronautics, MIT 33-314, 77 Massachusetts Ave. Corresponding author: [email protected] Journal of Nano Systems & Technology Impedance of Aligned Carbon Nanotube Arrays and Polymeric Nanocomposites JNST.2009.002 2 Vol. 1 | No. 1 | October. 2009 | www.JNST.org © Copyright 2009, Massachusetts Institute of Technology.