Electrical Impedance Analysis of Carbon Nanotube-Polyelectrolyte Thin Film Strain Sensors Jerome P. Lynch 1,2 , Kenneth J. Loh 1 , and Nicholas Kotov 3 1 Department of Civil and Environmental Engineering 2 Department of Electrical Engineering and Computer Science 3 Department of Chemical Engineering University of Michigan, Ann Arbor, MI 48109-2125 Abstract Single wall carbon nanotubes (SWNT) are an impressive material at the center of the nanotechnology revolution; in particular, SWNT posses a unique array of physical properties including high stiffness and good chemical reactivity. SWNTs are explored for inclusion in polymer matrices to produce thin films with electrical properties that exhibit sensitivity to strain loading. A layer-by-layer (LbL) fabrication procedure is detailed for the manufacture of free-standing thin films defined by interdigitated phase homogeneity and high strength. A characteristic feature of the proposed SWNT-polyelectrolyte (SWNT-PE) thin films is that nano-scale constituents can be deliberately varied to attain desired macroscopic electromechanical properties. Concentrations of SWNT and PE are varied to yield thin film strain sensors with high gage factors. SWNT-PE thin film strain sensors are patterned upon substrates as inductive coil antennas that can be wirelessly interrogated by a remote wireless reader. To fully characterize the wireless read-out mechanism of the patterned SWNT-PE antennas, electrical impedance spectroscopic (EIS) analysis of the thin films is performed. Keywords: Nanotechnology, carbon nanotubes, strain sensors, structural health monitoring 1. Introduction Nanotechnology has emerged as a promising technology that offers an unprecedented opportunity to design materials at the molecular level. One fundamental building block of the nanotechnology field is the carbon nanotube. The carbon nanotube was first discovered by Iijima (1991) and consists of carbon atoms bonded in a helical crystalline structure. Carbon nanotubes can be manufactured in multiple forms including single-wall (SWNT) and multi-wall (MWNT) nanotubes. SWNT are defined geometrically by one cylindrical lattice of carbon atoms (Figure 1) while MWNT are comprised of multiple helical lattices concentrically positioned. SWNT have diameters ranging from 0.7 to 10 nm and can be grown to long lengths offering aspect ratios as large as 10 5 (Saito et al., 1998). Carbon nanotubes posses a unique set of physical properties that heighten interest in their use. For example, SWNT have incredible stiffness with an elastic modulus experimentally verified to be greater than 1000 GPa (Treacy et al., 1996). Electrically, SWNT can exhibit conductivity properties consistent with conductors and semi-conductors depending upon the orientation of the carbon atoms in the tube’s lattice structure. In recent years, a plethora of work has been published regarding the chemical modification of carbon nanotubes (termed functionalization) to render their use in a broad array of chemical processes and applications (Lin et al., 2003). A new generation of smart structure technologies (specifically, sensors and actuators) can be designed by adopting the “bottom-up” approach offered by the tools and processes associated with the nanotechnology field. Within the smart structures and structural health monitoring communities, a number of researchers have begun to explore the adoption of nanotechnology as a means of designing sensing materials that exhibit measurable changes in their electrical properties due to mechanical and chemical stimuli. Dharap et al. (2004) report on the use of homogenous SWNT films formed by vacuum filtration of SWNT solutions as strain sensors. The SWNT film, commonly referred to as buckypaper, exhibits linear changes in conductivity when mechanically strained to levels as large as 400 μm/m. One limitation of buckypaper is the weak van der Waals attraction between individual SWNT resulting in the brittle failure of the film at low strains. Kang et al. (2006) propose reinforcing SWNT with a polymer matrix to provide a SWNT-based strain sensor with improved strain capacity. Their method first disperses SWNT in solution using dimethyl formamide (DMF) solvent and a polymer binding agent (polymethyl methacrylate) before casting in a mold. The result is a strain sensor exhibiting a linear (a) (b) Figure 1. Single-wall carbon nanotubes (SWNT): (a) metallic and (b) semi-conducting (adaptation of figures provided by Dr. Vin Crespi, Penn State) Source: Proceedings of the US-Korea Workshop on Smart Structures Technology for Steel Structures, Seoul, Korea, 2006