Deformation dependent electrical resistance of MWCNT layer and MWCNT/PEO composite films
Abstract
It has been well documented that the electrical properties of a carbon nanotube (CNT) can be either metallic or semiconducting depending upon the tube's chirality. Theoretical aspects of the unique electrical properties of CNTs are reviewed. Based upon the fundamental understanding of this special feature, the deformation-dependent electrical resistance of multiwalled carbon nanotube (MWCNT) layer and MWCNT/polyethylene oxide (PEO) composite in the macroscopic scale are investigated considering both experimental and theoretical aspects. In the first set of experiments, a MWCNT layer was grown by plasma enhanced chemical vapor deposition (PECVD) process on a surface of copper substrate and a copper probe was applied to this surface inducing compressive deformation onto the MWCNT layer. It was found that the electrical resistance of the MWCNT layer under compression was reduced by 80 percent. The possible mechanisms for electrical resistance reduction were analyzed and suggested. Also, the MWCNT-enhanced surface showed a finite slope of electrical resistance as a function of contact force, thereby making possible the use of this arrangement as a small-scale force or pressure sensor. However, there is limitation on the direct use of MWCNT grown directly onto copper substrates for real applications due to the easy separation of the MWCNTs from the copper surface and the low yield of MWCNTs by the given metal deposition and PECVD system. The processing method developed for the second set of experiments uses intentional coagulation of dispersed MWCNT in polymer solution. This process is simple and effective to fabricate MWCNT-filled polymer films. MWCNT/PEO composite was selected after comparative resistivity measurement and microstructure analysis. The percolation threshold of MWCNT/PEO was determined experimentally to be between 0.14 to 0.28 vol% of MWCNT. Films having MWCNT content above the percolation threshold were conductive and exhibited repeatable values of electrical conductivity. Unique and repeatable relationships of resistance versus strain were obtained for multiple samples with different volume fractions of MWCNT. The overall pattern of electrical resistance change versus strain for the samples of each volume fraction of MWCNT consists of linear and non-linear regions. A model to describe the combination of linear and non-linear modes of electrical resistance change as a function of strain is suggested. The unique characteristics in electrical resistance change for different volume fractions implies that nanotube-based composites can be used as tunable strain sensors for application into embedded sensor systems in structures.
Degree
Ph.D.
Advisors
Kim, Purdue University.
Subject Area
Aerospace engineering|Electrical engineering|Materials science
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