Researchers expect carbon nanotube (CNT) yarns to be an alternative to metallic materials because of CNT fibers’ low density (1.93 g/cm3) and low-cost mass-production. By the hexagonally orientated structure of carbon atoms and strong covalent bonds generated by sp2-hybridized orbitals, individual CNTs exhibit excellent electrical properties, such as high electrical conductivity (108 S/m), ballistic conduction, and lack of a skin effect. However, the growth of individual CNTs is limited to a few centimeters; thus researchers require a method to create continuous CNT yarns for macroscale applications. A simple dry-spinning method, to horizontally draw CNTs out of CNT arrays that comprise vertically aligned multi-walled carbon nanotubes (MWCNTs), was developed to overcome the growth limit. CNT yarns produced by this method are held together by van der Waals forces to form a bundle, reaching a few meters in length. Though, the electrical properties of CNT yarns, such as electrical conductivity and current capacity, remain relatively low (104 S/m and 107 A/m2, respectively), which necessitates further improvement.The cause for low electrical properties can be attributed to low crystallinity—because of structural defects (such as five-membered rings) and impurities (such as amorphous carbon, voids, and contact resistances between the CNT bundle interfaces). Researchers have developed several post-synthesis treatments to overcome these problems, such as densification, thermal treatment, and chemical doping. Kim et al. impregnated CNT yarns into an organic solvent mixture as a densification treatment. The dense structure obtained by impregnation reduced the contact resistance between the CNT bundle interfaces, which enhanced the electrical conductivity. Thermal treatment, another post-synthesis approach, purifies the crystalline structure of CNT yarns. Dini et al. annealed CNT yarns at 2010°C, 2250°C, and 2450°C in inert gas. They reported enhanced repair of defects and crystallinity by increasing annealing temperature, which decreased the resistivity and enhanced the electron transport in the CNT yarns. A further post-synthesis treatment is to increase the electron carrier density by chemically doping semiconducting CNTs. Inserting electron-rich n-type dopants or electron-poor p-type dopants increases the number of electron carriers in the conduction and valence bands, respectively—which improves the electrical conductivity.The effect of individual treatment has been studied as of great importance, however, researchers have not sufficiently evaluated a combination of these treatments under optimal conditions (i.e., annealing temperature, dopant), although we would expect additional enhancement.In this study, we fabricated CNT yarns by dry-spinning and performed several post-synthesis treatments—such as polymer solution impregnation, graphitization (GT), and iodine monochloride/dichloromethane (ICl/DCM) doping—to enhance the electrical properties, compared with no treatment. Polymer solution impregnation increased the electrical conductivity 1.82 times and the current capacity 1.58, attributable to densification of the CNT yarn bundles. GT increased the electrical conductivity 2.58 times and the current capacity 1.83, attributable to the purification of the crystalline structure. ICl/DCM doping increased the electrical conductivity 1.79 times and the current capacity 1.31, attributable to the insertion of dopants that increased the electron carrier density. We achieved further enhancement by a two-step treatment—GT and ICl/DCM doping—which results in a 4.88 times increase in conductivity and reaches a maximum of 5.12×105 S/m. We observed a greater doping effect after GT, which increased the electrical conductivity 1.89 times, whereas doping for pristine CNT yarn only increased the electrical conductivity by 1.78. The X-ray photoelectron spectra of CNT yarn in each step indicated a positive relationship between the peak area of the π–π＊transition component and the electrical conductivity.Also, diameter measurement using the field-emission scanning electron (FE-SEM) revealed that the bundle diameter is reduced after graphitization. Therefore, we hypothesize that the purification of the crystalline structure and the decreased diameter of the CNT bundle structure increase the electron carrier density and the doping effect by doping. Although revealing the exact mechanism will require further research, our post-synthesis combination treatment enables us to take a further step in achieving electrical properties comparable to copper.

Electrical Property Enhancement of Dry-Spun Carbon Nanotube Yarns by Combination of Post-Synthesis Treatment

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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