Mechanical and electrical properties of cross-linked carbon nanotubes Seung I. Cha a , Kyung T. Kim b , Kyong H. Lee c , Chan B. Mo c , Yong J. Jeong c , Soon H. Hong c, * a International Center for Young Scientist, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan b Leibniz-Institut fu ¨ r Festko ¨rper- und Werkstoffforschung Dresden, Institute for Complex Materials, P.O. Box 270116, D-01171, Germany c Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-Dong, Yuseong-Gu, Daejoen 305-701, Republic of Korea ARTICLE INFO Article history: Received 4 July 2007 Accepted 13 December 2007 Available online 1 January 2008 ABSTRACT Several macroscopic assemblies of carbon nanotubes(CNTs), consisting only of CNTs, have been developed for the maximum utilization of their characteristic properties for macro- scale structures. These include bucky paper, freestanding films, and fibres. However, these materials have limited performance, because the CNTs are assembled via weak interac- tions, such as van der Waals forces, or the entanglement of CNTs. Here, we report an iso- tropic bulk form of randomly oriented CNTs in which they are cross-linked with each other through chemical reaction between functional groups on them. The reaction is included by spark plasma sintering. This chemical cross-linking provides a strength of 100 MPa with improved electrical conduction. Compared to a CNT pellet before sintering, the electric conductivity of cross-linked CNT increases by one order from 2 S/cm to 24 S/cm at room temperature and at the same time, the activation energy for electric conduction is decreased. The increased electrical conductivity is related to increased inter-CNT transport of charge carriers, with the cross-links providing bridges for them. The enhancement of mechanical strength and electric conduction by this cross-linking allows wider applica- tions of macroscopic assemblies of CNTs as sensors, transistors, electrodes, actuators and fibres. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Recently, many attentions are given to macroscopic assem- blies of carbon nanotubes(CNTs), including bucky-papers, fibers, pellets and thin films, in order to utilize the character- istic properties of a single CNT in macroscopic scale. They have huge potential applications for structural and functional components such as hybrid solar cells, supercapacitors, transparent electrodes, chemical sensors, artificial muscles and high-surface-area electrodes [1–12]. These attempts are inspired by successful fabrication of carbon nanotube nano- composites, which enhanced the performance of polymer, ceramic and metal matrix considerably by addition of rela- tively small amount of CNTs [13,14]. However, the actual per- formance of macroscopic assemblies of CNTs, such as electric conductivity and mechanical strength, are still far below the expectation. For example, the strength of CNT fibres are infe- rior even compared to conventional carbon fibres; the strength of CNT fibres are around 1 GPa without polymer binders, while the strengths of carbon fibres are ranged from 1 to 4 GPa [15]. These inferior mechanical properties of macroscopic assembly of CNTs are induced from the weak interaction be- tween CNTs. The supporting forces for these materials are 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.12.023 * Corresponding author: Fax: +82 42 869 3310. E-mail address: shhong@kaist.ac.kr (S.H. Hong). CARBON 46 (2008) 482 – 488 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon