Mechanical and electrical properties of cross-linked carbon nanotubes

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 interactions, such as van der Waals forces, or the entanglement of CNTs. Here, we report an isotropic 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 applications of macroscopic assemblies of CNTs as sensors, transistors, electrodes, actuators and fibres. (c) 2008 Elsevier Ltd. All rights reserved.