(Digital Presentation) Thermoelectric and Electronic Transport Studies of Ultrahigh-Conductivity Aligned Carbon Nanotube Assemblies

Abstract

Macroscopic assemblies of aligned carbon nanotubes (CNTs) have been doubling in conductivity every three years and have now surpassed 10 MS/m [1]. They are promising for replacing copper- or aluminum-based electrical cables in applications where flexibility or weight savings are critical considerations. Understanding of transport processes in these ordered CNT assemblies is critical towards further conductivity improvement; yet, fiber-level transport is still poorly understood. Here, we studied thermoelectric and electrical properties of aligned CNT fibers and bundles produced by solution spinning. We first measured thermoelectric properties while tuning the Fermi energy and demonstrated a giant thermoelectric power factor [2]. We then performed temperature- and magnetic field-dependent conductivity measurements. In contrast to the majority of transport studies of CNT networks [3,4], our aligned CNT fibers exhibited a metallic behavior in a wide temperature range (30-300 K), i.e., conductivity monotonically increasing with decreasing temperature, which we attribute to the excellent sample morphology. At temperatures below 30 K, we observed a gradual decrease of conductivity with decreasing temperature, together with negative magnetoresistance, consistent with the weak localization theory for disordered metals. We determined the dimensionality and coherence lengths of carriers via analysis of the weak localization behavior. In addition to macroscopic CNT fibers with diameters of ~10 μm, we also conducted conductivity measurements on individual crystalline CNT bundles (with diameters ~ 50 nm and lengths ~ 30 μm) that constitute the fibers.