LAUSR

DOI: 10.1002/aelm.201800811 and environmentally friendly conductive cables or wires as a replacement for copper. From this view, carbon-based nanomaterial is a potential candidate. Carbon related nanomaterials including fullerenes, carbon nanotubes (CNTs), and graphene are promising due to their exceptional conductive and electronic transport properties, which may accelerate the practical and potential applications for various kinds of novel engineering areas spanning from electronics, energy storage, and advanced materials to nanotechnology and biotechnology. Among the family of carbon nanomaterials, CNTs have been a particularly attractive material since its discovery in 1991 by Iijima,[1] due to their nanoscale 1D shape, excellent mechanical properties, tunable electrical properties either metallic or semiconducting, high current carrying capacity, and many other exciting properties. These properties have highlighted the potential of CNTs use in a plethora of applications, including electrically conductive fillers in polymer composites, flexible and transparent conductive films, microelectronics (transistors, interconnectors, heat dissipaters), and lightweight conducting wires and cables.[2] Figure 1 points out the forecast presented by Endo et al.[3] on the present, near future, and long term applications of CNTs in various fields. An interesting potential application of CNTs is the long-term electrical conductors, which are able to transmit power from plants to plants or households, as well as be used in electronic devices. Compared to conventional copper cables or wires, CNT based cables have several advantages including 1) a lower density of 1.3 g cm−3 for single-walled carbon nanotubes (SWCNTs)[4] and 2.1 g cm−3 for multiwalled carbon nanotubes (MWCNTs),[5] both of which are much lower than that of copper, 8.96 g cm−3;[6] 2) environmental stability, which can stand with severe conditions including high pressure, large temperature changes, etc.; 3) excellent mechanical performance with a Young’s modulus and strength in the ranges of 1.0 TPa and 50 GPa, respectively;[7] 4) ultrahigh electrical conductivity as high as 108 S m−1 for SWCNTs, which is higher than that of copper (≈107 S m−1)).[8] Furthermore, the limited amount of conventional conductive metal resources in nature and their soaring market price greatly increased the need for a desirable alternative solution that are abundant in nature, low-cost, and The lack of progress to obtain commercially available large-scale production of continuous carbon nanotube (CNT) fibers has provided the motivation for researchers to develop high-performance bulk CNT assemblies that could more effectively transfer the superb mechanical, electrical, and other excellent properties of individual CNTs. These wire-like bulk assemblies of CNTs have demonstrated the potential for being used as electrical conductors to replace conventional conductive materials, such as copper and aluminum. CNT conductors are extremely lightweight, corrosive-resistive, and mechanically strong while being potentially cost-effective when compared to other conventional conductive materials. However, great technical challenges still exist in transferring the superior properties of individual CNTs to highly conductive bulk CNT assemblies, such as continuous wires, cables, and sheets. This paper gives an overview of the state-of-the-art advances in CNT-based conductors in terms of fabrication methods, characterization, conduction mechanisms, and applications. In addition, future research directions and possible attempts to improve performance are analyzed. The opportunities and challenges for related nonmetal competitive conductors are also discussed.