High-Ampacity Power Cables of Tightly-Packed and Aligned Carbon Nanotubes

wileyonlinelibrary.com carbon fi bers with the specifi c electrical conductivity of metals (“specifi c”: normalized by the linear mass density). [ 1 ] These macroscopic CNT fi bers hold the promise to replace traditional metals for many applications including making stronger and lighter power transmission cables or electronic interconnections, [ 2 ] as well as durable fi eld emission or thermionic emission sources. [ 3 , 4 ] These applications require the fi ber to operate under high current, which leads to natural questions about the fi ber's ability to carry such a current without being damaged. Traditionally, current carrying capacity (CCC), or often called ampacity, is used to quantify this ability. CCC is defi ned as the maximum amount of current a cable (including any insulating layer) can carry before immediate or progressive damages; sometimes, it is more convenient to use the current density, especially when making comparisons among different types of cables. Also, for weight-critical applications, for instance, in the aerospace industry, specifi c CCC (CCC normalized by the linear mass density) is usually considered. Owing to the strong C–C bond, the CCC of individual CNTs can exceed 10 13 A m −2 without damage by electromigration, [ 5–7 ] which is 2 to 3 orders of magnitude greater than the electromigration limit of copper. [ 8 ] However, such superb CCC (limited by intrinsic optical phonon emission) becomes unapproachable when many CNTs are packed together to form a macroscopic CNT fi ber or bundle. The unavoidable inter-tube transport signifi cantly increases the resistivity, and the resultant Joule heating at high current densities raises the temperature, inducing damages and ultimately breaking the fi ber. Thus, the competition between current-induced Joule heating and cooling by thermal environments becomes the determinant of the CCC, as in metal cables; this competition scales with the volume-to-surface ratio, which increases with increasing cable diameter, making Joule heating progressively more problematic for larger diameter cables. So far, the most widely studied case for CNT networks is their immediate breakdown (usually in seconds or less) when carrying high current. The damage usually initiates around the hottest spot, particularly if associated with defects, kinks, or impurities. [ 9–12 ] The corresponding current limit can be defi ned as the failure current density (FCD), similar to the fuse current limit for metal cables. On the other hand, to be used as a power cable, CNT wires must operate below a regulations-specifi ed High-Ampacity Power Cables of Tightly-Packed and Aligned Carbon Nanotubes