Carbon nanotube (CNT) fibers are next-generation materials with potential to displace traditional metals used as conductors. The constituent CNT that compose these fibers have remarkably high conductivity and low density; the current challenge is translating the desirable nanoscale properties to a macroscale material. One method for increasing the conductivity of produced fibers is to dope them by introducing chemical species that donate or withdraw electron density from the CNT network. In p-type doping, electron density is removed from the nanotubes, which increases the charge carrier density in the nanotubes as well as improving charge transport between adjacent tubes.<br/><br/>Through halogen intercalation, a record-high conductivity of 16.8 MS/m has been achieved in CNT fibers. However, the parameter space of CNT fiber doping, including the effects of CNT aspect ratio, thermal annealing, dopant concentration, temperature, pressure, and dopant species remains unoptimized. This is particularly important in the context of fiber density, as excess dopant material within the fiber structure increases the fiber mass without making substantial improvement to the material conductivity. Navigation of this parameter space is further obfuscated by the difficulty in obtaining a family of as-prepared CNT fibers that differ only in constituent CNT aspect ratio and not other parameters such as CNT chirality or diameter distribution, purification level, or fiber density.<br/><br/>CNT fibers in this study were prepared using an established method of spinning CNT-superacid solutions into a coagulation bath while applying an axial draw on the fiber. The raw CNT starting material was the same for all fibers prepared; the aspect ratio was altered by shortening the CNTs by applying a high shear to the CNT-superacid solution before extruding the solution into fibers. The aspect ratio was verified using an established method for relating solution extensional viscosity to the average aspect ratio of rigid rods in solution. For each fiber of a given aspect ratio, the effects of dopant concentration, temperature, and pressure were studied using iodine monochloride as the dopant of interest. The trends in specific electrical conductivity as a function of these process parameters identify the experimental conditions required to adequately penetrate the CNT network without leaving an excess of dopant material within the fiber structure.

P-Type doping of carbon nanotube fibers: Process optimization to maximize specific electrical conductivity

ACS Spring 2022

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