Understanding the Direct Spinning of CNT Fibers in Terms of the Thermodynamic and Kinetic Landscape: A Personal View

Abstract

Abstract The process for the direct spinning of CNT fibers has been made to work well on a laboratory scale by the diligence of teams at Cambridge over 15\xa0years. However, in finding the process “sweet spot” in multidimensional parameter space, it is clear that there are a number of potentially critical factors that determine the ability to spin continuous fiber. When spinning is achieved, there is then a fine-tuning scenario that determines the structure of the fibers, including nanotube type, and ultimately the property spectrum of what is a remarkable new yarn-like carbon fiber. This chapter takes a critical path through a mass of research results from many laboratories. It leans particularly heavily on data from the Cambridge groups active in this area, as these can be and have been interrogated retrospectively. The background thermodynamics are addressed but against the understanding that the formation of a solid from gaseous components requires a nucleation stage and thus potentially a delay within the timescale of the continuous process. The possibility of the material from the ceramic reactor tubes affecting the nucleation of the catalyst particles is also addressed, as it opens up the possibility of further process control by the insertion of heterogeneous nuclei. A significant surprise is the prediction that most of the injected iron, once available on the breakdown of the precursor molecule (ferrocene), and some of the carbon would be lost as “plating out” on the reactor walls in the temperature range 750–1000°C. The primary role of sulfur, a necessary addition to ensure continuous spinning, is seen as one that, as a reducer of surface energy, enables homogeneous nucleation of the iron particles at 750°C without an energy barrier. However, the question is addressed as to whether sulfur also influences the actual growth of nanotubes from the catalyst particles. Its presence is known to favor graphite edge growth over “c” axis growth (laying down of successive new graphitic layers), and this can have profound effects on controlling the distribution of graphite in gray cast irons. Whether sulfur is critical to the carbon nanotube growth, mechanism remains to be seen, but it is certainly an additional factor. Industrial pilot plant development, continually informed by this improving understanding, is progressing in the hands of Tortech Nano Fibers Ltd., with 2–3 orders of magnitude increase in throughput achieved to date from a scaled-up version of the research reactors.