S.J. Bauman, J. Clendenin, P. Cole, M. Leftwich
Keywords: Carbon Nanotubes, CNTs, Ultra-long CNTs, carbon nanostructures, semi-continuous extrusion
Summary:Significant research has gone into combining many short (20nm – 300nm long) carbon nanotube (CNT) strands together to form CNT yarns. Unfortunately, up to 90% of the benefits of utilizing pure CNT material can be lost in the end product due to the weaker interactions between individual CNT molecule walls versus a continuous molecular run of pure CNT material. Both physical and electronic characteristics are significantly different between CNT yarns versus pure, continuous CNT molecular strands. Pure, continuous, ultra-long (UL) CNTs are needed in order to reap optimal industrial benefit and to reveal true benefits over current carbon fiber reinforced composites. The UL-CNT synthesis process must be able to selectively produce a high percentage of metallic CNTs. Conveniently, production costs for short CNT strands (< 300nm) is now at an all-time low (~$2/gm for 70% SWCNT as of 2016) providing further justification that CNT production is maturing to the point that low-cost manufacturing is achievable. The next step is scale-up of UL-CNT production. Via a reactive CNT extrusion technique, growth of CNTs has been achieved directly on metal catalysts over a large area. Preliminary development of the technique using a chemical vapor deposition (CVD) system in an academic research setup has resulted in um-length CNTs, and simulations have revealed the potential to achieve pure CNTs in meter lengths within relatively short periods of time. Since the transfer of the technique to the nanotechnology strategic business unit of Nanomatronix, LLC, optimization of the growth method has been underway using a specially modified plasma-enhanced CVD (PECVD) system. While prior work studying the growth of ultra-long CNTs often claim the necessity of thin (typically, less than 20 nm) metal catalyst layers on substrates, Nanomatronix has demonstrated the ability to grow CNTs directly on bulk Ni catalyst substrates. Many parameters – sample pretreatment conditions, growth chemistry conditions, feed-gas flow rates, temperature, growth time, etc. – serve as tuning knobs by which to adjust the average CNT density, length, diameter, and the amount of amorphous carbon build-up relative to nanotube density. Dense CNT forests with average lengths on the order of 15 µm have been demonstrated, however, further development of the method is expected to produce high-density ultra-long forests (> mm lengths). The presented technique is novel in regards to high-volume production due to the fact that most nanotube growth processes with high-volume production potential and utilizing heterogeneous catalysis methods, such as those disclosed in literature, have been performed with catalysts suspended within a continuous gas stream (fluidized bed) or supported on a substrate within CVD-furnace systems. Neither of those processes allow for maximized-volume, multi-staged production units. In addition to the increased potential for high-volume production, the herein disclosed process may effectively eliminate the need for further refinement to remove catalytic material from the nanotube products, also referred to as nanotube purification, as is required in the typical HiPCO® and CoMoCat® procesess.