(688f) Rapid Feedback Experimentation for Floating Catalyst Carbon Nanotube Growth

Carbon nanotubes (CNTs) are among the most promising candidates for the creation of next generation engineering materials, as well as innumerable applications across diverse fields such as medicine and communications. Moreover, they provide a pathway to fix carbon from fossil or biological sources into long-term, useful solid materials. Adoption into large scale application such as composites or cabling, however, is still stifled by the limited synthesis scale of long, highly crystalline few walled CNTs (FWCNTs) needed to attain strength and conductivity at in macroscale applications. Despite major cost-reduction over the past decade, continuous production techniques such as floating catalyst chemical vapor deposition (FCCVD) have not yet attained the requisite the efficiencies to allow economic process scale-up. This is partially due to the large parameter space of the FCCVD process, which includes diverse carbon sources, catalyst chemistries, and process configurations. This makes systematic optimization challenging due to the large number of experimental combinations possible and extremely labor-intensive nature of such work.

To address these issues, the autonomous research system (ARES) was developed at the Air Force Research Laboratory and has proved invaluable by providing rapid feedback experimentation coupled with autonomous or semi-autonomous operation. ARES dramatically decreases the experimental effort required for each condition tested, allowing for both rapid, broad exploration of the CNT growth parameter space as well as repetition for the collection of meaningful statistics on process variability.

Thus far, ARES has been used to optimize surface-bound CTN growth. Here, we apply the ARES to FCCVD growth (FC-ARES) with the goal to develop an autonomous, closed loop research system for FCCVD reactors. Our custom built FCCVD reactor has an inline Raman spectrometer enabling rapid iterations through the FCCVD parameter space with near instantaneous feedback on the crystallinity and composition of the produced CNTs. We present two test cases to demonstrate the utility of this system. The first is a set of 179 experiments exploring how differing molecular structures of related carbon sources (ethanol, 1-propanol, and 2-propanol) impact the morphology of produced CNTs while controlling for flow rates as well as various interatomic ratios. The second is a set of 360 experiments which examines the effect of sulfur to iron ratios on the produced CNTs. It also examines the secondary effect of the oxidative or reducing nature of the system, as controlled by the addition of CO₂ and H₂ respectively. We expect FC-ARES to interrogate the large parameter space and identify impactful parameters that increase process efficiency, yield, and improved control over morphology and crystallinity, thereby defining pathways to geometric scaling of FCCVD reactors.

Figure Caption:

CNTs grown on the FC-ARES show diameter differences depending on the carbon source, as determined from the RBM region’s spectral centroid.