Aerosol carbon nanotube production via scalable microplasma synthesis of catalyst nanoparticles

Abstract

The exceptional properties of individual carbon nanotubes (CNTs) have long indicated their potential for a range of practical applications. Yet, the challenge of synthesizing ordered assemblies of high quality CNTs limits the ability to fully translate those properties into macroscale structures. Floating catalyst chemical vapor deposition (FC-CVD) has emerged as the most promising process for large-scale production with an increasing number of commercial uses. Yet, FC-CVD still requires improvements in control (i.e., reliability and CNT size) and quality (i.e., defects and impurities) to overcome the current trade-off between CNT quality and process intensity and enable the full potential of CNT-based materials.

This thesis describes the design, construction, and implementation of a lab-scale system that achieves end-to-end control of catalyst generation and aerosol CNT growth for the purpose of understanding these processes and assessing potential scalability. First, a stand-alone microplasma reactor is designed and fabricated, and used for synthesis of iron-carbon aerosols from a ferrocene vapor precursor. The microplasma approach achieves precise particle diameter control in the 1--5 nm range and aerosol concentrations an order of magnitude higher than previously published approaches; this is explained by a charge-mediated formation mechanism enabled by the us-scale residence time. The influence of operating conditions on process stability and run-time is investigated, and a dielectric gradient focusing technique is developed to reduce variability and extend the lifetime of operation. Second, a FC-CVD system is built and integrated with the microplasma reactor and used to explore CNT synthesis on iron-carbon catalyst aerosols. Controlling temperature, gas chemistry, and flow conditions at which the catalyst aerosol and carbon precursor streams mix is shown to be critical for enabling CNT nucleation, controlling CNT diameter, and limiting iron and amorphous carbon impurities. Synthesis of highly-graphitized single-wall CNTs is demonstrated over a range of operating conditions by a Pareto front analysis with production rates of ~1 mg/hr. Based on these findings, an outlook is presented on the limiting factors and criteria for the scale-up of high quality CNT production.