(680f) Atmospheric Pressure Plasma Nanoparticle Generator for Scalable CNT Production in a Fccvd Process

Carbon nanotubes (CNTs) are one of the most promising candidates for next-generation materials for applications ranging from high-strength, light-weight construction materials to field emission transistors. Unfortunately, wide-scale adoption of CNTs is impeded by the high production cost of crystalline, high aspect ratio CNTs needed for high performance in applications. Floating catalyst chemical vapor deposition (FCCVD) is the most promising synthesis method for scale-up due to its continuous nature and ability to directly produce powders, fibers, films, or foams. However, the FCCVD process currently exhibits low process efficiency and poor reactant utilization leading to production costs that are prohibitively high for most applications.

One major source of process inefficiency is from the catalyst nanoparticles used to grow CNTs. CNT diameter and number of walls is known to be proportional the catalyst nanoparticle diameter. To grow high-quality CNTs with few walls, catalyst nanoparticles need to be below 2 nm in diameter. Larger catalyst nanoparticles (diameters greater than 5-10 nm) will not be able to efficiently grow the desired CNTs. Catalyst utilization, defined as the proportion of catalyst material that successfully grows a CNT, is below 1% in current reactors. This leads to low productivity, low feedstock conversion, and additional post-processing steps to remove residual catalyst. Introducing the catalyst is generally done by the injection of metal precursors, such as ferrocene or iron pentacrabonyl, which decompose in-flight. This process introduces a high degree of complexity to the reaction system as the catalyst formation and CNT growth are occurring simultaneously and are poorly controlled since process parameters are optimized for CNT growth, not catalyst formation. Additionally, the metal precursors contain carbon that is released during thermal decomposition leading to a second carbon source, introducing potential competitive reaction pathways.

In this work, we describe an atmospheric pressure, radio-frequency plasma catalyst nanoparticle generator used to produce a scalable, consistent, and controllable stream of catalyst nanoparticles with mass flowrates that can enable high CNT productivity. Designing a preformed catalyst generator decouples the catalyst formation and CNT growth, allowing for process parameters to be optimized for both separately. A process schematic can be seen in Figure 1. A mixture of argon and hydrogen is used as a plasma gas, and methane is used as a carbon feedstock (in the dilution stream). This process utilizes an elemental iron source, in the form of a wire inserted into the plasma, for the catalyst particles, thus removing extraneous carbon from reaction management. The process geometry also plays a key role, in particular the length of the plasma and growth region before dilution. We present our results in optimizing this process to improve productivity, catalyst utilization, and reactant conversion to few walled, high aspect ratio CNTs.

This work was supported by the US Department of Energy award DE-AR0001015 (Advanced Research Projects Agency - Energy)