Carbon nanotube synthesis and spinning as macroscopic fibers assisted by the ceramic reactor tube

TL;DR

This study reveals that the ceramic reactor tube actively influences CNT fiber synthesis in floating catalyst CVD, doubling yield and enabling better control over the process by decoupling precursor decomposition from CNT extrusion.

Contribution

It uncovers the active catalytic role of ceramic reactor tubes, especially mullite, in CNT growth, improving yield without affecting fiber quality, and suggests new control strategies for CNT fiber synthesis.

Findings

Ceramic reactor tubes, especially mullite, double CNT yield.

Reactor tube material affects thermal decomposition and carbon species.

Decoupling precursor decomposition from CNT extrusion offers new synthesis control.

Abstract

Macroscopic fibers of carbon nanotubes (CNT) have emerged as an ideal architecture to exploit the exceptional properties of CNT building blocks in applications ranging from energy storage to reinforcement in structural composites. Controlled synthesis and scalability are amongst the most pressing challenges to further materialize the potential of CNT fibers. This workshows that under floating catalyst chemical vapor conditions in the direct spinning method, used both in research and industry,the ceramic reactor tube plays an unsuspected active role in CNT growth, leading for example to doubling of reaction yield when mullite (Al4+2xSi2-2xO10-x(x = 0.4)) is used instead of alumina (Al2O3), but without affecting CNT morphology in terms of number of layers, purity or degree of graphitization. This behaviour has been confirmed for different carbon sources andwhen growing either predominantly single-walled or multi-walled CNTs by adjusting promotor concentration. Analysis of large Si-based impurities occasionally found in CNT fiber fabric samples, attributed to reactor tube fragments that end up trapped inthe porous fibers, indicate that the role of the reactor tube is in catalyzing the thermal decomposition of hydrocarbons, which subsequently react with floating Fe catalyst nanoparticles and produce extrusion of the CNTs and formation of an aerogel. Reactor gas analysis confirms that extensive thermal decomposition of the carbon source occurs in the absence of Fe catalystparticles, and that the concentration of different carbon species (e.g. carbon dioxide and ethylene) is sensitive to the reactor tube type. These finding open new avenues for controlled synthesis of CNT fibers by decoupling precursor decomposition from CNT extrusion at the catalyst particle.