Development of an internally heated FC-CVD reactor for CNT synthesis

Abstract

Carbon nanotubes (CNTs) are a highly applicable material in many different applications ranging from energy storage systems to electromagnetic interference shielding devices. The macro assemblies of CNTs come in different forms including powders, textiles and sheets, which are synthesised through different techniques. One method to synthesise CNT textiles is known as floating catalyst chemical vapor deposition (FC-CVD) which generates some of the highest quality CNTs directly into the final form from the reactor. However, it was found that FC-CVD is not readily scalable with a low production rate per volume of reactor (< 2 kg/hr/m3reactor). Furthermore, the current reactors use ceramic synthesis tubes which are unfeasible to scale-up with due to the difficult operation, interchangeability with existing parts and high cost. As such, the global aim of this work was to improve the productivity of CNT aerogel through a new FC-CVD process utilising an internal heating system and a method to reduce the volume of reactor required to synthesise the CNTs. To understand the necessary steps in achieving a successful scale-up of FC-CVD, this work first investigated the effect that increasing hydrocarbon and catalyst concentration had on the process. It was discovered that CNTs can radially grow once formed, independent of the catalyst particles. The results implied that the radial growth was a consequence of molecular bombardment of the CNT surface with C2 species which had a significant presence at high hydrocarbon concentrations. The major outcome of this work completed the development of a new reactor design which increased CNT production density by decreasing the volume of the reactor. The new system utilised an internal heating element surrounded by a packed bed of particles to pre-heat the carrier gas (H2) and hydrocarbon source (CH4). In addition, the new system proved that the H2–CH4 delivery system could be de-coupled from the catalyst precursor injections whilst the aerogel product retained its high crystalline quality. Finally, this design solved an issue of scaling FC-CVD via reactor diameter by providing internal heating, which localised the thermal energy inside the reactor volume. With the increase in scale and usability of CNTs there is an incremental risk of exposure to the material. As a result, it is in the interest to quantify this risk and design solutions that ensure safety. This work also explored the types of particulate matter that could be emitted from the surface of a CNT mat whilst inducing thermal and mechanical stresses. It was found that under thermal stress CNT mats could release CNT bundles, however, most of the released particulate matter was composed of soot. Mechanical abrasion of the CNT mats induced CNT bundle pull-out from the mat, releasing the fibrils. Both release methods showed that interaction with CNTs bundles could be possible, but the interaction with an individual CNT was unlikely.