Extrapolating the properties of individual CNTs into macro-scale CNT materials using a continuous and cost effective process offers enormous potential for a variety of applications.The floating catalyst chemical vapor deposition (FCCVD) method discussed in this paper bridges the gap between generating nano-and macro-scale CNT material and has already been adopted by industry for exploitation. A deep understanding of the phenomena occurring within the FCCVD reactor is thereby key to producing the desired CNT product and successfully scaling up the process further. This paper correlates information on decomposition of reactants, axial catalyst nanoparticle dynamics and the morphology of the resultant CNTs and shows how these are strongly related to the temperature and chemical availability within the reactor. For the first time, in-situ measurements of catalyst particle size distributions coupled with reactant decomposition profiles and a detailed axial SEM study of formed CNT materials reveal specific domains that have important implications for scale-up.A novel observation is the formation, disappearance and reformation of catalyst nanoparticles along the reactor axis, caused by their evaporation and re-condensation and mapping of different CNT morphologies as a result of this process.
The floating catalyst chemical vapor deposition (FC-CVD) process permits macro-scale assembly of nanoscale materials, enabling continuous production of carbon nanotube (CNT) aerogels. Despite the intensive research in the field, fundamental uncertainties remain regarding how catalyst particle dynamics within the system influence the CNT aerogel formation, thus limiting effective scale-up. While aerogel formation in FC-CVD reactors requires a catalyst (typically iron, Fe) and a promotor (typically sulfur, S), their synergistic roles are not fully understood. This paper presents a paradigm shift in the understanding of the role of S in the process with new experimental studies identifying that S lowers the nucleation barrier of the catalyst nanoparticles. Furthermore, CNT aerogel formation requires a critical threshold of FexCy > 160 mg/m3, but is surprisingly independent of the initial catalyst diameter or number concentration. The robustness of the critical catalyst mass concentration principle is proved further by producing CNTs using alternative catalyst systems; Fe nanoparticles from a plasma spark generator and cobaltocene and nickelocene precursors. This finding provides evidence that low-cost and high throughput CNT aerogel routes may be achieved by decoupled and enhanced catalyst production and control, opening up new possibilities for large-scale CNT synthesis.
The floating catalyst chemical vapour deposition (FC-CVD) method is unique in providing the capability for continuous carbon nanotube (CNT) synthesis at an industrial scale from a one-step continuous gas-phase process. Controlling the formation of the iron-based catalyst nanoparticles is widely recognized as a primary parameter in optimizing both CNT product properties and production rate. Herein the combined influences of pyrolytic carbon species and catalytic nanoparticles are both shown to influence CNT aerogel formation. This work studies the source of carbon in the formed CNTs, the location of aerogel formation, the in-situ behaviour of catalyst nanoparticles and the correlated morphology of the resultant CNTs.Axial measurements using isotopically-labelled methane (CH 4 ) demonstrate that carbon within all CNTs is primarily derived from CH 4 rather than some of the early-forming CNTs being predominantly supplied with carbon via thermal decomposition of catalytic precursor components. Quantification of CNT production along the axis of the reactor definitively 2 dispels the notion that injection parameters influence CNT formation and instead shows that bulk CNT formation occurs near the reactor exit regardless of the carbon source (CH 4 , toluene or ethanol). Supply of carbon to different reactor locations indicates that CNT aerogel formation will occur even when carbon is delivered near the exit of the reactor so long as the carbon source reaches a sufficient temperature (>1000 °C) to induce pyrolysis. These results give an indication of how future large-scale CNT reactors may be optimized and controlled by modifying downstream catalyst and carbon delivery.
Carbon nanotubes are regarded by many as being the next generation in high performance materials due to their unique properties. The Cambridge Process was developed to utilize these unique properties by directly spinning carbon nanotube fibres drawn from an aerogel sock. The sock is formed from carbon nanotubes grown via a catalytic chemical vapour deposition (CVD) process. Due to the nature of CVD, the process is readily scalable. Kilometres of fibre can be made at a rate of 20 m/min. Altering process parameters (catalyst concentration, feedstock injection rate, furnace temperature, and gas flow rate) allows the production of nanotubes of a desired morphology. The fibres have been characterized with scanning electron microscopy, transmission electron microscopy, Xray diffraction, and Raman Spectroscopy in order to confirm nanotube composition and orientation. The fibre possesses mechanical and electrical properties that rival or exceed those of present-day materials. Mechanical properties can be enhanced by increasing the degree of orientation of the nanotubes with the long axis of the fibre and by overall densification. These effects can be accomplished through drawing the fibre and solvent treatment.
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