Development and Application of Gas-phase Raman Spectroscopy Techniques for Analysis of Carbon Nanotube Synthesis Processes

Abstract

Carbon Nanotubes (CNTs) are structures with outstanding chemical and physical properties, making them an attractive candidate for multiple industrial applications. They can be produced via different techniques, with the floating catalyst chemical vapour deposition (FC-CVD) being one of the most attractive solutions for the mass production of CNTs. The current production rate of high-quality CNT material with tailored properties falls short of the rising industrial demand. One of the major limiting factors for high volume production is insufficient knowledge of underlying chemical processes that result in the CNTs growth via FC-CVD, making the rate-limiting factors of the system unknown. Another limiting factor in the manufacture of high quality CNTs is the lack of on-the-fly monitoring of synthesis parameters. In this study, two Raman-based spectroscopic techniques were tested for the realisation of in situ measurements in a FC-CVD reactor. Cavity-enhanced Raman spectroscopy (CERS) was found to be a powerful low-cost high-resolution tool for analysis of non-reacting gaseous mixtures. Based on experimental results, its application for analysis of reacting flows was deemed to be limited due to localised changes in refractive index in reacting flows. A comprehensive model was derived, which was able to calculate an expected amplification of Raman signal, compared to a more typical free space spontaneous Raman scattering setup. Quality of the model was confirmed by experimental measurements. Pulsed Laser Raman spectroscopy setup was designed, constructed and implemented to perform in situ measurements of the CNT synthesis process via FC-CVD. A bespoke cross-shaped reactor with a purging system was developed and applied to provide continuous optical access to the reaction volume of FC-CVD reactor. Created Fluent-based computational fluid dynamics model was used to analyse fluid flow pattern and molar fraction distribution of chemicals inside the reactor in order to develop a suitable calibration procedure. Constructed Raman spectroscopy setup was able to perform in situ quantitative analysis of thiophene thermal decomposition at small quantities in H2 environment as a function of thiophene concentration and reactor temperature. Methane, H2S, Si-H and Si-H2 were detected as products of thiophene decomposition, followed by further decomposition of methane observed from quantitative measurements of H2 molar fraction inside of the FC-CVD reactor. The developed pulsed laser Raman setup successfully performed first of its kind in situ Raman measurements in a reaction volume of FC-CVD reactor during CNT synthesis at 1100 °C. The system was able to successfully provide key information of the reaction gas phase parameters which are vital for on-the-fly control synthesis process and quality of the CNT product. The developed reactor and Raman system can be applied for in-depth analysis of not only CNT synthesis but other reacting flows.