Carbon nanotube synthesis by impinging flame Fabrício Lucas Tavares Monteiro Thesis to obtain the Master of Science Degree in Mechanical Engineering Supervisors: Prof. Edgar Caetano Fernandes Dra. Luísa Maria Leal da Silva Marques Examination Committee Chairperson: Prof. Carlos Frederico Neves Bettencourt da Silva Supervisor: Prof. Edgar Caetano Fernandes Member of the Committee: Prof. Maria de Fátima Grilo da Costa Montemor March 2019 ii Resumo Desde o início da sua descoberta em 1991, os Nanotubos de Carbono (“Carbon Nano Tubes – CNTs”) têm atraído grandes quantidades de interesse devido às possíveis aplicações. Nos últimos anos a síntese através de chama despoletou um novo rumo de investigação pois é energeticamente eficiente, escalável, rentável, rápido e contínuo, em que a chama fornece as espécies químicas para a nucleação de CNTs. Pretende-se sintetizar CNTs recorrendo à combustão e avaliar qual o papel desempenhado pelo estado da superfície do substrato. O propano é a fonte de carbono para a produção de nanotubos e os parâmetros de controle são o Reynolds (Re) e a razão de equivalência (φ), onde se obteve um ponto ótimo com Re = 700 e φ = 1.6. Os materiais colocados sobre a ação da chama são essencialmente compostos por Fe, Ni e Fe/Cr/Ni sendo que as microestruturas encontradas são caracterizadas com um microscópio eletrónico (FESEM) que ajuda a visualizar as estruturas e pelo raio-X (XRD) que providencia dados sobre a composição. Geraram-se CNTs com variadas dimensões sendo que os maiores obtidos possuem diâmetros na ordem dos 500 nm e os menores cerca de 18 nm. Juntamente com os nanotubos formaram-se estruturas metálicas de Perlite e Austenite a temperaturas características de 850ºC. É possível concluir que a temperatura é um fator chave no crescimento de CNTs, que a configuração tipo impinging dá as condições suficientes e necessárias à formação sólida de nanotubos e que o recozimento do aço produz Perlite que não permite a deposição de CNTs. Palavras-chave: Nanotubos de Carbono, síntese de chama, nanocristais de ferro, 304 aço inoxidável, ferro, rede de níquel. iii iv Abstract Since the beginning of its discovery in 1991, Carbon Nano Tubes (CNTs) have attracted large amounts of interest due to the possibilities of application. In recent years, flame synthesis has triggered a new direction of research. Flame synthesis is energy efficient, scalable, cost-effective, fast and continuous, in which the flame provides the chemical species indispensable for the nucleation of CNTs. This work aims to synthesise CNTs using combustion and to evaluate which role played by the state of the surface of the substrate. Propane is the source of carbon to produce nanotubes, and the control parameters are the Reynolds (Re) and the equivalence ratio (φ). An optimal point is attained with Re = 700 and φ = 1.6. CNT growth is observed on several substrates composed of Fe, Ni and Fe/Cr/Ni. The obtained microstructures are characterised by an electron microscope (FESEM) which helps to visualise the structures and by the X-ray (XRD) that provides data on composition. CNTs with varying dimensions were generated. The biggest nanotubes acquired have diameters in the order of 500 nm, and the smallest ones can have up to 18 nm. Together with the nanotubes, metallic structures of Perlite and Austenite at characteristic temperatures of 850ºC were formed. It is possible to conclude that temperature is a critical factor in the growth of CNTs and the impinging flame configuration gives enough and necessary conditions for the stable formation of nanotubes. The annealing of the steel (Fe/Cr/Ni) produces Perlite that does not allow the CNT deposition. Keywords: Carbon Nano Tubes, flame synthesis, iron nanocrystals, 304 stainless steel, iron, nickel mesh. v vi Contents Resumo ........................................................................................................................................... iii Abstract ............................................................................................................................................v Contents .............................................................................................................. vii List of Figures .................................................................................................................................. ix List of Tables .................................................................................................................................. xiii Nomenclature .................................................................................................................................. xv 1 Introduction ......................................................................................................... 1 1.1 Study motivation ......................................................................................................................... 2 1.2 Research objective ..................................................................................................................... 3 1.3 Outline of the present work ......................................................................................................... 3 2 Literature survey ................................................................................................ 5 2.1 Overview ..................................................................................................................................... 5 2.2 Carbon nanotube groundwork .................................................................................................... 5 2.2.1 Carbon nanotube structure .................................................................................................. 6 2.2.2 Properties of CNTs ............................................................................................................... 7 2.2.3 Summary of CNT features ................................................................................................. 11 2.3 Applications of nanotubes ......................................................................................................... 11 2.3.1 Worldwide projected nanotube growth ............................................................................... 11 2.3.2 Carbon nanotubes in energy .............................................................................................. 12 2.3.3 Carbon nanotubes in healthcare ........................................................................................ 13 2.3.4 Carbon nanotubes in the environment ............................................................................... 14 2.3.5 Carbon nanotubes in effecting materials ........................................................................... 14 2.3.6 Carbon nanotubes in electronics........................................................................................ 17 2.3.7 Other applications .............................................................................................................. 19 2.4 Carbon nanotube formation mechanism................................................................................... 20 2.5 Synthesis techniques ................................................................................................................ 23 2.5.1 Arc discharge ..................................................................................................................... 23 2.5.2 Chemical vapour deposition ............................................................................................... 23 2.5.3 Laser ablation ..................................................................................................................... 23 2.5.4 Flame synthesis ................................................................................................................. 24 2.6 Large-scale production ............................................................................................................. 28 2.6.1 Purification of CNTs ........................................................................................................... 29 2.6.2 Health, environment and safety consideration of CNTs .................................................... 30 vii 3 Experimental setup........................................................................................... 31 3.1 Combustion system ................................................................................................................... 31 3.1.1 Burning system................................................................................................................... 32 3.1.2 Sampling system ................................................................................................................ 34 3.1.3 Acquisition system ............................................................................................................. 35 3.2 Experimental procedure ............................................................................................................ 36 3.2.1 Standard procedure for premixed flames ........................................................................... 36 3.2.2 Sample preparation ............................................................................................................ 36 3.2.3 Working conditions ............................................................................................................. 37 3.2.4 Flame documentation ........................................................................................................