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

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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.

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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.

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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

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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 ...... 38 3.3 Characterisation of CNTs ...... 40

3.4 Uncertainties ...... 41

4 Results and Discussion ...... 45 4.1 Burner analysis ...... 45

4.2 Impinging flame ...... 47

4.2.1 Effect of the state of the substrate on the synthesis of CNTs ...... 48 4.2.2 Nanotube formation zone ...... 51 4.2.3 CNT structure ...... 53 4.2.4 Effect of inert, oxygen and time on nanotube synthesis ...... 57 4.2.5 Influence of the catalyst on the morphology of CNTs ...... 60 4.2.6 Growth mechanism and Reynolds impact ...... 63 4.3 Summary of materials characterisation conclusions ...... 65

5 Concluding remarks ...... 67 5.1 Summary of contributions ...... 68

5.2 Recommendations ...... 69

Bibliography ...... 71 A Complementary data on CNT ...... 87

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List of Figures

1.1: Evolution of the number of publications of CNTs issued annually from 2000 to 2010, from [5]...... 1 2.1: The structures of eight allotropes of carbon. Modified from [18]...... 5 2.2: High-resolution transmission electron microscopy (HRTEM) imageries of typical SWNT ((a), (d)) and MWNT ((b), (c)). Adapted from [27]...... 6 2.3: The scheme is showing how a hexagonal sheet of graphene is rolled to form a CNT with different chirality (A: armchair; B: zig-zag; C: chiral). Modified from [28]...... 6 2.4: a) Illustration showing the transformation of 4 adjacent hexagons into a 5-7-7-5 defect or Stone- Thrower-Wales defect, and (b) HRTEM presentments showing two 5-7-7-5 defects located on the edges (red circles) of a hole in a graphene surface [41]...... 8 2.5: Temperature dependence of the thermal conductivity for a carbon nanotube. Adapted from [8]. ... 9 2.6: Left: scheme of a field emission display with incorporated CNTs. (portrait by courtesy of Dr Choi of SAIT, [47])...... 10 2.7: Global CNT market estimates and prediction [55], by application, 2012 - 2022 (in Tons)...... 11 2.8: Energy application for CNTs. Modified from [67]...... 12 2.9: An instantaneous graph that relates the toughness of the ceramic material according to the presence of CNTs. Adapted from [89]...... 15 2.10: On the left we have the representation of the strength of CNT reinforced nanocomposites versus diameter. On the right, strength versus stiffness of matrices. Modified from [92]...... 15 2.11: CNTs sheets and yarns can take their places in data cables and electromagnetic shielding material. [pictures of Nanocomp Technologies, Inc.]. Adapted from [13]...... 16 2.12: (a) Cross section of CNTs dispersed in an epoxy resin and (b) an example of application on a lightweight CNT-fibre composite boat hull for maritime safety boats [13]...... 16 2.13: Nanocomposite eliminates the reflection from the tank and may cloak from guided munitions system. Modified from [101]...... 17 2.14: Photograph of a flexible circuit fabricated. Adapted from [104]...... 18 2.15: CNT loudspeaker. Modified from [116]...... 19 2.16: Bullet impact by .177 air rifle. Adapted from [118]...... 19 2.17: (a) Corresponds to the root augmentation and (b) to the tip progression. Modified from [125]. .. 20 2.18: On these illustrations, it is possible to see photos acquired by SEM. On the left the root growth model and on the right, it can be seen the tip growth model. Adapted from [126]...... 21 2.19: TEM portrayal of a nanotube, in this specific situation it is possible to see a combination of root and tip growth methods. Modified from [126]...... 21 2.20: Plot of growth rate and diameter as a function of temperature, by CVD. Adapted from [133]. .... 22 2.21: Presentment of the most common blaze types...... 24 2.22: Example of hydrogen sulphide and oxygen mixture. Modified from [196]...... 29 2.23: Life cycle assessment of CNT products. Adapted from [198]...... 30 3.1: The schematic diagram of the experimental setup involving the use of a co-flow premixed flame, Burner#2...... 31

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3.2: Real representation of the work environment...... 32 3.3: Cross section of the burner’s nozzle and its respective measurements. The portrait on the left is adapted from [200] and front view of the jet flow burner on the right, contemplating Burner#1...... 32 3.4: Burner#2 before mounting the experimental setup...... 33 3.5: The thermocouple on the top and measurements of temperature in real probationary conditions on the bottom with Burner#1...... 35 3.6: Morphology variation with equivalence ratio. Adapted from [203]...... 38 3.7: A series of FESEM images assessing soot and the respective sample (a) of the naked eye perspective. Minimal structures distinguish the soot...... 38 3.8: A representation of the flame at working conditions, ɸ = 1.6 and Re = 700...... 39 3.9: Image processing. Flame before (on the left) and later on (on the right) binarisation...... 39 4.1: Spatial location of the temperatures of the Burner#1...... 45 4.2: Vertical data from the highest temperature of the section, the centre of the burner...... 46 4.3: Evolution of the temperature with radius (r) in Burner#1 at z = 40mm...... 46 4.4: FESEM micrographs of bare 304 SS in the as-received (a), after annealing as pretreatment (b) and completely polished states (c)...... 48 4.5: In this assembly of imageries, it is witnessed the pattern acquired by the analysis of thresholding harnessed to a given area (red boxes) of Figure 4.4...... 49 4.6: FESEM images at 5K magnification with CNT deposition. On the top side, as-produced sample (case A). In the middle, the sample after annealing as a pretreatment (case B) and at the base, the completely polished sample after annealing as a pretreatment (case C)...... 50 4.7: Macroscopic visualisation of carbon nanotubes on case A...... 51 4.8: Better magnifications (30K) of the deposited nanotubes. Case A on the left and case C on the right for ɸ = 1.6, t = 15 min, Re = 700...... 52 4.9: FESEM presentments of MWNTs grown on 304 SS at ɸ = 1.6, t = 15 min, Re = 700. As-received (a) and polished (b) samples...... 54 4.10: FESEM acquired photos of a cross-section...... 54 4.11: MWNTs found in the scratched sample, case B. Subject to ɸ = 1.6, t = 15min, Re = 700...... 55 4.12: On the top, XRD pattern of MWNTs as well as 304 SS, at IST. On the bottom, the assessment adapted from the work [187] (pictured with the blue colour of the legend), by IDF in 304 SS...... 56 4.13: Effect of the inert addition, x-axis, on the inner flame temperature, y-axis. Study of two points, at 27 (blue line) and 30.5 mm (orange line) in radius centred on the same circumference and without a sample...... 57 4.14: Standard conditions with the influence of nitrogen at 1 SLPM, typifying Sample 3...... 58 4.15: Standard conditions without nitrogen for Sample 2 and Sample 5...... 59 4.16: A series of FESEM images for a plain (no catalyst deposit) nickel mesh immersed within the flame gases supplied by a propane flame at standard conditions...... 61 4.17: FESEM magnification of 60K for both imageries. Even though the MWNTs from the present study (a) are larger in diameter than (b), the structure similarity accomplished is irrefutable. Photograph (b) adapted from [176]. Left side worked at ɸ=1.6, t = 15 min, Re = 700 in the nickel mesh...... 62

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4.18: MWNTs are discerned from the material produced by flame contact and the iron substrate. Standard conditions are applied for the growth of nanotubes...... 63 4.19: FESEM portrayal of the morphology of MWNTs in Sample 7 with Re = 500...... 64 A.1: FESEM shows the microscopic reproduction of the formed black CNT powder, removed from case A (Figure 4.6) and the obtained structure was as follows...... 87 A.2: Standard conditions for 67%Fe/23%Cr/10%Al alloy...... 87

A.3: Working conditions of ɸ = 1.6, Re = 700, N2 = 0.2 SLPM, HAB = 8mm and 30 minutes. Material 304 SS for Sample 6...... 87 A.4: At standard conditions, MWNTs are built over Sample 7...... 88 A.5: Standard conditions for 304 SS with the exception of Reynolds which assumes the value of 600. In Sample 8, there are magnifications of 5K and 60K, in that order...... 88

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List of Tables

3.1: Classification of the substrates required for the work, dimensions in millimetres...... 37 3.2: Flame parameters for the primary working condition (optimal point), al values in millimetres...... 40 3.3: Summarizes the list of equipment operated to characterise the growth of CNTs...... 40 3.4: Uncertainty and relative errors for each flow meter at the optimal point...... 42 4.1: The following table ponders all samples inspected in the discussion of the results in order of appearance...... 47 4.2: Representation of the window opportunity in fabricating nanotubes when φ = 1.6...... 47 4.3: A selected sample and its respective mass with and without CNTs...... 53 4.4: The behaviour of the composition of carbon as a function of nitrogen. Four samples with four quantities of Nitrogen. All tests are done at standard conditions...... 58 4.5: Carbon solubility in selected transition metals, modified from [246]. Note that on the table only has values for pure elements...... 60 4.6: Qualitative examination of the bulk sample of Figure 4.6, case C...... 60 4.7: Qualitative inquiry of one the 150 interstices of the nickel mesh...... 63 4.8: Screenshot of the work done on the flame synthesis...... 65 A.1: Synopsis of decisive events in the carbon element. Adapted from [17]...... 88 A.2: SWNT vs MWNT, modified from [234]...... 89 A.3: Microscopic approach on properties of CNTs and other materials, adapted from [235]...... 89 A.4: Applications on metal reinforcement, properties commercial limiting and challenges for some of the most sought metal matrices. Modified from [245]...... 90 A.5: Homemade comparison contemplating all the four methods of CNT synthesis...... 91

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Nomenclature

Roman symbols

푄𝑖 Volumetric flow rate.

푒𝑖 Relative error.

푚̇ Mass flow rate.

푥𝑖 Mole fraction.

A Arrhenius pre-exponential factor.

Ea Activation energy.

L Length.

R Ideal gas constant.

Re Reynolds number. t Residence time in minutes.

T Temperature.

U Velocity.

Subscripts

푚 Measured value.

푚푎푥 Maximum.

푚푖푥 Mixture.

푠푡 Stoichiometric conditions.

Chemical species

C2 Dicarbon.

C3H8 Propane.

CO Carbon monoxide.

H2 Hydrogen molecule.

N2 Nitrogen molecule.

O2 Oxygen molecule.

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Greek symbols

⍵̇ Rate constant.

훼𝑖 Experimental uncertainty.

µ Dynamic viscosity.

2ϴ Scattering angle.

ɸ Equivalence ratio.

ϴ Chiral angle.

ρ Density.

휆 Wavelength.

Acronyms

1D One-dimensional.

2D Two-dimensional.

304 SS AISI 304 stainless steel.

3D Three-dimensional.

AD Arc discharge.

CDF Counterflow diffusion flame.

CMC Carbon nanotube ceramic matrix composite.

CNF Carbon nanofiber.

CNT Carbon nanotube.

CVD Chemical vapour deposition.

EDS Electron diffraction spectroscopy.

FESEM Field emission scanning electron microscopy.

HAB Height above burner.

HRTEM High-resolution transmission electron microscopy.

IDF Inverse diffusion flame.

LA Laser ablation.

LCA Life cycle assessment.

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MFC Mass flow controller.

MMC Carbon nanotube metal matrix composite.

MWNT Multi-wall carbon nanotube.

NDF Normal diffusion flame.

NSM Nanostructured material.

PF Premixed flame.

PMC Carbon nanotube polymeric matrix composite.

SEM Scanning electronic microscopy.

SLPM Standard litre per minute (298K, 1 atm).

SWNT Single wall carbon nanotube.

TEM Transmission electronic microscopy.

γ-Fe Austenite.

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1 Introduction

Our world is in constant revolution, decade by decade, year by year, day by day. Change is something that happens continuously, and now we are perhaps moving to a new Era, the carbon Era, where graphene, Carbon Nano Tubes (CNTs) and the Carbon Nano Fibers (CNFs) are in charge of that.

This work is based on premonitions that started on the early '90s by Sumio Iijima [1] when he fortuitously discovered the carbon nanotubes. Since then, many papers and technologies have been grown [2]. Scholars and experts in the field point out that this tiny, robust and versatile material is going to impact our world in ways that the Industrial Revolution, for instance, impelled our society.

1 The discovery of buckminsterfullerene (C60) in 1985, by Kroto [3], sentenced the beginning of an era of carbon nanostructured materials. The recognition came by the award of the Nobel Prize in Chemistry (1996). After a few years, in 2010, Konstantin Novoselov and Andre Geim, both from Russia, were able to write themselves into the annals of science. They won the Nobel Prize in Physics (2010) [4] after being recognised for their paper from 2004 in this new material, graphene. They have succeeded in producing, isolating, identifying and characterising graphene. The unique compound of which much is spoken, CNTs, is composed by a layer of atoms of carbon, whose name is graphene. It is the structural element of many other allotropes of carbon, even as graphite, diamond and Single- Walled Carbon Nanotube (SWNT). Moreover, now, thanks to this robust growth (Figure 1.1), it is common sense that when it can be produced on a large scale, the economy and the technology will grow exponentially. All of this is predictable because of the excellent features whenever we try to compare it with any other stuff.

Figure 1.1: Evolution of the number of publications of CNTs issued annually from 2000 to 2010, from [5].

Flame synthesis can maybe afford this scalability. It is easy to manage, cheaper than the methods already known and has proven to manufacture other nanoparticles. This technique can also provide hot temperatures, oxidising/carbonising surroundings that are vital components for the CNT

1Buckminsterfullerene is an allotrope of carbon with 60 atoms. It is represented of Figure 2.1 (D).

1 synthesis. The production volume of the flame industry has been growing each year, right now it can be declared that is at least of the order of 100 metric tons per day [6].

Carbon nanofibers and nanostructures are strongly known by their unusual and unmatchable properties. As an example, the 4th state of matter emerged as water trapped inside a carbon nanotube, and it does not act as a solid, liquid, or gas [7].

1.1 Study motivation

Nanoscience and nanotechnology are the study and application of minuscule things and can be consumed across all the other science fields. Carbon nanotubes are made on the nanoscale and right now are one of the most exciting materials. Nanotubes are great conductors [8], have remarkable structural properties such as abnormal Young’s modulus [9], have great flexibility [10] and high aspect ratio [11].

In our days the evolving impact of something is directly proportional to the amount of money generated by it. The production of CNTs around the world rises by 60% each year [12] since the late 1990s, where growth has intensified in the past decade [13]. Concerning the demand of CNTs, the total market value started at USD 1.13 billion in 2014 and the projection for the year 2023 is approximately USD 6.1 billion [14]. It is estimated by the World’s Biggest Carbon Nanotube Plant [15], LG Chem, an increase of world's carbon nanotube market from 2016 estimated 824 tons to 1,335 tons in 2020.

Currently, technology is progressing at such an extraordinary speed, so some of the main problems that exist when trying to manipulate CNTs can be solved. It is projected, soon, to keep improving the mode of fabricating carbon nanotubes, reducing their associated costs, managing the purity and understanding how to produce CNTs according to the specified functional needs. Simplistically speaking, it is needed a source of carbon, a source of heat the presence of metallic catalyst particles to assemble graphene or carbon nanotubes.

Chemical vapor deposition (CVD) is a conventional production method utilised along with plasma-arc discharge (AD) and laser ablation (LA). None of them is widespread on account of economic viability and incongruities. It is believed that this problem can be solved by using flame synthesis. When making materials using combustion usually, the outcomes are an exalted volume of products and reduced costs.

What most intrigues the scientific community is whatever carbon nanotubes can do that is not yet known or applicable on a large scale. The uprise of CNT production [12] probably is the answer, so the efforts have been herculean in this direction. One fact that validates this outgrowth is the nomination by the scientific community of the carbon nanotubes on “The top ten advances in materials science” [16] of the last 50 years.

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1.2 Research objective

There is a large gap amid the real situation and the ideal situation regarding the spread of CNTs, so this thesis emerged. The present thesis uses flame synthesis as a technique. While several flame configurations are operated for the progress of CNTs, the only configuration accomplished throughout the project is the premixed flame burner.

The objective of this dissertation is to set up the optimised conditions for CNT synthesis using a flame configuration.

Three tasks are considered in the thesis in order to achieve the stated objective:

1. Comprehend how CNTs work by carrying out an extensive bibliographic review to frame the subject, in its genesis; 2. Assembly and adaptation of the experimental setup 3. It is intended to produce nanotubes and metallic nanostructures their respective characterisation

The chemical and physical categorisation is supported via Field Emission Scanning Electron Microscopy (FESEM), X-ray Diffraction (XRD) and Electron Diffraction Spectroscopy (EDS). FESEM focuses on the sample’s surface producing views from the structure invisible to the naked eye whereas XRD provides the details about internal composition, structure and physical properties. Lastly, EDS justifies the elements on the sample by a composition analysis that contemplates atomic and mass of each detected element.

1.3 Outline of the present work

This dissertation is distributed into 5 Chapters. In Chapter 1, the thematic related to the nanotubes is contextualised, also presents the reasons that motivated the present research. During Chapter 2, a theoretical basis for understanding CNTs is granted. One focus is allocated to current applications, where carbon nanotubes are the differentiating factor. This Chapter also aims at flame synthesis covering almost every single aspect of this routine and ends with an approach to the large-scale production of CNTs revealing the life cycle of a nanotube. All the experimental methods and techniques associated with the work developed are stated in Chapter 3. The culmination of the thesis stays on Chapter 4, containing all the pertinent data obtained in the process. Possible conclusions are discussed, and a parallel is made with what exists in the literature. Finally, Chapter 5 highlights the relevant conclusions and prospects for future work.

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2 Literature survey

2.1 Overview

Carbon is one of the most abundant materials in the Earth´s crust and owing to this fact carbon exhibits an outstanding ability to form a wide range of structures, as spotted in Figure 2.1. Table A.1 (inserted in Appendix A) summarises the remarkable events that have taken place in the history of carbon.

The connection between the atoms of carbon impacts immediately the properties and the geometry of the material. Entirely configurations are well-differentiated, ordered according to the orbital space around a central carbon atom with several types of hybridisation2 (sp3, sp2, sp), addressed at work [17]. Particularly, the connections established by the nanotubes resembles in the hybridisation sp2.

Figure 2.1: The structures of eight allotropes of carbon: (A) Diamond [3D, network covalent structure], (B) Graphite [2D, covalent plates], (C) Lonsdaleite, (D) C60 [0D, molecules] (Buckminsterfullerene or buckyball), (E) C540 Fullerene, (F) C70 Fullerene, (G) Amorphous carbon, (H) Single-Walled Carbon Nanotube [1D, tubes]. Modified from [18].

In Figure 2.1 it is presented many allotropes of carbon, where we can find and classify each structure according to their dimensions 0D, 1D, 2D and 3D. CNTs are hybridised in 1D and graphene in 2D. CNTs are categorised as 1D because of their small diameter and large aspect ratio.

2.2 Carbon nanotube groundwork

Carbon nanotubes are members of the fullerene structural family, portrayed as cylindrical fullerenes. Their name arises from their elongated hollow structure with the walls formed by one-atom-thick sheets of carbon entitled graphene as seen in Figure 2.1 (H).

Despite having graphene as main structural, CNTs and graphene have different properties and behaviours. There are different types of CNTs structures, but the most discussed and relevant are

2 Hybridisation befalls when atomic orbitals are mixed turning into new hybrid orbitals with new intermediate levels of energy, appropriate to pair electrons to form chemical bonds in valence bond theory.

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Single-Walled Carbon Nanotubes (SWNTs) and Multi-Walled Carbon Nanotubes (MWNTs). Together, SWNTs and MWNTs, examined in Figure 2.2, represent almost 80% of publications for the publications for the period of 2000-2010 [5]. Both can be organised in various shapes [19].

A SWNT have only a single graphene sheet and, by contrast, a MWNT owns multiple graphene sheets. The average diameter for SWNT and MWTN is between characteristically 0.9 and 1.5 nm [20], [21] and 10-60 nm [22], [23], respectively. For MWNTs, the spacing among the graphene layers is ~3.4 Å [24], see Figure 2.2. Regarding the length, in the year 2014 was published the most extensive MWNT recording 21.7 mm [25] and one sample of as-grown SWNTs strands with a length of 20 cm [26]. More details on SWNT and MWNT on Table A.2, Appendix A.

Figure 2.2: High-resolution transmission electron microscopy (HRTEM) imageries of typical SWNT ((a), (d)) and MWNT ((b), (c)). Closed MWNT tips in panel (c) and SWNT tip panel (d), shown by arrows. The interior space corresponds to the diameter of the inner hollow in the tube. The separation between the closely spaced fringes in the MWNT ((b), (c)) is 3.4 Å. Adapted from [27].

2.2.1 Carbon nanotube structure

The structure of the SWNT is determined by how close they are to themselves in the hexagonal grating network. They often have closed ends (Figure 2.2) but can be open-ended as well. There are also cases in which the tube diminishes in diameter before closing off. Generally, SWNTs, exemplified in Figure 2.3, are classified according to three possible crystallographic configurations: zig-zag, armchair, and chiral.

Figure 2.3: The scheme is showing how a hexagonal sheet of graphene is rolled to form a CNT with different chirality (A: armchair; B: zig-zag; C: chiral). There are two basics vectors a1 and a2. Ch is the chiral vector and ϴ the chiral angle. Modified from [28].

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The atomic structure pattern relies upon the way that the graphene sheet is enfolded, to define this pattern it is utilised a pair of indices (n,m). The integers (n,m) refers to the number of steps along the chiral vector Ch, also known as a roll-up vector. After that, the chiral angle, ϴ, is the gauge that measures the amount of “twist” in the tube and is measured among Ch and a1 [29]. In cooperation, the terms constitute the helicity of the tube and are represented in Figure 2.3. Zig-zag nanotubes correspond to (n, 0) or (0, m) and have a chiral angle of (ϴ = 0°), and if n = m (ϴ = 30°), the nanotubes are identified as armchair nanotubes. Otherwise, they are named chiral (0°< ϴ < 30°). The vector of the nanotube also delineates the diameter since the inter-atomic spacing of the carbon atoms is stipulated [28].

The chirality of CNT and affects the optical, mechanical and electronic properties. This basic structure for CNTs can be incremented by an advanced technology called functionalization of CNTs [30] providing relevant changes in the structure of the nanotube and by that, proper dispersion, appropriate interfacial adhesion can be achieved giving countless applications for them. Surface functionalization of carbon nanotubes might be grouped into covalent and noncovalent functionalization [31].

2.2.2 Properties of CNTs

Carbon nanotubes have been impacting the nanotechnology with the incredible properties, caused by the atomic arrangements of carbon. Numerous theoretical and probationary works have been dedicated to studying the science behind this material. Table A.3, in Appendix A, mentions some useful values of CNT properties to understand the current and potential application range of CNTs. The parameters evaluated are Young's modulus, tensile strength, electrical resistivity and thermal conductivity compared to other materials such as copper, steel and diamond.

Although the scientific community accepts the current standards, it has been reported values above those of Table A.3 that are yet to be proven. For instance, theoretically, there is Young´s modulus of 5TPa [32] for SWNT. Consequently, after analysing the properties, it is assessed that the material in question (SWNT & MWNT) can be nine times stiffer, has a tensile strength of 200 times than the stainless steel and, beyond all this, at 1/6th the weight of ordinary stainless steel [33], [34].

Electric conductivity As mentioned before, the structure of a CNT determines how conductive the nanotube is. For example, a metallic SWNT is considered a highly conductive material. In MWNTs the process is quite complicated as their interactions are not uniform, and the current is transferred randomly over individual tubes. Luckily, in SWNT, an undeviating distribution of current happens [35]. The higher the conductivity of the material means that the collisions inside, between electrons and atoms, are minimised. Amongst carbon atoms exists strong bonds that allow CNT to bear electric currents way above than copper can support.

At the nanoscale, as reported by Wang et al. [36] that individual nanotubes can transmit nearly 1,000 times more electrical current density than metals such as silver and copper. It is important to refer that the electron transport only befalls along the axis of the tube as we are talking about 1D material. Frank et al. [37] study the current density as well as the conductance of CNTs. For the first, they have

7 got extremely high stable values of current, over 107 A/cm2 and for the second it was found out to be quantised.

Strength and elasticity CNTs, by reason of their bond between carbons (C-C) in the sp2 hybridisation, have a higher tensile strength than almost any other material on earth, that includes any form of steel or even Kevlar. Nanotubes are candidates to be harder and have a robust structure. The bond found in the resistant diamond, sp3 hybridisation, is weaker when compared with sp2.

Adopting transmission electron microscopy (TEM) to test the natural thermal vibrations of an anchored tube, Treacy et al. [38], as pioneers in this study, obtained an average value of 1.8 TPa for Young’s modulus of MWNTs. Under stress, the tip of a nanotube can be suffering bend without any secondary effects or tear on the nanotube. Then, by removing applied force, the nanotube returns to its original shape. SWNTs have shown unbelievable superplasticity, evolving into something 280% longer and 15 times thinner before splintering [39], which is explained as to the nucleation and motion of kinks in the structure.

The elasticity of a nanotube has limits, so a high amount of stress might input permanent deformation although it is not usual. Rahimian-Koloor et al. [40] specify that CNTs can have two types of crystallographic defects in the structure, vacancy and Thrower-Stone-Wales (TSW). Figure 2.4 manifest the TSW defect.

Figure 2.4: a) Illustration showing the transformation of 4 adjacent hexagons into a 5-7-7-5 defect or Stone-Thrower- Wales defect, and (b) HRTEM presentments showing two 5-7-7-5 defects located on the edges (red circles) of a hole in a graphene surface [41].

These defects may modify the volume, the length and the radius of the nanotube and consequently reduce up to 25% of Young´s modulus of the original structure. The strength of a chain depends on the weakest link in the chain, similarly for nanotubes. Additionally, CNTs have a specific strength of up to 48,462 kN.m/kg, compared to high carbon steel’s 154 kN.m/kg, is the best of known materials. Nevertheless, CNTs are not nearly as strong under compression. Because of their hollow structure and high aspect ratio, they tend to collapse when placed under compressive, torsional, or bending stress [42]. This limitation may possibly be conditional on the storage capacity and handling of the tubes. In the present dissertation, some of the constructed nanotubes were removed from the sample and the structure obtained is attached in Figure A.1, in Appendix A.

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Thermal conductivity and expansion All CNTs are anticipated to be very decent thermal conductors along the tube span. The strength of the atomic forces consents them to withstand high temperatures, as we can settle on Table A.3, in the last column. There seems to be some disagreement in the scientific community when it is time to specify the exact nature of the thermal conductivity of CNTs, even though most affirm that thermal conductivity changes with temperature, current and with defects.

Back in the year 1999, Hone et al. [43] agreed that thermal conductivity was temperature dependent, and almost a linear relationship. A year later Berber et al. [44] present a counter-argument, in Figure 2.5, to this supposed linearity. Both pieces of research compare the thermal conductivity of a hypothetical isolated layer of graphite or diamond to CNT’s thermal conductivity.

Figure 2.5: Temperature dependence of the thermal conductivity for a (10,10) carbon nanotube. Adapted from [8].

Known as chemically and structurally stable, CNTs are stable up to 2800 °C in a vacuum and 750 °C in the air [34]. Recent trial measurements [33], [45] suggest that the CNT embedded matrices are stronger in comparison to bare polymer matrices. Therefore, projections [13] exclaim that the nanotube could also meaningfully improve the mechanical and thermal properties of the composite materials.

Aspect ratio This feature separates nanomaterials, namely, CNTs and CNFs from every other material that we have known until today. They are the highest ever found since the disparity on their dimensions is bizarre.

The average high aspect ratio put forward in CNTs (length to diameter) of 1000 to 1 [46], means that even with reduced loadings of CNTs, it is attainable the same electrical conductivity of other conductive additives. This property is thrilling, as it preserves some of the performance properties of the matrix resin, for example. The most extended aspect ratio ever [26] got has an order of 2 x 108 to 1.

Field emission Under the application of an electric field, tunnelling of electrons between the metal tip and vacuum originates a field emission phenomenon. The small diameter and high aspect ratio will favour field emission. The field emitters must be superior to traditional electron sources, and in our days, they are spent in flat-panel displays. For MWNTs, the field emission properties happen attributable to the emission of electrons and light. Without applied potential, the luminescence and light emission occurs

9 through the electron field emission and visible part of the spectrum, respectively. It is noteworthy that the Samsung Advanced Institute of Technologies (SAIT) accomplished a very bright colour display (look at Figure 2.6), which will be soon commercialised using this technology.

Figure 2.6: Left: scheme of a field emission display with incorporated CNTs. Right: prototype assembled by Samsung (portrait by courtesy of Dr Choi of SAIT, [47]).

Kinetic properties MWNTs have the particularity of being associated concentrically. By reason of this, they show an extraordinary telescoping property wherein an inner nanotube core may collide, inside its outer nanotube shell, thus creating an atomically perfect linear or rotational bearing.

An actual example of molecular nanotechnology associated with nanoscience is the accurate positioning of atoms by creating valuable apparatuses. Thanks to this incredible asset, the smallest electrical motor ever was created [48], and it consists of just one molecule. According to Guinness World Records [49], this motor has barely one nanometer.

Adsorbent Adsorption expresses a unit of quantity of gas concerning a unit of quantity of adsorbent. CNTs are microporous carbon macromolecules and have been increasing their value as adsorbers as a result of their lightweight, enormous flexibility, high specific surface, high mechanical strength and superior electrical properties. Fortunately, CNTs emerge out as a supreme candidate as a filter in gas [50], air and water [46]. In 1997, CNTs were reported [51] to store large amounts of hydrogen gas with uptake capacities in the range of 5-10% by weight (wt%).

Chemical properties Pure graphite is one of the most chemically inert materials. It is resistant to most acids, alkalies and corrosive gases [17]. The heightened chemical reactivity of CNTs, compared with graphite, derive from the curvature induced σ-π hybridisation on the surface that changes the energy band dispersion close to the Fermi energy, along with the presence of large topological defects (e.g. Stone-Wales defect in Figure 2.5).

After some experiments, Tsang et al. [52] described a general routine that allows carbon nanotubes to be opened at the end and filled with a substantial amount of metal oxides using chemical techniques. Consequently, there are proofs that nanotubes in the presence of an oxidizer will

10 preferentially see their tube ends etched away since the sidewalls are less reactive. Bringing out the excellent prospect of using CNTs as nanosized vessel systems or as unique templates for creation of novel nanomaterials. The names of this phenomenon are capillarity and wetting [53]. Finally, it is important to highlight that the nature of the bonding of a CNT is depicted by applied quantum chemistry, specifically, orbital hybridisation [18].

2.2.3 Summary of CNT features

All properties have one thing in common. They are all defined by the carbon nanotube features. It is possible to determine what influences the properties of nanotubes and these are: atomic arrangement (armchair, zig-zag and chiral), diameter, length, morphology, alignment, purity, yield, number of walls. Each of this technical feature can be modified by changing the investigational parameters during synthesis.

2.3 Applications of nanotubes

Now in this chapter, some of the properties of CNTs will be put to the test. Many efforts have been made to demystify these question marks, which leads to successive advances and retreats. It starts with the projected growth for the CNTs and proceeds with real examples of applications. This next section meets the needs of understanding the purpose of nanotubes and explains why it is essential to dissect every single CNT feature. The high cost, the lack of control of the method (type and diameter of the CNT) and the limitations of the purification process are some of the drawbacks, now, for most of the applications of nanotubes.

2.3.1 Worldwide projected nanotube growth

In 2010, 1317 consumer goods were already estimated in the market (with incorporated nanomaterials), and this number has been intensifying every year [13], [54]. In Figure 2.7, the marketplace of nanotubes multiplies in separate areas.

Figure 2.7: Global CNT market estimates and prediction [55], by application, 2012 - 2022 (in Tons).

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Five factors primarily drive the global CNTs market [56]:

✓ Potential to replace fragile and expensive materials like indium coated films [57]. ✓ The escalation in production capacities. ✓ Applicability in a large variety of sectors [58]. ✓ Leads most materials in physical properties. ✓ The exponential progression of technology.

2.3.2 Carbon nanotubes in energy

Researchers at North Caroline State University, Fu et al. [59], have demonstrated the use of silicon coated nanotubes in anodes for Li-ion batteries where the capacity can multiply up to 10 times. Numerous other works have been developed on batteries, for example [60], [61].

Academics at Rice University have developed a Li-ion battery that can be painted on virtually any surface. Singh et al. [62] said in the work that the batteries were quickly charged with a small solar cell. An air-breathing bio-battery has been constructed by investigators [63] in Warsaw, and it is used to power medical implants. It consists of biofuel cells consuming substances naturally transpiring in the human body.

In 2015, Han and his co-workers [64] made possible a flux capacitor and had verified increased energy density for capacitors. Supercapacitors have their place using CNTs [65], [66]. Figure 2.8 is an illustration of the work from Sotawa et al. [85] as an energy apparatus by the supplement of nanotubes.

Figure 2.8: Energy application for CNTs. Acetylene black and nanotubes (CNT powder) additives between LiCoO2. Modified from [67].

The world's first all-carbon solar cell [68] was built at Stanford in 2012. As these are electrically transparent, they consent light to pass through them delivering photocurrent on the top layer of carbon nanotube films.

A study published in the American Chemical Society's journal Nano Letters [69] declares that thermocells based on carbon nanotube electrodes should be exploited for generating electrical energy from heat dropped by chemical plants, automobiles and solar cell farms, in other words, from waste heat.

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2.3.3 Carbon nanotubes in healthcare

As said by literature, carbon nanotubes have a high potential of finding unique applications in broad areas of medicine. Moreover, the encapsulation of other materials in the carbon nanotubes would open a prospect for their bio applications in medicine such as biomedical applications, artificial implants, tissue engineering, cell cancer identification and drug and gene delivery.

In the area of dentistry, it was matured a transparent nanomaterial [70] that can battle infection, improve healing, and benefit dental implants last a lifetime: titanium dioxide nanotubes.

According to scientists at the University of Texas at Dallas [71], new artificial muscles can lift weights that are nearly 200 times heavier than natural muscles of the same size.

MIT members [72] found out that sensors using CNTs can be useful to identify nitric oxide (NO) in the bloodstream of a patient. Usually, NO indicates inflammation, allowing easy monitoring or inflammatory diseases.

The nanotubes might be the future of cancer treatment one day. Research on mice [73] revealed that the tumours shrank and completely disappeared in 80% of the mice. Xiao et al. [74] took advantage of two unique optical properties of carbon nanotubes to detect and then obliterate breast cancer cells. In less than an hour, more precisely in 50 minutes, it is already possible to detect with 98% of precision the presence of oral cancer thanks to Malhotra and his co-workers [75].

The University of Michigan engineers [76] exposed a work where an optoacoustic lens can be employed for micro-scale ultrasonic fragmentation of solid materials; this is denominated in medicine as an invisible knife for noninvasive surgery.

Mooney et al. [77] are using CNTs to transform adult stem cells into a cell that may help repair damaged heart tissue. On the event Nano Summit 2017, in Luxembourg, was presented a treatment for ventricular tachycardia that restored the heart conduction velocity in a sheep that was alive. The heart attack was induced in an anaesthetised sheep, and the CNT fibres helped to achieve excellent results, closer to a healthy heart.

Nanotubes play a pivotal role in bone regenerative medicine [78] because they can be moulded consistently with the requirements. Due to the work of Gonçalves et al. [79], a composite based on CNTs in a polymeric matrix of polycaprolactone, by 3D printing, was made with the intention of regenerating bones.

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2.3.4 Carbon nanotubes in the environment

Nanotubes also hold promise for cleaning up polluted environments. CNTs are very effective at absorbing chemicals from their surroundings and have possible applications in water filtration and air filters, such as smokestacks.

For instance, Hashim et al. [80] orchestrated a sponge that can soak up oil in water with unparalleled efficiency. Further experiments showed this strongly oleophilic sponge, which is visible to the human eye, is incredibly efficient at absorbing oil in contaminated seawater because it attracts oil and repels water. What makes this discovery something genuinely epic is his reusable sorbent properties.

Assistants at the Technische Universität München [81] formulated a low-cost method of spraying CNTs onto many substrates to produce sensors. They quickly detect small changes in the concentration of gases (NH3, CO2, NO2) and will be behaved as detectors for spoiled food.

Over the past few decades, advances have been made in the groundwork of reverse osmosis membranes from different materials [82]. Afterwards, it was projected that CNTs membranes could effectively work in water desalinisation. Now, as per a report from the United Nations, by 2025, two- thirds of the world population possibly will be under water stress conditions, and 1800 million people will be facing absolute water scarcity. An answer to that is given by Ihsanullah [83], in 2018, in a very realistic approach. Other pollsters [84] are using carbon nanotubes to create small, portable, inexpensive water purification and desalination devices needed in developing countries.

2.3.5 Carbon nanotubes in effecting materials

Nanotubes are widely expended as a reinforcement material for polymers, metals and ceramics.

Ceramics CNTs have been well-thought-out as a new promising strengthening for Carbon Nanotube Ceramic Matrix Composites (CMCs) over the last decade [85], owing to their exceptional properties. CMCs were created to overcome the intrinsic brittleness and mechanical insecureness of monolithic ceramics [86].

A wide range of reinforcing fibres have been tested, including those based on SiC, carbon, alumina (Al2O3) and mullite. In Figure 2.9 it is represented some of the works that show how the properties of a ceramic material can be heightened in function of the concentration of nanotubes in their composition.

Zhan et al. [87] demonstrate with their preliminary study the resultant properties of SWNT- reinforced Al2O3, as compiled in Figure 2.9. They vary the weight concentration up to 4.5% of nanotubes at sintering temperatures of 1150°C, and they got an augment of 194% in fracture toughness (9.7 MPa.m1/2), nearly thrice more than pure nanocrystalline alumina. Colloidally processed MWNT/silica composites [88] are sintered in the temperature range 950-1050 °C and the properties of the material suffered 160% (about 2.74 MPa.m1/2) of increased value in fracture toughness.

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Figure 2.9: An instantaneous graph that relates the toughness of the ceramic material according to the presence of CNTs. Adapted from [89].

Nevertheless this new class of materials, the CMCs, has applications in different high- technology areas such as aerospace and nuclear energy [90]. Azarniya et al. [91] give a terrific review of ceramic matrix composites.

Polymers and metals Most of the literature focuses on these Carbon Nanotube Metal Matrix Composites (MMCs) and Carbon- Nanotube Polymer Matrix Composites (PMCs). In this segment, various numbers of applications both for polymers and for metals are included. Table A.4 summarises MMCs.

Ying Sun, in his PhD [92], explains how to do every type of CNT composite (PMC, MMC and CMC) and approaches the bonding between CNTs and Cu matrix. Figure 2.10 analyzes the effect of CNT diameter and the variation of the stiffness of the matrix in the function of volume fraction of CNTs (Vf). Some applications are transparent materials so that windshields may perhaps be produced. They are 50 times stronger than steel and more laborious than glass. However, they will not be brittle or break under stress. CNTs are ideal to make pipelines, military cockpit canopies and bulletproof glass.

Figure 2.10: On the left we have the representation of the strength of CNT reinforced nanocomposites versus diameter. On the right, strength versus stiffness of matrices. Modified from [92].

The first flight of an aircraft using CNTs for enlarged fuselage strength was made in 2008 by Unidym. Avalon Aviation's Giles G-200 aircraft utilised an engine cowling stuffed with nanotubes,

15 reducing the weight without weakening the component. It is appreciated in [93] some of the CNT usages like aircraft structures, interiors, paint, brakes, repair, flywheels, engines and coatings.

Coaxial cables for data transmission are ubiquitous in telecommunications, aerospace, navigation automotive and robotics industries. These cables, in Figure 2.11, can be made 50 per cent lighter with an innovative nanotube-based outer conductor developed by Mirri and their co-workers [94].

Figure 2.11: CNTs sheets and yarns can take their places in data cables and electromagnetic shielding material. [pictures of Nanocomp Technologies, Inc.]. Adapted from [13].

Scientists have found that nanotubes may well fill the voids that occur in conventional concrete. Voids are one of the main reasons for water penetration followed by cracking. Ultra-high-performance concrete already exists [95], and the main advantages are flexibility, high strength, halt crack propagation, reduction on time and costs of construction. The most impressive change was water absorption, decreased by 17% at 28 days curing [96]. This report opens the possibility for CNT to replace steel in suspension bridges.

Kim and Davis [97] have developed a coating made with carbon nanotubes that reduce about 35% the flammability of foam replicated in furniture. By opposition, CNTs can also be handled to isolate and prevent ice build-up with a “heatable paint” [98]. This coating is appropriate for fire protection.

Reinforcement of CNTs into epoxy can be used for industrial applications such as sports goods (e.g. badminton rackets, archery, golf and hockey sticks, boat hull (Figure 2.12), ski poles, kayaking).

Figure 2.12: (a) Cross section of CNTs dispersed in an epoxy resin and (b) an example of application on a lightweight CNT-fibre composite boat hull for maritime safety boats [13].

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CNTs will penetrate about 3.6% within vehicular composites [99], and this means that the automobile production is affected mainly by PMCs.

Timothy Imholt [100] has built a composite coating illustrated in Figure 2.13. This coating can absorb infrared radiation (IR). By lowering the reflection of IR radiation, the composite coating might prevent the laser-guided munition system from spotting or pointing the coated body.

Figure 2.13: Nanocomposite eliminates the reflection from the tank and may cloak from guided munitions system. Modified from [101].

2.3.6 Carbon nanotubes in electronics

Nanoelectronics holds some answers on how to expand the capabilities of electronics devices. As said by Mishra in [102], nanoelectronics was exposed to drastic changes, for example, reduction in size, improvement in the fabrication of circuits, increasing the speed of data transmission, reduction in power consumption, building better displays and better memory units. On the next paragraphs, it is going to be enumerated some of the examples in each area mentioned before.

Reduction in size and improvement in the fabrication of circuits CNRS and CEA researchers [103] have developed a transistor that can mimic the main functionalities of a synapse.

The University of Pennsylvania has proved that nanocrystals of the semiconductor cadmium selenide can be “printed” or “coated” on flexible plastics to produce high-performance electronics. Using this process, the researchers [104] assembled three kinds of circuits to test the nanocrystals performance for circuit applications: an inverter (Figure 2.14), an amplifier and a ring oscillator.

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Figure 2.14: Photograph of a flexible circuit fabricated. Adapted from [104].

Although there are imperfections inherent in CNTs, the first computer built entirely using CNT field-effect transistors (CNTFETs) is the most complex carbon-based system yet realised. In this work elaborated by Shulaker et al. [105], it is possible to find a little scheme of the CNT computer, a characterisation of CNTFETs subcomponents and the program implementation.

The increase in the speed of data transmission IBM’s CMOS nanophotonics technology [106] demonstrates technology breakthrough, the result of more than a decade of pioneering research. Silicon nanophotonics can answer Big Data challenges by their capability to multiplex large data streams at high data rates and delivering connection between various sections of large systems.

The investigators [107] recently accomplished unique spectral purity as a direct consequence of the incorporation of a nanoscale corrugation inside the multi-layered construction of the laser. The purer the tone, the more information it may carry. This laser is managed for a faster Internet.

Reduction in power consumption and good quality displays The power consumption of the heavy metal tantalum magnets [108] is up to 10,000 times lower than the general nanomagnetic computing and may perhaps be effectively managed as a replacement for the existing transistors. In 2005, Motorola Labs [109] debuted first ever nano emissive flat screen display prototype.

Enhanced memory units Carbon nanotube memory (NRAM) is a high-density next-generation memory constructed by Imec and Nantero [110] with a size of 20 nm. This technology has peak advantages, for instance, the write speed is as fast as three nanoseconds, the endurance is unlimited (over a trillion cycles), a low operating power requirement and superior high-temperature retention. It is the substitute for dynamic random- access memory.

Magnetoelectric Random Access Memory (MeRAM) is up to 1,000 times more energy- efficient than any technology known. This UCLA product [111] will be the next foundation for memory chips on the next generation for electronic applications as smartphones, tablets, computers and microprocessors or even data storage.

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2.3.7 Other applications

Hydrogen storage: Although the issue is controversial, there are many pollsters in the scientific community [112] and some companies who argue that CNTs and can be exercised for this purpose.

Light bulb: CNTs is capable of supersede tungsten in incandescent bulbs [113].

Magnets: SWNTs can generate a magnetic field and create a nano-dipole antenna [114], for electromagnetic communications.

Electroacoustic: Loudspeakers operates in a super-aligned thin-film of multi-walled carbon nanotube to produce sound, represented in Figure 2.15. There is already a microphone made of CNTs [115], it has the ability to segment the vowels from the consonants and separate them.

Figure 2.15: CNT loudspeaker. (a) A one-layer thin film attached to two springs that behave as electrodes. (b) The thin film doubled the original width while remaining perpendicular and aligned. Modified from [116].

Body armour: CNT fibres can produce bulletproof vests [117] capable of absorbing ten times greater than the fibre materials customarily used in soft body armour. In Figure 2.16, experiments with Kevlar, one of the primary constituents of everyday military products.

Figure 2.16: Bullet impact by .177 air rifle. Kevlar with CNTs on the left and neat Kevlar on the right and their respective displacement graphs depending on the load applied. The yarn pullout test (in a graph form) is related to the impact resistance. Adapted from [118].

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2.4 Carbon nanotube formation mechanism

To produce any CNTs, it is frequently required three basilar columns [119]: (I) a carbon source, (II) a source of heat, (III) along with the presence of catalytic metals.

This interaction between these elements have been studied for a long time, so there are various revisions, both practical and theoretical. They define how all the process goes on, since the carbon atom itself up to the accretion in nanotubes. From a book that addresses and composes the formation mechanism of CNTs it is known only a few chapters, the rest stills uncovered. Ordinarily, the models carry out catalytic action, diffusion and nucleation as processes to produce nanotubes. Within the descriptions about the mechanism of carbon nanotube formation in a catalytic synthesis process will be given prominence to one of the most popular and the most accepted growth model.

The process can be expressed by a dissociation-diffusion-precipitation model authored by Baker et al. [120]. Catalytic nanoparticles (e.g. Fe, Ni and Co) are either floating or bolstered in a substrate. The decomposition of a carbon precursor (e.g. CO, CH4, C2H6 and C3H8) is presumed to happen on the surface of the metal catalyst, in the form of carbon atoms. Several carbon elements suffer deposition/precipitation that forces the particle away from the substrate, forming the hollow channel of carbon.

In 1964, Wagner and Ellis [121] suggested an original manner, the vapour-liquid-solid (VLS). However, the most accepted growth model derives from this one, the vapour-solid-solid (VSS). Adopting the VSS model [122], the decomposed carbon precipitates at the surface an diffuses into the catalyst, and then the nanotube is nucleated within the boundaries of the catalyst. Two phenomena may occur when the culmination of the CNT formation process arrives, and they are predicated on the relative position of the metal particle. In the first place, the root or extrusion growth model is covered in Figure 2.17 (a) and explained by Hofmann et al. [123]. Secondly, the tip growth model is described in Figure 2.17 (b) and presented by Puretzky et al. [124]. Scanning Electron Microscopy (SEM) images are in Figure 2.18. Both mechanisms know how to coexist, symmetrically or asymmetrically as in Figure 2.19.

Figure 2.17: (a) Corresponds to the root augmentation and (b) to the tip progression. Modified from [125].

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Figure 2.18: On these illustrations, it is possible to see photos acquired by SEM. On the left the root growth model and on the right, it can be seen the tip growth model. Adapted from [126].

Figure 2.19: TEM portrayal of a nanotube, in this specific situation it is possible to see a combination of root and tip growth methods. Modified from [126].

Although these described models are simple and easy to understand, in cooperation with other parameters, are final core pieces for the synthesis of CNTs. The literature is loaded with information about this topic and Naha et al. [129] approach a model for the flame synthesis of CNTs and CNFs. Naha took it to another level attacking this subject in his PhD thesis [125] where he broadens the study on modelling and gives a summary of the modelling approach, the catalyst usage, the source of carbon, temperature, mode of synthesis and of course, the authors of each work.

It is important to clarify a few things, the presence of a catalyst is necessary but not enough condition to form SWNTs [119]. MWNTs can be created without metals [130]. There will be, naturally, many pathways on the art of sculpting nanotubes nonetheless using a transitional metal is expressly efficient and accurate.

The subsequent equations underline the interactions of solid carbon production including the crack of the leading fuel and intermediate species (Equation 2.1) [127], hydrogenation (or reduction) reaction (Equation 2.2) and Boudouard equation (Equation 2.3) [128].

푦 퐶 퐻 → 푥퐶 + 퐻 퐶푟푎푐푘푖푛푔 (2.1) 푥 푦 푠표푙𝑖푑 2 2

퐶푂 + 퐻2 → 퐶푠표푙𝑖푑 + 퐻2푂 푅푒푑푢푐푡푖표푛 (2.2)

2퐶푂 → 퐶푠표푙𝑖푑 + 퐶푂2 퐷푖푠푝푟표푝표푟푡푖표푛푎푡푖표푛 (2.3)

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Modified Arrhenius law combines the concepts of activation energy and the Boltzmann distribution law into one of the most indispensable relationships in physical chemistry cited in Equation 2.4.

퐸 − 푎 ⍵̇ = 퐴푇푛[푓푢푒푙]푎[표푥푖푑푖푧푒푟]푏푒 푅푇 (2.4)

In which A is the pre-exponential factor [units depend on the order of the reaction], 푇푛 [K] makes clear the temperature dependence of the pre-exponential factor,푛 is a dimensionless correction factor,

-1 -1 퐸푎 represents the activation energy [cal.mol ], 푅푇 measures the average kinetic energy [cal mol ] and ⍵̇ is the reaction rate [units depend on the order of the reaction]. [푓푢푒푙]푎 and [표푥푖푑푖푧푒푟]푏 are molar concentrations of fuel (C) and oxidizer (O) per unit volume, with 푎 and 푏 are partial orders of reaction. Together, 푎 and 푏, depends on the equivalence ratio and the reaction mechanism, but generally are not equal to the stoichiometric coefficients. Little is known of the mechanism of the reaction, which made it impossible to progress in this area all through the experimental activity of the present work.

Besides all of this, there are a few questions that are far from being answered, like: What is the ignition moment that starts the process? Following precipitation, how does the nanotube stabilise? What determines the finalisation of the process? And so forth. In just three questions it is possible to realise that the field of uncertainties is gigantic.

Influence of temperature One of the most dominant parameters conditioning the CNT growth has been the temperature. Lee et al. [131] demonstrate in Figure 2.20 that the growth rate and the average diameter increase nonlinearly with the surrounding temperature. The CVD study produced vertically aligned CNTs in the range of 800- 1100°C. They finish their work by concluding that the growth rate, the diameter, the structure, and the degree of crystallinity of CNTs depend strongly on the growth temperature. The diffusion energy of carbon in bulk γ-phase Fe is close to 32-35 kcal/mol.

Figure 2.20: Plot of growth rate (µm/min) and diameter (nm) as a function of temperature, by CVD. A linear function comes from Arrhenius plot, granting them 퐸푎 of 30 ±3 kcal/mol for the growth of CNTs. Adapted from [131].

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2.5 Synthesis techniques

Despite being relatively simple to do CNTs, it is imperative to choose how to do them. Several methods can perform CNT synthesis but the most scalable at the moment are arc discharge (AD), chemical vapour deposition (CVD), laser ablation/pulsed laser vaporisation (LA) and flame synthesis. Other processes to synthesize CNTs consist of: decomposition of SiC [132], solar energy [133], pyrolysis [134], electrolysis [135], synthesis from bulk polymer [136], mechano-thermal [137], liquid phase [138], high pressure catalytic decomposition of carbon monoxide [139] , carbon monoxide disproportionation [140], hydrothermal [141] and plasma torch [142].

Table A.5, which is in Appendix A, represents and explains the most consumed and explored routines to produce CNTs.

2.5.1 Arc discharge

The arc discharge is the most common way to produce CNTs, and it is the easiest way as well. In order to understand how it works there is a representative explanation in Table A.5. While AD played a crucial role in the evolution of CNTs, it is not reliable for industrial production (tonnage level).

Arora and Sharma [143] have an intense review of the AD method. In this work, it is communicated the procedure, a brief comparison amid options in power supply (AC, DC and pulsed). All relevant works related are cited. They conclude on the effect of a catalyst in CNT formation and provide a great insight into the impact of each parameter in the size and yield of the CNT.

2.5.2 Chemical vapour deposition

In 1996, chemical vapour deposition arose as a well-established routine to produce nanotubes. Because it is a technique with high scientific oversight, there are 12 schemes to do CVD [144]. Their classification depends upon the energy source. In Table A.5 it is characterised the most usual way to do CVD.

One of the most explicit reviews on this transformational method is presented by Shah [145]. This review has an extensive table on CVD experiments pointing out new factors as the quality, gases involved, time, temperature, catalyst, substrate, carbon source, type of process and some excellent remarks about each approach.

2.5.3 Laser ablation

A brief interpretation of the process and the products of the synthesis are in Table A.5. In recent years, laser methods are frequently utilised to acquire high-quality CNTs in small quantities with relatively low metallic impurities. Nevertheless, lasers are quite a bit expensive and have scale-up issues.

Assuredly the work proposed by Das et al. [146] is one of the finest reviews of this system to produce carbon nanotubes. It is offered a blueprint for all the dominant factors and some rate-limiting steps to regulate the intensification of these nanomaterials.

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2.5.4 Flame synthesis

The development of nanostructured materials (NSMs) are becoming increasingly important and over the last few years the flame synthesis of 1D NSMs, primarily CNTs, has been explored by numerous investigators [147] around the world. Every synthesis technique designated up to now resort on external heating of some nature: high current for AD process, laser and an oven for LA and the furnace for the CVD techniques. Synthesising nanotubes with blaze can offer advantages such as the right temperature for its formation. As time goes by, the growing number of works have demonstrated that flame synthesis can be applied as a relatively inexpensive, however robust method to produce NSMs.

The literature concentrates on four different configurations (Figure 2.21) where CNTs can be made, for instance, premixed flame (PF) [148], normal diffusion flame (NDF) or co-flow diffusion flame [149], inverse diffusion flame (IDF) [150] and counterflow diffusion flame (CDF) [151]. They are labelled as whichever diffusion or premixed flame depending on the haul of the oxidizer.

Furthermore, although it is on a smaller scale, it is possible to find other configurations as well as pyrolysis flame combined with a premixed flame [152], multiple inverse diffusion flame (m-IDF) [153] or even unconventional methods such as diesel engine [154]. Important to refer that any of the configurations can exist side-by-side, in other words, there are no limitations for combinations. There is a multi-stage study [155] where it is possible to find diffusion along with premixed flame configurations.

Figure 2.21: Presentment of the most common blaze types: a) Premixed, b) Normal diffusion, c) Inverse diffusion, Counterflow diffusion flame located d) on the oxidizer side and e) on the fuel side.

Either one of the mentioned techniques has a fuel-rich flame providing elevated temperatures, flow velocity and carbon-rich surroundings to produce CNTs if the necessary catalyst metal is present in the process. An illustrated summary of burner schemes can be found at [156]. Now, a closer look for the configurations from Figure 2.21.

CNT in premixed flames Premixed flame is a solution to produce CNTs since 1994. Howard et al. in 1994 [157] and 1995 [158] achieved the synthesis of MWNTs structures and claimed to have resorted to various fuels. The low pressure (20-97 torr) with diluent concentrations (from 0 to 44%) and peak temperatures of near 1800˚C engendered MWNTs oscillating from 2 nm to 30 nm in length, 1 nm to 10 nm in diameter with 5 to 20

24 walls. The studied flames were considered ‘sooting’ flames because of the abnormal equivalence ratio, values starting at Φ = 2.15 up to Φ = 3.21. Other studies in similar conditions are presented by Richter et al. [159] in 1996, and Diener et al. [160] in 2000. However, they capitalised all the attention on SWNTs evolution and concluded that absence of SWNTs in benzene flames happens owing to the slower dissociation rate when compared to acetylene and ethylene.

Later, in 2001, Vander Wal and his co-workers [161] extensively studied premixed flames including different combinations of fuels, catalysts and techniques and synthesised SWNTs, MWNTs and CNFs. Regularly they manage a sintered burner with a diameter of 50 mm bounded by a 5 mm annular ring that provides an inert gas flow [162], known as a McKenna burner. Cobalt catalyst in a stainless steel mesh [163] and a temperature of 800˚C are the conditions where the growth of MWNTs in different rich environments, for example, ethane, ethylene, acetylene, propane and methane are discussed. One of the main concerns was on how to have CNT progression directly on a supporting substrate to be used in the final application. Fortunately, there is an illustration of this possibility in [164].

Height et al. [20] in 2004, examine conditions for SWNTs formation in premixed acetylene/oxygen/15 mol% argon flames in cooperation with a floating catalyst, Iron pentacarbonyl

(Fe(CO)5). It is seen that the nanotubes are intensely made between 40 and 70 mm because on the spot there is the perfect amount of iron atoms and iron oxide molecules needed for the coagulation of the carbon atoms. The SWNTs escalation achieved roughly 0.9 to 1.5 nm in diameter. Finally, thermophoretic sampling was applied to gather the CNTs at diverse temperatures, 1200˚C to 1500˚C, and equivalence ratio amid 1.5 and 1.9. The best results fell on φ = 1.6.

The electric field is one of the components that can shape the alignment of the nanotubes, and a study was performed by Bao et al. [165] in 2005. As a result of the electrostatic force implemented at the beginning of the growing period, the nanotubes grew aligned; it was stated that applying DC after would lead to a random and disordered expansion.

A variation of the stagnation wall technique, that also produces nanotubes in a trumpet-shaped (or cool central core) ethylene flame with Ni as a catalyst [166], was anticipated by Woo et al. [167], [168] via a double-faced wall stagnation flow burner configuration which is suitable for massive production of MWNTs with improved quality.

A study from Hall et al. [169], dating the year of 2011, compares the influence of ethylene, ethylbenzene and ethyl alcohol on the flame synthesis of CNTs. Hall et al. investigated using a 50 mm sintered burner and a 10 mm concentric ring of nitrogen shield gas. They wind up saying that the presence of CO and H2 compound may be a prerequisite for triggering the enlargement of CNFs.

Over the year of 2017, Chong and his co-workers [170], [127] bet on nickel catalyst coated on a substrate of different types of stainless steel wire mesh (from 60 to 100 mesh number), equivalence ratio between 1.4 and 2.2, temperatures near 500˚C and a propane flame to produce CNTs. After some purification methods, the maximum yield achieved was 85% with a value of 50-60 nm in diameter whereas the length was in the order of microns [171].

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CNT in normal diffusion flames Formation of filamentous carbon in NDF was first disclosed by Saito et al. [172] despite the fact of the objective of the work was soot characterisation, especially on methane-air diffusion flames [173].

In the early 2000s, Vander Wal [174] reported MWNTs growth by aggregating metal nitrate nanoparticles on TiO2 powder. Vander Wal worked with other groups [155], [175] to synthesise SWNTs due to metallocene precursor and high-temperature hydrocarbon environment with nitrogen as a diluent of the fuel. SWNTs exhibited a middling diameter of 1.4 nm and up to hundreds of nanometres in length.

Entangled MWNTs, with 20-60 nm in diameter, were shaped by the addition of a methane flame into a Ni-Cr wire during the work from Yuan et al. [176], in the course of 2001. The typical linear fuel flow was 16.3 cm/s through a 1.1 cm diameter stainless steel tube, which was walled by an average linear co-flow air of 63 cm/s over a 5 cm diameter tube. A few months later the same group made a similar study [23], but they switch the fuel to ethylene. Then, after obtaining analogous MWNTs from methane flame, they inserted in the mixture more nitrogen that formed well-aligned MWNTs with 20 nm diameter and 10 µm long at a yield rate of 3 mg/min.

At the beginning of 2002, Hu et al. [177] invested in a Si-substrate with porous anodic aluminium oxide (AAO) template coated with cobalt to produce well-aligned MWNTs, they were grown from a laminar ethylene-air co-flow configuration. To complete the work, they stated that the diameter and length of the nanotube could be regulated using the AAO template. Li et al. [178] did similar research, later in 2009, but exploited methane as a carbon source and added another study object, a Ni-alloy substrate.

Platinum, nickel and stainless steel supports were submitted to the same conditions in 2005. Arana et al. [179] detected more substantial carbon deposits in the Ni wire. In 2007, Camacho et al.

[180] deposited nanotubes on galvanised steel using a broad number of fuels (CH4, C3H8, C2H2) and it was attainable to analyse quantitatively and qualitatively using techniques such as TEM and SEM. In the same document, it was recognised that the capability of a fuel to produce nanotubes is, to some extent, associated to its molecular arrangement over its ‘sooting’ tendency being that the higher the soot yield, the higher the production of nanotubes for a wide range of flow rate.

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CNT in inverse diffusion flames Perhaps this is the least spoken topic of the big four; there are only a few researches conducted on nanotube formation through IDF. One of the first works took place in 2004 by Lee et al. [181], [182], where it was verified the progress of MWNTs 5 to 7 mm from the flame centre in the radial direction in the temperature range of 700˚C to 1000˚C.

From 2006, Xu et al. [150] have a study on IDF where they produce MWNTs with uniform diameters (15 nm) on the range of heights (z) from 9 to 15 mm. They assert that at z = 6 mm there are no conditions to produce CNTs and the optimal distance for the advance would be z = 12 mm. The Spontaneous Raman Spectroscopy (SRS) is exercised to measure the temperature, the concentration of the species to understand how they influence the CNT grown around the flame. Other parameters such as substrate composition and the sampling position have their part on CNT growth as well.

Using an oxy-ethylene IDF and ferrocene, commanded by Unrau et al. [183] in 2007, it was possible to reach the production of SWNT with arithmetic mean length of 1 µm. It was determined that this flame offers a possible method to large-scale synthesis of SWNT because the nanotubes are not exposed to oxygen and thus, can be collected downstream.

Oxygen concentration, mixed fuel and sampling positions were designated vital components for obtaining CNTs, carbon nano-onions (CNOs) or both. Hou et al. [184], during 2012, made a radial distribution evaluation of the gas phase compound which maintains that low levels of CO and C2H2 united with low substrate temperature (under 400˚C) cannot form any carbon nanomaterial. For CNTs, the appropriate temperature goes up to 1000˚C. A nickel mesh with a thickness of 0.2 mm and a diameter of 3 mm was used as a catalyst and a substrate at the same time. It was concluded that CNTs were detected regularly in the section outside the soot zone on the fuel side of the coaxial burner.

In 2018, CNTs and iron-oxide nanoparticles (γ-Fe2O3) were yielded directly on stainless steel substrates (304 and 316L) using an open-atmosphere flame synthesis technique, under a globally-rich environment (near Φ=3) as well as time-varying between 5 to 20 min. Hong et al. [185] emphasised the importance of the substrate temperature (500˚C to 850˚C) in the production of nanotubes.

CNT in counterflow diffusion flames Likewise, the inverse diffusion flame, counterflow diffusion flame has not received to same attention as other techniques for CNT production. In the 80s, CDF cropped up in the nucleation of oxide nanomaterials [186].

The application through CNTs came a few decades after, by the year 2002, with Merchan- Merchan et al. [187]. In this pioneering study, MWNT are synthesised at an oxygen enrichment of 50% (bottom nozzle) and methane (top nozzle), surrounded by the supply of co-flowing nitrogen. No further catalyst was employed. A year later, the same group of researchers already supplied larger quantities on a Ni-alloy probe with composition 73%Ni/17%Cu/10%Fe [188], and a thick layer (35-40 μm) of vertically aligned MWNTs when applied an external electric field [189].

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Afterwards investigating IDF, Xu and his co-workers [151] also looked at non-sooting methane/air CDF, in 2007. It has experimented several metal-allow compositions (i.e., Fe, Fe/Cr, Ni/Cu, Ni/Ti, Ni/Cr, Ni/Cr/Fe) and the morphology of the obtained MWNTs are demonstrated in their work. They examine two specific flame conditions, the first one is a non-sooty flame and the second has a very-thin visible soot layer. An essential remark of the work is that acetylene (C2H2) is found to encourage direct fusion of vertically well-aligned MWNT.

Although one of the limitations of the CDF flame is the inconsistency regarding temperature and concentrations gradients, Li et al. [190] achieved very trustworthy trial conditions to produce relatively straight, astronomical quality and size-controllable CNTs over Si-substrates with porous AAO templates and a Ni-alloy wire.

Several studies on the influence of the oxygen have been made by Hu and his co-workers [191], [192]. In the denouement of their work, dated 2009, it is affirmed that oxygen concentrations may possibly affect the architecture of the carbon-based products with a threshold of 30% differentiating the construction of CNT (below 30%) or CNO (above 30%).

2.6 Large-scale production

As already mentioned, the large-scale flame synthesis has been extended to SWNT [193] and potentially to MWNT also [168]. During the past years, there has been much interest in the commercialisation of nanotubes. Allied to this happened an exponential augment on research, urging the first studies on environmental, health and safety consideration of CNTs. It is significant to know the ecological footprint left by nanotubes as well as their life cycle.

In fundamental research, it is favoured to get nanotubes as pure as possible, nevertheless, in all the CNT preparation techniques, the product itself contains common impurities such as carbonaceous materials, metal particles and other unwanted species.

Purity in nanotubes is the subject discussed in the first section. After that, it will be presented in the next section, as shortly as possible, what is known about the life cycle assessment (LCA) of CNT products.

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2.6.1 Purification of CNTs

These impurities might inhibit the performance of CNTs partially, modifying their properties. An ideal purification would eradicate all trace elements of metal particles, carbonaceous materials and other species while maintaining the quality of the nanotube. Thereby, three primary modus [46] are handled to purify the CNTs, and these are a gas phase, liquid phase, and intercalation. Each method covers several techniques (e.g. oxidation, acid treatment, microfiltration) that can be found in [18] by Aqel and his co-workers.

To separate all the impurities, one can choose from structure-selective and size-selective processes. The first separates the CNTs from the contaminants and the second process must give a uniform diameter or size distribution. Generally, separation and purification techniques are combined in order to improve the quality and to remove impurities from the nanotubes at the same time.

Recently, Richter et al. [193] removed impurities from CNT produce by flame synthesis. It was spent hydrochloric acid. The conclusion was that in as-produced and purified SWNT were typically 40 and >96% in purity, respectively. Another example of purification (acid treatment and thermal oxidation) is given in Figure 2.22; purity beyond 95% was achieved on the arc-discharge grown SWNTs.

Figure 2.22: Example of hydrogen sulphide and oxygen mixture. SWNTs as-produced (left) and purified SWNTs (right), Arc discharge method. Modified from [194].

Depending on the method to synthesise CNTs different purification and separation should be operated. There are advantages (e.g. improvement of nanotube stability, reduction of waste materials) and disadvantages (e.g. losing some nanotubes during oxidation, decrease in size [195], contamination of the final substance) in recurring on these processes, so is essential to produce cleaner nanotubes shortly.

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2.6.2 Health, environment and safety consideration of CNTs

Are the nanotubes toxic or harmful when exposed to humans? Can they be hazardous for the environment? What happens at the end of life of a CNT product? To answer that, Zhang et al. [12] extensively talk about the environment, health, ecological and safety considerations for CNTs. The baseline of the work is how to produce CNTs sustainably. Figure 2.23 declares each phase of the LCA.

Figure 2.23: Life cycle assessment of CNT products. Adapted from [196].

Other researchers have been studying the potential risks and adverse effects of the CNT toxicity in health care [197]. More research is mandatory to see how CNTs can be a help to our modern society.

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3 Experimental setup

This chapter contains all the information regarding experimental setup and controlled techniques throughout this study. It has a description and comments segments to enhance the comprehension of the premixed flame configuration along with the burning system. Two jet flame burners (Burner#1 & Burner#2) were managed being only the second affected by a co-flow environment. Reference is made to the working conditions for CNT fabrication and the respective dimensions of the flame. The characterisation techniques that were framed include Field Emission Scanning Electron Microscopy (FESEM), Electron Diffraction Spectroscopy (EDS) and X-ray Diffraction (XRD). At the end of this unit, it is given a brief explanation of the system behaviour to collect the samples followed by a detailed inquiry of the uncertainties arising from the manipulated instrumentation.

3.1 Combustion system

This investigational gear is mainly composed of a burning system, a sampling system, an acquisition and processing system. Posteriorly, there is a dedicated field where each element is scrutinised. The schematic diagram of this setup is represented in Figure 3.1, and a picture of the real apparatus is available in Figure 3.2.

Figure 3.1: The schematic diagram of the experimental setup involving the use of a co-flow premixed flame (Burner#2). MFC stands as a mass flow controller.

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Figure 3.2: Real representation of the work environment. It is simple to discern the systems: burning, sampling, acquisition and processing.

3.1.1 Burning system

Jet flow burners, mass flow controllers, gases and the premixed flame also comprise the burning system.

Jet flow burners Jet flow burners are known by the constant velocity profile nozzle, laminar and plug flow which gives advantages in mass nanotube production to the extent that there is a greater area subject to the same conditions. The Burner#1 is classified in Figure 3.3, and the Burner#2 in Figure 3.4.

Figure 3.3: Cross section of the burner’s nozzle and its respective measurements. The portrait on the left is adapted from [198] and front view of the jet flow burner on the right, contemplating Burner#1.

The controlled Burner#1 was similar the one used by [198], divided into two main parts (top and bottom) connected by bolts. It has a roughness on the internal wall lesser than 0.4 µm, and the total volume is roughly V = 150 cm3. The bottom part of the concentric brass tube receives the fuel and the air by injection. On the top, a convergent cylindrical nozzle with a 6.53 area concentration ratio with an outer diameter D = 18mm and thickness under 0.1 mm. This thin wall diminishes heat losses to the

32 environment. The burner is attached to a burner cavity filled with a stainless steel grid to assist the progress of uniform flux distribution of premixed gases introduced on the bottom of the cavity and help to quench any flame flashback. Note that the setup is open to atmospheric conditions and the tube is adequately long to create a fully developed laminar profile at the burner exit, then enabling the formation of a premixed flame. A drawing of the burner nozzle is exhibited in Figure 3.3. According to the crossed section by the line of symmetry of the burner, the inner and outer walls are as well as the refined construction of the edge. The Burner#2 (co-flow jet flow burner) is similar to the presented above; the main distinctions stuck between them are the existence of another concentric tube and naturally, the dimensions. The central tube is stainless steel with a 10 mm diameter, a wall thickness of 0.2 mm and a length of 10 cm. The annular ring is filled with 2 mm glass beads to spread the gas leakage, producing a consistent velocity profile. Contrary to the first assembly, now the exterior stream has not one, but two outward- facing openings, the central tube containing the premixture of fuel/oxidizer and the annular ring encloses the inert gush. Furthermore, the burners managed were easily movable, facilitating not only the change in working conditions but also the thermal analysis in the premixed flame. Real photograph of the Burner#2 is on Figure 3.4.

Figure 3.4: Burner#2 before mounting the experimental setup. The co-flow system is fed by four tubes, where the homogeneous mixture of nitrogen and the subsequent projection on the burner follow.

The subsystem of flow monitoring and calibrating Mass flow controllers (MFCs) from Alicat Scientific were required to regulate the gas outflow rate precisely. The models for this work were two MC-5SLPM3-D (max. capacity of 5 SLPM) and one MC- 20SLPM-D (max. capacity of 20 SLPM) for propane, nitrogen and air, respectively. All MFCs had uncertainty in reading (±0.8%) and scale (±0.2%).

It is mandatory to define the desired reactive composition and flow rate of the mixture. For the present work, it is imposed the ɸ and Reynolds number (Re) to the flow meters’ software. A computer program utilising LabVIEW is responsible for controlling the MFCs, which guarantees a certain degree of accuracy of the trial activity and significantly reduces any experimental errors. The parameters φ and

3SLPM - standard liters per minute.

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푅푒 are mutally correlated with the quotient of 퐶3퐻8 and dry air and the kinetics of the reactive mixture. The formula employed to calculate the equivalence ratio is Equation 3.1,

ṁ퐶 퐻 /ṁ푎𝑖푟 ɸ = 3 8 (3.1) (ṁ퐶3퐻8/ṁ푎𝑖푟)푠푡

additionally, to calculate the Reynolds is Equation 3.2,

휌 푈 퐿 푅푒 = 푚𝑖푥 푚𝑖푥 푐ℎ푎푛푛푒푙 (3.2) µ푚𝑖푥

where ṁ퐶3퐻8 defines the mass flow rate of propane and ṁ푎𝑖푟 the mass flow rate of the oxidizer used, the subscript 푠푡 denotes the quantities in stoichiometric conditions. µ푚𝑖푥 is the mixture dynamic viscosity, 푈푚𝑖푥 assumes the velocity of the mixture and 퐿푐ℎ푎푛푛푒푙 is the width of the channel, 푤 = 2 mm.

Gases The gases expended in the present work possessed high purity and were conducted over the tubes by needle valves sealed with teflon in threaded connections. Usually, the purity grades were greater or equal to 99.95% in volume. Propane and air were the species consumed for the main flame issuing from the central tube and nitrogen emanate from the annular ring. The air comes from the atmosphere, after being sucked with an air compressor. Propane gas is allocated in a 45 Kg bottle. The same ensues for nitrogen but in another bottle.

Some researches have proven that NSMs (including CNTs) are attainable with this hydrocarbon,

C3H8. The relation among the production of well-resolved NSMs in the flame environment is proportional to the ‘sooting’ propensity [199], by that propane was selected.

3.1.2 Sampling system

The plate, in Figure 3.3, simulates the position where the metal substrate, other probes or even the thermocouple were placed. Each referred position is at a Height Above Burner (HAB) represented by the letter H in the z-axis. A metal claw was operated to support the samples above the flame. After exposing the material to the flame front, the sample was taken for FESEM exploration.

Three-way translators A three-way manual translator (x, y and z-axes) made of gears and a lead precision screw, allows measurements to be taken at various HAB. The vertical range of movement (z) is close to 60 cm. The horizontal range of movement has boundaries of 40 cm (for both axes, x and y). Any of the burners may well be deliberately manipulated to any position within a certain range of values by recurring the translator. Moreover, the thermocouple can be mounted in an optical three-way translator to evaluate blaze temperature profiles either radially or vertically accurately.

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3.1.3 Acquisition system

The acquisition system is composed of a type R thermocouple, an analytical balance, a camera and the Data Translation DT 9828 acquisition board and a thermocouple software (QuickDAQ 2013).

Thermocouple The thermal examination is sustained by the interaction of a 65 µm Pt/Pt-13%Rh type R thermocouple and the flame; equally represented in Figure 3.5. This resistant thermocouple was involved in the collection of the thermal data. Temperatures under 1700°C are suitable for the type-R thermocouple which turns out to be superior to any value measured in flames usage.

Figure 3.5: The thermocouple on the top and measurements of temperature in real probationary conditions on the bottom with Burner#1.

Analytical balance KERN ABT 120-5DNM is the analytical balance model utilised for measuring purposes. Samples were weighed prior to the experiment and then compared to weight before deposition. It is qualified equipment and works under ISO 9001 certification [200].

Camera and lens Image acquisition was made by a Canon EOS-7D camera with an EF-S 18-135mm f/3.5-5.6 17 IS USM lens set up in several moments. However, there is only one situation that the photos were taken with the specific idea to get and scale all measurements from the flame. The camera was positioned 190 cm above ground level and exactly 30 cm from the centre of the burner. all photos were captured at one time in order to reduce associated errors, such as parallax errors. In section 3.2.4 is where the flame documentation appears, and at that moment all the necessary information will be given that allowed the sizing of the flame of the experimental setup. For each condition, the burner was restarted, and the photo from that condition was taken two minutes after the ignition of the flame to guarantee the complete system stabilisation.

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3.2 Experimental procedure

Two main configurations are addressed in this work: premixed flame with (Burner#2) and without co- flow (Burner#1). Because the only difference in the process of co-flowing is the addition of inert only a roadmap is engendered below. Summarily, it is delivered below each step that guides to a successful generic procedure to produce CNTs utilising Burner#2.

3.2.1 Standard procedure for premixed flames

(1) Open the handle of compressed air and all the needles.

(2) Repeat the process for the propane and the nitrogen cylinder, regulate the pressure for 20 psi in both cases.

(3) Turn on the power of the mass flow controllers (MFCs) and connect them to the computer.

(4) Open the LabVIEW program: set the values of flow frequency for each air and propane in SLPM.

(5) Set the flow meter of 0.5 SLPM for nitrogen, analogically.

(6) Introduce the geometric values of the burner and the boundaries of MFCs.

(7) Click on the RUN button in the LabVIEW to initiate the software.

(8) Put the substrate braced by the metal claw in the three-way translator; choose the height and the radial position.

(9) Ignite the blaze.

(10) Run the set for 15 min.

(11) With the deposition on the sample, release it and prepare it for FESEM and get ready to explore it.

(12) When the experimental sets finish, close off the propane source and let the flame disappear.

(13) Let the air gush cleans up all the fuel from the tubes and needles; Close off the air and the nitrogen.

(14) Shut off all the valves in the experiment.

(15) Turn off the MFCs power and close the LabVIEW program.

3.2.2 Sample preparation

Many different types of samples were investigated during this work. Every sample can be placed above and parallel to the burner, impinging the leakage on the substrate. For nanotube evolution, acetone was used to clean all the surfaces, and in some cases, sandpaper was exploited to rectify the surface. The tested items are listed in Table 3.1.

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Table 3.1: Classification of the substrates required for the work, dimensions in millimetres.

Sample Thickness Length/Diameter Observations

Rectangular 304 0.5 50 Different surface states are presented stainless steel Foil during the experimental work

Circular 150 µm 0.2 3 Meshes as-produced nickel mesh

Rectangular Fe/Cr/Al4 0.5 50 Only for demonstration, will not be alloy appraised as the other substrates.

Iron substrate - - Plate (99.5%Fe)

To clarify the real impact of the substrate were added variables of relevance. Three types of stainless samples were evaluated: a brand-new sample (sample A), a plate after annealing as a pretreatment (sample B) and a completely sanded plate after annealing as a pretreatment (sample C).

3.2.3 Working conditions

The ambiguity on working conditions in the literature is extensive; this is owing to the complexity of the process of combustion where CNT augmentation occurs under special restrictions. A summary of the working conditions (where CNTs evolved) is in the table format, Table 4.2 on section 4.2.

Nanotubes are synthesised with a Reynolds value between 500 and 900 accompanied by a φ of 1.6. The HAB (situated in the z-axis) is 8 mm for all experiments, and the nitrogen has a value of 0.5 SLPM, applied as a co-flow. Regarding residence time (t), all the probes were in contact with the blaze for 15 minutes. The sum of these restrictions translates into standard conditions for this work. The optimal point gauged resembles velocity (Re = 700) and ɸ equal to 1.6. Throughout the presentation of the probationary results, some variables (inert, substrate and residence time) may perhaps change. When it happens, it will be signalised.

From the work of Height et al. [201], the obtained value of greater production is obtained, thus corroborating the value of ɸ = 1.6. On the left side of the next Figure 3.6, is shown the structures witnessed after some trials with equivalence ratios from 1.4 to 2.0. On the other hand, the right side of the image contains the enlargement ratio of nanotubes and the unique values to produce them.

4This iron alloy was subjected to several tests. However, a specific analysis was not introduced to this substrate since it did not contain relevant information that fell within the scope of this thesis. Despite this, it was introduced because it includes pertinent data and there are no studies for CNT production based on 67%Fe/23%Cr/10%Al alloys. In Appendix A (Figure A.1) there are some of the curious data of this alloy.

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Figure 3.6: Morphology variation with φ, CNTs from the flame method. Adapted from [201].

After all, the velocity is seen as kinetic energy empowering collision amongst species and facilitating coalescence of nanotubes, it also modifies the final product. The burner speed downstream at the optimal point is approximately 1.13 m/s. Woo et al. [168] and Nakazawa et al. [166] developed studies with similar velocities in the order of magnitude of the present work.

As referred in chapter 2.5.4 in the section of CNTs in PF, revisions [157] are purposing ‘sooting’ flames (2.15 < φ < 3.21) with CNT production. Nevertheless, for most of the literature where there is soot, there is no possibility to produce nanotubes in modest quantities.

Although in the literature there are some references to produce soot (Figure 3.6), a soot-based study is presented below in Figure 3.7. It is anticipated that the structures will be similar since to the naked eye the layer formed may have a similar thickness, however, after taking the respective samples to the techniques of microscopy it is verified that structural difference is significant.

a) b) c)

1µm

Figure 3.7: A series of FESEM images assessing soot and the respective sample (a) of the naked eye perspective. The magnification for (b) is 5K and for (b) is 30K. Minimal structures distinguish the soot. Conditions of soot elaboration: t =1 min, φ = 2.7, Re = 350 and HAB = 40 mm.

3.2.4 Flame documentation

In this section, the information of the impinging configuration with a rich premixed flame is discriminated. All the imageries hereby will serve as a baseline to estimate some parameters and characterise the morphology of the flame for this specific work. The documented flame corresponds to Burner#2 from Figure 3.4.

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Flame display Local temperature and growth-related species are decisive in the synthesis of CNTs. The impinging flame can be studied as a constant stagnation flow problem where the mixture flux impinges against a wall, represented by the pan, to form a continuous stagnation surface, with a premixed flame established in the interface. The aluminium pan filled with water kept boiling, at atmospheric pressure simulates a boundary condition for the system.

Flame dimensions

This flame on Figure 3.8, is usually characterised by a jet diameter Dj and the distance H1 meanwhile, because the context is not the typical stoichiometric conditions there are other variables added represented in Figure 3.9. On the next series of images, it is designated the exact position of each sample and his relative standings based on the origin of the reference (x0, y0, z0).

Figure 3.8: A representation of the flame at working conditions, ɸ = 1.6 and Re = 700. Looking at the left portrayal is observable the sampling system and the metal claw, useful as an intermediate between the sample or the thermocouple and the impinging flame. Most of the samples are placed in the origin of the reference where x, y and z-axis intersect. Every experiment was done with the sample leaning against the pan at a HAB = 8 mm, which corresponds to a value on the z-axis.

The proportion H/D for this flame is 0.8 since turbulence, vorticity nor noise are desirable [202]. On Table 3 is documented every structure for getting this flame. The camera operated for capturing photographs was 30 centimetres from the centre line of the burner nozzle. H1 value was used to compute the image scale, thus reducing parallax errors and pixel uncertainty at curved edges. The borders of the scale were the aluminium pan and the visible edge from the burner.

Figure 3.9: Image processing. Flame before (on the left) and after binarisation (on the right). The z-axis and the x- axis are expressed in the flame.

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For the image processing, the parameters of each flame were determined using the ImageJ software [203]. After acquiring the image by a photo shooting the flame (Figure 3.9, left), the first step in image processing consists in binarizing the image in order to convert the image to a grey scale. All the images are embodied with noise because of the permanent nature of the flame. Thus, the binarised image (Figure 3.9, right) contains fewer deviations, providing more precise information of the flame front and consequently reducing the systematic errors.

Table 3.2: Flame parameters for the primary working condition (optimal point), al values in millimetres.

Nozzle diameter Jet diameter Jet Jet Larger Smaller Annular (Dn) (Dj) distance height ring ring thickness

(H1) (H2) (D1) (D2) (T) 10.00 11.09 8.00 6.77 49.27 26.98 4.42

3.3 Characterisation of CNTs

There are many different tools to characterise the nanotubes, that includes in-situ and ex-situ techniques. The techniques applied for of the CNTs formed are described in the following Table 3.3. These processes are essential to quantify and qualify the CNTs being therefore possible to determine the properties because its applications require certification of features and function [18].

Table 3.3: Summarizes the list of equipment operated to characterise the growth of CNTs.

Technique Equipment Field Emission Scanning Electron Microscopy/ Energy Dispersive X- JEOL JSM-7001F/ Oxford 250 Ray Spectrometry X-ray Diffraction Bruker D8 ADVANCE Powder Diffractometer

All the practised techniques are considered non-destructive.

Scanning electron microscopy A Scanning Electron Microscopy (SEM) is one of the electron microscopes, readily available on the market that acquires images of a sample by scanning over it. Nowadays it is a necessary tool for modern nanotechnology and gives an estimation of the size and yield of nanostructures at various magnification, ranging from 1000 to 50000. Generally, it has an ultimate lateral resolution of a nanometer (depends on the type of the electron beam) and resolves to range from 1-20 nm in size. An intense energy beam of electrons strikes the electrons on the sample producing other particles; these contain information about the topography and composition of 1D nanostructures moulded from flames [204].

40

Emitter type is the main difference in the middle of SEM and FESEM. All thermionic emitters use electrical current to hot up the filament, as it happens in SEM. A Field Emission Gun, from FESEM, produces a cleaner photo with substantially fewer distortions, has a whole range of acceleration voltage and does not heat the filament, meaning 3 to 6 times better overall resolution than SEM. FESEM exercises Electron Diffraction Spectroscopy (EDS) that computes the intensity of the X-ray as a function of energy. The FESEM magnifications applicated on the thesis span from 500 to 60000 times.

X-Ray powder diffraction This analytical technique, XRD, is responsible for giving information on the range of structural properties of crystalline phases. Sometimes the measure characteristics of the sample might include composition and purity, grain size, orientation, interlayer spacing, number of walls (for MWNTs), mean tube diameter, chirality and film thickness. X-rays are produced in an x-ray tube with two metal electrodes inserted in a vacuum chamber. The intensity of the radiation diffracted from the sample is measured as a function of incident angle [205]. Therefore, the identification of the given crystal structure is regularly completed by correlating the diffraction pattern attained with standard diffraction archives because each mineral has a set of unique patterns.

Diffractograms were taped at room temperature by Bragg-Brentano geometry (4 kW Cu K α radiation source and a 휆 = 1.54 Å), and the diffractor saved a spectrum over a 2훳 range of incidence from 10° to 90° along a step size of 0.03° (2훳).

3.4 Uncertainties

This section has considerable relevance, to discuss some of the uncertainties accompanying the investigational work and to measure the impact of these in the results obtained. The uncertainty from the flow meters, thermocouple measurements, height and distance dimensions and data treatment are available in this chapter. Furthermore, an evaluation of the maximum values of uncertainty in each integral part of the trial work is computed to challenge the reliability of the absolute results. For this subchapter remember that N2 is working in co-flow, not perturbating the uncertainties from the mixture fuel/air. Only the optimal point (Re = 700 and ɸ = 1.6) to produce CNTs is appraised.

Flow monitoring All mass flow controllers (MFCs) exposed on Figure 3.2, were a core piece during this work, despite this, they have uncertainties when the outflow speed is measured. The total uncertainty linked with the overall flow rate, 훼푄, is premeditated by the sum of two contributions, uncertainty from reading (0.8%) and the scale (0.2%). These parameters are foreseen in the general formula, which is prearranged by

𝑖 훼푄 = ±(0.008푄푚 + 0.002푄푚푎푥) (3.3)

41

𝑖 where 푄푚 represents the measured value of the flow rate for a specific working condition and 푄푚푎푥 is the maximum operating flow rate of flow controller 푖. The variable 푄푡표푡푎푙 gathers the propane flow (푄퐶3퐻8) and the dry air flow (Qair) and is titled total flow. It is conceivable to use 휶푸 to estimate the error linked with the flow measured, denominated 풆푸. The outcomes for 휶푸 and 풆푸 for the optimal point are presented in Table 3.4.

Table 3.4: Uncertainty (휶푸) and maximum relative error (풆푸) for each flow meter at the optimal point.

Gas 푸풎 휶푸 풆푸 [SLPM] [SLPM] [%] Propane 0.34 ±0.01272 3.74 Air 4.95 ±0.07960 1.61 Nitrogen 0.50 ±0.01400 2.80

Propane + Air (푄푡표푡푎푙) 5.29 ±0.09232 1.75

This MFCs uncertainty will undoubtedly have implications on both the downstream velocity (Re) and ɸ anticipated for each burning condition. Thereat, the Taylor Series Method outlines [206] an error propagation for the standard uncertainty in the result to ascertain the Re and ɸ. The procedure to do so is deemed below.

Equivalence ratio, 휶ɸ The description of this non-dimensional number is in Equation 3.1. For practical purposes, the Equation can be rearranged in function of volumetric flow rates 푄퐶3퐻8 and Qair. For this, it ought to be contemplated 푎𝑖푟 the average molar fraction of oxygen 푥푂2 in the atmosphere at sea level, and 푎 the stoichiometric coefficient of oxygen for complete propane combustion. Subsequently, our equivalence ratio shall be delivered through Equation 3.4.

푎푄퐶 퐻 ɸ = 3 8 (3.4) 푥푎𝑖푟푄푎푖푟 푂2

푎𝑖푟 Where 푎 = 5 and 푥푂2 = 0.2095 acquire constant values [207] for propane-air complete combustion. The uncertainties related with the dynamic viscosity of the premixed gas µ was neglected. Finally, it is possible to estimate the relative errors because the uncertainty of ɸ is fixed by Equation 3.5,

5 푄퐶 퐻 훼푄 훼 = ± √훼2 + ( 3 8 푎푖푟 )2 (3.5) ɸ 푄퐶 퐻 0.2095푄푎𝑖푟 3 8 푄푎𝑖푟

42

Being that 훼ɸ is the equivalence ratio uncertainty. Attending to the working condition discriminated in Table 3.4, and Equation 3.5, the value of 훼ɸ and the maximum equivalent relative error

푒ɸ. Computed results are disposed in Table 3.5.

Table 3.5: Maximum uncertainty and relative error in ɸ for the optimal point.

Flow conditions 휶 휶 ɸ 휶 풆 푸푪ퟑ푯ퟖ 푸풂풊풓 ɸ ɸ [SLPM] [SLPM] [%] Optimal point ±0.01272 ±0.07960 1.6 ±0.066755 4.17

After all the examination performed for ɸ, it is possible to substantiate that the relative error of all the measurements made never exceeded 5%.

Reynolds number, 휶푹풆 Equation 3.2 can also be re-written to explicit 푅푒 as a function of the total flow frequency. Considering

푄푡표푡푎푙 = 푈푚𝑖푥퐴푐ℎ푎푛푛푒푙 where 퐴푐ℎ푎푛푛푒푙 contains the channel area value, the Equation generated is given by Equation 3.6.

휌푚𝑖푥퐿푐ℎ푎푛푛푒푙 푅푒 = 푄푡표푡푎푙 (3.6) 퐴푐ℎ푎푛푛푒푙µ푚𝑖푥

The uncertainty for Re is given by 훼푅푒. This relation can be expanded to uncertainties, by substituting 훼푅푒 as a replacement for 푅푒 as a function of 훼푄푡표푡푎푙 instead of 푄푡표푡푎푙. Mutually, 푅푒 uncertainties and relative errors are stated in Table 3.6.

Table 3.6: Uncertainty and relative errors interrelated with Re for the optimal point.

Flow conditions 휶푸풕풐풕풂풍 푹풆 휶푹풆 풆푹풆 [SLPM] [%] Optimal point ±0.09232 700 ±12.22 1.75

Regarding the uncertainty analysis, the maximum obtained relative error for the Reynolds is never superior to ~2% (±14), and for ɸ is no bigger than ~5% (±0.08). This means that for the higher value used of Re = 700 to produce CTNs the boundaries would be 714 and 686 for the real Reynolds input. Likewise, for the individual parameter of ɸ = 1.6, can be reached limiting values of 1.52 and 1.68.

To diminish these uncertainties, it could be applied other MFCs with substantial less amplification in flow rate, for instance, a 1 and 5 [max. SLPM] for propane and air, in that order.

43

Temperature Every temperature measured by the type R thermocouple usually has an inaccuracy of ±0.5%, according to [208].

Heat transfer transpires by three mechanisms: conduction (푄̇ 푐표푛푑) [W/m2], convection (푄̇ 푐표푛푣) [W/m2], and radiation (푄̇ 푟푎푑) [W/m2]. Equation 3.7 denotes the processes of heat transfer amid the

3 thermocouple and the external environment. The specific mass is 휌 [Kg/m ], 푇푏 refers to the temperature of the bead [K], and 퐶푣 stands for the specific heat capacity of the thermocouple [Wm/Kg.K]. However, choosing a thermocouple with a ratio length to diameter over 200 times makes the processes of convection and radiation more relevant as detected by Bradley et al. [209].

휕푇 휌퐶 푏 = 푄̇ 푐표푛푑 + 푄̇ 푐표푛푣 + 푄̇ 푟푎푑 (3.7) 푣 휕푡

Thanks to radiation, there is a gap flanked by the temperature measured and the real value. This phenomenon is observed, by Heitor et al. [210], as the body begins to emit by radiation may cause an underestimation of the mean temperature by nearly 100 K. All empiric data was subjected to processing in MATLAB, by a domestic program. A probabilistic enquiry was carried out with 12000 measurements (during 20 seconds at a sample acquisition pace of 600 Hz) of the temperature permits evaluating the most probable temperature at that specific circumstance, suitable to the number of natural oscillations. For the present work, the same margin of uncertainty is engaged because there is a relation between units Kelvin (K) and Celsius (ºC), then ±100ºC for the temperature measurements. As an example, in Figure 4.2, the highest data for φ = 1.9 and T = 1229°C is about 1129ºC for the minimal and 1329ºC for the maximum temperature possible.

Mass, height, diameter and thickness The KERN balance has a readout of 0.01 mg. The precision of the balance [211], by the construction company, is defined by five times the scale interval. In the end, the uncertainty associated with the measurements is about ±0.05 mg.

The flame heights, sampling positions and dimensions are measured with a stick, and the measurement is recreated to nearly ±0.05 mm. The three-way translators employed allows movements with a spatial precision of ±0.1 mm.

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4 Results and Discussion

All the tentative outcomes are introduced and amply examined. Starting with quantitative and qualitative consideration based on the experimental results. The focus of the chapter is to explain what happens behind the scenes and to highlight the mechanisms, the properties and construct some of the preliminary verdicts. Many variables can intervene in the process of CNT manufacturing. The only underlined trial parameters are substrate status, temperature, nitrogen addition, the presence of different catalysts (iron and nickel) and residence time.

4.1 Burner analysis

Both burners referred in this work establish premixed flame at their outlet. For Burner#1 a conical flame is characterised with a stronger blue, and the second reaction cone defined as a nebula of less bright colours as visible in Figure 3.7. These two reaction cones possess unlike conditions, and the exterior is abundant in oxidation phenomena. Burner#2 allowed to stratify these two zones which in turn exposed the flame reaction zone.

This work was intended to create carbon nanotubes with non-noble materials (e.g. SS and Fe), although there are authors who demonstrate the obtaining of nanotubes with the type of flame of the Burner#1. Note all works are developed with noble metal catalytic substrates Ni [170], [212] or Co [163], but in this work and within this range of velocities and equivalence ratios was not possible for propane. Being that with the imposed stratification one got to the production of tubes. With this change of configuration, it is verified that the flame has everything that is necessary for the elaboration of nanotubes, but the adequate form comes with the stratification.

Briefly, it is known that the first study outlined in Figure 4.1 defines the characteristic flame temperatures of the order of 1000ºC that match what is presented in the literature [148], [164] and the second study separate and laminates the flame allowing the stable maturation of CNTs.

a) b)

Figure 4.1: Spatial location of the temperatures of the Burner#1. Image (a) designates the segmentation of the burner nozzle, with a view from the top, for radial dimensions. Figure 4.1 (b) determines the vertical zone where the temperatures were investigated, at the point of peak intensity. The z-axis and the r-axis are expressed.

45

A thermal examination on Burner#1, approached by the type R thermocouple, was made radially and vertically. The outcome of this study revealed that only a simple change in millimetres could induce up to 300°C of temperature variance. Therefore, to sustain this claim, several heights were studied. Figure 4.2 portrays experiences completed in the centre of the flame, varying only the distance and the ɸ. On the contrary, in Figure 4.3 a radial analysis of the flame is made, elevated temperatures correspond to the centre of the flame, and the lower ones to the periphery. The research was terminated for the layer outside the indicated ones as it did not bring any experimental relevance due to the low characteristic temperature. The step utilised was always one millimetre.

] 1250 °

[C 1200

1150

1100

1050

1000

Temperature Temperature 35 37 39 41 43 45 z [mm]

Φ = 1.6 Φ = 1.7 Φ = 1.8 Φ = 1.9

Figure 4.2: Vertical data (on the z-axis) from the highest temperature of the section, the centre of the Burner#1.

1200

] °

1000 [C 800

600

400

200

Temperature Temperature 0 0 1 2 3 4 5 6 7 8 Φ=1.6 1098 1024 929 860 806 648 356 247 124 Φ=1.7 1104 1095 1005 907 772 513 411 380 134 Φ=1.8 1143 1073 939 945 655 494 475 328 178 Φ=1.9 1146 1104 980 921 773 628 445 317 213 r [mm]

Figure 4.3: Evolution of the temperature with radius (r) in Burner#1 at z = 40 mm. The number zero represents the centre of the nozzle of the burner being the diametric progress from the inside out.

46

4.2 Impinging flame

In this subchapter, there are thirteen samples evaluated. Among them, there is a nickel mesh (Ni), AISI 304 stainless steel (Fe/Cr/Ni) samples and an iron substrate (Fe). From now on only Burner#2 results are discussed. A road map of the samples is posted at the beginning of the discussion and presentment of all results. Table 4.1 lends the big picture of the trials with flame synthesis by localising the samples throughout the sections and adding value in the column dedicated to comments.

Table 4.1: The following table ponders all samples inspected in the discussion of the results in order of appearance. Standard conditions (HAB = 8 mm; N2 = 0.5 SLPM (co-flow); t = 15 min; φ = 1.6; Re = 700.) for all samples. All 304 SS and the iron substrate are completely polished excluding A and B. Any variant is mentioned in the last column.

Section Subject matter Sample Observations

4.2.1 State of substrate A, B, C A – as-produced sample (304 SS); B – sample after annealing as a pretreatment (304 SS); C – sample completely sanded plate after annealing as a pretreatment (304 SS).

4.2.2 Conditions for CNT A, B, 1 1 – only sample where the yield is calculated. production (zone, temperature and yield)

4.2.3 Structure of nanotubes A, C, B -

4.2.4 N2 & O2 and Residence 2, C, 3, 4, 5, 6 2 (N2 = 0 SLPM); 3 (N2 = 1 SLPM); 4 (N2 = 1.5 SLPM); time 5 (N2 =0 SLPM and t = 30 min); 6 (N2 = 0.2 SLPM and t = 30 min).

4.2.5 Catalyst (Ni & Fe) Ni mesh, C, Ni mesh – as-produced mesh; C – with ~72% Fe; iron iron substrate – composed by ~99.5% Fe. substrate

4.2.6 Growth mechanism and Ni mesh, 304 In the case of 304 SS, there is no mention of a specific Reynolds impact SS, 7, A, 8 sample, but the growth mechanism is generalised from case A; 7 (Re = 500); 8 (Re = 600).

A summary of the working conditions (where CNTs evolved) is in the table format, in Table 4.2.

Nanotubes are synthesised in some of these conditions, and the legend enunciates a quantitative evaluation. The letter X means that there was no visual evidence (to the naked eye) of production of CNTs. FESEM evaluated only samples in the window of 500 and 700 in Reynolds value.

Table 4.2: Representation of the window opportunity in fabricating nanotubes when φ = 1.6.

Re 500 600 700 800 900 1200 CNT evidence low medium excellent small stains trace elements X

It can be concluded from the adjacent Table 4.2 that the optimal point gauged resembles velocity (Re = 700) and ɸ equal to 1.6.

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4.2.1 Effect of the state of the substrate on the synthesis of CNTs

State of the substrate FESEM surface micrographs of samples before CNT development in Figure 4.4. The complete absence of CNTs is evident. EDS scan points out for AISI 304 stainless steel. The presence of Fe (71.56 norm. at. %5), Cr (20.75 norm. at. %), and Ni (7.68 norm.at. %) was attained for the stainless steel.

Micrographs from Figure 4.4 signalises that protruding grains could be differentiated at the surface if as-received samples, while after polishing the morphology becomes flatter, smooth and featureless. Figure 4.5 shows a pattern, after thresholding pictures from Figure 4.4. This indicates the variation in rugosity, in (a) the range of values are assigned by more than 160 units of measure as opposed to 85 units for (c). The pattern is compiled by plotting a profile of a restricted area with ImageJ.

a) b) c)

10 µm 1 µm Figure 4.4: FESEM micrographs of bare 304 SS in the as-received (a), after annealing as a pretreatment (b) and completely polished states after annealing as a pretreatment (c) with magnifications of 1, 5 and 0.5K, one-to-one. Image (a) has a zoomed area at 5K in the upper right corner. The lower right-hand ruler indicates the scale.

Pattern concerning the variation of roughness is outlined accompanied by the zone where the evaluation of the same one was given. All photos have different qualities of their sizing, in this way although visually the area evaluated is not identic ends up corresponding to the same number of evaluated pixels. On Figure 4.4 (b) it is presented some metallic structures that are formed after the annealing as a pretreatment. According to the iron-carbon phase diagram, it is formed Perlite.

The influence of the substrate on nanotube synthesis can be weighted. Commonly, to create the ideal interface of the nanotube deposition, it is often used stainless steel, iron, catalysts grid and metal alloys with and without the addition of catalysts. Hamzah et al. [14] reviewed a vast amount of the pertinent studies in this subject.

In this investigation, the effect of the substrate is cautiously analysed. Is the quality of the substrate relevant to the deposition of nanotubes? If it is, how can the state of the substrate influence the carbon precipitation?

5 The normalized atomic percentage (norm. at. %) portrays the percentage of a given substance towards the whole quantity of atoms present in the region analyzed.

48

Figure 4.5: In this assembly of imageries, it is witnessed the pattern acquired by the analysis of thresholding harnessed to a given area (red boxes) of Figure 4.4. Designated at the top are the data for the as-received sample (Figure 4.4 (a)) and on the bottom those relating to the polished sample (Figure 4.4 (c)).

The substrate and CNT morphology Three types of 304 SS were handled at standard conditions6. When referring to each one, it is said case A, B or C. Under the three conditions, carbon nanotube synthesis is perceptible, details at the macro and microscopic level will be considered. Marked areas identify the zone deemed by FESEM. The black area of sample A, with a rectangular shape, is only a high strength adhesive tape that has been glued to the plate.

Regarding the soot and CNT comparison, it is noteworthy that carbon organises itself in a radically different way. Note the structural discrepancies between soot (Figure 3.6) and CNTs (Figure 4.6). Samples with similar depositions in the sample can have wholly disproportionate values, one without any benefit (soot sample) and another with a higher value (CNT sample).

As seen in Figure 4.6, CNTs are grown on stainless steel substrate when exposed to 15 minutes. The advance of micro- and nano-scale carbon fibres and tubes are detected on stainless steel. A large- scale expansion is observed all over the substrate. However, the tubes have a propensity to be twisted, entangled and coiled. All the magnifications are equal, translating to better comparisons.

6 HAB = 8 mm; N2 = 0.5 SLPM applied as a co-flow; Residence time (t) = 15 min; φ = 1.6; Re = 700.

49

A

Metallic

structures

B

C

Figure 4.6: FESEM images at 5K magnification with CNT deposition. On the top side, as-produced sample (case A). In the middle, the sample after annealing as a pretreatment (case B) and at the base, the completely polished sample after annealing as a pretreatment (case C). Detailed areas reveal razor cuts in B sample. Metallic nanoparticles are hosted on FESEM image from case B. Red stars on cases B and C signals the areas where there was no CNT deposition, exposing the 304 SS.

In case A, the dispersion and decoupling of the nanotubes yielded are more explicit, the material looks less clustered and purer. By plotting a profile in the grey analysis (something similar to what was done to analyse the state of the surface in Figure 4.5), it is possible to verify that the production of nanotube was bulkier in A than in the others. On the other hand, examining the previous pictures from Figure 4.6, there are some red stars identifying the sample itself, where the CNT deposition was not intense enough to cover the surface. This fact is supported by the EDS analysis with a value of C (100.00 norm.at. %).

According to B, it is possible to identify not only the CNTs on the left but also some stable metallic nanoparticles on the right side. The deposition of CNTs becomes discontinuous or non-existent

50 in some points due to the existence of Perlite (Fe3C + α-Fe) after annealing as a pretreatment. Upon careful evaluation, an unexpected deposition of nanotubes in the grooves marked by the knife is visualized (Figure 4.6, case B). This cut created an intermediate situation between the pretreatment and the polished state, thus allowing the synthesis of nanotubes.

On C, the nanotube synthesis was conducted to different structures. Note that small clusters of nanotubes are formed over the plate, they have a strong tendency to bundle together in “ropes” due to Van der Walls forces. The nature of a polished state allowed a uniform deposition of nanotubes, despite the heat treatment.

In the images related to the samples (Figure 4.6, on the right side), it is noted that the band geometry varies slightly from case to case. Reinforcing the concept that having a flame configuration which produces CNTs (standard conditions) the substrate is a bond that "closes" this interaction, so the material that constitutes the surface and its respective topographic state are active contributors in the process.

4.2.2 Nanotube formation zone

Figure 4.7 stated the mapping of CNTs on the stainless substrate per flame incidence districts. Precipitated carbon can be detected in the area in contact with the flames.

It was achieved a dense, solid and black film of nanotubes distributed radially from 22.5 to 26 mm corresponding to location 2 from Figure 4.7. The location 1 in between 0 and 22.5 mm in radius, has not been subjugated to the flame reaction zone and for this reason, including no CNTs. On the left side of Figure 4.8, shows the morphology of the structures for location 2. In the outer zone pronounced 3, 26 to 33.83 mm, corresponding to the length of the flame, no CNTs were spotted. In this zone of the last ring, there is no fuel since it has already reacted with the oxidant.

Figure 4.7: Macroscopic visualisation of carbon nanotubes on case A. 1 - pre-reaction zone (0 to 22.5 mm in radius); 2 - flame zone or reaction zone (22.5 to 26 mm in radius); 3 - post-reaction zone (26 to 33.83 mm in radius).

Some deviations are visible on CNT deposition on samples in Figure 4.6; still, the best and precise deposition was obtained in A. With this description, it follows that nanotube progression befalls in areas that relate to flames and where the reaction takes place.

51

Figure 4.8: Better magnifications (30K) of the deposited nanotubes. Case A on the left and case C on the right for ɸ = 1.6, t = 15 min, Re = 700.

Initially, the high-temperature growth condition is inspected on stainless steel. The CNT evolution on stainless steel has been already reported using flame synthesis [213] and also CVD [214]. Nanotubes have been observed to form in an extensive range of temperatures being that MWNTs are favoured from 500 to 1000°C and SWNTs above 900°C.

In the present work, it ensued, at a characteristic temperature of 850°C, the development of micro- and nano-scale nanotubes. These temperatures from propane burning under these conditions are confirmed a little throughout the literature, notably by Hong et al. [185]. Underneath particular terms, the carbonic species under the action of the flame undergo dissociative adsorption and diffuse through the nanoparticles of Fe thus providing the augmentation of CNTs.

In CVD research Lee et al. [131] examined the temperature dependence of nanotube enlargement and affirmed that both, diameter and growth speed, swell with increasing temperature. Contrary information was given by Kumar et al. [215] over having experienced a reverse output. With this, it is possible to infer that by changing some base parameters, such as carbon source or catalyst, it can lead to different optimal temperatures.

In the flame zone reaction of this dissertation, the maximum temperature collected during the characterisation of the impinging flame was slightly above 1500°C. So as to minimise the thermal exchanges amid the substrate and the pan, this last one was placed at 100°C degrees and proved to be worthy. Without this frontier condition, the nanotube yield happened, albeit in reduced quantities. Figure 3.8 clarifies the trial experiment and elucidates the position of the pan.

Whereas the HAB and the actual distance of the sample are not the same, considering the thickness of the sample, measures from the nanotube formation sector were taken. For case B, concerning the inner tier the temperature varied about 40°C, and in the outer layer, the temperature changed 70°C, at most. The minimal and maximal temperature, for the case A, to form nanotubes in the present study is 800 and 893°C corresponding to outer and inner portions of the annular formation district represented in Figure 4.7, one-to-one.

The number of carbon nanotubes supplied per unit of input material is a valuable factor for evaluating the flame configuration as a nanotube synthesis path relative to the ensemble of competing methods. Besides, this parameter must be assessed to understand the practical feasibility as well as

52 the massification of the process. Establishing the yield for nanotubes is yet tremendously tricky, caused by a paucity of quantitative characterisation techniques and some limitations on nanotube separation.

An estimation to produce nanotubes using impinging flame configuration comes from weighing, before and after, the sample. The variation in mass from the probes comes from the deposition of carbon raw material on the surface, the mass that has nanotubes and undesirable products of combustion. Proceeding to the purification (a topic discussed in subsection 2.6.1) would provide a more efficient and more realistic measurement of the total deposition of nanotubes. Unfortunately, this possibility of measuring the particles came after the best results (A, B, C) still have a beneficial outcome. Five measurements were taken for the Probe 1 at the optimal point, resulting in better excellence in result acquire. Table 4.3 only incorporate the mass average of the sample with and without deposition plus the agreeing mass of CNT delivered, ensuing from the subtraction of the previous ones.

Table 4.3: A selected sample and its respective mass with and without CNTs, the representative unit is gram.

Sample With CNTs No CNTs CNT mass 1 1.88015 1.87886 0.00129

To calculate the yield, the area of the sample as well as the area of the flame that covers the sample is measured. The amount formed in the sample is extrapolated with the respective ratio to a full flame. Finally, the amount of propane and carbon is reached that has been transformed into CNT raw material. The quantified data and calculations are listed thereunder:

1.29 푚푔 표푓 퐶푁푇푠 푑푒푝표푠𝑖푡𝑖표푛 Yield per C3H8 = ≈ 1.99% 푦푖푒푙푑 푝푒푟 C3퐻8 64.697 표푓 퐶3퐻8 표푓 푓푒푒푑푠푡표푐푘

Yield per C ≈ 2.44% 푦푖푒푙푑 푝푒푟 퐶

The estimated yield of nanotubes as a rejoinder for the introduced carbon is a little bit more than 2%. At first sight, it may be a little bit low, but it is necessary to remember that this work is not endeavouring the ideal situation or most exceptional performance. Note that it is easy to improve this percentage by just adding a methodical approach that converges to maximum yield.

From the study, a mass production rate can be deduced translating into a deposition quotient of 0.0191 mg/s. The estimated yield area for CNT in Sample 1 is roughly 13%.

4.2.3 CNT structure

The 304 SS sample produces helical, spiral, zig-zag, coiled and entangled nanotubes when placed under the action of the premixed flame. As seen in Figure 4.8, there is a noticeable difference in the structure of CNTs engendered at the same circumstances but with substrates in different surface states. In Figure 4.9 (a), the average diameter of the as-grown nanotubes for case A is 55 nm being that the maximum and minimum values are around 86 and 32 nm, individually. As regards the illustration of Figure 4.9 (b) for case C, has thin CNTs from 16 to 32 nm, while averaging 21 nm. After a cross-section of case A, affixed in Figure 4.10, an approximate value for the thickness was indicated, thereabout 9

53

µm. The length of the nanotubes is situated on the tens of micrometre for the as-received substrate, for the others, it is difficult to estimate by the small dimensions that they present.

Figure 4.9: FESEM presentments of MWNTs grown on 304 SS at ɸ = 1.6, t = 15 min, Re = 700. As-received (a) and polished (b) samples. Case A for (a) image and case C for the (b) image. Magnification of 60K for both images. Helical structures are zoomed in the upper right corner (b).

Diameters of 1.5 nm characterise SWNTs and to produce them efficiently it is required a gas phase catalyst method [160]. This method provides finer catalysts, ideal for SWNTs. It would be challenging to have SWNTs after knowing this, so it can be inferred for the current work that MWNTs were originated. Both techniques exploited in the present study does not allow to take definite conclusions in the number of walls of the as-grown MWNTs or their exact length, merely with TEM, it is possible to do it accurately.

Hashempour et al. [214] notified the expansion of CNTs by CVD in the as-received state (achieving 55 nm in CNT average diameter) and in the polished state (accomplishing 75 nm in CNT average diameter) of AISI 316 stainless steel (316 SS). Contrary to the previous researchers, arithmetic mean diameters of 55 and 21 nm were reached for 304 SS for the same surface states (Figure 4.9), in the same order. To answer this, maybe it is exposure of the sample to different environments (CVD process or flame synthesis) that generates differentiated growing circumstances. A heating cycle and complementary pretreatment were exercised on 316 SS making significant changes of the surface topography and manipulating the original surface. Hashenpour et al. underlined the direct relation of the nanostructures conceived to the size of the nano-features as a result of each pretreatment.

Figure 4.10: FESEM acquired photos of a cross-section with the magnification of 1.5K on the left and 30K on the right. The relative inclination of Sample A was 68°, due to this, a scaling should be made in order to propose a maximum ordinary height. The indicated parameter, height of the CNT mass, contains this compensation. The nanotube progression from the substrate base is displayed in the portrait on the right-hand side.

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Metallic nanomaterials Notwithstanding the manufacture of the CNTs, as the growth ensues, there are other materials and nanostructures which usually form. Combustion processes always originate more than a product at the end, above all when facing a rich mixture (ɸ > 1). After some research on phase diagrams of the steel alloys [216], it was concluded that the proposed working conditions could lead to topological modifications in the steel structure.

Opportunely, the use of FESEM technique, on the matching sample from case B of Figure 4.6, discovered the emergence of crystallographically modified structures. The tubes produced came with an intermediate level of thickness, as illustrated in Figure 4.11 (a). Dimensions of case B are in between case A and case C with averages of 45 nm and a ceiling of 26 to 67 nm in diameter. Geometrically, nanotubes present themselves as coiled, entangled and spiralled. Perlite (Fe3C + α-Fe) and other nanostructures such as Austenite (γ-Fe) crop out in many shapes and sizes; these are gently introduced in Figure 4.11 (b). This information is also available at any iron-carbon phase diagram, as viewed in [216], remembering that the temperature has thresholds of 800 and 893°C and the percentage of carbon in the 304 SS is close to the value of C (0.08 norm.at. %). The metal structures are then evaluated based on the XRD outcomes.

Figure 4.11: MWNTs found in the sample after annealing as pretreatment, case B. Magnifications of 60K and 10K for images (a) and (b), separately. Different structures, coiled and spiral, accrue from the state of the surface. On (b), structures with a more defined profile (spiral nanotube) and some sharp edges (from metallic presence) were incorporated into the microscopy photo. Subject to ɸ = 1.6, t = 15min, Re = 700.

As introduced, XRD indicates the structure and microstructure of thin-films based on a small area of incidence and so complement the EDS.

Two XRD samples, from the present study, were collected. The first one refers to the immaculate sample of 304 SS. The second XRD spectra come from a raw CNT sample, Sample B at the optimal point (ɸ = 1.6 and Re = 700). Here in Figure 4.12, it is discussed the original data, in the first graph, and by comparisons with what already exists in the literature, blue line of the second graph.

In the FESEM inspection, metallic nanoparticles intervened with the nanotube analysis. Therefore, there will be the manifestation of a catalyst, an oxide or a carbide in the signal intensity curve drawn by the XRD system. Before proceeding to this scrutiny, it is recalled that Miller indices (hlk) are used to identify different planes of atoms [217]. Bragg’s law is the foundation to classify diffraction peaks. Although the window of values obtained in XRD examination was between 10 to 90°, a new range of

55 variation (20 to 70°) was adopted in a second occurrence, allowing direct analogy with the literature by Hong et al. [185] which worked in inverse diffusion flame (IDF).

304 SS pattern

MWNTs pattern

[a.u.] Intensity Intensity

20 30 40 50 60 70 80 90 Scattering angle, 2ϴ [°]

Hong et al. [185]

MWNTs pattern

[a.u.] Intensity Intensity

20 30 40 50 60 70 Scattering angle, 2ϴ [°]

Figure 4.12: On the top, XRD pattern of MWNTs (synthesised by flame method) as well as an immaculate 304 SS, at IST. Here, incident X-ray wavelength (λ) =1.54 Å. On the bottom, the assessment adapted from the work [185] (pictured with the blue colour of the legend), by IDF in 304 SS. The quality of the results makes them comparable.

Diffraction peaks arrive at the incidence 2훳 angles of 35.46°, 43.74°, 44.43°, 50.89° and 74.67° while showing an intensity equal or above 200 for the MWNTs pattern (yellow line). All peaks match perfectly the investigational data from Figure 4.12, on the bottom, where it was found Austenite (γ-Fe). The spectrum from the flame-generated sample has a clear peak in 43.74°, this value denunciates the presence of γ-Fe (111) as a face-centred cubic crystalline structure according to the American Mineralogist Crystal Structure Database [218]. At 50.89° it is formed γ-Fe (200) and 74.67° meets the form γ-Fe (220). The main differences, when conferring Figure 4.12 on the bottom, are the predominant formation of nanocrystalline γ-Fe (111) instead of γ-Fe (200). Other crystallographic structures are shaped in lesser quantities at the peak of 35.46°.

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Temperature is a peremptory component for this achievement consisting of the production of

MWNTs and γ-Fe side by side, in identical circumstances (Figure 4.12, the second graph). Knowledge in phase diagrams is sufficiently strong to help the nanotube synthesis with energy activation or elements necessary to form metallic structures. Reports prove growing of Ferrite (α-Fe) and γ-Fe in NDF [219] but not in PF.

Frequently the organisation of the structures in any material is affected by its treatments, with a higher incidence of thermal treatments. 304 SS spectrum is fixed in Figure 4.12 (first graph, orange line), it is noteworthy the changes by consulting the XRD patterns from MWNTs and 304 SS. For all literature, characteristic peaks of hexagonal graphite are observed around 26° in the study [220], embodying the C (002) reflection, and close to 44.5° for the C (101) crystallographic plane, as in [214]. So, the value of 44.43° implies the presence of graphite, something that always ensues with the formation of CNTs/CNFs.

4.2.4 Effect of inert, oxygen and time on nanotube synthesis

In regard to the impact of the inert a system was mounted. A study of the temperature due to the nitrogen presence and EDS analysis are critical components in the subsection.

An inert gas might intervene in two ways, dilution or co-flow. It was appreciated the usefulness of the dilution method. However, this procedure does not fix the reentry of air located during the study of Burner#1. The use of N2 eventually causes lift-off and blow-out [221]. So, it is anticipated that the share of mixture/diluent may well not be too large.

From the literature, although scarce in this area, different types of inert like nitrogen and argon tend to proportionate identical outcomes [175]. The impact of the nitrogen on temperature is summed up in Figure 4.13, which supports a more pronounced temperature fluctuation for outer layers. Conclusions are consistent with the CDF study defended by Li et al. [190].

C] 750 27 mm 30.5 mm

° [ 650

550

450

350 Temperature 0 1 2 3 4 5 Inert absolute value [SLPM]

Figure 4.13: Effect of the inert addition, on the x-axis of the graph, on the inner flame temperature, on the y-axis of the chart. Study of two points, at 27 (blue line) and 30.5 mm (orange line) in radius centred on the origin of the reference (x0, y0, z0) and without a sample. All the dimensions come from z = 8 mm, see Figure 3.8. The structure of the study follows the annular regions stated in Figure 4.7.

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Nitrogen dilution is a force in well-aligned MWNT synthesis by straightening the entangled and curved tubes [23], but unfortunately, this change is not quantified. For this reason, a series of values with nitrogen flow rates were applied to the impinging flame configuration, on Burner#2. EDS methodology ensures information on the constant change in composition, intensifying the atomic percentual of elemental carbon until (44.74 norm. at. %) as well as a slight shift in oxygen, in Table 4.4.

Table 4.4: The behaviour of the composition of carbon as a function of nitrogen. Four samples with four quantities of Nitrogen. All tests are done at standard conditions, and the composition is provided by EDS.

Sample Nitrogen [SLPM] Carbon [Atomic%] Oxygen [Atomic%] 2 0 24.59 14.02 C 0.5 40.68 14.40 3 1 44.74 11.42 4 1.5 0.00 23.99

Despite the persistent intensification of nitrogen, it is understood that the adaptation of carbon for each situation ends in a hiatus. In the first interval, from 0 to 0.5 SLPM, the gains outstrip 16%, and in the second, from 0.5 to 1 SLPM, it is drastically disparaged to a quarter of the value (4%). The best condition must be placed amid 1 and 1.5 SLPM. When increasing the percentage of nitrogen mole fraction added to the premixed flame the diameter of the as-produced MWNTs is uniformly larger. At current working conditions, the regular diameters coming from 0, 0.5 and 1 SLPM nitrogen rate flow are considered in the range of 18 (Figure 4.15 (a)), 21 (Figure 4.9 (b)) and 43 nm (Figure 4.14 (a)), in that order. From work [190], identic assertions were made using dilution instead of co-flow arrangement. On Figure 4.14 (b), metallic nanomaterials and nanotubes coexist in the same spot. Metallic nanoparticles can have dimensions equal to 400 nm.

Above a nitrogen flow rate of 1.5 SLPM occurs inhibition of the formation of CNTs, since the presence of nitrogen makes it impossible to adhesion the carbon to the catalytic metal Fe, which does end up in the non-formation of nanotubes. There is talk throughout the literature of possible inert effects on lowering the concentration of gas-phase carbon in the flame and the dissociation of nitrogen in flames. Nonetheless, Particle Image Velocimetry (PIV) technique would be required to help to discover the exact cause of inhibition.

Figure 4.14: Standard conditions with the influence of nitrogen at 1 SLPM, typifying Sample 3. In (a) is shown tubes with a middling diameter of 43 nm, oscillating between 30 and 63 nm. On image (b) two materials are obtainable, both metallic structures and carbon nanotubes. The magnification for (a) is 60K and 30K for the image (b).

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Of all experiences ended and submitted to the analytical technique EDS, there is a pattern in the generic quantity of oxygen for the samples with noticeable enlargement of CNTs. To produce CNTs, the upper limit for the atomic percentual of oxygen verified is about 14%. For this configuration, there is a critical threshold of oxygen concentration, a value somewhere between O (14.40 and 23.99 norm. at. %). Any other situation leads to no elaboration of CNTs. Reportedly, if the oxygen concentrations are too extreme more soot is produced, and other nanostructures can be formed, as carbon nano-onions for 30% of oxygen [184].

Overall, it ought to be a unique appreciation of residence time. The effects are evident, although they are not conclusive. In the literature, all the possible scenarios are bestowed. The variables often observed are the yield, morphology, length, diameter, where might remain the same or proliferate, depending on the flame configuration and fuel used.

Residence time has been doubled; it went from 15 to 30 minutes. From Table 4.4, the valuable data of C (24.59 norm. at. %) is reused for the operation with no input of nitrogen, matching to the

Sample 2. Afterwards, the new experiment at working conditions of ɸ = 1.6, Re = 700, N2 = 0 SLPM and t = 30 min, and the EDS synopsis generated a total amount of C (72.65 norm. at. %) for the Sample 5. Side by side, some differences in deposition are distinguished with the naked eye, although few. Where it was noticed the superior difference was diametrical that went from 18 (Figure 4.15 (a)) to 27 nm (Figure 4.15 (b)). By comparison, time evaluation suggests that the influence of the residence time in the original work fell only the diameter and the density of the MWNTs stratum. On the other hand, elemental oxygen came down impressively, to virtually half of the value for 15 minutes, O (7.48 norm. at. %).

Even though adjustments in residence time shows that it is an impacting parameter, the change in temperature shows more dominant effects [175].

On Appendix A it is exhibited another experiment, called Sample 6, at 30 minutes at standard conditions (Figure A.3), differing only in the amount of nitrogen inserted in the system, in this case, N2 = 0.2 SLPM.

Figure 4.15: Standard conditions without nitrogen magnified at 60K. On (a) the average diameter is 18 nm for 15 minutes of exposure distinguishes Sample 2. Aimed at (b) the typical diameter is 27 nm with 30 minutes contacting the flame results pinpointing to Sample 5.

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4.2.5 Influence of the catalyst on the morphology of CNTs

CNTs grow on substrates (304 SS, 150 µm nickel mesh, Fe sample) in the range of values corresponding to the flame zone (as shown in Figure 4.7). In Table 4.5 are listed the constituent elements of the substrates. For this sub-chapter, only Sample C, the nickel mesh (Figure 4.16) and the iron substrate (Figure 4.18) are in vogue. The synthesis process of CNTs using a catalyst depends upon the melting temperature, carbon solubility and carbon diffusion ratio.

Table 4.5: Carbon solubility in selected transition metals, modified from [246]. Note that on the table only has values for pure elements.

Name Melting point Carbon solubility Carbon diffusion on metal 2 -1 (푇푚) (K) (%) at 푇푚 (m s ) at 1000°퐶

Iron 1808 20.2 1.5×10-11

Nickel 1726 10.7 2×10-11

Carbon solubility of a neat metal increases as the temperature goes up and decreases as the particle diameter raises, as is attributed in [222]. Preferably, binary (Ni/Cr) or trinary (Ni/Cu/Fe) alloy metals have been employed because they are more active on nanotube growing. The literature says that, for the most part, pure metals may have small concentrations in oxygen in the vicinity of catalytic particles to emphasise the breakage of C-H bonds.

Recall that EDS investigation revealed the presence of Fe (71.56 norm. at. %), Cr (20.75 norm. at. %) and Ni (7.68 norm. at. %) for the 304 SS. After the deposition of nanotubes, from our optimal condition, the composition of the polished state (Sample C) grew from nothing to an astronomical 40.68% atomic per cent of carbon, as can be seen in Table 4.6. Oxidation phenomena originate the oxygen element, an essential component in the stabilization of the CNT structure. Nanotubes cannot grow deprived of oxygen, and with an exceeding quantity, it is impossible to build them. This last conclusion derives from the Burner#1.

Table 4.6: Qualitative examination of the bulk sample of Figure 4.6, case C.

Element Atomic [%]

C 40.68

O 14.40

Cr 9.43

Fe 31.66

Ni 3.83

Total 100.00

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Figure 4.16: A series of FESEM images for a plain (no catalyst deposit) nickel mesh immersed within the flame gases supplied by a propane flame at standard conditions. (a) and (c) represent the nickel mesh without contact with the flame, no CNT deposition at magnifications of 180 and 1K, correspondingly. (b), (d) and (e) it is glaring the presence of this nanomaterial with magnifications of 180, 1K and 10K, one-to-one. Note that some of the deposited stuff corresponds to amorphous carbon. The image (f) is only to give a perspective on where the nickel substrate was placed. The radial position of Ni mesh is in the middle of the reaction zone 2 (22.5 to 26 mm in radius) found in Figure 4.7, i.e. at x = 0, y = 24.25 mm and HAB = 8 mm.

EDS spectrum of the non-deposited nickel mesh revealed the presence of Ni (88.78 norm. at. %), O (10.52 norm. at. %) and Fe (0.70 norm. at. %). As expected, the fabric is not 100% pure, even after proper cleaning. Table 4.7 has post-experience outcomes.

In 150 µm nickel mesh, wide MWNTs were matured with an utmost diameter of 160 nm and a bare minimum of 59 nm, with a weighted average of 86 nm (ranked in Figure 4.17). There is a similar average value from [171], where they manage propane and Ni catalyst. Some structures in Figure 4.17 (a) detach themselves and are somewhat unusual. In some studies, it is asserted that Ni particles can produce SWNTs, for example [223]. Nonetheless, it is thought that is not the case since, firstly, the diameters can range from 9 to 33 nm (above what is tolerable for SWNTs) and secondly, no catalysts were added through the gas phase. Smaller agglomerates of short, thin MWNTs have a strong tendency

61 to bundle together in “ropes”. Often, the “ropes” denunciates the presence of the catalytic particle (light contrast at the tips of some nanostructures), in other words, it suggests that the nanotube began its formation from the tip towards the substrate. By way of reinforcement, Tan et al. [212] ascertained internal diameters for MWNTs of nearly 9 nm.

Figure 4.17: FESEM magnification of 60K for both imageries. Even though the MWNTs from the present study (a) are larger in diameter than (b), the structure similarity accomplished is irrefutable. Photograph (b) adapted from [176]. The left side worked at ɸ=1.6, t = 15 min, Re = 700 in the nickel mesh.

EDS estimation for nickel mesh with CNT deposition is in Table 4.7. Fundamentally, by equating the captured dimensions of the nanotubes produced with the Fe from Sample C (~21 nm) and Ni from nickel mesh (~86 nm), the carbon diffusion ratio is tested once their diffusivity rates are analogous. Carbon diffuses on top of nickel mesh to produce above average nanotube yield (C 90.97 norm. at. %) compared to the Sample C (C 40.68 norm. at. %), permitting thicker structures with Ni. Though contradictory to earlier observations [224], it sustains the significant roles portrayed by the surface reactivity of elemental iron [20] and the alloy composition.

The presence of Cr may possibly deactivate or lower the catalytic activities of Fe in consequence of the appearance of the stable chromium carbide (Cr4C) [225]. Cr4C can compete with the inbound carbon, delaying the carbon saturation and slowing down the diffusion rate in the catalysts. These actions may inhibit nanotube nucleation in the 304 SS. Zabetta et al. [226] describe a more massive and crystalline nanotube augmentation in Ni when contrasted with Fe.

The unprecedented rise in the percentage of elemental carbon, from 0% to almost 91%, occurs through the unbridled advance of nanotubes, as validated in Figure 4.16 (b). The oxygen element appears from the oxidation equations of metals. Although unexpected, the emergence of the copper element has an explanation. The production of CNTs in the actual study requires a sampling system, which has a metal claw that holds the nickel mesh (Figure 3.8). The chief agent of the sampling system is a metal claw with copper on their composition, so it is acceptable for copper to turn up in the analysis of the chemical composition due to contamination.

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Table 4.7: Qualitative inquiry of one the 150 interstices of the nickel mesh, designated in Figure 4.16 (b).

Element Atomic [%]

C 90.97

O 7.29

Fe 0.89

Ni 0.67

Cu 0.19

Total 100.00

The alloy composition of 304 SS is only about 72% in Fe. To verify what was to happen with a higher iron percentage an iron substrate (Fe 99.50 norm. at. %) was utilised. The structures obtained are massive when compared with both advertised. Thus, serves as corroboration for the Cr influence and the relation that would point for a more significant development for Fe over Ni because of the diffusion rate for pure metal (Table 4.5). Figure 4.18 possesses two distinct nanostructures, macro- MWNTs with ~500 nm and micro-MWNTs with ~125 nm relative to the diameter. The data is in line with the remarks from Xu et al. [150] in the inverse diffusion flame by applying methane as a fuel. On Figure 4.18 (a) is first observed (in this thesis) an aligned nanotube with this impinging flame configuration. Hence for a substrate of iron (Fe 99.50 norm. at. %), it is created MWNTs with a greater diameter than with Ni (~86 nm).

1µm

Figure 4.18: MWNTs are discerned from the material produced by flame contact and the iron substrate. Standard conditions are applied for the growth of tubes. Relatively aligned nanotubes are obtained as shown in the image (a). Magnification of 10K for (a) and 30K for (b). The characteristic diameters of MWNTs are expressed in (b).

4.2.6 Growth mechanism and Reynolds impact

The CNT growth rate depends on the extent of the catalyst particle, chemical nature of the catalyst, the pressure of gaseous carbon source and characteristic temperature. Adding to this, with the present thesis it can be said that heat transfer phenomena (experienced by the pan as a border condition) also influenced the growth rate.

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Growing can be achieved through the tip or root mechanism. In this experimental setup, it is experienced both mechanisms. Tip-based growth for nickel mesh (deliberated in the previous subsection 4.2.5) and root-based growth for the 304 SS, which will be debated below.

The 304 SS has the iron catalyst incorporated. As a pure metal, the bonds will be robust than deposited catalysts over any surface. By reason of that, it is safe to say that the root mechanism is prevailing, and the nanotubes start forming upwards from the metal catalyst which remains coupled to the substrate. Figure 4.10 contains a cross-section from the nanotube that tries to cast a visual image of expected growth via the root growth mechanism.

With the aiming of spreading the working conditions and the optimal point mentioned in Table 4.2, an effort was made to realise how incisive the velocity is in promoting nanotubes.

Is perceived that velocity is significant for the union of the elemental carbon and the catalyst. By default, the batch of nanotubes in the aftermath must be inferior because of shortening the reaction zone. Reynolds of 500, in standard conditions, the composition collected on EDS previse a quantity of C (67.04 norm. at. %). A qualitative map was personified in the following Figure 4.19, indicating upshots from Sample 7.

Figure 4.19: FESEM portrayal of the morphology of MWNTs in Sample 7 with Re = 500. Grade scale to uncover the residence of the elements carbon (stamped on red colour) and oxygen (branded as green). Magnification of 5K for the reported image.

At the point of maximum interest of this blaze impinging configuration, an EDS examined the sample of case A and the maximum possible value was displayed, one hundred per cent, literally. Hereupon, the velocity creates coalescence additionally by bringing the particles together. An explanation for the C (100.00 norm. at. %) is the thickness generated and the high density, both, were enough to cover up the sample and to forbid the access of the EDS to the nanocrystals.

All over the literature, there seems to be a compromise between the Re and the φ. This relationship depends exclusively on the configuration and technique exploited. Finding some relationship that would tie one condition to the other would be advantageous for the study of carbon nanotubes.

Experiment with Re equal to 500 (Figure A.4, Sample 7) is in Appendix A together with the structures obtained at Reynolds level of 600 (Figure A.5, Sample 8)

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4.3 Summary of materials characterisation conclusions

Ultimately, and before passing through the final remarks, it was created a table which segment the vision of the objective enunciated at the beginning of the thesis and clarify the analytical phases of the discussion. Already in Table 4.8, is contextualised the range of experimental values gathered as well as the tool that allowed conducting the research.

The iron element was presented in the form of stainless steel (Fe 71.56 norm. at. %) and pure metal (Fe 99.50 norm. at. %). The nanotubes formed for each situation have significant differences, and the largest dimensions are for the case of the iron substrate as the catalytic metal.

A brand-new sample of 304 SS allowed for the growth of CNTs with almost 10 micrometres in height, more than any other metal used.

The element nickel provided a smooth growth of the tubes, thus conferring the reason why this metal is manipulated in more than 70% of studies with carbon nanotubes.

The Fe/Cr/Al alloy has been shown to be a suitable substrate for deposition of CNTs, but little is discussed in the literature about aluminium element.

The metallic nanoparticles obtained in the form of Pearlite and Austenite favour the study of nanotubes, since they are formed under similar conditions.

Table 4.8: Screenshot of the work done on the flame synthesis.

Properties after characterisation FESEM EDS XRD Structural differences in MWNTs (e.g. helical, spiral) X Diameters 59 to 160 nm for MWNTs in 150 µm nickel mesh X X Diameters 18 to 86 nm for MWNTs in 304 SS X X X A thickness of 9.33 µm for MWNTs in 304 SS X Diameters 22 to 89 nm in 67%Fe/23%Cr/10%Al alloy X X Diameters of 125 and 500 nm for MWNTs in the iron substrate (99.5%Fe) X

Perlite (Fe3C + α-Fe) and Austenite (γ-Fe) growth associated with CNT X X X

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5 Concluding remarks

The head objective of the work was to materialise CNTs. Some research has also been done on the role they have developed in our current society, and it can be concluded that the repercussion of this material reaches most areas of human interest. The number of applications tends to a value that is not measurable, yet. With this, it is possible to define precisely the reason for this research.

To achieve the head objective, it was obligatory to cognise the process of forming the nanotubes, to demystify the influence of each parameter and to uncover the possibility of reproducing them in our laboratory in the IST. The underlined trial parameters were extensively studied through a set of experiments, such as are substrate status, temperature, nitrogen addition, the presence of different catalysts (Fe, Ni and Fe/Cr/Ni) and residence time.

All the work began the understanding of the flaring mechanisms of premixture and the configuration necessary. Burner#1 was essencial to understand the characteristic temperatures of the flame, and Burner#2 allowed to stratify two zones of the flame which in turn exposed the flame reaction zone where the stable maturation of CNTs occurred.

Rich flames with ɸ = 1.6 as well as moderate velocities Re = 700 with relative error inferior to 2% and 5%, respectively. This combination of equivalence ratio and Reynolds attested to be the most indicated in an impinging flame configuration, using Burner#2. The mapping of working conditions was carried out, qualitatively identifying the deposition of CNTs, in Table 4.2. The structure of the flame was documented since it is difficult to find this type of information throughout the literature. Table 4.1 systematise the samples involved in the discussion and Table 4.8 serves as a screenshot of the work. This last table from the previous page contains the most relevant results of the work where it was verified that with the iron substrate (Fe 99.50 norm. at. %) larger CNTs are obtained, which are around 500 nanometres.

The surface state was crucial because after the annealing of the SS 304 it was not possible to deposit nanotubes due to the metallic perlite structures formed. However, after the removal of the same structures, the deposition is done without a problem.

In order to stimulate this comparison of the results attained, the techniques of FESEM and XRD were fundamental, consenting a better overview of the role of CNT production parameters that translated into scientific contributions.

Subsequently, the entire thesis is reduced in scientific contributions. Some guidelines are given for future work based on observations made throughout the process.

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5.1 Summary of contributions

Without further ado, the conclusions of this dissertation are delivered briefly:

1. Impinging type flame (Burner#2) allows locating the production of carbon nanotubes and stratifying the conic zones from a standard configuration then supplying a better control of the obtained results.

2. The existence of thermal cycles and pretreatment proved to be important. A direct relation to the microstructure of the surface and the nanotubes originated can be made. One is more structured, as-produced Sample A and the other promotes a higher range of yield in terms of radius, for the sanded Sample C. In the case of B, the annealing of the 304 SS produces Perlite that does not allow the CNT deposition, only after the exposure of the virgin material the production of nanotubes happened.

3. An absence of the nitrogen resulted in the dominance of encapsulation of the Fe, giving a diminished nanotube yield. The excess of inert caused inhibition of nanotubes, something that until now had not been observed in the literature. In this way the formation conditions are disturbed, they separate the particles in such a way that they cannot approach the catalytic metal Fe and cease to have the energy to start their formation.

4. Analytical scales can be operated to regulate and increase the yield in the production of nanotubes; few studies provide mass production values.

5. Oxidation phenomena are more vital than what is presented in the literature. No CNTs were observed when the oxygen was above 14.40%, by EDS — validating why no CNTs were made with the Burner#1.

6. The production of metallic nanomaterials (Perlite and Austenite) simultaneously with CNTs in a flame environment is in the embryonic stage but provides advances on comprehension of the activation of energy field can propel a step forward in science because it contributes to perceive the Arrhenius equation.

7. The boundary condition related to the boiling water on the pan is also relevant. Without it, there was only two outcomes or no CNT production at all or small quantities of it.

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5.2 Recommendations

The work established in this thesis was preceded to extensive research on CNT activation by flame synthesis. All results achieved point to possible further research subjects and improvements, guided by the awakening of interest and successive advances in diversified areas. These are registered below:

1. The production of nanotubes was inhibited with the increase of the inert maximum limit (1.5 SLPM). It would be productive to carry out studies of Particle Image Velocimetry (PIV) to try to answer to the actual inert effect in addition to reducing the flame temperature. The velocity profile varies along the laminar flow providing different velocities for zones close to the wall.

2. With Figure A.1, some doubts arise. Hence, it is proposed some survey that tries to join what is already established from the other practices (CVD, AD and LA), with the intention of growing the expertise in synthesis by flame. The powder of CNT's raw material must be considered, as it will be another way of knowing the extent of the properties of this nanomaterial.

3. Based on the direct relationship amid soot formation and the ease and affinity to form nanotubes based on the study [199], it is suggested to be performed with butane. Butane has already served in CNT production for CVD and has competitive ratings in soot propensity when the compared element is propane.

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A Complementary data on CNT

Figure A.1: FESEM shows the microscopic reproduction of the formed black CNT powder, removed from case A (Figure 4.6) and the obtained structure was as follows. A wooden spatula cut off the CNT powder. Several issues are posed, as little is documented in the literature. The image on the left has a magnification of 10K, and on the right, it is subjected to 30K.

1µm

Figure A.2: Standard conditions for 67%Fe/23%Cr/10%Al alloy. Different sizes (from 22 nm to 89 nm in diameter) and heterogeneity in nanotube synthesis. Magnification of 10K and 60K, in turn.

1µm

Figure A.3: Working conditions of ɸ = 1.6, Re = 700, N2 = 0.2 SLPM, HAB = 8mm and 30 minutes. Material 304 SS for Sample 6. An average diameter of 31 nm where the maximum 51 nm and the minimum 23 nm. Magnifications of 10 K and 60K, one-to-one.

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1µm

Figure A.4: At standard conditions, MWNTs are built over Sample 7. The material for growth is 304 SS with Re = 500. Magnifications of 5K (left) and 60K (right).

1µm Figure A.5: Standard conditions for 304 SS with the exception of Reynolds which assumes the value of 600. In Sample 8, there are magnifications of 5K and 60K, in that order.

Table A.1: Synopsis of decisive events in the carbon element. Adapted from [17].

First "lead" pencils 1600's Discovery of the carbon composition of diamond [227] 1797 First carbon electrode for electric arc 1800 Graphite recognised as a carbon polymorph 1855 First carbon filament 1879 Chemical vapour deposition (CVD) of carbon patented [228] 1880 Production of first moulded graphite (Acheson process) 1896 Industrial production of pyrolytic graphite The 1950s Industrial production of carbon fibres from rayon The 1950s Flame synthesis of tube-like structures [229] 1958 Discovery of low-pressure diamond synthesis The 1970s First observation of CNTs [120] 1972 Development of diamond-like carbon (DLC) The 1980s Discovery and development of carbon nanotubes (CNTs) [1] 1991 Industrial production of CVD diamond 1992 4 cm long single-wall nanotube (SWNT) [230] 2004 Breakthrough of graphene [231] 2004 Classification of CNTs regarding their size and properties 2006 CVD production of large graphene films (FGL) 2010 A new class of carbon allotropes: novamene [232] 2017

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Table A.2: SWNT vs MWNT, modified from [233].

SWNT MWNT Constituted by one layer or graphene Multiple layers of graphene Catalyst/transitional metal is required for synthesis Can be produced with and without catalyst (e.g. Fe, Co, Ni) Purity is weak but can be easily improved Purity is high and tricky to improve Less yield and accumulation in the surface High yield and accumulation in the surface Easy to characterise Difficult to characterise because of its complexity Ideal for electronic applications Ideal for mechanical applications A defect can happen during functionalization Defects are rare Mass production is difficult as it requires proper control Mass production is easy Expensive to produce Cheaper to produce Effortlessly twisted Hard to twist

Table A.3: Microscopic approach on properties of CNTs and other materials, adapted from [234].

Material Young’s modulus Tensile strength Electrical Thermal (GPa) (GPa) resistivity conductivity (Ωm) (W/mK) SWNTa 1000 - 3600 [235], [236] 13 - 52 [237] 10−6 [235] 1750 - 5800 [43] MWNTa 2437 (270 - 950 GPa for 109 (11 - 63) GPa 10−6 [234] >3000a [238] the outermost layer) [9], outer layer) [44], [236] [237] Graphene 1000 [239] 130 [240] 10−4 [241] 4840 - 5300a [242] Diamonda 1000 1.2 1.0×1012 to 1000 - 2600 1.0×1018 [243] Steela (lain 200 0.4 12.0×10−8 to 43 carbon low alloy) 170×10−8 Coppera 110 0.413b 1.70×10−8 to 390 2.65×10−8

a experiments room temperature conditions; b Hard drawn.

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Table A.4: Applications on metal reinforcement, properties commercial limiting and challenges for some of the most sought metal matrices. Modified from [244].

Metal Applications Advantages Challenges References Al Automotive, sports Low density (2.7 g/cm3), Low mechanical properties, [245] equipment, abundance elongation diminishes aerospace drastically Cu Building material, Showed real-world High oxidation, decreasing [246], [247] conductor of heat applicability and were easily properties and electricity soldered into functional circuits. Ni Corrosion resistant Promotes homogeneous High density (8.9 g/cm3), [248] coating corrosion low mechanical properties Steel Construction, High compressive strength High density (7.9 g/cm3), [249], [250] (Fe + C) automobiles, and hardness value, an weak in high temperature aerospace, oilfield abundance of Fe and high-pressure environment

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Table A.5: Homemade comparison contemplating all the four methods of CNT synthesis. Gives a closer look to every single variable in the technique, all the values are given are standardised.

Process Arc discharge Chemical vapour Laser ablation Flame deposition synthesis

Who Sumio Iijima, Endo et al., Shinshu Smalley’s group, Rice, Saito et al., NEC University, Japan 1995 USA 1995 [252] University of Corporation, [251] Kentucky, USA Japan 1991 [1] 1991 [172]

How The plasma Metal catalyst deposited Illumination of a carbon A fuel-rich flame generated from on a substrate inside the target with a laser light delivers a high an electric pre-heated reactor that may vary in pulse temperature, discharge followed by the slow frequency or power. appropriate between two addition of hydrocarbon This laser vaporises carbon graphite rods gas. The gas starts the the graphite target, and availability in the (anode and decomposition and the nanotubes develop environment if cathode) spaced liberates carbon atoms, on the cooling surfaces catalytic metals a few millimetres which, when added of the reactor, as move into the apart together, form CNTs condensation occurs. system

Usual Yield % <30% 20-90% >50% 13% (from the present work)

SWNTs Short tubes, a Long tubes, a diameter Long (5-20µm) with a Depends on the diameter between 0.6-4nm diameter of 1-2nm catalyst between 0.6- nanoparticle, but 1.4nm generally <5nm

MWNTs Inner and outer Inner and outer tube Possible, but way too Depends on the tube diameter, 1- diameter, 0.7-2.6 nm expensive catalyst 3nm and 10nm and 240nm, one-to-one nanoparticle, but respectively generally <20nm

Production Small quantities Big quantities Small quantities Big quantities

Quality of CNTs Low Medium High (90%) High

Defects SWNT with MWNT riddled with Low quantity Low quantity defects defects MWNT without catalyst

Operating 800-5800°C 500-1200°C 1200°C 800-1500°C Temperature

Conditions 100A, 20V, 50- Low-pressure inert gas Presence of inert gas, Flame + fuel + 500Torr (argon) room temperature to inert gas 1000°C, 200-400Torr

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Per unit design High Medium High Low cost

Nanotube Low High Low High selectivity

Availability of Difficult, pure Easy, fossil-based and Difficult, solid Easy, raw material graphite botanical hydrocarbon graphite/carbon gas Hydrocarbon gas /Source of carbon

Purification of High Medium to high High Medium CNTs

Process nature Batch Batch/Continuous Batch Continuous

Process Difficult Easy Difficult Easy parameter control

Energy High Low High Low requirement

Reactor design Difficult Medium Difficult Easy

Production Low High Low Medium to high Rate

Time 1ms-120 min 15–60 min 5 min 10ms -15min

Advantages Simple, high- Low temperature, large- The reaction product is Efficient, quality scale production, aligned quite pure, accessible to a nanotubes growth possible Room temperature scale-up, fast, in- synthesis situ catalyst, supply gas has a dual purpose

Disadvantages Tubes tend to be Usually MWNT, crude Highly tangled CNTs A limited amount short with product purification form, mixed with an of literature random sizes required, the time unwanted form of and alignments, required for the process carbon (soot) or often we get catalysts tangled CNTs, high quantity of impurities (soot) when compared with other methods

References [143], [146] [215], [235] [146], [253] [14]

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