This diploma thesis deals with the concept of use of nanotechnology for cement composites and UHPC. In the theoretical part of this diploma thesis there are theoretical principles described for successful design of high performance concrete and characteristics of main feedstock and its production. Furthermore, there is a focus on nanoparticles, especially the ones with carbon base in the form of carbon nanotubes CNT, in addition to that, graphenes GN that are new on the market, moreover, graphene oxide GO and its application in cement composites to mechanical characteristics and its durability. The first part of the practical part devotes in trying to find out an optimal parameter for ultrasonic mix with a suitable surface active substance of three graphenes. The other two parts of the practical part are about influence of graphenes on mechanical characteristics, cement paste microstructure, and application in the mix of concrete C 35/45 and UHPC

Diese Arbeit beschäftigt sich mit der Entwicklung und Implementierung eines Algorithmus zur Berechnung des diffusiven elektronischen Transportes durch Kohlenstoffnanoröhrchen-Feldeffekttransistoren (CNTFETs) unter Verwendung des Drift-Diffusions-Modells. Als Grundlage dient ein bekannter, eindimensionaler Algorithmus für klassische Halbleiter, durch welchen das elektrostatische Potential im stationären Zustand berechnet werden kann. Dieser Algorithmus wird erweitert, um die geometrischen und physikalischen Besonderheiten von CNTFETs, insbesondere die Quasi-Eindimensionalität, zu berücksichtigen. Wichtige Kenngrößen des CNTFETs werden berechnet und deren Abhängigkeit von den Bauteilparametern wird untersucht.:1. Einleitung 2. Theoretische Betrachtungen 2.1. Kohlenstoffnanoröhrchen 2.1.1. Graphen als Baustein für CNTs 2.1.2. Eigenschaften von CNTs 2.2. Drift-Diffusions-Modell 2.2.1. Drift-Diffusions-Gleichungen 2.2.2. Kontinuitätsgleichungen 2.2.3. Poisson-Gleichung 3. Implementierung 3.1. Modell für klassische Halbleiter 3.1.1. Herleitung der dimensionslosen Bewegungsgleichungen 3.1.2. Umformung der Drift-Diffusions-Gleichungen 3.1.3. Iterative Lösung des Gleichungssystems 3.2. Anwendung des Modells auf Kohlenstoffnanoröhrchen 3.2.1. Betrachtetes Modell 3.2.2. Separationsansatz und Poisson-Gleichung 3.2.3. Anpassung der Drift-Diffusions-Gleichungen 3.2.4. Gate-Spannung 3.2.5. Intrinsische Ladungsträgerdichte und Ladungsträgerrandbedingungen 3.2.6. Dielektrizität 3.3. Numerik 3.3.1. Berechnung der Ladungsträgerdichten 3.3.2. Lösung der Poisson-Gleichung 3.3.3. Iterative Veränderung von Parameterwerten 3.3.4. Überprüfung der Konvergenz des Gitters 4. Auswertung 4.1. Literaturmodelle 4.2. Ergebnisse 4.2.1. Potentialverlauf 4.2.2. Potentialplateau 4.2.3. Abschirmlänge 4.2.4. Stromfluss 4.2.5. Rechenzeit 5. Zusammenfassung Anhang A. Herleitung der Drift-Diffusions-Gleichungen aus der Boltzmann-Transportgleichung B. Herleitung der eindimensionalen Poisson-Gleichung aus dem Separationsansatz

Graphene and Carbon nanotubes (CNTs) have been researched for more than a decade due to their extraordinary properties and advantages towards applications like electronics, structural re-enforcements, thermal management and energy storage. Graphene-CNT hybrid structures have been predicted to further enhance the exceptional properties and overcome some of the shortcomings of the individual materials. Advantages of a structure consisting of vertically aligned carbon nanotubes (VACNTs) covalently bonded with graphene layers have been predicted to be especially favourable for applications like TIM, supercapacitors and battery electrodes. This project investigates two growth mechanisms for obtaining Graphene-VACNT structures using scalable processes. Shortcomings of previously done research on similar structures like graphene transfer and bad CNT alignment is solved. A novel growth mechanism is also investigated to set the foundation for research into a new approach to grow Graphene-CNT hybrid structures in the future. Chemical Vapor Deposition (CVD) was the method used to grow the graphene and CNT structures. The characterization was done using optical microscopy, Scanning electron microscopy (SEM) and Raman spectroscopy. / Grafen- och kolnanorör (CNT) har forskats i mer än ett decennium på grund av deras extraordinära egenskaper och fördelar gentemot applikationer som elektronik, strukturförstärkning, termisk hantering och energilagring. Grafen-CNT hybridstrukturer har förutspåtts ytterligare förbättra de exceptionella egenskaperna och övervinna några av bristerna i de enskilda materialen. Fördelar med en struktur som består av vertikalt inriktade kolnanorör (VACNT) som är kovalent bundna med grafenskikt har förutspåtts vara särskilt fördelaktiga för applikationer som TIM, superkondensatorer och batterielektroder. Detta projekt undersöker två tillväxtmekanismer för att erhålla Graphene-VACNT-strukturer med hjälp av skalbara processer. Brister i tidigare utförd forskning om liknande strukturer som grafenöverföring och dålig CNT-anpassning är lösta. En ny tillväxtmekanism undersöks också för att lägga grunden för forskning om ett nytt tillvägagångssätt för att växa Graphene- CNT hybridstrukturer i framtiden. Chemical Vapor Deposition (CVD) var metoden som användes för att odla grafen- och CNT-strukturerna. Karakteriseringen gjordes med optisk mikroskopi, Scanning electron microscopy (SEM) och Raman-spektroskopi.

The control of low dimensional materials holds potential for revolutionizing the electronic, thermal, and thermoelectric materials engineering. Through strategic manipulation and optimization of these materials, unique properties can be uncover which enable more efficient and effective materials development. Towards the determination of nanoscale strategies to improve the electronic and phononic devices, computational simulations of modified low dimensional materials have been carried in this research. First, the electronic properties of chemically func tionalized phosphorene monolayers are evaluated with spin-polarized Density Functional Theory, as a potential method to tune their electronic properties. The functionalization not only leads to formation of additional states within the semiconducting gap, but also to the emergence of local magnetism. The magnetic ground state and electronic structure are investigated in dependence of molecular coverage, lattice direction of the molecular adsorption and molecule type functionalization. Furthermore, the physical and transport properties of phosphorene grain boundaries under uniaxial strain are evaluated by the use of Density Functional based Tight Binding method in combination with Landauer theory. In both grain boundary types, the electronic bandgap decreases under strain, however, the respective thermal conductance is only weakly affected, despite rather strong changes in the frequency-resolved phonon transmission. The combination of both effects results in an enhancement in the thermoelectric figure of merit in the phosphorene grain boundary systems. Finally, the thermoelectric properties of carbon nanotubes peapod heterostructures are studied and compared to pristine nanotubes using also the Density Functional based Tight Binding method and Landauer theory. It is found that the fullerene encapsulation modifies the electron and phonon transport properties, causing the formation of electronic channels and the suppression of vibrational modes that lead to an improvement of the thermoelectric figure of merit. The results of this thesis highlight the potential of strategic manipulation and optimization of low dimensional materials in improving their unique electronic and thermal properties, revealing promising avenues for improving electronic and phononic devices.:ABSTRACT i ZUSAMMENFASSUNG ii ACKNOWLEDGEMENT iv LIST OF FIGURES ix LIST OF TERMS AND ABBREVIATIONS xviii 1 Introduction 1 1.1 Motivation 1 1.2 Objectives and outline 6 2 Computational Methods 8 2.1 Density Functional Theory 8 2.1.1 The Many-Body System Hamiltonian and the Born-Oppenheimer approximation 9 2.1.2 Thomas-Fermi-Dirac approximation model 10 2.1.3 The Hohenberg-Kohn theorems 12 2.1.4 The Kohn-Sham orbitals equations 13 2.1.5 Exchange-correlation functionals 15 2.2 Density Functional Based Tight Binding method 16 2.2.1 Tight-binding formalism 17 2.2.2 From DFT to DFTB 20 2.2.3 Parametrization 22 2.3 Atomistic Green’s functions 23 2.3.1 Non-Equilibrium Green’s functions for modeling electronic transmission 23 2.3.2 Non-equilibrium Green’s function for modeling thermal transmission 27 3 Tuning the electronic and magnetic properties through chemical functionalization 3.1 Introduction 33 3.1.1 Black phosphorus as a 2D material 33 3.1.2 Chemical Functionalization of low dimensional systems 35 3.1.3 Bipolar Magnetic Semiconductors 36 3.2 Computational approach 38 3.3 Interface effects in phosphorene by OH functionalization 39 3.3.1 Single molecule functionalization 39 3.3.2 Lattice selection 43 3.3.3 Coverage 45 3.4 Chiral functionalization effect in phosphorene 48 3.5 Functionalizing phosphorene towards BMS 51 3.6 Summary 53 4 Tuning transport properties through strain and grain bound-aries 4.1 Introduction 54 4.1.1 Strain in low dimensional materials 54 4.1.2 Grain boundaries 56 4.2 Computational approach 58 4.2.1 Molecular systems 58 4.2.2 Electron and phonon transport and thermoelectric figure of merit 58 4.3 Structural modification by strain in GB systems 60 4.4 Electronic structure modification by strain in GB systems 63 4.5 Thermal transport modification by strain in GB systems 65 4.6 Thermoelectric figure of merit of strained GB systems 68 4.7 Summary 71 5 Tuning transport properties through hybrid nanomaterials: CNT peapods 73 5.1 Introduction 73 5.1.1 Carbon-based nanostructures 73 5.1.2 CNT peapods as hybrid nanomaterials 76 5.2. Computational details 77 5.2.1 CNT peapod model 77 5.2.2 Quantum transport methodology 78 5.3 Structural properties of CNT peapods 79 5.4 Electronic properties of CNT peapods 80 5.5 Thermal properties of CNT peapods 83 5.6 Thermoelectronic properties of CNT peapods 85 5.7 Summary 88 6 Conclusions and outlook 91 Appendices Appendix A Supplementary information to phosphorene functionalization A.1 Spin resolved density of states of 1-OH system 96 A.2 Spin valve model 97 Appendix B Supplementary information to phosphorene grain boundaries 98 B.1 Projected Phonon Density of States in GB1 98 B.2 Thermoelectric transport properties of GB2 99 Appendix C Supplementary information to CNT peapods 101 C.1 Geometry optimization of CNT peapods with larger CNT diameter 101 C.2 Additional analysis of electron transport properties 102 C.3 Phonon band structure of different CNT structures 104 C.4 Additional analysis of thermoelectric performance 105 REFERENCES 105 LIST OF PUBLICATIONS 131 PRESENTATIONS 132 / Die Kontrolle niedrigdimensionaler Materialien birgt das Potenzial für eine Revolutionierung der elektronischen, thermischen und thermoelektrischen Technologien. Durch strategische Manipulation und Optimierung dieser Materialien können einzigartige Eigenschaften aufgedeckt werden, die eine effizientere und effektivere Materialentwicklung ermöglichen. Um Strategien im Nanobereich zur Verbesserung elektronischer und phononischer Bauelemente zu ermitteln, wurden in dieser Forschungsarbeit rechnerische Simulationen modifizierter niedrigdimensionaler Materialien durchgeführt. Zunächst werden die elektronischen Eigenschaften von chemisch funktionalisierten Phosphoren-Monoschichten mit Hilfe der spinpolarisierten Dichtefunktionaltheorie als potenzielle Methode zur Abstimmung ihrer elektronischen Eigenschaften bewertet. Die Funktionalisierung führt nicht nur zur Bildung zusätzlicher Zustände innerhalb der halbleitenden Lücke, sondern auch zum Auftreten von lokalem Magnetismus. Der magnetische Grundzustand und die elektronische Struktur werden in Abhängigkeit von der molekularen Bedeckung, der Gitterrichtung der molekularen Adsorption und der Funktionalisierung des Moleküls untersucht. Darüber hinaus werden die Transporteigenschaften von Phosphoren-Korngrenzen unter uniaxialer Belastung mit Hilfe der auf Dichtefunktionen basierenden Tight-Binding-Methode in Kombination mit der Landauer-Theorie untersucht. In beiden Korngrenzentypen nimmt die elektronische Bandlücke unter Dehnung ab, die jeweilige Wärmeleitfähigkeit wird jedoch nur schwach beeinflusst, trotz ziemlich starker Änderungen in der frequenzaufgelösten Phononentransmission. Die Kombination bei der Effekte führt zu einer Erhöhung der thermoelektrischen Leistungszahl in den Phosphorkorngrenzensystemen. Schließlich werden die thermoelektrischen Eigenschaften von Kohlenstoffnanoröhren-Peapod-Heterostrukturen untersucht und mit denen von reinen Nanoröhren verglichen, wobei auch die auf Dichtefunktionen basierende Tight-Binding-Methode und die Landauer-Theorie verwendet werden. Es wird festgestellt, dass die Fullereneinkapselung die Elektronen- und Phononentransporteigenschaften modifiziert und die Bildung von elektronischen Kanälen und die Unterdrückung von Schwingungsmoden bewirkt, was zu einer Verbesserung der thermoelektrischen Leistungszahl führt. Die Ergebnisse dieser Arbeit verdeutlichen das Potenzial der strategischen Manipulation und Optimierung niedrigdimensionaler Materialien zur Verbesserung ihrer einzigartigen elektronischen und thermischen Eigenschaften und zeigen vielversprechende Wege zur Verbesserung elektronischer und phononischer Bauteile auf.:ABSTRACT i ZUSAMMENFASSUNG ii ACKNOWLEDGEMENT iv LIST OF FIGURES ix LIST OF TERMS AND ABBREVIATIONS xviii 1 Introduction 1 1.1 Motivation 1 1.2 Objectives and outline 6 2 Computational Methods 8 2.1 Density Functional Theory 8 2.1.1 The Many-Body System Hamiltonian and the Born-Oppenheimer approximation 9 2.1.2 Thomas-Fermi-Dirac approximation model 10 2.1.3 The Hohenberg-Kohn theorems 12 2.1.4 The Kohn-Sham orbitals equations 13 2.1.5 Exchange-correlation functionals 15 2.2 Density Functional Based Tight Binding method 16 2.2.1 Tight-binding formalism 17 2.2.2 From DFT to DFTB 20 2.2.3 Parametrization 22 2.3 Atomistic Green’s functions 23 2.3.1 Non-Equilibrium Green’s functions for modeling electronic transmission 23 2.3.2 Non-equilibrium Green’s function for modeling thermal transmission 27 3 Tuning the electronic and magnetic properties through chemical functionalization 3.1 Introduction 33 3.1.1 Black phosphorus as a 2D material 33 3.1.2 Chemical Functionalization of low dimensional systems 35 3.1.3 Bipolar Magnetic Semiconductors 36 3.2 Computational approach 38 3.3 Interface effects in phosphorene by OH functionalization 39 3.3.1 Single molecule functionalization 39 3.3.2 Lattice selection 43 3.3.3 Coverage 45 3.4 Chiral functionalization effect in phosphorene 48 3.5 Functionalizing phosphorene towards BMS 51 3.6 Summary 53 4 Tuning transport properties through strain and grain bound-aries 4.1 Introduction 54 4.1.1 Strain in low dimensional materials 54 4.1.2 Grain boundaries 56 4.2 Computational approach 58 4.2.1 Molecular systems 58 4.2.2 Electron and phonon transport and thermoelectric figure of merit 58 4.3 Structural modification by strain in GB systems 60 4.4 Electronic structure modification by strain in GB systems 63 4.5 Thermal transport modification by strain in GB systems 65 4.6 Thermoelectric figure of merit of strained GB systems 68 4.7 Summary 71 5 Tuning transport properties through hybrid nanomaterials: CNT peapods 73 5.1 Introduction 73 5.1.1 Carbon-based nanostructures 73 5.1.2 CNT peapods as hybrid nanomaterials 76 5.2. Computational details 77 5.2.1 CNT peapod model 77 5.2.2 Quantum transport methodology 78 5.3 Structural properties of CNT peapods 79 5.4 Electronic properties of CNT peapods 80 5.5 Thermal properties of CNT peapods 83 5.6 Thermoelectronic properties of CNT peapods 85 5.7 Summary 88 6 Conclusions and outlook 91 Appendices Appendix A Supplementary information to phosphorene functionalization A.1 Spin resolved density of states of 1-OH system 96 A.2 Spin valve model 97 Appendix B Supplementary information to phosphorene grain boundaries 98 B.1 Projected Phonon Density of States in GB1 98 B.2 Thermoelectric transport properties of GB2 99 Appendix C Supplementary information to CNT peapods 101 C.1 Geometry optimization of CNT peapods with larger CNT diameter 101 C.2 Additional analysis of electron transport properties 102 C.3 Phonon band structure of different CNT structures 104 C.4 Additional analysis of thermoelectric performance 105 REFERENCES 105 LIST OF PUBLICATIONS 131 PRESENTATIONS 132

No description available.

A novel electrochemical method to generate nm-scale bubbles at the tips of CNTs can temporarily block the membrane. A 92% blocking efficiency is achieved when the bubbles are stabilized in 30-60 nm diameter „wells‟ at the tips of CNTs. This well is formed by the electrochemical oxidation of the conductive CNTs partially into the polymer matrix of the membrane. Meanwhile, the nanoscale bubbles can be removed with 0.004 atm pressure to recover the transport through the CNT membrane. The CNT membrane with nanoscale bubble valve system was used to demonstrate electrochemical energy storage. Uniform ultrathin Pt films were electrodeposited onto an aligned array of carbon nanotubes (CNTs) for high-area chemically stable methanol fuel cell anodes. Electrochemical treatment of the graphitic CNT surfaces by diazonium benzoic acid allowed for uniform Pt electroplating. The mass activity of the Pt thin film can reach 400 A/g at a scan rate of 20 mV/s and in a solution of 1 M CH3OH/0.5 M H2SO4. A novel programmed pulse potential at 0 V was also seen to nearly eliminate the effects of carbon monoxide poisoning on catalyst Pt. Furthermore, the Pt monolayer was deposited on buckypaper by replacing the precursor Cu monolayer coated on CNTs by the underpotential deposition. The electrochemical surface modification of graphite CNTs by fluorinated benzoic acid was critical to coordinate Cu ions for monolayer formation. The mass activity of the monolayer can be improved to the record value of 2711 A/g. This is about 13 times higher than that of the ~10 nm thick Pt film coated on MWCNTs. Besides the high mass activity, the Pt monolayer coated on buckypaper can be used as catalyst for fuel cells with several advantages such as low cost, high surface area, flexibility, mechanical robustness and enhanced pressure flow. Finally, a new strategy has been developed toward electrochemical water oxidation with Ir complexes catalyst, which was grafted on buckypaper by direct binding to enhance catalyst activity. The TOF (turn over frequency) of the Ir catalyst for water splitting was 7.9 s-1 at the constant potential of 1.4 V vs Ag/AgCl.

Esta tese trata da formação de professores da área de Ciências da Natureza e suas Tecnologias (CNT), a partir do Estudo de Caso do projeto de extensão: Ciclos Formativos para o Ensino de Ciências da Universidade Federal da Fronteira Sul - Campus Cerro Largo/RS. O estudo decorre de uma inquietação oriunda das relações estabelecidas entre professores, licenciandos e formadores de Biologia, de Física e de Química em processo de investigação-ação, tendo como questão central: quais sentidos são atribuídos a conceitos atinentes à docência na área de CNT? A história da formação de professores da área de CNT e o estudo das racionalidades em processos de formação contribuíram para defender a perspectiva crítica e dialética na constituição dos sujeitos. No ensejo de investigação dos movimentos formativos utilizou-se os aportes epistemológicos do racionalismo aplicado de Bachelard e as categorias de estilos de pensamento e coletivo de pensamento fleckianos. A pesquisa, de abordagem qualitativa, teve como objetivo investigar como os processos de formação na área de CNT promovem o desenvolvimento do coletivo de pensamento desta área. Os estilos de pensamento, expressos pelos participantes, apontam para indícios de movimentos formativos que deflagram a extensão e a transformação dos estilos por meio da mudança de sentido atribuída aos conceitos de autonomia, reflexão, linguagem e área do conhecimento. Os instrumentos usados para a coleta de dados foram as degravações de encontros de formação e as escritas nos diários de bordo, realizadas no decorrer do ano de 2015. A discussão foi ancorada na análise microgenética sob o paradigma indiciário. Esta etapa da pesquisa apontou que os discursos decorrem de uma perspectiva utilitarista para uma humanista acerca dos processos formativos e evidenciaram a presença de dois estilos de pensamento, sendo um conservador e outro transformador. Nos Ciclos Formativos para o Ensino de Ciências, os encontros contemplam o compartilhamento de experiências e a leitura de referenciais teóricos que promovem a constituição do sujeito no coletivo instaurado e em extensão. Diante da quantidade de informações coletadas, foram selecionados turnos e episódios de fala e relatos escritos nos diários de bordo para compor a tese. A análise permitiu apontar dimensões epistemológicas, que podem contribuir para o desenvolvimento de uma racionalidade mais plural e a construção de novos processos formativos em que os sujeitos compartilhem conhecimentos teóricos e práticos para constituírem-se área de ensino. / 203 f.

Le carbure de silicium (SiC), grâce à sa légèreté, son caractère réfractaire, sa tolérance à l’oxydation et sa faible absorption des neutrons, constitue un candidat intéressant pour des applications comme l’aéronautique, l’aérospatiale ou le nucléaire du futur. Cependant, son comportement fragile est un inconvénient majeur qu’il convient de dépasser pour ces applications. La réduction de la taille des grains à une échelle nanométrique pourrait contribuer à améliorer son comportement mécanique pour être utilisé sous forme monolithique, en tant que revêtement ou bien encore en tant que matrice dans un compositeCette thèse s’inscrit dans ce contexte, et s’intéresse à l’élaboration et à l’étude des propriétés thermomécaniques de composites à matrice SiC nanostructurée renforcée par des nanotubes de carbone. Dans un premier temps les travaux se sont portés sur l’élaboration et l’étude de l’effet de la nanostructuration de la matrice seule de SiC frittée sans ajout de frittage. Des échantillons nanostructurés (taille moyenne des grains de 100 nm) et denses à plus de 95,5% ont été obtenus grâce à la mise en place d’un procédé de dispersion efficace et à l’étude des effets des paramètres de frittage. Associés à ces hautes densités, de très bonnes duretés (jusqu’à 2200 Hv) et des ténacités convenables (3,0 MPa.m1/2) ont été atteintes. Ces travaux ont également montré l’importance primordiale de l’effet de polluants fréquemment rencontrés dans les poudres (oxygène et carbone) sur la microstructure et les propriétés mécaniques du SiC. Si l’oxygène, présent sous forme de silice ou d’oxycarbure de silicium semble favoriser les mécanismes de densification, un excès de carbone libre (3,5%m) provoque au contraire une diminution de la taille des grains et des densités. Les propriétés mécaniques (950 Hv et 2,4 MPa.m1/2) sont également sensiblement affectées. Une telle dégradation est expliquée par la localisation spécifique du carbone structuré entre les grains de SiC. La deuxième partie des travaux s’est concentrée sur l’ajout de Nanotubes de Carbone (NTC) dans la matrice SiC nanostructurée dans le but d’améliorer les propriétés mécaniques et de compenser la forte réduction de la conductivité thermique, détériorée par l’augmentation importante de la densité de joints de grains. Des crus chargés jusqu’à 5%m en NTC individualisés et répartis de façon homogène ont été réalisés. De manière similaire au carbone libre, la localisation spécifique des NTC cause une diminution de la taille des grains. L’établissement d’un réseau de percolation de NTC au-dessus de 1%m, associé à la méthode de frittage non conventionnel SPS, permet cependant d’améliorer les densités jusqu’à 97%. L’apport de NTC contribue également à l’obtention de dureté (jusqu’à 2550 Hv) et de ténacité (4,0 MPa.m1/2) plus élevées. Malgré les bonnes propriétés thermiques des NTC, l’augmentation de la densité des joints de grains amoindrit la conductivité thermique de nos composites. / Ceramic carbides materials such as SiC, due to their refractory nature and their low neutron absorption are believed to be promising candidates for high temperature nuclear or aerospace applications. However, SiC brittleness has limited its structural application. In this context this work examines in a first part the possibilities to perform dense nanostructured SiC matrix by SPS without the use of sintering additive. Indeed a reduction of grain size (below 100 nm) accompanied by a high final density seem to be the solutions to counteract the brittleness and thus to improve mechanical properties. Dense (95%) and nanostructured (grain size around 100 nm) SiC samples were obtained thanks to the realization of an effective dispersion technique and the study on the sintering parameters effect. High hardness (2200 Hv) and decent fracture toughness (3.0 MPa.m1/2) were achieved. This first work also showed the preponderant influence of recurrent pollutants (oxygen and carbon) found in SiC powders on the final microstructure and mechanical properties of sintered samples. The oxygen as silica or silicon oxycarbide seems to promote densification mechanisms while free carbon (3.5%wt) causes lower grain size and densification state. Mechanical properties with carbon are also negatively impacted (950 Hv and 2.4 MPa.m1/2). Such degradation is due by the specific localization of carbon structure between the grains. In return of the expected mechanical properties improvement by reducing the grain size, the thermal conductivity is drastically decrease of due to the phonon scattering at the grain boundaries. With the aim of reducing this effect, a second study was initiated by introducing multiwalled carbon nanotubes (MWCNTs) into the SiC matrix. The MWCNTs by exhibiting a high toughness could also help to enhance the mechanical properties. Green bodies with different amounts of well dispersed MWCNTs (0%wt to 5%wt) were realized. Like free carbon, MWCNTs are located between the grains and induce a reduction of grain size. However the appearance of CNTs percolation for an amount above 1%wt, with the SPS sintering technique, allows an improvement of densification up to 97%. Hardness (up to 2550 Hv) and fracture toughness (4.0 MPa.m1/2) are also achieved with the SiC/NTC composites. Despite the good thermal properties of MWCNTs, the increase of grain boundary decreases the thermal conductivity of these composites.

Since the discovery of X-rays over a century ago the techniques applied to the engineering of X-ray sources have remained relatively unchanged. From the inception of thermionic electron sources, which, due to simplicity of fabrication, remain central to almost all X-ray applications at this time, there have been few fundamental technological advances. The emergence of new materials and manufacturing techniques has created an opportunity to replace the traditional thermionic devices with those that incorporate Field Emission electron sources. One of the most important attributes of Field Emission X-ray sources is their controllability, and in particular the fast response time, which opens the door to applying techniques which have formerly been the preserve of optical systems. The work in this thesis attempts to bridge the gap between the fabrication and optimisation of the vacuum electronic devices and image processing aspects of a new approach to high speed radiographic imaging, particularly with a view to addressing practical real-world problems. Off the back of a specific targeted application, the project has involved the design of a viable field emission X-ray source, together with the development of an understanding of the failure modes in such devices, both by analysis and by simulation. This thesis reviews the capabilities and the requirements of X-ray sources, the methods by which nano-materials may be applied to the design of those devices and the improvements and attributes that can be foreseen. I study the image processing methods that can exploit these attributes, and investigate the performance of X-ray sources based upon electron emitters using carbon nanotubes. Modelling of the field emission and electron trajectories of the cathode assemblies has led me to the design of equipment to evaluate and optimise the parameters of an X-ray tube, which I have used to understand the performance that is achievable. Finally, I draw conclusions from this work and outline the next steps to provide the basis for a commercial solution.

A theoretical methodology and study of charge transport through GNRs, as well as in metallic and semiconducting CNTs, with randomly distributed functional groups covalently attached to the system surface is presented. By resorting to both first principles calculations, to obtain a suitable parametrization of the electronic structure, and a fully ab initio transport approach calculation to explore conduction regimes through large and disordered systems. The quantum transport modeling is based on the Green function formalism, combining an iterative scheme for the calculation of transmission coefficients with the Landauer formula for the coherent conductance. The results describe how the conductance of the hybrid systems is altered as a function of incident electron energy and molecules coverage density. Comparing two different types of functional groups, transport regimes are explored. Phenyls and hydroxyl groups induce a local orbital rehybridization of the CNTs and GNRs anchor carbon atoms from sp2-type to sp3-type yielding a localized transport regime. On the other hand, carbene groups do not disrupt the original sp2 network of armchair and small diameter zigzag CNTs which allows for good conductance preservation.