Der steigende weltweite Energiebedarf treibt die Entwicklung sauberer Energiequellen voran, die dazu beitragen werden, den Verbrauch fossiler Brennstoffe zu reduzieren. Brennstoffzellen und Metall-Luft-Batterien sind vielversprechende Alternativen, um traditionelle fossile Energie zu ersetzen und durch die Reduzierung von O2 an der Kathode grünen Strom zu erzeugen. Aufgrund der langsamen Reaktionsraten der Sauerstoffreduktionsreaktion (ORR) ist hierfür jedoch elektrokatalytisches Material mit geringen Kosten und hoher Effizienz erforderlich. In den letzten Jahrzehnten wurde eine Vielzahl von Materialien als Nicht-Pt-Katalysatoren getestet, von metallfreien Katalysatoren bis hin zu Katalysatoren auf Übergangsmetallbasis. Aufgrund des mangelnden Verständnisses des Reaktionsmechanismus und der Wechselwirkung zwischen Elektrolyt und Elektrokatalysator befinden sich neue Designs stickstoffhaltiger Katalysatoren auf Kohlenstoffbasis jedoch noch in der Entwicklungsphase. Zu diesem Zweck wurden verschiedene (in situ) spektroskopische und elektrochemische Techniken eingesetzt, um die Wechselwirkung zwischen N-dotiertem Kohlenstoff und Elektrolyten sowie die katalytischen Mechanismen zu verstehen. Darüber hinaus weisen die neu entwickelten Katalysatoren für die ORR eine überlegene elektrokatalytische Leistung auf, die in dieser Dissertation ausführlich diskutiert wird. Die Struktur-Leistungs-Beziehung unserer ORR-N-dotierten Kohlenstoffkatalysatoren wurde gründlich untersucht. Diese Forschung zeigt, wie die Kombination fortschrittlicher Spektroskopietechniken, einschließlich In-situ-Spektroskopie und elektrochemischer Charakterisierung, ein tieferes Verständnis der Katalysator-/Elektrolyt-Wechselwirkung, des katalytischen Mechanismus und der optimierten elektrokatalytischen Leistung stickstoffhaltiger Kohlenstoffmaterialien, ORR-Katalysatoren, insbesondere nanoporöser N-dotierter Kohlenstoff, fördern kann Eisen-Stickstoff-codotierte Kohlenstoffmaterialien. / Increasing global energy demand drives the development of clean energy sources that will help reduce the consumption of fossil fuels. Fuel cells and metal-air batteries are promising alternatives to replace traditional fossil energy to generate green electricity by reducing O2 at the cathode. However, due to sluggish reaction rates of oxygen reduction reaction (ORR), this requires electrocatalytic material with low cost and high efficiency. Over the last few decades, a variety of materials have been tested as non-Pt catalysts, from metal-free catalysts to transition metal-based catalysts. However, due to the lack of understanding of the reaction mechanism and the interaction between electrolyte and electrocatalysts, new designs nitrogen-containing carbon-based catalysts are still under the development stage. To this aim, a variety of (in situ) spectroscopic and electrochemical techniques to understand N-doped carbon electrocatalysts/electrolyte interaction and catalytic mechanisms have been employed. Moreover, the newly-designed catalysts for ORR demonstrate superior electrocatalytic performance which are discussed in detail in this dissertation. The structure-performance relationship for our ORR N-doped carbon catalysts has been thoroughly investigated. This research highlights how the combination of advanced spectroscopy techniques including in situ spectroscopy and electrochemical characterization may promote a deeper understanding of catalyst/electrolyte interaction, catalytic mechanism and optimized electrocatalytic performance of nitrogen-containing carbon materials ORR catalysts, especially nanoporous N-doped carbon and iron-nitrogen-co-doped carbon materials.

Dans cette thèse, la spectroscopie Raman résonante UV (SRRUV) est appliquée pour la première fois à l'étude ‘in situ’ de matériaux carbonés à très haute température (> 2000 K) ou à haute pression (< 1 GPa).La thèse est constituée de trois parties. La première aborde notre travail de caractérisation en SRRUV (1) de films semi conducteurs de diamants ultra-nano-cristalins, (2) des kérogènes issues de météorites chondritiques et de charbons, et (3) des tholins, échantillons de carbone-hydrogène-azote, synthétisés comme analogues de l'atmosphère de Titan.L’intérêt pour ces phases du carbone en planétologie et en science des matériaux nous a poussé à mettre en œuvre leur étude ‘in situ’ en SRRUV. La seconde partie de la thèse est consacrée au développement d'une platine chauffante, grâce à laquelle les spectres Raman du graphite sous sa forme pyrolitique et HOPG ont été mesurés jusqu'à 2700 K. Ces données valident les modèles anharmoniques théoriques d’interaction électron-phonon et phonon-phonon. Le spectre Raman du graphite a été étalonné en fonction de la température et devient un « thermomètre » à très haute température.Dans la troisième partie de cette thèse, une presse à enclumes opposées a été modifiée pour suivre en SRRUV les changements structuraux de cristaux moléculaires très luminescents. Les vibrations intramoléculaires du cristal de pérylène sont étudiées sous pression par SRRUV. Ce composé est un cristal formé de molécules organiques polyaromatiques, avec des propriétés de semi-conducteur. Les effets de la pression sur certains modes de vibrations sont non linéaires et mettent en évidence des changement structuraux et de planéité de la molécule. / I applied UV resonant Raman spectroscopy (UVRRS) to an ‘in situ’ study of carbon materials at very hight temperature (> 2000 K) or at high pressure (< 1 GPa).The advantages of UVRRS are presented in the first part of this PHD thesis, and used to investigate details of the composition and structure of disordered carbon materials such as: (1) n-type nanocrystalline films, (2) carbonaceous matter in chondrites and (3) tholins, HCN synthetic samples of Titan 's atmosphere.‘In situ’ Raman studies are limited to 2000 K by the visible black-body emission. I designed a high temperature cell to perform UVRRS above this limit. The second part of the manuscript presents Raman spectra of pyrolitic graphite and HOPG up to 2700 K. This data are consistent with anharmonic models up to 900 K, and show the coupling effects of electron-phonon and phonon-phonon. The last one dominates the anharmonicity above 1000 K. The Raman spectra was calibrated as a function of temperature and became a “thermometer” up to 2700 K.For high pressure measurements in the third part, I modified an anvil cell to study by UVRRS, the vibrational changes induced by pressure on very luminescent molecular organic crystals. I present an analysis at 244 nm of resonant Raman modes of perylene crystal under hydrostatic pressure up to 0.8 GPa. Some of them have a non linear feature under pressure, revealing structural and planar modifications of the molecules.

Field emission is the emission of electrons from a solid under an intense electric field, of the order of 109 V/m. Emission occurs by the quantum mechanical tunneling of electrons through a potential barrier to vacuum. Field emission sources offer several attractive features such as instantaneous response to field variation, resistance to temperature fluctuation and radiation, a high degree of focusing ability in electron optics, good on/off ratio, ballistic transport, and a nonlinear current-voltage relationship. Carbon nanotubes (CNTs) are potential candidates as field emitters since they possess high aspect ratio and are chemically inert to poisoning, and physically inert to sputtering during field emission. They can carry a very high current density and do not suffer field-induced tip sharpening like metallic tips. In addition, the CNT field emitters have the advantage of charge transport through 1D channels and electron emission at the sharp tips due to large enhancement. But the injection of electrons from the back contact remains a technical challenge which requires binding of CNT emitters to metallic substrate. Also, detachment of the CNT from the substrate tends to occur with time. The electrically conducting mixtures of CNTs and polymer can provide an alternative route to address these issues in the field emission of CNTs. The composites can be casted on any substrate in desired shape and the polymer matrix provides necessary support. The research work reported in this thesis includes the preparation of high quality multiwall carbon nanotubes (MWCNTs), MWCNT-polystyrene (PS) composites, and experimental investigation on field emission properties of MWCNT¬PS composites in two different configurations. Electrical conductivity and percolation threshold of the MWCNT-PS composites are also investigated to ensure their high quality prior to the field emission studies. The study has been further extended to reduced graphene oxide (rGO) coated on polymer substrate. The main results obtained in present work are briefly summarized below. This thesis contains eight chapters. Chapter 1 provides an overview of basics of field emission, and the potential of CNT and CNT-polymer composites as field emitters. Chapter 2 deals with the concise introduction of various structural characterization tools and experimental techniques employed in this study. Chapter 3 describes the synthesis of MWCNTs and characterization by using electron microscopy and Raman spectroscopy. MWCNTs are synthesized by chemical vapor deposition (CVD) of toluene [(C6H5) CH3] and ferrocene [(C5H5)2 Fe] mixture at 980 °C. Here toluene acts as carbon source material and ferrocene provides catalytic iron (Fe) particles. The MWCNT formation is based on the thermal decomposition of the precursor mixture. Scanning electron microscopy (SEM) characterization shows that the MWCNTs are closely packed and quite aligned in one direction. The average length of MWCNTs is about 200 μm and outer diameter lies in the range of 50-80 nm. The high quality of as-prepared MWCNT sample is confirmed by Raman spectroscopy. The as-grown MWCNTs are encapsulated with catalytic Fe nanoparticles, revealed by transmission electron microscopy. The Fe nanoparticles trapped within the MWCNT serve as fantastic system for studying the magnetic properties. Three types of MWCNT samples filled with Fe nanoparticles of different aspect ratio (~10, 5 and 2) are synthesized by varying the amount of ferrocene in the precursor material, and their magnetic properties are investigated. Enhanced values of coercivity (Hc) are observed for all samples, Hc being maximum (~2.6 kOe) at 10 K. The enhancement in Hc values is attributed to the strong shape anisotropy of Fe nanoparticles and significant dipolar interactions between Fe nanoparticles. Chapter 4 deals with the field emission studies of MWCNT-PS composites in the parallel configuration. By incorporating as-prepared MWCNTs in PS matrix in a specific ratio, composites with varying loading from 0.01-0.45 weight (wt.) fraction are prepared using solution mixing and casting. High degree of dispersion of MWCNTs in PS matrix without employing any surfactant is achieved by ultrasonication. Low percolation threshold (~0.0025 wt. fraction) in the MWCNT-PS composites ensures the good connectivity of filler in the fabricated samples. Field emission of MWCNT¬PS composites is studied in two different configurations: along the top surface of the film (parallel configuration) and along the cross section of the sample (perpendicular configuration). In this chapter field emission results of the MWCNT-PS composites in parallel configuration are presented. The effect of charge transport in limiting the field emission of MWCNT-PS composite is discussed. Field emission results of MWCNT-PS composites in parallel configuration indicate that the emission performance can be maximized at moderate wt. fraction of MWCNT (0.15). The obtained current densities are ~10 µA/cm2 in the parallel configuration. Chapter 5 presents the study of field emission characteristics of MWCNT¬PS composites of various wt. fractions in the perpendicular configuration. Till date most studies using nanotube composites tend to have the nanotubes lying in two dimensional plane, perpendicular to the applied electric field. In the perpendicular configuration, the nanotubes are nearly aligned parallel to the direction of the applied electric field which results in high field enhancement, and electron emission at lower applied fields. SEM micrographs in cross-sectional view reveal that MWCNTs are homogeneously distributed across the thickness and the density of protruding tubes can be scaled with wt. fraction of the composite film. Field emission from composites has been observed to vary considerably with density of MWCNTs in the polymer matrix. High emission current density of 100 mA/cm2 is achieved at a field of 2.2 V/µm for 0.15 wt. fraction. The field emission is observed to follow the Fowler– Nordheim tunneling mechanism, however, electrostatic screening plays a role in limiting the current density at higher wt. fractions. Chapter 6 highlights the field emission response of rGO coated on a flexible PS film. Field emission of rGO coated PS film along the cross section of the sample is studied in addition to the top film surface of the film. The effect of geometry on the improved field emission efficiency of rGO coated polymer film is demonstrated. The emission characteristics are analyzed by Fowler–Nordheim tunneling for field emission. Low turn-on field (~0.6 V/µm) and high emission current (~200 mA/cm2) in the perpendicular configuration ensure that rGO can be a potential field emitter. Furthermore, stability and repeatability of the field emission characteristics are also presented. Chapter 7 deals with the synthesis, characterization, and field emission of two different kinds of hybrid materials: (1) MWCNT coated with zinc oxide (ZnO) nanoparticles (2) ZnO/graphitic carbon (g-C) core-shell nanowires. The field emission from the bucky paper is improved by anchoring ZnO nanoparticles on the surface of MWCNT. A shift in turn on field from 3.5 V/µm (bucky paper) to 1.0 V/µm is observed by increasing the ZnO nanoparticle loading on the surface of MWCNT with an increase in enhancement factor from 1921 to 4894. Field emission properties of a new type of field emitter ZnO/g-C core-shell nanowires are also presented in this chapter. ZnO/g-C core/shell nanowires are synthesized by CVD of zinc acetate at 1300 °C. Overcoming the problems of ZnO nanowire field emitters, which in general possess high turn on fields and low current densities, the core-shell nanowires exhibit excellent field emission performance with low turn on field of 2.75 V/µm and high current density of 1 mA/cm2. Chapter 8 presents a brief summary of the important results and future perspectives of the work reported in the thesis.

Les carbones poreux ont été largement utilisés et étudiés ces dernières années. Ce travail de recherche porte sur la préparation de matériaux carbonés mésoporeux modifiés ou non par de l'azote. Tout d'abord, un carbone mésoporeux pur a été synthétisé. Puis des atomes d'azote ont été introduits dans ce carbone mésoporeux par deux méthodes de dopage ("in situ" et "post-synthèse" respectivement). La comparaison des propriétés acido-basiques dans des conditions différentes pour les trois types de matériaux mésoporeux carbonés, avec ou sans diazote, a été faite grâce à des techniques calorimétriques. Ces carbones mésoporeux ont aussi été utilisés dans l'adsorption de polluants, le stockage de l'hydrogène et en tant que supports pour les métaux précieux et l'oxyde de fer en catalyse. Les différentes performances dans ces applications ont été mises en relation avec les différentes propriétés structurelles et surfaciques causées par le dopage au diazote / Porous carbon materials are widely used and studied in recent years. In this work, three kinds of mesoporous carbon materials were prepared. Firstly, cost-effective pure mesoporous carbon was synthesized. Then nitrogen atoms were introduced into the mesoporous carbon by “in situ” and “post” doping methods respectively. The comparisons of the acid-base properties in different conditions of the three kinds of mesoporous carbon materials with or without nitrogen were studied and revealed by different calorimetric techniques. The three kinds of mesoporous carbons were also applied in pollutants adsorption, hydrogen storage and as supports of precious metals and iron oxide in catalysis. The different performances in applications were related to the different structural and surface properties caused by the N-doping

Diplomová práce se zabývá teoretickými výpočty a návrhem struktury pro přenos tepla, která je součástí Miniaturizovaného tepelného spínače podle zadaných požadavků Evropské Kosmické Agentury. Základními parametry jsou nízká hmotnost a vysoká tepelná vodivost. Práce navazuje na spínač navržený firmou Arescosmo, který nesplňoval požadované limity zejména v oblasti hmotnosti a tepelné vodivosti. Pomocí teoretických výpočtů hmotnosti a tepelné vodivosti bylo ověřeno 49 variant ve třech základních konceptech – Mechanická struktura, flexibilní struktura složená z drátků a foliová struktura. Z hlediska tepelné vodivosti jako nejlepší struktury vycházejí ty, které jsou založené na použití ochranných kovových opletů. Z dostupných zdrojů byly rovněž navrženy technologie, které by bylo možné využít pro výrobu těchto struktur. Pro splnění požadavků, bude v další fázi projektu nutné vyrobit experimentální vzorky na kterých budou teoretické výpočty a vybrané technologie ověřeny.