Abstract: Micro­Raman Spectroscopy Investigation of Hard Coatings Diamond, silicon carbide, and boron nitride have attracted great interest in the last years, due to their excellent material properties. Especially the extreme hardness and the high thermal con­ ductivity of these materials favour them as protective layers. The very large hardness gave these materials, deposited as films on various substrates, their name: hard coatings. In contrast to di­ amond, silicon carbide and boron nitride can be n­ as well as p­doped, making them promising candidates for high speed and high temperature electronic applications. Contrarily to the materials mentioned above, carbon nitride was obtained in crystalline form just very recently. Up to now the deposited films mainly consist of amorphous or nanocrystalline, carbon­rich material. For all these material systems inelastic light scattering (Raman spectroscopy) has been already applied for the material properties investigation. However, these investigations usually were restricted to only one of the various Raman spectroscopy tools, described in this work: Incident laser light energy varia­ tion, temperature variation, utilizing the selection rules, measurements at varying sample positions, two­dimensional mappings and one­dimensional scans in the conventional plane­view and the addi­ tional cross­sectional sample geometry. In contrast to this, this work demonstrates the improvement of the information about the investigated material and/or the sample heterostructure obtained by using the combination of all the above mentioned techniques. In the case of the diamond material system, films deposited on silicon substrates were investigated and an interfacial graphitic layer of 2nm thickness was found by scanning across the interface, which was obscured in the conven­ tional plane­view sample geometry. Similar to this an ultra­thin top layer and buried intermixed regions were identified in the silicon carbide material system utilizing the cross­sectional sample geometry. In addition to this, the temperature and the incident laser light energy dependences for 5 SiC polytypes (3C, 4H, 6H, 15R, and 21R) were measured. A resonance enhancement for the 3C and the 21R polytype was found corresponding to their fundamental bandgaps at 2.46eV and ß2.8eV, respectively. For the other polytypes no resonance enhancement was found, due to their larger fundamental bandgap. In the boron nitride material system the spatial correlation model for Raman lineshape analysis was applied for the first time and the values of the asymmetric broad­ ening and the frequency downshift for decreasing crystal sizes were evaluated. This was measured for single crystals of different size and for films deposited on silicon substrates. The correlation lengths in the ten nanometer region found for the deposited films corroborate the nanocrystalline nature of these films. Additionally incident laser light energy was measured, revealing the 488.0nm (Ar + ) and 482.5nm (Kr + ) laser lines as the optimum laser lines for the boron nitride investigation. Furthermore the dependence of the phonon feature parameters was investigated depending on the incident laser light power. A maximum power of 5­10mW for the micro­Raman spectroscopy setup was found to avoid any laser light induced heating of the investigated material. Two­dimensional mappings of the deposited boron nitride films were performed to improve the information about the material system. In the case of carbon nitride for the first time distinct phonon features were measured in a wide spectral range contrarily to most of the other investigations, which usually show only broad bands.

Mo-Si-B alloys are a leading candidate for the next generation of jet turbine engine blades and have the potential to raise operating temperatures by 300-400°C. The alloys of interest are a three-phase mixture of the molybdenum solid solution (Moss) and two intermetallic phases, Mo3Si (A15) and Mo5SiB2 (T2). A novel powder metallurgical method was developed which uses the reaction of molybdenum, silicon nitride (Si3N4) and boron nitride (BN) powders to synthesize a fine dispersion of intermetallics in a Moss matrix. The covalent nitrides are stable in oxidizing environments up to 1000ºC, allowing for fine particle processing. The process developed uses standard powder processing techniques to create Mo-Si-B alloys in a less complex and expensive manner than previously demonstrated. This powder metallurgy approach yields a fine dispersion of intermetallics in the Moss matrix with average grain sizes of 2-4μm. Densities up to 95% of theoretical were attained from pressureless sintering at 1600°C and full theoretical density was achieved by hot-isostatic pressing (HIP). Sintering and HIPing at 1300°C reduced the grain sizes of all three phases by over a factor of two. Microstructure examination by electron back-scatter diffraction imaging was used to precisely define the location of the phases and to measure the volume fractions and grain size distributions. Microstructural quantification techniques including two-point correlation functions were used to quantify microstructural features and correlate the BN reactant powder size and morphology to the distribution of the intermetallic phases. High-temperature tensile tests were conducted and yield strengths of 580MPa at 1100°C and 480MPa at 1200°C were measured for the Mo-2Si-1Bwt.% alloy. The yield strength of the Mo-3Si-1Bwt.% alloy was 680MPa at 1100°C and 420MPa at 1300°C. A review of the pertinent literature reveals that these are among the highest yield strengths measured for these compositions. The oxidation resistance in air at 1000 and 1100°C was examined. The protective borosilicate surface layer formed quickly due to the close spacing of intermetallic particles and pre-oxidation treatment was developed to further limit the transient oxidation behavior. An oxidation model was developed which factors in the different stages of oxidation to predict compositions that minimize oxidation.

Comme le quatrième élément le plus abondant dans l’univers, le carbone joue un rôle important dans l’émergence de la vie sur la terre comme nous la connaissons aujourd’hui. L’ère industrielle a vu cet élément au cœur des applications technologiques en raison des différentes façons dont les atomes forment les liaisons chimiques, ce qui donne lieu à une série d’allotropies chacun ayant des propriétés physiques extraordinaires. Par exemple, l’allotrope le plus thermodynamiquement stable du carbone, le cristal de graphite, est connu pour être un très bon conducteur électrique, tandis que le diamant, très apprécié pour sa dureté et sa conductivité thermique, est néanmoins considéré comme un isolant électrique en raison de sa structure cristallographique différente par rapport au graphite. Les progrès de la recherche scientifique ont montré que les considérations cristallographiques ne sont pas le seul facteur déterminant pour une telle variété dans les propriétés physiques des structures à base de carbone. Ces dernières années ont vu l’émergence de nouvelles formes allotropiques de structures de carbone qui sont stables dans les conditions ambiantes, mais avec dimensionnalité réduite, ce qui entraîne des propriétés largement différentes par rapport aux structures en trois dimensions. Parmi ces nouvelles classes d’allotropes il y a le graphene, qui est le premier matériau à deux dimensions. L’isolation réussi de monocouches de graphène a contesté une croyance établie depuis longtemps en physique : le fait que les matériaux purement 2D ne peuvent pas exister dans les conditions ambiantes parce qu'ils sont instables en raison de l’augmentation des fluctuations thermiques lorsqu’ils se prolongent dans les 2D. Afin de minimiser son énergie, un matériau se brisera en îlots coagulées. Le graphène arrive cependant à surmonter cette barrière en formant des ondulations continues sur la surface du substrat et est stable même à température ambiante et pression atmosphérique. Une grande intention dans la communauté scientifique a été donnée au graphène, après les premiers résultats publiés sur les propriétés électroniques de ce matériau. Les propriétés fondamentales et mécaniques du graphène sont fascinants. Grace aux atomes de carbone qui sont emballés dans un mode sp2 hybridé, formant ainsi une structure de réseau hexagonal, le graphène possède le plus grand module de Young et la plus grande capacité d’étirement, en même temps des centaines de fois plus dur que l’acier. Il conduit la chaleur et l’électricité de manière très efficace. L’aspect le plus fascinant à propos du graphène est surement la nature de ses porteurs de charge à basse énergie. En effet, le graphène présente des bandes d’énergie linéaires au point de neutralité de charge, donnant aux porteurs de charge une nature relativiste. De nombreux phénomènes observés dans ce matériau sont des conséquences de la nature relativiste de ses porteurs. Transport balistique, conductivité optique universelle, absence de rétrodiffusion, et une nouvelle classe d’effet Hall quantique sont de bons exemples de phénomènes nouvellement découverts dans ce matériau. Il est cependant encore trop tôt pour affirmer que toutes les propriétés physiques du graphene sont bien comprises. Dans cette thèse, nous avons mené des expériences de spectroscopie magnéto-Raman pour répondre à certaines des questions ouvertes dans la physique du graphène, notamment l’effet de couplage de Coulomb sur le spectre d’énergie du graphène, et le changement dans les propriétés physiques du graphène multicouche en fonction de sa cristallographie. Nos echantillions ont été soumis à de forts champs magnétiques, appliqués perpendiculairement aux plans atomiques. Le spectre d’excitation sous champ magnétique montre un couplage entre ces excitations et les modes de vibratoires. Cette approche expérimentale permet de remonter à la structure de bande du graphene en champs nul, ainsi que de nombreuses autres propriétés du matériau. / As the fourth most abundant element in the universe, Carbon plays an important rolein the emerging of life in earth as we know it today. The industrial era has seen this element at the heart of technological applications due to the different ways in which carbon forms chemical bonds, giving rise to a series of allotropes each with extraordinary physical properties. For instance, the most thermodynamically stable allotrope of carbon, graphite crystal, is known to be a very good electrical conductor, while diamond very appreciated for its hardness and thermal conductivity is nevertheless considered as an electrical insulator due to different crystallographic structure compared to graphite. The advances in scientific research have shown that crystallographic considerations are not the only determining factor for such a variety in the physical properties of carbon based structures. Recent years have seen the emergence of new allotropes of carbon structures that are stable at ambient conditions but with reduced dimensionality, resulting in largely different properties compared to the three dimensional structures. Among these new classes of carbon allotropes is the first two-dimensional material: graphene.The successful isolation of monolayers of graphene challenged a long established belief in the scientific community: the fact that purely 2D materials cannot exist at ambient conditions. The Landau-Peierls instability theorem states that purely 2D materials are very unstable due to increasing thermal fluctuations when the material in question extends in both dimensions. To minimize its energy, the material will break into coagulated islands, an effect known as island growth. Graphene happens to overcome such barrier by forming continuous ripples on the surface of its substrate and thus is stable even at room temperature and atmospheric pressure.A great intention from the scientific community has been given to graphene, since 2004. Both fundamental and mechanical properties of graphene are fascinating. Thanks to its carbon atoms that are packed in a sp2 hybridized fashion, thus forming a hexagonal lattice structure, graphene has the largest young modulus and stretching power, yet it is hundreds of times stronger than steel. It conducts heat and electricity very efficiently, achieving an electron mobility as high as 107 cm−2V−1 s−1 when suspended over the substrate. The most fascinating aspect about graphene is the nature of its low energy charge carriers. Indeed, graphene has a linear energy dispersion at the charge neutrality, giving the charge carriers in graphene a relativistic nature. Many phenomena observed in this material are consequences of this relativistic nature of its carriers. Ballistic transport, universal optical conductivity, absence of back-scattering, and a new class of room temperaturequantum Hall effect are good examples of newly discovered phenomena in thismaterial. Graphene has become an active research area in condensed matter physics since 2004. It is however still early to state that all the physical properties of this material are well understood. In this thesis we conducted magneto-Raman spectroscopy experiments to address some of the open questions in the physics of graphene, such as the effect of electron-electron coupling on the energy spectrum of monolayer graphene, and the change in the physical properties of multilayer graphene as a function of the crystallographic stacking order. In all our experiments, the graphene-based systems have been subject to strong continuous magnetic fields, applied normal to the graphene layers. We study the evolution of its energy excitation spectra in the presence of the magnetic field, and also the coupling between these excitations and specific vibrational modes that are already in the system. This experimental approach allows us to deduce the band structure of the studied system at zero field, as well as many other lowenergy properties.

Bien que proposant des avantages importants par rapport à d'autres matériaux, les céramiques présentent un défaut récurrent, qui est leur plus ou moins grande fragilité due à des défauts de structure ou à des impuretés dans les réseaux structuraux. On s’affranchit de ces contraintes en améliorant la pureté des matériaux de base, en maîtrisant mieux les processus de fabrication, en les renforçant et en nanostructurant le matériau. C’est ce qui a donné naissance aux méthodes chimiques d’élaboration dites de « Bottom-up » qui reprennent le schéma de principe de la conception de la céramique naturelle en s’adaptant à la démarche des chimistes : des briques élémentaires représentant une architecture moléculaire sont assemblées pour former un composé macromoléculaire dont la composition est contrôlée à l’échelle atomique. Ce composé est mis en forme, durcit pour être transformé par cuisson en une céramique dont la composition est directement liée à la structure moléculaire des briques. Cette démarche est à l’origine de la voie dite des « polymères précéramiques ». Cette voie chimique s’adapte aux exigences des domaines de l’énergie. Notamment et afin d’exploiter et de saisir les opportunités que constituent l’apparition de nouveaux besoins en matériaux et/ou l’établissement de cahiers des charges stricts au regard des propriétés des matériaux dans ce domaine, la présente étude a pour objet d’élaborer des matériaux à base de BN comme les composites à renforts fibreux, les nanocomposites et les mousses.Après une introduction générale, le chapitre 1 décrit l’état de l’art de BN. Il s’intéresse à la littérature sur les propriétés des différentes formes du BN. La voie PDCs est détaillée et son application à l’élaboration du h-BN. Les différents types de précurseurs et de polymères sont décrits et l’accent est mis sur le borazine et le polyborazylène. La dernière partie concerne l’élaboration des composites à renforts fibreux, les nanocomposites et les mousses à base de BN qui sont considérés.Le chapitre 2 s’intéresse à l’élaboration de C/BN composites à partir de polyborazylènes qui est un projet de recherche sur ITER. Après un rappel sur le contexte de CEA, les différentes étapes liées au procédé d’élaboration des composites sont décrites et étudiées à l’aide d’outils de caractérisation comme la RMN solide,TGA, XRD et SEM. Le chapitre 3 s’intéresse à des nanocomposites qui se caractérisent par des phases nanocristallines de nitrure métallique parmi le nitrure de titane, de zirconium et d’hafnium dispersés dans une matrice de BN faiblement cristallisée. L’accent est mis sur la chimie moléculaire et sur la synthèse de polymétalloborazines qui permettent de conduire par pyrolyse à la formation directe de ces nanocomposites par croissance in-situ de la phase nanocristalline dans la matrice BN. Une étude préliminaire sur la possibilité de mettre en forme les polyméres en vue d’élaborer des structures massives nanocomposites est abordée.Le chapitre 4 se consacre à deux procédés de préparation de mousses. Le premier procédé qui combine la voie PDCs à la chimie intégrative vise à élaborer des mousses BN à porosité hiérarchisée. Le second procédé consiste à mélanger PMMA avec polyborazylènes pour subir des étapes de compactage et de pyrolyse générant des mousses. Pour ces deux types de matériaux, des mesures texturales comme BET et la porosimétrie mercure sont entreprises.Une conclusion générale termine le manuscrit. Elle fait un rappel des travaux entrepris dans chacun des trois chapitres et donne des perspectives liées aux trois types de matériau étudiés pendant la thèse. / Energy developments have brought hexagonal boron nitride-based materials increasing interest for future materials and technologies. The objective of this thesis concerns the preparation of BN shapes for energy applications including fiber-reinforced BN composites, BN-based nanocomposites and BN foams. Fiber-reinforced BN composite and BN nanocomposites display potential as tiles for protection limiters for the Ion Cyclotron Range Frequency antennas in fusion nuclear reactors. Porous BN materials have interests as host material for hydrogen storage and as catalyst supports. The Polymer-Derived Ceramics route which offers new preparation opportunities in chemistry and ceramic sciences is applied to manufacture shaped BN-based materials.Firstly, in the context of C/BN composite, polyborazylene vacuum-assisted infiltration and pyrolysis process was successfully introduced. We focused on the design, elaboration and properties of the C/BN composite through the study of the (1) synthesis and polymerization of borazine, (2) the polyborazylene-to-boron nitride conversion, (3) the morphological texture and mechanical properties of derived C/BN composites. We firstly demonstrated that it is possible to obtain dense-derived C/BN composites (density: 1.773 g cm-3, open porosity: 5.09%) by tuning the viscosity of polyborazylene in the infiltration process. SEM observation presented a very strong bonding between fibers and matrix. TGA under air analysis confirmed the improved oxidation resistance property of C/BN composite compared with C fiber.Secondly, we investigated the design, processing, and properties of transition metal-containing boron nitride nanocomposites from polymetalloborazine. With proper choice of boron nitride precursor, and by controlling the B/M ratio (M = Ti, Zr, Hf), a set of representative polymetalloborazines has been prepared as precursors of nanocomposites. In the reaction of BN source with metal precursor leading to polymetalloborazines, two main mechanisms are mainly concerned: N-H and B-H units of BN percursor react with N-alkyl groups presented in metal precursors. After its pyrolysis under ammonia up to 1000 oC then nitrogen from 1000 to 1500oC, the derived nanocomposites reveal the presence of metal nitride nanocrystales with an average diameter of 6.5 nm homogeneously embedded in a poorly crystallized boron nitride matrix. A preliminary study is presented on the preparation of monolith-type nanocomposites from selected polytitanoborazines. Finally, we applied two PDCs route-based strategies to prepare hierarchically porous and micro cellular BN foams. In the first strategy, monolith-type BN foams with a hierarchical porosity were synthesized from polyborazylene using an integrative chemistry combined-based sequence set-up that consists of the impregnation of silica and carbonaceous templates followed by pyrolysis process and elimination of the template. These novel porous BN architectures display hierarchical and high porosity (76 %) with an open-cell interconnected macroporosity and a surface area up to 300 m2g-1. In the second strategy, a sacrificial processing route has been proposed to fabricate micro cellular BN foams with a porosity of 79 % from a mixture of polyborazylene and poly(methylmethacrylate) (PMMA) microbeads by warm-pressing followed by pyrolysis consisting of the burn-out of PMMA while polyborazylene is converted into BN. These novel BN foams display potential as catalyst supports and host material for hydrogen storage.

Made available in DSpace on 2016-08-18T18:19:26Z (GMT). No. of bitstreams: 1 Nilton Ferreira Frazao.pdf: 1298809 bytes, checksum: 5aca0b703a735a7fb605b7bdee658bdd (MD5) Previous issue date: 2009-03-30 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Recently, the existence of nanoribbon of Boron Nitride with finite size was discovered experimentally, in porous nanoesferas of BN (100-400 nm of diameter), synthesized for the reaction of B2O3 with carbon spheres contend nanoporos filled for Nitrogen to a temperature of 17500C. However a theoretical inquiry of the properties of this nanometerial did not exist. Then, in this present work, we carry through simulation of molecular mechanics using field of universal force (forcefild) to optimize the structure of some of these nanoribbon of Boron Nitride, with objective to find a conformation more steady, that is, of lesser energy for these nanostructures. Later, we investigate the electronic properties of these nanoribbon of Boron Nitride (NRBN) in the finite form (not-periodic) of two types: nanoribbon of Boron Nitride of the type to armchair (a-NRBN) and nanoribbon of Boron Nitride of the type zigzag (z-NRBN). The study of these properties they had been carried through of calculations of first principles based in the Density Functional Theory, with the local density approximation (LDA). Through our calculations, we observe that all the nanoribbon are metallic when we made the analysis of the density of states (DOS). Result not waited, but surprising, therefore of literature we know that material nanostructuralized of Boron Nitride they are always semiconductors. However, our calculations had shown that as much a-NRBN as z-NRBN had presented a conducting electronic character. The simulations had been carried through for many cases of nanoribbon of width (L) and length (C), forming a pair of indices (L, C), with the objective to facilitate the identification of these nanostructures. However we will present the results of but twelve of these, being: (1,3), (1,6), (1,9), (2,3), (2,6) e (2,9) in such a way of the types a-NRBN and z-NRBN. / Recentemente, foi descoberto experimentalmente a existência de Nanofitas de Nitreto de Boro (BN) de tamanho finito, em nanoesferas porosas de BN (100-400 nm de diâmetro), sintetizada pela reação de B2O3 com esferas de carbono contendo nanoporos preenchidos por Nitrogênio a uma temperatura de 17500C. No entanto não existia uma investigação teórica das propriedades desses nanocompósitos. Então, neste presente trabalho, realizamos simulações de mecânica molecular usando campo de força universal (forcefild) para otimizar a estrutura de algumas destas nanofitas de Nitreto de Boro, com objetivo de encontrar uma conformação mais estável, ou seja, de menor energia para essas nanoestruturas. Depois, investigamos as propriedades eletrônicas dessas nanofitas de Nitreto de Boro (NRBN) na forma finita (não-periódica) de dois tipos: nanofitas de Nitreto de Boro do tipo armchair (a-NRBN) e nanofitas de Nitreto de Boro do tipo zigzag (z-NRBN). O estudo destas propriedades foram realizados através de cálculos de primeiros princípios baseados na Teoria do Funcional da Densidade, com a aproximação da densidade local (LDA). Através de nossos cálculos, observamos que todas as nanofitas são metálicas quando fizemos a análise da densidade de estados eletrônicos (DOS). Resultado não esperado, mas surpreendente, pois da literatura sabemos que materiais nanoestruturados de Nitreto de Boro são sempre semicondutores. No entanto, nossos cálculos mostraram que tanto as a- NRBN como as z-NRBN apresentaram um caráter eletrônico condutor. As simulações foram realizadas para muitos casos de nanofitas de largura (L) e comprimento (C), formando um par de índices (L, C), com o objetivo de facilitar a identificação dessas nanoestruturas. No entanto apresentaremos os resultados de apenas doze dessas, sendo: (1,3), (1,6), (1,9), (2,3), (2,6) e (2,9) tanto dos tipos a-NRBN e z-NRBN.

This thesis presents experimental and related theoretical studies of pyrochlore titanate oxides, boron nitride nanotubes, and multiferroic bismuth ferrite. We have investigated these systems at high pressures and at low temperatures using Raman spectroscopy. Below, we furnish a synoptic presentation of our work on these three systems. In Chapter 1, we introduce the systems studied in this thesis, viz. pyrochlores, boron nitride nanotubes, and multiferroic BiFeO3, with a review of the literature pertaining to their structural, electronic, vibrational, and mechanical properties. We also bring out our interests in these systems. Chapter 2 includes a brief description of the theory of Raman scattering and infrared absorption. This is followed by a short account of the experimental setups used for Raman and infrared measurements. We also present the technical details of high pressure technique including the alignment of diamond anvil cells, gasket preparation, calibration of the pressure, etc. Chapter 3 furnishes the results of our pressure-and temperature-dependent studies of pyrochlore oxides which has been divided into eight different parts. In recent years, magnetic and thermodynamic properties of pyrochlores have received a lot of attention. However, not much work has been reported to address the quasiparticle excitations, e.g., phonons and crystal-field excitations in these materials. A material that shows exotic magnetic behavior and high degree of degenerate ground states can be expected to have low-lying excitations with possible couplings with phonons, thereby, finger-printing various novel properties of the system. Raman and infrared absorption spectroscopies can, therefore, be used to comprehend the novel role of phonons and their role in various phenomena of frustrated magnetic pyrochlores. Recently, there have been reports on various novel properties of these systems; for example, Raman and absorption studies [Phys. Rev. B 77, 214310 (2008)] have revealed a loss of inversion symmetry in Tb2Ti2O7 at low temperatures which has been suggested as the key reason for this frustrated magnet to remain in spin-liquid state down to 70 mK. Powder neutron-diffraction experiments [Nature 420, 54 (2002)] have shown that an application of isostatic pressure of about 8.6 GPa in spin-liquid Tb2Ti2O7 induces a long-range magnetic order of the Tb3+ spins coexisting with the spin-liquid phase ascribing this transition to the breakdown of the delicate balance among the various fundamental interactions. Moreover, Raman and x-ray studies have shown that Tb2Ti2O7,Sm2Ti2O7,and Gd2Ti2O7 undergo a structural transition followed by an irreversible amorphization at very high pressures (~ 40 GPa or above) [Appl. Phys. Lett. 88, 031903 (2006)]. In this chapter, therefore, we present our temperature-and pressure-dependent Raman studies of A2Ti2O7 pyrochlores, where ‘A’ is a trivalent rare-earth element (A = Sm, Gd,Tb, Dy,Ho, Er,Yb, and Lu; and also Y). Since all the group theoretically predicted Raman modes of this cubic lattice are due to oxygen vibrations only, in Part (A), we revisit the phonon assignments of pyrochlore titanates by performing Raman measurements on the O16 /O18 − isotope based Dy2Ti2O7 and Lu2Ti2O7 and find that the vibrations with frequencies below 250 cm−1 do not involve oxygen atoms. Our results lead to a reassignment of the pyrochlore Raman phonons thus proposing that the mode with frequency ~ 200 cm−1, which has earlier been known as an F2g phonon due to oxygen vibration, is a vibration of Ti4+ ions. Moreover, we have performed lattice dynamical calculations using Shell model that help us to assign the Raman phonons. In Part (B), we have explored the temperature dependence of the Raman phonons of spin-ice Dy2Ti2O7 and compared with the results of two non-magnetic pyrochlores, Lu2Ti2O7 and Y2Ti2O7. Our results reveal anomalous red-shift of some of the phonons in both magnetic and non-magnetic pyrochlores as the temperature is lowered. The phonon anomalies can not be understood in terms of spin-phonon and crystal field transition-phonon couplings, thus attributing them to phonon-phonon anharmonic interactions. We also find that the anomaly of the disorder activated Ti4+ Raman vibration (~ 200 cm−1) is unusually high compared to other phonons due to the large vibrational amplitudes of Ti4+-ions rendered by the vacant Wyckoff sites in their neighborhood. Later, we have quantified the anharmonicity in Dy2Ti2O7. We have extended our studies on spin-ice compound Dy2Ti2O7 by performing simultaneous pressure-and temperature-dependent Raman measurements, presented in Part (C). We show that a new Raman mode appears at low temperatures below TC ~ 110 K, suggesting a structural transition, also supported by our x-ray measurements. There are reports [Phys. Rev. B 77, 214310 (2008), Phys.Rev.B 79, 214437 (2009)] in the literature where the new mode in Dy2Ti2O7 at low temperatures has been assigned to a crystal field transition. Here, we put forward evidences that suggest that the “new” mode is a phonon and not a crystal field transition. Moreover, the TC is found to depend on pressure with a positive coefficient. In Part (D), we have presented our results of temperature-and pressure-dependent Raman and x-ray measurements of spin-frustrated pyrochlores Gd2Ti2O7, Tb2Ti2O7,and Yb2Ti2O7. Here, we have estimated the quasiharmonic and anharmonic contributions to the anomalous change in phonon frequencies with temperature. Moreover, we find that Gd2Ti2O7 and Tb2Ti2O7 undergo a subtle structural transition at a pressure of ~ 9 GPa which is absent in Yb2Ti2O7. The implication of this structural transition in the context of a long-range magnetically ordered state coexisting with the spin-liquid phase in Tb2Ti2O7 at high pressure (8.6 GPa) and low temperature (1.5 K), observed by Mirebeau et al. [Nature 420, 54 (2002)], has been discussed. As we have established in the previous parts that the anomalous behavior of pyrochlore phonons is due to phonon-phonon anharmonic interactions, we have tuned the anharmonicity in the first pyrochlore of the A2Ti2O7 series, i.e., Sm2Ti2O7,by replacing Ti4+-ions with bigger Zr4+-ions, presented in Part (E). Our results suggest that the phonon anomalies have a very strong dependence on the ionic size and mass of the transition element (i.e., the B4+-ion in A2B2O7 pyrochlores). We have also observed signatures of coupling between a phonon and crystal-field transitions in Sm2Ti2O7. In Part (F), we have studied spin-ice compound Ho2Ti2O7 and compared the phonon anomalies with the stuffed spin-ice compounds, Ho2+xTi2−xO7−x/2 by stuffing Ho3+ ions into the sites of Ti4+ with appropriate oxygen stoichiometry. We find that as more and more Ho3+-ions are stuffed, there is an increase in the structural disorder of the pyrochlore lattice and the phonon anomalies gradually disappear with increasing Ho3+-ions. Moreover, a coupling between phonon and crystal field transition has also been observed. In Part (G), we have examined the temperature dependence of phonons of “dynamical spin-ice” compound Pr2Sn2O7 and compared with its non-pyrochlore (monoclinic) counterpart Pr2Ti2O7. Our results conclude that the anomalous behavior of phonons is an intrinsic property of pyrochlore structure having inherent vacant sites. We also find a coupling between phonon and crystal-field transitions in Pr2Sn2O7. In the last part of this chapter, Part (H), we present our Raman studies of Er2Ti2O7. Here, we show that in addition to the anomalous phonons, there are modes that originate from photoluminescence transitions and some of these luminescence lines show anomalous temperature dependence which have been understood using the theory of optical dephasing in crystals, developed by Hsu and Skinner [J. Chem. Phys. 81, 1604 (1984)]. Temperature dependence of a few Raman modes and photoluminescence bands suggest a phase transition at 130 K. In Chapter 4, we furnish our pressure-dependent Raman studies of boron nitride multi-walled nanotubes (BNNT) and hexagonal boron nitride (h-BN) and compare the results with those of their carbon counterparts. Using Raman spectroscopy, we show that BNNT undergo an irreversible transition at ~ 12 GPa while the carbon counterpart, multi-walled carbon nanotubes, show a similar transition at a much higher pressure of ~ 51 GPa. In sharp contrast, the layered form of both the systems (i.e. h-BN and graphite) undergo a hexagonal to wurtzite phase at nearly similar pressure (~ 13 GPa of h-BN and ~ 15 GPa for graphite). A molecular dynamical simulation on boron nitride single-walled nanotubes has also been undertaken that suggests that the polar nature of the B−N bonds may be responsible for the irreversibility of the pressure-induced transformations. It is interesting to see that in hexagonal phase both the systems have almost similar mechanical property, but once they are rolled up to make nanotubes, the property becomes quite different. Chapter 5 presents the temperature dependence of the Raman modes of multiferroic thin films of BiFeO3 and Bi0.7Tb0.2La0.1O3. Though there have been several Raman investigations of BiFeO3 in literature, here we emphasize the observation of unusually intense second order Raman phonons. Our results have motivated Waghmare et al. to suggest a theoretical model to explain the anomalously large second order Raman tensor of BiFeO3 in terms of an incipient metal-insulator transition. In Chapter 6, we summarize our findings on the three different systems, namely, pyrochlores, boron nitride nanotubes, and BiFeO3 and highlight a few possible experiments that may be undertaken in future to have a better understanding of these systems.

L’objectif de cette thèse est de développer une nouvelle génération de matériaux céramiques de confinement plasma pour les moteurs à effet Hall en mettant en oeuvre la voie PDCs pour polymer-derived ceramics ou voie des polymères précéramiques.Un état de l’art des différents matériaux de confinement et un bilan des travaux précédemment menés sur ce sujet ont permis de déterminer les paramètres essentiels de tels matériaux et de s’orienter vers l’utilisation des polymères précéramiques commeprécurseurs des matériaux envisagés. Après avoir détaillé les différents protocoles permettant de modifier chimiquement un polymère commercial avec le bore, de mettre en forme les composés obtenus puis de réaliser la pyrolyse pour générer la céramique, des pièces denses céramiques Si-B-(C)-N ont été réalisées avec une teneur variable en bore. Une étude complète allant de la structure chimique des polymères jusqu’aux propriétés des céramiques résultantes a permis de sélectionner la formulation optimale du polymère comme précurseur de céramique. Des composites c-BN/Si-B-(C)-N ont ensuite été préparés parajout de charges, puis mis en forme et caractérisés avant de procéder à un changement d’échelle visant à préparer des bagues céramiques de taille moteur. La dernière partie consiste en une ouverture sur l’utilisation des polymères précéramiques pour la réalisation de pièces denses Si-Al-(C)-N de composition contrôlée avec en particulier une étude sur l’impact de l’aluminium sur les propriétés des polymères et des céramiques. / The main objective is here to develop a new generation of ceramic materials used for plasma confinement in Hall-effect thrusters using the PDCs (polymer-derived ceramics) route. A state of the art of the different confinement materials and a review of the previous work done on this topic allowed to determine the key parameters of such materials and to move towards the use of preceramic polymers as precursors of the materials envisaged. After having detailed the various protocols used to chemically modify a commercial polymer with boron, to shape the compounds obtained and then to convert the polymers into ceramics bypyrolysis, Si-B-(C)-N dense ceramic pieces have been produced with various boron content. A complete study from the chemical structure of the polymers to the properties of the resulting ceramics allowed selecting the optimal formulation of the polymer as a ceramic precursor. c-BN/Si-B-(C)-N composites were then prepared by filler addition, then shaped and characterized before a scale-up to prepare engine-size ceramic rings. The last part consists of an opening on the use of the PDCs route for the realization of dense Si-Al-(C)-N pieces of controlled composition, with a study of the impact of aluminum on the polymers andceramics properties.

The direct epitaxial growth of multilayer BN by atomic layer deposition is of critical significance forfo two-dimensional device applications. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) demonstrate layer-by-layer BN epitaxy on two different substrates. One substrate was a monolayer of RuO2(110) formed on a Ru(0001) substrate, the other was an atomically clean Ni(111) single crystal. Growth was accomplished atomic layer deposition (ALD) cycles of BCl3/NH3 at 600 K substrate temperature and subsequent annealing in ultrahigh vacuum (UHV). This yielded stoichiometric BN layers, and an average BN film thickness linearly proportional to the number of BCl3/NH3 cycles. The BN(0001)/RuO2(110) interface had negligible charge transfer or band bending as indicated by XPS and LEED data indicate a 30° rotation between the coincident BN and oxide lattices. The atomic layer epitaxy of BN on an oxide surface suggests new routes to the direct growth and integration of graphene and BN with industrially important substrates, including Si(100). XPS and LEED indicated epitaxial deposition of h-BN(0001) on the Ni(111) single crystal by ALD, and subsequent epitaxially aligned graphene was deposited by chemical vapor deposition (CVD) of ethylene at 1000 K. Direct multilayer, in situ growth of h-BN on magnetic substrates such as Ni is important for spintronic device applications. Solid-state electrolytes (SSEs) are of significant interest for their promise as lithium-ion conducting materials but are prone to degradation due to lithium carbonate formation on the surface upon exposure to atmosphere, adversely impacting Li ion conduction. In situ XPS monitored changes in the composition of the SSE Li garnet (Li6.5La3Zr1.5Ta0.5O12, LLZTaO) upon annealing in UHV and upon Ar+ ion sputtering. Trends in core level spectra demonstrate that binding energy (BE) calibration of the Li 1s at 56.4 eV, yields a more consistent interpretation of results than the more commonly used standard of the adventitious C 1s at 284.8 eV. Annealing one ambient-exposed sample to >1000 K in UHV effectively reduced surface carbonate and oxygen, leaving significant amounts of carbon in lower oxidation states. A second ambient-exposed sample was subjected to 3 keV Ar+ ion sputtering at 500 K in UHV, which eliminated all surface carbon, and reduced the O 1s intensity and BE. These methods present alternative approaches to lithium carbonate removal than heating or polishing in inert atmospheres and are compatible with fundamental surface science studies. In particular, the data show that sputtering at mildly elevated temperatures yields facile elimination of carbonate and other forms of surface carbon. This is in contrast to annealing in either UHV or in noble gas environments, which result in carbonate reduction, but with significant remnant coverages of other forms of carbon.

Diploma thesis is intended on a polycrystalic diamond and a pocrystalic cubic boron nitride from the standpoint of their description, physical, mechanical and cutting properties, use and contemporary trends of development at renowned producers of tools and tool materials. The aim of the work was a complex working of conditions (type of cutting material, cutting conditions), which are recommended by choice producers for the effective turning applications their kind of superhard tool materials.

The master’s thesis is aimed at superhard cutting materials (polycrystalline diamond and polycrystalline cubic boron nitride) and their effective utilization. It describes the characteristics and production of these materials and evaluates product ranges of the most significant producers of tools and tool materials in given area. It further compares working conditions of selected world's producers which are related to the growing utilization of these materials in machining process. The thesis also deals with the cutting power of tools and provides a technical-economic assessment of the discussed issue.