Doctor of Philosophy / Department of Chemical Engineering / James H. Edgar / Hexagonal boron nitride (hBN) has recently been envisioned for electronic, optoelectronic, and nanophotonic applications due to its strong anisotropy and unique properties. To realize these applications, the ability to synthesize single crystals with large size and low defect density is required. Furthermore, a detailed mechanistic understanding of hBN growth process is helpful for understanding and optimizing the synthesis technique for high quality crystals. In this dissertation, the production of large-scale, high-quality hBN single crystals via precipitation from metal solvents, including Ni-Cr and Fe-Cr, was demonstrated. The use of Fe-Cr mixture provides a lower cost alternative to the more common Ni-Cr solvent for growing comparable crystals. The clear and colorless crystals have a maximum domain size of around 2 mm and a thickness of around 200 μm. Detailed characterizations demonstrated that the crystals produced are pure hBN phase, with low defect and residual impurity concentrations. The temperature-dependent optical response of excitons showed that the exciton-phonon interaction in bulk hBN is in the strong-coupling regime. A new growth method for monoisotopic hBN single crystals, i.e. h¹⁰BN and h¹¹BN, was developed, by which hBN single crystals were grown using a Ni-Cr solvent and pure boron and nitrogen sources at atmospheric pressure. The chemical bonding analysis revealed that the B-N bond in h¹¹BN is slightly stronger than that in h¹⁰BN. The polariton lifetime in our monoisotopic hBN samples increases threefold over the naturally abundant hBN, and the isotopic substitution changes the electron density distribution and the energy bandgap of hBN. The ability to produce crystals in this manner opens the door to isotopically engineering the properties and performance of hBN devices. Atomistic-scale insights into the growth of hBN were obtained from multiscale modeling combining density functional theory (DFT) and reactive molecular dynamics (rMD). The energetics and kinetics of BN species on Ni(111) and Ni(211) surfaces were calculated by DFT. These DFT calculations data were subsequently used to generate a classical description of the Ni-B and Ni-N pair interactions within the formulation of the reactive force field, i.e., ReaxFF. MD simulations under the newly developed potential helped reveal the elementary nucleation and growth process of an hBN monolayer - nucleation initiates from the growth of linear BN chains, which further evolve into branched and then hexagonal lattices. In the end, molecular dynamics simulations demonstrated that the thermodynamic preference of hBN geometries varying from triangle to hexagonal can be tuned by B to N molar ratios, and gas phase N₂ partial pressure, which is also supported by quantum mechanics calculations. The modeling confirms that the nitrogen species indeed plays an important role in dictating sizes and edge terminations of hBN sheets.

Master of Science / Department of Chemical Engineering / James H. Edgar / Hexagonal boron nitride has been used as an inert, refractory material with excellent resistance to thermal decomposition and oxidation for more than fifty years. In the past few years, hBN has been targeted for potential electrical and optical devices such as neutron detectors, ultraviolet light emitters, deep ultraviolet light detectors, and substrates for graphene and other atomically-thin two-dimensional materials. All of these potential applications benefit from high quality, single crystals, with thicknesses varying from nanometers to microns. This research was undertaken to investigate four aspects of hBN crystal growth and recovery. (1) In an effort to optimize hBN crystal growth from a nickel-chromium flux, a series of stepped cooling experiments were undertaken. The temperature profile was stepped in a way as to promote growth in both the a and c directions, at their optimal growth conditions. Crystals were found to be typically 100-500 µm across and thickness of approximately 20-30 µm with a pyramid-like crystal habit. (2) A method for the removal of hBN crystals prior to freezing of the metal flux was demonstrated using a specialized hot pressed boron nitride crucible capable of removing hBN crystals from the flux in situ. (3) Growth of isotopically pure hBN crystals was undertaken. By modifying the crucible material for solution growth, enrichment of hBN crystals over 90% was accomplished. (4) Exfoliation of hBN has many potential applications, specifically as graphene-hBN heterostructures where layers approaching thicknesses of single atoms are most effective surface to interact with graphene as an electronic device. Several methods were tested toward exfoliating a single crystal resulting in few-layered hexagonal boron nitride nanosheets. As a result of these investigations a greater understanding of hBN bulk growth, its isotopic enrichment, its recovery, and its exfoliation was obtained.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this work was performed a theoretical study on the electronic and magnetic properties of hexagonal ferrites LuFeO3 (h-LuFeO3). Was employed the Full Potential Linearized Augmented Plane Wave (FPLAPW) electronic structure calculation method, based on the density functional theory (DFT) and implemented in Elk code. The calculations were performed with experimental ferroelectric h-LuFeO3 structure. First, we simulated the collinear magnetic spin configurations as ferromagnetic (FM) and the antiferromagnetic of type A (A-AFM) and of type G (G-AFM). The electronic exchange and correlation effects was treated by LSDA approach and was accounted the spin-orbit coupling (SOC). The G-AFM state was found to be the ground state between later three collinear magnetic structures simulated (FM, A-AFM and G-AFM). This result is in agreement with that previously published. Based on this ground state magnetic structure, we analyzed which is the best + method (around mean field (AMF), fully-localized limit (FLL) and of the interpolation (INT)) to be applied to our system. The INT method was found as more appropriated to treat the double counting correction in + method for the h-LuFeO3 system. As a second step of calculation in this work were simulated the , , , e non-collinear magnetic structures that have been pointed in the literature as the ground state of h-LuFeO3. The electronic XC effects were treated by LSDA approach and was accounted the SOC. The latter was responsible for canting of the spin moment in magnetic structures that this feature is allowed by symmetry ( , , e ). The spin configuration should be the ground state by our DFT- LSDA+SOC calculations. The total and partial density of states revealed that Fe 3d and O 2p states are strongly hybridized in the top of valence band and bottom of the conduction band in h-LuFeO3 compound. Was not found significate difference between the electronic structure of G-AFM and states. This conclusion is result of higher degree of magnetic frustration of the studied system. Was verified that the band gap energy is dependent on spin configuration, besides of dependence with the electronic XC scheme employed. / Neste trabalho foi realizado um estudo teórico e computacional de propriedades eletrônicas e magnéticas da ferrita multiferróica hexagonal LuFeO3 (h-LuFeO3). Foi utilizado o método de cálculo de estrutura eletrônica denominado de Full Potential Linearized Augmented Plane Wave (FPLAPW) que é baseado na DFT e implementado no código computacional Elk. Os cálculos foram realizados com a estrutura ferroelétrica do h-LuFeO3. Primeiro, foram simuladas as estruturas magnéticas colineares do tipo ferromagnética (FM), antiferromagnética do tipo A (A-AFM) e do tipo G (G-AFM). Os efeitos de troca e correlação eletrônica foram tratados pela aproximação LSDA. A interação spin-órbita foi considerada. Os cálculos mostraram que a configuração de spin G-AFM é a de menor energia em relação às três colineares simuladas (FM, A-AFM e G-AFM). Este resultado está em concordância com aqueles previamente publicados. Diante desse fato, foram avaliados os diferentes métodos + (around mean field (AMF), fully localized limit (FLL) e o da interpolação (INT)) na base da configuração de spin do tipo G-AFM. Foi encontrado que o método INT é o mais apropriado para tratar as correções de dupla contagem em métodos + para o composto h-LuFeO3. Na segunda etapa dos cálculos deste trabalho, foram simuladas as estruturas magnéticas não colineares , , , e Essas são as possíveis candidatas a serem a do estado fundamental do h-LuFeO3, apontadas pela literatura experimental. Os efeitos de troca e correlação eletrônica foram tratados pela LSDA. Foi incluída a interação spin-órbita em todas as estruturas magnéticas simuladas. Verificou-se que essa interação é a responsável pelo canting dos momentos magnéticos nas estruturas ( , e ) que por simetria ele é permitido. Os resultados dos cálculos mostraram que é a configuração de spin de mais baixa energia. A análise das densidades de estados total e parcial revelaram que os estados 3d do Fe são hibridizados com os 2p dos O’s e predominam no topo da banda de valência e fundo da banda de condução do material. Não foi verificada nenhuma diferença significativa entre a densidade de estados no cálculo G-AFM e do tipo , resultado da frustação magnética do sistema. Verificou-se que existe dependência da energia de gap do material com a configuração magnética simulada, além da dependência com o funcional de troca e correlação eletrônico empregado. / São Cristóvão, SE

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this work, was theoretically studied the structural, electronic and magnetic properties of multiferroic hexagonal manganite LuMnO3. For this compound, was simulated the crystal structure with P63cm and P63 crystallographic space group (CSG). Was employed the formalism of Density Functional Theory and the Full Potential Linearized Augmented Plane Wave (FP-LAPW) method implemented in Elk code. The exchange and correlation electronic effects were simulated by mean of LSDA. To better describe these effects, was employed the +U method (LSDA+U). Test with different +U method was performed. First, for both P63cm and P63 CSG, three collinear magnetic structures (ferromagnetic (FM), antiferromagnetic of type A (A-AFM) and of type G (G-AFM) were simulated. According to our results, the P63cm CSG with G-AFM state was found the most energetic favorable. This result was independent of either LSDA or LSDA+U method applied. In the second stage of the calculations, 2,3,4 and 3+4 non-collinear magnetic structures were performed. For these simulations, was employed only the LSDA scheme. Again, the P63cm CSG was found as the ground state for LuMnO3. Between the non-collinear magnetic structure simulated, the 3+4 was the lowest energy. For this magnetic structure, the band gap energy was underestimated (0.7 eV) in relation to experimental value (1.1 eV). However, the calculated spin magnetic (3.1uB) moment agree with experimental value (3.3 uB). / Neste trabalho, foram estudadas teoricamente as propriedades estruturais, eletrônicas e magnéticas da manganita hexagonal multiferróica LuMnO3. Para esse composto, foram simuladas as estruturas cristalinas com grupos espaciais cristalográficos (GEC) P63cm e P63. Foi empregado o formalismo da Teoria do Funcional da Densidade utilizado o método de cálculo de estrutura eletrônica denominada de Full Potential Linearized Augmented Plane Wave (FP-LAPW) implementado no código Elk. Os efeitos de troca e correlação eletrônica foram simulados através da LSDA. Para melhorar a descrição desses efeitos, foi empregado o método +U (LSDA+U). Avaliação do método +U mais apropriado foi realizado. Primeiro, para os GEC P63cm e P63, foram simuladas três configurações magnéticas colineares: ferromagnética (FM), antiferromagnética do tipo A (A-AFM) e do tipo G (G-AFM). De acordo com os resultados, o grupo espacial P63cm com a configuração magnética G-AFM é a energeticamente favorável. Este resultado foi independente da aplicação dos métodos LSDA ou LSDA+U. Na segunda etapa dos cálculos, para os dois GEC, foram simuladas as configurações de spin não colineares do tipo 2,3,4 e 3+4. Nessas simulações foi empregado apenas o método LSDA. Novamente, obteve-se que o GEC P63cm é o estado fundamental para o composto. Entre as configurações magnéticas simuladas, a de menor energia foi a 3+4. A energia de band gap encontrada para essa configuração de spin é subestimada (0,7 eV) em relação ao valor experimental (1,1 eV). Porém, o momento magnético calculado (3,1uB) concordou bem com o valor experimental (3,3uB). / São Cristóvão, SE

Le fluotournage est un procédé de fabrication de tubes sans soudures par déformation plastique à froid. Il consiste à réduire l'épaisseur d'une préforme tubulaire montée sur un mandrin en utilisant plusieurs molettes en translation parallèlement au tube. Grâce à la conicité des molettes, l'épaisseur du tube diminue, donc celui-ci s'allonge dans la direction de fluotournage. Le TA6V est un alliage de titane de type α-β. Ses très bonnes propriétés mécaniques et sa faible densité en font l'alliage de titane le plus utilisé dans l'industrie aéronautique. Toutefois, le fluotournage d'un tel alliage représente un vrai défi, car il est connu pour avoir une faible ductilité à température ambiante. Afin d'optimiser les conditions opératoires, permettant ainsi de très grandes déformations du TA6V, le matériau fluotourné a d'abord été analysé, tant du point de vue microscopique (MEB, EBSD) que macroscopique (propriétés mécaniques). De plus, un fluotour expérimental instrumenté a été conçu afin d'analyser les mécanismes de déformation lors de la mise en forme. Celui-ci a permis d'identifier les phénomènes microstructuraux à l'origine de la déformation et de déterminer une loi de comportement valide pour la simulation numérique. Cette dernière a enfin servi à l'optimisation des conditions opératoires. / Flowforming is a means to produce seamless tubes by plastic deformation at room temperature. It consists in a tubular part, mounted on a mandrel, and a couple of rollers. While the mandrel is rotating, the roller translate along the tube axis. Because of the conical shape of the roller, the tube thickness reduces by cold plastic deformation, thus the tube elongates. Thanks to the high compressive stresses and to the incremental nature of the deformation process, flowforming can lead to a high thickness reduction and thus to high elongation of the deformed tubes. Ti-6Al-4V (also known as Ti64) is an α-β titanium alloy. Because of its high strength and its low density, Ti64 is the most widely used titanium alloy in aerospace industry. Nevertheless, flowforming of this alloy is a great challenge, considering its well known poor ductility at room temperature. In order to increase the "tube spinnability" of Ti64, first the flowformed material has been extensively investigated at microscopic scale (SEM, EBSD) as well as at macroscopic scale (mechanical properties). In addition, an experimental flowforming machine, equipped with load sensors, has been designed. It has provided valuable informations about the strain mechanism occuring during flowforming; furthermore, this experimental set-up has allowed to buid an adequate constitutive law for the plastic flow. Then, this law has been used to optimize the processing parameter, in order to avoid failure during the process.

The SRV test rig was used to evaluate the friction and wear properties of a lubricant in a laboratory setup. Normally, the coefficient of friction and the amount of wear that occurred are measured while the wear scar surface is also evaluated. Special attention was paid to factors that affect the repeatability. The test fluid was subjected to a friction and wear test on the SRV test rig in order to determine what factors affect the repeatability of the coefficient of friction, the amount of wear that occurred and the wear scar appearance. The test fluid used was based on rapeseed oil and white mineral oil. The fluid also contained an extreme pressure additive in the form of sulphurised ester. This was also compared for the same test fluid with dispersed hexagonal-boron nitride (h-BN) nanoparticles. The standard test method as described by ASTM D 6425, was used as test method. Instead of the standard temperature, the block temperature was increased to 100 °C in order to simulate harsher operating environments. The load was set at 200 N It was found that:  The rapid load increase from 50 to 200 N at the end of the running-in period (as described in the standard test method) caused poor repeatability. The test was modified with a more gradual load application for the duration of the running-in period (30 N/min), which resulted in improvement in the repeatability of the tests conducted.  The moisture content in the atmosphere also affected the repeatability of the friction and wear tests. This was most likely due to the formation of a corrosion layer that involves water and by keeping the relative humidity constant, a further improvement in the repeatability was observed. The addition of the h-BN nanoparticles resulted in an improvement of the repeatability of the coefficient of friction (COF), wear scar surface (WSS) and wear scar volume (WSV), since the wear scar surfaces indicated that the particles remove the corrosion layers. This could have led to more consistent wear surfaces for the duration of the test.  The particles also influenced the corrosion layer formation. For both fluids, Raman spectroscopy indicated that greigite (Fe3S4) and goethite (α-FeOOH) were found on the surface, while additional corrosion products were found on the wear scar surface for the test fluid with dispersed particles. These compounds were melanterite (FeSO4.7H2O) and rozenite (FeSO4.4H2O). All these corrosion products were most likely formed due to the reaction of iron from the specimens with sulphurised esters in the test fluid. / Dissertation (MEng)--University of Pretoria, 2015. / tm2015 / Chemical Engineering / MEng / Unrestricted

The addition of hexagonal boron nitride (h-BN) has introduced a comprehensive reinforcing effect to the mechanical and electrochemical properties of commercial concrete, including fiber reinforced concrete (FRC) and steel fiber reinforced concrete (SFRC). Although this has been proven effective and applicable, further investigation and study is still required to optimize the strengthen result which will involve the exfoliation of h-BN into single-layered nano sheet, improving the degree of dispersion and dispersion uniformity of h-BN into concrete matrix. There is currently no direct method to test the degree of dispersion of non-conductive particles, including h-BN, in concrete matrix, therefore it is necessary to obtain an analogous quantification method like SEM, etc. The reinforcing mechanism on concrete, including FRC and SFRC is now attracting a great number of interest thanks to the huge potential of application and vast demand across the world. This study briefly describes the reinforcing mechanism brought by h-BN. In this study, different samples under varied conditions were prepared according to the addition of h-BN and dispersant to build a parallel comparison. Characterization is mainly focused on their mechanical properties, corrosive performance and SEM analysis of the cross-section of post-failure samples.

Due to the multi-tasking and miniaturization of electronic devices, faster heat transfer is required from the device to avoid the thermal failure. Die-attached polymer adhesives are used to bond the chips in electronic packaging. These adhesives have to hold strong mechanical, thermal, dielectric, and moisture resistant properties. As polymers are insulators, heat conductive particles are inserted in it to enhance the thermal flow with an attention that there would be no electrical conductivity as well as no reduction in dielectric strength. This thesis focuses on the characterization of polymer nanocomposites for thermal and electrical properties with experimental and computational tools. Platelet geometry of hexagonal boron nitride offers highly anisotropic properties. Therefore, their alignment and degree of orientation offers tunable properties in polymer nanocomposites for thermal, electrical, and mechanical properties. This thesis intends to model the anisotropic behavior of thermal and dielectric properties using finite element and molecular dynamics simulations as well as experimental validation.

京都大学 / 0048 / 新制・課程博士 / 博士(医学) / 甲第21679号 / 医博第4485号 / 新制||医||1036(附属図書館) / 京都大学大学院医学研究科医学専攻 / (主査)教授 上本 伸二, 教授 戸井 雅和, 教授 上杉 志成 / 学位規則第4条第1項該当 / Doctor of Medical Science / Kyoto University / DFAM

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