京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19730号 / 工博第4185号 / 新制||工||1645(附属図書館) / 32766 / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 江口 浩一, 教授 安部 武志, 教授 陰山 洋 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Conversion material electrodes such as FeF2 possess the potential to deliver transformative improvements in lithium ion battery performance because they permit a reversible change of more than one Li-ion per 3d metal cation. They outperform current state of the art intercalation cathodes such as LiCoO2, which have volumetric and gravimetric energy densities that are intrinsically limited by single electron transfer. Current studies focus on composite electrodes that are formed by mixing with carbon (FeF2-C), wherein the carbon is expected to act as a binder to support the matrix and facilitate electronic conduction. These binders complicate the understanding of the electrode-electrolyte interface (SEI) passivation layer growth, of Li agglomeration, of ion and electron transport, and of the basic phase transformation processes under electrochemical cycling. This research uses thin-films as a model platform for obtaining basic understanding to the structural and chemical foundations of the phase conversion processes. Thin film cathodes are free of the binders used in nanocomposite structures and may potentially provide direct basic insight to the evolution of the SEI passivation layer, electron and ion transport, and the electrochemical behavior of true complex phases. The present work consisted of three main tasks (1) Development of optimized processes to deposit FeF2 and LiPON thin-films with the required phase purity and microstructure; (2) Understanding their electron and ion transport properties and; (3) Obtaining insight to the correlation between structure and capacity in thin-film microbatteries with FeF2 thin-film cathode and LiPON thin-film solid electrolyte. Optimized pulsed laser deposition (PLD) growth produced polycrystalline FeF2 films with excellent phase purity and P42/mnm crystallographic symmetry. A schematic band diagram was deduced using a combination of UPS, XPS and UV-Vis spectroscopies. Room temperature Hall measurements reveal that as-deposited FeF2 is n-type with an electron mobility of 0.33 cm2/V.s and a resistivity was 0.255 Ω.cm. The LiPON films were deposited by reactive sputtering in nitrogen, and the results indicate that the ionic conductivity is dependent on the amount of nitrogen incorporated into the film during processing. The highest ionic conductivity obtained was 1.431.9E-6 Scm-1 and corresponded to a chemical composition of Li1.9PO3.3N.21. FeF2/LiPON thin films microbatteries were assembled using a 2032 coin cell configuration and subjected to Galvanostatic cycling. HRTEM and EELS spectroscopy where performed across the FeF2/LiPON interface of samples cycled once 15 times in their lithiated and delithiated states to understand the relationship between microstructural evolution and capacity. The EELS measurements provided evidence of a three-phase conversion reaction over the first discharge described by FeF2 +2e-+2Li+↔Fe +LiF, and of incomplete reconversion back to FeF2 after the 1st cycle resulting in new Fe0 and LiF phases in delithiated samples. This incomplete conversion results in (a) a smaller phase fraction of FeF2 participating in the conversion process subsequently and (b) the formation of LiF which is resistive to both electron and ion transport. This results in the observed drastic drop in capacity after the1st cycle. More study to understand the reconversion reaction pathways is required to fully exploit the potential of FeF2 and other conversion materials as cathodes in Li ion batteries.

In this work, a mathematical model is reported and validated, which describes the performance of porous electrodes under low and high rates of discharge. This porous battery model can be used to provide researchers a better physical understanding relative to prior models of how cell morphology and materials affect performance due to improved accounting of how effective resistance change with morphology and materials. The increased understanding of cell resistances will enable improved design of cells for high-power applications, such as hybrid and plug-in-hybrid electric vehicles. It was found electronic and liquid-phase ionic transport resistances are strongly coupled to particle conductivity, size, and distribution of particle sizes. The accuracy of determining effective resistances was increased by accounting for how particle's size, volume fraction, and electronic conductivity affect electronic resistances and by more accurately determining how cell morphology influences effective liquid-phase transport resistances. These model additions are used to better understand the cause for decreased utilization of active materials for relatively highly loaded lithium-ion cathodes at high discharge rates. Lithium cobalt and ruthenium oxides were tested and modeled individually and together in mixed-oxide cathodes to understand how the superior material properties relative to each other can work together to reduce cell resistances while maximizing energy storage. It was found for lithium cobalt oxide, a material with low electronic conductivity, its low rate (1C) performance is dominated by local electronic resistances between particles. At high rates (5C or higher) diffusional resistance in the liquid electrolyte had the greatest influence on cell performance. It was found in the mixed-oxide system that the performance of lithium cobalt oxide was improved by decreasing its local electronic losses due to the addition of lithium ruthenium oxide, a highly conductive active material, which improved the number of electron pathways to lithium cobalt oxide thereby decreasing local electronic losses.

L’apparition des fissures macroscopiques peut manifestement se déclencher à plusieurs étapes du processus de la vie d’une cathode dans une cuve d’électrolyse. Diverses causes peuvent engendrer un tel surgissement. Au préchauffage électrique, par exemple, la répartition spatiale irrégulière des gradients thermiques peut conduire à la naissance d’une ou plusieurs fissures. L’objectif principal de ce travail se résume à la modélisation et la simulation de la propagation de fissures dans un contexte multiphysique, thermo-électro-mécanique. L’approche se base sur une philosophie de développement numérique monolithique, du couplage fortement non linéaire avec la prise en compte des échanges thermoélectriques à travers les lèvres de fissures, par la méthode des éléments finis étendus (XFEM). Ce projet a été dressé sous forme de trois sous-projets. En premier lieu, un nouveau modèle de la propagation de fissures en contexte thermomécanique dans un domaine fissuré a été développé. Une stratégie d’enrichissement géométrique par XFEM a été élaborée avec succès pour modéliser la propagation de fissures ainsi que pour atteindre le taux optimal de convergence espéré sur les réponses physiques et sur le calcul des facteurs d’intensité de contraintes. Trois benchmarks ont été examinés et validés à partir des résultats existants dans la littérature. Ensuite, un nouvel exemple de propagation de fissures et défauts multiples a été conçu. La comparaison des résultats obtenus montre la robustesse ainsi que la précision du développement numérique. En second lieu, une approche originale de la modélisation du couplage fort thermoélectrique, en présence d’une discontinuité matérielle forte statique et en tenant compte des échanges thermique et électrique à travers ses lèvres, a été développée. Tout d’abord, un modèle a été élaboré et validé dans le cas d’une fissure capacitive, où les propriétés du diélectrique, entre les lèvres de la fissure, jouent un rôle déterminant pour la quantification de son intensité d’échange. De même, un modèle a été confirmé pour le choix d’un milieu gazeux remplissant la région entre les deux lèvres, assurant l’aspect résistif d’échange thermique. Le modèle thermoélectrique a été mis en œuvre par XFEM, en implémentant la version améliorée, et en tenant compte du traitement des parasites numériques dus aux éléments de transition. Trois cas d’études ont été proposés, le premier a été appliqué pour valider la mise en œuvre numérique du modèle proposé via un patch test développé. Une bonne convergence et précision des résultats numériques ont été observées. Le second a porté sur le cas d’une fissure courbée, moins fréquente dans la littérature, qui peut être rencontré dans de nombreuses applications, et représente un défi numérique notable. Le troisième est une étude élaborée sur la cathode. L’analyse de l’impact de la présence des fissures sur l’efficacité énergétique a été soulevée aussi pour le cas de fissures multiples. En troisième lieu, une originalité numérique de la propagation de fissures en couplage thermo-électro-mécaniques (TEM) avec les échanges thermique et électrique entre les lèvres de la fissure a été exposée. La fissure n’est pas statique, cette fois-ci. L’aire générée par le déplacement des lèvres est prise en considération dans les expressions physiques des coefficients d’échanges thermique et électrique. Cela est transcrit par la quantification du saut de déplacement à travers les lèvres. Cet aspect rend le problème mécanique non linéaire, aussi, par le biais des échanges, et par conséquent le système global TEM est fortement non linéaire. Une approche originale du point de vue de la technicité d’intégration a été implémentée. Elle est fondée sur une technique de sous-triangulation barycentrique par une ’toile d’araignée’ pour les éléments de surface coupés par la fissure. Une autre technique basée sur l’intégration par ’bras’ de fissure réservée pour les éléments d’échanges thermique et électrique. Deux cas d’études essentiels ont été envisagés. Le premier est un exemple de propagation en mode mixte d’une fissure inclinée en TEM avec les échanges thermique et électrique en fonction du saut de déplacement. Le deuxième a été réservé pour un cas d’étude industrielle d’une cuve en opération, en contexte TEM. L’impact de la présence de la fissure sur les différentes réponses physiques est analysé. Comme constatation, en expansion thermoélectrique du bloc cathodique, la fissure a plus tendance à rejoindre la surface supérieure pour mettre, probablement, fin à la vie de la cathode. / The outbreak of visible cracks can be triggered at several stages in the life of a cathode block in an electrolysis cell. Various matters can prompt such an upsurge. Under electrical pre-heating, for example, the random spatial distribution of thermal gradients can lead to the rise of one or more cracks. The main objective of this work is to model and simulate the crack propagation in a multi-physical, thermo-electro-mechanical, context. The approach is based on a monolithic numerical development philosophy of a strongly nonlinear coupling, with the consideration of thermoelectric exchanges through the crack lips using the extended finite element method (XFEM). This project was sketched essentially on three sub-projects. Firstly, a new model of crack propagation in a thermomechanical fashion over a cracked domain has been developed. A geometrical enrichment strategy by XFEM has been successfully performed to model crack growth as well as to achieve the expected optimal convergence rate in physical responses and the computation of stress intensity factors. Three benchmarks were examined and validated from existing results in the literature. Then, anew example of the propagation of multiple cracks and multiple defects was designed. The comparison of the results obtained shows a good agreement with the reported works as well as the robustness and the accuracy of the numerical development. Secondly, an original approach to the modeling of full thermoelectric coupling, in the presence of a strong static material discontinuity and taking into account the thermal and electrical exchanges through its lips, has been elaborated. First, a model has been established and validated in the case of a capacitive crack, where the properties of the dielectric, between the inter-crack-lips, play a determining role for the ascertainment of its exchange intensity. Similarly, a model has been settled for the choice of a gaseous medium filling the gap between the two rims, ensuring the resistive heat exchange aspect. The thermoelectric model has been implemented via XFEM by performing the enhanced version and taking into account the treatment of numerical noise due to the blending elements. Three case studies were intended, the first one was employed to validate the numerical implementation of the stated model via a developed patch test. Good convergence and accuracy of numerical outcomes have been noted. The second focused on the case of a curved crack, less considered in the literature, which can be encountered in many applications, and represents a significant numerical challenge. The third is an elaborate study on the cathode. The analysis of the impact of cracks on energy efficiency was also raised for the case of multiple cracks. Thirdly, numerical originality of crack propagation in the context of the full thermo-electro-mechanical (TEM) coupling combined with thermal and electrical exchanges between the crack lips has been displayed. The area formed by the displacement of the two crack bank sis carried out in the physical expressions of the heat and electrical coefficients. The quantification of this gap is rendered by the resulting displacement jump across the lips. This aspect makes the mechanical problem nonlinear as well through exchanges, and therefore the global TEM system is strongly nonlinear. An original integration strategy, from a technical point of view, has been realized. It is based on a technique of barycentric sub-triangulation through a ’spider-web’ for the surface elements cut by the crack. Another procedure based on integration by crack ’arms’ reserved for heat and electrical exchanges elements. Two case studies have been examined. The first one is an example of the mixed-mode growth of a sloped crack in TEM context with thermal and electrical exchanges as a function of the displacement jump. The second was held for an industrial case of an electrolysis cell in operation and TEM circumstances. The impact of the presence of crack on several physical responses is analyzed. As an outcome, due to the thermoelectric expansion of the cathode block, the crack is expected to join the upper surface and lead to the end of the cathode’s life.

Ni-rich layered oxide materials have gained significant attention due to the ongoing advances and demands in energy storage. The energy revolution continues to catapult the need for improved battery materials, especially for applications in portable electronic devices and electric vehicles. Lithium batteries are at the frontier of energy storage. Due to geopolitical concerns, there is a growing need to understand the chemistries of Co-free, Ni-rich layered oxide materials which are cost-efficient and possess increased practical capacity. The challenge to studying this class of materials is their inherent electronic and structural fragility. The fragility of these materials is facilitated by a cooperation of metal cation migration, lattice oxygen loss, and undesirable oxide cathode-electrolyte interfacial reactions. Each of these phenomena contribute to complex electrolyte decomposition pathways and oxide cathode structural distortions. Structural instability leads to poor battery performance metrics including specific capacity fading and decreased Coulombic efficiency. Electrolyte decomposition occurs at the oxide cathode surface, but it can lead to bulk electronic and structural changes, chemomechanical breakdown, and irreversible phase transformations in the material. The work in this dissertation focuses on understanding some of the chemistries associated with degradation of representative Ni-rich layered oxides, specifically LiNiO2 (LNO) and LiNixMnyCozO2 (NMC) (where x+y+z =1) materials. Chapter 1 provides a comprehensive review of the interfacial chemistries of fragile, Ni-rich layered oxide materials with carbonate-based liquid electrolytes. These reactions are key in deducing mechanistic pathways that promote thermal runaway. Uncontrollable oxygen loss and electrolyte oxidation leads to catastrophic battery fires and explosions. The chapter highlights the material properties that become perturbed during high states-of-charge which complicate the materials chemistry associated with Ni-rich layered oxides. Lastly, a few strategies to mitigate undesired, structurally detrimental reactions at the Ni-rich layered oxide cathode surface are provided in Chapter 1. To obtain the technical data detailed in this dissertation, a variety of analytical methods are employed. Chapter 2 introduces the working principles of the X-ray techniques, electron microscopy, and other quantification methods. X-ray techniques including synchrotron X-ray absorption spectroscopy (XAS), and its components XANES and EXAFS are discussed. Other X-ray techniques, including X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) are additionally included. Electron microscopy techniques, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM) are provided. Quantification methods, such as gas chromatography – flame ionization detection (GC-FID) and other electrochemical testing methods are also described. Detailed experimental information obtained using the analytical methods is provided in the technical chapters. In understanding the chemistry of Ni-rich layered oxides, exploring surface reconstruction is key. Surface reconstruction, a phenomenon caused by a collaboration between Li/Ni cation intermixing and lattice oxygen loss, is one of the major explanations for structural degradation in Ni-rich layered oxide materials. Chapter 3 explores surface reconstruction and deduces a mechanism by which lattice oxygen is loss in LiNi0.6Mn0.2Co0.2O2 (NMC622). By exploiting Li+ intercalation chemistry, the work emulates various states-of-charge to explore how delithiation impacts small, organic molecule oxidation. Benzyl alcohol serves as a good probing molecule. It is similar to an oxidizable, nonaqueous electrolytic species that undergoes oxidation at the oxide cathode surface. Structure-reactivity trends are defined to correlate electronic and structural changes, lattice oxygen loss, and small molecule oxidation. After studying a proxy molecule, a practical system is required to grasp the complexity of the cathode-electrolyte interfacial reactions that promote Ni-rich layered oxide degradation. In Chapter 4, an electrolyte stirring experiment is described. Stirring experiments provide an accelerated testing method which helps to deduce the influences of chemical electrolyte decomposition on structural degradation of LiNiO2 (LNO). X-ray techniques are used to illustrate electronic perturbations and structural distortions in the material after probing with EC/DMC w/w 3:7 LiPF6. Additionally, this dissertation chapter features a novel voltage oscillation experiment that is employed to quantify Ni-rich oxide cathode degradation at the phase transition regions. LNO has three charging plateaus – H1  M, M  H2, and H2  H3. The latter two plateaus have been largely associated with irreversible structural fragility in Ni-rich layered oxides. Cation intermixing and oxygen loss are two phenomena that are largely associated with decreased Li+ intercalation kinetics and increased undesired side reactions. Although researchers debate the chemical phenomenon that occur at each of the phase transitions, most agree that the H2  H3 transition is highly influenced by irreversible lattice oxygen loss. This dissertation chapter describes the studies used to explore the electronic changes and structural distortions that accompany the voltage oscillation electrochemical testing. While Ni-rich layered oxides are largely employed as lithium battery cathodes, this class of material is unique in that it is a reducible and electronically tunable. Electronically modifiable metal oxide materials provide a unique platform to lend information to other applications, such as catalysis. There is much debate surrounding the role of metal oxides on metal nanocatalyst performance for catalytically reductive pathways. Chapter 5 discusses the method of employing LiNiO2 and other NMC materials as electronically tunable metal oxides to determine the role of the reducible metal oxide support on the gold (Au) nanocatalyst for p-nitrophenol reduction to p-aminophenol. By obtaining a continuum of nickel (Ni) oxidation states using delithiation strategies, structural-activity relationship trends are provided. Conversion rates for each of the delithiated materials was calculated using pseudo first-order kinetics. Lastly, a detailed discussion on metal oxide reducibility and its influences on key mechanistic factors, such as the induction period is included. Chapter 6 in this dissertation provides conclusions for the technical work provided. It bridges the works together and describes the overarching findings associated with the chemistries of Ni-rich layered oxide materials. This dissertation lays the foundation for future experimentation and innovation in understanding the surface chemistry of Ni-rich layered oxides. Chapter 7 provides future perspectives for each of the technical works included herein. Additionally, the final chapter includes insights toward the future of lithium batteries and other cathode chemistries. As the world navigates the energy revolution, it is important to provide global perspectives expected to catapult a sustainable future with batteries towards a greener world. / Doctor of Philosophy / Rechargeable lithium batteries have gained a significant surge of interest due to the ongoing demands for portable electronic devices, as well as the global trend towards electric vehicles to decrease the carbon footprint. Lithium batteries reside at the pinnacle of the energy transition. Layered oxide materials are typically employed as the cathode in Li-ion batteries. Ni-rich layered oxides have gained much interest due to their low cost and good charge/discharge capabilities. As consumers want increased charging rates and longer lifetimes, researchers struggle to optimize the balance between incorporating Ni-rich cathodes and increased safety concerns caused by cathode structural fragility. The lack of structural robustness is largely due to the surface reactivity of Ni-rich layered oxide materials. Bonding arrangements and electron transfer pathways intrinsic to this class of material increases the complexity in understanding the surface chemistry and the associated degradation pathways. Oxygen loss is the major cause of the safety issues in lithium batteries such as battery fires and explosions. To mitigate the safety concerns, it is imperative to understand the chemistries that promote organic, liquid electrolyte decomposition, electronic and structural changes, chemomechanical breakdown, and irreversible phase transformations. Each of these components leads to decreased battery performance. The work in this dissertation describes model and practical platforms to probe and understand the chemistries associated with battery performance degradation. A variety of analytical methods were utilized to determine overall structure-activity relationship trends and are highlighted in Chapter 2. Chapters 3-5 is technical research providing insight on Ni-rich layered oxide degradation pathways and behaviors. The work advances the understanding of battery surface chemistry which will lead to improved cathode design. As batteries continue to grow, it is important to know other applications that benefit from the unique chemistry of Ni-rich layered oxide materials. By exploiting the lithium battery cathode chemistry, this dissertation highlights a method to utilize these materials to understand the role of metal oxides on Au nanocatalysts. Conclusions to the findings in this dissertation are provided in Chapter 6. Future perspectives on the technical research provided herein this dissertation is included in Chapter 7. Additionally, Chapter 7 details future perspectives for lithium batteries and how they can facilitate the global transition toward a sustainable future.

Li-ion battery technologies have transformed the consumer electronics and electric vehicles landscape over the last few decades. Single-crystal cathode materials with controllable physical properties including size, morphology, and crystal facets can aid researchers in developing relationships between physical characteristics, chemical properties, and electrochemical performance. High-voltage LiNi_0.5Mn_1.5O₄ (LNMO) materials are desirable as cathodes due to their low cost, low toxicity, and high capacity and energy density making them promising to meet increasing consumer demands for battery materials. However, transition metal dissolution, interfacial instability, and capacity fading plague these materials when paired with graphite, limiting their commercial capability. Furthermore, variation in Ni/Mn ordering can lead to complex multiphase co-existence and changes in Mn oxidation state and electrochemical performance. These properties can be adjusted during synthesis using a facile and tunable molten salt synthesis method. This dissertation focuses on the investigation of chemical and structural heterogeneities of LNMO prepared under different synthetic conditions at different length scales. In Chapter 2, the influences of molten salt synthesis parameters on LNMO particle size, morphology, bulk uniformity, and performance are evaluated revealing the difficulty of reproducible cathode synthesis. We utilize the X-ray nanodiffraction technique throughout this work, which provides high-resolution structural information. We develop a method to measure and relate lattice strain to phase distribution at the tens of nanometers scale. In Chapter 3, mapping lattice distortions of LNMO particles with varying global Mn oxidation states reveals inherent structural defects and distortion heterogeneities. In Chapter 4, we examine lattice distortion evolution upon chemical delithiation, Mn dissolution behaviors, and evaluate the chemical delithiation method as a means to replicate electrochemical cycling conditions. We further investigate lattice distortion spatially via in situ nanodiffraction during battery cycling in Chapter 5, illustrating the capabilities of the measurement to provide practical understanding of cathode transformations. From intra-particle to electrode materials level, heterogeneities that arise in cathode materials can dictate performance properties and degradation mechanisms and are necessary for researchers to understand for the improvement of Li-ion battery systems. The development of the nanodiffraction measurements aids in our understanding of inherent and dynamic materials chemical and structural heterogeneities. / Doctor of Philosophy / The invention of rechargeable Li-ion batteries in the 1990s has undeniably revolutionized modern civilization. Cell phones, laptops, grid energy storage, and electric vehicles have become fundamental fixtures of the 21st century. As technologies improve and requirements for advanced renewable energy storage have increased, researchers have sought to design longer lasting, faster charging, and more lightweight batteries. Modifying and finding new positive electrode materials is one way to improve the capabilities of modern batteries as their properties are governed by fundamental chemistry. High-voltage LiNi_0.5Mn_1.5O₄ (LNMO) is one such material that can allow for fast charging and high energy storage capacity, but its commercialization is hindered by complex physical and chemical properties, which can limit its lifetime in batteries. Large, particles with well-defined shapes are desirable to improve the stability of the materials; however, understanding their defects and structural heterogeneities is vital to continued optimization and requires advanced characterization techniques. In this dissertation, we characterize the physical phases and chemical properties of LNMO samples prepared under different conditions resulting in different particle shapes, sizes, and chemical distributions. An advanced X-ray nanodiffraction technique is used to measure phase distributions within individual particles while lab-based analytical techniques and electrochemical testing can determine bulk properties and battery performance of materials. Overall, the aim of this work is to develop techniques to measure structural and chemical heterogeneities of cathode materials at different length scales and to understand how they influence properties and performance in batteries. This work provides valuable insights into the inherent and dynamic properties of high-voltage electrode materials useful to advance our understanding of how these materials fail and to aid researchers in creating design principles to develop stable, high-performing future generations of rechargeable batteries.

L’apparition des fissures macroscopiques peut manifestement se déclencher à plusieurs étapes du processus de la vie d’une cathode dans une cuve d’électrolyse. Diverses causes peuvent engendrer un tel surgissement. Au préchauffage électrique, par exemple, la répartition spatiale irrégulière des gradients thermiques peut conduire à la naissance d’une ou plusieurs fissures. L’objectif principal de ce travail se résume à la modélisation et la simulation de la propagation de fissures dans un contexte multiphysique, thermo-électro-mécanique. L’approche se base sur une philosophie de développement numérique monolithique, du couplage fortement non linéaire avec la prise en compte des échanges thermoélectriques à travers les lèvres de fissures, par la méthode des éléments finis étendus (XFEM). Ce projet a été dressé sous forme de trois sous-projets. En premier lieu, un nouveau modèle de la propagation de fissures en contexte thermomécanique dans un domaine fissuré a été développé. Une stratégie d’enrichissement géométrique par XFEM a été élaborée avec succès pour modéliser la propagation de fissures ainsi que pour atteindre le taux optimal de convergence espéré sur les réponses physiques et sur le calcul des facteurs d’intensité de contraintes. Trois benchmarks ont été examinés et validés à partir des résultats existants dans la littérature. Ensuite, un nouvel exemple de propagation de fissures et défauts multiples a été conçu. La comparaison des résultats obtenus montre la robustesse ainsi que la précision du développement numérique. En second lieu, une approche originale de la modélisation du couplage fort thermoélectrique, en présence d’une discontinuité matérielle forte statique et en tenant compte des échanges thermique et électrique à travers ses lèvres, a été développée. Tout d’abord, un modèle a été élaboré et validé dans le cas d’une fissure capacitive, où les propriétés du diélectrique, entre les lèvres de la fissure, jouent un rôle déterminant pour la quantification de son intensité d’échange. De même, un modèle a été confirmé pour le choix d’un milieu gazeux remplissant la région entre les deux lèvres, assurant l’aspect résistif d’échange thermique. Le modèle thermoélectrique a été mis en œuvre par XFEM, en implémentant la version améliorée, et en tenant compte du traitement des parasites numériques dus aux éléments de transition. Trois cas d’études ont été proposés, le premier a été appliqué pour valider la mise en œuvre numérique du modèle proposé via un patch test développé. Une bonne convergence et précision des résultats numériques ont été observées. Le second a porté sur le cas d’une fissure courbée, moins fréquente dans la littérature, qui peut être rencontré dans de nombreuses applications, et représente un défi numérique notable. Le troisième est une étude élaborée sur la cathode. L’analyse de l’impact de la présence des fissures sur l’efficacité énergétique a été soulevée aussi pour le cas de fissures multiples. En troisième lieu, une originalité numérique de la propagation de fissures en couplage thermo-électro-mécaniques (TEM) avec les échanges thermique et électrique entre les lèvres de la fissure a été exposée. La fissure n’est pas statique, cette fois-ci. L’aire générée par le déplacement des lèvres est prise en considération dans les expressions physiques des coefficients d’échanges thermique et électrique. Cela est transcrit par la quantification du saut de déplacement à travers les lèvres. Cet aspect rend le problème mécanique non linéaire, aussi, par le biais des échanges, et par conséquent le système global TEM est fortement non linéaire. Une approche originale du point de vue de la technicité d’intégration a été implémentée. Elle est fondée sur une technique de sous-triangulation barycentrique par une ’toile d’araignée’ pour les éléments de surface coupés par la fissure. Une autre technique basée sur l’intégration par ’bras’ de fissure réservée pour les éléments d’échanges thermique et électrique. Deux cas d’études essentiels ont été envisagés. Le premier est un exemple de propagation en mode mixte d’une fissure inclinée en TEM avec les échanges thermique et électrique en fonction du saut de déplacement. Le deuxième a été réservé pour un cas d’étude industrielle d’une cuve en opération, en contexte TEM. L’impact de la présence de la fissure sur les différentes réponses physiques est analysé. Comme constatation, en expansion thermoélectrique du bloc cathodique, la fissure a plus tendance à rejoindre la surface supérieure pour mettre, probablement, fin à la vie de la cathode. / The outbreak of visible cracks can be triggered at several stages in the life of a cathode block in an electrolysis cell. Various matters can prompt such an upsurge. Under electrical pre-heating, for example, the random spatial distribution of thermal gradients can lead to the rise of one or more cracks. The main objective of this work is to model and simulate the crack propagation in a multi-physical, thermo-electro-mechanical, context. The approach is based on a monolithic numerical development philosophy of a strongly nonlinear coupling, with the consideration of thermoelectric exchanges through the crack lips using the extended finite element method (XFEM). This project was sketched essentially on three sub-projects. Firstly, a new model of crack propagation in a thermomechanical fashion over a cracked domain has been developed. A geometrical enrichment strategy by XFEM has been successfully performed to model crack growth as well as to achieve the expected optimal convergence rate in physical responses and the computation of stress intensity factors. Three benchmarks were examined and validated from existing results in the literature. Then, anew example of the propagation of multiple cracks and multiple defects was designed. The comparison of the results obtained shows a good agreement with the reported works as well as the robustness and the accuracy of the numerical development. Secondly, an original approach to the modeling of full thermoelectric coupling, in the presence of a strong static material discontinuity and taking into account the thermal and electrical exchanges through its lips, has been elaborated. First, a model has been established and validated in the case of a capacitive crack, where the properties of the dielectric, between the inter-crack-lips, play a determining role for the ascertainment of its exchange intensity. Similarly, a model has been settled for the choice of a gaseous medium filling the gap between the two rims, ensuring the resistive heat exchange aspect. The thermoelectric model has been implemented via XFEM by performing the enhanced version and taking into account the treatment of numerical noise due to the blending elements. Three case studies were intended, the first one was employed to validate the numerical implementation of the stated model via a developed patch test. Good convergence and accuracy of numerical outcomes have been noted. The second focused on the case of a curved crack, less considered in the literature, which can be encountered in many applications, and represents a significant numerical challenge. The third is an elaborate study on the cathode. The analysis of the impact of cracks on energy efficiency was also raised for the case of multiple cracks. Thirdly, numerical originality of crack propagation in the context of the full thermo-electro-mechanical (TEM) coupling combined with thermal and electrical exchanges between the crack lips has been displayed. The area formed by the displacement of the two crack bank sis carried out in the physical expressions of the heat and electrical coefficients. The quantification of this gap is rendered by the resulting displacement jump across the lips. This aspect makes the mechanical problem nonlinear as well through exchanges, and therefore the global TEM system is strongly nonlinear. An original integration strategy, from a technical point of view, has been realized. It is based on a technique of barycentric sub-triangulation through a ’spider-web’ for the surface elements cut by the crack. Another procedure based on integration by crack ’arms’ reserved for heat and electrical exchanges elements. Two case studies have been examined. The first one is an example of the mixed-mode growth of a sloped crack in TEM context with thermal and electrical exchanges as a function of the displacement jump. The second was held for an industrial case of an electrolysis cell in operation and TEM circumstances. The impact of the presence of crack on several physical responses is analyzed. As an outcome, due to the thermoelectric expansion of the cathode block, the crack is expected to join the upper surface and lead to the end of the cathode’s life.

Ce projet de travail est divisé en deux études principales: (a) l'influence des certains additifs organiques sur la consommation d'énergie et la pureté du métal de zinc déposé dans le processus d'extraction électrolytique, et (b) l’électrodéposition des alliages binaires et ternaires de Fe-Mo et Fe-Mo-P sur des substrats d’acier doux afin d’agir comme cathodes pour la production de chlorate. (a) Parmi les sept différents additifs organiques examinés, les sels des liquides ioniques ont réussi à augmenter le rendement du courant jusqu'à 95,1% comparé à 88,7% qui a obtenu à partir de l'électrolyte standard en présence des ions de Sb3+. La réduction maximale de la consommation d'énergie de ~173 kWh tonne-1 a été obtenue en ajoutant de 3 mg dm-3 du chlorure de 1-butyl-3-méthylimidazolium dans le même électrolyte. La teneur en plomb dans le dépôt de zinc est réduite de 26,5 ppm à 5,1-5,6 ppm en utilisant les sels des liquides ioniques. (b) Des différents binaires Fe-Mo et ternaires Fe-Mo-P alliages ont été électrodéposés sur des substrats d’acier doux. Les alliages préparés ont une tenure en Mo entre 21-47 at.% et une tenure en P de 0 à 16 at.%. L'activité électrocatalytique de ces alliages vers la réaction de dégagement d'hydrogène (RDH) a été étudiée dans des solutions de chlorure de sodium. La réduction maximale de la surtension de RDH de ~313 mV a été obtenue par l’alliage ternaire préparé Fe54Mo30P16 par rapport à celle obtenue pour l'acier doux. La rugosité de surface et l'activité intrinsèque des revêtements de Fe-Mo-P peuvent être l'origine du comportement prometteur de ces électrocatalyseurs vers la RDH. / This work project is divided into two main studies: (a) the influence of certain organic additives on the power consumption and the purity of deposited zinc during electrowinning process, and (b) the electrodeposition of binary and ternary alloys of Fe-Mo and Fe-Mo-P on mild steel substrates to act as cathodes for chlorate production. (a) Among seven different examined organic additives, the ionic liquid salts succeeded to increase the current efficiency up to 95.1% compared to 88.7% obtained from standard electrolyte in presence of Sb3+ ions. Maximum reduction of power consumption of ~173 kWh ton-1 was observed by addition of 3 mg dm-3 of 1-butyl-3-methylimidazolium chloride to the same electrolyte. Lead content in the zinc deposit is reduced from 26.5 ppm to 5.1-5.6 ppm by using the ionic liquid salts. (b) Different binary Fe-Mo and ternary Fe-Mo-P alloys have been electrodeposited on mild steel substrates. The prepared alloys have Mo content between 21-47 at.% and P content from 0 to 16 at.%. The electrocatalytic activity of these alloys towards the hydrogen evolution reaction (HER) was investigated in sodium chloride solutions. The maximum reduction of HER overpotential of ~313 mV was achieved from the prepared ternary alloy Fe54Mo30P16 compared to that obtained from mild steel. The surface roughness and intrinsic activity of Fe-Mo-P coatings could be the origin of the promising behavior of these electrocatalysts towards the HER.

Microwave tubes are the choice of a wide range of high power and millimeter wave applications in radar, electronic warfare and communication systems. Advances in these devices are due to device innovation, improved modeling, and development of advanced materials. In a microwave tube, electron emitter is the source of electron beam and it is one of the vital components determining the life & performance of the device. High power, high frequency microwave tubes require electron emitters with high emission density. The present thesis aims at developing the materials for high emission density electron emitters. It is aimed to improve the emission density of thermionic cathodes for use in conventional microwave tubes and to develop cold emitters like ferroelectric cathodes for high power microwave devices. The work reported in the present thesis is a modest attempt of the author towards this aim. The thesis is organized in six chapters. Chapter 1 gives a brief introduction of thermionic and ferroelectric emitters. Different types of electron emission mechanisms and a brief background of thermionic and ferroelectricemitters are discussed in this chapter. The genesis of the problem taken up and its importance as well as the plan and scope of the work is also given in this chapter. In Chapter 2, the basic experimental techniques used in the present work are discussed. Preparation of mixed metal matrix and M- type dispenser cathodes and their characterization techniques has been discussed in this chapter. Subsequently, ferroelectric materials preparation and characterization for their material properties and electron emission has been discussed. A brief introduction to FEM software ANSYS, used for thermal analysis of dispenser cathodes and electrostatic field analysis of ferroelectric cathodes, has been given at the end of this chapter. Thermal analysis, development process, emission characterization, work function distribution, of W-Ir mixed metal matrix (MM type) cathodes and a simple innovative technique to estimate the barium evaporation rate from the emission data of the dispenser cathodes is presented in Chapter 3. Under normal microwave tube operating conditions, the cathode of the electron gun has to be heated up to 1050°C to obtain stable thermionic electron emission. Thermal analysis is a first step in the development process of cathodes, optimizing its structure for improved performance with respect to its operating power, warm-up time and efficiency. Thermal analysis of a dispenser cathode in electron gun environment using the FEM software ANSYS and its experimental validation are presented. Development of porous W-Ir mixed metal matrix material required for dispenser cathode applications has been discussed. Determination of pore size, pore density and pore uniformity has been carried out. The performance of the cathodes made with these pellets is at par with the results reported in the literature. The surface of mixed metal pellet is an inherently two-phase structure consisting of tungsten solid solution phase and W-Ir ε phase causing more spread in the spatial distribution of work function. W-Ir mixed metal matrix cathodes have been realized and their work function distribution has been determined form the measured I-V characteristics. Also in this chapter, a novel technique for estimation of barium evaporation rate for dispenser cathodes from their I-V characteristics is presented. Results of life test carried out on these cathodes are given at the end of the chapter. In Chapter 4, work carried out on enhancing the emission properties of mixed metal matrix cathodes by suitably modifying the impregnant mix is discussed. W-Ir MM type cathodes discussed in the previous chapter give a emission current density of ~ 7.5A/cm2 with a work function of 1.99 eV. Thesevalues are very close to that of B-type cathode. In this chapter, it is explored to suitably dope the 5BaO:3CaO:2Al2O3 impregnant mix to reduce the work function of W-Ir cathodes. Lithium and Scandium oxides have been added to the 5:3:2 imp regnant mix. Lithium oxide doped impregnated MM type cathodes have given more than 30 A/cm2 current density at 1050oC. For scandium oxide doped MM type cathodes current density has increased to 15 A/cm2 at the same temperature. In Chapter 5, Electron emission from the ferroelectric cathodes has been discussed. FEM simulation of Ferroelectric cathodes to study the electrical excitation effects on emission. Triple point electric field in FE Cathodes is very large and can lead to field emission from the metallic grid at triple points. FEM simulation has been carried out to find out the effect of grid thickness on triple junction electric field using ANSYS software. From FEM modeling it is also seen that if a dielectric layer of lower dielectric constant (εr≤10) is placed between the grid and the ferroelectric material the triple junction electric field increases three fold. Use of dielectric layer can also reduce the secondary electron coefficient (δ) and surface plasma generation. Lanthanum doped PZT has been chosen for the study and these materials have been tested in diode configuration for emission characterization in demountable vacuum systems. Repeatable electron emission has been achieved for all the three compositions of PLZT (x/65/35) material (x = 7, 8, 9). However, it has been observed that when the ferroelectric is subjected to repetitive unipolar electrical excitation, fatigue is set in and cathode material is cracking. To study the effect of domain switching on the residual stress in the ferroelectric material, XRD studies have been carried out. Shift in XRD peaks for fresh and emission tested samples has been used to calculate the residual stress developed in the samples. Details of High current switch realized using ferroelectric cathodes have been discussed. Chapter 6 gives the Summary of the work done and suggestions for further research on W-Ir mixed metal matrix cathodes and ferroelectric cathodes.

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