<div>For decades, the response of high-energy (HE) density materials at extreme conditions of pressure and temperature from strong insults like burning or impact have been studied in depth by the shock community. Shock physicists aim to develop a fundamental understanding for coupled chemical and physical processes across orders of magnitude spatial and temporal regimes. In order to succeed, this requires extensive collaboration between experiments and simulations, ranging from the electronic to the engineering scales. The end goals would be to develop predictive multiscale models capable of explaining ignition and initiation of HE systems and composites. The collected works in this thesis detail my contributions to the field of HE materials, specifically addressing the chemical reactivity at the atomistic level using reactive molecular dynamics (MD) simulations.</div><div><div>Through this endeavor, we aim to develop a critical understanding for the decomposition processes of HE materials. We begin with a validation the reactive force field, ReaxFF, by addressing the very strong anisotropic shock sensitivity in 2,2-Bis[(nitrooxy)methyl]propane-1,3-diyl dinitrate (PETN) through direct comparison of time-evolved spectra between experiments and simulations. Such strong orientation dependence is thought to relate to the initial decomposition events. Therefore we compare spectra at three different shock pressures, where we observe similar timescales for the disappearance of the NO2 symmetric and antisymmetric stretch modes. A more detailed chemical species analysis indicates that the NO2 molecular species could be considered the primary intermediate which initiates the decomposition process. Furthermore, these results suggest that the combination of explicit MD simulations and ultrafast spectroscopy will be key to the development of a detailed understanding of chemistry at extreme conditions.</div></div><div><div>Following the validation study, we further our understanding of reactivity in HE systems by investigating the differences in kinetics between an ordered and disordered system. It has been shown that shocked material is often severely strained, causing a loss in crystalline order. This in turn results in the disordered materials, such as amorphous solids, having</div><div>faster reactivity due to their higher internal energy and/or lower thermal conductivity. Our results indicate that extra energy is required to break the long-range order in bulk crystalline systems, thus resulting in slower decomposition rates. Further analyses of thermal hotspots point towards slightly faster chemical propagation in the amorphous samples due to lower thermal conductivity. These results provide an understanding for how molecular disorder can be attributed to increased reactivity.</div></div><div><div>After developing an understanding for the initial decomposition processes of HE materials, we turn our attention to a growing interest in the community which is the developing reduced order chemistry models for use in multiscale efforts. Many schemes report mechanisms that are obtained from experiments, which can have large error bars depending on the apparatus and/or extraction technique, or from gas phase simulations, which may not be relevant at shock conditions. To circumvent these issues, we develop a coarse-grained chemical kinetics model from all-atom reactive MD simulations by taking advantage of an unsupervised dimensionality reduction machine learning technique called non-negative matrix factorization. Doing so allows us to represent the overall decomposition chemistry as latent concentrations akin to reactants, intermediates, and products, which we then use to extract kinetics parameters and heats of reaction. These values are implemented into a continuum model, where we could simulate the criticality of thermal hotspots at regimes beyond the reach of MD, as well as verify how uncertainties in the parameters vary as a function of hotspot sizes.</div></div><div><div>Finally, we close with significant progress made towards on-going and future work, where we address two of the most challenging ideas in the field of HE materials: 1) developing definitive chemistry models at extreme conditions, and 2) improving coarse-grained descriptions for multiscale modeling.</div></div>

Dans le cadre de la fusion par confinement inertiel, une capsule contenant le combustible de deutérium-tritium est implosée soit par irradiation laser (attaque directe, interaction laser – cible de numéro atomique faible), soit par un rayonnement de corps noir émis par une cavité convertissant le rayonnement laser (attaque indirecte, interaction laser – cible de numéro atomique élevé).Dans les deux cas, une modélisation correcte du transport électronique est cruciale pour avoir des simulations hydro-radiatives prédictives. Cependant, il a été montré très tôt que les hypothèses d'un mécanisme de transport linéaire ne sont pas applicables dans le cadre de l'irradiation d'une cible solide par un laser de puissance (I~10^14 W/cm²). Cela est dû d'une part à des gradients de température très importants (effets cinétiques dits « non-locaux ») ainsi qu'à la présence d'un champ magnétique auto-généré par effet thermo-électrique. Enfin, le flux de chaleur et le champ magnétique sont fortement couplés au travers de deux mécanismes : le transport du champ magnétique par le flux de chaleur (effet Nernst) et la rotation et inhibition du flux de chaleur par la magnétisation du plasma (effet Righi-Leduc).Dans le présent manuscrit, nous commencerons par exposer les différents modèles de transport électronique, et en particulier le modèle non-local avec champ magnétique, implémenté dans le code hydro-radiatif FCI2. Par la suite, nous chercherons à valider ce modèle par des comparaisons avec un code cinétique, puis avec une expérience lors de laquelle le champ magnétique a été mesuré par radiographie proton. Cela fait, nous utiliserons le code FCI2 pour expliquer la source et le transport du champ, ainsi que son effet sur l'interaction.Enfin, nous étudierons la reconnexion du champ magnétique, lors de l'irradiation d'une cible par deux faisceaux lasers. / In the framework of the inertial confinement fusion, a pellet filled with the deuterium-tritium fuel is imploded, either through laser irradiation (direct drive, laser – low atomic number target interaction) or by the black body radiation from a cavity converting the laser radiation (indirect drive, laser – high atomic number target interaction).In both cases, a correct modeling of the electron transport is of first importance in order to have predictive hydro-radiative simulations. Nonetheless, it has been shown early on that the hypothesis of the linear transport are not valid in the framework of a solid target irradiated by a high power laser (I~1014 W/cm²). This is due in part to very steep temperature gradients (kinetic effects, so-called « non-local ») and because of a magnetic field self-generated through the thermo-electric effect. Finally, the heat flux and the magnetic field are strongly coupled through two mecanisms: the advection of the field with the heat flux (Nernst effect) and the rotation and inhibition of the heat flux by the plasma's magnetization (Righi-Leduc effect).In this manuscript, we will first present the various electron transport models, particularly the non-local with magnetic field model included in the hydro-radiative code FCI2. Following, in order to validate this model, we will compare it first against a kinetic code, and then with an experiment during which the magnetic field has been probed through proton radiography. Once the model validated, we will use FCI2 simulations to explain the source and transport of the field, as well as its effect on the interaction.Finally, the reconnection of the magnetic field, during the irradiation of a solid target by two laser beams, will be studied.

Tato práce je zaměřena na problematiku výpočtového a experimentálního modelování deformačně napjatostních stavů měkkých tkání se zaměřením na riziko ruptury u výdutě břišní aorty (AAA). V první části (kap. 1) je stručně nastíněn současný stav dané problematiky. Tato část shrnuje důležité poznatky publikované v dostupné literatuře. Pozornost je věnována zejména klíčovým faktorům pro stanovení rizika ruptury AAA. V další kapitole (kap. 2) je stručně popsána histologie cévní stěny a její výsledné mechanické chování, jakož i její patologie, především AAA. Druhá část práce (kap.3) je věnována experimentálnímu vyhodnocování deformačně napjatostního chování měkkých tkání, které je nutným předpokladem k věrohodnému výpočtovému modelování tohoto chování. V této kapitole je stručně popsáno experimentální zařízení speciálně vyvinuté pro testování měkkých tkání a typy zkoušek, které lze na tomto zařízení provádět. Dále jsou shrnuty klíčové faktory ovlivňující deformačně napjatostní chování měkkých tkání a experimentální ověření těchto faktorů na vzorcích z prasečích hrudních aort. V závěru této kapitoly jsou shrnuty nové poznatky vyplývající z experimentálního testování. Třetí část disertační práce (kap.4) je zaměřena na matematický popis deformačně napjatostního chování měkkých tkání, stručný popis používaných konstitutivních vztahu a postup při identifikaci parametrů pro tyto konstitutivní modely určované na základě provedených experimentálních zkoušek. Poslední část disertační práce (kap.5) je věnována výpočtovému modelování deformačně napjatostního chování AAA. V této kapitole jsou nejdříve shrnuty klíčové faktory a předpoklady pro vytváření modelů a pro vyhodnocování výsledku a dále jsou uvedeny materiálové parametry pro konstitutivní modely implementované do programu ANSYS. Byly provedeny testovací výpočty při použití hypotetické zjednodušené geometrie AAA, na kterých byly vyhodnoceny vlivy změny geometrie a vliv změny konsitutivního modelu na extrémní napětí ve stěně AAA. U reálné geometrie AAA byla navržena a otestována metoda výpočtu nezatížené geometrie z reálných CT snímků. Dále byl testován vliv zvýšení vnitřního tlaku jako rizika ruptury AAA. V závěru práce jsou shrnuty poznatky a možnosti výpočtového modelování a návrhy na další práce.

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Philosophiae Doctor - PhD (Chemistry) / Lithium ion capacitors are new and promising class of energy storage devices formed from a combination of lithium-ion battery electrode materials with those of supercapacitors. They exhibit better electrochemical properties in terms of energy and power densities than the above mentioned storage systems. In this work, lithium manganese oxide spinel (LiMn2O4; LMO) and lithium manganese phosphate (LiMnPO4; LMP) as well as their respective nickel-doped graphenised derivatives (G-LMNO and G-LMNP) were synthesized and each cathode material used to fabricate lithium ion capacitors in an electrochemical assembly that utilised activated carbon (AC) as the negative electrode and lithium sulphate electrolyte in a two-electrode system. The synthetic protocol for the preparation of the materials followed a simple solvothermal route with subsequent calcination at 500 - 800 ?C. The morphological, structural and electrochemical properties of the as prepared materials were thoroughly investigated through various characterisation techniques involving High resolution scanning electron microscopy (HRSEM), High resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD), Small-angle X-ray scattering (SAXS), Electrochemical impedance spectroscopy (EIS), Cyclic voltammetry (CV) and Galvanostatic charge/discharge.

Thèse en cotutelle (avec l'Université Toulouse 3 - Paul Sabatier) en Science des matériaux et Electrochimie / Avec le développement des systèmes électroniques embarqués se pose la question de la miniaturisation des dispositifs de stockage d’énergie. De nos jours, cette fonction est principalement assurée par des micro-batteries. Ces composants possèdent cependant une faible puissance disponible, une durée de vie limitée et un domaine de fonctionnement en température restreint. Les “micro-supercondensateurs” sur puce permettraient de s’affranchir de ces limitations, mais ils ne sont aujourd’hui qu’au stade de la recherche universitaire avec des densités d’énergie bien inférieures à celles des micro-batteries. L’énergie et la puissance stockées dans un supercondensateur sont proportionnelles au carré de la fenêtre de potentiel, qui dépend elle-même de la stabilité électrochimique de l’électrolyte utilisé. L’électrolyte joue ainsi un rôle prépondérant sur les propriétés des supercondensateurs (tension, gamme de température, courant de fuite, durée de vie…). Cette thèse vise à développer des liquides ioniques protiques et aprotiques dédiés aux micro-supercondensateurs pseudocapacitifs à base d'oxydes métalliques (RuO2, MnO2). Les électrolytes à base de liquides ioniques présentent des propriétés intéressantes, notamment une faible pression de vapeur saturante, une stabilité aux hautes températures, ainsi qu’une large fenêtre de potentiel. Ils contribuent ainsi à améliorer la densité d’énergie surfaciques, principal problème rencontré par les micro-supercondensateurs actuels. Les liquides ioniques étudiés ont été conçus sur la base de leurs structures et leurs propriétés physico-chimiques. Des caractérisations électrochimiques ont été réalisées avec des micro-supercondensateurs à base d’oxyde de ruthénium et d’oxyde de manganèse. De très bonnes performances ont été obtenus en utilisant des collecteurs de courant poreux à grande surface spécifique. Les électrolytes liquides constituant cependant un verrou technologique à la réalisation de micro-supercondensateurs fonctionnels compatible avec les procédés de microfabrication, des ionogels composés d’une matrice solide dans laquelle a été confinée le liquide ionique ont également été réalisés. / The rising growth of smart and autonomous microelectronic devices in the IoT (Internet of Things) era urges the development of advanced microscale energy sources with tailor-made features and customized energy/power requirements. Micro-supercapacitors (MSCs) emerged as potential energy storage devices complementing micro-batteries to power ubiquitous sensor networks needed to foster the development of IoT. However, the low cell voltage and low energy density remain major bottleneck that prevents their application at a large scale in real devices. To mitigate this issue, several studies have been devoted to the engineering of MSC electrode materials and structural architecting of current collectors to enhance the surface area and areal energy density by considering the limited available footprint area. This, however, has associated challenges such as a complex synthesis route, poor interfacial and mechanical stability of the electrode, and electrolyte compatibility issues, among others. Another key challenge to solve for reaching high energy density values in MSCs is the limited electrochemical stability window (ESW) of the electrolytes used as energy stored is directly related to the square of the cell voltage. The electrolytes play a major role in deciding the ESW and liquid-state electrolytes commonly used are troublesome for the microfabrication process due to leakage, evaporation, and safety issues. Therefore, it’s imperative to develop alternative electrolytes including solid-state electrolytes reconcilable to the target application of MSCs. This thesis aims at developing novel ionic-liquid (IL)-based electrolytes (both protic and aprotic) suitable for pseudocapacitive metal oxide (e.g., RuO2, MnO2)-based micro-supercapacitors (MSCs). IL-based electrolytes exhibit key properties including low vapor pressure, high temperature stability, low melting point, etc. with a wide ESW and help improve energy density performance, overcoming the major bottleneck faced by current MSCs. During this project, ILs are rationally designed based on their physicochemical properties. The detailed structure-property and electrochemical characterization studies were done using RuO2 and MnO2-based MSCs. We demonstrate state-of-the-art performance by developing high surface area porous current collectors with enhanced mass loading and solid-state devices using ionogel electrolytes, enabling their feasible integration with microelectronics to power connected IoT sensor networks.

Resolving the mismatch between the practical potential window (PPW) and the available capacitive potential window of supercapacitor electrodes provides a feasible way to expand the operating voltage of supercapacitors, which further boosts energy density. Here, our research unveils a unique approach to manually control the PPW of the corresponding carbon-based supercapacitors (CSCs) by rational functionalization with amino groups. The extra pair of electrons from amino N atoms naturally adsorbs cations in the electrolyte, which rationalizes the surface charge of the carbon electrode and adjusts the PPW. A remarkable voltage expansion is achieved for CSCs, from 1.4 V to its maximum limit, 1.8 V, correspondently resulting in an approximately 1-fold increase in the energy density. Importantly, such a simple strategy endows our CSCs with an outstanding maximum energy density of 7.7 mWh cm⁻³, which is not only among the best values reported for thin-film CSCs but also comparable to those reported for Li thin-film batteries. These encouraging results are believed to bring fundamental insights into the nature of potential control in energy storage devices.

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Graphite-based dual-ion batteries (GDIBs) represent a promising battery concept for large-scale energy storage on account of low cost, high working voltage, and sustainability. The electrolyte concentration plays a critical role in determining the energy density and cycle life of GDIBs. However, the concentrated electrolytes show low Lithium ions (Li+) transport kinetics, reducing their intercalation and solid electrolyte interface (SEI) formation abilities. Moreover, the GDIBs in the high cut-off voltage suffer from electrolyte degradation, and corrosion of the current collector. Herein, we report a highly concentrated electrolyte formulation based on hybrid lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI) salts with a super-wide electrochemical stability window (6 V) and the ability to form SEI and passivation layer on graphite anode and current collector, respectively. By regulating the concentrated LiFSI electrolyte with LiPF6 and solvent additive, the coulombic efficiency of the graphite cathode can be further improved to ∼98%. As a result, GDIB pouch cell exhibits a capacity of 21 mAh g−1 (cell level) at 50 mA g−1, and 98.2% capacity retention after 300 cycles. The resultant battery offers an energy density of 90.3 Wh kg−1, along with a high energy efficiency of 87% and average discharge voltage of 4.3 V.