Cette thèse de doctorat porte sur l'étude théorique de la conductivité thermique du réseau dans le bismuth semi-métallique et sur les stratégies pour réduire la conductivité thermique en vue d'applications pour réduire l'échauffement dans les circuits électroniques, et pour la thermoélectricité. J'ai utilisé des méthodes avancées de résolution de l'équation de transport de Boltzmann pour les phonons, et de calcul ab initio des éléments de matrice de l'interaction phonon-phonon. J'ai calculé la dépendance en température de la conductivité thermique du réseau dans le matériau en volume en excellent accord avec les rares expériences disponibles. J'ai obtenu une description très précise, à l'échelle microscopique, du transport de la chaleur et j'ai quantifié la contribution des porteurs de charge à la conductivité thermique totale. J'ai démontré que la nano-structuration et la photo-excitation sont des moyens très efficaces dans le bismuth de contrôler la diffusion des phonons qui portent la chaleur, respectivement par interaction avec les bords de l'échantillon, et par interaction phonon-phonon. En contrôlant l'équilibre entre ces deux derniers effets, j'ai prédit de façon exhaustive l'effet de réduction pour différentes températures et tailles de nanostructures, pour des mono et poly-cristaux, semi-conducteurs ou semi-métalliques. Enfin, j'ai étudié l'élargissement anharmonique des phonons acoustiques et optiques, et j'ai déterminé pour chacun les interactions majeures qui contribuent à l'élargissement. L'atténuation du son a été prédite dans le bismuth pour de futures expériences. L'approximation des grandes longueurs d'ondes [long-wave approximation (LWA)] a été validée pour le bismuth et ses limites ont été déterminées. / This work is devoted to the theoretical investigation of the heat conduction in bulk bismuth and the possible strategies for its reduction. Thermal properties of Bi are extremely interesting because of its low thermal conductivity that makes this material suitable for the thermal management applications. Moreover, bismuth is an excellent model substance for the study of thermoelectricity and bismuth-based compounds such as Bi2 Te3 and Bi2 Se3 which are typical thermoelectric materials used in industrial applications.In collaboration with L. Paulatto (IMPMC), G. Fugallo (Ecole Polytechnique), F. Mauri(IMPMC) and M. Lazzeri (IMPMC) I have applied the recently developed advanced methods of the solution of the Boltzmann transport equation (BTE) and of the phonon-phonon matrix elements calculation to describe thermal transport in bismuth. I have obtained the temperature dependence of the lattice thermal conductivity which is in excellent agreement with experiment. Moreover I am able to predict the lattice thermal conductivity (LTC) at temperatures at which it has not been measured. I have found that most of heat is carried by the acoustic phonons. However, the optical phonons were shown to play an important role by modulating the magnitude of the acoustic-optical phonon interaction (AOPI) and thus the value of the lattice thermal conductivity. Furthermore, I have shown that the available experimental data for the lattice thermal conductivity for polycrystalline thin-films are remarkably explained by my calculations, which enables me to predict the effect of the LTC size reduction for various temperatures and nanostructure shapes and sizes.The methods I use contain no empirical fitting parameters and give a direct insight into the microscopic mechanisms determining the transport and anharmonic properties of the materials. This allows me to analyze the anharmonic broadening that is inversely proportional to the phonon lifetime, for the various phonon modes along the high symmetry directions in the Brillouin zone and show what are the major scattering channels for coalescence/decays of phonons that govern the thermal transport in Bi.

Les propriétés de solvatation jouent un rôle important dans les problèmes chimiques et biochimiques. La théorie fonctionnelle de la densité moléculaire (MDFT) est l'une des méthodes frontières pour évaluer ces propriétés, dans laquelle une fonction d'énergie libre de solvatation est minimisée pour un soluté arbitraire dans une boîte de solvant cubique périodique. Dans cette thèse, nous travaillons sur l'évaluation du terme d'excès de la fonctionnelle d’énergie libre sous l’approximation du fluide de référence homogène (HRF), équivalent à l'approximation de la chaîne hypernettée (HNC) dans la théorie des équations intégrales. Deux algorithmes sont proposés: le premier est une extension d'un algorithme précédent, qui permet de traiter le cas d'un solvant moléculaire à trois dimensions (en fonction de trois angles d'Euler) au lieu d'un solvant linéaire (selon deux angles); L'autre est un nouvel algorithme qui intègre le traitement de la convolution angulaire de l'équation Ornstein-Zernike (OZ) moléculaire dans MDFT, et en fait développe la densité du solvant et le gradient fonctionnel en harmoniques sphériques généralisées (GSHs). On montre que le nouvel algorithme est beaucoup plus rapide que le précédent. Les deux algorithmes sont appropriés pour des solutés arbitraires tridimensionnel dans l'eau liquide, et pour prédire l'énergie libre et la structure de solvatation d'ions et de molécules. / Solvation properties play an important role in chemical and bio-chemical issues. The molecular density functional theory (MDFT) is one of the frontier numerical methods to evaluate these properties, in which the solvation free energy functional is minimized for an arbitrary solute in a periodic cubic solvent box. In this thesis, we work on the evaluation of the excess term of the free energy functional under the homogeneous reference fluid (HRF) approximation, which is equivalent to hypernetted-chain (HNC) approximation in integral equation theory. Two algorithms are proposed: the first one is an extension of a previously implemented algorithm, which makes it possible to handle full 3D molecular solvent (depending on three Euler angles) instead of linear solvent (depending on two angles); the other one is a new algorithm that integrates the molecular Ornstein-Zernike (OZ) equation treatment of angular convolution into MDFT, which in fact expands the solvent density and the functional gradient on generalized spherical harmonics (GSHs). It is shown that the new algorithm is much more rapid than the previous one. Both algorithms are suitable for arbitrary three-dimensional solute in liquid water, and are able to predict the solvation free energy and structure of ions and molecules.

abstract: The fuel cell is a promising device that converts the chemical energy directly into the electrical energy without combustion process. However, the slow reaction rate of the oxygen reduction reaction (ORR) necessitates the development of cathode catalysts for low-temperature fuel cells. After a thorough literature review in Chapter 1, the thesis is divided into three parts as given below in Chapters 2-4. Chapter 2 describes the study on the Pt and Pt-Me (Me: Co, Ni) alloy nanoparticles supported on the pyrolyzed zeolitic imidazolate framework (ZIF) towards ORR. The Co-ZIF and NiCo-ZIF were synthesized by the solvothermal method and then mixed with Pt precursor. After pyrolysis and acid leaching, the PtCo/NC and PtNiCo/NC were evaluated in proton exchange membrane fuel cells (PEMFC). The peak power density exhibited > 10% and 15% for PtCo/NC and PtNiCo/NC, respectively, compared to that with commercial Pt/C catalyst under identical test conditions. Chapter 3 is the investigation of the oxygen vacancy (OV) effect in a-MnO2 as a cathode catalyst for alkaline membrane fuel cells (AMFC). The a-MnO2 nanorods were synthesized by hydrothermal method and heated at 300, 400 and 500 ℃ in the air to introduce the OV. The 400 ℃ treated material showed the best ORR performance among all other samples due to more OV in pure a-MnO2 phase. The optimized AMFC electrode showed ~ 45 mW.cm-2, which was slightly lower than that with commercial Pt/C (~60 mW.cm-2). Chapter 4 is the density functional theory (DFT) study of the protonation effect and active sites towards ORR on a-MnO2 (211) plane. The theoretically optimized oxygen adsorption and hydroxyl ion desorption energies were ~ 1.55-1.95 eV and ~ 0.98-1.45 eV, respectively, by Nørskov et al.’s calculations. All the configurations showed oxygen adsorption and hydroxyl ion desorption energies were ranging from 0.27 to 1.76 eV and 1.59 to 15.0 eV, respectively. The site which was close to two Mn ions showed the best oxygen adsorption and hydroxyl ion desorption energies improvement with the surface protonation. Based on the results given in Chapters 1-4, the major findings are summarized in Chapter 5. / Dissertation/Thesis / Doctoral Dissertation Systems Engineering 2019

No description available.

No description available.

Molecular electronics is envisioned as a possible next step in device miniaturization. It is usually taken to mean the design and manufacturing of electronic devices and applications where organic molecules work as the fundamental functioning unit. It involves the easurement and manipulation of electronic response and transport in molecules attached to conducting leads. Organic molecules have the advantages over conventional solid state electronics of inherent small sizes, endless chemical diversity and ambient temperature low cost manufacturing. In this thesis we investigate the switching and conducting properties of photochromic dithienylethene derivatives. Such molecules change their conformation in solution when acted upon by light. Photochromic molecules are attractive candidates for use in molecular electronics because of the switching between different states with different conducting properties. The possibility of optically controlling the conductance of the molecule attached to leads may lead to new device implementations. The switching reaction is investigated with potential energy calculations for different values of the reaction coordinate between the closed and the open isomer. The electronic and atomic structure calculations are performed with density functional theory (DFT). It is concluded that there is a large potential energy barrier separating the open and closed isomer and that switching between open and closed forms must involve excited states. The conducting properties of the molecule inserted between gold leads is calculated within the Non Equilibrium Green Function theory. The transmission function is calculated for the two isomers with different basis sizes for the gold contacts, as well as the electrostatic potential, for finite applied bias voltages. We conclude that a Au 6s basis give qualitatively the same result as a Au spd basis close to the Fermi level. The transmission coefficient at the Fermi energy is around 10 times larger in the closed molecule compared to the open. This will result in a large difference in conductivity. It is also found that the large difference in conductivity will remain for small applied bias voltages. The results are consistent with earlier work. / QC 20101119

This thesis is part of an effort to enable large-scale Hartree-Fock/Kohn-Sham (HF/KS) calculations. The objective is to model molecules and materials containing thousands of atoms at the quantum mechanical level. HF/KS calculations are usually performed with the Self-Consistent Field (SCF) method. This method involves two computationally intensive steps. These steps are the construction of the Fock/Kohn-Sham potential matrix from a given electron density and the subsequent update of the electron density usually represented by the so-called density matrix. In this thesis the focus lies on the representation of potentials and electron density and on the density matrix construction step in the SCF method. Traditionally a diagonalization has been used for the construction of the density matrix. This diagonalization method is, however, not appropriate for large systems since the time complexity for this operation is σ(n3). Three types of alternative methods are described in this thesis; energy minimization, Chebyshev expansion, and density matrix purification. The efficiency of these methods relies on fast matrix-matrix multiplication. Since the occurring matrices become sparse when the separation between atoms exceeds some value, the matrix-matrix multiplication can be performed with complexity σ(n). A hierarchic sparse matrix data structure is proposed for the storage and manipulation of matrices. This data structure allows for easy development and implementation of algebraic matrix operations, particularly needed for the density matrix construction, but also for other parts of the SCF calculation. The thesis addresses also truncation of small elements to enforce sparsity, permutation and blocking of matrices, and furthermore calculation of the HOMO-LUMO gap and a few surrounding eigenpairs when density matrix purification is used instead of the traditional diagonalization method. / <p>QC 20101123</p>

This thesis describes an accurate and scalable computational method designed to perform nanoelectronic structure calculations. Built around the FEAST framework, this method directly addresses the nonlinear eigenvalue problem. The new approach allows us to bypass traditional approximation techniques typically used for first-principle calculations. As a result, this method is able to take advantage of standard muffin-tin type domain decomposition techniques without being hindered by their perceived limitations. In addition to increased accuracy, this method also has the potential to take advantage of parallel processing for increased scalability. The Introduction presents the motivation behind the proposed method and gives an overview of what will be presented for this thesis. Chapter 1 explains how electronic structure calculations are currently performed, including an overview of Density Functional Theory and the advantages and disadvantages of various numerical techniques. Chapter 2 describes, in detail, the method proposed for this thesis, including mathematical justification, a matrix-level example, and a description of implementing the FEAST algorithm. Chapter 3 presents and discusses results from numerical experiments for Hydrogen and various Hydrogen molecules, Methane, Ethane, and Benzene. Chapter 4 concludes with a summary of the presented work and its impact in the field.

TiP4N8 was obtained from the binary nitrides TiN and P3N5 upon addition of NH4F as a mineralizer at 8 GPa and 1400 °C. An intricate interplay of disorder and polymorphism was elucidated by in situ temperature- dependent single-crystal X-ray diffraction, STEMHAADF, and the investigation of annealed samples. This revealed two polymorphs, which consist of dense networks of PN4 tetrahedra (degree of condensation k= 0.5) and either augmented triangular TiN7 prisms or triangular TiN6 prisms for α- and β-TiP4N8, respectively. The structures of TiP4N8 exhibit body-centered tetragonal (bct) framework topology. DFT calculations confirm the measured band gaps of α- and β-TiP4N8 (1.6–1.8 eV) and predict the thermochemistry of the polymorphs in agreement with the experiments.

No description available.