Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第16854号 / 工博第3575号 / 新制||工||1540(附属図書館) / 29529 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 邑瀬 邦明, 教授 中村 裕之 / 学位規則第4条第1項該当

New layered cathode materials, Li₀.₉Ni₀.₄₅Ti₀.₅₅O₂, were synthesized by means of ion-exchange from Na₀.₉Ni₀.₄₅Ti₀.₅₅O₂. The degree of cation disordering in the material depends critically on the synthesis conditions. Longer times and higher temperatures in the ion-exchange process induced more cation disordering. However, the partially disordered phase showed better capacity retention than the least disordered phase. First principles calculations indicated this could be attributed to the migration of Ti⁺⁴ into the Li layer during the electrochemical testing, which seems to depend sensitively on the Ni⁺² -Ti⁺⁴ configuration in the transition metal layer. The poor conductivity of this material could also be the reason for its low specific capacity according to the Density of States (DOS) obtained from first principles calculations indicating that only Ni participates in the electronic conductivity. / Singapore-MIT Alliance (SMA)

No description available.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第18983号 / 工博第4025号 / 新制||工||1620 / 31934 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 宇田 哲也, 教授 白井 泰治 / 学位規則第4条第1項該当

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18983号 / 工博第4025号 / 新制||工||1620(附属図書館) / 31934 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 宇田 哲也, 教授 白井 泰治 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

No description available.

Unlike the binary compound, where the simple charge transfer between cation and anion, or the ternary compound, which is composed of two binary compound semiconductors with a common cation or anion and whose electronic structures usually can be derived from those of the two constituent binary compounds with some modifications, the electronic property of quaternary compound is quiet complicated and interesting because of its complex charge transfer due to the electronegativity differences of its composed atoms. In this thesis, the first-principles pseudofunction (PSF) method and the first-principles molecular dynamics (MD) method are used to investigate the complicated variations of the electronic properties of three kinds of quaternary compounds, namely titanates [Ba1-xSrxTiO3 (BSTO) and Pb1-xSrxTiO3 (PSTO)], manganites [La1-xSrxMnO3 (LSMO) and La1-xCaxMnO3 (LCMO)], and (SiC)1-x(AlN)x. First, for BSTO and PSTO titanates, the first-principles calculation results and O K-edge x-ray absorption near edge structure (XANES) measurements are used to study their electronic structures. Because the valence band maximum (VBM) and conduction band minimum (CBM) are composed of O-p and Ti-d partial densities of states (PDOS), respectively, the bowing upward of calculated band gaps are related to the bowing downward of the Ti-O bond lengths for both of BSTO and PSTO, though for PSTO Pb-p PDOS also contributes to states near CBM. The substitutions of Sr by Ba in BSTO and by Pb in PSTO are quiet different, and it is because Pb atom has two extra valence electrons and a larger electronegativity than other cations. Second, we provide a new interpretation of the insulator-like to metal-like and anti-ferromagnetic to ferromagnetic transitions with Sr and Ca doping concentrations of La1-xSrxMnO3 and La1-xCaxMnO3, which is based on the variations of the Sr and Ca induced delocalization of the Mn majority-spin eg subband and the lowering of the Mn minority-spin t2g subband down to the Fermi energy (EF). Moreover, this study also suggests that the magnetic properties of manganites result from a detailed balancing between the O-mediated super-exchange mechanism that favors anti-ferromagnetism and the delocalized-state mediated Mn-spin coupling that favors ferromagnetism. Third, for (SiC)1-x(AlN)x superlattice, where SiC and AlN layers arranged alternatively along a common c-axis, our analysis shows subtle charge transfer among Si, C, Al and N ions and the band gap is not linear but bows downwards with respect to x. The calculated results suggest that the direct band gap of (SiC)1-x(AlN)x can be tuned over a wide range from 2.97eV to 6.28eV. Thus, (SiC)1-x(AlN)x is potentially useful for optoelectronic applications. It can be inferred from the calculated electronic properties of the above three kinds of quaternary compounds, the subtle charge transfer is because of differing electronegativities of constituent atoms, especially cations, and the relative valence state of the dopant with respect to the host cation. The subtle charge transfer also influences the magnetic properties of these hole-doped manganites. Moreover, the quaternary compounds have four kinds of atoms with different electronegativities and relative orbital energies, the complicated competition and balancing between the occupation of orbitals and charge transfer render the electronic properties of these material unable to be predicted from constituent binary oxides/semiconductors or even ternary compounds. For example, even though Pb substitutes Sr only in Pb1-xSrxTiO3, the effective charges of Ti and O are significantly altered.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This work presents a theoretical study about the interaction between semiconducting carbon nanotubes (8,0) single-wall (SWNT), with selenium nanostructures by first principles calculations using the density functional theory. To simulate the adsorption of selenium atoms on SWNTs are done with three initial configurations with the selenium atoms in positions (i) BC (C-C bond); (ii) HC (hexagonal center site) and (iii) TOP (above the carbon atom). For these configurations we analyze the adsorption of atomic, dimmer and wire selenium on SWNTs and for some structures it was observed a physical adsorption (weak interaction) while for others a chemical adsorption (strong interaction) is observed depending on how these structures were functionalized. In the configuration on which a physical interaction is achieved, no considerable changes on the electronics properties are seen, as a consequence of the absence of hybridization between the SWNTs and selenium orbitals. Never the less, in the arrangements with chemical interactions significant alterations are verified on the electronic properties, in other words, a relevant hybridization of the selenium levels and the tube band near to the Fermi energy is noticed. However, analyzing the structural properties, we also observed changes on the final distances of selenium atoms, dimmer and wires when compared with the initials distances. In that way, experimental literature works had demonstrated the importance of a theoretical understanding of structures and electronics properties of selenium interacting with SWNTs, making relevant the connection between the theoretical results and the experiments. / Nesta dissertação apresentamos o estudo teórico da interação de nanotubos de carbono semicondutores (8,0) de paredes simples (SWNT) com nanoestruturas de selênio utilizando cálculos de primeiros princípios fazendo uso da teoria do funcional da densidade. Para a adsorção dos átomos de selênio nos SWNTs escolhemos três possíveis configurações iniciais com os átomos de selênio nas posições (i) BC (centro da ligação); (ii) HC (centro do hexágono) e (iii) TOP (em cima do átomo de carbono). Para essas configurações analisamos a adsorção de selênio atômico, dímeros e fios nos nanotubos, sendo que em algumas estruturas tivemos uma adsorção física (interação fraca), enquanto que em outras tivemos uma adsorção química (interação forte), dependendo de como essas estruturas foram funcionalizadas. Nas configurações que tiveram interações físicas, não observamos mudanças significativas nas propriedades eletrônicas não ocorrendo hibridização entre os orbitais do nanotubo e dos átomos de selênio. Entretanto, nas configurações que tiveram interações químicas observamos alterações significativas nas propriedades eletrônicas, ou seja, nas bandas de energia percebemos uma hibridização relevante dos níveis de selênio e do tubo próximos ao nível de Fermi. Entretanto, na análise das propriedades estruturais, também observamos mudanças nas distâncias finais dos selênios atômicos, dímeros e fios quando comparadas com as distâncias iniciais considerando o selênio mais próximo ao carbono do SWNT. Dessa forma, trabalhos experimentais da literatura têm demonstrado a importância de um entendimento teórico das propriedades estruturais e eletrônicas de Se interagindo com SWNTs, o que nos leva a querer conectar resultados teóricos com experimentais.

No description available.

High-entropy alloys (HEAs) represent a special group of solid solutions containing five or more principal elements. The new design strategy has attracted extensive attention from the materials science community. The design and development of HEAs with desired properties have become an important subject in materials science and technology. For understanding the basic properties of HEAs, here we investigate the magnetic properties, Curie temperatures, electronic structures, phase stabilities, and elastic properties of paramagnetic (PM) body-centered cubic (bcc) and face-centered cubic (fcc) AlxCrMnFeCoNi (0 ≤ x ≤ 5, in molar fraction) HEAs using the first-principles exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) for dealing with the chemical and magnetic disorder. Whenever possible, we compare the theoretical predictions to the available experimental data in order to verify our methodology. In addition, we make use of the previous theoretical investigations carried out on AlxCrFeCoNi HEAs to reveal and understand the role of Mn in the present HEAs. The theoretical lattice constants are found to increase with increasing x, which is in good agreement with the available experimental data. The magnetic transition temperature for the bcc structure strongly decreases with x, whereas that for the fcc structure shows a weak composition dependence. Within their own stability fields, both structures are predicted to be PM at ambient conditions. Upon Al addition, the crystal structure changes from fcc to bcc with a broad two-phase field region, in line with the observations. Bain path calculations suggest that within the duplex region both phases are dynamically stable. Comparison with available experimental data demonstrates that the employed approach describes accurately the elastic moduli of the present HEAs. The elastic parameters exhibit complex composition dependences, although the predicted lattice constants increase monotonously with Al addition. The elastic anisotropy is unusually high for both phases. The brittle/ductile transitions formulated in terms of Cauchy pressure and Pugh ratio become consistent only when the strong elastic anisotropy is accounted for. The negative Cauchy pressure of CrMnFeCoNi is found to be due to the relatively low bulk modulus and C12 elastic constant, which in turn are consistent with the relatively low cohesive energy. Our findings in combination with the experimental data suggest anomalous metallic character for the present HEAs system. The work and results presented in this thesis give a good background to go further and study the plasticity of AlxCrMnFeCoNi type of HEAs as a function of chemistry and temperature. This is a very challenging task and only a very careful pre-study concerning the phase stability, magnetism and elasticity can provide enough information to turn my plan regarding ab initio description of the thermo-plastic deformation mechanisms in AlxCrMnFeCoNi HEAs into a successful research.