In this study, we employ molecular statics to construct the structure of amorphous zinc oxide. First, we find out the first number of higher energy structures in all local stable structures by Basin-Hopping algorithm, which are separated into different ratio of crystalline/amorphous zinc oxide structures, and then we judge the type of zinc oxide structure by radial distribution function. In addition, we use coordination number to analyse the interatomic bond length and bond angle in the structures. Furthermore, we employ molecular dynamics to increase the temperature of amorphous zinc oxide structures, and then use the distribution of coordination number, bond length and bond angle between zinc and oxygen atom to analyse the change of the local structure of amorphous zinc oxide during the heating process.

Hopping is considered a mass-spring model movement in which the leg supports the center of mass. There is a preferred hopping frequency and hopping outside of that frequency is more difficult and requires more energy. Leg stiffness has been shown to be an important factor when hopping at different frequencies in young adult populations. The purpose of this study was to observe how a still-developing preadolescent population would modify leg stiffness while hopping at different frequencies and if they have similar motor control strategies compared to young adults. The subjects hopped on their dominant leg to the beat of a metronome at one of four frequency conditions based on their calculated preferred frequency, MP (preferred frequency), MM (20% increase), MF (40% increase), and MS (20% decrease). It was found that this population could change their hopping frequency and they achieved this by manipulating their leg stiffness. At the higher frequency conditions there was less movement of the toe and the center of mass in both the vertical and horizontal directions, including decreased hopping height, decreased COM displacement and COM range of motion. Preadolescents demonstrated an adult-like ability to increase leg stiffness and modulate movement of the toe and the COM while adapting to a range of hopping frequencies. This ability could translate into other mass-spring model movements such as running and jumping.

Stuttgart, Univ., Diss., 2003.

University, Diss., 2004--Konstanz.

Clusters e nanoclusters têm recebido grande atenção devido à suas propriedades físicas e químicas, as quais divergem bastante dos materiais na fase bulk. Essas propriedades podem variar de acordo com a composição e tamanho do cluster. Uma compreensão da evolução das propriedades em relação a estes parâmetros é de grande importância para potencializar diversas aplicações, entretanto, esse entendimento permanece insatisfatório. Este trabalho foi dividido em duas etapas, em que a primeira busca investigar parâmetros energéticos, por meio do cálculo da energia de excesso, e estruturais, analisando parâmetro de ordem química, função de distribuição radial central, comprimento médio de ligação e número de coordenação efetiva, dos sistemas ZrnCum-n, ZrnAlm-n, CunAlm-n e ZrnCunAlm-2n para n = 55 e 561 átomos com o incremento n tomado de 1 em 1 para o sistema de 55 átomos e de 20 em 20 para os sistemas de 561 átomos. A segunda etapa consiste de investigar como variam as propriedades energéticas e estruturais do sistema ZrCu em função da evolução do tamanho do sistema. Para alcançar os objetivos propostos, neste trabalho foi usado o algoritmo de otimização global de clusters e nanopartículas basin-hopping Monte Carlo revisado. O potencial de interação atômica utilizado é o método do átomo imerso, que é bastante utilizado na descrição de sistemas metálicos. Os resultados obtidos sugerem que: (i) os sistemas puros apresentaram energia de coesão mais alta que seu análogo material na fase bulk, sugerindo que estes tendem a aglomerar-se formando estruturas bulk. Para os sistemas binários e ternários, foi identificado que todas as composições são energeticamente estáveis devido aos valores negativos obtidos pelo excesso de energia e, para o sistema ZrCu verificou-se a presença de efeitos de tamanho. (ii) Com relação à estrutura, as composições puras estudadas apresentaram simetria icosaédrica. Para o estudo da evolução do tamanho do sistema, Zr e Cu apresentaram estrutura com simetria icosaédrica até a composição de 561 átomos, além deste tamanho a simetria icosaédrica é quebrada. Para os sistemas binários e ternários foi obtido que os átomos tendem a distribuir-se dentro do nanocluster além de apresentarem quebra da simetria icosaédrica apresentando ausência de camadas atômicas ordenadas acompanhada de redução da coordenação efetiva. Os sistemas ZrCu e ZrAl demonstraram seguir a lei de Vegard, enquanto que os sistemas CuAl e ZrCuAl apresentaram desvio da lei de Vegard providos por efeitos eletrônicos, além de apresentarem a presença de efeitos de tamanho. / Clusters and nanocluster have attracted great attention due to their physical and chemical properties, very different from their analogous bulk. These properties can vary with composition and size cluster. An understanding of the properties evolution with respect these parameters is essential to improve several applications. However, this understanding is not complete. This study was piecemeal in two stage, being the first the investigation of energetic properties, by excess energy analisys, and structural properties, by chemical order parameter, radial distribution function, effective coordination number and average bond length, from ZrnCum-n, ZrnAlm-n, CunAlm-n and ZrnCunAlm-2n systems, where n = 55, 561 atoms and the increment n vary in one unit for 55-atoms system and twenty unit for 561-atoms system. The second stage is the investigation of how vary the energetic and structural properties from the size evolution ZrCu system. To do this study, was employed the global optimization algorith for cluster and nanoparticle Revised basin-hopping Monte Carlo, were this method use the classical calculation to determine the total energy of the system. The interatomic potential used was the embedded atom method, that was very usefull to describe metallic systems. Our results suggest: (i) the unary systems present cohesive energy higher than their analogous bulk, that indicate the trend of clusters to form bulk. To the binary and ternary systems, we had that all systems are favorable to form nanoalloys by negative value of excess energy. From ZrCu system, the stability decrease when increase the size of system. With respect the structure, the unary compounds present icosahedral symmetry. From the size-evolution study, the unary compounds present icosahedral symmetry until 561-atoms composition, after this size the icosahedral symmetry is broken. To binary and ternary systems, the atoms trend form mixture into the nanocluster, the icosahedral symmetry is broken with respect the unary compounds and presenting absence of ordered layers followed by effective coordination reduction. The ZrCu and ZrAl systems follow the Vegard law, while the CuAl and ZrCuAl systems present deviation from Vegard law, because electronic effects.

The properties and behaviors of charge transport mechanisms in highly insulating polymers are investigated by measuring conduction currents through thin film samples of low density polyethylene (LDPE). Measurements were obtained using a constant voltage method with copper electrodes inside a chamber adapted for measurements under vacuum and over a wide range of temperatures and applied fields. Field-dependent behaviors, including Poole-Frenkel conduction, space charge limited current (SCLC), and Schottky charge injection, were investigated at constant temperature. These field-dependent mechanisms were found to predict incorrect values of the dielectric constant and the field dependence of conductivity in LDPE was not found to be in agreement with SCLC predicted behavior. A model of thermally assisted hopping was a good fit at low applied fields and produced activation energies within the accepted range for LDPE. Low applied field measurements over the range of 213 K to 338 K were used to investigate two prominent hopping conduction mechanisms: thermally assisted hopping and variable range hopping. The observed temperature dependence of LDPE was found to be consistent with both thermally assisted hopping and variable range hopping. Activation energies determined for the range of temperatures were consistent with values reported in the literature for LDPE under similar conditions. A third aspect of charge transport behavior is a bulk response with time dependence. Conductivity behavior is examined in relation to transient current behavior, long time decay currents, and electrostatic discharge. Comparing charging and discharging cycles allowed qualitative separation of polarization and multiple trapping behaviors.

No description available.

No description available.

High-performance functional electrode materials are critical for the development of electrochemical energy conversion and storage technologies. Among various advanced materials, three-dimensional (3D) porous structures have attracted extensive interest due to their high surface area and capability for efficient mass transport. Metal-organic frameworks (MOFs) are a novel class of porous coordination polymers constructed with organic linkers connected by inorganic nodes. Their extraordinarily high surface area, permanent pores/channels, good thermal and chemical stabilities have made MOFs one of the most promising materials for various electrochemical applications, including electrocatalysis, supercapacitors, Lithium-ion batteries, chemical sensors, etc. The present dissertation focuses on the investigation of charge transfer mechanism in MOF films so as to establish design rules for future MOF design, and the exploration of MOF-based materials for electrochemical and photoelectrochemical applications. To promote the use of MOF-based materials in electrochemical applications, efficient charge transfer is a necessity. In redox-active MOFs, charge transfer can happen through redox hopping, i.e. site-to-site electron hopping coupled to diffusion of counter ions to balance electroneutrality. While the apparent diffusion coefficient (Dapp) has been employed to describe the overall charge transfer efficiency, independent elucidation of electron and ion diffusion is crucial for providing insights into the mechanism of charge transfer in MOFs. In Chapter 2, we investigated the MOF pore size effect on electron and ion diffusion. Three redox-active ferrocene-doped MOF (Fc-MOF) films with different pore sizes immobilized on conductive substrates were prepared, and electron and ion diffusion coefficients and rate constants were quantified by applying a theoretical model to chronoamperometric responses. Increasing MOF pore size led to an increase in ion diffusion rate constant and a decrease in electron diffusion rate constant. The overall charge transfer rate constant increased when MOF pore size increased, implying the ability of promoting efficient charge transfer through control of MOF pore size. As charge transfer via redox hopping proved to be feasible, Chapter 3 focused on the application of a ruthenium(II)-polypyridyl doped MOF film immobilized on a conductive substrate, UiO-67-Ru@FTO, for solid-state electrochemiluminescence (ECL). In the presence of tripropylamine as a coreactant, UiO-67-Ru@FTO exhibited higher ECL intensity and better reproducibility compared to corresponding solution-based ECL system. Subsequently, UiO-67-Ru@FTO was successfully used for dopamine detection, highlighting the great potential of using MOF-based materials as solid-state ECL detector for practical applications. Covalent-organic frameworks (COFs) are a recently emerging family of crystalline organic polymers constructed with organic building blocks linked by covalent bonds. In addition to advantages including high surface area and high porosity that are similar to MOFs, COFs possess low density due to the constitution of light-weighted elements and excellent stability owing to the robust covalent bonds. Therefore, it is of our interest to investigate the properties and potential applications of COFs. Two-dimensional (2D) COFs are composed of conjugated organic layers stacked via - interactions. Chapter 4 focused on understanding the effects of intraplanar -conjugation and interplanar -stacking on the photophysical properties of a 2D COF, TpBpy. Compared to the two building blocks, TpBpy exhibited a red-shifted emission, due to the - stacking. Density functional theory (DFT) calculations were performed on energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). It was found that the extended structure of the framework resulted in a decrease in the HOMO-LUMO gap. The experimental and computational studies reveal the important influence of intraplanar and interplanar interactions on photophysical properties in 2D COFs. In Chapter 5, we modified the COF TpBpy with nickel(II) and investigated its application as an electrocatalyst for 5-hydroxymethylfufural (HMF) oxidation. Unlike TpBpy characterized in Chapter 4, TpBpy thin films were prepared by an interfacial crystallization strategy. The films were transferred to conductive substrates and then post-synthetically modified by nickel acetate. Similar to redox-active MOFs, the resulting TpBpy-Ni COF film exhibited redox conductivity. TpBpy-Ni showed good catalytic activity for HMF oxidation under basic conditions. This study suggests the great potential of functionalized COFs for electrochemical applications. / Doctor of Philosophy / The increasing demand for clean and efficient energy has triggered a great deal of research interest in developing novel energy conversion and storage technologies. In particular, electrochemical (EC) systems including supercapacitors, Lithium-ion batteries, artificial photosynthetic system, fuel cells, etc. have drawn significant attention. The key component in high-performance EC energy conversion and storage devices is the functional electrode materials. Three-dimensional (3D) porous nanostructures have been widely applied as advanced electrode materials due to their high surface area that enables more liquid/solid interfacial interactions, and pores/channels that allows efficient mass diffusion and transport. Metal-organic frameworks (MOFs), made of organic ligands bridged by inorganic nodes, are a novel kind of porous materials with extraordinarily high surface area and permanent porosity. As a result, there is great potential in developing MOF-based electrode materials for EC applications. As the name itself suggests, EC systems rely on electrochemical reactions that involve transfer of charges (i.e. electrons and ions). Therefore, efficient charge transfer is vital for achieving high performance. While MOFs used for gas separation and storage have been reported, their electrochemical applications are still in early stages. The fundamental understanding of charge transfer in MOFs is in its infancy. As a result, there is an urgent demand for understanding the nature of charge transfer in MOFs. In this dissertation, we investigated the mechanism of charge transfer by independent quantification of electron and ion transfer rate constants. With a better understanding in hand, we also explored two electrochemical applications in MOFs, electrocatalysis and electrogenerated chemiluminescence.

Our goal is to investigate protein networks conserved between different organisms. Given the protein interaction networks for two species and a list of homologous pairs of protein in the two species, we propose a model for measuring whether two subnetworks, one in each protein interaction network, are conserved. Our model separately measures the degree of conservation of the two subnetworks and the quality of the edges in each subnetwork. We propose an algorithm for finding pairs of networks, one in each protein interaction network, with high conservation and high quality. When applied to publicly-available protein-protein interaction data and gene sequences for baker's yeast and fruit fly, our algorithm finds many conserved networks with a high degree of functional enrichment. Using our method, we find many conserved protein interaction networks involved in functions such as DNA replication, protein folding, response to heat, protein serine/threonine phosphatase activity, kinase activity, and ATPase activity. / Master of Science