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1
Terna, Stefano. "Strong Coupling Analysis of D=2 and D=4 Maximally Supersymmetric YM Theories." Doctoral thesis, SISSA, 2000. http://hdl.handle.net/20.500.11767/4330.
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Cannon, James J. "Non-equilibrium molecular dynamics simulations of flow through carbon nanotubes." Thesis, University of Surrey, 2009. http://epubs.surrey.ac.uk/843548/.
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With increasing pressure on resources, filtration plays a key role in the lives of everyone on this planet. This can, for example, be in terms of desalination to obtain clean drinking water, hydrocarbon separation for gasoline production, or gas separation to meet stringent environmental regulations. In the design of membranes for applications such as these, fast transport of atoms combined with high selectivity is desired, and aligned carbon nanotube membranes show great promise in this respect. In order to design and tailor nanotube-membranes for optimum performance in specific applications, an understanding of the fundamental non-equilibrium flow properties through nanotubes is crucial. The aim of the research described in this thesis is to study these fundamental properties in a systematic fashion, in order to gain a better understanding of the potential that carbon nanotubes have for use in filtration and separation technologies. This research presents a new non-equilibrium molecular dynamics simulation written by this author, capable of reproducing and analysing long-time-scale non-equilibrium flow through carbon structures. Counter-intuitive flow dynamics as well as enhanced flux are demonstrated, and the reasons behind such phenomena are explained. The ability to manipulate the flow on the nano-scale through mechanical means is limited, and this research shows the potential role that nanotubes can play in manipulating the flow and inducing usual flow phenomena. The flow of point-particles is contrasted with that of diatomic molecules, and the level down to which macro-scale laws apply are discussed. Finally, the role of charge on the flow is also highlighted by considering the entry of water into nanotubes. By making these studies, a much greater insight into the potential role of nanotubes in filtration and separation applications is obtained. This research demonstrates that nanotube- membranes have great potential to help overcome the challenges facing the world, today as well as in the future.
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Martin, Willis. "Simulation study of non-covalent hybridization of carbon nanotubes by single-stranded DNA in water." Thesis, Boston College, 2010. http://hdl.handle.net/2345/1169.
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Thesis advisor: Goran Krilov<br>Solubilization and separation is an important step in utilizing both the unique mechanical and electrical properties of carbon nanotubes (CNTs). Due to different possible chiralities of CNTs, which can have drastically different electrochemical properties, it is also necessary to have a method of separation that will distinguish between these different species. Recent discovery of single-stranded DNA (ssDNA) absorption onto CNTs have shown high affinity towards forming soluble hybrids in polar solvents. The interactions between the ssDNA and CNTs as well as the geometry of the hybrid structure are not well understood. In order to study these phenomena we have implemented multiple all-atom replica exchange simulations. Simulations are carried out in an aqueous environment and vary in single-stranded decamer composition as well as nanotube chirality. The oligonucleotides readily adsorb onto the carbon nanotube surface and immediately following begin a slow structural rearrangement. Dependent upon both oligonucleotide composition and nanotube chirality, the ssDNA is found to form several unique backbone geometries as defined by both local and global order parameters. In contrast to the multiple geometries the backbone may form to, the nucleotide bases are found to organize themselves into either parallel or anti-parallel conformation with a high degree of orientational order. Binding appears to be mainly driven by π-stacking interactions between DNA bases onto the carbon nanotube surface, equilibrium of the structures is also controlled by a complex mixture of forces including DNA conformational strain and solvent interactions. The result of this is the free energy landscape is found to have multiple minima occupied at room temperature which are separated by high energy barriers<br>Thesis (MS) — Boston College, 2010<br>Submitted to: Boston College. Graduate School of Arts and Sciences<br>Discipline: Chemistry
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Evteev, Alexander V., Elena V. Levchenko, Irina V. Belova, and Graeme E. Murch. "Molecular dynamics study of carbon diffusion in cementite." Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-193325.
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Steele, Brad. "Non-equilibrium melting and sublimation of graphene simulated with two interatomic potentials." Scholar Commons, 2013. http://scholarcommons.usf.edu/etd/4586.
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The mechanisms of the sublimation of graphene at zero pressure and the condensation of carbon vapor is investigated by molecular dynamics (MD) simulations. The interatomic interactions are described by the Reactive Empirical Bond Order potential (REBO). It is found that graphene sublimates at a temperature of 5,200 K. At the onset of sublimation, defects that contain several pentagons and heptagons are formed, that are shown to evolve from double vacancies and stone wales defects. These defects consisting of pentagons and heptagons act as nucleation sites for the gaseous phase. The influence of the interatomic interactions on the sublimation process are also investigated by comparing the results using the REBO potential with the Screened Environment Dependent (SED)-REBO potential. Two-dimensional MD simulations are also performed, and it is found that graphene melts at a much higher temperature and forms many more point defects than in three dimensions. It is also observed that carbon chains make up the two-dimensional molten state.
The isothermal equation of state of gaseous and liquid carbon, as well as the coexistence of the two phases is calculated at 6,000 K and up to a few GPa. The analysis shows that the material that forms immediately following the phase transformation in graphene is actually a coexistence of liquid and gaseous phases, but it is primarily two-fold coordinated, so it is mostly a gas, hence the identification of the phase transformation as sublimation. The coexistence pressure for liquid and gaseous carbon is found using the Maxwell Construction to be 0.0365 GPa at 6,000 K. It was previously believed that carbon vapor consists exclusively of carbon chains. We find that under compression, at a pressure lower than the coexistence pressure, carbon vapor develops a small amount (6 %) of sp2 bonds indicating a slight non-chain bonding character. The diffusion coefficient of this dense gas is calculated to be in between that of the liquid and gaseous phases.
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Onyestyák, György, Zsolt Ötvös, Imre Kiricsi, and Lovat V. C. Rees. "The sorption dynamics of C 3 hydrocarbons over carbon nanotubes." Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-194893.
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Carbon nanotubes obtained by the catalytic vapor deposition (CVD) method were purified by various procedures to obtain samples in pure and well-defined forms. The frequency response (FR) technique was employed to determine the sorption dynamics of C3-hydrocarbons in these samples. Two parallel sorption processes were found, characterized by different sorption capacities and time constants and were assigned to sorption on the convex and concave surfaces of the carbon tubes, respectively. Over a carbon nanotube sample at low coverage the mass transport is little influenced by the different chemical nature of the various C3 hydrocarbons. However the dynamic sorption
properties of carbon nanotube preparations were shown to be strongly affected by the absence or presence of surface functional groups on the outer and inner surfaces of the tube walls. Different rate-determining mechanisms were observed for the samples functionalized to different extents.
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Onyestyák, György, Zsolt Ötvös, Jozsef Valyon, Imre Kiricsi, and Lovat V. C. Rees. "The sorption dynamics of C 3 hydrocarbons over carbon nanotubes." Universitätsbibliothek Leipzig, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-196636.
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Tognolini, S. "TRACKING THE INTERFACE STATES DYNAMICS AT CARBON-BASED NANOSTRUCTURES ON METAL BY NON-LINEAR PHOTOEMISSION SPECTROSCOPY." Doctoral thesis, Università degli Studi di Milano, 2016. http://hdl.handle.net/2434/362868.
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After single-layer graphene isolation in 2004, the scientific community has concentrated its efforts in the investigation of single-atom thin materials, due to their unique electronic properties, not shown by their bulk counterpart. Among this promising class of materials, two-dimensional carbon-based nanostructures have encountered a growing interest in nanotechnology, due to the huge variety of potential applications in which they are involved, from optoelectronics and photonics to energy generation, storage and solar cells. For the industrial realization of these devices a comprehensive understanding of the interaction between 2D carbon nanostructures and metallic substrates is necessary. An excellent tool to accomplish this aim is represented by the study of the electronic properties and dynamics of the states localized at the carbon nanostructures/metal interfacial region, being extremely sensitive to any modification at the surface. The investigation of these unoccupied electronic states has been accomplished applying femtosecond, high intensity laser pulses in the near-UV ranges to photoemission techniques.
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Cavalli, D. "CARBON AND NITROGEN DYNAMICS IN SOILS AFTER REPEATED MANURE APPLICATIONS: MEASUREMENTS AND MECHANISTIC SIMULATIONS." Doctoral thesis, Università degli Studi di Milano, 2011. http://hdl.handle.net/2434/152898.
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The first part of this Ph. D. Thesis (Chapters 1 and 2) is relative to the calibration of a soil organic matter simulation model (the CN-SIM model) by applying it to an experimental data-set, using two different methodologies, in order to verify its conceptual structure and obtaining parameters to describe the partitioning of C and N within the manures. Based on the optimization results, it emerged that the low model performance could be attributed to the lack of some processes involving mineral nitrogen transformations into the soil. In order to better understand the nitrogen dynamics following manure application to the soil, an incubation experiment was build up. In the experiment, two different manures were applied to two soils of different texture, and, in order to estimate the contribution of manure recalcitrant fractions to the mineralization dynamics of C and N in the soil (residual effect of added manure carbon and nitrogen), each manure was applied four times on the same soil (Chapter 3). In the incubation experiment were included measurements of non-exchangeable ammonium, with the aim of better understand if the lower recoveries of added manure ammonium, often found on the clay soils, were due to microbial biomass immobilization or ammonium clay-fixation. Measurements obtained in the incubation experiment were the used to perform a preliminary modification of the CN-SIM model to take into account the ammonium fixation and release processes (Charter 4).
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Kreier, Daniel. "Ultrafast single-electron diffraction at 100 keV and investigation of carbon-nanotube dynamics." Diss., Ludwig-Maximilians-Universität München, 2015. http://nbn-resolving.de/urn:nbn:de:bvb:19-183901.
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Time-resolved electron diffraction is a powerful tool to observe ultrafast structural dynamics in materials and molecules with atomic spatial as well as temporal resolution. Due to Coulomb repulsion, however, the use of only single-electrons or few-electrons per pulse is inevitable to reach the shortest pulse durations. Electrons have rather high scattering cross sections and thus experiments in transmission require ultrathin samples in the nanometer-range, making sample preparation very challenging. Up to now, ultrafast single-electron diffraction was only demonstrated at an electron energy of 30 keV; these measurements were performed in our group at the “UED1-beamline”.
This work introduces our second-generation beamline, “UED2”, where the electron
acceleration voltage is upgraded from 30 to 100 kV, which allows the investigation of significantly thicker samples. This is decisively widening the range of complex materials that can be studied. In the experiment, electron pulses are generated by a two-photon photoemission process and the long-term stability of the source is shown. The samples can be placed in transmission as well as grazing-incidence geometry. To achieve phase-matching between the optical and electron
pulses, tilted optical pulses can be applied. We figured out that to avoid temporal distortions in tilted pulses, a geometry must be chosen in which the propagation direction of the tilted pulses is perpendicular to the grating’s surface. Furthermore, temporal distortions for ultrashort electron pulses caused by misaligned magnetic lenses are examined. It is found that a displacement or tilt of the lens causes significant temporal aberrations on a femtosecond time scale and pulse-lengthening is only minimized if the beam travels precisely on the symmetry axis. An experimental procedure detailed here for aligning lens-position and -tilt reduces the aberrations to less than one femtosecond.
For the “UED2-beamline”, a new laboratory was established and a first time-resolved electron diffraction experiment at this beamline performed. Anisotropic ultrafast atomic motion in carbon-nanotubes was observed, revealing the nature of the system’s chemical bonds, which vary from relatively weak van der Waals to strong covalent interactions.
In summary, it is thus shown that ultrafast electron diffraction at 100 keV with single/few electrons per pulse is an excellent method to study ultrafast atomic-scale dynamics even in complex solid samples with the highest possible resolution in space and time.<br>Zeitaufgelöste Elektronenbeugung ist ein mächtiges Werkzeug, um ultraschnelle strukturelle Dynamik in Materialien und Molekülen sowohl mit atomarer räumlicher als auch zeitlicher Auflösung zu beobachten. Wegen der Coulombabstoßung ist allerdings die Verwendung von nur einem Elektron, oder wenigen Elektronen, pro Puls notwendig, um die kürzesten Pulsdauern zu erreichen. Elektronen haben recht hohe Streuquerschnitte und deshalb braucht man für Experimente in Transmission sehr dünne Proben, die nur wenige Nanometer dick sind; dies verursacht große Schwierigkeiten bei der Probenherstellung. Bisher wurden ultraschnelle Einzel-Elektronen-Beugungsexperimente nur bei einer Elektronenenergie von 30 keV gezeigt; diese Messungen wurden in unserer Gruppe an der „UED1-Beamline“ durchgeführt.
Diese Arbeit stellt unsere Zweite-Generation-Beamline, „UED2“, vor. Die Beschleunigungsspannung der Elektronen wurde hierbei von 30 auf 100 kV erhöht, was die Untersuchung signifikant dickerer Proben ermöglicht. Im Experiment werden Elektronenpulse durch einen Zweiphotonen-Photoprozess erzeugt und die langfristige Stabilität der Quelle wird gezeigt. Die Proben kann man sowohl in Transmission als auch im streifenden Elektronen-Einfall (grazing-incidence) platzieren und um Phase-Matching zwischen den optischen und Elektronenpulsen zu erreichen, können verkippte optische Pulse verwendet werden. Um zeitliche Verzerrungen (Distorsionen) in den verkippten Pulsen zu vermeiden, muss die Geometrie für deren Erzeugung so gewählt werden, dass die Ausbreitungsrichtung der verkippten Pulse senkrecht zur Gitteroberfläche ist. Dass und wie dies möglich ist, wird hier gezeigt. Außerdem werden zeitliche Distorsionen in ultrakurzen Elektronenpulsen, die durch schlecht justierte Magnetlinsen verursacht werden, untersucht. Es zeigt sich, dass eine Verschiebung oder Verkippung der Magnetlinse zu signifikanten zeitlichen Aberrationen im Femtosekundenbereich führt und diese Pulsverlängerungen nur minimiert werden können, wenn der Strahl genau auf der Symmetrieachse der Linse liegt. Ein hier experimentell gezeigtes Verfahren zum Ausrichten von Linsen-Position und Neigung verringert die Aberrationen auf weniger als eine Femtosekunde.
Für die „UED2-Beamline“ wurde ein neues Labor eingerichtet und ein erstes zeitaufgelöstes Elektronenbeugungs-Experiment an dieser Beamline durchgeführt. Eine anisotrope Bewegung der Atome in Kohlenstoff-Nanoröhrchen wurde beobachtet, welche die Natur der chemischen Bindungen in diesem System widerspiegelt, die zwischen relativen schwachen van der Waals und starken kovalenten Wechselwirkungen variiert.
Insgesamt ist damit gezeigt, dass ultraschnelle zeitaufgelöste Elektronenbeugung bei 100 keV mit einzelnen/wenigen Elektronen pro Puls eine hervorragende Methode darstellt, ultraschnelle atomare Dynamik auch in komplexen Festkörperproben mit höchstmöglicher Auflösung in Raum und Zeit untersuchen zu können.
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Carrasco, Barea Lorena. "Soil and vegetation carbon dynamics (stocks and fluxes) and litter decomposition in a Mediterranean non-tidal salt marsh." Doctoral thesis, Universitat de Girona, 2020. http://hdl.handle.net/10803/670976.
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Salt marshes are considered key ecosystems for climate regulation because they have a high carbon sequestration capacity. However, most studies of carbon sequestration capacity of these ecosystems have been performed in tidal salt marshes, mainly in North America, being studies from non-tidal salt marshes, such as those of the Mediterranean Basin, really scarce. In this thesis it has been analyzed the amount of carbon stored in vegetation and soil, the fluxes of carbon dioxide in vegetation and the fluxes of carbon dioxide and methane in soil, as well as the litter decomposition of the dominant plant species in a non-tidal Mediterranean salt marsh. Results from this thesis contribute to increase the knowledge on carbon dynamics and litter decomposition in Mediterranean Basin salt marshes.<br>Las marismas halófilas se consideran ecosistemas clave en la regulación climatica ya que presentan una alta capacidad de secuestro de carbono. Sin embargo, la mayoría de estudios sobre la capacidad de secuestro de carbono de estos ecosistemas se han realizado en marismas halófilas mareales, principalmente de Norte América, siendo escasos los estudios realizados en marismas no marelaes, como las de la cuenca de Mediterráneo. En esta tesis se ha analizado la cantidad de carbono almacenado en vegetación y suelo, los flujos de dióxido de carbono en vegetación y de dióxido de carbono y metano en suelo, así como la descomposición de la hojarasca de las especies vegetales dominantes en una marisma Mediterránea no mareal. Los resultados de esta tesis contribuyen a incrementar el conocimiento sobre la dinámica del carbono y la descomposición de la hojarasca en las marismas halófilas de la cuenca del Mediterráneo.
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Mi, Xi, and Yunfeng Shi. "On the measurement of surface diffusivity in disordered nanoporous carbon via molecular dynamics simulations." Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-185894.
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Gas diffusion in nanoporous carbon includes distinct diffusion modes: surface diffusion on the pore walls and non-surface diffusion away from pore walls. We have performed molecular dynamics (MD) simulations of Ar diffusion in disordered nanoporous carbon. The main objective of this work is to separate the surface diffusion and non-surface diffusion contributions so as to comprehend their respective dependency on the porous structure and testing conditions. The nanoporous carbon sample is obtained using the quenched molecular dynamics (QMD) method. Gas flows are generated by applying a constant external force on each Ar atom, mimicking a constant chemical potential gradient. The contributions from surface diffusion and non-surface diffusion are separated according to the distance of the gas atoms away from the pore wall. We show that the surface diffusivity is affected by the interactions between gas atoms and the pore walls, the temperature, and the loading.
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Sridharan, Harini. "COUPLED DYNAMICS OF HEAT TRANSFER AND FLUID FLOW IN SHEAR RHEOMETRY." University of Akron / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=akron1597346164780318.
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Forrester, David Ian, and davidif@unimelb edu au. "Mixed-species plantations of nitrogen-fixing and non-nitrogen-fixing trees." The Australian National University. Faculty of Science, 2005. http://thesis.anu.edu.au./public/adt-ANU20050202.164252.
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Mixed-species plantations of eucalypts and acacias have the potential to improve stand productivity over that of respective monocultures through the facilitative effect of nitrogen-fixation by acacias, and increased resource capture through above- and belowground stratification. However, growth in mixed-species plantations may not be improved compared to that of monocultures when competitive interactions outweigh the effects of improved nutrient availability and resource capture. Careful selection of sites and species is therefore critical to successfully improving stand productivity using mixed-species plantations. This study set out to examine some of the processes and interactions that occur in mixed-species plantations, and the effect nutrient and water availability can have on the growth of mixtures.
In three out of four mixed-species field trials examined in this study, growth was not increased in mixtures compared to monocultures. However, in the fourth field trial, heights, diameters, stand volume and aboveground biomass were higher in mixtures of E. globulus and A. mearnsii from 3-4 years after planting.
The range in outcomes from mixing species in these four trials shows that a fundamental understanding of the underlying processes is required to enable a greater predictive capacity for the circumstances under which mixtures will be successful. Therefore the growth dynamics, processes and interactions were examined in the mixtures of E. globulus and A. mearnsii. The difference in productivity between mixtures and monocultures in this trial increased with time up to age 11 years, when 1:1 mixtures contained twice the aboveground biomass of E. globulus monocultures. The positive growth response of trees in mixture compared to monocultures was the result of accelerated rates of nutrient cycling, a shift in C allocation and reductions in light competition through canopy stratification.
Nitrogen contents of foliage and soil clearly showed that A. mearnsii influenced the N dynamics in this trial. If these changes in N contents were due to N fixation by A. mearnsii, then about 51 and 86 kg N ha-1 yr-1 was fixed in the 1:1 mixtures and A. mearnsii monocultures, respectively. Nitrogen fixation was also examined using the natural abundance method. The delta15N values of foliage collected at 10 years were grouped according to the mycorrhizal status of the host plant. Therefore the discrimination of 15N during transfer from mycorrhizae to the host plant appeared to vary with mycorrhizal status, and the natural abundance of 15N was not used to quantify N fixation.
Rates of N and P cycling in litterfall were significantly higher in stands containing at least 25% A. mearnsii (more than 31 kg N ha-1 yr-1 and more than 0.68 kg P ha-1 yr-1) compared to E. globulus monocultures (24 kg N ha-1 yr-1 and 0.45 kg P ha-1 yr-1). Rates of litter decomposition and N and P release were about twice as high in 1:1 mixtures compared to E. globulus monocultures and were even higher in A. mearnsii monocultures. It is therefore important to select N-fixing species that are capable of cycling nutrients quickly between the plant and soil, and that have readily decomposable litter.
The total belowground C allocation was not significantly different between mixtures and monocultures (14 to 16 Mg C ha-1 yr-1). However, since aboveground net primary production was greater in 1:1 mixtures, the changes in nutrient availability appears to have increased total productivity (both above- and belowground), and reduced the proportion of C allocated belowground in mixtures compared to E. globulus monocultures.
In a pot trial containing mixtures of E. globulus and A. mearnsii both species grew larger in mixture than in monoculture at low N levels, and mixtures were more productive than monocultures. However, at high N levels, E. globulus suppressed A. mearnsii and mixtures were less productive than E. globulus monocultures. Similar effects were found for high and low levels of P.
Therefore resource availability can have a strong influence on the interactions and growth of mixtures. The productivity of mixtures may only be increased on sites where the resource for which competition is reduced in mixture is a major limiting growth resource. For example, if N is not a limiting growth factor then an increase in N availability from N-fixation may not increase growth, and the N-fixing species may compete for other resources such as soil P, moisture or light.
This study has shown that mixtures containing a N-fixing trees and a non-N-fixing trees can be more productive than monocultures, but that this increase in productivity will only occur on certain sites. Examination of the growth, interactions and processes that occurred in mixtures in this study provide useful information that can aid the selection of species combinations and sites.
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Koch, Juliane. "Humus Dynamics along Forest Conversion Sequences in the Lowland and Ore Mountain Region of Saxony, Germany." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2007. http://nbn-resolving.de/urn:nbn:de:swb:14-1185189101438-82589.
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Vor dem Hintergrund der steigenden CO2-Konzentration in der Atmosphäre gewinnt die Rolle des Waldes als C-Speicher zunehmend an Bedeutung. Eine besonders wichtige Funktion kommt hierbei dem Boden zu, denn Böden speichern weltweit mehr C als Vegetation und boden zusammen (Brady and Weil, 2002). Im Sinne einer nachhaltigen Waldbewirtschaftung werden in Sachsen derzeit großflächig die bestehenden Nadelforsten in naturnahe, strukturierte Laub- und Laubmischwälder umgewandelt. Ziel dieser Arbeit war es daher, baumarten- und bewirtschaftungsspezifische Effekte auf den Humus und die C-Speicherung im Waldboden aufzuzeigen. Die Untersuchungsflächen wurden entlang von Waldumbausequenzen gruppiert, welche die Entwicklung von konventionell bewirtschafteten Kiefern- und Fichten-Reinbeständen zu mehr oder weniger strukturierten Rotbuchen- und Rotbuchen-Traubeneichenbeständen über die Stufe der Voranbauten widerspiegeln. Die Untersuchungen wurden im Mittleren Erzgebirge und im Nordsächsischen Tiefland durchgeführt. Zusammenfassend zeigte die Untersuchung (i) einen um 24 % höheren C-Input über den Streufall unter Laubholz, (ii) eine höhere C-Freisetzung durch die C-Mineralisierung, welche im Laufe des Jahres im Of-Horizont unter Laubholz (BuEi) um 68 % höher war als unter Ki und (iii) eine höhere Akkumulation von C unter laubholzbestockten Beständen. Die Oh-Lagen unter den untersuchten Voranbauten und Laubholzbeständen enthielten entsprechend hohe Anteile von 61 % (Bu) und 40 % (BuEi) des in der organischen Auflage gespeicherten C und auch die oberen Mineralböden enthielten deutlich höhere C-Mengen unter LH als unter NH. Die Mechanismen, welche zur C-Sequestrierung in der Oh-Lage unter BuEi führen, konnten in der vorliegenden Arbeit detailliert durch die Humusdynamik in der organischen Auflage im verlauf eines Jahres erklärt werden. Im Gegensatz indizieren die Ergebnisse unter Ki einen verzögerten, aber vergleichbar intensiven Streuabbau im L-Horizont, gefolgt von einer Phase relativer Stagnation in der Of-Lage und einer wiederum aktiven Umsatzphase im vergleichsweise geringmächtigen Oh-Horizont. Die Menge bzw. der Anteil der langfristig gespeicherten organischen Substanz wird wesentlich durch die Streuqualität bestimmt, d.h. die Qualität der Streu steuert die bestandesspezifische Humusdynamik. Neben den standörtlichen und klimatischen Faktoren ist der Einfluss der Bewirtschaftung hingegen ein Faktor, welcher sich primär auf die Abbauaktivität und somit auf die Menge des akkumulierten C auswirken. Die für das Tiefland dokumentierten Effekte werden vermutlich auch im Erzgebirge wieder relevant sein, wenn die Kalkung an Wirkung verliert. Aus der vorliegenden Arbeit kann die Schlussfolgerung gezogen werden, dass im Vergleich zur Ki in der organischen Auflage unter LH (BuEi) größere Mengen C aktiv umgesetzt werden und in Folge dieser spezifischen Humusdynamik größere Mengen C in der Oh-Lage und im Mineralboden gespeichert werden können<br>Against the background of the increased CO2 concentration of the atmosphere the role of the forest as C store gains interest. An especially important function comes up to soil as soils sequester more C than vegetation and atmosphere together on a global scale (Brady and Weil, 2002). In the sense of a sustainable forest management of Saxony the vast areas covering coniferous stands are currently converted to semi-natural and structured deciduous and mixed forests. It was thus, the aim of this study to show species- and management-specific effects on humus dynamics and to evaluate top soils as possible C sink. All study sites involved were arranged along sequences representing the development from pure and conventionally managed Scots pine or Norway spruce stands to more or less structured European beech or European beech/Common oak stands via advanced plantings. The study was performed in the Ore mountain region and lowland of Saxony. In sum, the study revealed (i) a by 24 % higher litter-derived C input in the deciduous stand, (ii) a higher release of C by potential C mineralisation, that was in mean of one year by 68 % significantly higher in the F layer under beech/oak than under pine, and (iii) a higher accumulation of C under deciduous stands. The H layer under the studied advanced plantings and deciduous stands contained higher portions of 61 % (beech) and 40 % (beech/oak) of the total C accumulated in the organic layer and also upper mineral soil held evidently higher C under deciduous than under the pine stand. The specific mechanisms of C storage in the H layer under the beech/oak stand were explained in detail by explaining humus dynamics in the different horizons throughout the year. In contrast, the results under pine indicate a retarded, but as intensive decomposition in the L layer, followed by a phase of relative stagnancy (F layer) and in turn again active turnover phase in the comparatively thin H layer. The amount and portion of the long-term sequestered organic matter is substantially affected by the quality of the litter, that is that litter quality rules the stand-specific humus dynamics. Besides the site- and climate-specific factors, forest management in contrast is a factor, that affects turnover activity and thus, the amount of C accumulated. The effects documented for the lowland will presumably be relevant in the Ore Mountain region once the lime looses its effect. It can be concluded that in comparison to pine in the organic layer under deciduous trees (i.e., beech/oak) a higher amount of C is actively turned over and subsequently of this specific humus dynamics a higher amount of C is sequestered in the H layer and in mineral soil
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Smerat, Sebastian. "Ground state and dynamical properties of the finite Kondo lattice model and transport through carbon based nanodevices." Diss., lmu, 2011. http://nbn-resolving.de/urn:nbn:de:bvb:19-129416.
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Fugaciu, Florin. "Molekulardynamische Simulation der Stabilität und Transformation von Kohlenstoff-Nanoteilchen." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2000. http://nbn-resolving.de/urn:nbn:de:swb:14-994153395343-03649.
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Ziel der Arbeit ist die theoretische Analyse von Kohlenstoff-Clustern der Größe 100 - 500 Atome. Die experimentellen Beobachtungen sind bei dieser geringen Anzahl der Atome schwierig. Anderseits sind Kenntnisse über solche Cluster sehr wichtig, z.B. für die Keimbildung von Diamant auf Substraten, oder für die Kohlenstoff-Nanotechnologie (Fullerene, Nanotubes), oder für strukturelle Defekte in Kohlenstoff-Systemen. Es wurden gekrümmte Grenzflächen im Diamant simuliert. Zuerst mit einem empirischen Potential. Es wurde danach eine Methode entwickelt, bei der die schwach gestörten Gebiete einem empirischen Potential gehorchen, und die stark gestörten Gebiete, wo eine genaue Berechnung erforderlich ist, durch eine quantenmechanische Näherung beschrieben wurden. Somit kann man mit guter Genauigkeit große Systeme, bestehend aus einigen 10 (hoch)4 Atomen, simulieren, bei denen nur lokal quantenmechanische Methoden erforderlich sind. Mit diesem Hybrid-Code wurden weiterhin Diamantkeime auf Silizium gerechnet. Es wurden Aussagen bezüglich der Stabilität des Diamants auf dem Siliziumsubstrat, der kritischen Keimgröße, der Änderungen, die der Keim erfährt, gemacht. Ein anderes Gebiet ist die molekulardynamische Simulation bezüglich der Stabilität und des Transformationsverhaltens von Kohlenstoff-Nanoteilchen. Es wurden als »Rohstoffe» sowohl Diamant- und Graphitkristalle sphärischer, ellipsoidischer oder quadratischer Form benutzt, als auch amorpher Kohlenstoff. Es wurde demonstriert, daß sich Diamant unter höherer Temperatur und Bestrahlung in Kohlenstoffzwiebeln transformiert. Es wurde der innere Kern, bestehend aus zwei Schalen, der Kohlenstoffzwiebel simuliert. Es wurde, nach meinem Wissen, zum ersten Mal gezeigt, daß zwischen den Schalen der Kohlenstoffzwiebel Quer-Verbindungen (cross-links) existieren. Diese waren von den Experimentatoren vermutet worden. Sie bilden die Initiatoren der Diamantkeime der Kohlenstoffzwiebel bei ihrer ohne äußeren Druck möglichen Transformation in Diamant. Die Zentren der Kohlenstoffzwiebeln befinden sich bereits in der Entstehung der Zwiebel unter einem Selbstdruck. Bei den größeren Kohlenstoffzwiebeln beträgt der experimentell bestimmte Abstand zwischen den Schalen von außen nach innen von 3.34 Å bis 2.2 Å. Anlagen: nano1.mpg (91,8 MB); nano2.mpg (131 MB) Nutzung: Referat Informationsvermittlung der SLUB<br>The scope of this work is the analysis of carbon clusters of about 100 - 500 atoms. The experimental studies are at such small clusters heavy. Knowledges about thus clusters are very important, for example in the field of the nucleation of diamond on substrates, or for the carbon nano-technology (fullerene, nanotubes), or for local defects in carbon systems. There were simulated curved interfaces in diamond. Firstly with an empirical potential. Than I developed a method, in wich the defects and the structure around them are treated by a quantum mechanical algorithm and the rest with a near to ideal structure with an empirical potential. So, it is possible an accurate calculation of great systems of about 10 (high)4 atoms on wich only locally quantum mechanical methods are necessary. With this hybrid-code diamond nuclei on silicon substrate were simulated. The stability of the diamond nuclei on the silicon substrate, the critical radius of the nuclei and the changes of the nuclei during his transformation was investigated. Another field of investigations is the molecular dynamics simulation of free carbon clusters. The initial structures had spherical, ellipsoidical or square form and consists of diamond and graphite or a free form in the case of amorphous carbon. It was demonstrated that diamond transforms at higher temperatures and under irradiation in carbon onions. The genesis of the nucleus of a carbon onion with two shells was here for the first time simulated. The existence of the cross-links between the shells of a carbon onion was demonstrated. These existence was expected from the experimentators. The cross-links are the initiators of the transformation of carbon onions to diamond. The center of carbon onions is under self-pressure, because the distance between the outer shells is about 3.34 Å and between the inner shells about 2.2 Å. Appendix: nano1.mpg (91,8 MB); nano2.mpg (131 MB) Usage: Referat Informationsvermittlung/ SLUB
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Seriani, Nicola. "First-principles simulations of the oxidation of methane and CO on platinum oxide surfaces and thin films." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2006. http://nbn-resolving.de/urn:nbn:de:swb:14-1163174398225-14223.
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The catalytic oxidation activity of platinum particles in automobile catalysts is thought to originate from the presence of highly reactive superficial oxide phases which form under oxygen-rich reaction conditions. The thermodynamic stability of platinum oxide surfaces and thin films was studied, as well as their reactivities towards oxidation of carbon compounds by means of first-principles atomistic thermodynamics calculations and molecular dynamics simulations based on density functional theory. On the Pt(111) surface the most stable superficial oxide phase is found to be a thin layer of alpha-PtO2, which appears not to be reactive towards either methane dissociation or carbon monoxide oxidation. A PtO-like structure is most stable on the Pt(100) surface at oxygen coverages of one monolayer, while the formation of a coherent and stress-free Pt3O4 film is favoured at higher coverages. Bulk Pt3O4 is found to be thermodynamically stable in a region around 900 K at atmospheric pressure. The computed net driving force for the dissociation of methane on the Pt3O4(100) surface is much larger than on all other metallic and oxide surfaces investigated. Moreover, the enthalpy barrier for the adsorption of CO molecules on oxygen atoms of this surface is as low as 0.34 eV, and desorption of CO2 is observed to occur without any appreciable energy barrier in molecular dynamics simulations. These results, combined, indicate a high catalytic oxidation activity of Pt3O4 phases that can be relevant in the contexts of Pt-based automobile catalysts and gas sensors.
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Voloshina, Elena, Denis Usvyat, Martin Schütz, Yuriy Dedkov, and Beate Paulus. "On the physisorption of water on graphene: a CCSD(T) study." Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-138776.
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The electronic structure of the zero-gap two-dimensional graphene has a charge neutrality point exactly at the Fermi level that limits the practical application of this material. There are several ways to modify the Fermi-level-region of graphene, e.g. adsorption of graphene on different substrates or different molecules on its surface. In all cases the so-called dispersion or van der Waals interactions can play a crucial role in the mechanism, which describes the modification of electronic structure of graphene. The adsorption of water on graphene is not very accurately reproduced in the standard density functional theory (DFT) calculations and highly-accurate quantum-chemical treatments are required. A possibility to apply wavefunction-based methods to extended systems is the use of local correlation schemes. The adsorption energies obtained in the present work by means of CCSD(T) are much higher in magnitude than the values calculated with standard DFT functional although they agree that physisorption is observed. The obtained results are compared with the values available in the literature for binding of water on the graphene-like substrates<br>Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich
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Hartmann, Steffen. "Thermo-Mechanische Charakterisierung von Grenzflächen zwischen Einwandigen Kohlenstoffnanoröhren und Metallen mittels Auszugsversuchen." Doctoral thesis, Universitätsbibliothek Chemnitz, 2016. http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-197660.
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Vor dem Hintergrund zukünftiger Sensoren, basierend auf dem piezoresistiven Effekt von einwandigen Kohlenstoffnanoröhren (SWCNT), werden in dieser Arbeit umfangreiche Ergebnisse zum mechanischen Verhalten von Grenzflächen zwischen SWCNTs und edlen Metallen am Beispiel von Pd und Au präsentiert. Im Fokus steht dabei die Synergie von rechnerischen und experimentellen Methoden Molekulardynamik (MD), nanoskalige Tests und Analytik , um (1) mit guter Genauigkeit maximale Kräfte von gezogenen SWCNTs, welche in Metall eingebettet sind, vorauszuberechnen und (2) einen wertvollen Beitrag zum Verständnis der zu Grunde liegenden Fehlermechanismen zu liefern. Es wurde ein MDModell eines in eine einkristalline Matrix eingebetteten SWCNTs mit Randbedingen eines Auszugsversuchs entwickelt. Mit diesem Modell können Kraft-Weg-Beziehungen und Energieverläufe für einen quasistatischen verschiebungsgesteuerten Auszugsversuch errechnet werden. Das Modell liefert kritische Kräfte bei Versagen des Systems. Des Weiteren können mit diesem Modell der Einfluss des SWCNT-Typus, der Einbettungslänge, der Temperatur, von intrinsischen Defekten und Oberflächengruppen (SFGs) auf das Grenzflächenverhalten untersucht werden. Zum Vergleich wurden kritische Kräfte experimentell durch in situ Auszugsversuche in einem Rasterelektronenmikroskop bestimmt. Es wurde eine sehr gute Übereinstimmung von rechnerischen und experimentellen Daten festgestellt. Der vorherrschende Fehler im Experiment ist der SWCNT-Bruch, jedoch wurden auch einige SWCNT-Auszüge beobachtet. Mit Hilfe der MD-Simulationen wurde gefunden, dass die SFGs als kleine Anker in der umgebenden metallischen Matrix wirken und somit die maximalen Kräfte signifikant erhöhen. Diese Grenzflächenverstärkung kann Zugspannungen verursachen, die genügend hoch sind, so dass SWCNT-Bruch initiert wird. Im Gegensatz dazu zeigten Simulationen von Auszugstests mit idealen SWCNTs nur kleine Auszugskräfte, welche meistens unabhängig von der Einbettungslänge des SWCNTs sind. Dieses Verhalten wird mit einer inkommensurablen Konfiguration der Kristallstrukturen an der Grenzfläche von SWCNTs und der einbettenden Edelmetalle interpretiert. Zur Qualifizierung der Existenz von carboxylatischen Oberflächengruppen auf dem genutzten SWCNT-Material wurden analytische Untersuchungen mittels Fluoreszenzmarkierung von Oberflächengruppen durchgeführt. In Übereinstimmung mit Literaturstellen zum gesicherten Nachweis von SFGs, bedingt durch technologische Behandlungen, weisen diese Experimente stark auf das Vorhandensein von carboxylatischen Oberflächengruppen auf dem genutzten SWCNT-Material hin. Demnach kann der dominante SWCNT-Bruch Fehler durch die Grenzflächenverstärkung auf Grund von SFGs erklärt werden<br>In the light of future sensors, that are based upon the piezoresistive effect of singlewalled carbon nanotubes (SWCNTs), this work presents comprehensive results of studies on the mechanical behavior of interfaces between SWCNTs and noble metals using the examples of Pd and Au. With this contribution, the focus is on a synergy between computational and experimental approaches involving molecular dynamics (MD) simulations, nanoscale testing, and analytics (1) to predict to a good degree of accuracy maximum forces of pulled SWCNTs embedded in a noble metal matrix and (2) to provide valuable input to understand the underlying mechanisms of failure. A MD model of a SWCNT embedded in a single crystalline matrix with pull-out test boundary conditions was developed. With this model, force-displacement relations and energy evolutions for a quasi-static displacement controlled test can be computed. The model provides critical forces for failure of the system. Furthermore, the influence of SWCNT type, embedding length, temperature, intrinsic defects and surface functional groups (SFGs) on the interface behavior can be studied using this model. For comparison, critical forces were experimentally determined by conducting pull-out tests in situ, inside a scanning electron microscope. A very good agreement of computational and experimental values was discovered. The dominant failure mode in the experiment was a SWCNT rupture, although several pull-out failures were also observed. From MD simulations, it was found that SFGs act as small anchors in the metal matrix and significantly enhance the maximum forces. This interface reinforcement can lead to tensile stresses sufficiently high to initiate SWCNT rupture. In contrast, pull-out test simulations of ideal SWCNTs show only small pull-out forces, which are mostly independent on SWCNT embedding length. This behavior is interpreted with an incommensurate configuration of crystal structures at the interface between SWCNTs and embedding noble metals. To qualify the existence of carboxylic SFGs on the used SWCNT material, an analytical investigation by means of fluorescence labeling of surface species was performed. In agreement with literature reports on the secured verification of SFGs due to necessary technological treatments, these experiments strongly indicate the presence of carboxylic SFGs on the used SWCNT material. Thus, the dominant SWCNT rupture failure is explained with an interface reinforcement by SFGs
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Bauersfeld, Sindy. "Dynamische Modellierung des Gaspfades eines Gesamt-IGCC-Kraftwerkes auf Basis des SFG-Verfahrens." Doctoral thesis, Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2014. http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-148639.
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Im Rahmen der vorliegenden Arbeit werden dynamische Modelle eines IGCC-Kraftwerkes mit CO2-Abtrennung unter Verwendung des Modellierungstools Modelica/Dymola entwickelt. Dabei liegt der Schwerpunkt auf dem Gaspfad der Gasreinigung. Es ist vorteilhaft, für verschiedene Aufgaben, Modelle mit unterschiedlicher Tiefe zu verwenden. Mit den detaillierten Modellen werden Simulationen der Teilprozesse durchgeführt. Für den Aufbau eines Gesamtsystems werden vereinfachte Modelle verwendet. Anhand des Gesamtsystems werden drei Regelkonzepte (Gleitdruckregelung, Leistungsregelung der Gasturbine, Leistungsregelung des Vergasers) untersucht und bewertet. Des Weiteren werden drei Störfallszenarien (Ausfall des Sättigers im Brennstoffsystem, Betriebsstörung in der Vergaserinsel, Unterbrechung der Stickstoffzumischung im Brennstoffsystem) getestet.
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Touzik, Andrei. "Simulation der Nanostrukturbildung in Alkali-dotierten Fullerenschichten." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2004. http://nbn-resolving.de/urn:nbn:de:swb:14-1081344263703-58568.
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This work presents theoretical background for the investigation of nanostructure formation in alkali-metal doped fullerene layers. A number of computational methods are used to describe structural transformation in the fullerene layer. They include tight-binding molecular dynamics, empirical molecular dynamics, Monte-Carlo calculations as well as other methods. The doped fullerene layers show the highest superconducting critical temperature among organic superconductors. A new electrochemical method of synthesis of potassium and rubidium fullerides has been recently developed by Professor Dunsch and coworkers in the department of electrochemistry and conductive polymers at IFW Dresden. The process of electrochemical doping is accompanied by several side effects, and one of them is nanostructure formation at the surface of the fullerene layer. In the present work an explanation is given for the nanostructure formation observed recently by scanning tunnel microscopy. The corresponding model is based on the concept of spontaneous phase separation that has been realized by kinetic Monte Carlo calculations. These calculations predict instability of initially homogeneous alkali-doped fullerene layers. Due to the significant gap in the Madelung energy formation of an alkali-poor and an alkali-reach phase is expected. The results of the Monte Carlo simulations point out that the particle size of the corresponding phases remains in the nanometer range. Interpretation of experimental data for metal deposition on fullerene substrates can be easily given in the framework of the phase separation concept as well. Metal clusters of the size order 50 to 100 nm emerge in course of electrochemical copper deposition on alkali-doped fullerene layers. The electrically conductive paths through the insulating fullerene layer are probably responsible for the inhomogeneous copper deposition under electrochemical conditions. A novel computer program has been developed in course of this work, which is designed as a distributed application. It can be used for diverse conventional and kinetic Monte Carlo calculations<br>Die vorliegende Arbeit präsentiert theoretische Arbeiten, die das Ziel haben, die Nanostrukturbildung in dotierten Fullerenschichten zu verstehen. Diverse Rechenmethoden wurden verwendet, um die strukturellen Umwandlungen in der Fullerenschicht zu beschreiben. Die Tight-Binding-Molekulardynamik, die empirische Molekulardynamik und Monte-Carlo-Berechnungen sowie andere Methoden sind eingeschlossen. Die dotierten Fullerenschichten zeigen die höchste supraleitende kritische Temperatur unter den organischen Supraleitern. Eine neue elektrochemische Methode der Synthese von Kalium- und Rubidium-Fulleriden wurde vor kurzem von Professor Dunsch und Mitarbeitern in der Abteilung Elektrochemie und leitfähigen Polymere am IFW Dresden entwickelt. Der Prozess der elektrochemischen Dotierung wird von mehreren Nebenprozessen begleitet, und einer davon ist die Nanostrukturbildung an der Oberfäche der Fullerenschicht. In der vorliegenden Arbeit wird eine Erklärung für die Herausbildung der Nanostrukturen, die mit Hilfe von Rastertunnelmikroskopie beobachtet wurden, gegeben. Das entsprechende Modell basiert auf dem Konzept der spontanen Phasenentmischung und wird durch kinetische Monte-Carlo-Simulationen realisiert. Diese Simulationen sagen Instabilität der zunächst homogenen Alkali-dotierten Fullerenschichten voraus. Wegen des wesentlichen Unterschieds in der Madelungenergie ist die Herausbildung einer Alkalimetall-armen und einer Alkalimetall-reichen Phase zu erwarten. Die Ergebnisse der Monte-Carlo-Simulationen weisen darauf hin, dass die Teilchengröße der entsprechenden Phasen im Nanometer-Bereich bleibt. Im Rahmen des Phasenentmischungskonzepts können auch experimentelle Daten zur Metallabscheidung auf Fullerensubstraten problemlos interpretiert werden. Bei elektrochemischer Kupferabscheidung auf Alkali-dotierten Fullerenschichten entstehen Metallcluster der Größenordnung von 50 bis 100 nm. Die elektrisch leitfähige Pfade, die in einer isolierenden Matrix auftreten, sind wahrscheinlich für die ungleichmäßige Kupferabscheidung unter elektrochemischen Bedingungen verantwortlich. Ein neuartiges Computerprogramm wurde im Rahmen dieser Arbeit entwickelt, das als eine verteilte Anwendung entworfen ist. Damit können diverse konventionelle und kinetische Monte-Carlo-Simulationen durchgeführt werden
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Alaghemandi, Mohammad. "Thermal Conductivity and Thermal Rectification in Carbon Nanotubes - Reverse Non-Equilibrium Molecular Dynamics Simulations." Phd thesis, 2010. http://tuprints.ulb.tu-darmstadt.de/2226/1/Thesis.pdf.
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The purpose of this research is an investigation of the thermal conductivity () and thermal rectification of carbon nanotubes as well as the different factors which have an influence on these quantities. As computational tool we have used reverse non-equilibrium molecular dynamics (RNEMD) simulations. In chapter 1 we have briefly discussed the importance of research in nanoscale science. Furthermore the motivation for this work has been explained. In chapter 2 we have investigated the thermal conductivity of single-walled and multi-walled carbon nanotubes by RNEMD as a function of the tube length (L), temperature and chiral index. We found that the thermal conductivity in the ballistic-diffusive regime follows a L law. The exponent is insensitive to the diameter of the carbon nanotube; at room temperature has been derived for short carbon nanotubes. The temperature dependence of the thermal conductivity shows a peak between 250 and 500 K. We have also defined and shortly discussed the phenomenon of thermal rectification in mass-graded and extra-mass-loaded nanotubes. In chapter 3 the thermal rectification in nanotubes with a mass gradient has been studied in more detail. We predict a preferred heat flow from light to heavy atoms which differs from the preferential direction in one-dimensional (1D) monoatomic systems. This behavior of nanotubes is explained by anharmonicities caused by transverse motions which are stronger at the low mass end. The present simulations show an enhanced rectification with increasing tube length, diameter and mass gradient. Implications of the present findings for applied topics are mentioned concisely. In chapter 4 we have extended our work on thermal rectification from mass-graded quasi-one-dimensional nanotubes to the other model systems. Mass-graded polyacetylene-like chains behave like single-file chains as long as the mass gradient is hold by the backbone atoms. The thermal rectification in nanotubes with a gradient in the bond force constant (kr) has been studied, too. They show a preferred heat transfer from the region with large kr to the domain with small kr. Thermal rectification has been studied also in planar (2D) and 3D mass-graded systems where the heat flow followed a preferred direction similar to that observed in nanotubes. Additionally, a more realistic system has been implemented. Here a different number of carbon nanotubes have been grafted on both sides of a graphene sheet. We have found that the transfer of the vibrational energy as well as the generation of low-energy modes at atoms with large masses is responsible for the sign of the thermal rectification. In chapter 5 the thermal conductivity of carbon nanotubes (CNTs) with chirality indices (5,0), (10,0), (5,5) and (10,10) has been studied by reverse non-equilibrium molecular dynamics simulations as a function of different bondlength alternation patterns (r). The r dependence of the bond force constant (krx) in the MD force field has been determined with the help of an electronic band structure approach. From these calculations it follows that the r dependence of krx in tubes with not too small diameter can be mapped by a simple linear bondlength–bondorder correlation. A bondlength alternation with an overall reduction in the length of the nanotube causes an enhancement of while an alternation scheme leading to an elongation of the tube is coupled to a reduction of the thermal conductivity. This effect is more pronounced in CNTs with larger diameters.
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Alaghemandi, Mohammad [Verfasser]. "Thermical conductivity and thermal rectification in carbon nanotubes : reverse non-equlibrium molecular dynamics simulations / eingereicht von Mohammad Alaghemandi." 2010. http://d-nb.info/1004389701/34.
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Yu-WeiLo and 羅友威. "The Study on Thermal Transport Behavior of Perfect and Branched Carbon Nanotubes using Non-Equilibrium Molecular Dynamics Simulations." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/53700786734472509292.
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碩士<br>國立成功大學<br>機械工程學系<br>104<br>Thermal transport behavior of perfect and branched carbon nanotubes (CNTs) was investigated using non-equilibrium molecular dynamics (NEMD) simulation method. First, we introduced NEMD and reverse non-equilibrium molecular dynamics (rNEMD) simulation methods, and based testing of simulation parameters on theory of statistical mechanics. In the NEMD simulations with periodic and fixed boundary conditions for perfect CNTs, we compared our results, the relation of the length of perfect carbon nanotube and its thermal conductivity, with other literature. Furthermore, we observed thermal transport behaviors of various branched CNTs, which were composed of different length and diameter, using symmetric and asymmetric temperature-controlled NEMD simulations, and we explained these behaviors with the ballistic and diffusion transport of phonon.
In this study, we established a heat transfer simulation procedure of investigating perfect and branched CNTs, and we proposed lots of suggestions about NEMD simulation parameters. This research could pave the way to study the thermal transport behavior of nanostructures in future works.
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Ashek-I-Ahmed and Ashek-I-Ahmed. "Spectroscopic insight into the role of surface non-diamond carbon in reaction dynamics and charge distribution of nanodiamond." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/84br8j.
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博士<br>國立東華大學<br>物理學系<br>106<br>Nanodiamond (ND) refers to the diamond particles at a nanoscale level ranging from a single digit to several hundred nanometers. Unlike bulk diamond, nanodiamond is a composite of sp3/sp2 carbon structure in which the amorphous and disordered sp2 carbons populate the surface of a crystalline sp3 core lattice. Due to the production and purification process, the surface of nanodiamond contains a variety of oxidized groups such as C-O-C, C=O, OH etc. as well as some elemental contents, for example, N, H, S etc. Hydrogenation of nanodiamond is a suitable or required process to stabilize the surface as well as for further functionalization. The C-H adsorption on the surface of nanodiamond is conventionally performed through the interaction of atomic hydrogen generated by high-temperature hot filament heating (2000 C) or hydrogen plasma treatment. Recently it was found that the detonation nanodiamond (4~5 nm size) can be hydrogenated via low-temperature annealing (500 C) in a molecular hydrogen atmosphere. The mechanism of it is an interesting and important subject. This work was devoted to investigate the subtle surface reaction.
It is revealed using in-situ residual gas analysis of ultra-disperse diamond (UDD) particles (4~5 nm) during low temperature annealing (< 500 C) shows the desorption of surface non-diamond carbons as a gas phase radical form of C3, which incites a free radical reaction through the reduction of introduced molecular hydrogen (H2) to atomic hydrogen. As a result, the obtained atomic hydrogen facilitates C-H adsorption on the surface of nanodiamond which was confirmed by Fourier Transform Infrared Spectroscopic (FTIR) measurement.
In-situ measured FTIR spectra also showed that low temperature (<500 o C) annealing of UDD in vacuum results in the formation of isonitrile/isocyanide (-N=C:) and nitrile functionalization (-C≡N) on the surface. During annealing at 780~800 oC, the obtained nitrile group (-C≡N) is further reduced to the primary amine (NH2) and isonitrile (-N=C:) turns itself into a saturated ( ) structure, which is an effect of hydrogen (H) adsorption. On exposure to air, the obtained isonitrile is transformed to an N-formyl derivative (Aryl/R-NH-CHO) structure via hydrolysis.
In addition, the effect of low temperature hydrogenation was shown in graphitized larger size nanodiamond (50 nm) which leads to unfold the role of non-diamond surface carbon in the positive zeta-potential/the surface charge distribution of nanodiamond. It was revealed that the positive zeta potential of nanodiamond is only exhibited whenever the sp2 carbon present as a form of graphitic basal plane at the surface.
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Chia-ShingLi and 黎加興. "The study on interfacial thermal resistance and thermal transport behavior of mass-graded carbon nanotube using non-equilibrium molecular dynamics simulation." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/2anbf6.
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Forrester, David Ian. "Mixed-species plantations of nitrogen-fixing and non-nitrogen-fixing trees." Phd thesis, 2004. http://hdl.handle.net/1885/46988.
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Mixed-species plantations of eucalypts and acacias have the potential to improve stand productivity over that of respective monocultures through the facilitative effect of nitrogen-fixation by acacias, and increased resource capture through above- and belowground stratification. However, growth in mixed-species plantations may not be improved compared to that of monocultures when competitive interactions outweigh the effects of improved nutrient availability and resource capture. Careful selection of sites and species is therefore critical to successfully improving stand productivity using mixed-species plantations. This study set out to examine some of the processes and interactions that occur in mixed-species plantations, and the effect nutrient and water availability can have on the growth of mixtures.
¶
In three out of four mixed-species field trials examined in this study, growth was not increased in mixtures compared to monocultures. However, in the fourth field trial, heights, diameters, stand volume and aboveground biomass were higher in mixtures of E. globulus and A. mearnsii from 3-4 years after planting.
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Santos, Rafael Alexandre Mota dos. "Effect of the Distance Between Impact Point and Hole Position and Non-Perpendicular Holes on the Impact Strength of Composite Laminates." Master's thesis, 2016. http://hdl.handle.net/10400.6/7703.
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The effect of the distance between impact point and hole position and the angle of the hole with the vertical axis was studied. In order to understand this effect, flexural tests were also performed to evaluate the bending strength of CFRP. In terms of distance of the hole, a maximum reduction of 29.7% on the bending load for a distance of 0 mm was found. This reduction was 22.3% on the impact load. In terms of angle of the hole, a maximum load reduction of 15.6% on the bending strength was found and for the impact load this value was found to be 7% for 20º. The fatigue resistance was also studied. An average reduction of 68.5% on the fatigue resistance of GFRP was obtained for an impact energy of 12 J, in the presence of a hole.<br>O efeito da distância entre o ponto de impacto e posição de um furo e o ângulo do mesmo com o eixo vertical foi estudado. Para avaliar este efeito, foram também realizados ensaios de flexão. Em termos de distância do furo, uma redução máxima da resistência à flexão de 29,7% foi verificada para uma distância do furo de 0 mm. Esta redução foi de 22,3% no carregamento de impacto. Em termos de ângulo do furo, a redução máxima do carregamento foi de 15,6% para a flexão e em relação ao impacto este valor foi de 7%, para um ângulo de 20º. A resistência à fadiga foi também estudada. Foi verificada uma redução média de 68,5% na resistência à fadiga em compósitos de fibra de vidro para uma energia de impacto de 12 J, na presença de um furo.
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Clarkin, OWEN. "Chemical Reaction Dynamics at the Statistical Ensemble and Molecular Frame Limits." Thesis, 2012. http://hdl.handle.net/1974/7456.
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In this work, experimental and theoretical approaches are applied to the study of chemical reaction
dynamics. In Chapter 2, two applications of transition state theory are presented: (1) Application of
microcanonical transition state theory to determine the rate constant of dissociation of C2F3I
after π∗ ← π excitation. It was found that this reaction has a very fast rate constant
and thus
is a promising system for testing the statistical assumption of molecular reaction dynamics. (2) A general
rate constant expression for the reaction of atoms and molecules at surfaces was derived within the statistical
framework of flexible transition state theory.
In Chapter 4, a computationally efficient TDDFT approach was found to
produce useful potential energy surface landscapes for application to non-adiabatic predissociative dynamics
of the molecule CS2 after excitation from the ground state to the singlet C-state. In Chapter 5, ultrafast
experimental results of excitation of CS2 to the predissociative neutral singlet C-state is presented. The
bandwidth of the excitation laser was carefully tuned to span a two-component scattering resonance with each
component differently evolving electronically with respect to excited state character during the quasi-bound
oscillation. Scalar time-resolved photoelectron spectra (TRPES) and vector time-resolved photoelectron
angular distribution (TRPAD) observables were recorded during the predissociation. The TRPES yield of
photoelectrons was found to oscillate with a quantum beat pattern for the photoelectrons corresponding to
ionization to the vibrationless cation ground state; this beat pattern was obscured for photoelectron energies
corresponding to ionization from the vibrationally excited CS2 cation. The TRPAD data was recorded for
two general molecular ensemble cases: with and without a pre-excitation alignment laser pulse. It was found
that in the case of ensemble alignment (Chapter 6), the “molecular frame” TRPAD (i.e. TRMFPAD) was
able to image the purely valence electronic dynamics of the evolving CS2 C-state. The unaligned ensemble
TRPAD observable suffers from excessive orientational averaging and was unable to observe the quantum
beat.
Engineering efforts were also undertaken to eliminate scattered light background signal (Chapter 7,
Appendix A) and improve laser stability as a function of ambient pressure (Appendix B) for TRMFPAD
experiments.<br>Thesis (Ph.D, Chemistry) -- Queen's University, 2012-09-11 22:18:20.89
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Sharma, Maya. "Microporous Membranes Derived using Crystallisation Induced Phase Separation in PVDF/PMMA (Polyvinylidene Fluoride/ Polymethyl Methacrylate) Blends in Presence of Multiwalled Carbon Nanotubes." Thesis, 2017. http://hdl.handle.net/2005/3266.
Full textAbstract:
Segmental chain dynamics in polymer blends is a very important topic, not only from a fundamental point of view but also from technological applications. Because of the difficulties in the commercialization of new polymers, industries have turned increasingly towards blending of polymers to optimise their end use (mechanical, rheological) properties. The design of tailor-made materials would be enormously facilitated by the understanding of the blending phenomena at a molecular level. The key question to address is to understand the dynamics of each component of the blend modified by blending? The thesis has systematically studied the effect of multiwalled carbon nanotubes on the chain dynamics, demixing temperature, structural properties and evolution of morphology in a classical miscible polymer blend system (PVDF/PMMA).
The thesis comprises of six chapters, Chapter 1 is an introductory chapter that outlines the fundamentals of polymer blends, crystallisation in polymer blends and the basics of dielectric spectroscopy. As one of the rationales of this work is to systematic study whether phase separated in these blends can be used as a tool to develop membrane for water purification. This chapter also gives an overview of the reported studies of ultrafiltration membrane fabrication, factors affecting membrane morphology and flux. In Chapter 2, the materials and methodology used to carry out experiments and the experimental procedures are discussed.
Chapter 3 discusses the effect of concentration of PMMA and amine functionalized multiwalled carbon nanotubes (MWNTs) on the crystallisation induced phase separation using FTIR, XRD, POM and shear rheology. Electron microscopy and selective etching confirmed the localisation of MWNTs in the PVDF phase of the blends. Blends with MWNTs facilitated in heterogeneous nucleation manifesting in an increase in crystallisation temperature. The crystallisation induced phase separation in PVDF/PMMA blends was observed to influence the interconnected network of MWNTs in the blends.
Chapter 4 discuss the effect of concentration of PMMA and MWNTs on the miscibility and the segmental relaxations was probed in situ by DSC and dielectric relaxation spectroscopy (DRS). The dynamic heterogeneity in the blends as manifested by the presence of an extra relaxation at a higher frequency at or below the crystallisation induced phase separation temperature was also discussed. We found that PVDF/PMMA blend (PVDF ≥ 80 wt%) exhibits three distinct relaxations; αc corresponding to crystalline PVDF, αβ segmental relaxation of PMMA and αm of amorphous miscibility whereas all relaxations overlap and constitute a single broad relaxation in PVDF/PMMA blend (PVDF ≤ 70 wt%). This confirms that there is a certain composition width in this blend wherein three distinct relaxations can be traced. This could due to many reasons like the width of crystal-amorphous interphase in the crystal lamellae, crystal size and morphology is strongly contingent on the concentration of PMMA. Relaxations are not very distinct in presence of MWNTs due to defective spherulites that shift the relaxations towards a higher frequency.
Chapter 5 has attempted to tune the microporous morphology of PVDF membranes using crystallisation induced phase separation in PVDF/PMMA blends. As PVDF/PMMA is a melt-miscible blend, the samples were allowed to crystallise and the amorphous PMMA phase, which isolates in the interlamellar or inter-spherulitic regions in the blends, was etched out to generate
microporous structures. The pore sizes can be tuned by varying the PMMA concentration in the blends. We observed that 60/40 PVDF/PMMA blends showed larger pores as compared to 90/10 PVDF/PMMA blends. We further modified PVDF membranes by sputtering silver on the surface. The bacterial cell viability was distinctly suppressed (99 %) in silver sputtered membranes. The ICP analysis suggests that slow Ag+ ions release from the sputtered membrane surface assisted in developing antibacterial surface. Our findings open new avenues in designing water filtration membranes and also help in understanding the crystallisation kinetics for tuning pore size in membranes.
Chapter 6 summarises the important results of this work. MWNTs act as hetero nucleating agent and specifically interact with PVDF thereby influences the dynamics of PVDF chains. MWNTs can also restrict the amorphous segmental mobility and can influence the intermolecular cooperativity and coupling. The crystallisation induced phase separation in various blends can result in various crystalline morphologies depending on the PVDF concentration. By selectively etching PMMA from the phase-separated blends, microporous morphology can be generated