Os diferentes tipos de treinamento de força (TF) apresentam distintas adaptações do tecido muscular como o desenvolvimento de força máxima, potência e/ou força reativa muscular. Dentre estes tipos de TF, estão os treinamentos de força Tradicional, Treinamento de Potência e de Treinamento de Força Reativa, em que está presente o ciclo-alongamento-encurtamento (CAE). A habilidade de produzir força rapidamente requer a participação intensa das fibras do tipo IIX, condição esta frequentemente reduzida em mulheres idosas. A força rápida tem importância significativa para homens e mulheres de todas as idades, destacando-se ainda mais em mulheres idosas. A prescrição de um tipo de TF que melhor desenvolva as atividades de vida diária (AVD) é primordial para a manutenção da independência física e qualidade de vida de idosas. Assim o objetivo principal do trabalho foi avaliar e comparar as adaptações neuromusculares, morfológicas e funcionais em mulheres idosas submetidas a três tipos específicos de treinamento de força. Na revisão de literatura a metodologia embasou-se em pesquisa bibliográfica, utilizando artigos atuais e clássicos da literatura sobre TF em mulheres idosas (publicados, principalmente, a partir de 2000) selecionados nas bases de dados Pubmed e Sportdiscus, que comparassem e analisassem pelo menos dois tipos específicos de TF. No segundo estudo, 58 mulheres idosas saudáveis sem a prática de treinamento de força foram randomizadas em grupo experimental (GE, n= 41) e grupo controle (GC, n=17), e submetidas a seis semanas de treinamento de força resistente. Foram realizadas avaliações da qualidade muscular e sua correlação com os testes funcionais (sentar e levantar em 30s e up foot and go). Como resultado o GE apresentou incremento significativo na qualidade muscular do quadríceps (14,8%) com alta correlação com os testes funcionais sentar e levantar (r=0,62, p< 0,001) e up foot and go (r= -0,72, p< 0,001). O terceiro estudo o GC foi preservado e o grupo experimental foi dividido em Treinamento de Força Tradicional (GT, n=14), Treinamento de Potência (GP, n=13) e Treinamento de Força Reativa (GR, n=14). Foram realizadas avaliações do 1RM extensão de joelhos, espessura muscular, ativação, onset e tempo de reação muscular, taxa de produção de força e testes funcionais como o sentar e levantar e o salto com contra movimento. Como resultados, observou-se que o GR foi mais efetivo que os grupos GT e GP no onset muscular do RC, da taxa de produção de força (0-150 ms), no tempo de reação muscular e nos testes funcionais (p< 0,05). Dessa forma, conclui-se que o treinamento de força reativa é mais efetivo para o desenvolvimento da produção de força rápida do músculo que os outros tipos específicos de treinamento de força, e por conseqüência disto, melhor desenvolve as capacidades funcionais de mulheres idosas. / Differents strenght trainning (ST) are associated with distincts muscles tissues responses that comprised maximum strenght, power and/or reactive force. Among these different ST programmes there are traditional strenght training (GT), power (GP) and reactive force (GR) which is present in stretch-shortening cycle (SSC). In this context, for fast force production is necessary that IIX type fibers being recruited, condiction that to be seem decreasing among elderly women population. Furthermore, fast production of force has significant importance for all ages, however this condition could be considerate more significant for elderly women population. Consequently, prescrition of ST associated with improving activities of daily living (ADL) have been evidenciated as a important pathway for physical independence, as well as, for elderly quality of life. Thus, the mayor aim of this study was to verifiy and compare neuromuscular, morphological and functional adaptations in a sample of elderly women which were submitted in three different ST programmes. The methodology used in the first study was a sistematic literature review included classics studies about ST in this population. These studies were selected by two international scientic database (Pubmed and Sportdiscus); were published since 2000 until now and to show a comparation between at least two different kinds of ST. For the second study, 58 inactivity elderly women was randomized between control (GC=17) and experimental group (GE=41) and after submitted over a six weeks of ST. The relationship between muscle quality assesment with up foot and go and 30- second chair stand functional test was realized. Subjects included in GE showed significant improvement of quadricips muscle quality (14.8%), as well as, muscle quality showed significant strong association with 30-second chair stand (r=0.62, p< 0.001) and up foot and go (r= -0.72, p< 0.001) functional tests. For the third study, the GC was maintenced while GE was again randomized between GT (14), GP (13) and GR (14). 1 RM of knee extention, muscles thickness, activation, onset, reaction time, rate development force tests and working out as the sit and stand and countermovement jumps were measured. Results of this study showed that GR was more effective than GT and GP when comprised rectus thigh muscle onset, initial periods of production of strenght rate, reaction time and muscles function tests (p <0.05). Finnaly, considerating results of these studies we concluded that GR training was more effective when considerated fast force production of muscle if compared with other specifics ST programmes. Consequently this study suggest GR as a better ST programm for improving functional capapcities in a sample of elderaly women.

Os diferentes tipos de treinamento de força (TF) apresentam distintas adaptações do tecido muscular como o desenvolvimento de força máxima, potência e/ou força reativa muscular. Dentre estes tipos de TF, estão os treinamentos de força Tradicional, Treinamento de Potência e de Treinamento de Força Reativa, em que está presente o ciclo-alongamento-encurtamento (CAE). A habilidade de produzir força rapidamente requer a participação intensa das fibras do tipo IIX, condição esta frequentemente reduzida em mulheres idosas. A força rápida tem importância significativa para homens e mulheres de todas as idades, destacando-se ainda mais em mulheres idosas. A prescrição de um tipo de TF que melhor desenvolva as atividades de vida diária (AVD) é primordial para a manutenção da independência física e qualidade de vida de idosas. Assim o objetivo principal do trabalho foi avaliar e comparar as adaptações neuromusculares, morfológicas e funcionais em mulheres idosas submetidas a três tipos específicos de treinamento de força. Na revisão de literatura a metodologia embasou-se em pesquisa bibliográfica, utilizando artigos atuais e clássicos da literatura sobre TF em mulheres idosas (publicados, principalmente, a partir de 2000) selecionados nas bases de dados Pubmed e Sportdiscus, que comparassem e analisassem pelo menos dois tipos específicos de TF. No segundo estudo, 58 mulheres idosas saudáveis sem a prática de treinamento de força foram randomizadas em grupo experimental (GE, n= 41) e grupo controle (GC, n=17), e submetidas a seis semanas de treinamento de força resistente. Foram realizadas avaliações da qualidade muscular e sua correlação com os testes funcionais (sentar e levantar em 30s e up foot and go). Como resultado o GE apresentou incremento significativo na qualidade muscular do quadríceps (14,8%) com alta correlação com os testes funcionais sentar e levantar (r=0,62, p< 0,001) e up foot and go (r= -0,72, p< 0,001). O terceiro estudo o GC foi preservado e o grupo experimental foi dividido em Treinamento de Força Tradicional (GT, n=14), Treinamento de Potência (GP, n=13) e Treinamento de Força Reativa (GR, n=14). Foram realizadas avaliações do 1RM extensão de joelhos, espessura muscular, ativação, onset e tempo de reação muscular, taxa de produção de força e testes funcionais como o sentar e levantar e o salto com contra movimento. Como resultados, observou-se que o GR foi mais efetivo que os grupos GT e GP no onset muscular do RC, da taxa de produção de força (0-150 ms), no tempo de reação muscular e nos testes funcionais (p< 0,05). Dessa forma, conclui-se que o treinamento de força reativa é mais efetivo para o desenvolvimento da produção de força rápida do músculo que os outros tipos específicos de treinamento de força, e por conseqüência disto, melhor desenvolve as capacidades funcionais de mulheres idosas. / Differents strenght trainning (ST) are associated with distincts muscles tissues responses that comprised maximum strenght, power and/or reactive force. Among these different ST programmes there are traditional strenght training (GT), power (GP) and reactive force (GR) which is present in stretch-shortening cycle (SSC). In this context, for fast force production is necessary that IIX type fibers being recruited, condiction that to be seem decreasing among elderly women population. Furthermore, fast production of force has significant importance for all ages, however this condition could be considerate more significant for elderly women population. Consequently, prescrition of ST associated with improving activities of daily living (ADL) have been evidenciated as a important pathway for physical independence, as well as, for elderly quality of life. Thus, the mayor aim of this study was to verifiy and compare neuromuscular, morphological and functional adaptations in a sample of elderly women which were submitted in three different ST programmes. The methodology used in the first study was a sistematic literature review included classics studies about ST in this population. These studies were selected by two international scientic database (Pubmed and Sportdiscus); were published since 2000 until now and to show a comparation between at least two different kinds of ST. For the second study, 58 inactivity elderly women was randomized between control (GC=17) and experimental group (GE=41) and after submitted over a six weeks of ST. The relationship between muscle quality assesment with up foot and go and 30- second chair stand functional test was realized. Subjects included in GE showed significant improvement of quadricips muscle quality (14.8%), as well as, muscle quality showed significant strong association with 30-second chair stand (r=0.62, p< 0.001) and up foot and go (r= -0.72, p< 0.001) functional tests. For the third study, the GC was maintenced while GE was again randomized between GT (14), GP (13) and GR (14). 1 RM of knee extention, muscles thickness, activation, onset, reaction time, rate development force tests and working out as the sit and stand and countermovement jumps were measured. Results of this study showed that GR was more effective than GT and GP when comprised rectus thigh muscle onset, initial periods of production of strenght rate, reaction time and muscles function tests (p <0.05). Finnaly, considerating results of these studies we concluded that GR training was more effective when considerated fast force production of muscle if compared with other specifics ST programmes. Consequently this study suggest GR as a better ST programm for improving functional capapcities in a sample of elderaly women.

Os diferentes tipos de treinamento de força (TF) apresentam distintas adaptações do tecido muscular como o desenvolvimento de força máxima, potência e/ou força reativa muscular. Dentre estes tipos de TF, estão os treinamentos de força Tradicional, Treinamento de Potência e de Treinamento de Força Reativa, em que está presente o ciclo-alongamento-encurtamento (CAE). A habilidade de produzir força rapidamente requer a participação intensa das fibras do tipo IIX, condição esta frequentemente reduzida em mulheres idosas. A força rápida tem importância significativa para homens e mulheres de todas as idades, destacando-se ainda mais em mulheres idosas. A prescrição de um tipo de TF que melhor desenvolva as atividades de vida diária (AVD) é primordial para a manutenção da independência física e qualidade de vida de idosas. Assim o objetivo principal do trabalho foi avaliar e comparar as adaptações neuromusculares, morfológicas e funcionais em mulheres idosas submetidas a três tipos específicos de treinamento de força. Na revisão de literatura a metodologia embasou-se em pesquisa bibliográfica, utilizando artigos atuais e clássicos da literatura sobre TF em mulheres idosas (publicados, principalmente, a partir de 2000) selecionados nas bases de dados Pubmed e Sportdiscus, que comparassem e analisassem pelo menos dois tipos específicos de TF. No segundo estudo, 58 mulheres idosas saudáveis sem a prática de treinamento de força foram randomizadas em grupo experimental (GE, n= 41) e grupo controle (GC, n=17), e submetidas a seis semanas de treinamento de força resistente. Foram realizadas avaliações da qualidade muscular e sua correlação com os testes funcionais (sentar e levantar em 30s e up foot and go). Como resultado o GE apresentou incremento significativo na qualidade muscular do quadríceps (14,8%) com alta correlação com os testes funcionais sentar e levantar (r=0,62, p< 0,001) e up foot and go (r= -0,72, p< 0,001). O terceiro estudo o GC foi preservado e o grupo experimental foi dividido em Treinamento de Força Tradicional (GT, n=14), Treinamento de Potência (GP, n=13) e Treinamento de Força Reativa (GR, n=14). Foram realizadas avaliações do 1RM extensão de joelhos, espessura muscular, ativação, onset e tempo de reação muscular, taxa de produção de força e testes funcionais como o sentar e levantar e o salto com contra movimento. Como resultados, observou-se que o GR foi mais efetivo que os grupos GT e GP no onset muscular do RC, da taxa de produção de força (0-150 ms), no tempo de reação muscular e nos testes funcionais (p< 0,05). Dessa forma, conclui-se que o treinamento de força reativa é mais efetivo para o desenvolvimento da produção de força rápida do músculo que os outros tipos específicos de treinamento de força, e por conseqüência disto, melhor desenvolve as capacidades funcionais de mulheres idosas. / Differents strenght trainning (ST) are associated with distincts muscles tissues responses that comprised maximum strenght, power and/or reactive force. Among these different ST programmes there are traditional strenght training (GT), power (GP) and reactive force (GR) which is present in stretch-shortening cycle (SSC). In this context, for fast force production is necessary that IIX type fibers being recruited, condiction that to be seem decreasing among elderly women population. Furthermore, fast production of force has significant importance for all ages, however this condition could be considerate more significant for elderly women population. Consequently, prescrition of ST associated with improving activities of daily living (ADL) have been evidenciated as a important pathway for physical independence, as well as, for elderly quality of life. Thus, the mayor aim of this study was to verifiy and compare neuromuscular, morphological and functional adaptations in a sample of elderly women which were submitted in three different ST programmes. The methodology used in the first study was a sistematic literature review included classics studies about ST in this population. These studies were selected by two international scientic database (Pubmed and Sportdiscus); were published since 2000 until now and to show a comparation between at least two different kinds of ST. For the second study, 58 inactivity elderly women was randomized between control (GC=17) and experimental group (GE=41) and after submitted over a six weeks of ST. The relationship between muscle quality assesment with up foot and go and 30- second chair stand functional test was realized. Subjects included in GE showed significant improvement of quadricips muscle quality (14.8%), as well as, muscle quality showed significant strong association with 30-second chair stand (r=0.62, p< 0.001) and up foot and go (r= -0.72, p< 0.001) functional tests. For the third study, the GC was maintenced while GE was again randomized between GT (14), GP (13) and GR (14). 1 RM of knee extention, muscles thickness, activation, onset, reaction time, rate development force tests and working out as the sit and stand and countermovement jumps were measured. Results of this study showed that GR was more effective than GT and GP when comprised rectus thigh muscle onset, initial periods of production of strenght rate, reaction time and muscles function tests (p <0.05). Finnaly, considerating results of these studies we concluded that GR training was more effective when considerated fast force production of muscle if compared with other specifics ST programmes. Consequently this study suggest GR as a better ST programm for improving functional capapcities in a sample of elderaly women.

Due to the wide application of silica based systems ranging from microelectronics to nuclear waste disposal, detailed knowledge of water-silica interactions plays an important role in understanding fundamental processes, such as glass corrosion and the long term reliability of devices. In this dissertation, atomistic computer simulation methods have been used to explore and identify the mechanisms of water-silica reactions and the detailed processes that control the properties of the water-silica interfaces due to their ability to provide atomic level details of the structure and reaction pathways. The main challenges of the amorphous nature of the silica based systems and nano-porosity of the structures were overcome by a combination of simulation methodologies based on classical molecular dynamics (MD) simulations with Reactive Force Field (ReaxFF) and density functional theory (DFT) based ab initio MD simulations. Through the development of nanoporous amorphous silica structure models, the interactions between water and the complex unhydroxylated internal surfaces identified the unusual stability of strained siloxane bonds in high energy ring structure defects, as well as the hydroxylation reaction kinetics, which suggests the difficulty in using DFT methods to simulate Si-O bond breakage with reasonable efficiency. Another important problem addressed is the development of silica gel structures and their interfaces, which is considered to control the long term residual dissolution rate in borosilicate glasses. Through application of the ReaxFF classical MD potential, silica gel structures which mimic the development of interfacial layers during silica dissolution were created A structural model, consisting of dense silica, silica gel, and bulk water, and the related interfaces was generated, to represent the dissolution gel structure. High temperature evolution of the silica-gel-water (SGW) structure was performed through classical MD simulation of the system, and growth of the gel into the water region occurred, as well as the formation of intermediate range structural features of dense silica. Additionally, hydroxylated silica monomers (SiO4H4) and longer polymerized silica chains were formed in the water region, indicating that glass dissolution is occurring, even at short time frames. The creation of the SGW model provides a framework for a method of identifying how interfacial structures which develop at glass-water interfaces can be incorporated into atomistic models for additional analysis of the dissolution of silicates in water.

No description available.

Siliziumnanodrähte (SiNWs) bieten eine aussichtsreiche Grundlage zur Entwicklung neuartiger nanoelektronischer Bauelemente, wie Feldeffekttransistoren oder Sensoren. Dabei ist insbesondere die Oxidation der Drähte interessant, weil diese weitreichenden Einfluss auf die elektronischen Eigenschaften der Bauelemente hat, die aus den SiNWs gefertigt werden. Die Größe der untersuchten Strukturen erfordert eine atomistische Analyse des Oxidationsprozesses. In der vorliegenden Arbeit wird der bisher wenig verstandene Beginn der Oxidation dünner Drähte molekulardynamisch simuliert, wobei als Potential ein reaktives Kraftfeld dient. Dabei wird sich intensiv mit dem Transfer elektrischer Ladungen zwischen Atomen unterschiedlicher Elektronegativitäten während der Simulationen auseinandergesetzt. Desweiteren werden Strukturen, die während der Oxidation von SiNWs der Orientierungen <100> und <110> bei Temperaturen von 300 K und 1200 K entstehen, untersucht. Ein Fokuspunkt dieser Untersuchungen ist die Analyse der Anzahl am Draht adsorbierter Sauerstoffatome während der frühen Oxidationsphase. Darüber hinaus wird die Dichte der entstehenden Strukturen beleuchtet. Dies geschieht mit einer hohen radialen Auflösung und erstmalig während der gesamten Simulation. Hierbei zeigt sich, dass während des Übergangs von kristallinem Silizium zu amorphem Siliziumdioxid zwischen den Siliziumatomen Sauerstoff eingelagert wird, die Kristallstruktur des Siliziums sich zunächst jedoch noch nicht auflöst. Dadurch entsteht ein charakteristisches Muster hoher und niedriger Dichten, das von der ursprünglichen Kristallstruktur des SiNW abhängt.

Gasphasenabscheidungen werden zur Produktion dünner Schichten in der Mikro- und Nanoelektronik benutzt, um eine präzise Kontrolle der Schichtdicke im Sub-Nanometer-Bereich zu erreichen. Elektronische Eigenschaften der Schichten werden dabei von strukturellen Eigenschaften determiniert, deren Bestimmung mit hohem experimentellem Aufwand verbunden ist. Die vorliegende Arbeit erweitert ein hochparalleles Modell zur atomistischen Simulation des Wachstums und der Struktur von Dünnschichten, welches Molekulardynamik (MD) und Kinetic Monte Carlo-Methoden (KMC) kombiniert, um die Beschreibung beliebiger Gasphasenabscheidungen. KMC-Methoden erlauben dabei die effiziente Betrachtung der Größenordnung ganzer Nano-Bauelemente, während MD für atomistische Genauigkeit sorgt. Erste Ergebnisse zeigen, dass das Parsivald genannte Modell Abscheidungen in Simulationsräumen mit einer Breite von 0.1 µm x 0.1 µm effizient berechnet, aber auch bis zu 1 µm x 1 µm große Räume mit 1 Milliarden Atomen beschreiben kann. Somit lassen sich innerhalb weniger Tage Schichtabscheidungen mit einer Dicke von 100 Å simulieren. Die kristallinen und amorphen Schichten zeigen glatte Oberflächen, wobei auch mehrlagige Systeme auf die jeweilige Lagenrauheit untersucht werden. Die Struktur der Schicht wird hauptsächlich durch die verwendeten molekulardynamischen Kraftfelder bestimmt, wie Untersuchungen der physikalischen Gasphasenabscheidung von Gold, Kupfer, Silizium und einem Kupfer-Nickel-Multilagensystem zeigen. Stark strukturierte Substrate führen hingegen zu Artefakten in Form von Nanoporen und Hohlräumen aufgrund der verwendeten KMC-Methode. Zur Simulation von chemischen Gasphasenabscheidungen werden die Precursor-Reaktionen von Silan mit Sauerstoff sowie die Hydroxylierung von alpha-Al2O3 mit Wasser mit reaktiven Kraftfeldern (ReaxFF) berechnet, allerdings ist weitere Arbeit notwendig, um komplette Abscheidungen auf diese Weise zu simulieren. Mit Parsivald wird somit die Erweiterung einer Software präsentiert, die Gasphasenabscheidungen auf großen Substraten effizient simulieren kann, dabei aber auf passende molekulardynamische Kraftfelder angewiesen ist.

Siliziumnanodrähte (SiNWs) bieten eine aussichtsreiche Grundlage zur Entwicklung neuartiger nanoelektronischer Bauelemente, wie Feldeffekttransistoren oder Sensoren. Dabei ist insbesondere die Oxidation der Drähte interessant, weil diese weitreichenden Einfluss auf die elektronischen Eigenschaften der Bauelemente hat, die aus den SiNWs gefertigt werden. Die Größe der untersuchten Strukturen erfordert eine atomistische Analyse des Oxidationsprozesses. In der vorliegenden Arbeit wird der bisher wenig verstandene Beginn der Oxidation dünner Drähte molekulardynamisch simuliert, wobei als Potential ein reaktives Kraftfeld dient. Dabei wird sich intensiv mit dem Transfer elektrischer Ladungen zwischen Atomen unterschiedlicher Elektronegativitäten während der Simulationen auseinandergesetzt. Desweiteren werden Strukturen, die während der Oxidation von SiNWs der Orientierungen <100> und <110> bei Temperaturen von 300 K und 1200 K entstehen, untersucht. Ein Fokuspunkt dieser Untersuchungen ist die Analyse der Anzahl am Draht adsorbierter Sauerstoffatome während der frühen Oxidationsphase. Darüber hinaus wird die Dichte der entstehenden Strukturen beleuchtet. Dies geschieht mit einer hohen radialen Auflösung und erstmalig während der gesamten Simulation. Hierbei zeigt sich, dass während des Übergangs von kristallinem Silizium zu amorphem Siliziumdioxid zwischen den Siliziumatomen Sauerstoff eingelagert wird, die Kristallstruktur des Siliziums sich zunächst jedoch noch nicht auflöst. Dadurch entsteht ein charakteristisches Muster hoher und niedriger Dichten, das von der ursprünglichen Kristallstruktur des SiNW abhängt.:Abbildungsverzeichnis Abkürzungsverzeichnis Symbolverzeichnis 1 Einleitung 2 Einführung zu Siliziumnanodrähten 2.1 Kristallstuktur von Silizium 2.2 Ideale Siliziumnanodrähte 2.3 Herstellung von Siliziumnanodrähten 3 Grundlagen der Molekulardynamik 3.1 Newtonsche Axiome 3.2 Einige grundlegende Begriffe der statistischen Physik 3.3 Molekulardynamik 3.4 Reaktives Kraftfeld 3.5 Methoden zur Beschreibung des Ladungstransfers 3.6 Thermostat und Barostat 3.7 Large-scale Atomic/Molecular Massively Parallel Simulator 4 Entwicklung des Modellsystems 4.1 Ausgangsstruktur 4.2 Vorrelaxation 4.3 Ablauf der Oxidation 4.4 Verwendeter ReaxFF-Parametersatz 4.5 Optimierung der Zeitschrittweite 4.5.1 Modellsystem, Relaxation und Oxidation 4.5.2 Festlegung der Zeitschrittweite 4.6 Optimierung der Systemlänge 4.6.1 Modellsystem, Relaxation und Oxidation 4.6.2 Festlegung der Systemlänge 4.7 Einfluss des globalen, instantanen Ladungstransfers auf die Simulation 4.7.1 Festlegung des Einsetzabstands 4.7.2 Vergleich mit Daten von Khalilov et al. 5 Variation von System- und Einsetztemperatur sowie Drahtorientierung 5.1 Variation von System- und Einsetztemperatur 5.1.1 Untersuchung des Oxidationsgrads 5.1.2 Untersuchung von Dichten und Grenzflächenpositionen 5.2 Variation der Drahtorientierung 5.2.1 Untersuchung des Oxidationsgrads 5.2.2 Untersuchung von Dichten und Grenzflächenpositionen 6 Zusammenfassung und Ausblick 6.1 Zusammenfassung 6.2 Ausblick Literaturverzeichnis

Siliziumnanodrähte (SiNWs) bieten aufgrund ihrer exzellenten elektrostatischen Kontrollierbarkeit eine gute Grundlage für die Entwicklung neuartiger Bauelemente, wie rekonfigurierbarer Feldeffekttransistoren (RFETs). Da SiNWs durch die Oxidation gezielt verzerrt werden können und diese Verzerrung die Bandstruktur des Siliziums verändert, bietet der Oxidationsprozess eine Möglichkeit, die Leitungseigenschaften der RFETs zu modulieren und eine symmetrische Transfercharakteristik zu erhalten. Die Untersuchung von SiNWs mit Durchmessern im einstelligen Nanometerbereich bedarf eines atomistischen Ansatzes. In der vorliegenden Arbeit wird mit einem reaktiven Kraftfeld die initiale Phase der Oxidation dünner SiNWs molekulardynamisch simuliert. Gegenstand der Untersuchungen sind die Temperaturabhängigkeit der Oxidation von <110>-SiNWs mit Anfangsradien von 10.2 Å sowie das Oxidationsverhalten von <110>- und <100>-SiNWs mit Anfangsradien von 5.1 Å. Dabei wird neben dem Sauerstoffanteil im Simulationssystem und der radial aufgelösten Dichte auch das radial aufgelöste Verhältnis zwischen Sauerstoff- und Siliziumatomen während der gesamten Simulationsdauer untersucht und ein Zusammenhang zur Dichte festgestellt. Darüber hinaus wird bei 300 K erstmals eine Analyse der Verzerrungsentwicklung während der initialen Oxidationsphase durchgeführt, bei der sich sowohl für <110>-SiNWs als auch für <100>-SiNWs eine tensile Verzerrung im unoxidierten Drahtkern einstellt. Wie eine Analyse der partiellen radialen Verteilungsfunktion zeigt, kommt es zu dieser Verzerrung, weil während der Oxidation die Grundstruktur des Siliziums im Oxid erhalten bleibt, durch die Einlagerung des Sauerstoffs allerdings der Bindungsabstand erhöht wird. Dieser erhöhte Bindungsabstand wird durch Bindungen zu Siliziumatomen im Oxid auch Siliziumatomen im unoxidierten Kern aufgezwungen.:Inhaltsverzeichnis Abbildungsverzeichnis Tabellenverzeichnis Abkürzungsverzeichnis Symbolverzeichnis 1. Einleitung 2. Theoretische Grundlagen 2.1. Molekulardynamik 2.2. Siliziumnanodrähte 2.3. Verzerrung und Verspannung 3. Modellsystem 3.1. Ausgangsstruktur 3.2. Vorrelaxation 3.3. Ablauf der Oxidation 4. Untersuchungsmethoden 4.1. Sauerstofffluenz, Oxidationsgrad und Oxidationsrate 4.2. Massendichte und Siliziumanteil 4.3. Radiale Verteilungsfunktion 4.4. Verzerrung 4.4.1. <110>-Draht 4.4.2. <100>-Draht 5. Ergebnisse und Diskussion 5.1. Festlegung des Einsetzintervalls 5.2. Temperaturvariation 5.2.1. Oxidationsgrad 5.2.2. Siliziumanteil 5.2.3. Massendichte 5.2.4. Radiale Verteilungsfunktion 5.3. Radius- und Orientierungsvariation 5.4. Verzerrung 6. Zusammenfassung und Ausblick 6.1. Zusammenfassung 6.2. Ausblick A. Festlegung des Einsetzintervalls Literaturverzeichnis

L’étude de la dissociation de molécules poly-atomiques en surface est d’une importance à la fois fondamentale et industrielle. La compréhension du mécanisme et la dynamique réactionnelle sous-jacents représente un défi. Comme un système modèle, la dissociation de méthane sur la surface de nickel a fait l’objet de nombreuses études pour élucider les chemins de réaction et le transfert d’énergie parmi les différents degrés de liberté durant la réaction. La mode-spécifique ou liaison-spécifique réactivité pour la dissociation de CH4 sur Ni(111) et Ni(100) ont été mise en évidence récemment par des expériences de pointe du jet moléculaire. Jusqu’à présent, les études théoriques de la dynamique réactionnelle ont été effectuées avec un modèle simplifié dans lequel CH4 est décrit comme une molécule pseudo-diatomique. Le concept d’un groupe méthyle spectateur introduit dans un tel modèle impose des contraintes drastiques. Par exemple, l’indiscernabilité des quatre liaisons C-H de méthane est violée par le fait que la liaison C-H capable de se dissocier se singularise par rapport aux trois autres liaisons inertes. En réalité, n’importe quelle des quatre liaisons est susceptible de se dissocier. Par ailleurs, l’unique mode vibrationnel du modèle pseudo-diatomique ne ressemble à aucun des quatre modes vibrationnels principaux du méthane, qui décrivent tous des mouvements collectifs de plusieurs atomes. Lorsque tous les degrés de liberté sont pris en compte, la dimensionnalité de la surface de l’énergie potentielle pour CH4/Ni(111) est très élevée (15 degrés de liberté pour CH4 et certains degrés de liberté du substrat). Construire une surface de l’énergie potentielle fiable à une telle grande dimension est, en soi, un grand défi. A notre connaissance, ce défi n’a jamais été tenté auparavant pour quelconque réaction d’une molécule poly-atomique sur une surface métallique. En utilisant un champ de force réactif, nous avons développé, dans le présent travail, une surface de l’énergie potentielle qui prend en compte tous les 15 degrés de liberté de CH4 ainsi que ceux des 3 premières couches de NI(111). Des simulations de dynamique moléculaire ont été effectuées pour étudier la dynamique réaction de CH4 sur Ni(111) aussi bien dans son état fondamental vibrationnel que dans un état excité. Ces simulations ont permis de révéler des comportements dynamiques inattendus et très intéressants. / In the present work, we undertook a challenging task, i.e., construction a full-dimension potential energy surface (PES) for a benchmark poly-atomic molecular surface reaction, CH4/Ni(111), by using a reactive force field. Careful appraisal of the PES was made in order to establish the validity of the PES. The differences between the results for the transition state (dissociation barriers and structures) given by our PES and those by DFT calculations do not exceed 15%. The molecular dynamics simulation results obtained by using our PES are compared to experimental results for CH4 dissociation probability on Ni(111). For the vibrationally excited state, v3 (v=1, J=0), the agreement between our simulation results and the experimental ones is excellent. For the ground state, the sticking coefficient is somehow over-estimated because of the under-estimation of the dissociation barrier by about 150 meV with our reactive force field. Nevertheless, the overall agreement between simulation and experiment is pretty good. Within the help of the full-dimensional PES, we have extensively studied some important aspects of reaction dynamics, e.g., the effects of surface impact position, surface temperature, vibrationally excited state, rotationally excited states etc. For CH4 in ground state (v=0, J=0), the investigation of the effect of CH4 impact position shows that the top site is the most reactive one. The surface temperature strongly affects the reactivity of methane, especially in the region of the low incident energy near to the dissociation threshold, while in the high incident energy region, the effect is less important. For CH4 v3 (v=1, J=0), an important coupling between rotation and vibration is found. The rotation of CH4 can enhance its reactivity in the following way. In its ground state (v=0, J=0), CH4 does not rotate during its flight to the surface. In this case, only one of the two lowest C-H bonds pointing initially toward the surface can be cleaved while the two other bonds never break. In v3 (v=1, J=0) vibrational state, due to the rotation induced by vibration-rotation coupling, any of the four H atoms can be dissociated even if it forms a C-H bond which has an unfavorable initial orientation (i.e., pointing away from the substrate). The rotation of CH4 induced by vibration-rotation coupling near the substrate allows for bringing an unfavorable initial orientation of C-H bond to the right one required by a transition state (TS) during the adsorbate’s approaching to the substrate. As the enhanced reactivity of vibrationally excited molecules is concerned, the intuitively limpid and overwhelmingly accepted explanation is that the vibration-induced bond stretching helps bond breaking. Our simulation results show clearly that the vibration-induced CH4 rotation contributes an important part to the enhanced reactivity of a v3 (v=1, J=0) vibrationally excited CH4. A series of simulations to determine the sticking curves for CH4 in the vibrational ground state (ν=0) but excited to higher rotational levels (J=0-12) have also been performed. Due to its small level spacing, the lowest rotational excited states (J=1-3) of CH4 do not affect its reactivity on Ni(111) as observed experimentally. We found that rotation enhances significantly CH4 reactivity on Ni(111) with a deposited rotational energy amounting only to 12% of the dissociation barrier. Moreover, in a hypothetic simulation, we found also very striking evidences that rotation can even promote better dissociation of CH4 on Ni(111) than vibration. In a vibrationally excited CH4, its C-H bonds undergo alternate stretching and compressing and the latter hinders dissociation. In this case, the reactivity is inevitably modulated by vibration phase. However, the centrifugal force due to rotation tends always to stretch the C-H bonds for CH4 in rotationally excited states. / 多原子气相分子的分解，不仅在物理，化学及相关学科有着基本的重要性，而且可以促进工业进程，如工业制氢气。对其涉及的反应，即化学键的断裂与形成，在理解其反应机制和动力学上更是一项挑战。作为多原子气相-固相化学反应中最为典型的反应，甲烷分子在金属镍表面的分解，已经被广泛地研究从而理解其在动力学过程中的能量转化和反应路径。最近，选态分子束实验报道了有关甲烷在镍表面分解反应的重要特征，即模式选择性和化学键选择性。从理论角度来看，以前大多数理论研究都是基于一个简化模型，即将甲烷分子看成是一个赝双原子分子(CH4=RH,其中R=CH3)。在该简化模型中，将甲基团当做一个“spectator”会导致严重的限制性，如四个碳氢键的不可分辨性就被破坏。因为在简化模型中，只有一个可分解的碳氢键而其他三个碳氢键则被保护起来；而在实际的分解反应过程中，甲烷分子的任何一个碳氢化学键应该都有概率被分解掉。此外，在该赝双原子分子模型中，单键伸长振动模式不能类比于甲烷的四个基本振动模式，因为其每种基本振动模式都涉及多个原子的复合运动。如果不将甲烷处理成赝双原子分子，那么该体系（CH4/Ni(111)）的势能面的维度会很高，即甲烷的15 个自由度加上部分基地原子的自由度。欲建立一个如此高维度而且又可靠的势能面，本身就是一个值得挑战的研究任务。据我们所知，目前对多原子分子在金属表面反应的高维度势能面的报道几乎没有。在本论文中，我们运用键序反应力场（REBO），为体系CH4/Ni(111)，首次建立起一个全维度的势能面。该势能面的维度包含甲烷的15 个自由度和3 层基地原子的自由度。在经典分子动力学（和准经典分子动力学）模拟下，我们研究了甲烷处于基态和激发态时在金属表面的分解活性，并发现了一些非常有趣的结果。本论文包含以下六章：第一章：简单介绍了甲烷在过渡金属表面分解的最新进展。在选态分子束试验报道中，我们介绍了一些有关该反应的重要特征，如模式选择性，化学键选择性，表面温度效应，空间效应，旋转激发效应等。在理论工作方面，主要介绍了两个理论研究小组近期在简化模型下的一些量子动力学结果。第二章：对本文所运用的理论方法和近似做了基本的介绍。这些方法主要归纳于两类：i)电子结构计算；ii) 分子动力学模拟。我们重点介绍了这些方法和近似的特征。第三章：我们运用二阶矩近似力场(SMA)和键序反应力场(REBO)模拟了氢分子在金属钯表面的分解反应, 从而验证反应力场在模拟表面化学反应的适用性。该章讨论了在参数化反应力场时的一些影响因素，如有效数据库大小，不同排斥势以及长程作用项等，为对复杂体系的研究提供了有效的帮助。第四章：基于键序反应力场(REBO)，我们首次为CH4/Ni(111) 体系建立起一个全维度势能面(PES)。同时我们对该势能面(REBO(PES))做了全面评估，如比较势能面(REBO(PES))与DFT计算得到的过渡状态结构和与之对应的分解势垒，比较两者对于不同形式相互作用给出的势能变化等。此外，我们还直接模拟了甲烷在基态时的活性，其模拟结果与实验有着很好的符合度，从而进一步地说明了该势能面(REBO(PES))的可靠性。第五章：在全维度势能面下，我们深入地研究了甲烷处于不同状态时在镍表面分解的反应活性，即基态(v=0,J=0)，反对称振动态v3 (v=1,J=0)和旋转激发态(v=0,J=1-12)。对于基态的甲烷，我们定性并定量地分析了表面碰撞位置，表面温度对其分解概率的影响。对于反对称振动态的甲烷，我们观察到振动激发态的甲烷分子反应活性比基态甲烷的反应活性要大大地增强。究其根源在于，平动能量不易转换至旋转自由度，而振动能量则非常容易转入到旋转自由度。我们利用三个定量参数详细地阐述了这种振动耦合转动的重要性。此外，对于甲烷处于旋转激发态时，我们发现其激发状态非常有利于甲烷的分解，尽管其旋转能量只有分解势垒的12%。更为惊奇的是，对于甲烷分子而言，其旋转激发态比振动激发态更有利于其分解。其相应的物理解释是，对于振动激发的甲烷，它的碳氢键处于伸长与收缩的交替中，而后者却阻止其分解。对于旋转激发中的甲烷，其离心力一直促使碳氢键的伸长。第六章：总结和展望。我们总结了本文的主要结论以及给出一些将来需要进行的工作，如同位素效应等。