Studies of Inorganic Layer and Framework Structures Using Time-, Temperature- and Pressure-Resolved Powder Diffraction Techniques

This thesis is concerned with in-situ time-, temperature- and pressure-resolved synchrotron X-ray powder diffraction investigations of a variety of inorganic compounds with twodimensional layer structures and three-dimensional framework structures. In particular, phase stability, reaction kinetics, thermal expansion and compressibility at non-ambient conditions has been studied for 1) Phosphates with composition MIV(HPO4)2·nH2O (MIV = Ti, Zr); 2) Pyrophosphates and pyrovanadates with composition MIVX2O7 (MIV = Ti, Zr and X = P, V); 3) Molybdates with composition ZrMo2O8. The results are compiled in seven published papers and two manuscripts. Reaction kinetics for the hydrothermal synthesis of α-Ti(HPO4)2·H2O and intercalation of alkane diamines in α-Zr(HPO4)2·H2O was studied using time-resolved experiments. In the high-temperature transformation of γ-Ti(PO4)(H2PO4)·2H2O to TiP2O7 three intermediate phases, γ'-Ti(PO4)(H2PO4)·(2-x)H2O, β-Ti(PO4)(H2PO4) and Ti(PO4)(H2P2O7)0.5 were found to crystallise at 323, 373 and 748 K, respectively. A new tetragonal three-dimensional phosphate phase called τ-Zr(HPO4)2 was prepared, and subsequently its structure was determined and refined using the Rietveld method. In the high-temperature transformation from τ-Zr(HPO4)2 to cubic α-ZrP2O7 two new orthorhombic intermediate phases were found. The first intermediate phase, ρ-Zr(HPO4)2, forms at 598 K, and the second phase, β-ZrP2O7, at 688 K. Their respective structures were solved using direct methods and refined using the Rietveld method. In-situ high-pressure studies of τ-Zr(HPO4)2 revealed two new phases, tetragonal ν-Zr(HPO4)2 and orthorhombic ω-Zr(HPO4)2 that crystallise at 1.1 and 8.2 GPa. The structure of ν-Zr(HPO4)2 was solved and refined using the Rietveld method. The high-pressure properties of the pyrophosphates ZrP2O7 and TiP2O7, and the pyrovanadate ZrV2O7 were studied up to 40 GPa. Both pyrophosphates display smooth compression up to the highest pressures, while ZrV2O7 has a phase transformation at 1.38 GPa from cubic to pseudo-tetragonal β-ZrV2O7 and becomes X-ray amorphous at pressures above 4 GPa. In-situ high-pressure studies of trigonal α-ZrMo2O8 revealed the existence of two new phases, monoclinic δ-ZrMo2O8 and triclinic ε-ZrMo2O8 that crystallises at 1.1 and 2.5 GPa, respectively. The structure of δ-ZrMo2O8 was solved by direct methods and refined using the Rietveld method.

Mesoscale dynamics and boundary-layer structure in topographically forced low-level jets

Two types of mesoscale wind-speed jet and their effects on boundary-layer structure were studied. The first is a coastal jet off the northern California coast, and the second is a katabatic jet over Vatnajökull, Iceland. Coastal regions are highly populated, and studies of coastal meteorology are of general interest for environmental protection, fishing industry, and for air and sea transportation. Not so many people live in direct contact with glaciers but properties of katabatic flows are important for understanding glacier response to climatic changes. Hence, the two jets can potentially influence a vast number of people. Flow response to terrain forcing, transient behavior in time and space, and adherence to simplified theoretical models were examined. The turbulence structure in these stably stratified boundary layers was also investigated. Numerical modeling is the main tool in this thesis; observations are used primarily to ensure a realistic model behavior. Simple shallow-water theory provides a useful framework for analyzing high-velocity flows along mountainous coastlines, but for an unexpected reason. Waves are trapped in the inversion by the curvature of the wind-speed profile, rather than by an infinite stability in the inversion separating two neutral layers, as assumed in the theory. In the absence of blocking terrain, observations of steady-state supercritical flows are not likely, due to the diurnal variation of flow criticality. In many simplified models, non-local processes are neglected. In the flows studied here, we showed that this is not always a valid approximation. Discrepancies between simulated katabatic flow and that predicted by an analytical model are hypothesized to be due to non-local effects, such as surface inhomogeneity and slope geometry, neglected in the theory. On a different scale, a reason for variations in the shape of local similarity scaling functions between studies is suggested to be differences in non-local contributions to the velocity variance budgets.

Dynamics of the cold surface layer of polythermal Storglaciären, Sweden

Polythermal glaciers, i.e. glaciers with a combination of ice at and below the freezing point, are widespread in arctic and subarctic environments. The polythermal structure has major implications for glacier hydrology, ice flow and glacial erosion. However, the interplay of factors governing its spatial and temporal variations such as net mass balance, ice advection and water content in the ice is poorly investigated and as yet not fully understood. This study deals with a thorough investigation of the polythermal regime on Storglaciären, northern Sweden, a small valley glacier with a cold surface layer in the ablation area. Extensive field work was performed including mapping of the cold surface layer using ground-penetrating radar, ice temperature measurements, mass balance and ice velocity measurements. Analyses of these data combined with numerical modelling were used specifically to investigate the spatial and temporal variability of the cold surface layer, the spatial distribution of the water content just below the cold surface layer transition, the effect of radar frequency on the detection of the surface layer, and the sensitivity of the cold surface layer to changes in forcing. A comparison between direct temperature measurements in boreholes and ground-penetrating surveys shows that the radar-inferred cold-temperate transition depth is within ±1 m from the melting point of ice at frequencies above ~300 MHz. At frequencies below ~155 MHz, the accuracy degrades because of reduced scattering efficiency that occurs when the scatterers become much smaller compared to the wavelength. The mapped spatial pattern of the englacial cold-temperate transition boundary is complex. This pattern reflects the observed spatial variation in net loss of ice at the surface by ablation and vertical advection of ice, which is suggested to provide the predominant forcing of the cold surface layer thickness pattern. This is further supported by thermomechanical modeling of the cold surface layer, which indicates high sensitivity of the cold surface layer thickness to changes in vertical advection rates. The water content is the least investigated quantity that is relevant for the thermal regime of glaciers, but also the most difficult to assess. Spatial variability of absolute water content in the temperate ice immediately below the cold surface layer on Storglaciären was determined by combining relative estimates of water content from ground-penetrating radar data with absolute determination from temperature measurements and the thermal boundary condition at the freezing front. These measurements indicate large-scale spatial variability in the water content, which seems to arise from variations in entrapment of water at the firn-ice transition. However, this variability cannot alone explain the spatial pattern in the thermal regime on Storglaciären. Repeated surveys of the cold surface layer show a 22% average thinning of the cold surface layer on Storglaciären between 1989 and 2001. Transient thermomechanical modeling results suggest that the cold surface layer adapts to new equilibrium conditions in only a few decades after a perturbation in the forcing is introduced. An increased winter air temperature since mid-1980s seems to be the cause of the observed thinning of the cold surface layer. Over the last decades, mass balance measurements indicate that the glacier has been close to a steady state. The quasi-steady state situation is also reflected in the vertical advection, which shows no significant changes during the last decades. Increased winter temperatures at the ice surface would result in a slow-down of the formation of cold ice at the base of the cold surface layer and lead to a larger imbalance between net loss of ice at the surface and freezing of temperate ice at the cold-temperate transition.

Structural investigation of Nb-based layer sulfides

In this work we have investigated the intercalation of electron-donors between NbS2 slabs in Nb-based layer sulfides. Two series of Sr substituted Nb-based misfit sulfides belonging to the 1.5Q/1H and 1Q/1H series of misfit layer compounds have been synthesised. For large lanthanides (Ln=La, Ce), only the 1Q/1H compounds formed whereas for smaller lanthanides and yttrium, both types of phases can be obtained. The crystal structure of misfit sulfide (Pr0.55Sr0.45S)1.15NbS2 has been refined using the composite approach. In the Q-slab, Pr-atoms are partly replaced by Sr with a random distribution over one cation position. The crystal structure of misfit sulfide [(Sm1/3Sr2/3S)1.5]1.15NbS2 belonging to the 1.5Q/1H series have also been determined. The obtained results suggest a preferred occupancy of the cation positions in the slab where Sr atoms mainly occupy positions on the exterior of the slab while Sm atoms are in the center of the slab. The (La1-xSrxS)1.15NbS2 solid solution (0.1<x<0.9) has also been studied. It was found that the maximum value of Sr substitution is 40-50% and therefore, the minimal value of charge transfer to stabilize this structure type is about 0.6ē per Nb atom. An attempt to synthesize SrxNbS2 (0.1≤x≤0.5) intercalates was made but single phases were not obtained and increasing the temperature from 1000оС to 1100оС leads to the decomposition of these intercalates. Single crystals of Sr0.22Nb1.05S2 and Sr0.23NbS2 were found and their structures were determined. The structures belong to two different types of packings with statistical distribution of Sr between layers. A new superconducting sulfide, "EuNb2S5", was investigated by ED and HREM and its structure model consisting of Nb7S14 and (Eu3S4)2 slabs alternating along the c-axis is suggested. An attempt to suggest a model for the structure of "SrNb2S5" by means of X-ray single crystal diffraction was made. The proposed structure consists of two types of slabs: a Nb7S14 and a [Sr6(NbS4)2S] slab with niobium in tetrahedral coordination. It is shown that "SrNb2S5" and "EuNb2S5" are have similar structures. For the first time, single crystals of the complex sulfide BaNb0.9S3 have also been studied by means of X-ray single crystal diffraction. The single crystal refinement and EDX analysis showed the existence of cation vacancies at the niobium position. BaNb0.9S3 has also been studied by ED and no superstructure was found which implies that and the vacancies are statistically distributed. No improvement of the magnetic properties of the studied compounds was observed in comparison to NbS2.

Coupling between upper ocean layer variability and size-fractionated phytoplankton in a non-nutrient-limited environment

[EN] We describe the coupling between upper ocean layer variability and size-fractionated phytoplankton distribution in the non-nutrient-limited Bransfield Strait region (BS) of Antarctica. For this purpose we use hydrographic and size-fractionated chlorophyll a data from a transect that crossed 2 fronts and an eddy, together with data from 3 stations located in a deeply mixed region, the Antarctic Sound (AS). In the BS transect, small phytoplankton (<20 μm equivalent spherical diameter [ESD]) accounted for 80% of total chl a and their distribution appeared to be linked to cross-frontal variability. On the deepening upper mixed layer (UML) sides of both fronts we observed a deep subducting column-like structure of small phytoplankton biomass. On the shoaling UML sides of both fronts, where there were signs of restratification, we observed a local shallow maximum of small phytoplankton biomass. We propose that this observed phytoplankton distribution may be a response to the development of frontal vertical circulation cells. In the deep, turbulent environment of the AS, larger phytoplankton (>20 μm ESD) accounted for 80% of total chl a. The proportion of large phytoplankton increases as the depth of the upper mixed layer (ZUML), and the corresponding rate of vertical mixing, increases. We hypothesize that this change in phytoplankton composition with varying ZUML is related to the competition for light, and results from modification of the light regime caused by vertical mixing.

Cross-layer optimizations in multi-hop ad hoc networks

Unlike traditional wireless networks, characterized by the presence of last-mile, static and reliable infrastructures, Mobile ad Hoc Networks (MANETs) are dynamically formed by collections of mobile and static terminals that exchange data by enabling each other's communication. Supporting multi-hop communication in a MANET is a challenging research area because it requires cooperation between different protocol layers (MAC, routing, transport). In particular, MAC and routing protocols could be considered mutually cooperative protocol layers. When a route is established, the exposed and hidden terminal problems at MAC layer may decrease the end-to-end performance proportionally with the length of each route. Conversely, the contention at MAC layer may cause a routing protocol to respond by initiating new routes queries and routing table updates. Multi-hop communication may also benefit the presence of pseudo-centralized virtual infrastructures obtained by grouping nodes into clusters. Clustering structures may facilitate the spatial reuse of resources by increasing the system capacity: at the same time, the clustering hierarchy may be used to coordinate transmissions events inside the network and to support intra-cluster routing schemes. Again, MAC and clustering protocols could be considered mutually cooperative protocol layers: the clustering scheme could support MAC layer coordination among nodes, by shifting the distributed MAC paradigm towards a pseudo-centralized MAC paradigm. On the other hand, the system benefits of the clustering scheme could be emphasized by the pseudo-centralized MAC layer with the support for differentiated access priorities and controlled contention. In this thesis, we propose cross-layer solutions involving joint design of MAC, clustering and routing protocols in MANETs. As main contribution, we study and analyze the integration of MAC and clustering schemes to support multi-hop communication in large-scale ad hoc networks. A novel clustering protocol, named Availability Clustering (AC), is defined under general nodes' heterogeneity assumptions in terms of connectivity, available energy and relative mobility. On this basis, we design and analyze a distributed and adaptive MAC protocol, named Differentiated Distributed Coordination Function (DDCF), whose focus is to implement adaptive access differentiation based on the node roles, which have been assigned by the upper-layer's clustering scheme. We extensively simulate the proposed clustering scheme by showing its effectiveness in dominating the network dynamics, under some stressing mobility models and different mobility rates. Based on these results, we propose a possible application of the cross-layer MAC+Clustering scheme to support the fast propagation of alert messages in a vehicular environment. At the same time, we investigate the integration of MAC and routing protocols in large scale multi-hop ad-hoc networks. A novel multipath routing scheme is proposed, by extending the AOMDV protocol with a novel load-balancing approach to concurrently distribute the traffic among the multiple paths. We also study the composition effect of a IEEE 802.11-based enhanced MAC forwarding mechanism called Fast Forward (FF), used to reduce the effects of self-contention among frames at the MAC layer. The protocol framework is modelled and extensively simulated for a large set of metrics and scenarios. For both the schemes, the simulation results reveal the benefits of the cross-layer MAC+routing and MAC+clustering approaches over single-layer solutions.

Modeling the gas and aqueous phase chemistry of the marine boundary layer

Ein eindimensionales numerisches Modell der maritimenGrenzschicht (MBL) wurde erweitert, um chemische Reaktionenin der Gasphase, von Aerosolpartikeln und Wolkentropfen zu beschreiben. Ein Schwerpunkt war dabei die Betrachtung derReaktionszyklen von Halogenen. Soweit Ergebnisse vonMesskampagnen zur Verfuegung standen, wurden diese zurValidierung des Modells benutzt. Die Ergebnisse von frueheren Boxmodellstudien konntenbestaetigt werden. Diese zeigten die saeurekatalysierteAktivierung von Brom aus Seesalzaerosolen, die Bedeutung vonHalogenradikalen fuer die Zerstoerung von O3, diepotentielle Rolle von BrO bei der Oxidation von DMS und dievon HOBr und HOCl in der Oxidation von S(IV). Es wurde gezeigt, dass die Beruecksichtigung derVertikalprofile von meteorologischen und chemischen Groessenvon grosser Bedeutung ist. Dies spiegelt sich darin wider,dass Maxima des Saeuregehaltes von Seesalzaerosolen und vonreaktiven Halogenen am Oberrand der MBL gefunden wurden.Darueber hinaus wurde die Bedeutung von Sulfataerosolen beidem aktiven Recyceln von weniger aktiven zu photolysierbarenBromspezies gezeigt. Wolken haben grosse Auswirkungen auf die Evolution und denTagesgang der Halogene. Dies ist nicht auf Wolkenschichtenbeschraenkt. Der Tagesgang der meisten Halogene ist aufgrundeiner erhoehten Aufnahme der chemischen Substanzen in die Fluessigphase veraendert. Diese Ergebnisse betonen dieWichtigkeit der genauen Dokumentation der meteorologischenBedingungen bei Messkampagnen (besonders Wolkenbedeckungsgrad und Fluessigwassergehalt), um dieErgebnisse richtig interpretieren und mit Modellresultatenvergleichen zu koennen. Dieses eindimensionale Modell wurde zusammen mit einemBoxmodell der MBL verwendet, um die Auswirkungen vonSchiffemissionen auf die MBL abzuschaetzen, wobei dieVerduennung der Abgasfahne parameterisiert wurde. DieAuswirkungen der Emissionen sind am staerksten, wenn sie insauberen Gebieten stattfinden, die Hoehe der MBL gering istund das Einmischen von Hintergrundluft schwach ist.Chemische Reaktionen auf Hintergrundaerosolen spielen nureine geringe Rolle. In Ozeangebieten mit schwachemSchiffsverkehr sind die Auswirkungen auf die Chemie der MBL beschraenkt. In staerker befahrenen Gebieten ueberlappensich die Abgasfahnen mehrerer Schiffe und sorgen fuerdeutliche Auswirkungen. Diese Abschaetzung wurde mitSimulationen verglichen, bei denen die Emissionen alskontinuierliche Quellen behandelt wurden, wie das inglobalen Chemiemodellen der Fall ist. Wenn die Entwicklungder Abgasfahne beruecksichtigt wird, sind die Auswirkungendeutlich geringer da die Lebenszeit der Abgase in der erstenPhase nach Emission deutlich reduziert ist.

Charakterisierung von S-Layer-Proteinen bei Prokaryoten

S-Layer sind kristalline Proteinschichten, die als Komponenten von Zellwänden in allen Zweigen der Bakterien und Archaebakterien vorkommen. Aus der Domäne der Archaea wurden die S-Layer-Proteine mesophiler, thermophiler und hyperthermophiler Methanococcales verglichen. Die Zellwand dieser Organismen besteht nur aus einer S-Layerschicht, die die Zellen vor äußeren Einflüssen schützt. Analog zu den Methanococcales wurden S-Layer-Proteine mesophiler und thermophiler Vertreter aus der Familie der Bacillaceae verglichen.Ziel dieser Arbeit war es, die S-Layer-Gensequenzen von Methanotorris igneus, Methanothermococcus thermolithotrophicus, Methanococcus vannielii, Geobacillus stearothermophilus, Bacillus sphaericus und Bacillus fusiformis zu ermitteln. Durch Vergleiche der Primärsequenzen mesophiler und (hyper)thermophiler S-Layer-Proteine sollten Hinweise auf thermostabilisierende Faktoren abgeleitet werden. Durch Verwendung geeigneter bio-chemischer und gentechnischer Arbeitsmethoden wurden die Gen- und Proteinsequenzen der S-Layer-Proteine ermittelt. Die unbekannten Genbereiche wurden durch die Entwicklung einer modifizierten Zwei- und Drei-Schritt-PCR ermittelt.Die Sequenzanalyse der archaebakteriellen und bakteriellen S-Layer-Proteine erbrachte nur für erstere Hinweise auf thermostabilisierende Faktoren. Die (hyper)thermophilen S-Layer-Hüllproteine aus der Ordnung Methanococcales zeigten gegenüber den mesophilen folgende Unterschiede:1. Zunahme von geladenen Resten;2. Abnahme von Alanin und unpolaren Resten3. Vorhandensein von Cystein4. Erhöhte Anzahl an N-glykosidischen BindungsstellenDas hyperthermophile S-Layer-Protein von Mcc. jannaschii wurde in Escherichia coli erfolgreich expremiert. Für das native Hüllprotein wurde, als ein möglicherweise weiterer thermostabilisierender Faktor eine Glykosilierung detektiert. Zudem wies das native S-Layer-Protein eine Konformationsänderung im Verlauf einer Temperaturerhöhung, bei verschiedenen pH-Werten und in Anwesenheit zweiwertigen Mangans auf. Auch das Expressionsprotein zeigte im Verlauf der Temperaturerhöhung und bei verschiedenen pH-Werten eine Konformationsänderung. Mn2+ hatte dagegen keinen Effekt und eine Glykosi-lierung war nicht nachweisbar.Die transkriptionellen und translationellen Erkennungsregionen der S-Layer-Gene aus der Ordnung Methanococcales wurden bestimmt. Basierend auf Sequenzähnlichkeiten und Gemeinsamkeiten der archaebakteriellen S-Layer-Proteine wurden diese in vier Gruppen eingeteilt.Aus dem bakteriellen Zweig wurde die Gen- und Primärsequenzen der S-Layer-Proteine von Gb. stearothermophilus DSM 2358, B. sphaericus DSM 396, B. fusiformis B3 und DSM 2898T ermittelt. Die Proteine wiesen z. T. eine geringere molekulare Masse auf als die be-kannten S-Layer-Hüllproteine aus der Familie der Bacillaceae. Das Protein von B. sphaericus DSM 396 zeigte eine Diskrepanz zwischen der theoretischen (85 kDA) und experimentell (120 kDa) ermittelten Molmasse. Erstmals wurde für die S-Layer-Proteine von B. fusiformis B3 und DSM 2898T eine vermutliche Dimerbildung festgestellt. Basierend auf den N-terminalen Sequenzähnlichkeiten wurden die S-Layer-Proteine aus der Familie der Bacillaceae in vier Gruppen eingeteilt.

Multilayer thin films by layer-by-layer (LBL) assembly of functional polyelectrolytes

Nanoscience aims at manipulating atoms, molecules and nano-size particles in a precise and controlled manner. Nano-scale control of the thin film structures of organic/polymeric materials is a prerequisite to the fabrication of sophisticated functional devices. The work presented in this thesis is a compilation of various polymer thin films with newly synthesized functional polymers. Cationic and anionic LC amphotropic polymers, p-type and n-type semiconducting polymers with triarylamine, oxadiazole, thiadiazole and triazine moieties are suitable materials to fabricate multilayers by layer-by-layer (LBL) self-assembly with a well defined internal structure. The LBL assembly is the ideal processing technique to prepare thin polymer film composites with fine control over morphology and composition at nano-scale thickness, which may have applications in photo-detectors, light-emitting diodes (LEDs), displays and sensors, as well as in solar cells. The multilayer build-up was investigated with amphotropic LC polymers individually by solution-dipping and spin-coating methods; they showed different internal orders with respect to layering and orientation of the mesogens, as a result of the liquid crystalline phase. The synthesized p-type and n-type semiconducting polymers were examined optically and electrochemically, suggesting that they are favorably promising as hole-(p-type) or electron-(n-type) transport materials in electronic and optoelectronic devices. In addition, we report a successful film deposition of polymers by the vacuum deposition method. The vapor deposition method provides a clean environment; it is solvent free and well suited to sequential depositions in hetero-structured multilayer system. As the potential applications, the fabricated polymer thin films were used as simple electrochromic films and also used as hole transporting layers in LEDs. Electrochemical and electrochromic characterizations of assembled films reveal that the newly synthesized polymers give rise to high contrast ratio and fast switching electrochromic films. The LEDs with vacuum deposited films show dramatic improvements in device characteristics, indicating that the films are promising as hole transporting layers. These are the result of not only the thin nano-scale film structures but also the combination with the high charge carrier mobility of synthesized semiconducting polymers.

Detection and Modeling of Boundary Layer Height

This thesis tackles the problem of the automated detection of the atmospheric boundary layer (BL) height, h, from aerosol lidar/ceilometer observations. A new method, the Bayesian Selective Method (BSM), is presented. It implements a Bayesian statistical inference procedure which combines in an statistically optimal way different sources of information. Firstly atmospheric stratification boundaries are located from discontinuities in the ceilometer back-scattered signal. The BSM then identifies the discontinuity edge that has the highest probability to effectively mark the BL height. Information from the contemporaneus physical boundary layer model simulations and a climatological dataset of BL height evolution are combined in the assimilation framework to assist this choice. The BSM algorithm has been tested for four months of continuous ceilometer measurements collected during the BASE:ALFA project and is shown to realistically diagnose the BL depth evolution in many different weather conditions. Then the BASE:ALFA dataset is used to investigate the boundary layer structure in stable conditions. Functions from the Obukhov similarity theory are used as regression curves to fit observed velocity and temperature profiles in the lower half of the stable boundary layer. Surface fluxes of heat and momentum are best-fitting parameters in this exercise and are compared with what measured by a sonic anemometer. The comparison shows remarkable discrepancies, more evident in cases for which the bulk Richardson number turns out to be quite large. This analysis supports earlier results, that surface turbulent fluxes are not the appropriate scaling parameters for profiles of mean quantities in very stable conditions. One of the practical consequences is that boundary layer height diagnostic formulations which mainly rely on surface fluxes are in disagreement to what obtained by inspecting co-located radiosounding profiles.

Surface layer formation in polymer melts and in solutions

In this work the surface layer formation in polymer melts and in polymer solutions have been investigated with the atomic force microscope (AFM). In polymer melts, the formation of an immobile surface layer results in a steric repulsion, which can be measured by the AFM. From former work it is know, that polydimethyl siloxane (PDMS) forms a stable surface layer for molecular weights above 12 kDa. In the present thesis, polyisoprene (PI) was investigated apart from PDMS, by a)measuring the steric surface interactions and b)measuring the surface slip in hydrodynamic experiments. If a polymer flows over a surface, the flow velocity at the surface is larger then zero. If case of a surface layer formation the flow plane changes to the top of the adsorbed layer and the surface slip is reduced to zero. By measuring the surface slip in hydrodynamic experiments, it is therefore possible to determine the presence of a stable surface layer. The results show no stable repulsion for PI and only a small decrease of the surface slip. This indicates that PI does not form a stable surface layer, but is only adsorbed weakly to the surface. Furthermore for 8 kDa PDMS the timescale of the formation of a surface layer was investigated by changing themaximal force the tip applied to the surface. With a repulsive force present, applying a higher force than 15 nN resulted in a destruction of the surface layer, indicated by attractive forces. Reducing the applied force below 15 nN then resulted in an increase of the repulsion to the former state during one minute, thus indicating that a surface layer can be formed within one minute even under the influence of continuous measurements. As a next step, mixtures of two PDMS homopolymers with different chain lengths have been investigated. The aim was to verify theoretical predictions that shorter chains should predominate at the surface due to their smaller loss in conformational entropy. The measurements where done in dependence of the volume fractions of short and long chain PMDS. The results confirmed the short chain dominance for all mixtures with less then 90 vol.% long chain PDMS. Surface layer formation in solution was investigated for superplasticizers which are industrially used as an additive to cement. They change the surface interaction between the cement grains from attractive to repulsive and the freshlymixed cement paste therefore becomes liquid. The aimin this part of the thesis was, to investigate cement particle interactions in a close to real environment. Therefore calcium silicate hydrate phases have been precipitated onto an AFM tip and onto a calcite crystal and the interaction between these surfaces have beenmeasured with and without addition of superplasticizers. The measurements confirmed the change from attraction to repulsion upon addition of superplasticizers. The repulsive steric interaction increased with the length of the sidechain of the superplasticizer, and the dependence of the range of the steric interactions on the sidechain length indicated that the sidechains are in a coiled conformation.

Abscheidung und Charakterisierung von Plasmapolymerschichten auf Fluorkohlenstoff- und Siloxan-Basis

In dieser Arbeit wurden Fluorkohlenstoff-basierte und siliziumorganische Plasmapolymerfilme hergestellt und hinsichtlich ihrer strukturellen und funktionalen Eigenschaften untersucht. Beide untersuchten Materialsysteme sind in der Beschichtungstechnologie von großem wissenschaftlichen und anwendungstechnischen Interesse. Die Schichtabscheidung erfolgte mittels plasmachemischer Gasphasenabscheidung (PECVD) an Parallelplattenreaktoren. Bei den Untersuchungen zur Fluorkohlenstoff-Plasmapolymerisation stand die Herstellung ultra-dünner, d. h. weniger als 5 nm dicker Schichten im Vordergrund. Dies wurde durch gepulste Plasmaanregung und Verwendung eines Gasgemisches aus Trifluormethan (CHF3) und Argon realisiert. Die Bindungsstruktur der Schichten wurden in Abhängigkeit der eingespeisten Leistung, die den Fragmentationsgrad der Monomere im Plasma bestimmt, analysiert. Hierzu wurden die Röntgen-Photoelektronenspektroskopie (XPS), Rasterkraftmikroskopie (AFM), Flugzeit-Sekundärionenmassenspektrometrie (ToF-SIMS) und Röntgenreflektometrie (XRR) eingesetzt. Es zeigte sich, dass die abgeschiedenen Schichten ein homogenes Wachstumsverhalten und keine ausgeprägten Interfacebereiche zum Substrat und zur Oberfläche hin aufweisen. Die XPS-Analysen deuten darauf hin, dass Verkettungsreaktionen von CF2-Radikalen im Plasma eine wichtige Rolle für den Schichtbildungsprozess spielen. Weiterhin konnte gezeigt werden, dass der gewählte Beschichtungsprozess eine gezielte Reduzierung der Benetzbarkeit verschiedener Substrate ermöglicht. Dabei genügen Schichtdicken von weniger als 3 nm zur Erreichung eines teflonartigen Oberflächencharakters mit Oberflächenenergien um 20 mN/m. Damit erschließen sich neue Applikationsmöglichkeiten ultra-dünner Fluorkohlenstoffschichten, was anhand eines Beispiels aus dem Bereich der Nanooptik demonstriert wird. Für die siliziumorganischen Schichten unter Verwendung des Monomers Hexamethyldisiloxan (HMDSO) galt es zunächst, diejenigen Prozessparameter zu identifizieren, die ihren organischen bzw. glasartigen Charakter bestimmen. Hierzu wurde der Einfluss von Leistungseintrag und Zugabe von Sauerstoff als Reaktivgas auf die Elementzusammensetzung der Schichten untersucht. Bei niedrigen Plasmaleistungen und Sauerstoffflüssen werden vor allem kohlenstoffreiche Schichten abgeschieden, was auf eine geringere Fragmentierung der Kohlenwasserstoffgruppen zurückgeführt wurde. Es zeigte sich, dass die Variation des Sauerstoffanteils im Prozessgas eine sehr genaue Steuerbarkeit der Schichteigenschaften ermöglicht. Mittels Sekundär-Neutralteilchen-Massenspektrometrie (SNMS) konnte die prozesstechnische Realisierbarkeit und analytische Quantifizierbarkeit von Wechselschichtsystemen aus polymerartigen und glasartigen Lagen demonstriert werden. Aus dem Intensitätsverhältnis von Si:H-Molekülen zu Si-Atomen im SNMS-Spektrum ließ sich der Wasserstoffgehalt bestimmen. Weiterhin konnte gezeigt werden, dass durch Abscheidung von HMDSO-basierten Gradientenschichten eine deutliche Reduzierung von Reibung und Verschleiß bei Elastomerbauteilen erzielt werden kann.