Drivers of forest dynamics: Joint effects of climate and competition
- The present dissertation thesis addresses different aspects of forest dynamics and possible changes due to climate change. Various modelling approaches are used to explore joint effects of climate and competition on forest dynamics with a focus on temperate forests. Motivated by scientific interest, this thesis is aimed at contributing to the establishment of fundamental knowledge for proper ecosystem management. Each of the research projects of this thesis explores a facet of forest dynamics. It appears that for projections of forest dynamics under climate change particularly, it is critical to consider competition among trees.
In the first study, the joint effect of climate and competition on forest dynamics in a mountain forest of New Zealand was investigated. The landscape simulation model LandClim was calibrated based on empirical data and applied to reproduce a 1700 years forest succession under stationary climate at the slope of Mt. Hauhungatahi, North Island. Although designed for European temperate forests, LandClim was capable of simulating NZ´s forest dynamics.
Under non-stationary climate, forests likely remain in disequilibrium with climate for some time due to the longevity of trees and competitive prevention of establishment. This aspect was investigated in a mixed beech-oak forest in Germany, using LandClim and the forest gap model SILVA in a cooperation study. Furthermore, a possible ‘climatic turning point’ was investigated, the point at which species dominances change due to changes in competitiveness caused by climate change. Both models projected a potential climatic turning point at a mean annual temperature of 11-12 °C and precipitation sum of 500‑530 mm. However, the change of species composition in existing mixed stands was much slower since the turning point also depended on inherited stand structure. Based on these projections the promotion of oak at dry sites seems advisable due to its superior resistance and resilience to drought.
The applied simulation models consider joint effects of climate and competition but no changes in species sensitivity to competition along climatic gradients. The Spanish National Forest Inventories provided a solid basis to develop a statistical model for the influence of climate and competition on tree growth. The results indicated that in Mediterranean forests the effect of competition increases with aridity potentially resulting in an additional disadvantage for drought sensitive oaks compared to pines under climate change.
Under the prerequisite that forest dynamics will be affected by climate change, different forest management strategies on adaptation are currently discussed. One option is the promotion of tree species that are better adapted to anticipated future climates (such as oak instead of beech in Germany) and also the introduction of non-local ecotypes of local species. This increase in biodiversity intuitively appears promising because it is in line with the insurance hypothesis and the portfolio effect theory. Within this thesis the effect of ecotype mixing as an increase of within stand diversity under consideration of self-thinning was assessed. It was shown that ecotype mixing in forest stands might lower the risk of yield losses and at the same time might exempt the portfolio effect from its drawback of lower chances for high yields.
Climate not only affects demographic rates of tree species but also all other ecosystem components. Disturbances are an important component of forest dynamics because they initiate successions and thereby influence species coexistence. Climate will alter disturbance regimes not only directly but also due to interactions among disturbances, for example an increased risk of insect outbreaks due to weak tree defence caused by severe drought stress. Most disturbance interactions have been observed to be positive, implying that increases of disturbances in quality and quantity due to climate change will be amplified. Furthermore, systems containing positive feedback loops are considered to be mostly unstable, which would result in forest collapse. A theoretical study on disturbance interactions showed why positive feedback loops of disturbances do not necessarily lead to a forest collapse. Disturbance interactions might cause only a minor part of disturbances, whereas direct changes due to climate change are of much higher importance.
The described studies reflect the diversity of the research field forest dynamics and innovative ecological methodology. Nevertheless, the present thesis is not an exhaustive discussion of drivers of forest dynamics under climate change. Forest dynamics and its drivers provide a range of open research questions posing a challenge for fundamental an applied research of high relevance for society.
Partikel und Kapseln auf Basis des rekombinanten Spinnenseidenproteins eADF4(C16)
- Natürliche Spinnenseide als Biomaterial wird aufgrund seiner einzigartigen mechanischen Eigenschaften und seiner guten Biokompatibilität schon seit Jahrhunderten vom Menschen genutzt. Hierbei sind insbesondere die Proteine des Abseilfadens der Gartenkreuzspinne A. diadematus von Interesse. Der Abseilfaden setzt sich aus mehreren Proteinkomponenten zusammen, eine dieser Komponenten ist das Protein ADF4, diesem nachempfunden ist das rekombinant hergestellte Protein eADF4. ADF4 besteht aus bis zu 100 Wiederholungen einer repetitiven Kernsequenz, welche von nicht-repetitiven terminalen Regionen flankiert wird. eADF4 besteht aus dieser, jedoch für E. coli optimierten, Kernsequenz (C-Modul). In dieser Arbeit lag das Hauptaugenmerk auf eADF4(C16). Dieses besteht aus 16 Wiederholungen des C-Moduls, das daraus resultierende Protein hat ein Molekulargewicht von ca. 47 kDa.
Die kontrollierte und gezielte Freisetzung von Wirkstoffen oder Enzymen (controlled drug delivery) ist ein wichtiges Forschungsgebiet sowohl in der Pharmazie als auch in der Medizin. Mittels controlled drug delivery sollen Probleme der herkömmlichen Medikamentation behoben werden. Hierzu zählen beispielsweise Nebenwirkungen die aufgrund einer sehr hohen lokalen Wirkstoffkonzentration nach Injektion des Wirkstoffs oder nach Auflösen einer Tablette am Wirkort auftreten können. Um dies zu minimieren, sollen Depotsysteme Medikamente kontrolliert über einen längeren Zeitraum am gewünschten Wirkort abgeben. Hierfür eignen sich insbesondere mobile Systeme, wie Partikel und Kapseln.
Bei Partikeln aus eADF4(C16) handelt es sich um Vollkugeln, welche durch Aussalzprozesse aus wässriger Lösung hergestellt werden. Sie eignen sich insbesondere für den Transport von wasserlöslichen Stoffen, was am Beispiel Rhodamin B gezeigt wurde. Die Beladung der Partikel erfolgt hierbei auf zwei verschiedene Arten. Zum einen durch Eindiffundieren der Wirkstoffe in die fertig ausgebildeten Partikel, zum anderen durch co-Präzipitation des Wirkstoffs zusammen mit dem Protein. Es konnte gezeigt werden, dass die Beladung der Partikel mit Wirkstoffen durch co-Präzipitation im Vergleich zu Diffusionsprozessen deutlich erhöht werden konnte. Zusätzlich sollte durch Vernetzungsprozesse (crosslinking) Einfluss auf die Wirkstofffreisetzung genommen werden. Dies konnte jedoch nicht in vollem Umfang erzielt werden. Es konnte jedoch gezeigt werden, dass chemisches Vernetzen einen deutlichen Einfluss auf die chemische Stabilität der Partikel hat und dass sowohl chemisches als auch physikalisches Vernetzen die Mechanik der Partikel stark beeinflusst. Zusätzlich konnte das Quellverhalten der Partikel analysiert werden, was einen entscheidenden Einfluss auf ihre Lagerstabilität aufweist. Sowohl die Mechanik als auch die Oberflächeneigenschaften der Partikel sind wichtige Parameter in Bezug auf weitere Anwendungsmöglichkeiten. Insbesondere im Bereich der Kompositmaterialien sind die Partikel vielseitig einsetzbar. Durch Inkorporation von eADF4 Partikeln in eine Matrix können die mechanischen Eigenschaften des Matrixmaterials möglicherweise stark verbessert werden und durch die Bioabbaubarkeit der Partikel ergeben sich weitere Möglichkeiten im Bereich von Knochenersatzmaterialien.
Bei Kapseln handelt es sich um flüssigkeitsgefüllte Systeme, die von einem dünnen, aber stabilen Seidenfilm umhüllt sind. Die Kapselherstellung erfolgt mittels Grenzflächenpolymerisation an der Grenzfläche zwischen Öl und Wasser. Das bisher etablierte System basiert auf der Verwendung von Toluol, welches in Hinblick auf eine medizintechnische Anwendung bedenklich ist. Kapseln als Transportsystem eignen sich sowohl für schlechtlösliche als auch für gut-wasserlösliche Wirkstoffe mit einem Molekulargewicht größer als 30 kDa. Durch die Herstellung von Kapseln mittels Silikonöl anstatt Toluol, konnte ihre Verwendung als Transportsystem für Enzyme erfolgreich gezeigt werden. Dies erfolgte mittels -Galaktosidase als Modellenzym. Die Substrate können, aufgrund der semi-permeablen Kapselhülle, in die Kapseln eindiffundieren, bestimmte Proteasen mit entsprechend großem Molekulargewicht werden jedoch abgehalten. Ein Vorteil der Spinnenseidenkapseln gegenüber anderen Systemen ist, dass auch inaktive Enzyme verkapselt und am Wirkort durch Zugabe des Aktivators gezielt aktiviert werden können. Um eine möglichst homogene Größenverteilung der Kapseln zu erzielen, wurde eine Herstellung mittels Mikrofluidik erprobt.
Um die Anwendbarkeit von eADF4 Proteinen im medizinischen oder pharmazeutischen Bereich sicherstellen zu können ist die Sterilität der Proteine ein entscheidendes Kriterium. Rekombinante Proteine, speziell solche, die in Bakterien produziert wurden, sind in der Regel auch nach Aufreinigung unsteril. Aus diesem Grund wurden in meiner Arbeit verschiedene Sterilisationsmethoden getestet und ihre Auswirkungen auf die rekombinanten Spinnenseidenproteine analysiert.
Development of an artificial silk protein on the basis of a lacewing egg stalk protein
- Silks are widely used in textile industry as clothing and furnishings due to their tensile strength, smoothness, soft texture, lustre, and drape. Most commonly silk of the mulberry silkworm Bombyx mori (B. mori) is used in such applications, however, silks evolved independently in many different arthropods for various purposes.1 During evolution the different silks were optimised for their task-specific uses over millions of years, e.g. adopting different mechanical properties. The mechanical properties mainly derive from the protein secondary structure and its higher order arrangement in silk fibres. Spider silk, for example, is known for its tensile properties surpassing nylon, Kevlar®, silkworm silk, and high-tensile steel.2-5 Beyond their mechanical properties, some silks are also reported to be biocompatible and non-immunogenic.6 One beneficial feature of silk proteins is the possibility to process them into various morphologies.7, 8
Several of these silk features make them interesting for material scientists, intending to produce silks with tuneable properties depending on the desired application, ranging from technical ones such as high performance fibres to medical ones such as drug delivery.
This thesis deals with the characterisation and reproduction of a less explored silk, the lacewing egg stalk silk. Mechanical testing revealed a strong dependence on the relative humidity. In the dry state at 30% relative humidity, the stalks are quite rigid and break at an elongation of 2% whereas at 70% and 100% relative humidity they elongate up to 434%. This extension is accompanied by a secondary structure change from cross-ß to parallel-ß. The cross-ß structure in unstretched stalks provides bending stiffness and rigidity to the stalk, and this bending stiffness gets lost when the stalks are stretched. In this thesis a model is proposed which explains these differences at various relative humidity on the molecular level, wherein changes in the strength of hydrogen bonds upon exposure to water (a hydrogen bond donor/acceptor) in combination with multiple disulphide cross-links (which are not affected by water) act together and are responsible for this behaviour.
Based on consensus sequences of published sequence data (derived from MalXB2 an egg stalk protein of Mallada signata (M. signata)),9 an engineered egg stalk protein named N[AS]8C was recombinantly produced.
To produce an artificial stalk, a droplet of a solution of purified N[AS]8C was placed on a substrate, and tweezers were used to pull out a fibre. After drying, and post treatment, the properties of the artificial stalks were investigated in comparison to the natural ones. Mechanical testing revealed similar behaviour at 30% relative humidity, but at 70% and 100% relative humidity the artificial stalks were not as extensible as the natural ones. This corresponds to the fact, that no cross-ß structure was formed, and, therefore, no rearrangement into parallel-ß structure was possible.
Subsequently, N[AS]8C was processed into non-fibrous morphologies. It was possible to produce capsules, hydrogels, foams, and films. The foams show an interesting micro and nano structure which differs from that of recombinant spider silk. The cavities are filled with a mesh of nano fibres building a 3D scaffold.
Films are a morphology with potential for application in cell culture. Fibroblast attachment on N[AS]8C films is quite poor. Therefore, we tried to induce guided fibroblast growth on patterned protein films. A first layer of the films was cast from ntagCysC16-c(RGDfK), an engineered spider silk protein coupled with the integrin recognition motif RGD to provide a protein layer to which fibroblasts attached well. The second protein layer was produced using a PDMS (polydimethylsiloxane) template and N[AS]8C. Fibroblasts grown on these films adhere only to the RGD modified spider silk and not to the N[AS]8C areas. A second feature of such films is to orient the fibroblasts on films with alternating lines of the two proteins. Such films might be useful for tissue engineering to control cell adhesion and get a structured cell pattern. This is essential for many tissues such as bones, muscles, and epithelia tissue. The low cell adhesion properties of N[AS]8C films might be interesting for coatings for applications where cell adhesion is not desired such as stents or catheters.
An Investigation of Organizational Level Continuance of Cloud-Based Enterprise Systems
- Cloud-based enterprise systems are a growing trend in today’s business software market. With a steadily expanding number of implementations, cloud service providers are now turning their attention from adoption issues towards retaining their existing customer base. The difficulties even established cloud players, like e.g. Salesforce.com, face in retaining their customers have been emphasized by tech bloggers, where the subscriptions of cloud-based enterprise systems are cancelled even at an early stage of adoption. This discontinuance of enterprise systems at an early stage is a rather new phenomenon, which is related to the subscription-based payment model of cloud services, which (theoretically) allows service cancellation without the customers having to fear financial penalties. In contrast, traditional on-premise systems (e.g. SAP ERP) are on a long term license base, where customers are contractually bound. Therefore the research question of the thesis is as follows: What factors influence the organizational level continuance intention of cloud-based enterprise systems? In an effort to answer this research question, the thesis presents five inter-related papers. The first paper develops a conceptual model to study the continuance of cloud-based enterprise systems. Building on this, paper two develops a formative measurement instrument to assess the success of operational cloud-based enterprise systems. The third paper quantitatively explores the influence of the variables identified in the conceptual model. Building on these findings, paper four conducts a stakeholder analysis to solve the problem of broad samples. Finally, the fifth paper uses the formative measurement instrument to test the final research model, which is a revision of the conceptual model developed in the first paper. The results show that continuance intention is influenced both, by information systems success variables, as well as continuance inertia. In addition, behavioral variables, such as attitude towards usage also explained a decent amount of variance in the dependent variable.
Monsoonal affected dynamics of nitrate and dissolved organic carbon in a mountainous catchment under intensive land-use
- In recent decades, complex mountainous landscapes have been increasingly under pressure by deforestation and intensified highland agriculture. This trend not only compromises the quality of water in the highlands, but also impacts the availability of water resources downstream. Hence, an effective and sustainable management of these mountainous regions is essential and needed to mitigate this risk. Developing sustainable management principles to ensure the freshwater supply, however, requires a profound understanding of decisive factors and processes that control the water quality in mountainous landscapes. Amongst other substances, nitrate and dissolved organic carbon (DOC) play a critical role in the ecosystem health of water bodies. The major focus of this dissertation is on determining the nitrate and DOC mobilization processes and dynamics in the Haean Catchment, a mountainous watershed located in South Korea, which is agriculturally productive and strongly influenced by the prevailing East-Asian monsoon. The primary objective was to identify decisive factors that control the nitrate and DOC mobilization processes and dynamics in such catchments.
To these ends, we conducted stream water quality and discharge measurements during a range of conditions, from monsoonal rainfall events to dry, baseflow conditions. Assessments were completed along the topographic elevation gradient of the catchment, from an upland deciduous forest, over areas intensively used for agriculture, down to the catchment outlet. In order to gain a better understanding of nutrient fate within the Haean Catchment, we investigated river-aquifer exchange fluxes. In addition to hydraulic gradient monitoring, we used heat as tracer. To resolve the river-aquifer exchange fluxes, we set up a two-dimensional flow and heat transport model using the numerical code HydroGeoSphere (HGS). Potential effects of river-aquifer exchange dynamics on local water quality were investigated by collecting both, river and groundwater samples. Furthermore, we determined the impact of the ridge and furrow cultivation that is commonly applied in South Korean dryland agriculture, on nitrate leaching and evaluated fertilizer best management practices (FBMPs) using a three-dimensional flow and solute transport model (HGS).
Our results show that DOC and nitrate sources as well as their mobilization differ between the forest and agricultural river sites. In the forest, elevated in-stream DOC concentrations during rainfalls were due to hydrologic flushing of soluble organic matter in upper soil horizons with a strong dependency on pre-storm wetness conditions. Nitrate contributions to the stream occurred predominantly along interflow transport pathways.
At the agricultural sites, in-stream DOC concentrations were considerably higher and supplied from adjacent rice paddies. The highest in-stream nitrate concentrations occurred in the lower agricultural part of the catchment. Through hydraulic gradient monitoring, we identified in this part of the catchment a distinct connection between the river and aquifer, and nitrate-rich groundwater inputs to the river elevated the in-stream nitrate concentration. In the mid-elevation portion of the catchment, however, a limited connectivity between the river and aquifer was identified and river water quality was consequently unaffected in these areas.
The results of our HGS modeling study show a high temporal and spatial variability in river-aquifer exchange fluxes with frequent flow reversals during the monsoon season. Our results also suggest that these flow reversals may enhance the natural attenuation of nitrate in the shallow groundwater below the stream. The simulation results on evaluating FBMPs demonstrate that applying a combination of several FBMPs such as an adapted placement and timing is recommended to minimize the risk of groundwater nitrate contamination.
Overall, this dissertation demonstrates that the hydrologic pathways resulting from the monsoon climate drive the in-stream DOC dynamics in the forested catchment, whereas sources and mobilization of DOC in downstream agricultural areas are mainly controlled by the specific land-use type. In particular, the widely used rice paddy “plot-to-plot” irrigation system in Korean highlands was shown to control in-stream DOC concentrations. Nitrate dynamics in streams and aquifers in the agricultural areas of the catchment reflect the combined effects of land-use type and monsoonal hydrology. Particularly, the highly variable river-aquifer exchange fluxes with frequent flow reversals, which may enhance the nitrate attenuation, are driven by monsoonal extreme precipitation events. Since it has been forecasted that global warming will increase the frequency and the intensity of extreme precipitation events also in other regions of the world, our results will become increasingly important in future water quality assessments and research.
Plasticity, Intraspecific Variability and Local Adaptation to Climatic Extreme Events of Ecotypes/Provenances of Key Plant Species
- Climate change, and especially an increase of magnitude and frequency of climatic extreme events such as drought periods or heatwaves, will alter growing conditions for plants in the future. Persistent ecosystems, with long-living organisms, such as forest or permanent grassland will be particularly impacted by this development. The velocity of these changes is likely to occur at a pace, which species may not be able to keep track with by natural dispersal or genetic adaptation. Agriculture, forestry and ecosystem management must develop counteracting practices to secure the persistence and functioning of these ecosystems and thus their provision of goods and services. Therefore it is important to develop a better understanding how species and ecosystems may respond to future climatic stressors. Impact assessments, e.g. via climatic envelope modelling are prone to misinterpretations of the adaptive capacity of species, as they do not incorporate the intraspecific genetic and phenotypic differences that exist within the populations accross the distribution range of a species.
Yet, intraspecific variation may exhibit potential tools for the development of climate change adaptation strategies. Here, I focus on key ecosystems in Central Europe. In particular the selective use of plant provenances or ecotypes may help to make ecosystems climate-resilient without a potentially more problematic introduction of exotic species. Especially provenances from warmer, drought-prone regions, with a current climate similar to the projected one for Central Europe recently came into focus as potential substitutes for local provenances, as they might have developed local adaptations to climate conditions at their location of origin. Insights about the response of these provenances to changing averages and extreme event regimes are crucial for a reasonable use of within-species diversity in climate change adaptation.
First, the concept of assisted colonization or migration of species or ecotypes and the role it can play as an adaptation strategy in agriculture, forestry or nature conservation is introduced (Manuscript1). It is suggested that a focus should be laid on keystone species that ensure ecosystem persistence and functioning as they govern the habitat structure and microclimate of a site. The assisted colonization of pre-adapted ecotypes of keystone species from climates similar to future projections for the target site is proposed.
Furthermore, provenances of selected grassland and forest key-species were exposed to drought and warming in two experiments in Bayreuth and Landau, and their ecological responses were analysed. Results suggest that local adaptations to climatic stressors exist. However, the magnitude and direction of responses strongly depend on species and climatic variables. For grassland species, e.g. differences in drought sensitivity could be demonstrated in some cases (Manuscript 4). Fagus sylvatica exhibited differences between the provenances in response to drought conditions, as well (Manuscript 3). It seems that marginal provenances, from the dry margins of the distribution range, show less increment reduction due to the drought treatment. Yet, under more favourable conditions of water supply these provenances did not yield the same high increment rates than more central provenances, indicating a trade-off between stability under stress and yield under non-stress conditions. A pine species that is generally considered to be rather drought-resistant, Pinus nigra, which is a potential substitute for climate-threatened conifers on dry sites in Central Europe, did not show any differences in response to drought and warming (Manuscript 2), maybe due to a weak selective pressure as a result of high drought-resistance across the whole distribution range. The impacts of drought on increment became not visible before the second year after the treatment, stressing the need for more long-time experiments in climate impact research.
Even in a generally warmer environment, cold extremes in winter or spring are expected still to prevail in the future. Therefore, the provenances of the selected species were tested for their cold-hardiness and late frost resistance (Manuscripts 5-7). Growth of the grassland species and F. sylvatica were negatively impacted by a late frost event and differences in late-frost sensitivity between provenances or ecotypes were identified. The (sub-) mediterranean species P. nigra showed differences between provenances in their winter cold hardiness. Correlations between performance under cold stress and winter conditions or late frost proneness of the places of origin could be established for almost all species. However, preceding climate experience, such as the warming or drought treatment of the plants altered their reaction to cold extremes compared to the control treatment, indicating the complexity of the interactive impacts of climate factors on ecosystem and plant performance.
The uncertainty of climate projections and the multitude of changing climatic stressors, though, make the prospect of an easy and rapid success in the search for single “best-adapted” provenances very questionable. In economics the portfolio effect shows that a diversification of investments decreases the risk of a total loss of profits. Hence, in a modelling procedure based on the increment data from the above mentioned experiment it was tested if a “portfolio investment” in several provenances in one stand decreases the risk of yield losses (Manuscript 8). Results indicate that the higher the number of provenances the higher the chance for a “best-performer” to be included in the set. So the likelihood of higher yields, under different climatic conditions increases, yet the risk of low yields stays stable.
Generally, it seems that the selective use of plant species and ecotypes in climate change adaptation can be a feasible tool to maintain ecosystem functionality and productivity. However, the uncertain projections, the multitude of climatic stressors and their interplay with other environmental factors and the potential impacts of assisted colonization of ecotypes on the genetic diversity within species and populations require further research.
Du texte au film : L'adaptation cinématographique de La visite de la vieille dame par Djibril Diop Mambéty
Cheikh Anta Babou
- Si certains cinéastes africains produisent des scénarios dont le fondement repose sur leur imagination, leurs expériences personnelles et/ou quotidiennes etc., d'autres, par contre, pour une raison ou une autre, puisent leur matière à filmer dans la littérature. En portant des œuvres littéraires à l'écran, ils opèrent un véritable changement de code qui passe du texte au film. Considéré comme un phénomène d'adaptation cinématographique, le passage du littéraire au filmique entraine, à cause de la différence qui existe entre les deux systèmes sémiotiques (texte et film), des transformations remarquables qui sont surtout d'ordre « transmédiatique » ou « transsémiotique ». Il est important de signaler que ces transformations sont d'autant plus manifestes lorsqu'il s'agit de la transposition d'une production littéraire, issue du monde suisse ou occidental, dans un contexte totalement différent comme celui du Sénégal. Une réécriture de cette nature impose également une métamorphose du cadre spatio-temporel, du contexte socioculturel et des personnages. L'objectif de cette présente contribution est d'aider à une meilleure compréhension des mécanismes qui sous-tendent une telle opération de transécriture. A cet effet, sur la base de Hyènes, l'adaptation de la pièce théâtrale de Friedrich Dürrenmatt (Der Besuch der alten Dame) par Djibril Diop Mambéty, nous allons, en premier temps, nous intéresser au rapport qui existe entre l'hypotexte et l'hypertexte. Dans un second temps, il s'agira de se pencher sur la métamorphose du titre du texte de référence et sur la portée symbolique qu'a le titre du filme dérivé. Le troisième volet de notre argumentation sera consacrée à une réflexion sur les mutations observables au niveau de l'incipit et de l'excipit du texte fondateur. Le quatrième point étudiera l'adaptation des références culturelles contenues dans la pièce de Dürrenmatt au contexte culturel sénégalais. La cinquième et dernière partie de cette contribution essayera de comprendre les entropies et ajouts perceptibles dans le film inspiré.
Effect of Dopants on the Local Atomic Structure and Sintering Behavior of Bismuth Sodium Titanate
- The most commonly used piezoceramic is lead zirconate titanate Pb(ZrxTi(1-x))O (PZT). It possesses outstanding piezoelectric properties which can be modified for numerous applications by the addition of dopants. However, because of environmental and health concerns regarding lead, lead-free alternatives are demanded by politics.
One of the two most promising lead-free replacement materials is the ferroelectric bismuth sodium titanate (Bi0.5Na0.5)TiO3 (BNT). Like PZT, it crystallizes in the perovskite structure.
Since the dielectric and piezoelectric properties of pure BNT ceramics are insufficient for application, BNT is often modified by the addition of dopants. These influence a great variety of material properties to different degrees, e.g. the sintering behavior, the dielectric and piezoelectric properties and their respective temperature stabilities. Doping of BNT aims to decrease the sintering temperature in order to avoid Bi vaporization, to increase the depolarization temperature and to enhance the piezoelectric coefficient.
The effects of numerous dopants on the resulting performance of BNT were studied extensively in the literature. However, so far little attention has been payed to the way in which dopants interact with the piezomaterial. Nevertheless, it is the understanding of these relationships that would make targeted modifications and improvements of BNT possible.
The primary goal of this study was to investigate and explain the effects of a model dopant -cobalt- on the phase formation, sintering behavior and microstructure of BNT as well as on the resulting dielectric and piezoelectric properties. In this regard, a core issue was to determine the preferred lattice site of Co in BNT.
BNT was synthesized from oxide powders using the classic solid-state route and sintered at temperatures ranging from 1000 °C to 1150 °C. Cobalt was added in concentrations between 0.1 mol% and 2.6 mol% Co prior to the calcination as Co3O4.
About one third of the total cobalt amount was incorporated into the BNT lattice on the perovskite B-site, that is, it substituted for Ti. The cobalt in BNT appeared to be in equilibrium with the secondary phase Co2TiO4, which invariably formed at cobalt concentrations greater than 0.1 mol% Co. For charge balancing reasons, oxygen vacancies were created in the lattice of cobalt-doped BNT. These markedly enhanced the diffusivity. As a result, the sintering temperature of doped BNT decreased with increasing cobalt concentrations, and high final densities were achieved. However, in highly doped BNT sample swelling occurred at elevated temperatures of the sintering cycle. This phenomenon was attributed to evaporating oxygen caused by the valence transition of Co3+ to Co2+.
Up to 950 °C, BNT was found to densify via solid state sintering mechanisms. Above this temperature, a small amount of liquid phase was present, which probably formed from decomposing BNT because of a slight Ti-deficiency due to doping. Increased Bi vaporization from the melt above 1000 °C appeared to have stabilized sodium cobalt titanate, an additional secondary phase.
The rotation of the iso-lines in the kinetic field diagram of doped BNT was interpreted such that the activation energy for grain growth was higher than the activation energy for densification. Possible reasons are the solute-drag effect and the pinning of domain walls by secondary phase particles.
Both the depolarization temperature and the piezoelectric coefficient d33 decreased with increasing cobalt concentrations. The dielectric properties deteriorated as well. This was attributed to the high electrical conductivity of the doped samples, which prevented full poling.
Polyelectrolyte Complexes and their Therapeutic Potential
- This thesis describes the preparation of polyelectrolyte nanostructures, the characterization of interpolyelectrolyte complexes (IPECs) made from these structures and their use in a therapeutic context. The therapeutic use of such IPECs connects the two major topics of this work: First, the delivery of genes into eukaryotic cells in vitro by means of new star-shaped polycations was explored. Second, the structure of ionic multicompartment micelles (MCMs) when complexed with polyions was studied and the performance of these nanostructures as delivery agents of anti-cancer drugs both in vitro and in vivo was tested.
To better understand structure-property relationships of polycations relevant for gene delivery, a library of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) homopolymers was synthesized. Star-shaped polymers with a different number of arms and molecular weights were created by using sugar-based or inorganic nanoparticles as core molecules with varying number of initiation sites. The cytotoxicity as well as transfection performance of polyplexes from these polymers and plasmid DNA was determined for different PDMAEMA-nitrogen/DNA phosphate ratios in CHO-K1 cells. A decrease of the cytotoxicity of polymers with a given molecular weight was observed with increasing degree of branching, i.e., with increased arm-number. Star-shaped PDMAEMA with roughly 20 arms (Si-PDMAEMA) showed exceptionally high transfection efficiency. The superior transfection behavior of this specific polymer was demonstrated in non-dividing or differentiated (C2C12 and human T lymphocytes) cell lines. Additionally, polymeric micelles were produced from a polybutadiene-block-PDMAEMA diblock copolymer in aqueous solution and subsequently used for gene transfection. Their transfection efficiency was in the same range as that of Si-PDMAEMA, hinting towards a general design principle for highly effective gene vectors. This consists of a star-shaped architecture of PDMAEMA chains emanating from a common center.
The second major part of this thesis deals with the structure of ionic MCMs complexed with diverse polycations as well as the drug delivery capabilities of some of these complex micellar structures. MCMs from polybutadiene-block-poly(1-methyl-2-vinyl pyridinium)-block-poly(methacrylic acid) (BVqMAA) triblock terpolymers were used as the basis for further structural modifications. These micelles exhibit a core-shell-corona morphology, where PB forms the core of the micelles, a discontinuous (patchy) shell consisting of an IPEC between P2VPq and PMAA is present and finally a corona of excess PMAA stabilizes the micelles in aqueous solution. At sufficiently high pH a portion of the corona carries negative charges, which were then used to form further IPECs with either cationic homopolymers or double-hydrophilic block copolymers featuring one positively charged block. If polycations other than poly(2-vinyl pyridine) such as quaternized PDMAEMA were used for the complexation, a new and distinguishable IPEC compartment was formed on top of the already existing P2VPq/PMAA IPEC. In the case of MCMs with a short to moderate block length of the corona, i.e., the degree of polymerization (DP) of PMAA was between 345 to 550 units, a layered arrangement of the newly formed IPEC compartment was found.
For BVqMAA micelles with a long PMAA corona (DP of PMAA = 1350) complexed with different quaternized homopolymers, a patchy arrangement of the newly formed IPEC compartment instead of a layered one was found. The formation of this new structure is due to an interfacial energy minimization between the new IPEC and the compartmentalized core of the micelles that became possible due to the exceptionally long PMAA corona of the precursor micelles.
Finally, MCMs from BVqMAA were tested for their capacity to deliver a hydrophobic anti-cancer drug for photodynamic therapy to lung cancer cells in vitro and in vivo. The influence of the corona composition was studied by forming complexes with a double-hydrophilic diblock copolymer, poly(L-lysine)-block-poly(ethylene glycol) (PLL-b-PEG). A new cylinder-on-sphere morphology was observed in electron microscopy for BVqMAA/PLL-b-PEG complex micelles at high complexation ratios between lysine and MAA units. Highest cytotoxicity and uptake were found for pure BVqMAA micelles, both decreasing with increasing amount of PLL-b-PEG attached to the micelles. In mice, a prolonged blood circulation time in the range of several hours was exclusively observed when fully PEGylated micelles were injected. Those micelles were the only ones showing an enhanced accumulation in a subcutaneous tumor model 24 h after intravenous injection. The amount of drug delivered to the tumor tissue by the micelles suppressed tumor growth for up to 21 days after a single dose injection and photoirradiation step. The potential of complex micelles on the basis of BVqMAA MCMs could thus be proven.
High-Resolution Modelling of Surface-Atmosphere Interactions and Convection Development at Nam Co Lake, Tibetan Plateau
- The Tibetan Plateau has recently become an area of increased interest for the atmospheric and environmental sciences. Surface-atmosphere interactions and specifically the exchange of momentum, turbulent energy and water vapour as well as the development of convection are not only important for the surface energy balance and local water resources, but also have influence on the evolution of the monsoon system and climate. High-resolution, numerical atmospheric models with a fully coupled surface model are a valuable tool for the systematic investigation of surface-atmosphere interactions. Nam Co Lake, located at the northern extent of the monsoon's influence, was selected as a complex system in order to study the interaction of the land and the lake in the generation of mesoscale circulations and the development from boundary-layer clouds to moist convection.
Turbulent fluxes estimated by eddy-covariance and atmospheric profiles measured by radiosondes are used in this work in conjunction with the ATHAM (Active Tracer High-resolution Atmospheric Model) and Hybrid models. Substantial model development is undertaken for both ATHAM and Hybrid. This means a more consistent formulation of tracer and heat transport in ATHAM and improved model stability. Hybrid has been modified with an extrapolated surface temperature, to be used for the calculation of turbulent fluxes. A quadratic temperature profile based on the layer mean and surface model base temperature is assumed in each layer and extended to the surface. Compared to eddy-covariance measurements and a Surface-Vegetation-Atmosphere Transport (SVAT) Model there is an overall reasonable model performance, when tested on four days for two sites with variable environmental conditions during the 2009 summer monsoon season. At the same time, errors are reduced by 40-60% compared to the unmodified Hybrid.
Subsequently, the coupled modelling system is used for 2-dimensional cross-sections through the Nam Co Lake basin with horizontal resolutions of
200 m and at least 150 vertical layers between the surface and the model top located in the lower stratosphere. The 2-dimensional modelling approach has a
tendency to overestimate convective strength due to the underestimation of dry air entrainment and cannot reproduce fully realistic flow fields. Nevertheless,
it provides a valuable tool for systematic investigations of environmental factors, where 3D simulations are prohibitively expensive.
In simulations with several background wind speeds it is found that the model adequately simulates the mesoscale circulation system between the lake and the surrounding mountain chains. Dependent on the geostrophic wind direction there are two different mechanisms for the triggering of convection: Convective triggering, when overflowing topography, and triggering due to convergence between the lake-breeze front and the background wind. It is concluded that coupled modelling setup is capable of reproducing the system's most important dynamics, such as realistic turbulent surface fluxes, mesoscale circulations and cloud evolution.
Thereafter, the influence of the atmospheric profiles of temperature and relative humidity and the uncertainty that arises from them is discussed.
Simulations are initialised with profiles based on direct measurements (radiosondes), NCEP-I and ERA-INT reanalysis and GFS-FNL analysis data on two
days during the summer of 2012. The simulated convection from radiosondes compares reasonably well with weather observations for the first day, but
less well for the second day, when large-scale synoptic effects, which are not included in the model, gain importance. The choice of vertical profile information leads to strongly differing convection development, causing modifications of the surface energy balance and thus of the energy and water
cycle for Nam Co Lake.
With respect to precipitation it is found that a large fraction of the precipitation that is generated in the simulations is deposited within the basin
and on the slopes of the surrounding mountain chains and thus locally recycled. This also means that a weather station in the centre of the basin is not representative of the system. Furthermore, Nam Co Lake may be of importance as a water supply for the region. Additionally, the choice of profile and the initial water vapour contents determine the amount of precipitation so that there are strong differences spanning one order of magnitude in the generated precipitation between the model simulations driven by different vertical profiles.
The findings from the thesis provide an example of the impacts of surface-atmosphere interactions, mesoscale circulations and convective evolution on
the Tibetan Plateau. Scaled to the entire plateau these processes are highly relevant to ecosystems, climate and the water cycle.