Supramolecular Nanofibers - Preparation, Structure-Property Relations, and Applications
- Conventional polymer nanofibers have gained tremendous interest in the last years in the fields of catalysis as templates, in medicine as tissue engineering, in functional textiles as protective suits, and especially in air filtration as filter media. Generally, nanofibers are prepared by top-down approaches. However, these processes feature several disadvantages. As consequence cost-effective alternative strategies are required. One strategy to this problem is the bottom-up approach – the self-assembly of small molecules. Therefore, this thesis covers different topics with respect to the preparation, structure-property relations, and application of supramolecular nanofibers:
To investigate the impact of the molecular structure on the stacking behavior in the self-assembly process, a set of pyrene-containing model compounds was synthesized. Here, the focus was set on the influences of sterical demanding side groups as well as hydrogen bonding motifs on the stacking of the pyrene units. These influences were, besides others, detected by excimer formation in dilute solution, in the aggregated state and in solid films. It was demonstrated that stacking of the pyrene units is the driving force of the self-assembly process in solution in this system. However, hydrogen bonds are required to obtain well-defined supramolecular nanofibers. The influence of the hydrogen bonding motif and the sterical hindrance on the pyrene stacking becomes more and more significant the closer the molecules are forced together. Hence, the columnar stacking is increasingly disturbed in solid films compared to solution.
The class of 1,3,5-benzenetrisamides is one of the simplest and most-versatile motifs in supramolecular chemistry. Within this thesis, two different self-assembly processing pathways of benzenetrisamides from solution; in particular self-assembly upon cooling at constant concentration and self-assembly during solvent evaporation at constant temperature were explored. One factor that determines the actual processing pathway is the solubility of the benzenetrisamide molecule. Exclusive self-assembly upon cooling takes place when the benzenetrisamide is almost completely insoluble in the used solvent at room temperature. The prerequisite for self-assembly during solvent evaporation is certain solubility of the BTAs at room temperature. In addition, these self-assembly pathways were compared with respect to control the supramolecular nanofiber morphology in view of homogeneity, fiber diameter, and fiber diameter distribution. Thereby, influences of external parameters such as temperature, solvent, and concentration were investigated in detail.
Especially in air filtration industry nanofibers are an important tool because of their beneficial effects due to their high surface-to-volume ratio. In industry, electrospinning is the standard technique to post-modify nonwoven filters with conventional polymer nanofibers on the filter surface. However, this process is limited to the surface of the scaffold. In this thesis, the principle utilization of supramolecular nanofibers in air filtration is demonstrated for the first time. Here, a solution-based immersion process was developed, which allows a successful in-situ formation of supramolecular nanofibers in nonwoven scaffolds. This results in a stable microfiber-nanofiber composite. The main advantage of this process is the effective incorporation of nanofibers in the volume of the nonwoven fabrics. For supramolecular systems, it was claimed that they are too fragile to be competitive with conventional polymers. But the herein prepared supramolecular nanofibers possess enough stability even upon applied airstreams of 3.0 meter per second. This stability is by far superior than it is required at standard vacuum cleaners which possess flow velocities of 0.25-0.40 meter per second at the filter element. First filtration tests revealed promising filtration efficiencies.
Building on these promising results a comprehensive study on structure-property relations at the preparation of microfiber-nanofiber composites in view of optimized filtration efficiencies was investigated. Depending on the selected benzenetrisamide, solvent, and concentration of the immersion solution, the filtration efficiency of the filters can be adjusted. By varying the thickness of the filters by means of double- and triple-layer filters, for supramolecular modified filters, excellent filtration efficiencies over 90 percent were obtained for aerosol particles with the size of 0.2 micrometer.
Challenging notions of development and change from everyday life in Africa
Mohamed A.G. Bakhit
Girum Getachew Alemu
- The fourth volume of BIGSASworks! seeks to present the interdependence of paradigms and practices in global, national, and local spheres from different disciplinary perspectives and foci - Social Anthropology, Geography, Media Studies, Political Sciences and Sociology. The contributing papers present various ways in which the daily livelihood activities of community people in different parts of Africa represent this interdependence. Together and in interlinked ways, the authors address the question of how global development paradigms affect people’s lives, what meanings are there in the everyday things people do to live that may synchronise or be at variance with these global paradigms. The contributing papers challenge us all to take another look at our approach to development in Africa and its entanglements with broader forces.
Quantifying water use by temperate deciduous forests in South Korea: roles of species diversity, canopy structure, and complex terrain
- About seventy percent of South Korea is covered with forests, most of which are found in the mountain regions since mountains receive more rainfall and are difficult terrains not suitable for agriculture. Because mountains are important water sources for cities and human population downstream, performing water balance for forest catchments has become a research priority. The ongoing shift from coniferous to species-rich deciduous forests due to a changing government policy and the anticipated changes in future climate, associated with increasing amount of rainfall and temperature will also impact forest water use, calling for an urgent need to understand how forests, in their current status, use water. The knowledge is vital for predicting water requirements for the future forest. The warm-deciduous temperate forests found in South Korea, however, have a high diversity of tree species, have multi-layered canopies and are mostly located on rugged mountainous terrains, which make it difficult to quantify forest water use, a basic requirement for catchment water budgeting. The main objectives of this study were to: (1) identify the roles of species diversity in tree and forest water use, (2) examine the impact of canopy structure on forest transpiration, and (3) evaluate the influence of terrain on forest water use.
Site-specific studies were carried out in three different natural deciduous forests, namely, Gyebang (GB), Gwangneung (GN) and Haean (HA) forest sites, representing the general structure of S. Korean forests. GB site is known for its high species diversity, GN site is an old forest growth at climax, with clearly defined understory and overstory canopy layers while the HA site was located with in a catchment, with strong elevation changes within short horizontal distances, rising from 400 to 1,000 m a.s.l., and in different aspects. Four locations with varying elevations and aspects were chosen in the HA site. Tree water use (TWU) and canopy transpiration (EC) were estimated from sap flux density measured with thermal dissipation probes. Understory transpiration (EU) was measured using stem heat balance while ecosystem evapotranspiration (Eeco) was determined using eddy covariance technique. Air temperatures (Ta), precipitation, solar radiation, vapor pressure deficit (VPD), wind speed were measured from weather stations and soil water content was measured from frequency domain reflectometry (FDR) sensors at the respective study sites. Vegetation surveys, including diameter at breast height (DBH), tree density, species composition, sapwood area (AS), and leaf area index were performed in all the sites. Canopy conductance (GC) and stomatal sensitivity to VPD were assessed based on transpiration and microclimate measured at each site.
A functional allometric relationship was established between AS and DBH, and also between TWU and DBH for all the study sites; first for single species and then combining all the species either in a single site or in all the sites. Irrespective of tree species, AS and maximum TWU were significantly correlated with DBH in a power function for AS (R2 = 0.77, P <0.0001) and both in power (R2 = 0.63, P <0.0001) and sigmoid functions (R2 = 0.66, P <0.0001) for TWU, for the co-occurring species as well as across the sites, suggesting that DBH can be a good predictor of stand AS and maximum TWU, based on the established allometric functions.
Early bud break and development of the understory compared to the overstory canopy resulted in an earlier onset of forest transpiration, with EU contributing 22% and 14% between April and May to the total forest transpiration. This high contribution was favored by high radiation and VPD in the understory, since the overstory was still undeveloped and open. Despite diminishing VPD and light conditions in the understory between June and August, the understory continued to transpire a substantial amount of water, contributing 10% of the total transpiration. The seasonal patterns of both EO and EU were synchronized to canopy development, while VPD and radiation determined daily trends. EO and EU accounted for 80% of Eeco in spring but only 60% during the monsoon period due to lowered radiation input, VPD, and plant area index (PAI). Thus, Eeco is largely influenced by transpiration rate and its seasonal variation and also canopy structure.
Early saturation of EC at relatively low VPD and also a rapid decrease in GC with increasing VPD were observed in the forest stand located at the highest elevation studied (950 m) in the HA site, compared to the GN and the other forest stands in HA. These differences in transpiration rates and stomatal response can be explained by greater stomatal sensitivity to VPD of 0.83 found at the 950 m site compared to 0.63–0.66 in the other study sites. However, the main controlling factor of the change in stomatal sensitivity at the 950 m stand is uncertain. Although maximum daily EC were correlated with AS of the forest stands at different sites (R2 = 0.78, P <0.01), annual EC declined with increasing elevation, i.e., 176 >175 >110 >90 mm year−1 at 340 >450 >650 >950 m, respectively. Decline in total EC was due to the decline in annual Ta, daytime VPD, and length of growing season at higher elevations. The GB site, which was located at 960 m elevation, however, did not display a same response pattern as those observed at the 950 m site. It is likely because these sites were under different environmental conditions, i.e., GB site is exposed to higher Ta and higher humidity, and is sheltered (lower wind speeds). These observations emphasize the complexity associated with estimation of transpiration in rugged terrains, since general principles do not always apply and the spatial patterns of forest transpiration are complex.
Complexity arising from multiple tree species composition when estimating forest water use can be reduced by applying functional allometric relationship linking tree size and water use. Forest canopy structure and physical location should be taken into account since they influence the way forests use water resources by altering microclimate and plant physiology. Based on our findings, estimation of forest water use on rugged terrains require repeated measurements at relatively small spatial scales since the driving factors change rapidly over very narrow vertical distances.
Structural and electronic properties of transition metal nanoalloys and magnetic compounds
- In transition metal clusters, potentially profitable technological applications and fascinating fundamental questions are closely connected. Bimetallic nanoalloys, e.g., have become increasingly popular as their performance in catalysis is often superior to their pure counterparts. Exemplary for this are gold-platinum (Au-Pt) nanoalloys that have been used as highly potent catalysts in electrocatalysis and in a variety of oxidation reactions. However, the mere existence of Au-Pt nanoalloys is astonishing, as Au and Pt cannot be mixed in bulk over a wide range of compositions. Furthermore, how a combination of Au and Pt in nanoalloys results in their special properties has not yet been determined conclusively.
It has been shown in empirical simulations and first-principles density functional theory (DFT) calculations that Au-Pt nanoalloys preferably arrange in a core-shell mixing pattern with Au forming a shell around a Pt core. This is in contradiction to many experimental studies that report the formation of solid solutions of Au and Pt. In the present work, this seeming discrepancy is addressed by simulating x-ray diffraction patterns that are experimentally used to characterize nanoalloys. It is shown that the interpretation of the diffraction patterns relies on questionable assumptions and therefore does not suffice as a definite characterization tool for Au-Pt nanoalloys.
To shed light on the special catalytic properties of Au-Pt nanoalloys under rather different experimental conditions, a thorough investigation of their electronic and structural properties has been carried out. It is found that features favorable for catalysis in Au-Pt nanoalloys emerge as a consequence of combining two fundamental properties: Pt contributes a high density of states close to the Fermi level, which promotes chemical activity. Au increases the structural flexibility of the Au-Pt system, which might be beneficial for the formation of active and element-specific binding sites as well as regeneration of the catalyst after the reaction.
Although DFT offers an attractive compromise between computational effort and accuracy for a theoretical description of Au-Pt nanoalloys, other transition metal compounds severely challenge existing DFT approximations. Manganese (Mn) doped silicon (Si) clusters represent an ideal model system to study the interaction of a single magnetic impurity with a semiconducting host both experimentally and theoretically. The transition from exohedral (lowly coordinated) to endohedral (highly coordinated) doping that occurs for Si clusters with more than ten atoms, is accompanied by complete quenching of the magnetic moment of Mn. We show that MnSi11+, the smallest endohedral cluster found in experiment, suffers strongly from a well-known general problem of most DFT approximations: the self-interaction error. Finally, a universal correlation between magnetic moment and the coordination of the Mn dopant is established that can be generalized to extended systems and suggests a route to stabilize the magnetic moment of bulk Mn-Si compounds.
Synthesis and Self-Assembly of Novel ABC Miktoarm Star Terpolymers
- A novel synthesis for ABC miktoarm star terpolymers and their self-assembly into complex superstructures in aqueous solution are described within this thesis. To this aim a modular route for such materials was developed, combining anionic polymerization and copper-catalyzed azide-alkyne cycloaddition. At the example of ABC miktoarm star terpolymers and an ABA’ miktoarm star copolymer containing a poly(N-methyl-2-vinypyridinium iodide) (P2VPq) segment, the counterion-mediated superstructure-formation of complex shaped aggregates was thoroughly investigated.
The key compound of the combinatorial synthesis is the newly synthesized 4-alkyne-substitued diphenylethylene derivative 1-[(4-(tert-butyldimethylsilyl)ethynyl)phenyl]-1-phenylethylene (“click-DPE”). This was applied in sequential anionic polymerization to prepare well-defined alkyne mid-functional diblock copolymers composed of polybutadiene (PB) as first and poly(tert-butyl methacrylate) (PtBMA), poly(2-vinylpyridine) (P2VP), or poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) as second block. The alkyne-midfunctional diblock copolymers were afterwards conjugated with azido-functional polystyrenes (PS), poly(ethylene oxide) (PEO), PtBMA and PDMAEMA to successfully obtain different novel ABC miktoarm star terpolymers with narrow molecular weight distributions.
For an ABC miktoarm star terpolymer consisting of arms of PB, PtBMA and P2VP it was demonstrated that after quaternization with methyl iodide (yielding BVqT) and dialysis to water the nature of the counterion allows for manipulation of the obtained structures. The miktoarm star architecture together with iodide as counterion is essential for this directed self-assembly. Transformation of iodide to triiodide, via the addition of iodine before dialysis to water, decreases the hydrophilicity of the P2VPq corona and therefore induces the directed self-assembly of spherical micelles with a PB/PtBMA core, into cylinders, superstructures thereof and finally barrel-shaped aggregates of up to 1 µm with an internal lamellar fine structure. Based on their appearance in transmission electron micrographs these were termed “woodlouse” aggregates. The compact particles consist of alternating lamellae of a partially demixed PB/PtBMA phase and a swollen P2VPq phase.
The general applicability of this counterion-mediated hierarchical self-assembly was furthermore demonstrated by using two other miktoarm star systems. For three ABC miktoarm star terpolymers of different composition, consisting of PB, PS and P2VPq segments (BVqS), a dependence of the morphology on the fraction of the hydrophilic block was determined, in analogy to diblock copolymers. For long P2VPq blocks stacked lamellar/disk-like structures evolve from micellar building units. In contrast, a short P2VPq segment yields multilamellar vesicles via fusion of vesicular primary building blocks. The vesicle walls are supposed to consist of a lamellar structure with the PB phase in the centre, shielded from the P2VPq corona by thin PS layers. At the example of one BVqS miktoarm star terpolymer the successful formation of nanohybrids containing gold nanoparticles within the P2VPq phase is demonstrated.
In the second system the low-Tg PB segment was replaced by a second PS block of different length (SVqS’). Even though vesicles serve as initial building units, the triiodide-induced superstructure formation leads to anisotropic aggregation of deformed vesicles, rather than to the fusion into multilamellar vesicles. This is attributed to the two glassy PS core blocks which minimize the dynamics during self-assembly and allow only minor rearrangement of the aggregated structures. Similar to the “woodlouse” aggregates from BVqT, lamellar structured particles of elongated shape were obtained from SVqS’, despite vesicles serving as primary building units. Consequently, the presented triiodide-directed self-assembly into complex superstructures is not restricted to miktoarm star polymers containing a low-Tg segment, as the rearrangement processes take place during the dialysis process, where the organic co-solvent enables sufficient mobility of the core-forming blocks.
Besides the introduction of a novel synthetic approach for the construction of miktoarm star terpolymers and the synthetic advance of the alkyne-functionalized DPE, the presented triiodide-mediated superstructure formation represents an interesting concept for directed self-assembly processes.
The Effect of Spatial and Environmental Drivers on Patterns in Species Richness and Composition
- This thesis includes eight manuscripts with methodological, empirical and theoretical contributions that aim to enhance the understanding of species richness and composition patterns and their underlying drivers. Islands and isolated systems are in the focus of this work.
Islands provide optimal conditions to study biogeographic patterns. Theoretical advances in ecology have been initiated by island biogeography. Theory on island biogeography has particularly been improved by a better representation of time related components including speciation and environmental change. Oceanic islands are not stable systems but follow a characteristic ontogeny. After the volcanic emergence over the sea surface, erosion processes, shaping the island first more heterogeneous and then flatter, transform islands. This thesis shows how particular characteristics of the classic theory of island biogeography can be included in the currently most advanced theoretical framework. While MacArthur & Wilson (1963) particulary focussed on processes (colonisation and extinction) for generating species richness patterns, current theory assumes a defined upper limit for species richness (“carrying capacity”). By reinforcing the importance of processes in the current theory, as suggested in this thesis, it is much simpler to formulate hypothesis that can be tested by empirical data. Carrying capacity is linked to “habitat heterogeneity”, both, in the meaning of topographic variability as well as the number of vegetation units that are present in a given area. This thesis demonstrates that a clear terminology is a prerequisite for a profound understanding of the effects of “heterogeneity” on species diversity patterns in general and the underlying biogeographic processes in particular. The heterogeneity of surfaces influences species diversity not only on scales larger than kilometres but also is important on very fine scales of meters and smaller. Novel methods to measure different aspects of surface variability are introduced and discussed and their effect on species richness and composition of plant species groups in different ecological systems is presented.
Furthermore, this thesis highlights the isolating effect of elevation (elevation-driven ecological isolation hypothesis). Environmental filtering along an elevational gradient differentiates ecosystems. Isolation increases with elevation, as comparable ecosystems are much farther apart at high elevations than is the case for lowland ecosystems. In addition, ecosystems on neighbouring islands or on the continent that serve as source regions for colonising species are smaller in area in high elevations in comparison to low elevation ecosystems. Consequently, an above average speciation rate reflected in a high percentage of endemic species can be expected for higher elevations on islands and high mountains. The elevation driven ecological isolation hypothesis is tested for a number of islands and a new hypothesis indicating a complex interaction with isolation is developed. The difference in isolation between low and high elevation ecosystem diminishes as the overall isolation of the island increases. Thus the relation between the percentage of endemic species and elevation should reverse with an increase in isolation. On very isolated islands, low and high elevation ecosystems are alike isolated but low elevation ecosystems should have an above average speciation rate as they provide more area and higher temperatures relative to the ecosystems above (e.g. metabolic theory of ecology).
The scale dependence of diversity patterns are attributed to ecological processes that operate differently over varying extents and grain sizes. This thesis demonstrates that scale dependencies in distance-decay analyses cannot be traced back to processes that are specific for the ecological scale, but can largely be attributed to sampling design and are highly sensitive to grain size and study extent. Distance-decay analyses are an adequate method to assess spatial turnover in species composition. However, this thesis shows that frequent practise of making comparisons among studies is not possible within the current methodological framework.
Finally, this thesis provides an overview on patterns in species richness and composition and elaborates interconnections between associated theories and underling drivers. Promising novel research questions and directions are identified in the field of island biogeography and in an adequate formalisation of a “heterogeneity” concept.
Impact of time and spatial averages on the energy balance closure
- Secondary circulations are large and relatively stationary eddies, which are caused by the surface heterogeneity and normally reside away from the ground. They are believed to be the cause of the energy balance closure problem at the earth's surface, because their contribution to the turbulent fluxes is missed by a fixed eddy-covariance tower measurement that has a typical averaging time of 30 minutes. In this thesis, data from the LITFASS-2003 experiment was used to investigate the impact of time and spatial averages on the energy balance closure. This data consisted of many observations over a large heterogeneous landscape that could generate secondary circulations; some of which might be still near the earth's surface.
For the time average analysis, the averaging time was extended to increase the possibility that secondary circulations were picked up by the sensor. Two approaches, which were the modified ogive analysis and the block ensemble average, were applied to analyze the data from ground-based measurements. The modified ogive analysis requiring a steady state condition, could extend the averaging time up to a few hours and suggested that an averaging time of 30 minutes was still overall sufficient for the eddy-covariance measurement over low vegetation. The block ensemble average, on the contrary, did not require a steady state condition, but could extend the averaging time to several days. However, this approach could only improve the energy balance closure for some sites during specific periods, when secondary circulations existed in the vicinity of the sensor. Based on this approach, it was found that the near-surface secondary circulations mainly transported sensible heat, which led to an alternative energy balance correction by the buoyancy flux ratio approach, in which the attribution of the residual depended on the relative contribution of the sensible heat flux to the buoyancy flux. The fraction of the residual attributed to the sensible heat flux by this energy balance correction was larger than in the energy balance correction that preserved the Bowen ratio.
In the spatial average analysis, two energy balance correction approaches, the buoyancy flux ratio and the Bowen ratio approaches, were applied to the area-averaged fluxes (composite fluxes) in order to include contribution from secondary circulations. These composite fluxes were aggregated from multiple ground-based measurements. The energy balance corrected fluxes were validated against the spatial average fluxes, which were measured by an aircraft and a large aperture scintillometer (LAS). In this validation, the backward Lagrangian footprint model was used to estimate the source area of the measurement. It was found that both energy balance correction approaches did improve the agreement between time and spatial averages fluxes. This suggested that the contribution from secondary circulations could be properly accounted by the energy balance correction.
All findings in this thesis, therefore, depict that secondary circulations significantly transport energy in the atmospheric surface layer. The energy balance correction, accomplished by using either the Bowen ratio approach or the buoyancy flux ratio approach, is necessary to estimate the actual vertical transport of energy at the earth's surface.
Direct Force Measurements on the Colloidal Scale: From Modified Electrodes to Particle Manipulation
- In this thesis the interfacial surface forces and mechanical properties of thin films have been studied by the colloidal probe technique. One central point is the combination of direct force measurements with an electrochemical setup in order to tune interfacial properties of an electrode modified with an organic layer. In particular the adhesion and ion adsorption have been studied, which are ubiquitous phenomena in the colloid science, electrochemistry, and biology. Moreover, a novel technique has been developed to fabricate chemically and mechanically stable colloidal probes for atomic force microscopy (AFM). Additionally, the elastic properties of polyelectrolyte multilayer films were locally resolved under controlled humidity.
The adhesive behaviour of colloidal particles on modified electrodes has been studied by direct force measurements with a micrometre-sized silica probe attached to an AFM-cantilever. By controlling the external potential applied to the modified electrode by means of a potentiostat, separate adhesion contributions at the modification layers in electrolyte solution were quantified. In particular, to determine the influence of the terminating functional groups, gold electrodes modified with self-assembled monolayers (SAMs) terminated in non-ionizable groups were used. It has been demonstrated that electrostatic double-layer forces dominate the adhesion of colloidal particles on hydrophobic and hydrophilic interfaces. In contrast to hydrophilic interface, for hydrophobic one forces due to the solvent exclusion play a significant role and leads to an offset in the adhesive force, which otherwise can be compensated by the external potential. However, the electrocapillarity is of minor importance and can be neglected.
To quantify the ion adsorption at organic interfaces a novel approach was followed, which is based on direct force measurements with silica colloidal probes on SAM-modified electrodes in electrolyte solutions. By variation of applied potential and concentration of specifically adsorbed ions, given by the solution’s pH, the charging behaviour of hydrophilic SAM-OH and hydrophobic SAM-CH3 has been determined. In difference to electrokinetic techniques, direct force measurements allow to probe the full range of the diffuse layer. The analysis of the diffuse layer potential as a function of externally applied potential provides important information. In particular, the shift of the potential of zero charge (pzc) indicates on the specific ion adsorption in the Stern layer as it alters the charging behaviour of the electrode’s interface. It has been demonstrated that hydronium and hydroxide ions adsorb on both the hydrophobic and hydrophilic interfaces. However, the presence of the background electrolyte (KCl) does not shift pzc and thus its ions have no specific affinity towards the interfacial adsorption. The adsorption of hydronium and hydroxide ions is stronger on hydrophobic, than on hydrophilic interface. This is in agreement with theoretical studies. The simple three-capacitor model based on a Langmuir-type adsorption isotherm provides semi-quantitative description of observed dependence of the diffuse double layer potential on applied potential.
A new technique for colloidal probe preparation was developed. A great challenge for the force measurements with the AFM is to ensure the cleanliness, chemical and mechanical stability of the used probes. The approach is based on high-temperature sintering of micrometer-sized silica particles to AFM cantilever with enhanced contact area. Due to a “neck” formed by nanometer-sized particles the increased mechanical stability of colloidal probes was achieved, which has been quantitatively determined by lateral force spectroscopy. The implementation of sintering procedure for silica colloids allowed the development of the highly stable colloidal probes, whose surface properties could be renewed by heating.
Finally, the mechanical properties of polyelectrolyte multilayer films have been determined by nanoindentation as a function of relative humidity. For these series of measurements again a colloidal probe has been used. It has been demonstrated that films containing polyglutamic acid have Young’s modulus, which depends on humidity. The change of stiffness with ambient humidity has reversible character.
Nostalgia, Home and Be-longing in Contemporary Postapartheid Fiction by Zakes Mda and Ivan Vladislavić
- Nostalgia, Home and Be-Longing in Contemporary South African Fiction: Zakes Mda and Ivan Vladislavić
The dissertation examines the representation of nostalgic longing and the question of home and belonging in selected South African novels by the contemporary writers Zakes Mda and Ivan Vladislavić. I argue that nostalgia is a crucial aspect to be explored in the South African context because it poses uncomfortable questions about one’s sense of belonging in a transforming society and one’s position in the present with regard to a history of colonialism and apartheid. The sentiment of nostalgia and the feeling of displacement bind the selected novels and characters and reveal a deep dissatisfaction with the present. In a comparative study, I show that nostalgia and the question of home have different impacts on the two authors. While the longing for a past home and time supports a reaffirmation of cultural identity in Zakes Mda’s novels, Ivan Vladislavić portrays nostalgia with a critical distance showing that the past is difficult to identify with. Both writers are critical of restorative forms of nostalgia (Boym 2001) and show that home and be-longing are a matter of inclusion and exclusion resulting from a legacy of segregation and Othering. I demonstrate how nostalgia can either contribute to constructively engage with the past and present and envision a future for the South African nation, or how it, on the contrary, can enforce the erection of borders and the (colonial) method of Othering within society.
Energy-domain synchrotron radiation Mössbauer source for physics under extreme conditions
- Iron is one of the most abundant elements on Earth, and it is an important component in minerals. Electronic and magnetic properties of iron-bearing materials significantly affect processes occurring in the deep interior of the Earth. In the materials that make up the Earth’s lower mantle iron may exist in different valence, spin states and crystallographic environments. Most of the existing experimental techniques either do not allow to separately follow evolution of different iron sites or are not suitable for measurements under high-pressure/high-temperature conditions. This makes studies of iron electronic structure under such conditions extremely challenging.
The current Ph.D. thesis is divided into two major parts. The first part is dedicated to the development of a Synchrotron Mössbauer Source (SMS). This device allows energy domain Mossbauer spectroscopy to be performed on a sample under pressures above 100 GPa in laser heated diamond anvil cells. The second part is dedicated to studying the behavior of iron in iron/alumina-bearing silicate perovskite under conditions of the Earth’s lower mantle.
1. Synchrotron Mössbauer Source
There are several techniques that allow magnetic and electronic properties of materials under extreme conditions to be probed: X-ray Emission Spectroscopy (XES), X-ray absorption near edge structure (XANES), Nuclear Resonance Spectroscopes, etc. For elements in which observation of Mössbauer effect is possible the most mature, sensitive, and suitable technique for studies of magnetic and electronic properties is energy-domain Mössbauer spectroscopy.
However, due to low brilliance of utilized radioactive sources and low natural abundance of iron in lower mantle minerals measurements using conventional energy- resolved Mössbauer spectroscopy require very long time and usually are limited to moderate pressures. The problem can be solved by combining the outstanding properties of synchrotron radiation (high brilliance, possibility for extreme focusing) with the energy-resolved approach. In brief, what is needed is a synchrotron source of Mössbauer radiation. Construction of such source was the primary task of my PhD work. The possibility to develop such a source was demonstrated at the Nuclear Resonance beamline ID18 at the European Synchrotron Radiation Facility (ESRF) by Smirnov et al. (1997). The source is based on pure nuclear reflections existing in antiferromagnetic 57FeBO3 crystals.
￼The major technical goals of my PhD work were to (a) construct a device that would be permanently ready for operation, and (b) optimize it to have the best possible resolution and highest possible intensity. In order to achieve these goals theoretical calculations were first conducted to understand how to best improve the performance. Second, several experiments were performed to confirm the theoretically predicted results. Third, several optical schemes of the SMS were tested in order to determine the optimal setup.
As a result of the research and development program a powerful Synchrotron Mössbauer Source (SMS) for high-pressure applications was constructed at the Nuclear Resonance beamline (ID18) of the ESRF. Using results obtained in the combined theoretical/experimental study of angular dependence of energy and temporal distributions of the pure nuclear reflections of iron borate crystal, the SMS was optimized for the highest possible intensity and best possible resolution. The bandwidth of radiation provided by the SMS is between 10-15 neV (2-3 Γ0, where Γ0 is a natural linewidth of Mössbauer resonance for Iron), the intensity is ~2.5×104 photons/s and the typical scanning velocity range is about ±12 mm/s (±0.6 μeV). In contrast to conventional radioactive sources, the SMS gives the possibility to focus the beam to tens of microns. SMS is the in-line monochromator, permanently located in the optics hutch and operational immediately after moving it into the incident beam position. The source can be used with all existing sample environments in the experimental hutches downstream of the beamline.
The implementation of this device opens the possibility for studying systems with complex hyperfine structure utilizing energy-resolved approach under various extreme conditions, for example at high-pressure. Furthermore, the SMS allows for very short collection times of only a few minutes, which enables data to be collected during laser heating. Several high-pressure and high-pressure/high-temperature studies that have already been performed are described in the second part of this Ph.D. thesis. The almost 100% recoilless resonant radiation delivered by the source and its high brightness allow a broad field of SMS applications. The SMS can be utilized in any mode of synchrotron storage ring operation.
2. Study of the spin state of Fe3+ ions in perovskite
Silicate perovskite (Mg,Fe)(Si,Al)O3 is the most abundant phase in the Earth’s lower mantle. Knowledge of its properties is indispensable for understanding lower mantle behavior. Dynamic, thermodynamic, and transport properties of silicate perovskite can be significantly affected by the valence and spin state of iron. Silicate perovskite with 5-10 mol% of Fe (where Fe3+/ΣFe ~50–75% (McCammon et al., ￼1997)) and Al, is dominant phase in Earth lower mantle (~75 vol%) (Zhang et al., 2006; Stackhouse et al., 2007). The behavior of Fe electronic properties under conditions close to those of the Earths lower mantle remains strongly controversial.
The second part of my Ph.D. work is dedicated to an investigation of the spin state of iron in Fe3+- rich silicate perovskite at high pressure. Four different silicate perovskite samples with different stoichiometry were studied using the Synchrotron Mössbauer Source. SMS spectra were collected at room temperature and pressures up to 122 GPa using diamond anvil cells, with or without laser annealing of the samples.
The hyperfine parameters, i.e., centre shift and quadrupole splitting, for the same phases, which were extracted from measured spectra for all perovskite samples studied in this work, are the same at each pressure within experimental error. Moreover, there is no change in Fe3+/ΣFe for individual samples over the entire pressure range of the experiment. The hyperfine parameters of the Fe3+ doublet are consistent with the high-spin state (Gütlich et al., 2011), and their smooth variation with pressure indicates that Fe3+ does not undergo spin crossover within the entire pressure range. All observed changes in the spectra are associated with abrupt changes in the electronic state of Fe2+. The hyperfine parameters of the low QS Fe2+ doublet correspond to the high-spin state (McCammon et al., 2008), while the doublet with high quadrupole splitting, whose intensity grows with pressure at the expense of the Fe2+ high-spin state, corresponds either to intermediate-spin (IS) Fe2+ (McCammon et al., 2008) or a distortion of the site occupied by high-spin Fe2+ (Hsu et al., 2010). Based on results presented in a work of Narygina (2010), we indentify changes in Fe2+ electronic structure as high-spin to intermediate spin transition. Irrespective of the interpretation of the Fe2+ spin state, conclusions regarding the absence of spin crossover in Fe3+ remain valid.
These results show that the previously reported spin crossover of Fe3+ ions does not occur when Fe3+ occupies the A-site. In both alumina-containing and alumina-free silicate perovskites Fe3+ ions remain in the high-spin state up to at least 122 GPa, i.e., almost up to the pressure corresponding to the lower mantle - outer core boundary. The results also indicate that Fe3+ ions do not diffuse from the A-site to the B-site in perovskite after high-temperature annealing at high pressure, Mössbauer spectra of before and after annealing are identical. There is also no evidence for high-spin to low-spin crossover of Fe3+ ions due to site change. In contrast, the results confirm that Fe2+ ions undergo a transition from a high-spin to an intermediate spin state, without reaching a low-spin state within the studied pressure range at room temperature. These results suggest that the seismic velocity anomalies in the lower mantle cannot be attributed to spin crossover in Fe3+.