84 search hits
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The Effect of Zr-Doping and Crystallite Size on the Mechanical Properties of TiO2 Rutile and Anatase
(2008)
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Eva Susanne Holbig
- TiO2 is an important technological material, used as white pigment, as wide band gap semiconductor in electrochemical dye solar cells, for photocatalysis and in photochemical energy-conversion processes. The most abundant phases are rutile, anatase and brookite. In addition, there are a number of metastable low density modifications. The compression behavior of anatase and rutile was studied for TiO2 and Ti0.9Zr0.1O2 starting materials with crystallite size in the micro- and also nanometer range. Quenched samples of rutile, anatase and high pressure polymorphs synthesized at up to 10 GPa adopt 10 mol% ZrO2, Zr-doped TiO2 starting materials therefore have the composition Ti0.9Zr0.1O2. Compression experiments were carried out in the diamond anvil cell and samples were characterized by in-situ X-ray diffraction, X-ray absorption and Raman spectroscopic measurements. A sol-gel route was developed for the synthesis of nanoscale anatase Tix:Zr1-xO2 with x=0.90 and 1.0, which was annealed at 1000°C to microscale rutile. In hydrothermal experiments, nanoscale anatase Ti0.9Zr0.1O2 was used as starting material for the synthesis of microscale Zr-doped anatase. Neither the incorporation of Zr nor the decrease of crystallite size to the nanometer range modifies the bulk modulus of rutile. These results are different from those of anatase, where a decreasse of crystallite size and doping with Zr leads to an increase of the bulk modulus. Second order EoS fits (K0’=4) resulted in a bulk modulus of microscale anatase of K0=178 GPa and K0=179 GPa. The nanoscale counterpart shows much higher values of K0=237 GPa and K0=243 GPa. In this study, it was found that microscale anatase Ti0.90Zr0.10O2 has K0=195 GPa, which is comparable to undoped material. Largest values were found for nanoscale anatase Ti0.90Zr0.10O2 with K0=258 GPa. Zr-doping thus reduces the compressibility of nanoanatase, even though ZrO2 polymorphs are more compressible than the corresponding TiO2 forms. For the Zr-doped nanoanatase, XRD analysis showed a significant change in compression behavior at pressures 4 GPa, suggested as a consequence of deviatoric stresses during experimental compression of the nanoscale material. Computations on supercells with different distances of neighboring Zr-atoms suggested cluster formation of Zr in the (Ti,Zr)O2 anatase. The resulting structural distortions can further augment the change in compression behavior. Zr-doped nanoanatase becomes stiffer upon multiple compression cycles. While the bulk modulus of the first compression was 211 GPa, after the sample was decompressed, the second compression showed a bulk modulus of 249 GPa. We suggest that partial pressure induced amorphization plays an important role for the observed stiffening. Anatase and rutile TiO2 transform to the MI phase upon compression. The transition pressure increases with a decreasing crystallite size from 12 GPa for microscale material to 18 GPa for crystallite size of 12 nm. For anatase, smaller particles transform to an amorphous phase at pressures of 20–24 GPa. Zr-doping does not seem to vary the transformation pressure. Ab-initio all-electron density functional electronic structure simulations on the ground state energetics of the TiO2 phases rutile, anatase, brookite, TiO2II and MI-phase were performed using the projector augmented wave and the linear augmented plane wave methods along with local density approximation (LDA) and two types of generalized gradient approximations (GGA), using the formulations by Perdew, Bunge and Enzerhoff, referred to as PBE, and by Wu and Cohen, reffered to as WC. The zero pressure volumes are predicted smaller by <3% in LDA computations and larger by 8 and 0.4% in PBE and WC computations. The stable structure at 0 GPa is baddeleyite for LDA computations and anatase for GGA computations, contradicting experimental results that determine rutile as the most stable phase. Rutile appears to have the highest energy in LDA computations and intermediate energy in GGA computations.
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Existence and stability of stellardynamic models
(2008)
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Achim Schulze
- We examine existence and stability of stationary solutions to the Vlasov-Poisson system. This system is used in stellardynmaics to describe the evolution of galaxies where collissions are neglected and the evolution is determined by the self-consistent gravitational field which is created by the particles, e.g. the stars . In the first part we examine steady states which decsribe static shells under the influence of a fixed point mass. These solutions can be used as a model for a galaxy with a massive black hole in its center. For the Vlasov--Poisson system under the influence of such a point mass, we prove a global existence result. In the second part, we construct axially symmetric solutions depending on Jacobis integral. The presented results are in accordance with the numerical examinations of the P.O. Vandervoort.
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Stability of flat galaxies
(2008)
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Roman Firt
- In this thesis we investigate the existence and properties of stationary solutions of the flat Vlasov-Poisson system. This system of partial differential equations can be used as a model of extremely flat astronomical objects and is a combination between the two-dimensional motion of particles and the three-dimensional interaction through their gravitational potential.
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Bounds for the minimum oriented diameter
(2008)
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Sascha Kurz
Martin Lätsch
- We consider the problem of finding an orientation with minimum diameter of a connected bridgeless graph. Fomin et. al. discovered a relation between the minimum oriented diameter an the size of a minimal dominating set. We improve their upper bound.
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The Top-Dog Index: A New Measurement for the Demand Consistency of the Size Distribution in Pre-Pack Orders for a Fashion Discounter with Many Small Branches
(2008)
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Sascha Kurz
Jörg Rambau
Jörg Schlüchtermann
Rainer Wolf
- We propose the new Top-Dog-Index, a measure for the branch-dependent historic deviation of the supply data of apparel sizes from the sales data of a fashion discounter. A common approach is to estimate demand for sizes directly from the sales data. This approach may yield information for the demand for sizes if aggregated over all branches and products. However, as we will show in a real-world business case, this direct approach is in general not capable to provide information about each branchs individual demand for sizes: the supply per branch is so small that either the number of sales is statistically too small for a good estimate (early measurement) or there will be too much unsatisfied demand neglected in the sales data (late measurement). Moreover, in our real-world data we could not verify any of the demand distribution assumptions suggested in the literature. Our approach cannot estimate the demand for sizes directly. It can, however, individually measure for each branch the scarcest and the amplest sizes, aggregated over all products. This measurement can iteratively be used to adapt the size distributions in the pre-pack orders for the future. A real-world blind study shows the potential of this distribution free heuristic optimization approach: The gross yield measured in percent of gross value was almost one percentage point higher in the test-group branches than in the control-group branches.
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Maximal integral point sets over Z^2
(2008)
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Sascha Kurz
Andrey Radoslavov Antonov
- Geometrical objects with integral side lengths have fascinated mathematicians through the ages. We call a set P={p(1),...,p(n)} in Z^2 a maximal integral point set over Z^2 if all pairwise distances are integral and every additional point p(n+1) destroys this property. Here we consider such sets for a given cardinality and with minimum possible diameter. We determine some exact values via exhaustive search and give several constructions for arbitrary cardinalities. Since we cannot guarantee the maximality in these cases we describe an algorithm to prove or disprove the maximality of a given integral point set. We additionally consider restrictions as no three points on a line and no four points on a circle.
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Function of the ATP-dependent Metalloprotease FtsH during sporulation in Bacillus subtilis
(2008)
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Thi Thuy Ai Le
- The analysis of the function of the ftsH gene of Bacillus subtilis started about ten years ago. It was shown at that time that an ftsH knockout was viable, but exhibited a pleiotropic phenotype. Cells are sensitive to salt and heat shock, exhibit filamentous growth, are difficult to be transformed and are almost unable to sporulate. Despite the severe phenotype caused by the absence of the ftsH gene, the precise functions of this protein remained unclear. This PhD thesis presents data to elucidate the function of ftsH during sporulation. Furthermore, it describes the construction of a cold-inducible expression system. The major finding of this thesis is that the FtsH protease interferes with the synthesis and/or phosphorylation of Spo0A, the master regulator during initiation of sporulation called phase 0. In the ftsH knockout, the amount of Spo0A is greatly reduced, and the small amounts present are inactive. When the wild-type spo0A allele was replaced by an IPTG-inducible allele coding for mutant Spo0A protein being fully active in the absence of phosphorylation (Spo0A-Sad67), spores were formed at a normal rate in an ftsH knockout. Again, this result indicates that FtsH is clearly involved in the formation of active Spo0A and that this protease is only essential during stage 0 of sporulation. To become active, Spo0A needs to be phosphorylated by the multi-component system called phosphorelay. Since no active Spo0A is present in an ftsH knockout, it was hypothesized that FtsH has to degrade one or more negative regulator(s) either preventing the phosphorylation of Spo0A or/and being involving in its rapid dephosphorylation. The further analysis focused on four antagonists of the phosphorelay, three Rap phosphatases being involved in the dephosphorylation of Spo0F~P, and Spo0E which targets Spo0A~P. When a null allele in any one of them was combined with the ftsH knockout, the wild-type amount of Spo0A was restored only in the case of the ftsH spo0E knockout and the sporulation frequency was increased by two to three orders of magnitude in all double knockouts, but remained below 1%. Since overexpression of Spo0E reduces the sporulation frequency and removal of the gene from the genome has an opposite effect, a direct interaction between FtsH and Spo0E was envisaged. In vitro proteolysis assays with purified GST-FtsH and GST-Spo0E showed that Spo0E is indeed a target of FtsH. In contrast, the two homologs of Spo0E, YisI and YnzD, remained stable upon incubation with FtsH. Since all three proteins are distinguished by a C-terminal extension of about 25 amino acids present in Spo0E, but not in the two other phosphatases, these additional amino acids could serve as a target for FtsH. When two mutant versions of Spo0E, Spo0E94 and Spo0E11, with truncated C-terminal ends were analyzed, they turned out to be stable in the presence of FtsH. When the C-terminal 25 amino acids was transferred to YnzD, this fusion protein became unstable when incubated with FtsH. In conclusion, the C-terminal end of Spo0E confers instability to this enzyme. Since a spo0E knockout in a wild-type background does not result in a sporulation frequency close to 100% and a combination of a spo0E and an ftsH knockout raises the sporulation frequency only close to 1%, it can be concluded that there are additional targets for FtsH interfering with the synthesis of active Spo0A. Moreover, it is likely that FtsH also exerts a function late during sporulation. It could be shown that SpoVM, a small peptide essential for spore morphogenesis, inhibits the proteolytic activity of the B. subtilis FtsH protease in vitro. It can be inferred that SpoVM also inhibits activity of FtsH during sporulation, and in the absence of SpoVM, FtsH will degrade at least one protein essential for successful completion of sporulation. When the intracellular proteomes of spoVM+ and spoVM- cells were compared, a total of 83 proteins were identified being either completely absent or present in reduced amounts in the absence of the peptide. Analysis of the expression of the spoVM gene revealed that cells started to synthesize the spoVM transcript at stage 2 while the SpoVM peptide accumulated at stage 4. The 5´ untranslated region of the spoVM transcript has been identified to act as a negative regulator of its own transcription or translation. Furthermore, a cold-inducible expression system has been constructed allowing intra- and extracellular production of recombinant proteins. This expression system makes use of a two-component signal transduction system, which senses changes in the fluidity of the cytoplasmic membrane.
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Vulnerability to Drought Risk and Famine: Local Responses and External Interventions among the Afar of Ethiopia, a study on the Aghini Pastoral Community
(2008)
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Ali Hassen Muhaba
- In East African countries drought-related famine has been a number one risk. Ethiopia is among those countries that are repeatedly stricken by recurrent famine. Agricultural and pastoral households have increasingly become vulnerable to famine. The successive Ethiopian governments attributed the recurring famine and hunger to natural events, particularly to droughts. However, though drought triggers famines, it does not necessarily lead to famine disaster in every context. This is the current tone of literature in disaster causation. Each famine has its own specific causes in each context and this requires exploring the causal factors thereof. This study in the Afar region, in north-east Ethiopia, attempts to explain the root causes of vulnerability to famine, and assess the local and external responses. The central argument of the research is that pastoralists’ vulnerability to famine and food crisis has increased overtime because of the complex interplay of multiple factors such as environmental or ecological degradation, socio-economic destabilization, and political processes. It is also stated that despite efforts of internal and external actors, vulnerability of the pastoral groups to famine has increased over time. In that respect the study attempts to explain how these factors have led to an increase of vulnerability and livelihood insecurity among the Afar pastoralists. Three specific arguments are addressed through analyzing both secondary and primary data. These are: The Afar pastoralists’ vulnerability to famine has increased over the past decades because of the combined effects of drought, ecological crisis and external pressures (encroachments, loss of key pastoral resources, violent conflict and political instability). Pastoral households/communities are currently less able to cope with stresses through their traditional coping and adaptive strategies. Consequently, pastoral households/communities have become more dependent on public transfer (food aid) to cope with recurring food crisis. The empirical research used both qualitative and quantitative data in addressing the basic research questions. The outline of the research is structured to suit an approach of presenting discussions at macro and micro levels. An assessment of factors both, at macro (regional/national) and micro (community) levels, is made on the basis of secondary and primary data respectively. Accordingly, factors related to ecological degradation, socio-political processes and recurrent droughts, etc. are examined. The extent of these problems at the macro level is assessed mainly based on secondary data, while the magnitude of these problems at micro (community) level is assessed on the basis of primary data gathered through a household survey, and individual and focus group interviews. Based on the analyses of both primary and secondary data the research attempts to answer the question why the Afar pastoralists’ vulnerability to famine has increased over time and how the local and external actors have responded to recurring famine. The search for explanations of vulnerability to famine focuses on highlighting the interplay of multiple causal factors at different levels within historical socio-political and economic processes overlapping with ecological crisis and recurrent drought. Accordingly the research highlights the major factors that have created vulnerability to famine. These include: External pressures which include state intervention, land alienation, encroachment by cultivators, loss of dry season/drought retreats, curtailment of mobility and unfavourable terms of trade. Stresses which include frequent drought, conflicts, political instability, weakening mutual support systems, lack of trust in formal government institutions due to non-participatory, lack of commitment, patron-client relationship, and corruption. Ecological/environmental crises which are reflected in terms of loss of key pastoral resources (grass, natural fodder vegetation, and water). The conclusion of the research is that famine and the increase of vulnerability are not primarily the consequences of drought, but of external domination and uneven development. This suggests that the genesis of food crisis (famine) must be understood as an interaction of institutional, economic and political variables. Natural events like droughts don’t necessarily lead to famine in all contexts. It is only when livelihood assets are eroded, opportunities are constrained and people are not well-prepared that the consequences of natural events develop into famine or food crisis. This suggests that production or yield failures caused by drought do not become famines unless other conditions are propitious. Therefore, the current approach in social science research with regard to disaster causation is to look at the interrelationship between natural risks and social vulnerability.
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Experimental Constraints on Silicate Perovskite Forming Reactions and Elastic Properties: Geophysical Implications for Chemical Heterogeneity in the Deep Mantle
(2008)
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Saikia Ashima
- Three experimental investigations have been performed in order to understand how the composition of the mantle may influence mineral stability and elastic properties and how these may influence seismic properties of the deep mantle. The phase relations of calcium perovskite have been studied in high pressure and temperature experiments to examine the effect of its formation on seismic discontinuity features at 520 km depth in the mantle. The effect of varying composition on the compressibility of magnesium silicate perovskite has been examined in order to understand the geophysical consequences of chemical heterogeneity in the lower mantle. Calorimetric measurements of the pyrope-majorite garnet solid solution have been made to provide essential thermodynamic data for modeling the formation reactions of both magnesium and calcium silicate perovskite.
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Dating Saharan dust deposits on Lanzarote (Canary Islands) by luminescence dating techniques and their implication for palaeoclimate reconstruction of NW Africa.
(2008)
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Hans von Suchodoletz
Markus Fuchs
Ludwig Zöller
- Lava flow dammed valleys (Vegas) on Lanzarote (Canary Islands) represent unique sediment traps, filled with autochthonous volcanic material and allochthonous Saharan dust. These sediments and the intercalated palaeosoil sediments document past environmental change of the last glacial-interglacial cycles, both on Lanzarote and in NW Africa. A reliable chronology must be established to use these sediment archives for palaeoclimate reconstructions. Owing to the lack of organic material and the limiting time range of the 14Cdating method, luminescence dating is the most promising method for these sediments. However, the fluvio-eolian character of these sediments is a major problem for luminescence dating, because these sediments are prone to insufficient resetting of the parent luminescence signal (bleaching) prior to sedimentation. To check for the best age estimates, we compare the bleaching behavior of (1) different grain sizes (coarse- versus fine-grain quartz OSL) and (2) different minerals (fine-grain feldspar IRSL versus fine-grain quartz OSL). The results show that owing to its bleaching characteristics, quartz is the preferable mineral for luminescence dating. On the basis of the fine- and coarse-grain quartz OSL age estimates, a chronostratigraphy up to 100 ka could be established. Beyond this age limit for OSL quartz, the chronostratigraphy could be extended up to 180 ka by correlating the vega sediments with dated marine sediment archives.
