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Item Open Access Entwicklung und Integration neuartiger Komponenten für Polymerelektrolytmembran- (PEM) Elektrolyseure(2018) Lettenmeier, Philipp; Friedrich, K. Andreas (Prof. Dr.)Item Open Access Wetting, de-icing and anti-icing behavior of microstructured and plasma-coated polyurethane films(2019) Grimmer, Philipp E. S.; Hirth, Thomas (Prof. Dr. rer. nat.)Ice build-up on surfaces, for example on wings of airplanes or on rotor blades of wind turbines, impairs the functionality of transportation vehicles or technical systems and reduces their safety. Therefore, functional anti-ice surfaces are being researched and developed, which shall enable an easy removal or reduce the amount of ice on the surfaces at risk. The starting hypothesis for this work is that superhydrophobic polyurethane (PU) films with microstructure base diameters of 35 µm or more reduce the wetting by water, show a low ice adhesion for easy removal of ice and reduce or delay icing. Superhydrophobic PU films for passive anti- and de-icing were created by hot embossing and plasma enhanced chemical vapor deposition (PECVD). The hot embossing process as well as the plasma coating and etching processes were analyzed for the dependence of the surface characteristics on different process parameters. The functionalized PU films were characterized for their surface topography, surface chemistry, stability against erosion, wettability, ice adhesion and icing behavior. For comparison, the ice adhesion and icing behavior were examined on relevant technical materials (aluminum, titanium, copper, glass, epoxy resin of carbon fiber reinforced polymer and other fluoropolymers) and on some commercial anti-ice coatings. The PU films were chemically analyzed by IR spectroscopy. As the first process step for functionalization, microstructures of cylindrical, elliptical or linear shape were imprinted in PU films by a hot embossing technique with different ns-pulsed laser-drilled stamps and characterized by several microscopy methods. The microstructures had heights of 15 µm to 140 µm, diameters or widths of 35 µm to 300 µm and distances (pitch values) of 50 µm to 500 µm. The embossing process was analyzed and optimized in terms of the process parameters temperature, pressure, time, PU film release temperature and reproducibility of the microstructures. In a second functionalization step (PECVD) the microstructured surfaces were coated with thin, hydrophobic plasma polymers using different fluorocarbon precursors (CHF3, C3F6 and C4F8) or hexamethyldisiloxane (HMDSO). Different process parameters for plasma coating and etching (Ar or O2 plasmas) were used in order to create various nanoscale roughness values. Electron spectroscopy for chemical analysis (ESCA), spectroscopic ellipsometry and atomic force microscopy (AFM) were used for analysis of the chemical composition, the thickness and the nanoroughness of the plasma polymers. The functionalizations, especially the plasma coatings, were completely worn off by a UV/water weathering test (1000 h, X1a CAM 180 Test, SAE J-2527), but showed sufficient stability against sand erosion (DIN 52348), in a long-term outdoor test for 13.5 months and against fivefold repeated pull-off of ice. The silicone-like plasma coatings were more stable than the fluorocarbon plasma coatings. The wetting behavior of water was determined by static, advancing and receding contact angle measurements. Static contact angle measurements with diiodomethane (DIM) were made for determination of the surface free energies of the relevant surfaces. Advancing contact angles of over 150° and very low contact angle hysteresis values below 10° were reached on some of the cylindrically and elliptically structured PU samples with microstructure base diameters in the range of 35 µm to 50 µm. The measured water advancing contact angles did not reach the theoretical values of the Cassie-Baxter state. Starting from a mixed wetting state near Cassie-Baxter in case of the superhydrophobic PU surfaces, they approached the Wenzel state with an increasing pitch/diameter (P/d) factor. Fluorescence laser scanning microscopy images were taken of some microstructured, uncoated or plasma coated samples during the wetting by a water drop containing a fluorescent dye. These images show the Wenzel state or a mixed wetting state by visualization of the interface between the water droplet and the surface. A new icing test chamber and a test setup were developed for characterization of the ice adhesion and the icing behavior. The tensile ice adhesion was measured at -20 °C by pull-off of ice cylinders (highly purified water, (<0.056 µS/cm, diameter of 4 mm, similar to the diameter of large raindrops) and compared to the theoretical values and the wetting behavior. The technical material surfaces measured for comparison showed a high ice adhesion, which led to cohesive fractures especially on the metal surfaces, whereas some of the commercial anti-ice coatings showed lower ice adhesion values. The flat, plasma coated PU surfaces showed adhesive fractures with a reduced ice adhesion compared to the technical material surfaces and uncoated PU and revealed a good correlation of the ice adhesion with the wetting behavior of water (work of adhesion). On the other hand, the microstructured PU surfaces showed a greatly increased ice adhesion in comparison to the flat PU and technical material surfaces which was enhanced even further by the plasma coatings and did not correlate with the wetting behavior. The reason for this is the wetting transition from the Cassie-Baxter to the Wenzel state during the cooling or freezing process, leading to an increased ice-surface contact area and mechanical interlocking of the ice with the micro- and nanostructures. The freezing of water drops was examined in thermodynamic equilibrium (static experiment) and under quasi-steady conditions (dynamic experiment). In the static experiment, 15 µl water drops (corresponding to medium to large raindrops) at room temperature were dispensed onto a cold surface at a constant temperature of -20 °C. The freezing delay times, the crystallization times and the total freezing times were measured and compared to calculated expected values. On the flat samples, the freezing delay times could be extended by the plasma treatments. On the microstructured samples, the freezing (nucleation) could sometimes be delayed even further, but not always reproducible because of an unstable Cassie-Baxter state. In the dynamic experiment, 25 µl water drops (corresponding to large raindrops) were cooled down in quasi-steady conditions with the surface and the surrounding atmosphere by a constant, low cooling rate of 1 K/min while the water drop temperature was measured by an IR camera for determination of the surface-specific nucleation temperature and crystallization time. A lower nucleation temperature could be measured on the flat, plasma coated PU surfaces compared to uncoated PU and the hydrophilic glass and metal surfaces. The superhydrophobic PU surfaces did not show a further reduction of the nucleation temperature because of an unstable Cassie-Baxter state. The resulting measured nucleation temperatures were compared to the expected values calculated with an enhanced nucleation theory including a quasi-liquid interfacial layer of the ice nucleus and a Poisson process. Overall, it is shown that hot embossing and PECVD are useful processes for creating superhydrophobic PU surfaces with regard to a roll-to-roll process. The flat, plasma coated PU films show a reduced ice adhesion and lowered nucleation temperature compared to the relevant technical material surfaces. The microstructured, plasma coated PU films are far more water repellent than the flat, plasma coated PU surfaces or the other technical materials. However, the microstructures with base diameters of 35 µm or more and the nanoroughness of the plasma coatings cannot stabilize the Cassie-Baxter state of a freezing water drop enough for a low ice adhesion or a significant decrease of the nucleation temperature. These superhydrophobic PU films are therefore not more icephobic than the flat, plasma coated PU films. In the outlook, the reduction of the geometrical parameters of the microstructures (diameter D, distance P) and nanostructures (curvature radius R) of the surface functionalizations for lower ice adhesion values and nucleation temperatures is proposed.Item Open Access Misfit-layered cobalt oxides for thermoelectric energy conversion(2017) Büttner, Gesine; Weidenkaff, Anke (Prof. Dr.)The conversion of waste heat into electrical current by a thermoelectric converter can significantly contribute to a more sustainable usage of our resources. The p-type misfit-layered [Ca2CoO3-δ][CoO2]1.62 is known for its promising conversion efficiency, which yet needs to be improved significantly for commercial applications. The efficiency of a material increases with the Figure of Merit ZT=σα^2/κ, with Seebeck coefficient α, electrical conductivity σ, and thermal conductivity κ. The aim of this thesis is to provide a better understanding of the electrical and the thermal properties of the complex [Ca2CoO3 δ][CoO2]1.62 and to use this understanding to improve the efficiency of converters. Accordingly, (i) the increase of ZT via cation substitution is shown; (ii) a better understanding of the electrical transport above room temperature is developed; (iii) the effect of stoichiometric defects and secondary phases on the thermoelectric properties is investigated. Finally, (iv) [Ca2CoO3 δ][CoO2]1.62 - CaMn0.97W0.03O3 δ - converters are fabricated and the efficiency is increased by a suitable converter design. More specifically, the unexplored influence of Ru and In substitution on the thermoelectric properties of the polycrystalline [Ca2CoO3 δ][CoO2]1.62 is investigated. While In does not have a positive effect, Ru for Co substitution increases ZT up to 20 %. This increase stems from a strong reduction of the thermal conductivity - which is probably induced by resonance scattering - while the decrease of the power factor α^2 σ is minor. The electrical transport mechanism of pure and Ru-substituted [Ca2CoO3 δ][CoO2]1.62 between room temperature and 800 K so far lacks a coherent theoretical model. Surprisingly, the framework of Anderson localization, which was developed to describe conduction in an impurity band of semiconductors, can be applied to the oxide. The Anderson model assumes that transport happens via charge-carrier hopping in a random Coulomb potential. For [Ca2CoO3 δ][CoO2]1.62, charges are considered to hop between Co sites in the CoO2 layer, while the random potential originates from interactions with the mismatched Ca2CoO3 δ layer. The presence of the ionized Ru atoms further alters the Coulomb potential, which increases the activation energy of the transport behavior. This understanding might contribute to the development of better theoretical models for the prediction of the thermoelectric properties of substituted [Ca2CoO3 δ][CoO2]1.62 compounds. A further improvement of the materials efficiency can be achieved by systematic introduction of stoichiometric defects and impurity phases. Here, the unexplored influence of the Co/Ca ratio on the thermoelectric properties of [Ca2 wCoO3 δ][CoO2]1.62, and the effect of Co3O4 impurity phase are investigated. It is shown that an increasing Co/Ca ratio in the [Ca2 wCoO3 δ][CoO2]1.62 phase leads to a larger figure of merit ZT induced by a strong resistivity drop. The decrease of resistivity stems from additional p-type charge carriers created by the formation of Ca vacancies. The Co3O4 impurity phase increases the thermal conductivity of the composite samples and leads to a reduction of ZT when the volume fraction of the Co3O4 phase is increased from 1% to 3%. Hence, the best figure of merit is expected close to the upper phase boundary of the [Ca2 wCoO3 δ][CoO2]1.62 phase. Not only the figures of merit of the materials, but also the design of a thermoelectric converter determines the device efficiency. In a converter, a p-type and a suitable n-type thermoelectric material are connected electrically in series and thermally in parallel. Here, [Ca2 wCoO3 δ][CoO2]1.62 is combined with the n-type CaMn0.97W0.03O3-δ and the device efficiency is improved by a variation of the ratio A_p/A_n of the cross section areas of the legs. The good agreement between the experimental values and the predictions of the compatibility model show the high quality of the fabricated devices and the value of the model for the optimization of the converter design. The adjustment of A_p/A_n improves the power output and the efficiency of the converters, where the best volume and area power densities exceed published high temperature values. The achieved efficiency of 1.08 % at a temperature of 1085 K at the hot side is close to the theoretical expected efficiency and can be further improved via ZT.Item Open Access Untersuchungen zur Belagsbildung und Charakterisierung der Ascheablagerungen in steinkohlegefeuerten Kraftwerken(2016) Babat, Selahattin; Scheffknecht, Günter (Prof. Dr. techn.)Bei der Verfeuerung fester Brennstoffe in Kraftwerken kann es durch Ascheablagerungen auf Heizflächen (Membranwände, Überhitzer, Zwischenüberhitzer, etc.) zur Verminderung des Wärmetransports und zu Betriebsstörungen und Schädigungen an Anlagenteilen kommen. Verantwortlich für diese Ablagerungen sind die mineralischen Bestandteile der Kohle, die beim Verbrennungsprozess zu schmelzflüssigen Phasen umgewandelt werden können. In einem Heizkraftwerk (730 MWth) wird für drei Steinkohlen das Verbrennungsverhalten im Hinblick auf Asche- und Belagsbildung sowie Belagscharakterisierung untersucht. Ein Schwerpunkt ist die Untersuchung des Belagaufbaus auf gekühlten und ungekühlten Depositionssonden. Mit der gekühlten Sonde wird die Initialschicht, die sich unmittelbar auf den Kesselrohren bildet, untersucht, während mit der ungekühlten Sonde eine Untersuchung im Hinblick auf die Deckschicht durchgeführt wird. Die Initialschichten zeigen für alle drei untersuchten Steinkohlen eine Anreicherung an Eisen im Vergleich zur Deckschicht und des Flugstaubes. Ursächlich für diese eisenreiche Grundschicht ist nach bisherigen Erkenntnissen das Mineral Pyrit. Neben den experimentellen und analytischen Untersuchungen werden mit der Software FactSage Gleichgewichtsberechnungen unter Berücksichtigung der im Brennstoff enthaltenen Mineralien (Pyrit, Illit, Kaolinit und Quarz) und der Analyse von Aschepartikeln unter oxidierenden und reduzierenden Verbrennungsbedingungen durch-geführt. Der Einfluss der Feuerraumatmosphäre auf das Ascheschmelzverhalten und die daraus hervorgehenden Ablagerungen wird mit diesen Berechnungen beurteilt. Die Ergebnisse zeigen, dass die Ablagerung eines Aschepartikels erheblich durch die Partikelhistorie beeinflusst wird. Abgesehen von der Partikelzusammensetzung haben insbesondere die Temperatur und die jeweilige Atmosphäre, die ein Partikel auf seiner Flugbahn bis zum Auftreffen auf die Heizfläche durchläuft, großen Einfluss auf die Mineralumwandlung und das Anhaften des Partikels.Item Open Access Zechstein Kupferschiefer at Spremberg and related sites : hot hydrothermal origin of the polymetallic Cu-Ag-Au deposit(2019) Spieth, Volker; Massonne, Hans-Joachim (Prof. Dr.)Copper-silver-gold-polymetallic (Cd, Hg, Mo, Co, Ni, Cr, V, Sb, U, Cs, Re, Pb, Zn, PGE) rich mineralization is present in the deposits containing Kupferschiefer black shale of the lowermost Zechstein Group of Late Permian (Lopingian) age in central Germany and southwestern Poland. Mineralized areas are near large shear lineaments at the border between the Saxo-Thuringian and Rheno-Hercynian zones. Polymetallic mineralization is contained in and geochemically transgresses the Upper Permian Rotliegend strata: the Weissliegend Sandstone, the Zechstein conglomerate, the Kupferschiefer sensu stricto, the Zechstein dolostone, and the overlying Werra carbonate rocks of the Zechstein Group. Hematitic alteration features of the Rote Fäule type are massively present. The mineralization occurs on a continental scale of more than 750 km in an east-westerly direction from eastern Poland to the Rhön near the Rhine valley in Germany in the so called European Copper Belt. The Spremberg-Graustein-Schleife Kupferschiefer deposit in the Lausitz of southeastern Germany has been newly explored and shows the high-grade metallic features of the typical Kupferschiefer deposits, e.g. in the Mansfeld area of Germany and the Lubin area of Poland. The deep drilling campaign from 2008 to 2010 produced much new sample material that became the basis for this scientific research undertaking, which is the first comprehensive study of its type in decades. The major focus of the study was to establish the nature and occurrence of the mineralization in its stratigraphic and ore depositional environment. The methodology employed was: (1) Geological mapping and sampling in the Spremberg-Graustein-Schleife deposit from the new drilling as well as from the drill repository of the LBGR Geological Survey of Brandenburg. This was also done in addition at the Rhön project, the Sangerhausen-Wettelrode deposit in Germany and the Konrad, Lubin, Polkowiecze-Sierosowiecze and Rudna deposits in Poland. (2) The SGS analytical services in Montreal, Canada, geochemically analyzed more than 800 rock powder samples of the exploration campaign 1956 to 1980 at Spremberg, as well as hundreds of new drill core assays were prepared from the new Spremberg exploration campaign. (3) Optical microscopy of more than 1,350 thin and polished rock sections were reviewed and the most important and significant ones were selected for detailed analysis. (4) Electron-microprobe (EMP) analytics, in which chemical compositions of minerals were determined. Textural relations were documented by back-scattered electron images. X-ray maps were produced to recognize the chemical zonation of minerals. 350 polished and thin sections from drill holes and underground locations were selected and analyzed. 626 measurements were taken of stoichiometric and non-stoichiometric metallic minerals, which resulted in new insights about their hot hydrothermal origin and depositional environment. Scanning electron microscope (SEM) studies with wavelength dispersive X-ray spectroscopy microprobe analyses (WDS) were conducted with the CAMECA SX 50. For the calculation of the mineral formulas and the mineral distribution diagrams, the Mincalc-5-program was used. (5) The Raman spectra were measured with the Horiba XpLora Raman microscope with confocal optics with laser wavelengths of 532 and 638 nm. The research focused on minerals and inclusions that were in the size fraction between 1 and 50 nm. Metallic minerals and hydrocarbon aggregates were identified and their intensity frequencies determined. (6) δ34S isotope analysis was conducted on 55 samples that were specifically selected to represent single sulfide aggregates to demonstrate the multi-phase nature of the mineralization. The mineral concentrates were analyzed with an EA-analyzer to SO2 at a reaction temperature of 1,050 °C. The S-isotopic composition was measured with a NC 2500 connected to a Thermo Quest Delta+XL mass spectrometer. The results confirmed the multi-phase nature of the deposit mineralization and supported the new model of origin. (7) Rock samples in historical and significant museum collections were reviewed and evaluated at the following places: Geological Collection at Universität Tübingen, Mansfeld Museum, Wettelrode Röhrigschacht Museum, German Federal Geological Survey Museum at Potsdam, Freiberg Bergakademie Mineralogical Museum, Polish Geological Museum, Warsaw, and Collection of the Mineralogical Institute of University Cracow, Poland. (8) Research progress was presented and discussed in-house and with national and international researchers at seminars, conferences and through publications. The new research results show that the high-grade, Upper Permian, Zechstein polymetallic deposits indicate strong chemical and paragenetic relationships that lead to a unified genetically linked model related to deep-sourced, hot hydrothermal, rift-related volcanism. Mantle heat during failed, intra-continental rifting of the Pangea supercontinent at the end of the Permian time released vast amounts of the exotic metal-rich, alkali-rich, silica-aluminum-rich, organic-rich, halogen-rich, high-density brines into deep-basement fractures, depositing them above the continental flysch Rotliegend sandstones and conglomerates. Detailed investigations show that the high-grade, exotic metal and hydrocarbon mineralization has a hot hydrothermal origin. These result in a micro-layered deposit that was extruded on the Upper Permian Rotliegend peneplain that may have been covered with a shallow Zechstein sea, which was very hostile to lifeforms, at the time of the Permian Mass Extinction. The mineral assemblies are unusual, often chemically non-stoichiometric and unique in their composition as they contain high-temperature and low-temperature minerals adjacent to each other. The stability fields of the sulfides indicate the temperature ranged between 72 °C and 557 °C and up to 1,120 °C for high digenite. Mineralogical results obtained through microscopy, microprobe, Raman spectroscopy, geochemistry and δ34S isotope analysis in this thesis show that: o The Kupferschiefer deposit type mineralization in its vast majority is somewhat monotonous, as it is made up in Spremberg and the European Copper Belt mainly of chalcocite (Cu2S), digenite (Cu1.75S5), covellite (CuS), bornite (Cu5FeS4), and chalcopyrite (CuFeS2), plus a high hydrocarbon content, which is significant as it occurs over a distance of more than 750 km in length. o Many of the copper minerals are of non-stoichiometric composition and unusual association. Bornite, chalcocite, chalcopyrite and pyrite occur as spherules, immiscible metallic drops in the slurry mud. Bornite of the Kupferschiefer sensu stricto T 1 layer often shows exsolutions of electrum (AuAg) and other solid state exsolutions with chalcopyrite and covellite, indicating pre-mixture in the rising metal-hydrocarbon mud slurry and rapid cooling after extrusion on the sea floor surface. o The microprobe element analysis of sulfide phases that are widespread in natural ores of the Kupferschiefer Cu-Ag deposits plot in a phase field that includes chalcocite, digenite, djurleiite, anilite, yarrowite (“blaubleibender” covellite), klockmannite, and krutaite. Klockmannite (CuSe) and krutaite (CuSe2) have a stability field of about 343 °C and 384 °C and thus document the high hydrothermal nature of the mineral deposition. o The δ34S sulfur stable isotopes are a unique feature to the Kupferschiefer sensu stricto and at Spremberg have a similar composition as those of the copper mineralization of the other deposits of the European Copper Belt. The δ34S sulfur stable isotopes are light to very light with values ranging from -31‰ to -40‰ (permille) in chalcocite-digenite and chalcopyrite samples of the lower Kupferschiefer sensu stricto. Given the high temperature of the sulfide mineralization, these low values cannot be explained by microbial reduction. As it is shown in published diagrams, deep-sourced systems of ultramafic to serpentinitic origin and composition can contribute brines with a similar δ34S sulfur stable isotope composition. o Geochemical major and trace element compositions are anomalous and are much enhanced compared to average global black shale. The Kupferschiefer sensu stricto analysis and geostatistical comparison diagrams demonstrate the interdependence of the base, precious and polymetallic mineralization with the hydrocarbon deposition in the Zechstein rocks. o Geometallurgical analysis of the available operational and scientific data proves the genetic association of the enriched ultrabasic-sourced elements PGE, Co, Ni, Cr, V, Se, Re, Os with the contemporaneously deposited hydrocarbons. o Geological observation and mineralogical analyses demonstrate that the hematitic “Rote Fäule” is a post Zechstein Kupferschiefer, pervasive alteration event. In places, the “Rote Fäule” may have two distinct phases, of which one might have added gold to the system, forming independent new deposits. The advancing “Rote Fäule” front creates a “TZ Transition Zone”, where existing base and precious metals are enriched to a higher grade. o The age of the Zechstein Kupferschiefer deposition is considered to be 252.5 M.y. This might vary slightly along the 750 km of the European Copper Belt. The age dating relies on illite and rhenium-osmium ages. Spremberg samples have been submitted to age dating. The mineralization has a multi-phase history with age dates spreading from 267.7 M.y. to the “Rote Fäule” alteration event date of 244.5 M.y. o Large, deep-reaching, continent-size rifting lineaments are known in the Zechstein mineralized area of the European Copper Belt. These NW-SE lineaments are disrupted by NE-SW faults. This tectonic pattern is common in all Kupferschiefer districts and has been demonstrated with a seismic exploration program at Spremberg. Geological observations and mapping in Sangerhausen-Wettelrode, Spremberg and the Lubin-Rudna district show that: o The Weissliegend sand is an injectite/extrudite, silica slurry of Zechstein age that mostly rests on top of the Permian Rotliegend peneplain and is covered in an undulating manner by Kupferschiefer sensu stricto. o The Weissliegend sands are cut by veins and veinlets of sulfides and hydrocarbon and Kupferschiefer-like black mud rock that may represent the feeder veins of an open, hot hydrothermal vent. o The Weissliegend sand hosts by far the majority in quality and quantity of the Kupferschiefer-type deposit mineable copper resources measured in 100s of million of tons. o These observations resulting in a high-temperature, hydrothermal emplacement model lead in their conclusion to a paradigm change that replaces the “obsolete” syn-sedimentary epigenetic model, with consequences: - future exploration and mine development, - can rely on parameters that are congruent with the scientific knowledge that in many aspects resembles Volcanic Submarine Massive Sulfide deposits, and - will assist in the finding and development of the so far termed(by the USGS) “undiscovered Kupferschiefer resources”. • The new model for the Zechstein Kupferschiefer deposits postulates a high-energy, hot-hydrothermal, extrusive environment not dissimilar to submarine “Black Smoker” and volcanogenic, submarine, metal-brine deposits. The metal-rich fluids ascended through deep-reaching faults and erupted as slurries in low-relief, mud volcanism above fractures in an open, shallow, inland sea. Metal sulfide deposition is systematically accompanied by the precipitation of silica, dolomitic carbonate, and illite, as well as primary copper chlorides, such as atacamite (CuCl2) and other brine minerals, such as anhydrite and sylvite. • The ultimate brine source is interpreted to be serpentinized peridotite in the lower crust near the Moho transition to the mantle. Dehydration of the serpentinite source to talc (steatization) by mantle heat during failed, intra-continental rifting of the Pangaea supercontinent at the end of Permian time released vast amounts of element-laden, high-density brines into deep basement fractures, depositing them above the continental flysch sediment Rotliegend sandstone and conglomerate peneplain in the shallow Kupferschiefer sea, which is analogous to the modern northern Caspian Sea and the Salton Sea of southern California, USA.