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Sol-gel preparation and characterization of corundum based ceramic oxidation protection coatingsDressler, Martin 08 December 2006 (has links)
The Ni-base superalloy, IN-718, has been coated with alumina sols. Coated surfaces, carrying alumina layers having thicknesses between 0.6 μm and 3.6 μm show a significantly reduced oxidation rate when compared with uncoated reference surfaces, even if heating temperature is increased up to 900 °C and heating time is extended to 800 h. Alumina layers were prepared via sol-gel processing using a modified Yoldas procedure to obtain alumina sols. No change in rheological sol behavior was observed for more than 1 year of aging under static conditions at room temperature. Depending on pH value, modified Yoldas sols contain a manifold of Al species, among them Al13 polycations. Thermal evolution of sol derived alumina powders depends on Al speciation of parent sols. Depending on sol composition, both gamma-Al2O3 and eta-Al2O3 occur as intermediate transition aluminas. Phase composition and gas phase velocity influence oxygen permeability of thin layers prepared with modified Yoldas sols.
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Bohrspülungen zur Erschließung mariner Gashydratlagerstätten - inhibierende und stabilisierende Additive sowie verbesserte rheologische Charakterisierung: Bohrspülungen zur Erschließung mariner Gashydratlagerstätten - inhibierende und stabilisierende Additive sowie verbesserte rheologische CharakterisierungSchulz, Anne 20 February 2015 (has links)
Gashydrate sind natürlich vorkommende feste Verbindungen aus Wasser und Gas, deren Erschließung als zukünftige Energiequelle von Interesse ist. Für die bohrtechnische Erschließung mariner Gashydratlagerstätten ist eine leistungsfähige Bohrspülung notwendig. Das vom Bohrmeißel gelockerte Sediment und darin enthaltenes Gashydrat werden durch die Bohrspülung nach übertage transportiert. Die Gashydratpartikel verlassen beim Aufsteigen im Ringraum in ca. 300 m Wassertiefe ihren Stabilitätsbereich und dissoziieren in Wasser und Gas. Um eine Verdünnung und eine Dichteerniedrigung der Bohrspülung zu verhindern, soll das Gashydratbohrklein stabilisiert werden. Gleichzeitig darf sich in der Bohrspülung bei Anwesenheit von freiem Gas in der Lagerstätte kein neues Gashydrat bilden. Die Arbeit beschäftigt sich mit der Suche nach Additiven, welche die Gashydratneubildung und -dissoziation gleichzeitig hemmen. Es wurde ein Schüttelautoklav genutzt, um die Dissoziationstemperatur von Methanhydrat bei ca. 85 bar zu ermitteln und die Verzögerung des Hydratzerfalls bei Anwesenheit verschiedener Additive zu vergleichen. Es konnte ein Additiv gefunden werden, das diese Anforderungen erfüllt. Des Weiteren wurden neue rheologische Untersuchungsprogramme für verschiedene Spülungstypen erarbeitet, die eine detaillierte Charakterisierung der Fließfähigkeit, Thixotropie und Geleigenschaften von Bohrspülungen erlauben.
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Mg-Al Layered Double Hydroxide: A Potential Nanofiller and Flame-Retardant for PolyethyleneCosta, Francis Reny 09 November 2007 (has links)
The presented research report deals with the investigation of magnesium aluminum based layered double hydroxide (LDH) as a potential nanofiller and flame-retardant for polymers with special reference to polyethylene. LDH is a mixed hydroxide of di- and trivalent metal ions that crystallizes in the form of mineral brucite. The basic reason for selecting LDH or more specifically magnesium-aluminum based LDH (Mg-Al LDH) is their typical metal hydroxide-like chemistry and conventional clay-like layered crystalline structure. The former is helpful in the direct participation in flame inhibition through endothermic decomposition and stable char formation. On the other hand, the later makes LDH suitable for polymer nanocomposite preparation, which can address the poor dispersibility problem associated with conventional metal hydroxide type fillers in polyolefin matrix. Besides, unlike layered silicate type clays (often reported for their capability to improve flame retardancy of polymers), LDH being reactive during combustion has higher efficiency to reduce the heat released during combustion of the composites. LDH clay with fixed Al:Mg ratio was synthesized using urea hydrolysis method and characterized. The organic modification of Mg-Al LDH using anionic surfactants has been studied in details. The main purpose of such modification is to enlarge the interlayer distance and to render it more organophilic. The surfactants were selected based on their functionality, chain length, etc and the modification was carried out by regeneration method. In the modified LDHs, the surfactants anions are arranged as a monolayer in the interlayer region and expand the interlayer distance according to their tail size. PE/LDH nanocomposites were prepared by melt-compounding method using a co-rotating tightly intermeshed twin-screw extruder and the morphological, mechanical and flammability properties of the nanocomposites were investigated in details. The X-ray diffraction analysis and electron microscopic analysis show a complex LDH particle morphology with hierarchy of particle size and shape starting from exfoliated particles fragments to particle aggregates over few hundred nm size. The exfoliated LDH platelets are distributed both in the vicinity of large particles and also in the bulk matrix. The melt rheological characterization of the nanocomposites also reflects the similar complex particle morphology. The dynamic oscillatory shear experiments showed that with increasing LDH concentration, the rheological behavior of the nanocomposite melts deviates strongly from that of the unfilled polyethylene. Thermogravimetric analysis (TGA) shows that LDH significantly improves the thermal stability of the polymer matrix in comparison to the unfilled polymer. The flammability studies of the PE/LDH nanocomposites have been reported in terms of various standard methods, like limited oxygen index (LOI), cone-calorimetry and UL-94 vertical and horizontal burn tests. The cone-calorimetric investigation shows that the nanocomposites have significantly lower burning rate and heat released during combustion. With increasing concentration of LDH though the LOI value of the nanocomposite increases marginally, the burning behavior, like dripping, rate of burning, etc are significantly improved. The flammability performance of LDH in combination with other commonly used flame-retardant (magnesium hydroxide) was also investigated. It has been observed that in polyethylene, a 50 wt% combination filler (4:1 weight ratio of magnesium hydroxide and LDH) can provide similar flammability ratings (like V0 rating in UL94 test, no dripping, etc) as that observed with 60 wt% magnesium hydroxide alone.
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Rheological and Mechanical behaviour of Block copolymers, Multigraft copolymers and Block copolymer NanocompositesThunga, Mahendra 18 June 2009 (has links)
Block copolymers are commercially significant and fundamentally interesting class of polymeric materials. The ability to undergo interfacial thermodynamics-controlled microphase separation from a completely disordered state in the melt to a specifically defined ordered structure through self-organization makes the block copolymers based materials unique. Block copolymer are strongly replacing many of the commercially available polymers due to their unique microstructure and properties. The most practical interests of block copolymers lie in the area of thermoplastic elastomers (TPEs). The objective of the present thesis work is to developing novel roots for enhancing the physical and mechanical properties in block copolymer and multigraft copolymers. Initially the properties are tailored by controlling chemical architecture at synthesis level and by selective blending at production level. This gives an easy access for improvement of the material properties and this is one of my major tasks in the present research modules.
Further the block copolymer based TPEs are cross-linked in presence of electron beam (EB) radiation for developing materials with superior properties. The electron beam radiation has the ability to alter material parameters at molecular level for enhancing the macroscopic properties. The desirable physical and chemical properties can be easily attained by varying the radiation beam parameters. In addition to that, controlling the material at nanometer scale is one of the greatest challenges for current nanocomposite research. In elastomeric materials it is very prominent to fill the rubber matrix with nano particles from carbon or silica by melt mixing technique for enhancing the material properties. Other than conventional melt mixing technique, sol–gel processing is also a versatile technique, which making it possible to produce a wide variety of materials and to provide existing materials with novel properties. A combination of in situ sol-gel reaction with electron beam cross-linking in TPEs from triblock copolymer has been demonstrated for the first time as one of the novel nanocomposite system in this work. The main advantage of this system lies in controlling the material behaviour by finely tuning the size of silica nano particle generated inside TPE during in situ sol-gel reaction. Finally, the various roots employed for enhancing the material behaviour in block copolymers in the above research module were secussfully employed on super elastic multigraft copolymers for improving their strength withour sacrificing the super elastic nature.
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Beiträge zur Entwicklung Wasser speichernder Materialien auf Basis von Stärke und LigninPassauer, Lars 07 July 2008 (has links)
Vor dem Hintergrund des weltweit wachsenden Bedarfes an Bodenverbesserungsmitteln, durch die Humus-, Nährstoff- und Wassermangel auf Problemstandorten kompensiert werden sollen, war es das Ziel der vorliegenden Arbeit, Bodenwasser-speicher auf Basis der nachwachsenden Biopolymere Stärke und Lignin zu entwickeln. Durch Derivatisierung der Stärke wurde deren Hydrophilie deutlich gesteigert, was Voraussetzung für die Bildung von Hydrogelen war. Es konnte gezeigt werden, dass durch Vernetzung der wasserlöslichen Stärkederivate Carboxymethylstärke und Monostärkemonophosphat mit Di-/Tricarbonsäuren quellfähige Hydrogele erzeugt werden, deren Quellungsvermögen und rheologische Eigenschaften über die Wahl des Vernetzers und die Vernetzerstoffmenge gezielt eingestellt werden können. Die Modifizierung von Lignin wurde durch Oxidation mit Wasserstoffperoxid, z.T. in Kombination mit Fe(II)- bzw. Mn(II)-Chloriden realisiert. Dadurch wurde die Vernetzbarkeit von Lignin deutlich verbessert, was auf oxidativ bedingte Strukturänderungen des Lignins zurückzuführen war. Diese bestanden im Wesentlichen in der Spaltung und Oxidation der Lignin-Seitenkette sowie der Hydroxylierung der Seitenkette und aromatischer Strukturen. Die Vernetzung von Lignin mit Poly-(ethylenglycol)-diglycidylether ergab quellfähige Hydrogele, deren Wasseraufnahmevermögen und rheologische Materialfunktionen von der eingesetzten Vernetzerstoffmenge abhängig sind. Es konnte gezeigt werden, dass durch den Einsatz von Stärkephosphat- und Lignin-Hydrogelen das Wasserspeichervermögen erhöht und die Evaporationsraten eines entsprechend behandelten Sandbodens verringert werden. Im Wachstumsversuch wurden die Frischmasseerträge von Gelbsenf (Sinapis alba) durch Behandlung eines Sandbodens mit Hydrogelen gesteigert. / Soil degradation leading to a lack of humus, nutrients and water especially on exploited sites causes the worldwide need in soil amendments. Aim of the work was the development of hydrogels from renewable biopolymers starch and lignin improving water retention especially in degraded soils. A significant increase of hydrophilic properties of starch was obtained by chemical modification with the objective of forming starch based hydrogels. Swellable hydrogels were formed by cross-linking of water soluble starch derivatives like carboxymethyl starch and monostarch monophosphates with di- and tricarboxylic acids. Swelling capacity and rheological properties of the starch gels were selective adjusted by variation of cross linking agent and whose amounts. Modification of lignin was realized by oxidation with hydrogen peroxide partly in combination with ferrous and manganese chlorides, respectively. In consequence of oxidative structural changes which were cleavage and oxidation of side chain as well as aliphatic and aromatic hydroxylation, gelation of lignin was improved significant. Lignin hydrogels with different swelling capacities and rheological functions were formed by cross-linking lignin with different amounts of poly (ethylene glycol) diglycidyl ether. Application of hydrogels based on starch and lignin causes increased water storing capacity/field capacity and decreased evaporation of a sandy soil as well as an increased biomass yield of yellow mustard (Sinapis alba).
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Propriétés d'écoulement de suspensions concentrées de particules de PVC et leur lien avec la physico-chimie du système / Flow properties of PVC-particles concentrated suspensions and their relation to physico-chemistry of the systemChatté, Guillaume 18 September 2017 (has links)
Nous étudions des suspensions concentrées de particules non-colloïdales de PVC. Ces suspensions, appelées plastisols, sont utilisées principalement pour la fabrication de revêtements de sols.Elles présentent notamment un phénomène de rhéoépaississement (hausse de la viscosité en cisaillant). Nous montrons que cela provient des forces de frottement entre particules. En effet, à l’aide d’un microscope à force atomique, nous sommes pour la 1ère fois capable de relier directement la contrainte macroscopique d’apparition du rhéo-épaississement à la contrainte microscopique d’apparition de la friction solide entre particules.Nous caractérisons la viscosité de la suspension jusqu’à 100 000 s-1 et nous observons qu’une plus grande polydispersité limite le rhéoépaississement. Les différences de contraintes normales N1 et N2 sont aussi mesurées. Par ailleurs, des mesures à l’aide de rayons X ou d’ultrasons ne montrent aucune migration de particules sous cisaillement.Nous montrons également que la géométrie a un fort impact sur l’écoulement de la suspension concentrée. Un entrefer plus petit provoque une baisse de la viscosité et retarde le rhéoépaississement. Une approche non-locale permet de rationaliser les résultats.La substitution de particules de PVC par des particules de CaCO3 modifie profondément la viscosité et la densité d’empilement maximum. Nous développons alors des modèles simples pour modéliser ces effets. En outre, nous mesurons l’impact sur la rhéologie d’un éventuel surfactant à la surface des particules.Nous avons pu finalement étudier des instabilités observées en étalant ces suspensions à haute vitesse. Une instabilité de surface est d’abord observée. A plus haute vitesse, un dépôt se forme en aval sur le couteau. Nous corrélons ces instabilités avec l’apparition de différences de contraintes normales. / Highly concentrated and non-colloidal suspensions consisting of micrometric PVC particles dispersed in a liquid phase, were studied. These suspensions, called plastisol, are mostly used in vinyl flooring manufacture.A key feature of these suspensions is shear-thickening, since viscosity greatly increases as a function of the applied shear rate. This phenomenon is explained as being related to frictional forces between particles. Indeed, using an Atomic Force Microscope, we were able, for the first time, to link the macroscopic stress, at which shear-thickening appears, with the microscopic stress needed to enter a frictional regime.We then characterize the suspension viscosity up to 100 000 s-1. We observed that shear thickening is lowered with a more polydisperse powder. Large normal stress differences N1 and N2 were also measured, along with shear thickening. In addition, using both X-ray radiography and ultrasound, no particle migration in the sheared suspension could be detected.We also found that geometry plays a major role in the features of the flow of concentrated suspensions. For a smaller gap, the viscosity is lower and shear-thickening is pushed to higher shear rates. A non-local approach accounts for our experimental results.Replacing a number of PVC particles with CaCO3 particles changes both the viscosity and the maximum packing fraction quite dramatically. For both of these, we developed simple models that matched quite well with the experimental data. Moreover, we elucidate the rheological changes resulting from adding surfactant at the surface of each particle type.Finally, we investigated some instabilities observed while coating at high speed. At a moderate speed, a ribbing phenomenon appears. At a higher speed, a deposit is formed on the knife (downstream). The appearance of these instabilities correlates with normal stress differences
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Control concept for a grease lubricated hydrostatic bearingMass, Igor, Hoppermann, Andreas, Murrenhoff, Hubertus 25 June 2020 (has links)
In industrial practice greases are mainly used as lubricants in hydrodynamic plain bearings and roller bearings. The use in hydrostatic bearings is avoided due to the difficult controllability. One reason is the complex non-Newtonian flow behavior of greases. The motivation for this paper is the use of greases to increase the efficiency of hydrostatic bearings. The assumption is that the so-called yield stress of consistent greases can lead to self-sealing behavior in the bearing under stationary operation conditions. Assuming a volume-flow-free operation of the bearing, a concept for the active control of the sealing gap height and thus the bearing stiffness was developed. The concept idea is the use of a second medium with Newtonian characteristic for pressure transfer. The grease and the pressure control fluid are structurally separated. The grease is induced in the shortest possible way into the bearing pocket to keep the pressure losses as low as possible. The results of test bench investigations indicate the feasibility of a gap height control with very high bearing stiffness under use of highly consistent greases and initiate further investigation on non-stationary operation.
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Flow-Induced Particle Migration in Concrete under High Shear RatesFataei, Shirin 18 August 2022 (has links)
The correlation between concrete rheological parameters and its pumping behavior under consideration of the so-called lubricating layer has been investigated for decades. In this thesis, flow-induced particle migration (FIPM) was studied in-depth, as the main underlying mechanism for the formation of the lubricating layer.
Conventionally vibrated and self-compacting concretes were chosen as the target mixtures. Furthermore, cementitious model concretes, containing colored glass beads, were proposed to obtain further insights into the FIPM and its impact on pumping. The mixtures were differentiating with regard to particle volume fractions, mortars composition, maximum particle size and particle size distributions. In the experiments, various established methods were used to characterize the rheological properties and the pumping behavior of the concretes. New methodologies for estimating the thickness of the lubricating layer and the particle distribution in pumped cross-sections were proposed as well.
The rheological properties and pumping behavior of real and model concretes were in agreement with the state-of-the-art literature. Based on the radial particle distributions, it was illustrated that the particle migration intensifies by increasing the particle size and decreasing the total volume fraction of solid particles. Furthermore, in highly-concentrated poly-dispersed model concretes, the particle concentration curves consist of a sudden jump close to the pipe wall and a relatively uniform distribution in the inner zone of the pipe. In these cases, the influence of FIPM on overall flow behavior cannot be neglected. Moreover, it was shown that the lubricating layer thickness is not constant for all concrete mixtures. For poly-dispersed suspensions, the shear-induced particle migration (SIPM) has a stronger impact than the wall for forming the lubricating layer. Finally, it was concluded that a solid-liquid threshold of 1 to 2 mm is a reliable choice when investigating concrete pipe flow. The faster the concrete is pumped or the lower the yield stress of the constitutive mortar is, the smaller is the solid-liquid threshold.
Based on the experimental findings, a first-order physical correlation between pumping parameters (pressure and discharge rate), particle properties (volume fraction and packing properties) and lubricating layer properties (thickness and viscosity) was proposed. Accordingly, for a simple flow topology with a shearing lubricating layer and an extruding yield stress concrete, the lubricating layer thickness was computed from the competition between shear-induced fluxes of particles. In agreement with the driving equations of SIPM, it was assumed that the first flux correlates with the internal stress gradient in the plug concrete whereas the counteracting particle flux correlates with the shear rate in the lubricating layer. The model was validated for two flow types, pipe flow in the Sliper and Couette flow in a co-axial tribometer. The proposed model can capture the observed main features and their evolutions despite the absence of any fitting parameters.
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Assessing and Modelling the Structural Build-up of Concrete in the Context of Digital FabricationIvanova, Irina 24 May 2023 (has links)
Nowadays, construction industry is rapidly moving towards digitalization and automation that should enable increased rates and efficiency of construction processes, as well as higher possibilities for customization and architectural freedom. Among all technologies under development, digital fabrication with concrete by means of layered extrusion appears to be one of the most promising for purposes of fast mass housing construction. It enables formwork-free production of structures via layer-by-layer concrete printing. Freedom from formwork potentially makes the construction process more cost- and time-saving, but poses multiple challenges to mix design and test methods, especially in terms of concrete rheology. A special focus must be put on the structural build-up of concrete at rest, which is related to its buildability, i.e. capacity of the material to retain the shape of the extruded layers under their own weight and the weight of the subsequently placed layers.
This research investigates into the structural build-up of cementitious materials, i.e. evolution of their strength and deformation properties over time at rest, and includes development and refinement of methodology to assess the structural build-up, as well as its modelling and prediction.
With respect to methodology, major attention was directed to the constant rotational velocity (CRV) test used for evaluation of the static yield stress development, and rationalization of its application under field conditions. Based on a large amount of experiments performed with two rheometers of different design, characteristic curves and points describing patterns in behaviour of cementitious materials during a CRV test were established. The experimental study also dealt with assessing the effects of alterations in main elements of the CRV test protocol, such as test approach (single- versus multi-batch), pre-shear regime, applied CRV, on the test results. Possible errors in CRV tests were addressed and methods to improve the procedures of testing and data evaluation were suggested. In particular, the single-batch approach was enhanced by implementation of the developed breaking criterion, a concept of zero measurement for non-pre-sheared samples was introduced, and a method for simplified evaluation of elasticity by a single-head rheometer was proposed. General methodological recommendations on the design of a CRV test protocol were formulated. The results are applicable for various cementitious materials and not limited to concretes for layered extrusion.
Furthermore, test methods for assessing the structural build-up of printable concretes were studied in terms of their applicability under field conditions, potential for automation, descriptiveness of obtained data and efficiency in predicting the buildability of printed concrete structures. The methods under investigation included CRV test, unconfined uniaxial compression test (UUCT), fast penetration test and newly proposed confined uniaxial compression test (CUCT); all tests were performed on extruded samples of eight printable concrete mixtures with various compositions and rheological properties. The corresponding results were juxtaposed in order to establish correlations between the data and compared to the results of buildability tests, in which hollow cylindrical structures were produced using a laboratory-scale 3D printer.
The developed methodology was further used to conduct an in-depth investigation into the influence of aggregates on the structural build-up of ordinary concrete. In the experimental program, the binder composition was kept constant while the aggregate was varied in terms of the volume fraction and the surface area (per unit volume of concrete); the main focus was put on compositions with elevated aggregate content (45−55 % by volume). A mechanism lying behind the effects of the aggregate properties on the structural build-up of concrete was discovered by studying the structure of constitutive paste in concrete. Aggregate-induced inhomogeneity of constitutive paste allowed to introduce a three-component model of fresh concrete. Furthermore, to find models capable of predicting the parameters of structural build-up of concrete, i.e. static yield stress and structuration rate, concrete was viewed as a suspension of aggregate particles in suspending medium. Three approaches were employed to define the correspondent components. Suspending medium was represented by plain cement paste, screened cement paste and fine mortar considered as a part of concrete comprising particles below 0.5 mm; a substantiation for such a definition was provided. Applicability and limitations of the models based on all three approaches were compared. The modelling approach was further extended to printable concretes with nearly identical aggregate compositions, but different properties of paste. Opportunities and challenges in modelling the structural build-up of printable concrete, including the problem of material dependency of the models and the relevancy of fitting coefficients, were discussed. A modified Chateau-Ovarlez-Trung model based on the definition of suspending medium as fine mortar was acknowledged as best suited to describe the structural build-up of both ordinary and printable concrete.
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Modellierung des Materialverhaltens Magnetorheologischer Fluide unter Verwendung der Fourier-Transformations RheologieBoisly, Martin 30 November 2018 (has links)
In dieser Dissertation wird das viskoplastische Schubverhalten eines magnetorheologischen Fluids (MRF) modelliert. Mithilfe eines phänomenologischen Modellierungsansatzes auf Basis nichtlinearer rheologischer Elemente können die gemessenen Fließkurven sowie Speicher- und Verlustmoduli abgebildet werden. Ein MRF ist ein Material mit fest-flüssig Übergang. Es besitzt von einem Magnetfeld abhängige Materialeigenschaften. Um diese beschreiben zu können, wird zunächst eine phänomenologische Stoffklassifizierung eingeführt. Auf deren Grundlage teilen sich Stoffe allgemein in Flüssigkeiten, Festkörper und Materialien mit fest-flüssig Übergang auf. Zur Beschreibung des Materialverhaltens von MRF werden drei viskoplastische Modelle formuliert und gegenübergestellt. Zur Identifikation der Materialparameter wird eine Identifikationsstrategie auf der Grundlage charakteristischer Punkte entwickelt. Charakteristische Punkte sind exklusive Punkte von Materialfunktionen, die analytisch beschrieben und ohne Weiteres experimentell ermittelt werden können. Analytische Ausdrücke für charakteristische Punkte der Speicher- und Verlustmoduli werden über das Analogieprinzip unter Verwendung von Lissajous Diagrammen abgeleitet. Infolgedessen können die Materialparameter durch das Auswerten algebraischer Zusammenhänge identifiziert werden, ohne nichtlineare Optimierungsverfahren anwenden zu müssen. Hierbei stellt die Fließspannung einen signifikanten Materialparameter dar. Deswegen werden die Standardverfahren zur Bestimmung der Fließspannung auf rheologische
Modelle angewendet und bewertet.
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