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The Global ETD Search service is a free service for researchers
to find electronic theses and dissertations. This service is
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Networked Digital Library of Theses and Dissertations.
Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
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Showing 11 to 19 of 19 (0.074 seconds)| 11 |
Einfluss von verschiedenen Erodierstrukturen auf den Reibwert von Kunststoff-Kunststoff PaarungenBergmann, André 21 December 2016 (has links)
Im Beitrag werden bei verschiedenen Lastkollektiven und zwei Reibgeschwindigkeiten die Erodierstrukturen K27, K36, K42 und die Ätzstruktur K36 mit glatten Probekörpern hinsichtlich des Reibwertes und des Verschleißverhaltens untersucht. Als strukturierte Probekörper kommen die Kunststoffe POM und PA und als Gegenkörper zwei verschiedene PE-UHMW´s zum Einsatz.
Die Ergebnisse zeigen eine deutliche Abhängigkeit des Reibwertes von der Oberflächenstruktur der Probekörper, jedoch ist die Senkung des Reibwertes stark von den Belastungsparametern Normalkraft und Reibgeschwindigkeit abhängig.
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Task-specific uncertainty of areal surface texture measurement using structured illumination microscopyLi, Zhen 31 May 2023 (has links)
Surface quality plays a vital role in controlling the function performance of the workpiece. With the development of the measuring technique, areal surface measurement has been widely applied in the industry. However, estimating the uncertainty of areal surface measurement is still a challenge. Except for the metrological characteristics of the measurement system, measurement conditions should be considered for uncertainty evaluation.
The dissertation investigates the influence of measurement settings on surface measurement. A silver-plated surface, three different rough grinding surfaces, and three different rough cylindrical grinding surfaces were measured using structured illumination microscopy. The measurements were at the different objective lenses, vertical scanning interval, exposure time, and sample tilt. The results show that the measurement settings influence the non-measured points, measurement noise, and areal surface texture parameters. Therefore, according to the investigation, the sample tilt and exposure time should also be included in the uncertainty budget.
An approach was proposed to investigate the influence of non-measured points on the areal surface texture parameters. The relation between the non-measured points ratio and measurement settings was investigated, and how the areal surface texture parameters changed due to the non-measured points was studied. Moreover, an approach based on the metrological characteristic method was proposed to estimate the uncertainty due to the measurement noise. This method can be extended to the uncertainty evaluation due to other metrological characteristics. Additionally, an approach based on the Monte Carlo Method was proposed to estimate the measurement uncertainty due to different influences. This approach was verified as feasible in the practical measurement.
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Register: 25 GraukartenBamberger, Jasper 17 November 2023 (has links)
Beton wird global als Werkstoff eingesetzt, weist als Natur-verwendendetes Material allerdings unterschiedliche regionale Spezifikationen auf. Hier entsteht ein Spannungsfeld zwischen Universalismus und Partikularismus. Die beim Absäuern offengelegten Sandkörner werden zum ortsbezogenen Naturpigment, die diese Ambivalenz sinnlich erfahrbar machen. Die Werkgruppe präsentiert in ihrer Uniformität ein Bild der Gemeinsam- und Verbundenheit. Gleichzeitig erheben die einzelnen Platten Anspruch auf spezifische, in ihrer Körnung auf partikularer Ebene abgebildete Eigenschaften der Diversität und Einzigartigkeit.
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Structural, electronic and optical properties of cadmium sulfide nanoparticles / Strukturelle, elektronische und optische Eigenschaften von Cadmiumsulfid NanoteilchenFrenzel, Johannes 08 March 2007 (has links) (PDF)
In this work, the structural, electronic, and optical properties of CdS nanoparticles with sizes up to 4nm have been calculated using density-functional theory (DFT). Inaccuracies in the description of the unoccupied states of the applied density-functional based tight-binding method (DFTB) are overcome by a new SCF-DFTB method. Density-functional-based calculations employing linear-response theory have been performed on cadmium sulfide nanoparticles considering different stoichiometries, underlying crystal structures (zincblende, wurtzite, rocksalt), particle shapes (spherical, cuboctahedral, tetrahedral), and saturations (unsaturated, partly saturated, completely saturated). For saturated particles, the calculated onset excitations are strong excitonic. The quantum-confinement effect in the lowest excitation is visible as the excitation energy decreases towards the bulk band gap with increasing particle size. Dangling bonds at unsaturated surface atoms introduce trapped surface states which lie below the lowest excitations of the completely saturated particles. The molecular orbitals (MOs), that are participating in the excitonic excitations, show the shape of the angular momenta of a hydrogen atom (s, p). Zincblende- and wurtzite-derived particles show very similar spectra, whereas the spectra of rocksalt-derived particles are rather featureless. Particle shapes that confine the orbital wavefunctions strongly (tetrahedron) give rise to less pronounced spectra with lower oscillator strengths. Finally, a very good agreement of the calculated data to experimentally available spectra and excitation energies is found.
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Brown coal char CO2-gasification kinetics with respect to the char structureKomarova, Evgeniia 11 September 2017 (has links) (PDF)
This research has been performed in the framework of the Virtuhcon project, which intends to virtualize high temperature conversion processes. Coal gasification is one of these processes, which is nowadays considered as a promising technology for the chemical industry. This study is devoted to the coal char physical structure, which is one of the most important parameters influencing coal gasification reaction.
First, this study presents the extensive literature review of the char physical structure role during its conversion. Collection of the char structural properties as well as their changes during char conversion are shown and discussed.
Literature review is followed by the experimental investigations. Chars prepared from two brown coals (Lusatian and Rhenish) were gasified in a laboratory scale fluidized bed reactor in CO2 at temperatures of 800, 850, 900, and 950 °C and atmospheric pressure. Char samples were gasified completely as well as partially in order to evaluate the reaction kinetics and char structural changes during the reaction, respectively. Complete gasification curves were evaluated by different methods, including application of three gasification models (the Random Pore Model, the Volume Reaction Model, and the Shrinking Reaction Model), instantaneous reaction rate approach as well as the self-developed surface-related reaction rate approach. The results of different approaches were compared.
This study also presents a comprehensive methodology to analyze coal char physical structure. The variety of measurement techniques (gas physical adsorption, mercury porosimetry, helium pycnometry, SEM, etc.) were applied to assess structural properties of the char, such as specific surface area, particle density, porosity, pore size and shape, structure morphology, etc. Problems associated with the choice of a proper measurement technique and the comparability of the data delivered by different techniques were discussed.
The main objective of the study was to link char structural changes to the char gasification kinetics. The specific task of this thesis was to investigate pore size in relation to their availability for the reaction. As such, specific surface areas of pores of different sizes (from sub-micro to mesopores) were correlated to the instantaneous reaction rates.
Both chars exhibit similar trends in their structural changes during gasification, although the absolute values differ, especially with respect to the pores of microscale. Furthermore, structural changes were caused not only by the reaction but also by the influence of the heat treatment, especially at the earlier stages of the reaction. The most reasonable correlation has been achieved between the instantaneous reaction rate and the specific surface area of mesopores. Sub-micro- and micropores did not govern the gasification reaction under given conditions.
Finally, kinetic parameters derived from different evaluation methods were reapplied in order to test their ability to predict the experimental data. Each of the method has its advantages and disadvantages as used for the kinetic evaluation.
The results of this study represent a substantive base of the experimentally derived data concerning physical structure and morphology of coal char. The findings can be used in numerical and simulation studies for development, validation, and improvement of the models which consider coal particle as a reactive porous solid.
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Brown coal char CO2-gasification kinetics with respect to the char structureKomarova, Evgeniia 14 August 2017 (has links)
This research has been performed in the framework of the Virtuhcon project, which intends to virtualize high temperature conversion processes. Coal gasification is one of these processes, which is nowadays considered as a promising technology for the chemical industry. This study is devoted to the coal char physical structure, which is one of the most important parameters influencing coal gasification reaction.
First, this study presents the extensive literature review of the char physical structure role during its conversion. Collection of the char structural properties as well as their changes during char conversion are shown and discussed.
Literature review is followed by the experimental investigations. Chars prepared from two brown coals (Lusatian and Rhenish) were gasified in a laboratory scale fluidized bed reactor in CO2 at temperatures of 800, 850, 900, and 950 °C and atmospheric pressure. Char samples were gasified completely as well as partially in order to evaluate the reaction kinetics and char structural changes during the reaction, respectively. Complete gasification curves were evaluated by different methods, including application of three gasification models (the Random Pore Model, the Volume Reaction Model, and the Shrinking Reaction Model), instantaneous reaction rate approach as well as the self-developed surface-related reaction rate approach. The results of different approaches were compared.
This study also presents a comprehensive methodology to analyze coal char physical structure. The variety of measurement techniques (gas physical adsorption, mercury porosimetry, helium pycnometry, SEM, etc.) were applied to assess structural properties of the char, such as specific surface area, particle density, porosity, pore size and shape, structure morphology, etc. Problems associated with the choice of a proper measurement technique and the comparability of the data delivered by different techniques were discussed.
The main objective of the study was to link char structural changes to the char gasification kinetics. The specific task of this thesis was to investigate pore size in relation to their availability for the reaction. As such, specific surface areas of pores of different sizes (from sub-micro to mesopores) were correlated to the instantaneous reaction rates.
Both chars exhibit similar trends in their structural changes during gasification, although the absolute values differ, especially with respect to the pores of microscale. Furthermore, structural changes were caused not only by the reaction but also by the influence of the heat treatment, especially at the earlier stages of the reaction. The most reasonable correlation has been achieved between the instantaneous reaction rate and the specific surface area of mesopores. Sub-micro- and micropores did not govern the gasification reaction under given conditions.
Finally, kinetic parameters derived from different evaluation methods were reapplied in order to test their ability to predict the experimental data. Each of the method has its advantages and disadvantages as used for the kinetic evaluation.
The results of this study represent a substantive base of the experimentally derived data concerning physical structure and morphology of coal char. The findings can be used in numerical and simulation studies for development, validation, and improvement of the models which consider coal particle as a reactive porous solid.
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Structural, electronic and optical properties of cadmium sulfide nanoparticlesFrenzel, Johannes 19 December 2006 (has links)
In this work, the structural, electronic, and optical properties of CdS nanoparticles with sizes up to 4nm have been calculated using density-functional theory (DFT). Inaccuracies in the description of the unoccupied states of the applied density-functional based tight-binding method (DFTB) are overcome by a new SCF-DFTB method. Density-functional-based calculations employing linear-response theory have been performed on cadmium sulfide nanoparticles considering different stoichiometries, underlying crystal structures (zincblende, wurtzite, rocksalt), particle shapes (spherical, cuboctahedral, tetrahedral), and saturations (unsaturated, partly saturated, completely saturated). For saturated particles, the calculated onset excitations are strong excitonic. The quantum-confinement effect in the lowest excitation is visible as the excitation energy decreases towards the bulk band gap with increasing particle size. Dangling bonds at unsaturated surface atoms introduce trapped surface states which lie below the lowest excitations of the completely saturated particles. The molecular orbitals (MOs), that are participating in the excitonic excitations, show the shape of the angular momenta of a hydrogen atom (s, p). Zincblende- and wurtzite-derived particles show very similar spectra, whereas the spectra of rocksalt-derived particles are rather featureless. Particle shapes that confine the orbital wavefunctions strongly (tetrahedron) give rise to less pronounced spectra with lower oscillator strengths. Finally, a very good agreement of the calculated data to experimentally available spectra and excitation energies is found.
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Stoffübertragung beim SpritzgießenHärtig, Thomas 22 March 2013 (has links) (PDF)
Das Fügen mehrerer Komponenten während des Spritzgießprozesses wird bei vielen Spritzgießsonderverfahren angewandt. Diese Arbeit beschäftigt sich mit der Verbundbildung zwischen einem kalten Einlegeteil und der einströmenden Kunststoffschmelze beim Spritzgießen, im Folgenden Stoffübertragung genannt. Ein Großteil der Untersuchungen findet an Zweikomponenten-Zugstäben statt, wobei erste und zweite Komponente aus dem gleichen Thermoplast gefertigt werden. Mögliche Einflussfaktoren auf die Verbundfestigkeit werden zunächst im Theorieteil vorgestellt und diskutiert. Eine Auswahl relevanter Prozess- und Materialparameter wird dann in praktischen Versuchen detailliert analysiert. Es wird nach korrelierenden Tendenzen sowohl zwischen unterschiedlichen Verfahren als auch zwischen verschiedenen Kunststoffen gesucht. Mittels statistischer Versuchsplanung werden die Spritzgießparameterkombinationen nach Größe des Einflusses auf die Verbundfestigkeit sortiert. Dies trägt zum Verständnis der bei der Stoffübertragung ablaufenden Grundmechanismen bei. Weiterhin werden die Einflüsse der Prozessparameter auf das neue Verfahren der In-Mold Oberflächenmodifizierung, bei dem ein funktionaler Modifikator während des Spritzgießprozesses übertragen wird, mit den Ergebnissen der Zweikomponenten-Verbundfestigkeit verglichen. Abschließend wird auf die Besonderheiten bei der selektiven Stoffübertragung eingegangen und das neue Verfahren des In-Mold Printing vorgestellt. / The joining of two components by the process of injection molding is state of the art, although adhesion phenomena are not fully understood yet. The formation of bonds between a cold material, which was inserted or applied onto the surface of the cavity before injection molding, and an injected polymer melt is studied in this work. Providing sufficient bond strength, the material is transferred from the surface of the mold to the injection molded part. Possibly influencing factors on the bond strength are first identified, theoretically discussed, later in experiments varied and finally analyzed. Thereby correlating tendencies between different polymers and different in-mold technologies are observed. The relevant material and processing parameters are put in order by their influence on the bond strength using design of experiments. This helps to understand the mechanisms of the formation of bonds. The majority of the experiments is concerned with two component injection molding by measuring the bond strength of two component tensile bars, produced under varying processing conditions. In each case, first and second components are made of the same thermoplastic polymer. The thermal energy of the melt can be used also to initiate chemical reactions. This permits bonding of a thin layer of a functional polymer, which is applied onto the surface of the mold before injecting the melt, to the surface of the molded part. In this way, process-integrated surface modification during injection molding becomes possible. In a further attempt, patterns of paint are printed onto the surface of the mold by pad printing. During injection molding the paint is transferred completely to the surface of the polymeric part. Using this new technology of In-Mold Printing, fully finished surface decorated parts can be produced by injection molding.
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Stoffübertragung beim SpritzgießenHärtig, Thomas 28 February 2013 (has links)
Das Fügen mehrerer Komponenten während des Spritzgießprozesses wird bei vielen Spritzgießsonderverfahren angewandt. Diese Arbeit beschäftigt sich mit der Verbundbildung zwischen einem kalten Einlegeteil und der einströmenden Kunststoffschmelze beim Spritzgießen, im Folgenden Stoffübertragung genannt. Ein Großteil der Untersuchungen findet an Zweikomponenten-Zugstäben statt, wobei erste und zweite Komponente aus dem gleichen Thermoplast gefertigt werden. Mögliche Einflussfaktoren auf die Verbundfestigkeit werden zunächst im Theorieteil vorgestellt und diskutiert. Eine Auswahl relevanter Prozess- und Materialparameter wird dann in praktischen Versuchen detailliert analysiert. Es wird nach korrelierenden Tendenzen sowohl zwischen unterschiedlichen Verfahren als auch zwischen verschiedenen Kunststoffen gesucht. Mittels statistischer Versuchsplanung werden die Spritzgießparameterkombinationen nach Größe des Einflusses auf die Verbundfestigkeit sortiert. Dies trägt zum Verständnis der bei der Stoffübertragung ablaufenden Grundmechanismen bei. Weiterhin werden die Einflüsse der Prozessparameter auf das neue Verfahren der In-Mold Oberflächenmodifizierung, bei dem ein funktionaler Modifikator während des Spritzgießprozesses übertragen wird, mit den Ergebnissen der Zweikomponenten-Verbundfestigkeit verglichen. Abschließend wird auf die Besonderheiten bei der selektiven Stoffübertragung eingegangen und das neue Verfahren des In-Mold Printing vorgestellt. / The joining of two components by the process of injection molding is state of the art, although adhesion phenomena are not fully understood yet. The formation of bonds between a cold material, which was inserted or applied onto the surface of the cavity before injection molding, and an injected polymer melt is studied in this work. Providing sufficient bond strength, the material is transferred from the surface of the mold to the injection molded part. Possibly influencing factors on the bond strength are first identified, theoretically discussed, later in experiments varied and finally analyzed. Thereby correlating tendencies between different polymers and different in-mold technologies are observed. The relevant material and processing parameters are put in order by their influence on the bond strength using design of experiments. This helps to understand the mechanisms of the formation of bonds. The majority of the experiments is concerned with two component injection molding by measuring the bond strength of two component tensile bars, produced under varying processing conditions. In each case, first and second components are made of the same thermoplastic polymer. The thermal energy of the melt can be used also to initiate chemical reactions. This permits bonding of a thin layer of a functional polymer, which is applied onto the surface of the mold before injecting the melt, to the surface of the molded part. In this way, process-integrated surface modification during injection molding becomes possible. In a further attempt, patterns of paint are printed onto the surface of the mold by pad printing. During injection molding the paint is transferred completely to the surface of the polymeric part. Using this new technology of In-Mold Printing, fully finished surface decorated parts can be produced by injection molding.
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