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Entwicklung von Leuchtstoffen zur Lichtumwandlung und Thermografie in Solarmodulen
Full text linkThe most significant parameter that impacts practically all industrial processes in many ways is temperature. Novel methods of temperature measurement, like phosphor thermometry, are very useful in many situations where conventional techniques have some limitations. Lack of measurement in two-dimensional (2D) space is a significant drawback of conventional techniques. Phosphor thermography has many advantages in 2D measurement in terms of resolution, accuracy, and cost in comparison with competing techniques like infra-red, thermal paints, and liquid crystals. The phosphor thermography method is based on the photoluminescence characteristics of a phosphor material that is affected by temperature in most phosphors. The search for a suitable phosphor material which can survive harsh measurement conditions with suitable optical characteristics is of prime importance. The focus of this work is to investigate suitable phosphor materials and develop a cost-effective lifetime-based technique.
At the start of the thesis, the motivation behind this work is given. In chapter 2, the reader is provided with the knowledge of conventional temperature measurement techniques and commonly employed 2D techniques other than phosphor thermography, along with their pros and cons. Chapter 3 starts with the development history of phosphors, and then the basic theory behind luminescence is discussed. Radiative and non-radiative processes occurring during absorption, emission and energy transfer are also discussed. Energy levels in transition metals and rare earth elements, which will be used often in the luminescence Radiography, are also given. Chapter 4 describes the different methods of phosphor thermography along with their advantages and disadvantages. It also describes the experimental factors that may contribute to the errors in the measurement.
Manganese-doped magnesium germanate phosphor is the most frequently used phosphor for thermography applications. However, there are various compounds possible with different stoichiometries of magnesium and germanium oxides. Fluoride ions may also replace some oxide ions in magnesium germanate and increase the emission many times. In my first paper of the appendix, the synthesis and characterization of different compositions of magnesium germanate and fluoro-germanate phosphors are discussed. A strikingly unique observation was made during the synthesis of magnesium fluoro-germanate phosphor that the moisture content during the synthesis can severely deteriorate the photoluminescence emission. This phenomenon is investigated in the second paper of the appendix. In the third paper attached, a new method is introduced for the determination of lifetime in 2D space using a conventional low-speed scientific grade camera. In contrast to the well-known present methods, the technique relies on capturing multiple images in multiples decay curves with progressively shifted integration time window after excitation. The obtained results indicate a relatively low standard deviation in 2D thermography in a wide temperature range
Eine berührungslose Temperaturbestimmung an Festkörperoberflächen mittels Phosphor
Full text linkThe main application addressed in this thesis is contactless solid surface temperature determination using luminescent characteristics of phosphors. This application motivates us to develop three platforms as calibration setups for temperature regions 253 K to 363 K in vacuum ambient, room temperature up to 573 K and room temperature up to 1123 K in a standard ambient atmosphere. All setups were automatized using LabVIEW. These platforms are used for phosphor screening to address compatible phosphor matching the camera red, green and blue sensitivity, right phosphor for thermography. Three methods of phosphor spectroscopy, dual-channel dual-phase lock-in, and phase-sensitive averaging imaging were applied on setups. Sufficient software using MATLAB GUI was developed to analyses the results.
In the first step of my research, utilizing three platforms as calibration setups, temperature-dependent luminescent emission for around thirty-one industrial and laboratory synthesized phosphors analyzed. I examined the temperature-dependent luminescence at an excitation wavelength of 405 nm. In the second step of my research, dual-channel dual-phase lock-in phosphor thermometry, which offers an option of accurate and contactless temperature determination under high environmental noise, practiced. YAG: Dy3+, YAG: Sm3+ and Ba3MgSi2O8: Eu2+, Mn2+ as thermographic phosphors at an excitation wavelength of 405 nm used. I tested the dual-channel lock-in approach under experimental conditions with a noise level of more than 2000 times higher than the excitation signal. Additionally, the effect of signal to noise ratio, lock-in frequency, and lock-in bandwidth on temperature determination accuracy studied.
In the final step of my research, a surface temperature determination method, based on the luminescence characteristics of Ba3MgSi2O8: Eu2+, Mn2+ studied. Three different imaging methods were developed and are presented together with the corresponding calibration curves. I successfully utilized all three methods to determine surface temperature distribution on a glass plate with a point heat source in the format of an image.Die Hauptanwendung, die in dieser Arbeit behandelt wird, ist die kontaktlose Temperaturbestimmung von Festkörperoberflächen unter Verwendung der Lumineszenzeigenschaften von Leuchtstoffen. Diese Anwendung motiviert mich zur Entwicklung von drei Plattformen als Kalibrieraufbauten für die Temperaturbereiche 253 K bis 363 K in Vakuumumgebung, Raumtemperatur bis 573 K und Raumtemperatur bis 1100 K in einer Standardumgebungsatmosphäre. Alle Aufbauten wurden mit LabVIEW automatisiert. Diese Plattformen werden für das Phosphor-Screening verwendet, um kompatiblen Phosphor anzusprechen, der mit der Empfindlichkeit der Kamera für Rot, Grün und Blau übereinstimmt, den richtigen Phosphor für die Thermografie. Drei Methoden der Phosphorspektroskopie, Zweikanal-Lock-in und phasenempfindliche Mittelwertbildgebung wurden auf die Setups angewendet. Zur Analyse der Ergebnisse wurde eine ausreichende Software mit MATLAB-GUI entwickelt.
Im ersten Schritt meiner Forschung wurden unter Verwendung von drei Plattformen als Kalibrierungs-Setups die temperaturabhängige Lumineszenzemission für etwa einunddreißig industriell und im Labor synthetisierte Phosphore analysiert. Ich untersuchte die temperaturabhängige Lumineszenz bei einer Anregungswellenlänge von 405 nm. Im zweiten Schritt meiner Forschung praktizierte ich die Zweikanal-Lock-in-Phosphorthermometrie, die eine Möglichkeit zur genauen und kontaktlosen Temperaturbestimmung unter hohem Umgebungsrauschen bietet. Dabei wurden YAG: Dy3+, YAG: Sm3+ und Ba3MgSi2O8: Eu2+, Mn2+ als thermographische Leuchtstoffe bei einer Anregungswellenlänge von 405 nm verwendet. Ich testete den Zweikanal-Lock-in-Ansatz unter experimentellen Bedingungen mit einem Rauschpegel, der mehr als 2000 Mal höher war als das Anregungssignal. Zusätzlich wurde die Auswirkung des Signal-Rausch-Verhältnisses, der Lock-in-Frequenz und der Lock-in-Bandbreite auf die Genauigkeit der Temperaturbestimmung untersucht.
Im letzten Schritt meiner Forschung wurde eine Methode zur Bestimmung der Oberflächentemperatur, basierend auf den Lumineszenzeigenschaften von Ba3MgSi2O8: Eu2+, Mn2+ untersucht. Drei verschiedene bildgebende Verfahren wurden entwickelt und werden zusammen mit den entsprechenden Kalibrierkurven vorgestellt. Ich habe alle drei Methoden erfolgreich eingesetzt, um die Oberflächentemperaturverteilung auf einer Glasplatte mit einer punktförmigen Wärmequelle im Bildformat zu bestimmen
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Wasserstofffreisetzung aus LOHC-Systemen: Dehydrierung mit brennerbasierter Direktbeheizung und Charakterisierung mittels Flüssigphasen-Thermometrie
Full text linkWhen using hydrogen as an energy carrier, liquid organic hydrogen carrier (LOHC) systems represent a particularly promising technology for its transport and safe long-term storage. The liquid organic compounds can be reversibly charged and discharged with hydrogen. Chemical bonding of hydrogen to a liquid offers the advantages of storage under ambient conditions in low-cost containers and easy handling while making extensive use of the existing infrastructure for liquid fuels in e.g. automotive applications. LOHC systems could be used to supply hydrogen at refueling stations in order to enable the accelerated development of a hydrogen infrastructure. However, the release of the hydrogen stored in the LOHC is accompanied with a high energy requirement (at least 27 % of the hydrogen stored in the LOHC is needed for this reaction with additional losses, e.g. for heating to reaction temperature). Thus, for an economic application of the LOHC technology, it is necessary to optimize the dehydrogenation reactor with respect to power density and system dynamics in order to achieve an energy efficient operation of the storage technology.
In the framework of the present work this goal is addressed in two different ways. On the one hand, the heat exchanger unit for the hydrogen release reaction will be heated directly with a porous media burner for the first time. At the moment, research has mostly focused on indirect heating of the dehydrogenation reactor. The approach of a direct heat input via a burner promises significant potentials for increasing the efficiency and the dynamics of the reaction compared to existing systems due to the reduced system inertia and the lower heat losses. Both direct heating of a dehydrogenation unit with a porous media burner and the associated optimization of the reactor geometry are in focus. On the other hand, an optical measurement technique will be used for the first time to detect temperature fields in the reaction volume during hydrogen release. So far, no comparable work exists in the literature. In particular, the spatial resolution of temperature fields is of great interest with regard to the strong endothermic reaction in order to localize and characterize existing cool spots and overheating areas and thus to optimize the heat supply in future dehydrogenation reactors. The measurement technique of laser-induced phosphorescence for temperature field measurements during the release of hydrogen from the LOHC material perhydro-dibenzyltoluene (H18-DBT) is therefore applied in a model system.
The work consequently considers optimization potentials in the dehydrogenation of a LOHC system and identifies ways to increase the dynamics and efficiency of the hydrogen release reaction
Optical sensor system for time-resolved quantification of methane concentrations: Validation measurements in a rapid compression machine.
No full textLowering greenhouse gas emissions is one of the most challenging demands of today's society. Especially, the automotive industry struggles with the development of more efficient internal combustion (IC) engines. As an alternative to conventional fuels, methane has the potential for a significant emission reduction. In methane fuelled engines, the process of mixture formation, which determines the properties of combustion after ignition, differs significantly from gasoline and diesel engines and needs to be understood and controlled in order to develop engines with high efficiency. This work demonstrates the development of a gas sensing system that can serve as a diagnostic tool for measuring crank-angle resolved relative air-fuel ratios in methane-fuelled near-production IC engines. By application of non-dispersive infrared absorption spectroscopy at two distinct spectral regions in the ν3 absorption band of methane around 3.3 µm, the system is able to determine fuel density and temperature simultaneously. A modified spark plug probe allows for straightforward application at engine test stations. Here, the application of the detection system in a rapid compression machine is presented, which enables validation and characterization of the system on well-defined gas mixtures under engine-like dynamic conditions. In extension to a recent proof-of-principle study, a refined data analysis procedure is introduced that allows the correction of artefacts originating from mechanical distortions of the sensor probe. In addition, the measured temperatures are compared to data obtained with a commercially available system equipped based on the spectrally resolved detection of water absorption in the near infrared
Imaging Techniques for Multi-Parameter Determination in Mixture Formation and Combustion Processes
Full text linkTracer-based measurement techniques such as laser-induced phosphorescence (LIP) and laser-induced fluorescence (LIF) are powerful means for multi-parameter imaging in the gas phase of thermodynamic systems and especially in reactive flows. The simultaneous measurement of temperature, species concentrations and velocity provides deep insights into the complex processes of mixture formation, combustion and pollutant emissions. This is relevant for many technical applications in power and process engineering, while the major field covered in this thesis are IC engines. The present habilitation thesis gives a review of LIP and LIF techniques for multi-parameter imaging and a summary of the research activities on refinement of these diagnostics conducted under the guidance of the author at LTT Erlangen.
In the area of LIP, the work focusses on high temperature thermometry and velocimetry for which new thermographic phosphor materials are developed. The main part of the work is the optimization of the optical properties of the phosphor YAG:Dy and its derivatives, which are especially designed for application under high temperature conditions. Co-doping and host material modification are conducted to enhance signal intensity and to increase temperature sensitivity.
The accuracy of tracer-LIF techniques is improved by identification of partial tracer decomposition and oxidation during LIF-signal calibration. A new tracer for diesel engine relevant conditions is chosen and calibrated in a wide temperature and pressure range. A novel approach for simultaneous measurement of fuel-air ratio (FAR) and temperature is proposed, and detailed calibration data was presented. Additionally, a novel “2-tracer” approach for simultaneous measurement of mixture temperature, FAR and exhaust gas concentration is developed in the framework of this thesis. For this purpose, the tracer-interactions are studied, which could potentially lead to larger measurement errors.
Finally, measurement examples are presented for fundamental and applied research including thermometry in a heated air jet, temperature and fuel concentration measurements in a diesel spray and a DISI spray as well as multi-parameter analysis of the mixture formation in a modern Direct-Injection Spark-Ignition (DISI) engine with exhaust gas recirculation (EGR). The tracer-LIF technique is finally applied to study the influence of ethanol admixture to gasoline on the evaporation and mixture formation characteristics, which delivered deep insights into soot formation in a DISI IC engine
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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