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JLUpub bietet Mitgliedern und Angehörigen der Universität die Möglichkeit neben wissenschaftlichen Dokumenten auch Forschungsdaten elektronisch zu veröffentlichen und dauerhaft zugänglich zu machen. Alle Veröffentlichungen erhalten einen Digital Object Identifier (DOI) und werden über nationale und internationale Bibliothekskataloge sowie Suchmaschinen nachgewiesen und auffindbar.
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Unraveling the interplay between composition, electronic band structure and electronic transport properties in n-type Mg2X (X: Si, Sn) materials
(2024-03-26) Sankhla, Aryan
Mg2X (X: Si, Sn) based solid solutions are state-of-the-art thermoelectric (TE) materials in the intermediate temperature range (300 K – 900 K). Particularly, the n-type Mg2Si1-xSnx (with x = 0.6 – 0.7) shows excellent TE properties. These materials possess features such as a low mass density, abundant and economic precursors, and less interference to human and environmental health. All these characteristics makes these materials a prominent choice for the fabrication of modules for power generation. Even though these materials show a high TE performance, there is an obvious need for further research to understand the composition-synthesis-microstructure-property relationships for these materials. The motivation comes from the fact that the employed synthesis routes usually take days-to-weeks to prepare one batch of these materials, poor reproducibility of TE properties, and a large difference (beyond measurement uncertainty) in the TE properties of samples with nominally the same composition, as per the literature reports. The discrepancy could arise from the employed method, where the systematics are poorly understood i.e. more than one synthesis conditions (duration, number of repetitions of high temperature heating or milling etc.) could be responsible for the cited deviation in the TE properties. These aspects need to be systematically addressed. Therefore, for the development of efficient Mg2Si1-xSnx modules, a synthesis route with optimized parameters should be established which can be achieved by utilizing a technique to prepare Mg2Si1-xSnx materials with excellent reproducibility in relatively less time, identifying the synthesis parameters which influence the TE properties, and analyzing the TE properties to identify the microscopic material parameters affected due to a variation in synthesis parameters. Besides, for a long-term functioning and sustenance of maximum efficiency of Mg2Si1-xSnx based modules, the high temperature stability of Mg2Si1-xSnx materials is a significant challenge to overcome. Previous studies reveal that the high temperature (800 K – 875 K) stability of n-type Mg2Si1-xSnx materials is compromised by Mg loss. However, these studies lack proper methodology as the high-temperature annealing causes material degradation beyond the actual composition, which results in the formation of secondary phases. As a result, a comparison of TE properties measured before and after annealing hold no significance as the properties measured after annealing are representative of mixed-phase material. These studies also lack a proper detailing of the degradation mechanism and no conclusions about the mechanism can be drawn from the reported microstructure. These investigations highlight the fact that the practically relevant topic of thermal stability of Mg2Si1-xSnx materials needs to be addressed well. In this regard, the measurement of TE properties should be conducted by simulating the operation conditions of TEGs made from these materials. A measurement of TE properties of these materials at high temperatures of annealing would show a direct effect of heat-treatment on the TE properties. Moreover, a modelling-based analysis of measured properties will provide the extent of changes in the microscopic material parameters. Such an investigation would enable a systematic understanding of the different stages of the degradation mechanism of Mg2Si1-xSnx materials.
Therefore, the focus of this work is to investigate the composition-synthesis-microstructure-property relationships in n-type Mg2X (X: Si, Sn) solid solutions, and the effects of long-term heat treatment on the thermoelectric transport properties of these materials, using semiconductor physics-based models.
Carrier concentration (n) is one fundamental parameter that needs to be optimized for achieving best TE properties of any material. Therefore, a pre-existing in-house Hall facility is successfully advanced for high temperature measurements (T=300 K – 723 K) of Mg2Si1-xSnx materials. The Hall facility is capable to simultaneously measure the Hall coefficient (R_H) and the electrical conductivity (σ) in a van der Pauw configuration. The uncertainty in the signals corresponding to the σ measurement was addressed, partially resolved and quantified. The effect of non-ideal contact geometry on the R_H measurement was investigated using finite element simulations. For certain relevant cases, R_H values deduced from simulation and analytical expression showed a relative deviation (R.D.) >5 %, and so, the results of the analytical expression could be furthermore corrected using simulation results. Room temperature and T-dependent measurement of R_H and σ on Mg2(Si,Sn) and p-type FeSi2, respectively, in the Hall facility at DLR and other international research labs showed good agreement in their absolute measured values with R.D.≤8 %.
For a better understanding of composition-synthesis-microstructure-property relationship, Sb-doped Mg2Si0.4Sn0.6 was synthesized using mechanical alloying. Employing an in-depth microstructure analysis, the formation mechanism of this material was studied in detail using mechanical alloying. The Mg-Sn phase forms readily followed by slow Si diffusion in the ductile matrix. However, the investigations showed that the powder sample was not phase pure even after extensive milling and therefore a high-temperature compaction step was added, enabling a successful synthesis of the desired composition. The effect of synthesis parameters (milling time, sintering time and temperature) was studied systematically. TE properties were hardly influenced by milling duration. However, carrier concentration reduced with an increase of sintering time and temperature which led to an observable effect on TE properties. Maximum TE performance, characterized by a TE figure-of-merit zT~1.4, was achieved for samples sintered at 973 K for 20 min.
A TEM based microstructural analysis was performed on the samples sintered for different durations (10 min, 20 min and 40 min) at 973 K. This analysis was done with an objective to identify the reason of the sensitivity of TE properties to the sintering duration. An increase in the sintering duration resulted in Mg-depleted grain boundaries and local compositional inhomogeneities in the material. The transport properties of these samples were analyzed upto 700 K by using a single parabolic band (SPB) model under the assumptions of acoustic phonon (AP) and alloy scattering (AS) mechanisms. Analysis revealed carrier loss, a lowering in carrier mobility (μ) and a reduction in lattice thermal conductivity (κ_lat) with increasing sintering duration. A lower mobility for the sample (sintered for prolonged duration 40 min) was due to a combined effect of increasing electron-phonon interaction (higher deformation potential constant E_Def) and local compositional inhomogeneities, which are both associated with Mg loss. The altered temperature dependence of electrical conductivity could be reproduced by considering grain boundary (GB) scattering together with AP and AS, thus, linking the observed changes of the microstructure with the TE transport. The compensation between both a lower κ_lat and μ of the sample sintered for 40 min led to a similar TE performance as short-term sintered samples, with 〖zT〗_max=1.3±0.18 for all samples. The absence of any strong influence of sintering duration on 〖zT〗_max indicates that Mg2Si1-xSnx materials, and hence the TE modules made from them, can accommodate some Mg loss without their performance being degraded.
Theoretical investigations have suggested scandium (Sc) as an impurity element for Mg2X materials that introduces resonant levels in the band structure, which could greatly enhance the Seebeck coefficient (S) and the overall TE performance. Correspondingly, the effect of the substitution of Mg by Sc in Mg2X binaries and their solid solutions was experimentally investigated. The TE properties showed no enhancement due to an addition of Sc to Mg2X material, thus, disproving the results of the theoretical study.
The effect of long-term annealing on the transport properties of Mg2Si1-xSnx was further investigated at different temperatures. Chosen temperatures were near and beyond the operation temperature range of Mg2Si1-xSnx based TE modules. It is clear from the last investigation that the Mg loss is inevitable. The missing part is the extent and the speed of Mg loss at higher temperatures. It is furthermore unclear whether the microscopic mechanism of degradation is temperature dependent or not. Considering this, the electrical transport properties of six thermoelectrically identical samples were measured in-situ in two different setups: S and σ (three samples), and R_H and σ (other three as-prepared samples) during annealing at 710 K, 773 K and 848 K. All samples were largely single phase with no hints of de-mixing after annealing for several hundred hours. In-situ measured properties were analyzed using a SPB and a two parabolic band (2PB, one conduction and one valence band) model. The analysis revealed a loss of majority carriers due to ongoing Mg loss as annealing progressed. Besides, a lowering of μ, due to an increase in E_Def, and a reduction in the density-of-states effective mass (m_D^*) is also observed. This change in said microscopic material parameters could be directly related to an ongoing Mg loss or a lifted degeneracy of conduction bands in Mg2Si0.4Sn0.6, respectively.
A microscopic, multi-step model of Mg loss was developed which qualitatively explains the Mg-transport from sample to the annealing atmosphere. Time constants (and rates) were estimated from the change of majority charge carriers and the electrical conductivity measured in situ at different temperatures. Rates were plugged in the Arrhenius expression to determine the activation energy of the complete degradation process. The activation energy was compared with the height of energy barriers associated with Mg-transport to judge the limiting step in the transport chain of multi-step model. Mg-transport was found to be dominated by Mg-vacancies owing to their low energy of formation and migration energy. With the established mechanism, the observed changes in the transport properties and the differences in rate and the frequency factor were found to be dependent on the design of measuring setups. This governs the degradation process by allowing one or more steps to act as a dominant mechanism of multi-step transport chain.
The findings in this thesis provide a description of previously unknown synthesis mechanism of high performance n-type Mg2(Si,Sn) solid solutions using mechanical alloying. The effect of synthesis parameters on material properties is analyzed which explains discrepancies in the material properties reported in different literature reports. An effect of high-temperature degradation of n-type Mg2(Si,Sn) is understood by analyzing in situ measured transport properties which revealed the affected microscopic material parameters and the extent of their deterioration. Finally, the kinetics of Mg-loss in n-type Mg2(Si,Sn) solid solutions is known by utilizing the change in the majority charge carrier concentration and a model of Mg-loss in n-type Mg2(Si,Sn) is developed.
Eine retrospektive Analyse postoperativer Kurz- vs. Langzeit Hörergebnisse nach Tympanoplastik Typ IIIa/b (PORP vs. TORP)
(2023) Steinbrika, Kristina
Die monozentrische, retrospektive Studie analysiert den Effekt der einzeitigen Cholesteatomsanierung und prothesengestützten Gehörknöchelchenrekonstruktion auf das Hörvermögen. Dabei wurden die Tympanoplastiken Typ IIIa (PORP) und IIIb (TORP) zu definierten Zeitpunkten anhand der Air-Bone Gap (ABG) miteinander verglichen.
In die Untersuchung wurden Patienten mit der Erstdiagnose eines Cholesteatoms aus der Klinik für Hals-Nasen-Ohrenheilkunde der Justus-Liebig-Universität Gießen eingeschlossen. Bei diesen wurde zwischen Januar 2012 und Dezember 2017 die Erstdiagnose eines Cholesteatoms gestellt und die operative Sanierung und Rekonstruktion im Sinne einer hörverbessernden Tympanoplastik Typ III durchgeführt. Dabei wurde die Air-Bone Gap (ABG) zu den Zeitpunkten präoperativ (T1), postoperativ nach drei Wochen (T2), 1-Jahreskontrolle (T3) und post second-look (T4) für die Frequenzen 0,5 kHz, 1 kHz, 2 kHz, 3 kHz und 4 kHz anhand der Reintonaudiometrien ermittelt. Zudem wurde der Einfluss der Operationstechnik (CWU und CWD) analysiert.
Insgesamt wurden 69 Patienten (M: n = 46, W: n = 23; mittleres Alter: 46,7 ± 15,8 Jahren) nach Prüfung der definierten Ein- und Ausschlusskriterien in die Analyse einbezogen. Davon wurden in 54 Fällen Tympanoplastiken Typ IIIa (PORP) und in 15 Fällen eine Typ IIIb (TORP) durchgeführt. Die Analyse der ABG ergab im Gruppenvergleich, dass diese in der PORP- Gruppe stets und für alle untersuchten Frequenzen kleiner als in der TORP-Gruppe war. Ein signifikanter ABG-Unterschied konnte nur für die Zeitpunkte prä- (T1) und postoperativ (T2) für beide Gruppen gefunden werden. Dieser signifikante Unterschied und Effekt bestand nicht mehr zur 1-Jahreskontrolle (T3). Bezüglich der Operationstechniken Canal-wall-up bzw. down konnte eine geringe ABG nur in der TORP-Gruppe für CWU erfasst werden. Kein relevanter Unterschied ergab sich in der PORP-Gruppe.
Die Untersuchung aus der universitären HNO-Klinik zeigt, dass eine einzeitige Cholesteatomsanierung und Hörverbesserung im Sinne einer Tympanoplastik IIIa/b möglich ist. Dabei ergibt sich im prä- und postoperativen Gruppenvergleich auch eine signifikante Verbesserung des Hörvermögens um rund 11 dB in der TORP und 4 dB in der PORP-Gruppe. Aus diesen Mittelwerten lässt sich eine objektive Verbesserung des Hörvermögens zwar ableiten, es kann aber keine Aussage über das Ausmaß der subjektiv empfunden Hörverbesserung getroffen werden. Zusammenfassend scheint die Operationsmethode in der PORP-Gruppe ohne relevanten Einfluss auf die Hörergebnisse zu sein. Im Gegensatz dazu kommt die Untersuchung zu dem Ergebnis, dass die gefundene geringere ABG in der TORP-Gruppe bei Anwendung der CWD-Technik mit einer Verbesserung des Hörvermögens verbunden ist. Diese Beobachtung sollte in nachfolgenden Studien weiter untersucht werden, um eine bestmögliche Rehabilitation bzw. den Erhalt des Hörvermögens der Patienten zu erreichen.
Register für Neuromuskuläre Erkrankungen und COVID-19
(2024-04-03) Worm, André
Die Promotion umfasst ein Register, welches die Daten neuromuskulär erkrankter Patient:innen mit durchgemachter SARS-CoV-2 Infektion integriert und die gegenseitigen Auswirkungen der COVID-19 Infektion und der neuromuskulären Erkrankung untersuchen soll.
Development of a chronology for open water reservoirs and subterranean cisterns in the Negev Highlands (Israel) using Optically Stimulated Luminescence : Construction - Utilization - Abandonment
(2024) Junge, Andrea
Ancient open water reservoirs and subterranean cisterns are frequently occurring archaeological installations in drylands like the Negev Highlands, Israel. The former type are round open water reservoirs, which were dug into impermeable layers in foot slope areas and wadis. They may reach 10 m to 12 m in diameter and 4 m to 5 m in depth and were lined internally with unhewn stones and clay or marl that served as an aquiclude. In comparison, subterranean cisterns are rock-cut, excavated into soft limestone or chalk formations with a capacity of up to several hundred cubic meters. These two types of installations to collect and store water can sustain settlements, agriculture, and nomadic pastoralism under dry conditions. Despite the large number of cisterns and reservoirs in the Negev Highlands, they have scarcely been studied systematically.
Crucial for investigating water harvesting systems and their sediments is the establishment of robust chronologies of their life cycles. When the cisterns and water reservoirs are constructed, material is removed and deposited on an adjacent spoil pile. During precipitation events surface runoff from adjoining slopes erodes and transports sediments, which are then deposited in the reservoirs and cisterns. As long as they are maintained, the material is removed and also deposited on the spoil pile. After the cessation of maintenance, the installations were filled up with sediments.
The transport and deposition of sediments enables the application of optically stimulated luminescence (OSL) at the preferred dating method to determine the time of construction, utilisation and abandonment of reservoirs and cisterns. The interpretation of OSL ages is supported by micromorphological analyses and provides insights into the life cycles of the different installations, enabling a discussion about their chronology, the circumstance of construction and their purpose.
The age determinations of ten studied installations allow the conclusion that the open water reservoirs were introduced into the Negev Highlands latest during the Intermediate Bronze Age with a continuing construction until Medieval times, while the construction of subterranean cisterns started during the Roman period, without interrupting the construction of further reservoirs. From this, it can be derived that the construction of different types of water collection and storage systems occurred in different contexts. While the construction of reservoirs could be carried out by small groups in a relatively short time, the building of cisterns required more technological knowledge, more individuals, and longer periods, facilitated by central authorities.
Crucial for investigating water harvesting systems and their sediments is the establishment of robust chronologies of their life cycles. When the cisterns and water reservoirs are constructed, material is removed and deposited on an adjacent spoil pile. During precipitation events surface runoff from adjoining slopes erodes and transports sediments, which are then deposited in the reservoirs and cisterns. As long as they are maintained, the material is removed and also deposited on the spoil pile. After the cessation of maintenance, the installations were filled up with sediments.
The transport and deposition of sediments enables the application of optically stimulated luminescence (OSL) at the preferred dating method to determine the time of construction, utilisation and abandonment of reservoirs and cisterns. The interpretation of OSL ages is supported by micromorphological analyses and provides insights into the life cycles of the different installations, enabling a discussion about their chronology, the circumstance of construction and their purpose.
The age determinations of ten studied installations allow the conclusion that the open water reservoirs were introduced into the Negev Highlands latest during the Intermediate Bronze Age with a continuing construction until Medieval times, while the construction of subterranean cisterns started during the Roman period, without interrupting the construction of further reservoirs. From this, it can be derived that the construction of different types of water collection and storage systems occurred in different contexts. While the construction of reservoirs could be carried out by small groups in a relatively short time, the building of cisterns required more technological knowledge, more individuals, and longer periods, facilitated by central authorities.
Entwicklung einer therapeutischen Strategie basierend auf CRISPR/Cas Genome Editing im Schweinemodell für Morbus Stargardt
(2023) Bucher, Michael
STGD1 ist eine der häufigsten genetisch bedingten Ursachen für Sehbehinderung. Verursacht durch autosomal rezessiv vererbte Mutationen im ABCA4 Gen kommt es zur Akkumulation von toxischen Produkten im retinalen Pigmentepithel und damit letztlich zu einer zunehmenden Einschränkung der Sehkraft beider Augen. Bisher gibt es keine klinisch zugelassene kausale Therapie. Eine der für STGD1 verantwortlichen humanpathogenen Mutationen liegt im Exon 43 des ABCA4 Gens. Mithilfe von Genome Editing ist es theoretisch möglich, diese ursächlich zu therapieren. Dabei wird zunächst mit einer Endonuklease, in dieser Arbeit CRISPR/Cas9, ein Doppelstrangbruch in der Nähe der Mutation erzeugt. Die Cas9 Endonuklease braucht dazu eine effektive guideRNA, welche sie spezifisch zu der gewünschten Stelle (sog. Targetstelle) im Genom führt. Der erzeugte Doppelstrangbruch soll dann von den zelleigenen Reparaturmechanismen unter Nutzung eines DNA-Templates, welches das entsprechende wildtypische DNA-Fragment des Gens enthält, so repariert werden, dass am Ende die wildtypische Sequenz des Gens im Genom vorliegt und das funktionstüchtige Genprodukt von der Zelle hergestellt werden kann. In dieser Arbeit wurde am Schweinemodell (Sus Scrofa) gearbeitet, d.h. es wurden die porzinen Sequenzen des ABCA4 Gens verwendet. Es konnten zunächst 10 verschiedene Plasmide, sog. gRNA/Cas9-Plasmide, welche die codierenden Sequenzen sowohl für gRNAs als auch für die Cas9-Enzyme enthalten, generiert werden. Die zugehörigen Targetsequenzen der gRNAs lagen dabei in den intronischen Bereichen um das Exon 43 des porzinen ABCA4 Gens. Anschließend wurden die gRNA/Cas9-Plasmide mit einem sog. BRET-Assay auf ihre Schneideaktivität in vitro überprüft. Dabei zeigte sich, dass 6 der 10 Plasmide eine signifikante Schneideaktivität besitzen, mit 4 davon wurde weitergearbeitet. Parallel dazu wurde ein Template entworfen und hergestellt, welches aneinandergereiht die wildtypischen Sequenzen der Exons 42, 43 und 44 des porzinen ABCA4 Gens enthält. Zudem wurde ein zweites Template (MMEJ-Template) entworfen und und in das BRET-Reporter-System integriert werden. Anschließend wurde versucht, mithilfe der gRNAs und dem Cas9 Enzym die DNA-Sequenz des WT-Templates in das MMEJ-Template einzubauen und dies anschließend mit einem sog. MMEJ-Assay zu überprüfen. Es zeigte sich, dass kein Einbau des WT-Templates stattgefunden hatte. Gründe und Lösungsstrategien hierfür wurden diskutiert.