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Item type:Publication, Access status: with files , Thermal stability of magnetron sputtered W/HfO₂ and Ir/HfO₂ thin films used as selective emitter in thermophotovoltaic applicationsHochtemperaturstabilität und Langlebigkeit eines selektiven Emitters sind die beiden entscheidenden Parameter, um hohe Strahlungsleistung und einen höheren Wirkungsgrad beim Einsatz in einem thermophotovoltaischen System (TPV) zu erzielen. Bei hohen Temperaturen sind thermisch-aktivierte Prozesse wie Diffusion, Kornwachstum, Phasentransformation, Erholung, chemische Reaktionen insbesondere Oxidation sowie Sublimation von Metalloxiden wirksam und somit stellt die thermische Stabilität eines Dünnschichtsystems eine besondere Herausforderung dar. In dieser Arbeit wurden die thermische Stabiliät und die Versagensmechanismen einer 1D-Metamaterial-Vielfachschicht als selektiver Emitter untersucht und analysiert. Zwei Dünnschichtsysteme: W/HfO2 und Ir/HfO2 standen aufgrund ihrer besonderen Materialeigenschaften (hoher Schmelzpunkt und niedriger Vakuumdampfdruck) im Fokus. Sie wurden mittels Magnetron sputtering hergestellt und durch Röntgenbeugung (XRD) untersucht. Die in-situ Hochtemperaturuntersuchungen (bis zu 2000 ⁰C) unter Verwendung von Heizbändern und die Abschätzung der Probentemperatur waren herausfordernd, dadurch wurde ein spezielles Wissen für die Hochtemperaturvalidierung unterschiedlicher Materialien aufgebaut. Der erste Teil der Arbeit zeigt die Heizexperimente an selektiven W/HfO2-Emitterschichten in verschiedenen Temperatur- (1000 ⁰C bis 1520 ⁰C) und Vakuumdruckbereichen (2 × 10-2 mbar bis 2 × 10-6 mbar). Eine selektive W/HfO2-Emitterschicht ist bis 6 h bei 1400 ⁰C und 2 × 10-5 mbar stabil, welches bisher einer der weltweit höchsten erreichten Werte ist. Darüber hinaus erfolgte eine detaillierte in-situ Röntgenanalyse am Schichtsystem HfO2/W/HfO2 zwischen 300 ⁰C und 1520 ⁰C im Hinblick auf die Mechanismen, die zu strukturellen und mikrostrukturellen Veränderungen wie Relaxation, Phasenumwandlung, Kornwachstum und schließlich zum Versagen (Degradation) führen. Der Hauptgrund des Versagens in einer selektiven W/HfO2-Emitterschicht ist auf die Oxidation des Wolframs gefolgt durch eine Sublimation des Suchtigen Wolframoxids zurückzuführen. Für diesen Prozess wurde eine Aktivierungsenergie von 1.2 eV abgeschätzt. Im Falle des HfO2 wurde eine polymorphe Phasentransformation beobachtet. Die strukturelle Umwandlung der Phasen verursacht einen Anstieg des Volumens der Einheitszelle, der wiederum zur Bildung von Leerstellen, Poren sowie der Entstehung von Transportkan alen entlang der Korngrenzen in der keramischen Schicht führt. Im zweiten Teil wird ein systematischer Ansatz verfolgt, um die thermische Stabilität des Metalls Iridium gegen Oxidation und die Lebensdauer der selektiven Ir/HfO2-Emitterschicht zu testen. Zunächst wurden in-situ XRD-Untersuchungen am Schichtsystem: HfO2/Ir/HfO2 zwischen 800 ⁰C - 1100 ⁰C bei 2 × 10-6 mbar durchgeführt. Iridium zeigte eine sehr gute thermische Stabilität bis zu 1000 ⁰C unterstützt durch HfO2 als hervorragende Deckschicht. Bei Temperaturen über 1000 ⁰C begann die Iridium-schicht zu agglomerieren, was bedeutet, dass die treibende Kraft eine Minimierung der Oberflächenenergie ist. Die thermische Stabilität wurde auch durch exsitu XRD-Untersuchungen bei unterschiedlichen Drücken von 1 bar bis zu 2 × 10-5 mbar im Schichtsystem HfO2/Ir/HfO2 verifiziert, welche bis zu 100 h bei 1000 ⁰C und 2 × 10-2 mbar stabil ist. Satellitenreflexe wurden nahe am primären (111) Ir-Peak beobachtet, die als wichtiger Hinweis für die thermische Stabilität dienen können. Ein finaler Vergleich erfolgte zwischen verschiedenen Metamaterialschichten bestehend aus W/HfO2 und Ir/HfO2 bei 1000 ⁰C und 2 × 10-2 mbar. Über 100 h weist Iridium eine exzellente thermische Stabilität im Vergleich zu Wolfram auf. Die in dieser Arbeit durchgeführten Hochtemperaturexperimente und - analysen bieten eine neue Perspektive auf die thermische Stabilität, die in Dünnschichtsystemen praktisch erreicht werden kann und ermöglichen die Entwicklung selektiver Emitter für TPV-Systeme der nächsten Generation.Publicationtype: Doctoral Thesis115 184 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Data-based analysis of nanoporous metals: from microstructural features to structure-property relationships(2023); In this thesis, a comprehensive workflow for an FE beam modeling approach was developed, which allows to predict the macroscopic elastic and plastic mechanical behavior of nanoporous structures. An in-depth characterization of various geometrical and topological descriptors provided detailed insights about the comparability of experimental nanoporoug gold and computer-generated structures. The combination of the macroscopic mechanical predictions by FE simulations and the structural descriptors derived for the over one hundred computer-generated structures enabled to propose an improved scaling law.Publicationtype: Doctoral Thesis178 187 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Atomistic simulation of dealloying-made microstructures(2023); This thesis explores the evolution of these microstructural features: residual silver, characteristic length scale, and topological connectivity in nanoporous gold (NPG). Kinetic Monte Carlo was used to simulate the dealloying and coarsening processes. Molecular Dynamics was used to simulate the shrinkage process of NPG structures. It is found that the dealloying occurs in two stages. In the primary dealloying, silver clusters are embedded in the ligaments; in the secondary dealloying, the number of silver clusters is reduced. The silver cluster size retains constant during the whole dealloying and scales with the initial ligament size established in the primary dealloying. The finding of coarsening law provides a conclusive confirmation for scattered experimental results. This thesis points out that the solid fraction plays a significant role in the evolution of the nanoporous network’s connectivity during the dealloying, coarsening and shrinkage processes.Publicationtype: Doctoral Thesis159 683 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Smart dispersion of carbon nanoparticle epoxy composites: from nano to application(2021-04-27); Incorporation of carbon nanoparticles in high performance polymer composites promises extensive potential for enhancement of their physical properties. Here the dispersion of the particles is the crucial factor. Only a defined dispersion results in the desired adjustment of properties. On the one hand, the market offers a high variety of industrially available carbon based nanoparticles. These are: Carbon blacks (CB), multi-walled carbon nanotubes (MWCNT), single-walled carbon nanotubes (SWCNT) and graphenes. On the other hand, the demand for nano-enabled products is given. Unfortunately, the bottleneck lies within the incorporation of these particles into the polymer, namely the carbon intermediate production. A commonly used effective technique for the dispersion of carbon-based nanoparticles into high performance epoxy resins (EP), in their liquid state, is the three roll milling (TRM) process. However, it is time consuming and requires a lot of manual handling of the material. In-line analysis enables the expansion of the technological bottleneck and leads to a cost-efficient and quality-controlled dispersion process for the future industrial application. In a first step, the carbon nanoparticles were characterized regarding their geometry, specific surface area and elementary composition. Rheological characterization of the different molecular weight epoxy resins built a starting point for further investigations. Rheo-electrical characterization of the nanocomposites revealed the applied strain amplitude as predominant driving force for dispersion and not the shear rate. Optical coherence tomography (OCT) allowed the direct evaluation of the particle size distribution over the dispersion process. Analysis of the dependency of the complex impedance on the frequency via impedance spectroscopy revealed the network formation on macro and even on micro scale. Next generation three roll mills are enabled to monitor occurring process forces. The analysis of the line forces during processing allows an assessment of the homogeneity of dispersion. The combination of these in-line quality-monitoring methods enables a process time reduction of 70 % for a demonstrator masterbatch production of typical batch size. The evaluated impact of the particle type, filler loading and epoxy resin on the viscosity during processing and the resulting thermo-mechanical, electrical and fracture behavior (mode I and II) revealed the potential for industrial applications. Here, SWCNT showed the best property enhancement regarding the electrical conductivity and fracture toughness without increase of viscosity at ultra-low filler loading of 0.01 wt.%. This allowed the manufacturing of a bright colored glass fiber reinforced polymer (GFRP) with antistatic discharge protection. Subsequently, the presented results enable the manufacturing of nano-intermediates in an efficient way with controllable dispersion quality.Publicationtype: Doctoral Thesis491 320 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Sicherheit bei der Implantation keramischer EndoprothesenkomponentenPublicationtype: Doctoral Thesis48 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , 48 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Active strain-sensing with a nanoporous-metal hybrid material(2019); This thesis presents first experiments on active strain-sensing with a nanoporous-metal hybrid material. The thermodynamic relation between elastic properties, sensing- and actuation parameters that is derived in this work leads to good agreement between experiment and theory. The experiments show that this class of material behaves phenomenologically equally to piezoceramics although the underlying microscopic mechanisms are completely different.Publicationtype: Doctoral Thesis445 513 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Herstellung von hochgefüllten Kupfer-Polymer-Kompositen mittels eines Strahlschicht-Beschichtungsverfahrens und ihre AnalysePublicationtype: Doctoral Thesis70 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Ultra-hoch gefüllte und orientierte CNT- und 3D-vernetzte Aerographit Epoxidkomposite: Synthese, Herstellung und EigenschaftenIn this work two different polymer nanocomposites (PNCs) with graphitic fillers in epoxy matrix are characterized. Both PNCs possess structural features, which are used to address open scientific questions in the field of carbon nanotube (CNT) based PNCs. The PNCs of this work use in-house synthesized nanoparticles, which are: mm-long, unidirectional aligned carbon nanotubes (CNTs) and covalently 3D interconnected graphite-based networks ('Aerographite', AG). For CNT-based PNCs (CNT-PNCs) a manufacturing method is introduced (patented), which allows to embed very high amounts of CNTs in epoxy matrix (Φ= 10-70 wt.%). Two different structures are given for these CNT-based PNCs: APNCs: PNCs with unidirectional alignment of mm-long, in-house produced CNTs (anisotropic). RPNCs: PNCs with randomly oriented CNTs (mm-long in-house produced CNTs or µm-long commercial CNT) (isotropic). AG-PNCs: Aerographite-based PNCs are PNCs with an isotropic, covalently interconnected graphite filler network embedded in epoxy matrix (Φ<1 wt.%) (isotropic). Here different orientations, different aspect ratios and different qualities of CNTs are directly compared. The scaling of electrical conductivity and Young's moduli in respect to filler loadings are in focus of the investigations (σ (Φ) , E(Φ)). The piezoresistive and thermoelectric behavior of CNT-based PNCs with filler loadings Φ >5 wt.% are investigated here for the first time. The findings for CNT-PNCs are combined with the results on AG-PNCs. The influence of inner network topologies on electrical conduction is discussed in detail. If the graphitic networks of AGs are infiltrated with epoxy, AG-PNCs of low filler loading and isotropic structure result. Inner graphite elements of AGs are covalently interconnected. Therefore AG-PNCs can be used to investigate the influence of reduced electron tunneling on the electrical conduction and piezoresistive behavior of PNCs. It can be shown, that highly filled APNCs can lead to highly improved material properties like electrical conductivities up to σ= 37000 S/m and Youngs's moduli up to E= 36 GPa. On the other hand APNCs are shown to follow similar scaling rules of electrical conduction (σ(Φ)), as they had been derived from dispersed CNT-PNCs in the last two decades of research. The inner interconnections of AG lead to - compared to all of today's CNT-based PNCs - effective electrical conduction (σ/Φ). Appendix: Aerographite (AG): In the year 2010 first time a metal oxide (ZnO) was shown to be a feasible substrate for graphite deposition via a chemical vapor deposition process (CVD). Technically no other synthesis offers following combination: (i) structuring by tailored ceramic networks & graphite deposition with in situ removal of template. The density and inner structure of AG can be altered by the CVD-process. Scientifically the details of nucleation and growth of graphite within the CVD-process is not yet fully understood. To continue the research, the appendix deal with following contents: Development of synthesis & outlook on structuring Statistics of CVD-processes (2010-2015) & introduction of one control parameter for selecting variant and density of AGs Structural analyses (SEM, TEM, SAEDs, XRD,…) & introduction of a hypothetical growth modelPublicationtype: Doctoral Thesis749 387 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Beschichtung, thermomechanische Charakterisierung und Spannungsanalyse von SchneidkeramikPublicationtype: Doctoral Thesis21