Ingenieurwissenschaften und zugeordnete Tätigkeiten
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Eingeladener Vortrag
- nein (1)
The optical properties of semiconductor nanocrystals (SCNC) are controlled by constituent material, particle size, and surface chemistry, specifically the number of dangling bonds favoring nonradiative deactivation, and hence also by particle synthesis. In this respect, the fluorescence properties of coreshell CdSe SCNCs with different shells and surface chemistries were studied on ensemble and single particle level, using steady state and timeresolved fluorometry and confocal microscopy with time correlated single photon counting detection. Special emphasis was dedicated to correlate ensemble photoluminescence (PL) quantum yields and decay kinetics with particle brightness, PL time traces, and the Ontime fraction of the single SCNCs. Additionally, the confocal PL images were correlated with AFM measurements in order to derive the amount of absorbing, yet nonemisssive ”dark” SCNCs, the presence of which leading to an underestimation of ensemble PL quantum yields. The results of this study can help to identify synthetic routes and surface modifications minimizing the fraction of dark SCNC, thereby closing the gap to the ultimate goal of colloidally and photochemically stable SCNCs with a PL quantum yield of close to unity.
The optical properties of semiconductor nanocrystals (SCNCs) depend on constituent material, particle size, and surface chemistry, with the size of the photoluminescence (PL) quantum yield (QY) and the PL decay kinetics being largely controlled by the number of dangling bonds, which have to be properly passivated for high quality materials. Hence, PL measurements can provide insight not only in SCNC photophysics, yet can be also used for quality control of SCNC synthesis and surface modification. In this respect, steady state and time-resolved fluorometry and confocal microscopy with time correlated single photon counting were used to study the PL properties of core-shell CdSe SCNCs with different shells and surface chemistries on ensemble and single particle level, thereby focusing on a correlation of ensemble PL QY and PL decay kinetics with particle brightness, PL time traces, and the On-time fraction of single SCNCs. Additionally, confocal PL images were correlated with AFM measurements in order to derive the amount of absorbing, yet non-emissive ”dark” SCNCs, the presence of which resulting in an underestimation of ensemble PL quantum yields. The results of this study can help to identify synthetic routes and surface modifications, which minimize the fraction of dark SCNCs.
Two polyethylene types with ultra high (UHMWPE) and high molecular weight (HMWPE), which are used as neutron radiation shielding materials in storage casks for radioactive waste, were subjected to gamma irradiation doses up to 600 kGy and subsequent thermal ageing at 125 °C for up to one year. One material was a medical grade UHMWPE and the other a HMWPE containing an antioxidant. Degradation effects in the materials were characterized using colorimetry, UV-Vis spectroscopy, MIR and FTIR measurements, DSC and, in the case of HMWPE, insoluble content determination. Both materials exhibited a yellowing upon irradiation. The discoloration of UHMWPE disappeared again after thermal ageing, which is why it was attributed to annealable color centers in the form of free radicals entrapped in the crystalline regions of the polymer that recombine during thermal ageing. Furthermore, oxidation species were observed with MIR and FTIR spectroscopy. For HMWPE, the yellowing occurred during both irradiation and thermal ageing and was correlated to antioxidant decomposition. Additionally, black spots were observed after thermal ageing of HMWPE that were attributed to reaction products of antioxidant derivatives and catalyst residues. While only little evidence of oxidation species was found in the light material parts, oxidation is expected to concentrate in the black spots as the catalyst residue promotes hydroperoxide decomposition and thus radical formation that initiate polymer oxidation
Pressurisation of full-containment pressure vessels in fire is known to be driven by thermal stratification. The predominant mode of heat transfer to the contents (convection from the shell to the liquid phase) results in formation of „hot“ boundary layers. Sub-cooled boiling may also be present. The warm layer rises to the surface through buoyancy and bubble flow, increasing the surface of the liquid above that of the bulk temperature, and hence driving a pressure rise.
For reliable prediction of the complex effects governing vessel pressurization a three-dimensional numerical model is required. Work is being undertaken on such a model by other institutions in cooperation with this project.
A new tank design for Hazmat
(2016)
Initial studies have shown pre-deformed honeycomb structure
pressure vessels to have comparable burst pressure to straight steel vessels.
It was shown that honeycomp structured tanks have a higher energy absorption than straight steel tanks.
Future studies will investigate optimization of the deformed plate, to determine if increased payloads or a reduced wall thickness can be achieved.
The arrangement of the measuring sensors allowed the
capture of air flow induced temperatures and material
temperatures on various points of the cylinder body. Four
significant temperature levels could be observed: Inside the cylinder (far from the wall, close to the wall), inside the aluminium liner and outer surface. Figure 5 shows the temperature distribution after a regular filling process with a filling time of ten minutes. The temperature sensors T1 - T9 are marked with different colors. The corresponding measurement points are shown in Figure 4. Due to the high thermal conductivity of the aluminum liner the induced temperature peaks are quickly derived. A flow-induced exceeding of the maximum approval temperature is not expected. Regarding the safety assessement the results show that the pressure and temperature profilesmustbe taken into account.
Ethylene-propylenene-diene copolymer rubbers (EPDM) are used in a wide range of sealing applications e.g. in automotive industry or in containers for dangerous goods. Investigations with regard to the ageing behavior and lifetime prediction of commercial EPDM rubber seals consisting of 48 % polypropylene (PP) and 4.1 % of 5-ethylidene-2-norbornene (ENB) were conducted [1]. However, structural parameters (monomer ratio, diene type, curing agent etc.) and additives (filler, plasticizer etc.) are known to affect the ageing behavior of rubber compounds [2, 3]. The aim of the current study was to elucidate the influence of each component on the overall deterioration of material properties after thermal ageing. Therefore, different EPDM mixtures were prepared at BAM, the reference being an EPDM with 48 % PP and 4.1 % ENB reinforced with Sillitin. Five more formulations were prepared in order to assess the effect of (i) filler type (carbon black or sillitin), (ii) curing agent (sulfur or peroxide), (iii) plasticizer, (iv) ethylene-propylene ratio and (v) ENB content. Initially, the properties of the unaged materials were investigated by tensile testing, hardness and density measurements, TGA, DSC and DMA. Sulfur vulcanization resulted in higher elongation at break due to the flexibility of the sulfidic crosslinks. Carbon black resulted in better reinforcement compared to Sillitin. A higher ethylene and ENB content lead to higher hardness due to higher crystallinity and higher crosslink density, respectively.
Thermal ageing for up to 30 days (with 10 days intervals) took place in circulating hot air ovens at 125 °C. Pronounced crosslinking was found to take place in all peroxide-cured materials, as the unreacted double bonds of the ENB units acted as starting points for oxidation and crosslinking. Increasing the ethylene content resulted in an increase in the ageing resistance of EPDM. This improvement was attributed to the higher crystallinity that inhibits oxygen diffusion and to the smaller number of chain scissions which occur in the PP units. Although significant loss of the plasticizer was observed, the remaining plasticizer adequately improved the tensile properties of the material. The filler type did not significantly affect the ageing behavior of EPDM. Finally, the lower thermal stability of the sulfidic crosslinks resulted in desulfurization and, thus, in an increase in the crosslinking density.
To sum up, it has been shown that the monomer composition, curing agent and additives used in EPDM formulations greatly influence the properties and ageing resistance of these materials.
Characterization of residual stress state by neutron diffraction and residual magnetic field mapping
(2016)
Based on the residual stress characterization of tungsten inert gas welded S235JRC+C plates by means of neutron diffraction, the evaluation of residual stress with high spatial resolution GMR (giant magneto resistance) sensors is discussed. The experiments performed indicate a correlation of residual stress changes and local residual magnetic stray fields.
Selective Laser Melting (SLM) technique allows to produce parts with complex geometry. Due to the rapid solidification and heat gradient during production, SLM results in presence of residual stress. The present study shows the influence of manufacturing parameters on surface residual stress of Ti-6Al-4V SLM parts. High tensile stresses in the front surface are found. Heat Treatment conditions relax residual stresses almost to zero. High scanning speed during manufacturing results in higher tensile stresses in the surface.
Additive Manufacturing (AM) by Selective Laser Melting (SLM) offers ample scope for producing geometrically complex parts as compared to the traditional subtractive manufacturing strategies. However, the residual stresses developed during the processing can reduce the load bearing capacity as well as induce unwanted distortion, limiting the application of SLM parts.
IN718 manufactured by SLM process can show high tensile residual stresses in the surface as high as the yield strength of the wrought alloy. On the other hand, residual stresses in the bulk Show considerably lower stress values, even in compression, indicating a stress re-distribution during deposition of the SLM layers.
We present an in situ investigation of the mechanochemical formation of cocrystals using synchrotron XRD and Raman spectroscopy. This combination allows to study milling processes on the level of the molecular and crystalline structure thus obtaining reliable data for mechanistic studies. Thereby, mechanochemical syntheses can be optimized to isolate new crystal structures.
We report on the in situ investigation of mechanochemical syntheses of metal phosphonates. The metal phosphonates are formed in milling reactions starting from a metal acetate and a phosphonic acid. The conversions are observed by synchrotron PXRD and Raman spectroscopy to shed light on the reaction mechanisms including possible intermediates.
Ceramic components with complex shape cannot be produced frequently by usual ceramic forming and sintering processes. Therefore, numerous joining methods were developed and introduced in industrial scale. Nowadays, multi-stage Mo-Mn-process and active brazing are preferentially used, if temperature-stable and gastight joints are required. Unfortunately, both processes involve cost-intensive thermal processes: hydrogenous atmosphere is essential for metallization in Mo-Mn-process and active brazing takes place under vacuum. Thermal processes can be drastically simplified by using Reactive Air Brazing (RAB). Joining under air atmosphere is an interesting alternative, especially to join oxide ceramic components among themselves. So far, main disadvantage of RAB is low strength of join connections.
Aim of this investigation was the development of high-strength, thermal shock resistant and gastight ceramic-ceramic joints by RAB. Therefore,
- commercial, silver and copper oxide containing RAB soldering composition was modified by addition of ceramic particles with low thermal expansion coefficients (TEC). Hence, thermal misfit between TEC of solder and ceramic components was significantly reduced.
- RAB soldering paste was replaced with newly developed RAB composite tapes, produced by ceramic “doctor blade” technology. Thereby, improved potential exist to tailor the brazing layer relating to composition, thickness and thickness uniformity.
Gastight alumina-alumina, alumina-zirconia and zirconia-zirconia joints with strongly improved strength were produced by novel composite tapes. No strength degradation of joints was observed after thermal cycling up to 700°C.
The wetting behavior of material surfaces can be controlled by surface structures. We functionalized inorganic material surfaces, such as steel, titanium alloy and silicon, to modify the wetting behavior using ultrashort laser pulses (fs- to ps-range). The laser processing was performed by scanning the laser beam across the surface of initially polished flat sample material. A combined experimental and theoretical study of the laser processing parameters (peak fluence, scan velocity, line overlap) allowed the identification of different regimes associated with characteristic surface morphologies (laser-induced periodic surface structures, grooves, micro cones, dimples, etc.). Analyses of the surface using optical as well as scanning electron microscopy allowed the identification of morphologies providing the optimum similarity to the natural skin of lizards. For mimicking skin structures of moisture-harvesting lizards towards an optimization of the surface wetting behavior, additionally, a two-step laser processing strategy was established for realizing hierarchical micro- and nanostructures. In this approach, a laser-generated regular array of small dimples was superimposed (step 2) to the micron-scaled capillaries processed before (step 1). Optical focus variation imaging measurements finally revealed the three dimensional topography of the laser processed surfaces derived from lizard skin structures. The functionality of these surfaces was analyzed in view of wetting properties.
Laser-induced periodic surface structures (LIPSS, ripples) were generated on steel and titanium surfaces upon irradiation with multiple linear polarized femtosecond laser pulses (pulse duration 30 fs, central wavelength 790 nm). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning geometry for the processing of large surface areas covered homogeneously by the nanostructures. The irradiated surface regions were subjected to optical microscopy (OM), white light interference microscopy (WLIM) and scanning electron microscopy (SEM) revealing sub-wavelength spatial periods. The nanostructured surfaces were tribologically tested under reciprocal sliding conditions against a sphere of hardened 100Cr6 steel at 1 Hz using paraffin oil and engine oil as lubricants. After 1000 sliding cycles at a load of 1.0 N, the corresponding wear tracks were characterized by OM and SEM. For specific conditions the laser-generated nanostructures endured the tribological treatment. Simultaneously, a significant reduction of the friction coefficient was observed in the laser-irradiated (LIPSS-covered) areas when compared to the non-irradiated surface, indicating the potential benefit of laser surface structuring for tribological applications.
Laser-induced periodic surface structures (LIPSS) were generated on titanium nitride (TiN) hardcoating surfaces (deposited on metallic substrates) upon irradiation with multiple linearly polarized femtosecond laser pulses in air (30 fs duration, 790 nm wavelength, 1 kHz pulse repetition rate). The conditions were optimized in a sample-scanning geometry for the processing of large surface areas (5 mm x 5 mm) covered homogeneously by nanostructures with sub-wavelength periods ranging between ~200 nm and 700 nm. For these nanostructures the coefficient of friction was characterized under reciprocating sliding condition against a ball of hardened steel at 1 Hz using different lubricants (regime of mixed friction). After 1000 cycles, the corresponding wear tracks were characterized by optical and scanning electron microscopy. High-resolution energy dispersive X-ray analyzes (EDX) allowed the visualization of chemical alterations within the wear tracks. For specific conditions, the nanostructures endured the tribological treatment. Our experiments provide a qualification of the tribological performance of the fs-LIPSS on TiN surfaces.
Large area periodic surface structures were generated on steel surfaces using 30-fs laser pulses at 790 nm wavelength. Two types of steel exhibiting a different corrosion resistance were used, i.e. a plain structural steel (corrodible) and a stainless steel (resistant to corrosion). Homogeneous fields of laser-induced periodic surface structures (LIPSS) were realized utilizing laser fluences close to the ablation threshold while scanning the sample under the focused laser beam in a multi-pulse regime. The nanostructures were characterized with optical and scanning electron microscopy. For each type of steel, more than ten dentical samples were laser-processed.
These samples were subjected to microbial adhesion tests, investigating bacterial adhesion behavior on the laser structures in comparison to polished reference surfaces. Short term experiments (<24h) were carried out to determine initial biofilm development. E. coli as a typical bacterium representing pathogenic bacteria and Shewanella putrefaciens as metal corrosive bacterium were used for biofilm development analyses. Bacterial cell adhesion was determined microscopically after DAPI cell staining (DNA staining). Comparison of the coverage areas between nanostructured and polished surfaces revealed differences in cell adhesion behavior and biofilm structure.
Das EU-Projekt HOMER befasst sich mit den sogenannten Home Made Explosives (HMEs) und frei verfügbaren Rezepten für deren Herstellung. Diese Selbstlaborate spielen in der heutigen Zeit vor allem im Zusammenhang mit terroristischen Aktivitäten eine bedeutende Rolle. Informationen zu HMEs und ihrer Herstellung werden einfacher denn je über das Internet verbreitet. Hier knüpft das im 7. EU‑Forschungsrahmenprogramm finanzierte Projekt an und bezweckt, die im Internet verfügbaren Quellen auszuwerten und so die Informationslage für Sicherheitskräfte und Ermittlungsbehörden zu verbessern. Im Rahmen des Projektes wird eine automatisierte Suche von Rezepten im Internet entwickelt. Die gefundenen sowie die im Projekt generierten Informationen über HMEs sollen im Endergebnis in Form einer Software (Knowledge Management Platform) informationstechnisch nutzbar gemacht werden. Diese Datenbank soll auch für mobile Geräte zur Verfügung stehen. Der Schwerpunkt der Projektbeteiligung des Fachbereichs 2.3 Explosivstoffe der BAM liegt in der Bewertung der Rezepturen und der auf ihrer Grundlage hergestellten Laborate. Es wurden HMEs entsprechend der vorgefundenen Rezepte und unter Beachtung aller notwendigen Sicherheitsvorkehrungen hergestellt und anschließend sicherheitstechnisch geprüft und bewertet. Dabei wurden sowohl Handhabungsparameter wie die Empfindlichkeiten gegen Schlag, Reibung und thermische Einflüsse ermittelt, als auch die Leistungsparameter wie Detonationsgeschwindigkeit und Luftstoßdruckwirkung gemessen. Die gewonnen Erkenntnisse fließen in die genannte Datenbank ein und sollen beispielsweise Einsatzkräften vor Ort helfen, angemessene Schutzabstände zu ermitteln. Auch das Personal in Laboren der Kriminalämter profitiert von der zusammengetragenen Information über HMEs bei der Analyse von unbekannten Explosivstoffspuren.
Wasserstoffbarrieren aus Glas erfahren zunehmend Bedeutung als Werkstoff in der Energietechnik. Trotz wachsender Anwendungen liegen überwiegend empirisch gewonnene Beziehungen zwischen Glaszusammensetzung und Wasserstoffpermeation vor, die im wesentlichen aus dem Verhalten von Kieselglas abgeleitet werden. In dieser Studie werden daher topologische Parameter der Glasstruktur identifiziert und mit thermodynamischen Parametern korreliert. Auf der Seite der Glastopologie steht das freie Volumen und der Vernetzungsgrad des Netzwerkes während auf der thermodynamischen Seite die Konfigurationsentropie bzw. die Konfigurationswärmekapazität Aufschluss über die Verteilung des freien Volumens geben kann. Dabei wird angenommen, dass für Temperaturen weit unterhalb der Glasübergangstemperatur die chemische Löslichkeit von H2 zu vernachlässigen ist. Um die eingeschränkte Datenlage für die Wasserstoffpermeation kompositionell zu erweitern, wurden zusätzlich Heliumpermeationsmessungen ausgewertet. Die Ergebnisse zeigen, dass für Silicatgläser ein charkteristisches Verhältnis von freiem Volumen zu Netzwerkaufspaltung herrschen muss, um permeationsstabile Gläser zu erhalten.