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Due to their chemical structure, a rigid disk-like aromatic core and flexible alkyl chains attached to the core, discotic liquid crystals (DLCs) can organize and stack themselves into columns in a hexagonal columnar mesophase, a mesophase in between the plastic crystalline and isotropicphase. The overlap of the π orbitals of the aromatic core in the hexagonal columnar mesophase leads to a high charge-carrier mobilitie along the column axis – these columns can be considered as “moleculer nanowires” making liquid crystals a promising class of materials for electronic applications. Previous studies on DLCs showed that the phase behavior of DLCs is susceptible to nano confinement. Here in this study, the thermotropic collective orientational order of 2,3,6,7,10,11 hexakis[hexyloxy] triphenylene (HAT6), a triphenylene based DLC, and the effects of nano confinment on of its phase behavior were investigated by broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). HAT6 was embedded into nanoporous anodic aluminum oxide membranes by melt infiltration in the isotropic phase under argon atmosphere. The filled membranes have the thickness of 80 µm and the diameter of 15mm and parallel aligned tubular nanopores having the pore diameters of 25, 40, 80 and 180 nm. The filling degree for each sample was checked by thermogravimetric analysis (TGA) in order to ensure complete filling. The bulk HAT6 forms, a hexagonal columnar phase between the isotropic phase above 371 K, and the plastic crystalline phase below 340 K. Unlike the bulk, the confined HAT6 forms two peaks in the heat flow during the plastic crystalline-to-hexagonal columnar phase transition, which might be the evidence of two different phase structures close to the wall and in the pore center. Moreover, the isotropic-to-columnar transition of the confined HAT6 shifts with decreasing pore diameter to lower temperatures.
Metal reducing bacteria (MRB) are able to utilize various materials such as iron, uranium and manganese as well as many organic compounds as electron acceptors. This process leads to the conversion of Fe(III) containing passive film species to soluble Fe(II) oxides and hydroxides. The reduction process triggers the acceleration of general and local corrosion processes. Electron transfer mechanisms are not yet fully understood. In literature it has been controversially discussed to which extend secreted electron shuttles contribute to the extracellular electron transfer (EET).
To understand the chemical and electrochemical interaction mechanisms of MRB with steel surfaces this project combines a variety of in-situ techniques. The changes in oxide chemistry on Fe/steel surfaces in the presence of biomolecules and MRB are under investigation using a newly designed electrochemical cell for in situ XANES (x-ray absorption near edge structure) spectroscopy. Electrochemical quartz crystal microbalance (eQCM) studies support the spectroscopic investigations to gain information about the kinetics of attachment processes and changes in biofilm viscosity. The biofilm structure and composition as well as cell viability are investigated by complementary ex situ spectroscopic and microscopic analysis.
Combining spectroscopic techniques and eQCM data with electrochemical measurements, biological processes and the resulting degradation of steel surfaces can be observed in a non-destructive manner. Selecting model systems and a defined biological medium allows the determination of individual effects of diverse surface and environmental parameters. The fundamental understanding of bacterial attachment mechanisms and initial steps of biofilm formation will contribute to the development of new antifouling strategies.
The role of metal reducing bacteria (MRB) in corrosion is being controversially discussed in the literature. They can utilize metals including iron, uranium and manganese as well as many organic compounds as electron acceptors. The reduction of Fe(III) compounds to soluble Fe(II) species leads to the destruction of passive films on steel, resulting in acceleration of general and local corrosion processes. Recent research shows that the secretion of electron shuttles like riboflavins also contribute to the extracellular electron transfer (EET).
The aim of this project is to understand the chemical and electrochemical interaction mechanisms of MRB with steel surfaces by means of combined in situ techniques. An electrochemical XANES (x-ray absorption near edge spectroscopy) cell has been designed to study the changes of passive film chemistry in the presence of biomolecules and MRB. Electrochemical quartz crystal microbalance (eQCM) is used for studying the kinetics of bacterial cell attachment and diffusion of biomolecules in model biofilms. In situ investigations are complemented by ex situ spectroscopic and microscopic analysis to investigate the biofilm structure, composition and cell viability.
Via the combination of electrochemical methods with spectroscopic techniques and QCM we are able to follow biological processes and resulting degradation of steel surfaces in a non-destructive manner. The selection of model systems and a defined biological medium allows the identification of the effects of individual surface and environmental parameters. The fundamental understanding of bacterial attachment mechanisms and initial steps of biofilm formation will contribute to the development of new antifouling strategies.
Many engineering structures are made of composite materials or metal foam. To simulate the deformational behaviour of these structures often requires a high number of discretisation elements. This in turn yields a very large system of linear equations that are extremely time and memory consuming or practically impossible to solve. It is therefore desirable to find an approach to overcome this obstacle.
Many engineering structures are made of composite materials or metal foam. To simulate the deformational behaviour of these structures often requires a high number of discretisation elements. This in turn yields a very large system of linear
equations that are extremely time and memory consuming or practically impossible to solve. It is therefore desirable to find an approach to overcome this obstacle.
The chemical industry has expressed great interest in using polyethylene terephthalate (PET) as material for packagings for the transport of dangerous goods. Due to the high strength and stiffness of PET, the wall thickness and weight of packagings can be reduced. This is a relevant cost factor. PET is a semi-crystalline thermoplast which is produced by polycondensation of terephthalic acid and ethylene glycol.
According to the European dangerous goods regulations RID and ADR, samples of PET design types of packagings for dangerous goods must be pre-stored with the original filling good for six months at 23°C, before the design type tests can be carried out. For packagings made of polyethylene (PE) tests to prove the stress cracking resistance by using laboratory methods are possible to reduce time and costs. Therefore, standard liquids, simulating the different types of damaging effects on PE are defined in RID and ADR. However, there is no information and research available about the damaging mechanisms on PET in comparison to PE, especially regarding to stress cracking resistance and test methods.
The aim of this work is to compare the stress cracking resistance of PE and PET with the Full Notch Creep Test (FNCT), which was developed by chemical industry and is described in the standards EN ISO 16101 and EN 15507. It was investigated whether testing specimens made of PE and PET with a full coplanar notch around the middle of the specimens show weakening after the impact of a tensile force in a wetting solution (Lutensol® FSA 10) at 50°C in a test device on the basis of the FNCT.
The test results confirmed the suitability of the method for eight PE grades. Unfortunately, this method couldn’t be used for PET because the specimens broke during notching. In addition the molding of the sheets and the following temper process for twelve hours are very time-consuming, and the specimens broke during notching due to the high brittleness of PET.
Another possibility to provide evidence of stress cracking resistance of PE packagings is laid down in BAM’s Dangerous Goods Rule BAM-GGR 015. This test was carried out with PE and PET bottles to compare both materials. Therefore 1l test bottles were filled with a 5% wetting solution and mounted with a clamping tool for 28 days at 40°C. Tensile test specimens were cut out afterwards from the middle of the bottles in the deformed areas. Tensile strength and breaking elongation of PE specimens were determined in comparison to the PET specimens. The tensile properties of the PET specimens couldn’t be determined due to the strength and the stiffness of the material.
In conclusion the only way to provide information about the stress cracking resistance of PET was to perform stacking tests. 1l bottles made of PE and PET were filled with a 5% wetting solution and stored with a stacking load for 28 days at 40°C according to EN ISO 16495. Design types of PE and PET both passed the stacking tests.
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.
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.