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The interest in porous anodic aluminum oxide (AAO) has been rapidly growing due to its numerous applications in separation, catalysis, energy generation and storage, electronics, and sensors. From the scientific point of view, AAO is a topical interest in soft matter fields. Spatial confinement of soft matter in nanoporous media influences its structure, thermodynamics, and mobility. Embedding polymers and liquid crystals into nanopores of AAO results in a 2D nanoconfinement of these materials. This confinement affects their properties, compared to the bulk, such as phase transition temperatures and enthalpies, molecular mobility, and architecture of the crystallization. On the other hand, discotic liquid crystals (DLCs) are a promising class of soft matter for electronic applications. This is due to their ability to organize and stack themselves into columns in a hexagonal columnar mesophase, a mesophase in between the plastic crystalline and isotropic phase, driven by the overlap of the π orbitals of their aromatic core. This leads to a high charge-carrier mobility along the column axis. Further, these columns could then be considered as “molecular nanowires”. In this study, 2,3,6,7,10,11 hexakis[hexyloxy] triphenylene (HAT6), a triphenylene based DLC, was confined into nanoporous AAO membranes. The structure, dynamics and the phase behavior of the confined HAT6 were investigated by broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC). HAT6 was embedded into nanoporous AAO membranes by melt infiltration in the isotropic phase under argon atmosphere. The membranes have parallel aligned cylindrical nanopores, with pore diameter of 10, 20, 25, 40, 80, 120 and 180 nm. The filling degree for each sample was checked by thermogravimetric analysis (TGA) in order to ensure complete filling. Bulk HAT6 forms a hexagonal columnar phase; in between the isotropic phase above 371 K and the plastic crystalline phase below 340 K. Unlike the bulk, the confined HAT6 split the plastic crystalline-to-hexagonal columnar phase transition in two, which might be interpret as two different phase structures; close to the wall and at the pore center. Moreover, the isotropic-to-columnar transition of the confined HAT6 shifted, with decreasing pore diameter, to lower temperatures. Furthermore, pore surfaces of a series of membranes were chemically modified, resulting in a more hydrophobic pore surface than the unmodified ones. HAT6 was embedded into the modified membranes by the same aforementioned preparation. The influence of the changed host-guest-interaction, on the structure, dynamics, and the phase behavior of HAT6 confined in the modified membranes, was also investigated by BDS and DSC.
The manufacturing of NbC with Ni binder with addition of titanium carbide by using a colloidal process for blending the different powders without a milling step were investigated. The the sintering process and formation of the resulting microstructures, the phase relations and the hardness of the produced cermet materials are characterized.
Ferritic alloys containing Cr form Cr5S6 precipitates along grain boundaries in hot SO2 environment. Grain boundary precipitates may influence the steel’s mechanical behavior. Cr5S6 precipitates were reconstructed three-dimensionally using FIB tomography analysis to determine the volume, number, and penetration depth. The volume growth of Cr5S6 precipitates increased only for the initial aging time of 3 h but became constant after 6 h. The number of precipitates increased linearly.
Acoustics is an important aspect for large rooms especially in schools, kindergartens and offices. Foam tiles made from melamine resins are often used for acoustic insulation because they show good fire resistance and good thermal insulation properties. But they can emit formaldehyde. In Germany there are cases with a resulting indoor air concentration of higher than 100 µg/m³. In this study samples from two schools and one office were investigated. Additionally, new tiles were purchased and tested to evaluate the general potential as formaldehyde source.
Upconversion nanoparticles are characterized by their ability to convert low energy near-infrared excitation in high energy emissions in the visible range. This is enabled by sequential absorption of two or more photons via energy transfer from the sensitizer lanthanide ion Yb3+ to an activator lanthanide ion, e.g. Er3+. Yb3+ sensitation has the advantage of 980 nm excitation achieved by commercial available low power cw laser modules.
Nevertheless, for some applications, especially in aqueous media, which is predominantely in bioanalytical applications, there is the drawback of a local Absorption maximum of water at this wavelength. Sample heating can occur, and excitation power density is strongly distance dependent.
To overcome this issue, the idea is to shift the excitation wavelength to 808 nm. This can be achieved by a tandem-excitation using Nd3+ and Yb3+ ions. Nd3+ gets excited by 808 nm irradiation and transfers the energy to Yb3+, which triggers the upconversion emission of the Er3+ ions. Therefore, it is mandatory to separate Nd3+ and Er3+ by core-shell architecture for reducing the energy backtransfer from Er3+ to Nd3+.
To further enhance the upconversion luminescence an additional inert shell formed by the host material NaYF4 reduces the quenching effects caused by surface defects and dangling bonds.
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.
The aim of this work was to find a laboratory test method for the determination of the stress cracking resistance of PET. One test method is the Full Notch Creep Test (FNCT), which was developed for polyethylene (PE) and is described in the standards EN ISO 13274 and EN 15507. It was investigated whether testing specimens made of 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 at 50°C. Unfortunately, this method couldn’t be used for PET because the specimens broke during notching due to the high brittleness of PET. The molding of the sheets and the following temper process for twelve hours are very time-consuming, Another possibility is laid down in BAM’s Dangerous Goods Rule BAM-GGR 015. This test was carried out with 1l PET bottles, which 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. The tensile properties of the PET specimens couldn’t be determined due to the hardness of the material.
In conclusion, the only way to provide information about the stress cracking resistance of PET was to perform stacking tests with PET design types of packagings. 1l bottles made of 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. The test bottles of PET passed the stacking tests.
An efficient energy provision using fuel cells is linked to effecitve hydrogen storage capabilities. Because of its low intrinsic hydrogen permeability, glass has a huge potential in modern concepts of hydrogen storage. Previous studies on oxidic glasses showed an empirical connection between glass composition and gas permeation, which was mainly derived from the behavior of silica glass. In this study we focus on the relationship between topologic (free volume; network polymerization) and thermodynamic (configurational entropy) parameters. The comparison of three glasses within the meta (earth-) alkaline alumino silicate system to silica glass shows that an increase of the atomic packing factor (APF) does not necessarily decrease the permeability. Furthermore, the results suggest a connection between ΔCp at Tg and hydrogen permeability of the glass. All experiments were performed assuming that chemical hydrogen solubiltity in glass is negligible at temperatures well below the glass transition temperature.
"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. This can lead to a distribution of photoluminescence Quantum yields (PL QY) amongst the SCNC particles, i.e., mixtures of “bright” and “grey” or “dark” SCNCs.
Particularly the number of absorbing, yet not emitting particles can have a significant effect on the PL quantum yield obtained in ensemble measurements, leading to ist underestimation. The “dark fraction” is not assessable in common ensemble measurements; it can be probed only on a single particle level using a confocal laser scanning microscope coupled with an AFM. Such a setup was used to study core‐shell CdSe SCNCs with different shells and surface chemistries. Special emphasis was dedicated to correlate brightness, blinking, dark fraction, and decay kinetics of the single SCNCs with the ensemble PL QY and the PL decay kinetics. The results of this study can help to identify new synthetic routes and surface modifications to colloidally and photochemically stable SCNCs with a PL QY of close to unity."
Compared to its intrinsic strength, the practical strength of glasses is substantially
lowered by microscopic flaws. Their sub critical growth from the surface is strongly
affected by ambient humidity.
A new device was designed to enable the observation of sub critical crack growth
(SCCG) in vacuum and controlled atmosphere. It allows measurements in transparent and
semi-transparent materials in double cantilever beam (DCB) geometry. The aim of the
present work is to study the effect of crystals on crack growth.