Zeitschriftenaufsätze und Buchbeiträge

The input of combustion heat in engines has a major impact on the piston friction and the resulting wear of the piston skirt. The new methodology presented here enables the simulation of combustion heat input during motored operation, and thus a detailed investigation of the piston friction under realistic piston temperature profiles of real engine operation is possible. For this purpose a standardized engine test bench for motored friction evaluations was expanded to include, among other things, a movable high-power diode laser with special defocusing optics. The setup of the test engine is based on the FEV teardown step methodology [1] and has open access to the engine piston from above due to a cylinder head dummy. Thus, the heat input by means of a high-power diode laser into the piston crown can be made. The reduced engine structure also enables a precise and highly accurate evaluation of the piston friction. A previously conducted validation process of the methodology ensures the most accurate possible replication of fired piston temperature profiles. The comparison between the piston temperatures measured in fired operation and those simulated in motored operation for a partial load operating point shows a maximum variance deviation of only 15˚C depending on the measuring point. The new methodology is also used in particular for the evaluation and detection of critical piston friction conditions. Experiments in this context are presented and discussed exemplary by using three measurement series at different operating temperatures and engine speeds. There is a gradual increase in the laser power for each series of measurements and thus in the heat input into the piston. The increase in heat input leads to a significant increase in friction at all operating points due to thermal expansion and the associated decrease reduction in piston clearance. Depending on the operating temperature and the engine speed, a critical piston friction condition is achieved and detected by the level of friction increase. The additional use of ultrasonic sensors and the knock sensor installed as standard makes a simultaneous measurement of the structure-borne sound signals possible. The increase in the acceleration levels of all sensors correlates here with the increase in piston friction. An evaluation of the noise, vibration, and harshness (NVH) measurement in both the frequency range and the crank angle (CA) range shows conspicuous high-frequency excitation levels that occur in the top dead center area. This correlation can be proven for all three measurement series. The results obtained here may open a path to an improved piston cooling strategy in the future.

There is an apparent mismatch between electron paramagnetic resonance and Mössbauer spectroscopy results on the charge and spin states of dilute Fe impurities in NaCl; Mössbauer spectroscopy data have been interpreted in terms of high-spin Fe2+, while electron paramagnetic resonance studies suggest low-spin Fe1+. In the present study, the charge and spin states of dilute substitutional Fe impurities in NaCl and LiF have been investigated with 57Mn&flech;57Fe emission Mössbauer spectroscopy. A scheme is proposed which takes into account the effects of nearest-neighbor distances and electronegativity difference of the host atoms on the Mössbauer isomer shift and allows for the unequivocal differentiation between high-spin Fe2+ and high/low-spin Fe1+ in Mössbauer spectroscopy. From these considerations, the Mössbauer results are found to be consistent with dilute Fe impurities in NaCl and LiF in a low-spin Fe1+ state. These conclusions are supported by theoretical calculations of isomer shifts and formation energies based on the density-functional theory. The experimental results furthermore suggest that charge compensation of dilute Mn2+ dopants in NaCl and LiF is achieved by Na vacancies and F− interstitials, respectively.

During the impregnation of reinforcement fabrics in liquid composite molding processes, the flow within fiber bundles and the channels between the fiber bundles usually advances at different velocities. This so-called “dual-scale flow” results in void formation inside the composite material and has a negative effect on its mechanical properties. Semi-empirical models can be applied to calculate the extent of the dual-scale flow. In this study, a methodology is presented that stops the impregnation of reinforcement fabrics at different filling levels by using a photo-reactive resin system. By means of optical evaluation, the theoretical calculation models of the dual-scale flow are validated metrologically. The results show increasingly distinct dual-scale flow effects with increasing pressure gradients. The methodology enables the measurability of microscopic differences in flow front progression to validate renowned theoretical models and compare simulations to measurements of applied injection processes.

MgF2-coated screws made of a Mg-2Y-1Mn-1Zn alloy, called NOVAMag® fixation screws (biotrics bioimplants AG), were tested in vitro for potential applications as biodegradable implants, and showed a controlled corrosion rate compared to non-coated screws. While previous studies regarding coated Mg-alloys have been carried out on flat sample surfaces, the present work focused on functional materials and final biomedical products. The substrates under study had a complex 3D geometry and a nearly cylindrical-shaped shaft. The corrosion rate of the samples was investigated using an electrochemical setup, especially adjusted to evaluate these types of samples, and thus, helped to improve an already patented coating process. A MgF2/MgO coating in the µm-range was characterized for the first time using complementary techniques. The coated screws revealed a smoother surface than the non-coated ones. Although the cross-section analysis revealed some fissures in the coating structure, the electrochemical studies using Hanks’ salt solution demonstrated the effective role of MgF2 in retarding the alloy degradation during the initial stages of corrosion up to 24 h. The values of polarization resistance (Rp) of the coated samples extrapolated from the Nyquist plots were significantly higher than those of the non-coated samples, and impedance increased significantly over time. After 1200 s exposure, the Rp values were 1323 ± 144 Ω.cm2 for the coated samples and 1036 ± 198 Ω.cm2 for the non-coated samples, thus confirming a significant decrease in the degradation rate due to the MgF2 layer. The corrosion rates varied from 0.49 mm/y, at the beginning of the experiment, to 0.26 mm/y after 1200 s, and decreased further to 0.01 mm/y after 24 h. These results demonstrated the effectiveness of the applied MgF2 film in slowing down the corrosion of the bulk material, allowing the magnesium-alloy screws to be competitive as dental and orthopedic solutions for the biodegradable implants market.

Total energy and electronic structure calculations based on density functional theory are performed in order to determine the atomic structure and electronic properties of clean and hydrogen-adsorbed Al0.5In0.5P(001) surfaces. It is found that most of the stable surfaces obey the electron-counting rule and are characterized by surface atom dimerization. The dimer-related surface states are predicted to occur in the vicinity of the bulk band edges. For a very narrow range of preparation conditions, ab initio thermodynamics predicts metal atomic wires formed by surface cations. A surface covered with a monolayer of buckled phosphorus dimers, where half of the phosphorus atoms are hydrogen saturated, is found to be stable for metal-organic vapor-phase epitaxy growth conditions. The occurrence of this structure is confirmed by low-energy electron diffraction and X-ray photoelectron spectroscopy data measured on epitaxially grown Al0.52In0.48P(001) epilayers lattice matched to GaAs.