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BTU
We apply soft x-ray emission spectroscopy (XES) to measure the electronic structure of crystalline silicon supersaturated with sulfur (up to 0.7 at. %), a candidate intermediate-band solar cell material. Si L2,3 emission features are observed above the conventional Si valence band maximum, with intensity scaling linearly with S concentration. The lineshape of the S-induced features change across the insulator-to-metal transition, indicating a significant modification of the local electronic structure concurrent with the change in macroscopic electronic behavior. The relationship between the Si L2,3 XES spectral features and the anomalously high sub-band gap infrared absorption is discussed.
Effect of Shot Peening on Surface Characteristics and Fatigue Properties of T5-treated ZK60 Alloy
(2009)
The aim of this work is to evaluate the influence of the n-heptane/toluene ratio on the reactivity of binary toluene reference fuels (TRFs), through a combined experimental and numerical work. Novel experimental ignition delay time (IDT) data of three binary TRFs of varying n-heptane/toluene ratios have been obtained in a high-pressure shock tube and in a rapid compression machine at conditions relevant to novel engine operation. Measurements have been performed at two pressures (10 and 30 bar), and at three fuel/air equivalence ratios (0.5, 1.0 and 2.0) for TRF mixtures of 50%, 75% and 90% by volume toluene concentration, over the temperature range of 650–1450 K. It was found that, increasing the n-heptane content, led to an increase in reactivity and shorter measured IDTs. Reduced sensitivity to the equivalence ratio was observed at high temperatures, especially for high toluene content mixtures. A …
A novel canning technology to forge gamma-TiAl alloys was developed at the BTU Cottbus. A TiAl specimen is encapsulated with multilayer stainless steel foil and glass. The steel foil layers prevent the heat loss through radiation and the glass layer reduces the temperature decrease through conduction. First, the effect of steel foil on the cooling rate was investigated. Cooling curves were recorded for TiAl specimens without steel foil layer, with 1, 2, 3 and 4 layers of steel foil, as well as with 3 coated steel foil layers, respectively. While the unprotected specimen cooled from 1200 °C to 1100 °C within 12 s, the specimen with 3 coated steel foil layers needed 52 s for the same temperature decrease. The efficiency of the glass layer was examined with forging of steel specimens. The cooling rate during forging of the specimen with a glass layer is only half of that without a glass layer. Based on the results, Ti-45Al-0.5Mo-0.5Cu-0.2Si specimens, canned with steel foil and glass, were successfully forged at strain rates of 0.1 s-1 and 0.04 s-1 with warm dies which were heated to 500 °C. Visual and metallographic examinations revealed no cracks, pores or micropores. The microstructures are fine-equiaxed grains.
Tensile and high cycle fatigue (HCF) properties of the hot-rolled Mg-12Gd-3Y (wt.%) magnesium alloyhave been investigated. The magnesium alloy exhibits a fatigue strength of about 150 MPa, which ismuch higher than that of the commercial Mg-8Al-Zn alloy AZ80. Aging heat-treatment (T5) improvedthe fatigue life of the Mg-12Gd-3Y alloy. Fatigue cracks nucleated at the intense slip bands in theas-rolled alloy. After T5 treatment, however, the fatigue crack nucleation site shifted to the phase boundariesbetween MgGdY particles and Mg matrix. T5 heat-treatment retarded the crack initiation and thusimproved the fatigue life of the Mg-12Gd-3Y alloy.
The valence band structures (VBS) of eight
transition metals (Fe, Co, Ni, Cu, Pd, Ag, Pt, Au) were
investigated by photoelectron spectroscopy (PES) using He I,
He II, and monochromatized Al Kα excitation. The influence
of final states, photoionization cross-section, and adsorption of
residual gas molecules in an ultrahigh vacuum environment are
discussed in terms of their impact on the VBS. We find that
VBSs recorded with monochromatized Al Kα radiation are
most closely comparable to the ground state density of states
(DOS) derived from quantum mechanics calculations. We use
the Al Kα-excited PES measurements to correct the energy scale of the calculated ground-state DOS to approximate the “true”
ground-state d-band structure. Finally, we use this data to test the d-band center model commonly used to predict the electronicproperty/
catalytic-activity relationship of metals. We find that a simple continuous dependence of activity on d-band center
position is not supported by our results (both experimentally and computationally).
Experimental Investigation of Tip Vortex Formation Noise Produced by Wall-Mounted Finite Airfoils
(2021)
Motivated by promising results from experiments on the noise reduction performance of finite wall-mounted airfoils with a porous tip in an aeroacoustic wind tunnel, corresponding measurements were performed in a dedicated rotor test rig. This rig features a 3 kW direct-drive electric motor with three rotor blades of 1.1 m diameter. In the experiments, a variety of different tip modifications was tested regarding their noise reducing performance, including different porous materials that were manufactured using rapid prototyping. Acoustic measurements were performed at different rotationals speeds and blade angles using a ring-shaped microphone array consisting of 60 evenly spaced microphones. The rotational speed was measured using a laser sensor. Different beamforming algorithms were used in post-processing to help analyzing the data. The results show that porous tips can lead to a notable noise reduction at low frequencies, but a small penalty at medium and high frequencies which may be due to the surface roughness of the porous tips. This trend seems more severe for lower blade angles than for higher ones. These findings agree well with those obtained from the previous wind tunnel measurements.