Failure behavior of modern double-layer thermal barrier coatings subjected to compression tests

Demands for reduced emissions and higher efficiency of stationary gas turbines and jet engines lead to the necessity for increased operating temperatures. Therefore, thermal barrier coating (TBC) systems deposited to high-temperature impinged parts, e.g. turbine blades and vanes, as well as combustor liners, containing an yttria-stabilized zirconia (YSZ) top layer are well established. Currently, surface temperatures of YSZ coated turbine parts are limited to approximately 1250 °C in long-term operation, due to the rapid degradation of YSZ caused by sintering and phase instability. Double-layer TBC based on gadolinium zirconate (GZO) applied on top of a 7 to 8 wt. % YSZ layer seem to be proper candidates for advanced coating architectures to withstand temperatures up to 1550 °C. The present work investigates the failure behavior and fracture process of double-layer thermal barrier coatings under uniaxial compressive loading conditions. Coating systems of type GZO and YSZ with low and high GZO porosity (LP, HP) were fabricated, to examine the influence of microstructure and spray process parameter on failure behavior and compressive strain energy. A conventional YSZ-HP single-layer coating serves as a reference. All systems were deposited via atmospheric plasma spraying (APS) on cylindrical rods made from CoNiCrAlY (LCO-22) coated, nickel-based, single crystal superalloy (PWA 1483). The total thickness of ceramic layers was about 600 µm. Effects of thermal ageing were taken into account by isothermal pre-oxidation at 1050 °C and dwell-times of 100, 500 and 1.500 hours and compared to cyclic annealed TBC systems (50 to 1050 °C, up to 500 cycles). Failure and cracking processes during compression tests were monitored by an acoustic emission (AE) system and piezo-electric, wideband sensors. Furthermore, a stereo camera system provides information about three-dimensional displacements and TBC surface strain. In as-sprayed condition, the stored volume related strain energy to failure of double-layer coatings is comparable to the referenced single-layer system. AE analysis indicates coating failure at earlier stages after thermal ageing. Consequently, pre-oxidation leads to reduced strain energies with increasing dwell-time in all investigated coating systems. Based on digital image correlations (DIC), the failure behavior of as-sprayed GZO/YSZ coatings has been identified to be similar to the YSZ single-layer system. A different behavior was observed for pre-oxidized coatings, where cracking and spallation of GZO occurs predominantly. Chair and Institute for Materials Technology, Technische Universität Darmstadt, 64283 Darmstadt, German

Tandem Solar Cell Concept Using Black Silicon for Enhanced Infrared Absorption

AbstractIn this work we present a novel tandem solar cell concept that is based on enhanced below band gap infrared absorption. The solar cell structure is based on silicon and infrared activated Black Silicon. Infrared active Black Silicon is produced by exposing silicon to fs-laser pulses. It features an enhanced IR absorption, when processed under a sulfur-containing atmosphere. Then sulfur is incorporated into the silicon lattice during laser processing providing energy states in the band gap. This silicon based tandem cell thus absorbs light with wavelengths beyond 1.1μm. This can potentially increase the overall efficiency. In this paper we present the first experimental realization of this concept. We use a standard aluminium-back-surface-field (Al-BSF) silicon solar cell and implement a Black Silicon solar cell on its rear side for enhanced IR absorption. Current and voltage measurements show the feasibility of our concept

Continuous infusion of an agonist of the tumor necrosis factor receptor 2 in the spinal cord improves recovery after traumatic contusive injury.

AimThe activation of the TNFR2 receptor is beneficial in several pathologies of the central nervous system, and this study examines whether it can ameliorate the recovery process following spinal cord injury.MethodsEHD2-sc-mTNFR2 , an agonist specific for TNFR2, was used to treat neurons exposed to high levels of glutamate in vitro. In vivo, it was infused directly to the spinal cord via osmotic pumps immediately after a contusion to the cord at the T9 level. Locomotion behavior was assessed for 6 weeks, and the tissue was analyzed (lesion size, RNA and protein expression, cell death) after injury. Somatosensory evoked potentials were also measured in response to hindlimb stimulation.ResultsThe activation of TNFR2 protected neurons from glutamate-mediated excitotoxicity through the activation of phosphoinositide-3 kinase gamma in vitro and improved the locomotion of animals following spinal cord injury. The extent of the injury was not affected by infusing EHD2-sc-mTNFR2 , but higher levels of neurofilament H and 2', 3'-cyclic-nucleotide 3'-phosphodiesterase were observed 6 weeks after the injury. Finally, the activation of TNFR2 after injury increased the neural response recorded in the cortex following hindlimb stimulation.ConclusionThe activation of TNFR2 in the spinal cord following contusive injury leads to enhanced locomotion and better cortical responses to hindlimb stimulation

Tailoring the Absorption Properties of Black Silicon

AbstractSamples of crystalline silicon for use as solar cell material are structured and hyperdoped with sulfur by irradiation with femtosecond laser pulses under a sulfur hexafluoride atmosphere. The sulfur creates energy levels in the silicon band gap, allowing light absorption in the infrared wavelength regime, which offers the potential of a significant efficiency increase. This Black Silicon is a potential candidate for impurity or intermediate band photovoltaics. In this paper we determine the laser processed sulfur energy levels by deep-level transient spectroscopy (DLTS). We present how the number of laser pulses per sample spot influence the sulfur energy levels and hence the DLTS spectra. Further we show that changing the laser pulse by splitting it with a Michelson interferometer setup results in altered absorption which is most likely due to altered sulfur energy levels. This contribution focuses on the possibility of controlling the sulfur in Black Silicon through manipulating the laser pulse shape. As a first step samples of microstructured silicon are fabricated with doubled laser pulses at two different laser pulse distances and the absorption spectra by integrating sphere measurements are compared

Modelling the growth of ZnO nanocombs based on the piezoelectric effect

In this work a model for the growth of ZnO nanocombs based on the piezoelectric character of ZnO is presented that explains the periodic growth of nanowire branches on the polar +(0001) surface of a ZnO nanobelt as a self catalytic growth process. In this model the perturbation and elasticity theory are applied to approximate the induced mechanical strain and piezoelectric potential distribution in the nanobelt under the growth kinetics. To implement a quantitative simulation of the periodic growth of ZnO nanobranches the induced piezoelectric charges in the ZnO nanostructure are calculated. These are responsible for the structural transformation from a nanobelt into a nanocomb. A comparison with nanocombs that are synthesized using the vapor-liquid-solid method shows good agreement between experimental and theoretical results

Extracting accurate capacitance voltage curves from impedance spectroscopy

We propose a method to obtain accurate capacitance-voltage (C-V) curves in the presence of multiple space charges. This method uses impedance spectroscopy to evaluate individual space charges separately. The advantage is that the knowledge of the exact equivalent circuit is not essentially needed. The comparison with other methods to calculate the doping concentration NA shows that our method is unaffected by series resistances and agrees best with the correct value of NA. The evaluation of the impedance spectra leads to a more thorough understanding of the respective Mott-Schottky plots

Laser Processed Black Silicon for Photovoltaic Applications

We present a femtosecond laser pulse process that induces a texture-like surface structure on silicon wafers and optionally incorporates sulfur into the silicon lattice for emitter formation depending on the processing atmosphere. Such laser processed Black Silicon provides an easily adjustable surface roughness for good light trapping in silicon solar cells. The structure is independent of the silicon crystal orientation and is easily applied on one wafer side only. A sulfur emitter can be formed within the laser structuring process, and allows electric current extraction from a solar cell structure manufactured from this material. Then the advantage is that no further emitter formation step like diffusion is necessary compared to other Black Silicon solar cell approaches, where the Black silicon is created wet chemically. By incorporating sulfur in the silicon crystal lattice, we can show that this Black Silicon absorbs in the infrared wavelength regime. This characteristic can potentially be used to better exploit the energy in the sun spectrum. We manufacture a laser processed Black Silicon solar cell prototype without any emitter diffusion step and achieve the highest efficiency of 4.5 % reported for this cell type

Tandem solar cell concept using Black Silicon for enhanced infrared absorption

In this work we present a novel tandem solar cell concept that is based on enhanced below band gap infrared absorption. The solar cell structure is based on silicon and infrared activated Black Silicon. Infrared active Black Silicon is produced by exposing silicon to fs-laser pulses. It features an enhanced IR absorption, when processed under a sulfur-containing atmosphere. Then sulfur is incorporated into the silicon lattice during laser processing providing energy states in the band gap. This silicon based tandem cell thus absorbs light with wavelengths beyond 1.1 μm. This can potentially increase the overall efficiency. In this paper we present the first experimental realization of this concept. We use a standard aluminium-back-surface-field (Al-BSF) silicon solar cell and implement a Black Silicon solar cell on its rear side for enhanced IR absorption. Current and voltage measurements show the feasibility of our concept

Essential protective role of tumor necrosis factor receptor 2 in neurodegeneration

Despite the recognized role of tumor necrosis factor (TNF) in inflammation and neuronal degeneration, anti-TNF therapeutics failed to treat neurodegenerative diseases. Animal disease models had revealed the antithetic effects of the two TNF receptors (TNFR) in the central nervous system, whereby TNFR1 has been associated with inflammatory degeneration and TNFR2 with neuroprotection. We here show the therapeutic potential of selective inhibition of TNFR1 and activation of TNFR2 by ATROSAB, a TNFR1-selective antagonistic antibody, and EHD2-scTNFR2, an agonistic TNFR2-selective TNF, respectively, in a mouse model of NMDA-induced acute neurodegeneration. Coadministration of either ATROSAB or EHD2-scTNFR2 into the magnocellular nucleus basalis significantly protected cholinergic neurons and their cortical projections against cell death, and reverted the neurodegeneration-associated memory impairment in a passive avoidance paradigm. Simultaneous blocking of TNFR1 and TNFR2 signaling, however, abrogated the therapeutic effect. Our results uncover an essential role of TNFR2 in neuroprotection. Accordingly, the therapeutic activity of ATROSAB is mediated by shifting the balance of the antithetic activity of endogenous TNF toward TNFR2, which appears essential for neuroprotection. Our data also explain earlier results showing that complete blocking of TNF activity by anti-TNF drugs was detrimental rather than protective and argue for the use of next-generation TNFR-selective TNF therapeutics as an effective approach in treating neurodegenerative diseases.</p