ALBERT

Description: Based on the application of OB considerations (Part I) to various major thermal barrier coating (TBC) compositions and two types of important calcium–magnesium–alumino–silicates (CMAS)—desert sand and fly ash—the 2 ZrO 2 · Y 2 O 3 solid solution (ss) TBC composition, with high CMAS-resistance potential, is chosen for studying molten-CMAS/TBC interactions. It is demonstrated that 2ZrO 2 ·Y 2 O 3 (ss) air plasma sprayed (APS) TBCs are highly resistant to high-temperature attack by both sand-CMAS and fly-ash-CMAS. Despite the differences in the compositions of the two CMASs, the overall CMAS-attack mitigation mechanisms in both cases appear to be similar, viz reaction between 2ZrO 2 ·Y 2 O 3 (ss) APS TBC and the CMAS, and the formation of main reaction products of Y-depleted c - ZrO 2 and nonstoichiometric Ca – Y apatite. Large differences in the OBs (ΔΛ) between the 2ZrO 2 ·Y 2 O 3 (ss) and the CMASs are good predictors of ready reaction between this TBC and these CMASs. While the details of the CMAS-mitigation mechanisms can depend critically on various other aspects, the OB difference (ΔΛ) calculations could be used for the initial screening of CMAS-resistant TBC compositions.

Description: The higher operating temperatures in gas-turbine engines enabled by thermal barrier coatings (TBCs) engender new materials issues, viz silicate particles (sand, volcanic ash, fly ash) ingested by the engine melt on the hot TBC surfaces and form calcium–magnesium–alumino–silicate (CMAS) glass deposits. The molten CMAS glass degrades TBCs, leading to their premature failure. In this context, we have used the concept of optical basicity (OB) to provide a quantitative chemical basis for the screening of CMAS-resistant TBC compositions, which could also be extended to environmental barrier coatings (EBCs). By applying OB difference considerations to various major TBC compositions and two types of important CMASs—desert sand and fly ash—the 2 ZrO 2 · Y 2 O 3 solid solution (ss) TBC composition, with the potential for high CMAS-resistance, is chosen for this study. Here, we also demonstrate the feasibility of processing of 2ZrO 2 ·Y 2 O 3 (ss) air-plasma sprayed (APS) TBC using commercially developed powders. The resulting TBCs with typical APS microstructures are found to be single-phase cubic fluorite, having a thermal conductivity 〈0.9 W·(m·K) −1 at elevated temperatures. The accompanying Part II paper presents results from experiments and analyses of high-temperature interactions between 2ZrO 2 ·Y 2 O 3 (ss) APS TBC and the two types of CMASs.

Description: The explosive fragmentation of the 2013 Chelyabinsk meteorite generated a large airburst with an equivalent yield of 500 kT TNT. It is the most energetic event recorded by the infrasound component of the CTBT-IMS, globally detected by 20 out of 42 operational stations. This study performs a station-by-station estimation of the IMS detection capability to explain infrasound detections and non-detections from short to long distances, using the Chelyabinsk meteorite as global reference event. Investigated parameters influencing the detection capability are the directivity of the line source signal, the ducting of acoustic energy and the individual noise conditions at each station. Findings include a clear detection preference for stations perpendicular to the meteorite trajectory, even over large distances. Only a weak influence of stratospheric ducting is observed for this low-frequency case. Furthermore, a strong dependence on the diurnal variability of background noise levels at each station is observed, favoring nocturnal detections.

Description: [1]  Three large-scale infrasound calibration experiments were conducted in 2009 and 2011 to test the IMS infrasound network and provide ground-truth data for infrasound propagation studies. Here we provide an overview of the deployment, detonation, atmospheric specifications, infrasound array observations, and propagation modeling for the experiments. The experiments at the Sayarim Military Range, Israel had equivalent TNT yields of 96.0, 7.4, and 76.8 tonnes (t) of explosives on 26 August 2009, 24 January 2011, and 26 January 2011, respectively. Successful international collaboration resulted in the deployment of numerous portable infrasound arrays in the region to supplement the IMS network and increase station density. Infrasound from the detonations is detected out to ~3500 km to the northwest in 2009 and ~6300 km to the northeast in 2011, reflecting the highly anisotropic nature of long-range infrasound propagation. For 2009, the moderately strong stratospheric wind jet results in a well-predicted set of arrivals at numerous arrays to the west-northwest. A second set of arrivals is also apparent, with low celerities and high frequencies. These arrivals are not predicted by the propagation modeling and result from unresolved atmospheric features. Strong eastward tropospheric winds (up to ~70 m/s) in 2011 produce high amplitude tropospheric arrivals recorded out to 〉1000 km to the east. Significant eastward stratospheric winds (up to ~80 m/s) in 2011 generate numerous stratospheric arrivals and permit the long-range detection (i.e. 〉1000 km). No detections are made in directions opposite the tropospheric and stratospheric wind jets for any of the explosions. Comparison of predicted transmission loss and observed infrasound arrivals gives qualitative agreement. Propagation modeling for the 2011 experiments predicts lower transmission loss in the direction of the downwind propagation compared to the 2009 experiment, consistent with the greater detection distance. Observations also suggest a more northerly component to the stratospheric winds for the 2009 experiment and less upper atmosphere attenuation. The Sayarim infrasound calibration experiments clearly demonstrate the complexity and variability of the atmosphere, and underscore the utility of large-scale calibration experiments with dense networks for better understanding infrasound propagation and detection. Additionally, they provide a rich dataset for future scientific research.

Description: [1]  Atmospheric pressure waves were recorded within five hours after the 2011 great Tohoku earthquake (Mw = 9.0) by sensitive microbarographs at four regional stations and eight International Monitoring System (IMS) stations at distances up to 6,700 km. While its apparent phase velocity between the regional stations is 341 m/s, the global stations indicate weak dispersive wave trains with low frequencies between 1.6 and 4.8 mHz, propagating with an average phase velocity around 364 m/s. The low-frequency waves may be interpreted as acoustic-gravity waves excited by upheaval and depression of the sea surface in the source region due to coseismic uplift and subsidence of the sea bottom during this great thrust earthquake. Assuming the source dimension and the average coseismic vertical displacements of the sea surface, referenced to tsunami observations, we calculate synthetic waveforms for some of the far-field stations, by incorporating a standard sound-velocity structure in the atmosphere up to an altitude of 220 km. The synthetics provide reasonable explanations to the general features of the observed waveforms, suggesting possible ranges for the source parameters generating these acoustic-gravity waves. Our analysis suggests that the average initial upheaval of the sea surface in the central zones of the source region may exceed 4 - 6 m, and the rise time of the coseismic deformation may be in the range between 3 and 4 min. In the eastern narrow zone adjacent to the Japan Trench, the deformation has significantly higher initial amplitude and shorter rise time.

Description: [1]  Monitoring stations around the globe routinely detect microbarom signals with a dominant frequency of ∼0.2 Hz from regions of marine storminess. International Monitoring System (IMS) infrasound array IS59 in Kailua-Kona, Hawaii recorded clear signals in close proximity of Hurricanes Felicia and Neki of 2009 for a first-hand investigation of the detailed source mechanism through a hindcast analysis. A spectral wave model describes the tropical cyclone and ambient sea states through a system of two-way nested grids with forcing from a blended data set of global, regional, and cyclonic winds. The computed wave conditions are validated with altimetry measurements and utilized in an acoustic model to estimate the intensity and spatial distribution of the microbarom source. The model results elucidate origins of infrasound signals from the tropical cyclone waves as well as their interactions with the ambient conditions consisting of swells, wind seas, and storm waves from nearby systems. The positive correlation between the IS59 observations and the theoretical microbarom estimates, and the saturation of recorded signals from high-energy sources support the use of infrasound signals for inference of tropical cyclone waves.

Description: [1]  The 2011 Mw 9.0 Tohoku earthquake generated infrasound that was recorded by nine infrasonic arrays. Most arrays recorded a back azimuth variation with time due to the expanse of the source region. We use ray tracing to predict group velocities and backazimuth wind corrections. A Japan accelerometer network recorded ground shaking in unprecedented spatial resolution. We back projected infrasound from arrays IS44 (Kamchatka) and IS30 (Tokyo) to the source region and compare these results with acceleration data. IS44 illuminates the complex geometry of land areas that experienced shaking. IS30 illuminates two volcanoes and a flat area around the city of Sendai, where the maximum accelerations occurred. The arrays and epicentral region define three source-receiver profiles. The observed broadband energy transmission loss (TL) follows an exponential decay law. The best-fitting model, which has parameters that are interpreted to include the effects of geometric spreading, scattering, and the maximum ratio of the effective sound speed in the stratosphere to that at the ground (accounts for stratospheric wind speed), yields a 65% variance reduction relative to predictions from a traditional TL relationship. This model is a simplified versionof the model of [47], which yields an 83% variance reduction for a single frequency, implying fine-scale atmospheric structure is required to explain the TL for stratospheric upwind propagation. Our results show that infrasonic arrays are sensitive to ground acceleration in the source region of megathrust earthquakes. The TL results may improve infrasonic amplitude scaling laws for explosive yield.

Description: Coastal back-barrier perched lakes are freshwater bodies that are elevated over sea-level and are not directly subjected to the inflow of seawater. This study provides a detailed reconstruction of the Doniños back-barrier perched lake that developed at the end of a small river valley in the rocky coast of the north-west Iberian Peninsula during the Holocene transgression. Its sequence stratigraphy was reconstructed based on a core transect across the system, the analyses of its lithofacies and microfossil assemblages, and a high-resolution radiocarbon-based chronology. The Doniños perched lake was formed ca 4.5 ka BP. The setting of the perched lake was favoured by Late Holocene sea-level stabilization and the formation of a barrier and back-barrier basin, which was contemporaneous with the high systems tract period. This basin developed over marine and lagoonal sediments deposited between 10.2 and 8.0 ka BP, during rapidly rising sea-level characteristic of the transgressive systems track period. At 1.1 ka BP, the barrier was breached and the perched lake was partially emptied, causing the erosion of the back-barrier basin sediments and a significant sedimentary hiatus. Both enhanced storminess and human intervention were likely to be responsible for this event. After 1 ka BP, the barrier reclosed and the present-day lake was reformed, with the water level reaching as high as 5 m above mean sea-level. The depositional evolution of the Doniños system serves as a model of coastal back-barrier perched lakes in coastal clastic systems that have developed over gently seaward-dipping rugged substrates at small distances from the shoreline and under conditions of rising sea-level and high sediment supply. A review of estuaries, back-barrier lagoons, pocket beaches and back-barrier perched lakes in the rocky coast of north-west Spain shows that the elevation of the bedrock is the main factor controlling the origin and evolution of these systems. This article is protected by copyright. All rights reserved.

Description: Internally cooled, hollow SiC-based ceramic matrix composites (CMCs) components that may replace metallic components in the hot section of future high-efficiency gas-turbine engines will require multilayered thermal/environmental barrier coatings (T/EBCs) for insulation and protection. In the T/EBC system, the thermally insulating outermost (top coat) ceramic layer must also provide resistance to attack by molten calcia-magnesia-aluminosilicate (CMAS) deposits. The interactions between a potential candidate for top coat made of air-plasma-sprayed (APS) 2ZrO 2 ·Y 2 O 3 solid-solution ( ss ) ceramic and two different CMASs (sand and fly ash) are investigated at a relevant high temperature of 1500°C. APS 2ZrO 2 ·Y 2 O 3 ( ss ) top coat was found to resist CMAS penetration at 1500°C for 24 hours via reaction products that block CMAS penetration pathways. In situ X-ray diffraction (XRD) studies have identified the main reaction product to be an Ca-Y-Si apatite, and have helped elucidate the proposed mechanism for CMAS attack mitigation. Ex situ electron microscopy and analytical spectroscopy studies have identified the advantageous characteristics of the reaction products in helping the CMAS attack mitigation in the APS 2ZrO 2 ·Y 2 O 3 ( ss ) coating at 1500°C. Finally, the Y 3+ solubility limit and transport behavior are identified as potential comparative tools for assessing the CMAS resistance ability of top-coat ceramics.

Description: ABSTRACT We present a 7-ka environmental reconstruction based on sedimentological and geochemical data from Lago Vichuquén, a coastal eutrophic lake in central Chile (34°48′S, 72°03′ W, 4 m a.s.l.). A relatively shallow and restricted marine environment with low productivity, high detrital input and dominant anoxic conditions in the Vichuquén Basin occurred from 7.0 to 6.5 cal ka BP. Rapid onset of a Holocene marine transgression at 6.5 cal ka BP favoured deeper and more biologically productive environments that existed until 2.8 cal ka BP. Bioproductivity changes during the mid-Holocene (6.5–4.2 cal ka BP) were related to upwelling dynamics controlled by the intensity of the South-east Pacific Anticyclone (SPA). Periods with lower organic productivity and dominant anoxic conditions reflect an increased intensity of SPA (increased upwelling) and decreased precipitation. A shift at ∼4.0 cal ka BP reflects the onset of modern Southern Westerly Winds and El Niño-Southern Oscillation patterns. Tectonic uplifting and geormorphological activity (dune advancement) are possible reasons behind the Vichuquén Basin closure at ∼1.2 cal ka BP, leading to a low bioproductivity lacustrine environment which has developed until the present.