41 research outputs found
Evolution of nano- and microstructure during the calcination of Bayer gibbsite to produce alumina
No full textThe microstructure of Metallurgical Grade Alumina (MGA) has extensive consequences on how it performs as a raw material for aluminium production. MGA is produced primarily in the Bayer process in which both the precipitation and the calcination steps play important roles for the development of alumina microstructure. Calciner technology is progressively moving from rotary kilns to more energy efficient gas suspension or circulating fluidised bed processes, which influences the intricate interplay of calcination parameters and therefore effects the development of structure and properties in the alumina. The evolution of structural features during the calcination of Bayer gibbsite is a complex process that involves both a dehydroxylation reaction and a rearrangement of the crystal lattice. Due to the highly agglomerated starting material, the pseudomorphic nature of the reactions and the incomplete transformation to the thermodynamically stable alpha alumina, a mixed phase meso- to macro porous material results. As a consequence of the wide range of particle sizes, rapid heating rates, short residence times and other inhomogenities in the modern calcination process a diversity and mixture of aluminium oxide products are formed. Deviations from the average transition alumina structures are also observed. In this thesis an integrated approach to evaluate alumina micro- and nanostructure is presented. Using a suite of modern analytical techniques, in conjunction with more traditional approaches, the structures that make up the MGAs are probed and insights into the development of structural features and the transformation reactions during the calcination process are obtained. It is shown that the alumina phase composition is influenced by the calcination conditions, which for the MGAs are dictated by the calciner technology, and that compositional variations are observed even within individual alumina grains. The results indicate that these compositional variations arise both due to the distribution of sodium and other impurities which catalyse the transition to alpha alumina, and as a result of the rapid heating rates. It is proposed that the rapid heating rates in modern calcination processes result in the formation of a boehmite-like structure within the particles. The internal boehmite formation may be promoted by desorption resistance of the product water, from surfaces and pore openings, due to the rapid decomposition rates. Electron microscopy investigations reveal that a lamellar and sponge-like network of interconnected pores and channels is formed with relatively large alpha alumina crystallites growing in the transition alumina matrix. Furthermore, it is shown that alpha alumina may be directly observed in cross sectioned alumina grains through Charge Contrast Imaging using an Environmental SEM. It is also demonstrated that multiple field Solid-state Magic Angle Spinning NMR can be used to quantitatively assess the phase composition of MGAs. The availability of high magnetic fields significantly aids this quantification. The NMR results indicate that the MGAs are relatively ordered on the short range, and that only small amounts of pentahedral aluminium are present. Furthermore, using X-ray Absorption Near-edge Structure (XANES) spectroscopy it was observed that the short range order is influenced by the precursor material and the calcination conditions. This offers a possible explanation for some of the discrepancy in the literature regarding the structural relationship between the gibbsite and boehmite derived transition aluminas. The results also indicate that changes in the intermediate range ordering occur with increasing calcination temperature. Even in the complex mixed phase MGAs some long range order is observed, which seems to be closely tied to the formation of alpha alumina in the samples. XANES is very sensitive to these long range structures. However quantitative or structural analysis is difficult due to the complex and overlapping resonances arising from multiple scattering interactions in the samples. The complex mixture of phases and the high level of disorder in the MGAs present significant challenges in understanding their structure, particularly with relation to the performance as a raw material for aluminium metal production. The structures that make up the MGAs transcend short-range ordering, on the nano-scale, and into longer range domains. Therefore, to examine MGA microstructure and better understand its impact in smelter operations, it is essential to evaluate structural ordering on several different scales. This requires advanced and often multiple analytical techniques
The nature and impacts of fines in smelter-grade alumina
No full textFines in smelter-grade aluminas art recognized (is a significant process problem in aluminum smelting. However understanding the nature of this fine material and how it impacts the reduction process arc, less clearly understood The combination of new analytical methods such as variable pressure scanning electron microscopy and very; high field solid state nuclear magnetic resonance provide new insights into the phases present and their spatial distribution within aluminas, and suggest how such fine materials are generated, particularly during calcination in the alumina refinery
How Many Answers Are Enough? Optimal Number of Answers for Q&A Sites
No full textWith the proliferation of the social web, questions about information quality and optimization attract the attention of IS scholars. Question-answering (QA) sites, such as Yahoo!Answers, have the potential to produce good answers, but at the same time not all answers are good and not all QA sites are alike. When organizations design and plan for the integration of question answering services on their sites, identification of good answers and process optimization become critical. Arguing that ‘given enough answers all questions are answered successfully,’ this paper identifies the optimal number of posts that generate high quality answers. Based on content analysis of Yahoo! Answers’ informational questions (n=174) and their answers (n=1,023), the study found that seven answers per question are ‘enough’ to provide a good answer
Estimation of Air Pollution with Remote Sensing Data: Revealing Greenhouse Gas Emissions from Space
No full textDescription
This dataset contains remote sensing data from the ESA Copernicus missions Sentinel-2 and Sentinel-5P (tropsopheric NO2 column density) in the 2018-2020 timespan. The satellite measurements each cover ~3100 locations in Europe and ~100 on the US Westcoast, each with a size of 1.2x1.2km. The locations are selected such that each measurement is centered at the location of an air quality measurement station on the ground (from the European Environment Agency or the US Environmental Protection Agency, measuring NO2). This makes it possible to analyze spatiotemporally aligned remote sensing and ground-based measurements.
The 13 Sentinel-2 bands are upsampled (bilinear) to 10m resolution and cropped to 120x120 pixel. For some locations multiple Sentinel-2 images are available. The images are stored as binary numpy `.npy` files organized into directories based on their locations.
The Sentinel-5P data was pre-processed by mapping the measurements from consecutive satellite overpasses onto a common rectangular grid of 0.05×0.05◦(∼5×5km) across Europe. To harmonize the Sentinel-2 (10m to 60m, upscaled to 10m) and Sentinel-5P (5×3.5km, rescaled to 5×5km) imaging resolutions, the Sentinel-5P data is linearly interpolated to 10m resolution and cropped to 120×120 pixel around the locations of interest. Additionally, all measurements with a QA flag (qa_value) below 75 were discarded, following ESA recommendations. The Sentinel-5P data are stored as `.netcdf` file, organized by location. For each location, three such files are available, containing averaged Sentinel-5P measurements at different temporal frequencies (2018-2020, quarterly, monthly).
The samples_{frequency}_{area}.csv files provide a list of observations with the corresponding file paths to a (cloud-free) Sentinel-2 image, the Sentinel-5P measurement, and the average NO2 concentration measurement by the EEA or EPA ground station. These files can be used for easy data-loading.
Content
The data is organized into the following files:
README.md - this file
sentinel-2-eea.tar.gz [33.1GB]
sentinel-5p-eea.tar.gz [80.1GB]
samples_2018_2020_eea.csv
samples_quarterly_eea.csv
samples_monthly_eea.csv
sentinel-2-epa.tar.gz [0.15GB]
sentinel-5p-epa.tar.gz [1.8GB]
samples_2018_2020_epa.csv
samples_quarterly_epa.csv
samples_monthly_epa.csv
Acknowledgement
If you use this data set, please cite our publication:
Scheibenreif, L., Mommert, M., Borth, D., "Estimation of Air Pollution with Remote Sensing Data: Revealing Greenhouse Gas Emissions from Space", Tackling Climate Change with Machine Learning workshop at ICML 2021.
Please refer to this publication for additional information on the data set.
This data set contains modified Copernicus Sentinel data acquired in 2018-2020, processed by ESA.
Responsible Author
Linus Scheibenreif
University of St. Gallen, Institute of Computer Science
Chair Artificial Intelligence and Machine Learning
linus.scheibenreif ( at ) unisg.c
Understanding how tissue geometry and signalling dynamics control cell fate patterning using an in vitro model of human anterior primitive streak
No full textCell fate patterning remains difficult to study, particularly when focusing on gastrulation and the anterior primitive streak (APS). The cells present at and surrounding the APS will give rise to anterior and nascent mesoderm, definitive endoderm, neuromesodermal progenitors and axial mesendoderm. Despite the latter two being crucial for axial elongation and formation of the spinal column, how these cells spatiotemporally pattern and interact remains to be fully understood. This is mainly due to the inherent complexity of the emerging patterning and the difficulties to investigate this in vivo.
In recent years, the use of 2D micropatterning technologies has gained traction as means to partially reconstruct cellular environments and decouple biological variables in quantitative studies. Indeed, 2D micropatterning techniques have shed light into the effect of cellular confinement on signalling dynamics and cell fate decisions. In this thesis, micropatterned mediated cellular confinement has been used to investigate how spatiotemporal signalling regimes lead to APS cell fate decisions and organisations, and how spatial organisation organisation influences the signalling environment that dictates APS-associated cell fate emergence and patterning.
In the first chapter of this thesis, I address the issue of investigating cell fate decisions during gastrulation. To address this, I have developed an in vitro system that mimics the anterior primitive streak. Culturing 2D confined human embryonic stem cells stimulated with the WNT pathway activator CHIR9901 and FGF2, elicits the self-organisation of human embryonic stem cells into a multi-tissue architecture, composed of an internal region of pluripotent cells surrounded by posterior mesoderm and a ring-domain of definitive endoderm at the periphery of the colony. Importantly, I show that cell fate patterning only arises under strict confinement, with size of micropattern controlling different morphological changes. I further show that whilst initially there is an emergence of anterior primitive streak cell fates, such as neuromesodermal progenitors, these are then lost in favour of definitive endoderm. Thus, showing the potential of using this model to study anterior primitive streak cell fate decisions.
By using this model, in the second chapter, I then look at understanding cell fate patterning with a focus on the role of biochemical signalling pathways. Via small molecule inhibition of the WNT and NODAL pathways, I show that endogenous TGFβ signalling dynamics downstream of CHIR9901, controls cell fate decision. I show that whilst NODAL inhibition is required for NMP specification, sustained NODAL signalling drives endoderm emergence, in accordance with in vivo data on when these cell types emerge during gastrulation. I also show evidence of a crosstalk between TGFβ inhibition and increase in WNT activity which correlates with notochord progenitor emergence. Additionally, I show that both adequate NODAL suppression and colony size control neuromesodermal progenitor-like cell emergence and axial elongation. Altogether, this work provides insights into the specification of APS cell fates during gastrulation, highlighting the importance of spatiotemporal signalling dynamics in their emergence.
Finally in my last chapter I investigate the effects of geometrical confinement in cell fate specification, more specifically, the effects of boundary curvature. I show that epithelial organisation depends on the boundary curvature (convex vs concave). These changes induced by curvature ultimately defines the domain in which SOX17+ endodermal and SOX2+ pluripotent cells arise in our patterns. Additionally, I show that whilst SOX2+ epiblast-like cells coalesce to the centre of the colony via an actomyosin-mediated mechanisms, SOX17+ do not pattern in the same manner. These results further show the complexity of definitive endoderm patterning, the importance of tissue geometry, and highlight the further need to understand how
does definitive endoderm migrate in vivo within the developing embryo.
Overall, this work has lead to a first author publication in development and provides insights into the regulatory biochemical and physical mechanisms that ultimately control cell fate patterning and population balance of the progenitors emerging at the APS during gastrulation. Whilst more research is required, this work provides an initial step in understanding definitive endoderm patterning as well as a starting point for the investigation of neuromesodermal and notochordal progenitor interactions. Thus, providing further understanding of APS cell fate patterning, useful for the generation of APS cell fates in vitro for further clinical and therapeutic applications
