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Otakaari 1 grandhall. Photo: Esa Kapila
 

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Recent Submissions

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Comparison of Simplistic System-Level RIS Models and Diffraction-Theory Solutions
(2024) Hao, Le; Cuesta, Francisco S.; Tretyakov, Sergei A.; Department of Electronics and Nanoengineering; Sergei Tretiakov Group; Vienna University of Technology
We discuss and compare two recently published path-loss models for reconfigurable intelligent surfaces (RIS) derived from the electromagnetic and communication theory points of view. The electromagnetic model considers the RIS as a whole, taking into account field interactions of all metasurface elements, whereas the communication model is based on an assumption that each array element is an independent relay. The estimations given by the two models are rather different but, surprisingly, we find that the simplistic model gives the same result as the electromagnetic calculation for perfectly functioning anomalous reflectors if one replaces the effective areas of the unit cells by their geometrical areas, scaled by the corresponding angular factors. Simulation results validate our analysis and show the numerical differences between the two models when we vary the frequency and the number of elements in RISs.
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Strain-based analysis on the influence of local undercut geometry on fatigue crack initiation life
(Elsevier Science Ltd., 2024-09) Niraula, Abinab; Remes, Heikki; Nussbaumer, Alain; Department of Mechanical Engineering; Marine and Arctic Technology; Ècole Polytechnique Fédérale de Lausanne
The local undercut defects at the weld toe provide a potential initiation site for fatigue cracks and significantly impact the structure’s fatigue strength. The influence of continuous undercut depth on fatigue performance is widely studied, but the research on the influence of local undercut geometry is limited. In this research, the influence of the 3D undercut geometry on the fatigue strength of welded joints is investigated using the elasto-plastic finite element analysis, with explicit consideration of geometry and plasticity effect. A simplified parametric model is created to represent the realistic undercut geometry based on high-resolution measurement data. The combination of the geometric parameters for the realistic undercut models was filtered from a full-factorial design of six undercut geometry parameters. The influence of the individual geometric parameters of the local undercut defect such as depth, radius, width, and length on the fatigue performance of weld is systematically investigated. Based on the results, a three-dimensional undercut geometry index is proposed in the current study and found suitable for estimating fatigue crack initiation life. Furthermore, the research provides insights into fatigue strength-based quality criteria for welds with local undercut defects.
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Synthesis, Characterization, and Polymorphism of [H3O][NbF6]: A Polar and Possibly Ferroelectric Oxonium Salt
(Wiley-VCH Verlag, 2024-05-22) Möbs, Martin; Sachs, Malte; Rolheiser, Konstantin; Pietzonka, Clemens; Karttunen, Antti J.; Kraus, Florian; Department of Chemistry and Materials Science; Inorganic Materials Modelling; University of Marburg
[H3O][NbF6] was obtained from the controlled hydrolysis of NbF5 in anhydrous liquid HF. It adopts a polar, orthorhombic crystal structure with space group Iba2 (no. 45, oI88) at room temperature. A first-order phase transition at 137 K leads to a cubic non-centrosymmetric polymorph in space group I213 (no. 199, cI88). This low-temperature modification results from a distinct rotation of the [H3O]+ cations canceling their polar orientation in the room temperature phase. Quantum-chemical calculations estimate a rotational barrier between 5.8 to 6.4 kJ/mol. At a temperature of 363 K, the compound adopts a centrosymmetric, cubic crystal structure in space group Pm (Formula presented.) m (no. 221, cP11) that shows rotational disorder of cations and anions. The transition from the polar phase at room temperature to the centrosymmetric phase at high temperature not only reveals the plastic nature of the high-temperature structure but also hints at potential ferroelectric properties, underscoring the multifaceted behavior of [H3O][NbF6] across different temperature regimes.
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Building consumption anomaly detection : A comparative study of two probabilistic approaches
(Elsevier Science Inc., 2024-06-15) Stjelja, Davor; Kuzmanovski, Vladimir; Kosonen, Risto; Jokisalo, Juha; Department of Mechanical Engineering; Energy Conversion and Systems
This paper investigates the performance of two probabilistic approaches, Ensemble batch Prediction Intervals (EnbPI) a conformal prediction approach and XGBoost Location, Scale and Shape (XGBoostLSS), in predicting building energy consumption and in detecting systemic anomalies with proposed alarm matrix. The research questions focus on the effectiveness of these models in providing both point and probabilistic predictions and their utility in identifying collective anomalies. Both models showed good point and distribution prediction performance. For example, the observed point prediction had CV-RMSE in the range of 9 to 17%, outperforming recommendations from ASHRAE. Furthermore, a post-processing stage, the alarm matrix, effectively flags collective, repetitive anomalies, thus shifting focus from conventional point anomalies. The EnbPI-based method yields higher recall rates, with a trade-off of having more false alarms, while XGBoostLSS-based method excels in precision, minimizing overlooked alarms. Moreover, a robustness analysis was carried out to evaluate how these models performed when faced with training datasets containing anomalies. The robustness analysis revealed that the EnbPI-based method was more prone to overfitting, meaning its performance actually improved when the training data included some noise. On the other hand, the XGBoostLSS-based method was more stable, performing well with low levels of noise with performance drop when the noise level was high. While the findings contribute significantly to building energy consumption prediction and anomaly detection, future research could address performance in dynamic environments for the methodology and explore continual learning strategies.
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Electrochemical cobalt oxidation in chloride media
(Elsevier Ltd, 2024-07-01) Makarava, Iryna; Vänskä, Jere; Kramek, Agnieszka; Ryl, Jacek; Wilson, Benjamin P.; Yliniemi, Kirsi; Lundström, Mari; Department of Chemical and Metallurgical Engineering; Department of Chemistry and Materials Science; Hydrometallurgy and Corrosion; Rzeszow University of Technology; Gdańsk University of Technology
The green transition, despite recent advances in cobalt-free battery technologies, is still highly dependent on the availability of critical cobalt-based materials. Consequently, there has been increasing interest towards the development of new methods that maximize critical metals recovery from industrial hydrometallurgical solutions. In the current study, direct anodic oxidation of cobalt species from cobalt chloride solutions was studied as one alternative future strategy for cobalt recovery. Electrochemical methods were used (cyclic voltammetry, potentiostatic anodic deposition) and the effect of pH, temperature, and the concentration of cobalt and chloride ions on cobalt precipitation were investigated. The increase of pH and temperature was shown to stabilize the electrochemical oxidation of cobalt, while a decrease in cobalt concentration had a negative effect on precipitation. Scanning Electron Microscope, Atomic Force Microscopy and X-ray Photoelectron Spectroscopy were exploited to evaluate the morphology, structure, and composition of obtained anodic product. Calculated for potentiostatic anodic deposition (at highest studied potential of 1300 mV vs. Ag/AgCl) nucleation mechanism shows that the rate of nucleation for oxygen-cobalt species is faster than the subsequent growth rate of nuclei (instantaneous mechanism). XPS results confirmed that mixed Co3O4/Co(OH)2/CoOOH precipitate could be obtained by optimized anodic potentiostatic deposition in the range from 900 to 1150 mV and pH from 3 to 6 at 60 °C.