Algorithm to suppress drift for micro-mirror and other intensity modulated hydrogen sensors

Here, a method to suppress drift in intensity-modulated sensors is presented that preserves the advantages of such sensors including simplicity and low-cost components. This method is illustrated using metal hydride-based optical hydrogen sensors that can reliably, accurately, and quickly sense hydrogen across a large concentration range. These sensors rely on a metal hydride-sensing material that reversibly absorbs hydrogen when a hydrogen concentration is present. In turn, this causes a change in the optical properties which can be probed to determine the hydrogen concentration. To do this, two major methods exist: intensity- and frequency-modulated sensors. While intensity-modulated sensors are typically simpler and cheaper to fabricate, they may suffer from drifting light sources and unstable alignments. Using the fact that exposure to hydrogen reduces (increases) the optical transmission (reflectivity) of a Ta-based sensing material for blue/green light while it increases (decreases) the transmission for (near) infrared (IR) light, it is possible to differentiate between a changing hydrogen concentration and a drifting light source: Whereas the signal of both wavelengths is positively correlated for a drifting light source, the signal is negatively correlated when the hydrogen concentration changes. Using this algorithm, the drift on the signal can be reduced by a factor of 5 for intensity-modulated sensors. In a more general perspective, the wealth of information in the wavelength-dependent optical response allows for more advanced approaches to improve the signal and accuracy of (optical) sensors.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.RID/TS/Instrumenten groe

Skyrmions and spirals in cubic chiral magnets

Magnetic skyrmions are two-dimensional topologicallyprotected spin textureswith particle like properties which may crystallize inlattices that are typically oriented perpendicular to the magnetic field. Theirfirst observation was in the archetype cubic helimagnet MnSi, where skyrmionlattices appear spontaneously in the A-phase located just below the transitiontemperature and under magnetic fields. Single crystal cubic helimagnets host, besideskyrmion lattices, a variety of helimagnetic phases that are stabilized bythree hierarchically ordered interactions, being the strongest ferromagnetic,the weaker Dzyaloshinsky-Moriya which is non-zero due to the absence ofinversion symmetry, and the weakest anisotropy interaction. The competitionbetween these and the Zeeman interaction results in a relatively generic phasediagram for cubic helimagnets. Albeit this relative generic phase diagram, eachof these systems have their own particularities. This thesis mainly presentsresults of magnetization, ac susceptibility and neutron scattering studies offour cubic helimagnets: MnSi, Mn1-xFexSi, Fe1-xCoxSi and Cu2OSeO3 RST/Neutron and Positron Methods in Material

Design and characterization of 2D MXene-based electrode with high-rate capability

MXenes, two-dimensional transition metal carbides and nitrides, are promising materials for electrochemical energy storage application due to their redox-active surface and flexible interlayer space. Among all reported MXene-based electrodes, some have shown significantly better high-rate energy storage capabilities. Hence, it is crucial to have a systematic understanding on the decisive factors of the rate capability in the MXene family. This article discusses the impact of material properties at three levels, including intralayer composition, interlayer space and morphology, on the charge transfer and ion transport, revealing all the possible rate-limiting factors of MXene-based electrodes. We also describe systematic methods to characterize MXene electrodes as a detailed fundamental understanding of the structural and chemical properties, and the charge storage mechanisms crucial for rationally designing MXene-based electrodes. Graphic abstract: [Figure not available: see fulltext.]RST/Storage of Electrochemical EnergyInstrumenten groe

Tantalum-Palladium: Hysteresis-Free Optical Hydrogen Sensor Over 7 Orders of Magnitude in Pressure with Sub-Second Response

Hydrogen detection in a reliable, fast, and cost-effective manner is a prerequisite for the large-scale implementation of hydrogen in a green economy. Thin film Ta1−yPdy is presented as an effective optical sensing material with extremely wide sensing ranges covering at least 7 orders of magnitude in hydrogen pressure. Nanoconfinement of the Ta1−yPdy layer suppresses the first-order phase transitions present in bulk and ensures a sensing response free of any hysteresis. Unlike other sensing materials, Ta1−yPdy features the special property that the sensing range can be easily tuned by varying the Pd concentration without a reduction of the sensitivity of the sensing material. Combined with a suitable capping layer, sub-second response times can be achieved even at room temperature, faster than any other known thin-film hydrogen sensor.Instrumenten groepChemE/AfdelingsbureauChemE/Materials for Energy Conversion & Storag

Hydrogenation Kinetics of Metal Hydride Catalytic Layers

Catalyzing capping layers on metal hydrides are employed to enhance the hydrogenation kinetics of metal hydride-based systems such as hydrogen sensors. Here, we use a novel experimental method to study the hydrogenation kinetics of catalyzing capping layers composed of several alloys of Pd and Au as well as Pt, Ni, and Ru, all with and without an additional PTFE polymer protection layer and under the same set of experimental conditions. In particular, we employ a thin Ta film as an optical indicator to study the kinetics of the catalytic layers deposited on top of it and which allows one to determine the absolute hydrogenation rates. Our results demonstrate that doping Pd with Au results in significantly faster hydrogenation kinetics, with response times up to five times shorter than Pd through enhanced diffusion and a reduction in the activation energy. On the other hand, the kinetics of non-Pd-based materials turn out to be significantly slower and mainly limited by the diffusion through the capping layer itself. Surprisingly, the additional PTFE layer was only found to improve the kinetics of Pd-based capping materials and has no significant effect on the kinetics of Pt, Ni, and Ru. Taken together, the experimental results aid in rationally choosing a suitable capping material for the application of metal hydrides and other materials in a hydrogen economy. In addition, the used method can be applied to simultaneously study the hydrogenation kinetics in thin-film materials for a wide set of experimental conditions.Instrumenten groepChemE/O&O groe

Metal Hydride Based Optical Hydrogen Sensors

Hydrogen is playing a key role in the transition to a sustainable economy and in a variety of industrial processes. For its safe handling, the detection of hydrogen in a fast, reliable and accurate manner is crucial. Thin film metal hydride based optical hydrogen sensors provide an attractive option to sense hydrogen in a variety of conditions and have an attractive safety benefit over other methods of detection: They do not require the presence of electrical leads near the sensing area. These sensors rely on a change of the optical properties arising from a change in the hydrogenation of the metal hydride sensing layer in response to a different partial hydrogen pressure in the environment of the sensor. In this paper, we review how the performance and characteristics of an optical hydrogen sensor are related to the material properties of the metal hydride sensing layer and we discuss previously considered materials and challenges and opportunities left for the future.Accepted Author ManuscriptInstrumenten groepChemE/Materials for Energy Conversion & StorageChemE/Afdelingsburea

Charge Carrier Dynamics in Co-evaporated MAPbI₃with a Gradient in Composition

Co-evaporation of metal halide perovskites by thermal evaporation is an attractive method since it does not require harmful solvents and enables precise control of the film thickness. Furthermore, the ability to manipulate the Fermi level allows the formation of a graded homojunction, providing interesting opportunities to improve the charge carrier collection efficiency. However, little is known about how these properties affect the charge carrier dynamics. In this work, the structural and optoelectronic properties of co-evaporated MAPbI3 films varying in thickness (100, 400, and 750 nm) with a gradient in composition are analyzed. The X-ray diffraction patterns show that excess PbI2 is only present in the thick layers. From X-ray photoelectron spectroscopy depth analysis, the I/Pb atomic ratio indicates methylammonium iodide deficiencies that become more prominent with thicker films, resulting in differently n-doped regions across the thick MAPbI3 films. We suggest that due to these differently n-doped regimes, an internal electric field is formed. Side-selective time-resolved microwave photo conductivity measurements show an elongation of the charge carrier lifetimes on increasing thickness. These observations can be explained by the fact that excess carriers separate under the influence of the electric field, preventing rapid decay in the thick films. ChemE/Opto-electronic MaterialsChemE/O&O groepInstrumenten groe

Evolution of helimagnetic correlations when approaching the quantum critical point of Mn1-xFexSi

We present a comprehensive investigation of the evolution of helimagnetic correlations in Mn1−xFexSi with increasing doping. By combining polarized neutron scattering and high resolution neutron spin echo spectroscopy we investigate three samples with x = 0.09, 0.11, and 0.14, i.e., with compositions on both sides of the concentration x∗∼0.11 where the helimagnetic Bragg peaks disappear and between x∗ and the quantum critical concentration xC∼0.17, where TC vanishes. We find that the abrupt disappearance of the long range helical periodicity at x∗ does not affect the precursor fluctuating correlations. These build up with decreasing temperature in a similar way as for the parent compound MnSi. Also the dynamics bears strong similarities to MnSi. The analysis of our results indicates that frustration, possibly due to achiral Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions, increases with increasing Fe doping. We argue that this effect explains both the expansion of the precursor phase with increasing x and the abrupt disappearance of long range helimagnetic periodicity at x∗.RST/Neutron and Positron Methods in MaterialsInstrumenten groe

Tuning the Properties of Thin-Film TaRu for Hydrogen-Sensing Applications

Accurate, cost-efficient, and safe hydrogen sensors will play a key role in the future hydrogen economy. Optical hydrogen sensors based on metal hydrides are attractive owing to their small size and costs and the fact that they are intrinsically safe. These sensors rely on suitable sensing materials, of which the optical properties change when they absorb hydrogen if they are in contact with a hydrogen-containing environment. Here, we illustrate how we can use alloying to tune the properties of hydrogen-sensing materials by considering thin films consisting of tantalum doped with ruthenium. Using a combination of optical transmission measurements, ex situ and in situ X-ray diffraction, and neutron and X-ray reflectometry, we show that introducing Ru in Ta results in a solid solution of Ta and Ru up to at least 30% Ru. The alloying has two major effects: the compression of the unit cell with increasing Ru doping modifies the enthalpy of hydrogenation and thereby shifts the pressure window in which the material absorbs hydrogen to higher hydrogen concentrations, and it reduces the amount of hydrogen absorbed by the material. This allows one to tune the pressure/concentration window of the sensor and its sensitivity and makes Ta1-yRuy an ideal hysteresis-free tunable hydrogen-sensing material with a sensing range of >7 orders of magnitude in pressure. In a more general perspective, these results demonstrate that one can rationally tune the properties of metal hydride optical hydrogen-sensing layers by appropriate alloying.RID/TS/Instrumenten groepChemE/O&O groepApplied SciencesChemE/Chemical Engineerin

In Situ Reflectometry and Diffraction Investigation of the Multiscale Structure of p-Type Polysilicon Passivating Contacts for c-Si Solar Cells

The integration of passivating contacts based on a highly doped polycrystalline silicon (poly-Si) layer on top of a thin silicon oxide (SiOx) layer has been identified as the next step to further increase the conversion efficiency of current mainstream crystalline silicon (c-Si) solar cells. However, the interrelation between the final properties of poly-Si/SiOx contacts and their fabrication process has not yet been fully unraveled, which is mostly due to the challenge of characterizing thin-film stacks with features in the nanometric range. Here, we apply in situ X-ray reflectometry and diffraction to investigate the multiscale (1 Å-100 nm) structural evolution of poly-Si contacts during annealing up to 900 °C. This allows us to quantify the densification and thinning of the poly-Si layer during annealing as well as to monitor the disruption of the thin SiOx layer at high temperature >800 °C. Moreover, results obtained on a broader range of thermal profiles, including firing with dwell times of a few seconds, emphasize the impact of high thermal budgets on poly-Si contacts' final properties and thus the importance of ensuring a good control of such high-temperature processes when fabricating c-Si solar cells integrating such passivating contacts. Overall, this study demonstrates the robustness of combining different X-ray elastic scattering techniques (here XRR and GIXRD), which present the unique advantage of being rapid, nondestructive, and applicable on a large sample area, to unravel the multiscale structural evolution of poly-Si contacts in situ during high-temperature processes.RST/Storage of Electrochemical EnergyInstrumenten groe