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Architecture for sub-100 ms liquid crystal reconfigurable intelligent surface based on defected delay lines
Get PDFReconfigurable intelligent surfaces, comprised of passive tunable elements, are emerging as an essential device for upcoming millimeter wave and terahertz wireless systems. A fundamental aspect of the device involves the tuning technology used to achieve reconfigurability. Among alternatives such as semiconductors and micro-electromechanical systems, liquid crystal offers advantages including cost- and power-effective large-panel scalability. In this context, conventional liquid crystal-based reconfigurable intelligent surface approaches face limitations in optimizing for bandwidth, response time and loss simultaneously, requiring trade-offs between them. Here we detail an architecture for a liquid crystal-based reconfigurable intelligent surface with compact defected delay lines that provide continuous, 360-degree tunability, enabling fast response time, wide bandwidth and low loss. A reconfigurable intelligent surface with a thin 4.6 μm liquid crystal layer is designed, fabricated, and characterized, exhibiting response times of 72 milliseconds, insertion losses below 7 dB, and a 6.8 GHz (10.9%) bandwidth at 62 GHz, all while utilizing a lossy glass substrate and gold as a conductor
Oxidation Behavior and Outward Diffusion of Al Along Oxide Grain Boundaries of FeCrAl Alloys Overdoped with Zr and Hf
Get PDFThe formation of the α-Al₂O₃ scale on reactive element (RE)-doped FeCrAl alloys is commonly believed to be primarily caused by inward oxygen transport along grain boundaries. However, this study suggests that metal ion outward diffusion also plays a role in the development of the oxide scales and their microstructural characteristics. The study examines the oxidation behavior and grain boundary outward diffusion of iron-chromium alloys containing ~ 10 at% aluminum and ~ 22 at% chromium, doped with an over-critical concentration of REs, i.e., Zr and Hf. All samples were investigated after thermal exposure at 1100 °C by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom probe tomography (APT). As a result of the overdoping, a considerable increase in oxide growth, an increase in the depth of internal oxidation, and RE-oxide formation near and at oxide grain boundaries (GBs) were observed as a consequence of increased inward and outward diffusion. The effect of overdoping manifests itself differently depending on the RE type and amount due to different solubility, ionic size, and electronic structure of alumina. The sample with Zr retained the adhesion of alumina to the alloy after the first and second thermal exposure, while Hf overdoping resulted in severe spallation after the second thermal exposure
What can machine learning help with microstructure-informed materials modeling and design?
Get PDFMachine learning (ML) techniques have been widely employed as effective tools in addressing various engineering challenges in recent years, particularly for the challenging task of microstructure-informed materials modeling. This work provides a comprehensive review of the current ML-assisted and data-driven advancements in this field, including microstructure characterization and reconstruction, multiscale simulation, correlations among process, microstructure, and properties, as well as microstructure optimization and inverse design. It outlines the achievements of existing research through best practices and suggests potential avenues for future investigations. Moreover, it prepares the readers with educative instructions of basic knowledge and an overview on ML, microstructure descriptors, and ML-assisted material modeling, lowering the interdisciplinary hurdles. It should help to stimulate and attract more research attention to the rapidly growing field of ML-based modeling and design of microstructured materials
In-situ probing of the Fischer-Tropsch reaction on Co single crystal surfaces up to 1 bar
No full textThe surface chemistry of the Fischer-Tropsch catalytic reaction over Co has still several unknows. Here, we report an in-situ X-ray photoelectron spectroscopy study of Co0001 and Co(1014), and in-situ high energy surface X-ray diffraction of Co0001, during the Fischer-Tropsch reaction at 0.15 bar - 1 bar and 406 K - 548 K in a H2/CO gas mixture. We find that these Co surfaces remain metallic under all conditions and that the coverage of chemisorbed species ranges from 0.4–1.7 monolayers depending on pressure and temperature. The adsorbates include CO on-top, C/-CxHy and various longer hydrocarbon molecules, indicating a rate-limiting direct CO dissociation pathway and that only hydrocarbon species participate in the chain growth. The accumulation of hydrocarbon species points to the termination step being rate-limiting also. Furthermore, we demonstrate that the intermediate surface species are highly dynamic, appearing and disappearing with time delays after rapid changes in the reactants’ composition
A high‐resolution monitoring station for the in situ assessment of nitrate‐related redox processes at an agricultural site
Get PDFBiogeochemical redox processes control the chemical behavior of many major and trace elements, making their comprehension crucial for predicting and protecting environmental health. Nitrogen (N) is especially susceptible to changes in soil redox conditions and affects the cycles of other redox-sensitive species. Elevated N concentrations, in nitrate form, in agricultural soils and associated freshwater ecosystems constitute a problem in many parts of the world. Although a wide variety of measures have been adopted, their assessment through concentration measurements in groundwater and surface water of the different monitoring networks has shortcomings. Nitrate, as a non-point pollutant, is subject to several processes (e.g., transformation and retardation) before it is detected, making it impossible to evaluate measurements’ effectiveness reliably. Thus, we designed and constructed a monitoring station featuring commercially available products and self-manufactured components at an agricultural site for the in situ assessment of nitrate-related processes by high-resolution monitoring of hydraulic (soil water content, matric potential, groundwater head) and hydrogeochemical variables (oxidation-reduction potential and groundwater and pore water chemistry) within the vadose zone and the shallow aquifer. The monitoring station has proven to be a reliable tool. Changes over depth and time of measured variables have been identified, allowing the detection of the transient behavior of the redox reactive zone and the interpretation of ongoing denitrification processes and other redox nitrate-triggered phenomena, such as uranium roll-front and selenium accumulation at the redox interface. Measuring both geochemical and soil water variables allows for the calculation of in situ solute inputs into the groundwater and their reaction rates
Collective Excitations in Nuclei with ab initio Methods
No full textCollective excitations offer a unique perspective of the structure of nuclei and give crucial insights into their properties. An elegant standard observable to probe collective excitations are transition strengths. They depend on the detailed form of the wave function, are accessible in experiments and, thus, allow for a verification of the applied theory and the underlying interactions.
Two different approaches for the investigation of collective excitations from low-lying excitations to the giant-resonance regime are employed throughout this thesis. The random-phase approximation (RPA), which was developed in the 1950s in quantum chemistry, has been established as a standard tool for the description of transitions strengths. The RPA is based on the particle-hole (ph) formalism, which allow for the construction of excited states via ph and hp excitations of the ground state in terms of excitation creation operators. The aforementioned operators serve as starting point for the derivation of conditional equations by utilization of the variational principle and the equations-of-motion method. An extension of the RPA, the so-called second-order RPA (SRPA), includes additional 2p2h (de-)excitations besides the ph (de-)excitations. Beyond the RPA, we address a second access to collective excitations, which combines the importance-truncated no-core shell model (IT-NCSM) with the Lanczos strength-function method by Whitehead. Within this framework, an eigenstate of a previous IT-NCSM calculation and the Hamiltonian serve as starting point. With the help of the simple Lanczos algorithm, a tridiagonal matrix is produced. Standard techniques are then utilized in order to diagonalize this matrix. Based on the obtained eigenvalues and states in the Lanczos basis, the strength distributions can be calculated.
In this thesis, we focus on the extension of the (S)RPA to different input single-particle bases and to the inclusion of ground-state correlations from the in-medium similarity renormalization group. We present the electric monopole, dipole, and quadrupole transition strengths of selected oxygen and calcium isotopes and examine the model space convergence for several truncation parameters for different single-particle bases. The second-order RPA shows the pathological behavior of an artificial energy shift to lower energies compared to first-order RPA, independently of the underlying single-particle basis. Moreover, the transition strengths from first- and second-order RPA with matrix elements from the in-medium similarity renormalization group lie at higher energies compared to RPA using standard single-particle bases, which causes the instabilities to disappear. Furthermore, the electric monopole and quadrupole responses of different oxygen and helium isotopes are explored with the Lanczos strength function method comparing different interactions from chiral effective field theory. Due to the sensitivity of strength distributions to the details of the wave function, those observables provide a testing ground for state-of-the-art chiral interactions. Both approaches are compared with regard to fragmentation, fine structure, and their limitations.
In recent years, the uncertainty estimation of theoretical observables has received close attention in order to achieve a better comparability to experimental data. Thus, we present an uncertainty estimation for the running sum of discrete electric monopole, dipole, and quadrupole strengths based on the chiral expansion of the interaction and rooted in Bayesian statistics
Tapered Cross‐Section Photoelectron Spectroscopy of State‐of‐the‐Art Mixed Ion Perovskite Solar Cells: Band Bending Profile in the Dark, Photopotential Profile Under Open Circuit Illumination, and Band Diagram
No full textThe purpose of this article is twofold. On the one hand the method of spacial resolved photoemission spectroscopy on small angle tapered cross‐sections (TCS) of complete devices is introduced to analyze simultaneously the chemical and electronic structure. On the other hand, a specific working principle of the analyzed cell type is revealed. Solar cells of 18% efficiency are prepared from a single precursor (FAPbI₃)₀.₈₅ (MAPbBr₃)₀.₁₅ with excess of 15% PbI₂. It is shown that TCS‐phototoelectron spectroscopy allows to determine the chemical composition as well as the potential distribution across the full device in the dark and in operation. The energy converting contact is the hole extraction back contact. Interestingly the photopotential in the analyzed cell type is predominantly created within the hole extraction layer and not in the n‐doped perovskite absorber. With the addition of measured core level to valence band maximum positions of the respective layers, TCS line scans lead to the band diagram for the full device. In addition, depth variations of the chemical composition are found: the bromide concentration increases while the iodide concentration is reduced near and within the mesoporous TiO₂ layer
When to Use Rectangular Waveforms in Dielectrophoresis Application to Increase Separation and Sorting Efficiency
No full textIn this study, the influence of using rectangular waveforms is comprehensively investigated on the separation and sorting efficiency of dielectrophoretic (DEP) processes. Besides positive effects on DEP experiments, cases of a diminished force due to rectangular waveforms are investigated and discussed. This investigation encompasses two primary experimental setups. First, microparticle‐focusing experiments are carried out using a pair of electrodes within a microfluidic channel. Second, separation experiments are performed using a macroscopic insulator‐based dielectrophoretic filter. The study reveals that harmonics of rectangular signals can have a positive impact on separation or sorting efficiency when compared to sinusoidal waveforms, provided that these harmonics contribute to the overall DEP force with the same sign. This positive effect is found to depend on the ratio between the applied fundamental frequency and the cross‐over frequency in the Clausius–Mossotti factor. However, violating related derived boundary conditions leads to negative effects and a decrease in the DEP net force
Numerical simulations of binary neutron star merger ejecta
No full textImproving numerical simulations of binary neutron star mergers (BNSM) is essential for advancing our understanding of and ability to interpret observations of these events.
We demonstrate that smoothed particle hydrodynamic simulations of BNSM yield differing outcomes for identical merger events. This thesis is the first study to explore these variations, considering the possibility that they may be physical stochastic effects arising from small initial perturbations. We perform simulations of a 1.35-1.35 solar mass binary system with the same physical and numerical setup and discuss the observed variation in ejecta properties and the post-merger gravitational wave signal. We find an antiproportional correlation between the ejecta mass and the amplitude of the main feature fₚₑₐₖ of the gravitational wave frequency spectrum. We show that these fluctuations depend on how the double core structure evolves after merging. Our simulations do not yield a reduction of these variations when increasing the resolution, which might be an indication that these fluctuations are a physical effect.
To make reliable predictions for the electromagnetic signal of merger events, it is necessary to evolve the mass-outflows on timescales which are significantly longer than the merger itself. Over this time, the ejecta dilutes and cools down over several orders of magnitude.
We present a method to extend tabulated equation of states (EoS) to encompass lower rest-mass densities and temperatures. We investigate the issue that outflowing matter evolve towards negative internal energies in regions of low resolution in our simulations. We improve the discretization of the general-relativistic energy evolution equation to reduce the occurrence of this issue.
Utilizing the extended EoS and the improved discretization of the energy equation, we perform a long-term BNSM simulation up to 250 milliseconds after merger. We estimate how much material reaches homologous expansion within this time. Our analysis indicates that the majority of the material expelled during the first 25 milliseconds nearly exhibits homologous expansion, and we estimate an upper bound of 10 % for the alteration in its radial velocity. For later ejecta, we observe that they attain a reduced velocity, requiring a duration on the order of seconds to reach homologous expansion. BNSM simulations of this length require a sufficient resolution of the expanding ejecta.
We present a method to efficiently increase the resolution of BNSM ejecta in smoothed particle hydrodynamic simulations. We implement particle splitting, test different splitting criteria, and conduct a comparative study between simulations with and without particle splitting. Our findings indicate that the resolution of the BNSM ejecta can be enhanced by a factor of five using our proposed method while maintaining similar computational expenses
Versatile Bunch Length Measurement Setup for Femtocoulomb Bunch Charges at the S-DALINAC
No full textFor optimization of the bunch length of the beam at the superconducting Darmstadt electron linear accelerator (S-DALINAC), a new measurement setup was designed and constructed. Aluminum-coated Kapton screens are inserted into the beamline to emit optical transition radiation (OTR) when impacted by electrons. A streak camera is used to measure the duration of light pulses created from these screens, which is identical to the bunch length of the respective electron bunches at large kinetic energies. An optical system based on mirrors is used to transport the light pulses to the streak camera and maintain the high resolution of the device.
Two commissioning experiments with the new bunch length measurement setup were performed close to the entrance of the main linear accelerator (linac). A sweep of the electric field amplitude of the buncher located in the injector section was performed, where the empirically determined setting for a minimum bunch length was verified. In the second experiment, the arc between the injector linac and main linac was investigated for its capabilities as a bunch compressor to achieve even smaller bunch lengths. For a specific injector linac setting as well as non-zero longitudinal dispersion of the arc, a minimum bunch length value with a low momentum spread of the electrons in the bunch could be determined. The achieved time resolution of the setup was found to be competitive with eleven other electron accelerators. With the versatility of the setup allowing for bunch length measurements at any location of the accelerator with an OTR screen, the presented measurement method was significantly improved. In the regime of low bunch charges and small bunch lengths, the merit of the new bunch length measurement setup was proven.
The S-DALINAC can be operated as an energy-recovery linac (ERL), which provides significant reduction in external power demand during operation. A proposed future accelerator with higher beam currents and kinetic energies per electron shall incorporate this technique. A concept study of an existing injector for this future ERL will be presented, where the impact of the buncher on the beam quality was investigated. Here, it was shown that the injector lattice fulfills the required specifications for the ERL and that the buncher can be used to fine-tune the beam quality