Inhomogeneous Loss Research Articles

Effect of droplet diameter on oblique detonations with partially pre-vaporized n–heptane sprays

Liquid fuels have been widely used in propulsion systems for their higher energy density and easier storage, but there are few studies on oblique detonation engines (ODEs) using the liquid fuels. In this study, oblique detonation waves (ODWs) in partially pre-vaporized n–heptane sprays are simulated, using a hybrid Eulerian–Lagrangian algorithm with a skeletal chemical mechanism, to facilitate liquid fuels applications in ODE. A novel phenomenon has been observed, illustrating that the ODW initiation lengths in pre-vaporized n–heptane sprays firstly increase and then decrease with increasing fuel droplet diameters. It is found out that an increase of droplet diameter can increase the transferred heat amount caused by droplets evaporation and lower the inflow temperature, so leading to the increase of ODW initiation length firstly. However, extremely large droplet diameter can markedly reduce the heat transfer rate of the droplet and its surrounding mixture, and the compressed gas-stream behind oblique shock wave has a higher temperature that accelerate the induction chemical reaction and results in a decrease in the ODW initiation length. Furthermore, the unsteady oscillations of ODW initiation structures are also observed even for a steady inflow, which involves the periodic hot spots, normal detonation waves and their evolutions. Some factors causing the unsteady behavior of ODWs are tested through a sensitivity analysis method of the initiation lengths. Through the analyzation, the fluctuation of the post-shock temperatures is the main factor causing the fluctuation of initiation lengths, revealing that inhomogeneous heat loss by evaporation of dispersed liquid droplets is the main reason causing the unsteady behaviors of ODWs and the competition between heat release by chemical reactions and heat loss by evaporation determines the steadiness of ODWs.

Wigner dynamics for quantum gases under inhomogeneous gain and loss processes with dephasing

Wigner dynamics for quantum gases under inhomogeneous gain and loss processes with dephasing

Acoustic circular dichroism in a three-dimensional chiral metamaterial

Circular dichroism (CD) is an intriguing chiroptical phenomenon associated with the interaction of chiral structures with circularly polarized lights. Although the CD effect has been extensively studied in optics, it has not yet been demonstrated in acoustic systems. Here, we demonstrate the acoustic CD effect in a three-dimensional chiral metamaterial supporting circularly polarized transverse sound. We find that the effect is negligible in the lossy metamaterial possessing ${C}_{4}$ rotational symmetry but can be strongly enhanced in the ${C}_{2}$-symmetric system with inhomogeneous loss. The phenomena can be understood based on the properties of the metamaterial's complex band structure and the quality factors of its eigenmodes. We show that the enhanced CD in the ${C}_{2}$-symmetric system is attributed to the polarization band gaps and the non-Hermitian exceptional points appearing near the Brillouin-zone center and boundaries. The results contribute to the understanding of chiral sound-matter interactions and can find applications in acoustic sensing of chiral structures and sound manipulations based on vector properties.

Single charge control of localized excitons in heterostructures with ferroelectric thin films and two-dimensional transition metal dichalcogenides.

Single charge control of localized excitons (LXs) in two-dimensional transition metal dichalcogenides (TMDCs) is crucial for potential applications in quantum information processing and storage. However, traditional electrostatic doping method by applying metallic gates onto TMDCs may cause inhomogeneous charge distribution, optical quenching, and energy loss. Herein, by locally controlling the ferroelectric polarization of the ferroelectric thin film BiFeO3 (BFO) with a scanning probe, we can deterministically manipulate the doping type of monolayer WSe2 to achieve p-type and n-type doping. This nonvolatile approach can maintain the doping type and hold the localized excitonic charges for a long time without applied voltage. Our work demonstrated that the ferroelectric polarization of BFO can control the charges of LXs effectively. Neutral and charged LXs have been observed in different ferroelectric polarization regions, confirmed by magnetic optical measurement. Highly circular polarization degree with 90% photon emission from these quantum emitters was achieved in high magnetic fields. Controlling the single charge of LXs in a non-volatile way shows a great potential for deterministic photon emission with desired charge states for photonic long-term memory.

Influence of Flow Rate on Catalyst Layer Degradation in Polymer Electrolyte Fuel Cells

With the U.S. Department of Energy (DOE) 2020 durability target for transportation applications of 5000 hours in mind [1], several studies have been performed to understand the factors influencing load cycle durability of the cathode catalyst layer subjected to various operating conditions. In general, high upper potential limit (> 1 V vs RHE), high temperature (> 90 °C) and high relative humidity (RH) during potential cycling have been shown to exacerbate degradation [2]. It is known that Electrochemical surface area (ECSA) loss occurs by Pt dissolution attributed to particle size growth via modified Ostwald ripening, crystal migration, detachment from carbon support due to carbon corrosion, and precipitation in the membrane by chemical reduction due to hydrogen crossover [3].Accelerated stress tests (ASTs) are often used to mimic material component degradation similar to real automotive driving conditions. Different cycle profiles have been shown to cause varying rates of degradation depending on the operating conditions. The influence of flow rate of reactant gases on degradation, however, has received little attention. Stariha et al. [4], showed that a relatively high flow rate AST had the highest degradation acceleration factor and resulted in non-uniform decay rates when compared to low flow rate conditions of nitrogen gas (N2) at the cathode. However, this phenomenon has not been further studied in detail.The current study is aimed at elucidating the effect of flow rate on cathode catalyst layer degradation subjected to a standard DOE square-wave AST protocol. Single cell studies with 5-cm2 active area were performed using Nafion® XL membrane (Ion Power Inc.) and SGL-22 BB gas diffusion electrodes (GDEs) as MEA materials and 1-serpentine flow field. All the MEA samples used in the present study had nominally identical Pt loading and were subjected to a consistent conditioning procedure prior to AST. A DOE square-wave cycling protocol [4] was executed: H2/N2 flows at anode/cathode, voltage cycling between 0.6 V and 0.95 V, and 3 seconds hold at each voltage. 30,000 cycles (total test time of 50 hours) were performed at 100% RH, 80°C, and atmospheric pressure. Three different cathode N2 flow rates of 20 sccm, 40 sccm and 80 sccm were used during the AST with fixed H2 flow of 40 sccm at the anode. ECSA loss was tracked using cyclic voltammograms at several intervals during the AST. Complete electrochemical characterization at beginning and the end of life (BOL and EOL), along with ex-situ diagnostic techniques such as TEM and GIXRD, were used to obtain particle size distribution and spatial degradation profiles. The results indicate higher performance loss (at EOL) for 80 sccm flow of N2 at the cathode compared to 20 sccm and 40 sccm flow rates, most likely due to inhomogeneous Pt loss from inlet to the outlet due to variations in local flow conditions by fully saturated N2 flow at different flow rates.

Local degradation and differential voltage analysis of aged lithium-ion pouch cells

Abstract Knowledge of the underlying degradation mechanisms in a lithium-ion cell is crucial for the development process and control of automotive lithium-ion battery packs. In this paper, a local post-mortem degradation analysis is performed on two differently cycled large-format pouch cells to improve the understanding of the underlying degradation mechanisms. For each cell, anode and cathode sheets are extracted and electrode coins are cut out from the sheets in an equally spaced, five-by-five pattern to assemble three-electrode test cells. Subsequently, the local capacities and differential voltage curves of the test cells are determined showing an inhomogeneous loss of lithium inventory for one cell and a capacity degradation of the cathode for the other cell. In a parallel connection of the test cells, the influence of the degradation mechanisms on the pouch cell’s differential voltage curve is shown and an approach is validated for calculating the resulting differential voltage curve by superimposing the single measured curves. In addition, the local discharge resistance is evaluated, revealing for both aged cells that the measured resistance increase at pouch cell level is the result of an increased resistance of the cathode and a decreased anode resistance.

Non-Hermitian multimode interference.

Multimode interference (MMI) and self-imaging are important phenomena of diffractive optics with major applications in signal processing, beam shaping, and optical sensing. Such phenomena generally arise from interference of normal modes in lossless dielectric guiding structures; however, the impact of spatially inhomogeneous optical gain and loss, which break mode orthogonality and symmetries, has been overlooked. Here we consider MMI in non-Hermitian optical systems, either graded-index or coupled optical waveguide structures, and reveal distinctive features, such as the absence of mirror images and strong sensitivity of self-imaging to perturbations, making MMI in non-Hermitian waveguides of interest in optical sensing.

Development of an Electropolishing Process for Additively Manufactured 316L Parts Under Direct and Pulsed Potential Control

The initial purpose of additive manufacturing was mainly rapid prototyping, but all industrial sectors have adopted this technique that has increased in profitability and allows innovative shapes in almost all alloys. Advances in technology have led to AM components whose mechanical and structural properties are equivalent to machined parts. However, a major impediment to the extension of the process is due to the bad surface finish of the as built parts, characterized by a high roughness (Ra up to 40 µm) and the presence of un-melted particles and weld beads. These defects are incompatible with the planned applications (like inner channels for fluid flow), and also induce a degradation of the material functional properties such as an increased sensitivity to fatigue and corrosion. Electropolishing, an electrochemical method for planarization of metals based on their anodic dissolution in an appropriate electrolyte, is a solution to consider. During this work a specific electropolishing process dedicated to 316 L AM parts was developed with the originality of being under potentiostatic control [1,2]. First, a preliminary study done at laboratory scale in a three electrodes electrochemical cell, allowed us to determine the process parameters including the working potential range. Then, the same type of arrangement was subsequently up-scaled in a 3 L reactor in order to electrochemically polish bigger parts ( ≈40 cm2). The potential control is made possible thanks to the introduction of a reference electrode in the pilot and by the use of a high current/high voltage potentiostat able to monitor both direct and pulse potentials. When transposing the process at pilot scale, new difficulties are emerging, due to the large initial roughness of the parts. If a satisfactory roughness decrease is easily achievable in less than one hour, it goes with an inhomogeneous loss of dimension. This geometric deformation of the parts needs to be considered as least as much as roughness parameters. In this context, pulsed potential are emerging as an attractive way to performed electropolishing, by controlling the dissolution while getting interesting final surface properties and limited deformation [3]. Samples present a very bright finishing and low roughness values (Fig.1) In this study a special attention was paid to the quantification of the deformation on especially designed grooved test samples by 3D optical microscopy and laser scanning [4]. Acknowledgments : the authors would like to thank the IRT-M2P program AFTER ALM for its financial support. [1] Rotty C., Mandroyan A., Doche M.-L., Hihn J. Y., Surf. Coat. Technol., 307 Part A, 125–135 (2016) [2] Rotty C, Doche M.-L, Mandroyan A.,Hihn J-Y., Montavon G., Comparison of electropolishing behaviours of TSC, ALM and cast 316L stainless steel in H3PO4/H2SO4, Surfaces and Interfaces, 6, 170-176 (2017) [3] Taylor EJ., Sequential electromachining and electropolishing of metals and the like using modulated electric fields, US Patent 6,558,231 (2003) [4] C.M. Sulyma, P.C Goonetilleke, D.Roy, Journal of materials processing technology, 1189-1198 (2009) Figure 1

Ring defects in n-type Czochralski-grown silicon: A high spatial resolution study using Fourier-transform infrared spectroscopy, micro-photoluminescence, and micro-Raman

We investigated ring defects induced by a two-step anneal in n-type Czochralski-grown silicon wafers using a combination of high spatial resolution Fourier Transform Infrared Spectroscopy (FTIR), micro-photoluminescence (PL) mapping, and micro-Raman mapping. Through FTIR measurements, we show the inhomogeneous loss in interstitial oxygen with a positive correlation with the inverse lifetime. Using high-resolution micro-PL mapping, we are able to distinguish individual recombination-active oxygen precipitates within the rings with a decreasing density from the center to the edge of the sample. The radial inhomogeneity of the oxygen precipitates is likely to be related to variations in the distribution of grown-in defects. We also demonstrate that micro-Raman mapping reveals the oxygen precipitates without the smearing effects of carrier diffusion that are present in micro-PL mapping.

Nonadiabatic Modal Dynamics Around Exceptional Points in an All-Lossy Dual-Mode Optical Waveguide: Toward Chirality-Driven Asymmetric Mode Conversion

We report a 1D planar optical waveguide with transverse distribution of inhomogeneous loss profile, which exhibits an exceptional point (EP). The waveguide hosts two leaky resonant modes; where the interaction between them in the vicinity of the EP is controlled by proper adjustment of the inhomogeneity in attenuation profile only. We study the adiabatic dynamics of propagation constants of the coupled modes by quasi-static encirclement of control parameters around the EP. Realizing such an encirclement with the inhomogeneous loss distribution along the direction of light propagation, we report the breakdown of adiabatic evolution of two coupled modes through the waveguide in presence of an EP. Here, during conversion the output mode is irrespective of the choice of input excited mode but depends on the direction of light transportation. This topologically controlled, robust scheme of asymmetric mode conversion in the platform of the proposed all-lossy waveguide structure may open up an extensive way-out for implementation of state-transfer applications in chirality driven waveguide-based devices.

Inhomogeneous active layer contact loss in a cycled prismatic lithium-ion cell caused by the jelly-roll curvature

Inhomogeneous active layer contact loss in a cycled prismatic lithium-ion cell caused by the jelly-roll curvature.

Photon-pair generation in a quadratically nonlinear parity-time symmetric coupler

Integrated nonlinear waveguide structures enable generation of quantum entangled photons. We describe theoretically the effects of spatially inhomogeneous loss on the creation of photon pairs through spontaneous parametric down-conversion in quadratically nonlinear directional couplers, where photons experience effective parity-time (PT) symmetric potential due to the presence of optical loss in one of the waveguides. We show that for losses below the PT-breaking threshold, the quantum photon states can be flexibly tuned similarly to conservative couplers, whereas for stronger losses, the correlations between two waveguide modes are suppressed. We also formulate a quantum-classical correspondence with sum-frequency generation for fast evaluation of device performance. These results can be applied for the design of quantum plasmonic circuits.

Analytical and numerical investigations of the crack behavior in thermal barrier coatings under the trip thermal load

Increasing the temperature of the combustion products in power plant turbines is one of the easiest way to enhance the efficiency and decrease the fuel costs. Therefore, using thermal barrier coating (TBC) to protect the load carrying components in the hot gas path is inevitable. In this manuscript, the damage caused by trip shutdown in power plants as an emergency thermal shock is investigated based on analytical and finite element (FE) analysis for in-plane stress condition. The used geometry is a thin disk under axisymmetric condition. The results show that as a consequence of a fast and inhomogeneous temperature loss, a thermal strain of more than 1.1% occurs in the TBC top layer. Similar behavior is observed in the stress and thus the stress intensity factor of a preexisting surface crack because it approaches the critical value of the ceramic top layer. This condition has a significant effect on the length and growth rate of the crack in comparison with heating and constant temperature period as the crack length is about one- third of the ceramic layer thickness at the end of a single cycle. In addition, the analytical and the finite element calculation results match together and the determined crack behavior is compatible with the other researcher's output.

Degradation Study by Start-Up/Shut-Down Cycling of Superhydrophobic Electrosprayed Catalyst Layers Using a Localized Reference Electrode Technique.

Degradation of a polymer electrolyte membrane fuel cell (PEMFC) with electrosprayed cathode catalyst layers is investigated during cyclic start-up and shut-down events. The study is carried out within a single cell incorporating an array of reference electrodes that enables measurement of cell current as a function of local cathode potential (localized polarization curves). Accelerated degradation of the cell by start-up/shut-down cycling gives rise to inhomogeneous performance loss, which is more severe close to the gas outlet and occurs predominantly during start-up. The degradation consists primarily of loss of cathode catalyst activity and increase in cell internal resistance, which is attributed to carbon corrosion and Pt aggregation in both anode and cathode. Cells with an electrosprayed cathode catalyst layer show lower degradation rates during the first 100 cycles, compared with those of a conventional gas diffusion electrode. This difference in behavior is attributed to the high hydrophobicity of the electrosprayed catalyst layer microstructure, which retards the kinetics of corrosion of the carbon support. In the long term, however, the degradation rate is dominated by the Pt/C ratio in the cathode catalyst layer.

Multiwavelength passively mode-locked fiber laser with ultra-broad bandwidth

We demonstrate a stable multiwavelength passively mode-locked erbium-doped fiber laser with ultra-narrow wavelength spacing and ultra-broad bandwidth. It is realized by exploiting an intracavity birefringence-induced comb filter and inhomogeneous loss mechanism based on nonlinear polarization rotation. Stable 808-line wavelength lasing with wavelength spacing of 0.07nm and 3-dB bandwidth of 55.75nm is achieved at room temperature. If 5-dB bandwidth of 86.15nm is considered, the number of the lasing lines reaches 1248. In addition, the lasing wavelength lines and spacing could be flexibly tuned by modifying polarization-dependent cavity loss via properly rotating the waveplates.

Modulational instability in a [formula omitted]-symmetric vector nonlinear Schrödinger system

A class of exact multi-component constant intensity solutions to a vector nonlinear Schrödinger (NLS) system in the presence of an external PT-symmetric complex potential is constructed. This type of uniform wave pattern displays a non-trivial phase whose spatial dependence is induced by the lattice structure. In this regard, light can propagate without scattering while retaining its original form despite the presence of inhomogeneous gain and loss. These constant-intensity continuous waves are then used to perform a modulational instability analysis in the presence of both non-hermitian media and cubic nonlinearity. A linear stability eigenvalue problem is formulated that governs the dynamical evolution of the periodic perturbation and its spectrum is numerically determined using Fourier–Floquet–Bloch theory. In the self-focusing case, we identify an intensity threshold above which the constant-intensity modes are modulationally unstable for any Floquet–Bloch momentum belonging to the first Brillouin zone. The picture in the self-defocusing case is different. Contrary to the bulk vector case, where instability develops only when the waves are strongly coupled, here an instability occurs in the strong and weak coupling regimes. The linear stability results are supplemented with direct (nonlinear) numerical simulations.

Changing black carbon transport to the Arctic from present day to the end of 21st century

AbstractHere we explore how climate warming under the Representative Concentration Pathway 8.5 (RCP8.5) impacts Arctic aerosol distributions via changes in atmospheric transport and removal processes. We modify the bulk aerosol module in the Community Atmosphere Model to track distributions and fluxes of 200 black carbon‐like tracers emitted from different locations, and we conduct idealized experiments with and without active aerosol deposition. Changing wind patterns, studied in isolation, cause the Arctic burdens of tracers emitted from East Asia and West Europe during winter to increase about 20% by the end of the century while decreasing the Arctic burdens of North American emissions by about 30%. These changes are caused by an altered winter polar dome structure that results from Arctic amplification and inhomogeneous sea ice loss and surface warming, both of which are enhanced in the Chukchi Sea region. The resulting geostrophic wind favors Arctic transport of East Asian emissions while inhibiting poleward transport of North American emissions. When active deposition is also considered, however, Arctic burdens of emissions from northern midlatitudes show near‐universal decline. This is a consequence of increased precipitation and wet removal, particularly within the Arctic, leading to decreased Arctic residence time. Simulations with present‐day emissions of black carbon indicate a 13.6% reduction in the Arctic annual mean burden by the end of the 21st century, due to warming‐induced transport and deposition changes, while simulations with changing climate and emissions under RCP8.5 show a 61.0% reduction.

Phosphatidylserine-targeted molecular imaging of tumor vasculature by magnetic resonance imaging.

Phosphatidylserine (PS), normally restricted to the inner leaflet of the plasma membrane, becomes exposed on the outer surface of viable endothelial cells in tumor vasculature, but not in normal blood vessels. In the present study, we report the use of PGN635, a novel human monoclonal antibody that specifically targets PS, for in vivo molecular MRI of tumor vasculature. The F(ab')2 fragments of PGN635 were conjugated to polyethylene glycol (PEG) coated iron oxide nanoparticles (IO). Targeting specificity of the PS-targeted Nanoprobe, IO-PGN635F(ab')2 was first confirmed by in vitro MRI and histological staining. In vivo longitudinal MRI was then performed before and after i.v. injection of IO-PGN635F(ab')2 into mice bearing 4T1 breast tumors. T2-weighted MR images at 9.4 T revealed inhomogeneous signal loss in tumor as early as 2 h post injection. Furthermore, ionizing radiation induced a significant increase in PS exposure on tumor vascular endothelial cells, resulting in significantly enhanced and sustained tumor contrast (p < 0.05). Spatially heterogeneous MRI contrast correlated well with histological staining of tumor vascular endothelium. Our studies suggest that PS exposed within the lumen of tumor vasculature is a highly specific and useful biomarker for targeted MRI contrast agents.

Manipulating solitons by antisymmetric inhomogeneous loss in the complex Ginzburg–Landau model

We study the dynamics of dissipative spatial solitons by using antisymmetric-type spatially inhomogeneous loss that locally modulates an optical medium described by the complex Ginzburg–Landau model. We reveal soliton dynamics by launching input solitons at different positions of the spatial modulation loss profile, including its zero value and positive value. The soliton transverse drift exhibits tilt straightaway propagation except for the oscillation in initial stage. The tilt angle of the soliton drift can be controlled by the modulation parameters. We also investigate the dependences of the tilt angle of the soliton drift on the amplitude of the inhomogeneous loss and the linear loss coefficient.

Switchable multi-wavelength erbium-doped photonic crystal fiber laser based on nonlinear polarization rotation

A multi-wavelength erbium-doped photonic crystal fiber (ED-PCF) ring laser based on nonlinear polarization rotation was demonstrated experimentally. To the best of our knowledge, this study is the first to use a polarization-dependent isolator with an ED-PCF to form an equivalent fiber filter. The intensity-dependent inhomogeneous loss induced by nonlinear polarization rotation was applied to reduce the mode competition caused by ED-PCF. A detailed theoretical analysis using the Jones matrix method to calculate the transfer function of the laser was presented. The theoretical data, which were confirmed by the experimental results, showed that a multi-wavelength with a spacing of approximately 2.26nm was achieved by controlling the polarization controller. The proposed fiber laser with a signal-to-noise ratio higher than 40dB achieved a low threshold power of 30mW. Therefore, stable multi-wavelength outputs with a power fluctuation of less than 2dB were generated at room temperature.