The Role of Channel-Land Architecture, Diffusion Media Transport Properties, and Aging Effects on Water Transport and Storage in Polymer Electrolyte Fuel Cells

Thermally driven transport of water vapor in polymer electrolyte fuel cells, also known as the heat-pipe effect or phase-change-induced flow, can transport several times the generated amount of water given enough temperature differentials. Understanding this transport process is necessary to properly engineer the water balance in the fuel cell to ensure high performance and long operational life. Channel-land architecture, diffusion media heat and mass transport properties, and operational age can all have an influence on thermally driven flow. High resolution neutron imaging was used to determine the steady-state water accumulation in various cell configurations to understand the influence of these parameters. A novel non-dimensional parameter was proposed to predict the influence of engineering parameters on water balance. The thermal transport number (TTN) compares the strength of anode and cathode thermally driven flow to determine a bias for transport to the anode. Channel-land architecture, specifically asymmetric flow field patterns with larger anode lands, was found to pump water to the anode and allow for large accumulations of water with saturation approaching 60%. Water transport was facilitated to the anode by a delta-T inversion caused by the insulating effect of the cathode gas channel located opposite of the center of the large anode land. To limit anode water accumulation, a new experimental high diffusion resistance anode diffusion media was evaluated. This material was found to be effective at reducing anode water accumulation primarily due to the increased tortuosity of the material but also due to the higher thermal conductivity reducing the thermal transport effect. Material age was shown to reduce cell water content due to increased hydrophilic nature and increased thermal conductivity that increased from inlet to outlet with greater effects on the anode. A parametric study was performed to determine what parameters have the strongest influence on thermally driven transport. It was found that porosity and tortuosity of the diffusion media, and channel-land architecture can be engineered to drive water balance in a favorable direction. Temperature and thermal conductivity primarily influence the strength of thermal transport. This work demonstrated methods to properly design thermal management for optimized and predictive water transport

Three Phase-Grating Moire Neutron Interferometer for Large Interferometer Area Applications

We demonstrate a three phase-grating moire neutron interferometer in a highly intense neutron beam as a robust candidate for large area interferometry applications and for the characterization of materials. This novel far-field moire technique allows for broad wavelength acceptance and relaxed requirements related to fabrication and alignment, thus circumventing the main obstacles associated with perfect crystal neutron interferometry. We observed interference fringes with an interferometer length of 4 m and examined the effects of an aluminum 6061 alloy sample on the coherence of the system. Experiments to measure the autocorrelation length of samples and the universal gravitational constant are proposed and discussed.U.S. Department of CommerceNational Institute of Standards and TechnologyCanada Excellence Research Chairs, Government of CanadaNatural Sciences and Engineering Research Council of CanadaU.S. Department of EnergyCanada First Research Excellence Fun

Far-field interference of a neutron white beam and the applications to noninvasive phase-contrast imaging

© 2017 American Physical Society, https://dx.doi.org/10.1103/physreva.95.043637The phenomenon of interference plays a crucial role in the field of precision measurement science. Waveparticle duality has expanded the well-known interference effects of electromagnetic waves to massive particles. The majority of the wave-particle interference experiments require a near monochromatic beam which limits its applications due to the resulting low intensity. Here we demonstrate white beam interference in the far-field regime using a two-phase-grating neutron interferometer and its application to phase-contrast imaging. The functionality of this interferometer is based on the universal moire effect that allows us to improve upon the standard Lau setup. Interference fringes were observed with monochromatic and polychromatic neutron beams for both continuous and pulsed beams. Far-field neutron interferometry allows for the full utilization of intense neutron sources for precision measurements of gradient fields. It also overcomes the alignment, stability, and fabrication challenges associated with the more familiar perfect-crystal neutron interferometer, as well as avoids the loss of intensity due to the absorption analyzer grating requirement in Talbot-Lau interferometer.U.S. Department of CommerceNIST Center for Neutron ResearchNational Institute of Standards and TechnologyCanada Excellence Research Chairs, Government of Canada: 215284Natural Sciences and Engineering Research Council of Canad

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign

Abstract: In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 109 M ⊙. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87’s spectrum. We can exclude that the simultaneous γ-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the γ-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded

Fabrication of Black Body Grids by Thick Film Printing for Quantitative Neutron Imaging

Neutron imaging offers deep penetration through many high-Z materials while also having high sensitivity to certain low-Z isotopes such as 1H, 6Li, and 10B. This unique combination of properties has made neutron imaging an attractive tool for a wide range of material science and engineering applications. However, measurements made by neutron imaging or tomography are generally qualitative in nature due to the inability of detectors to discriminate between neutrons which have been transmitted through the sample and neutrons which are scattered by the sample or within the detector. Recent works have demonstrated that deploying a grid of small black bodies (BBs) in front of the sample can allow for the scattered neutrons to be measured at the BB locations and subsequently subtracted from the total measured intensity to yield a quantitative transmission measurement. While this method can be very effective, factors such as the scale and composition of the sample, the beam divergence, and the resolution and construction of the detector may require optimization of the grid design to remove all measurement biases within a given experimental setup. Therefore, it is desirable to have a method by which BB grids may be rapidly and inexpensively produced such that they can easily be tailored to specific applications. In this work, we present a method for fabricating BB patterns by thick film printing of Gd2O3 and evaluate the performance with variation in feature size and number of print layers with cold and thermal neutrons

Data-driven simulations for training AI-based segmentation of neutron images

Abstract Neutron interferometry uniquely combines neutron imaging and scattering methods to enable characterization of multiple length scales from 1 nm to 10 µm. However, building, operating, and using such neutron imaging instruments poses constraints on the acquisition time and on the number of measured images per sample. Experiment time-constraints yield small quantities of measured images that are insufficient for automating image analyses using supervised artificial intelligence (AI) models. One approach alleviates this problem by supplementing annotated measured images with synthetic images. To this end, we create a data-driven simulation framework that supplements training data beyond typical data-driven augmentations by leveraging statistical intensity models, such as the Johnson family of probability density functions (PDFs). We follow the simulation framework steps for an image segmentation task including Estimate PDFs $$\,\rightarrow \,$$ → Validate PDFs $$\,\rightarrow \,$$ → Design Image Masks $$\,\rightarrow \,$$ → Generate Intensities $$\,\rightarrow \,$$ → Train AI Model for Segmentation. Our goal is to minimize the manual labor needed to execute the steps and maximize our confidence in simulations and segmentation accuracy. We report results for a set of nine known materials (calibration phantoms) that were imaged using a neutron interferometer acquiring four-dimensional images and segmented by AI models trained with synthetic and measured images and their masks

Simultaneous neutron and X-ray tomography for ex-situ 3D visualization of graphite anode degradation in extremely fast-charged lithium-ion batteries.

Extreme fast charging (XFC) of commercial lithium-ion batteries (LIBs) in ≤10-15 minutes will significantly advance the deployment of electric vehicles globally. However, XFC leads to considerable capacity fade, mainly due to graphite anode degradation. Non-destructive three-dimensional (3D) investigation of XFC-cycled anodes is crucial to connect degradation with capacity loss. Here, we demonstrate the viability of simultaneous neutron and X-ray tomography (NeXT) for ex-situ 3D visualization of graphite anode degradation. NeXT is advantageous because of the sensitivity of neutrons to Li and H and X-rays to Cu. We combine the neutron and X-ray tomography with micron resolution for material identification and segmentation on one pristine and one XFC-cycled graphite anode, thereby underscoring the benefits of the simultaneous nature of NeXT. Our ex-situ results pave the way for the design of NeXT-friendly LIB geometries that will allow operando and/or in-situ 3D visualization of graphite anode degradation during XFC