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The current demand of high powered portable electronics and electric vehicles has put significant strain on current battery technology. To improve performance of advanced battery technologies, such as lithium-air, it is critical to understand how reactive species transport and distribute throughout the cell at varying states of charge. Neutron and x-ray imaging techniques allow in-situ visualization of reactants within the battery. X-ray attenuation increases with increasing atomic number while neutron attenuation is more random with strong attenuation by lithium. These techniques can provide complementary data by providing critical microstructure information. To provide access to both techniques, NIST is developing a simultaneous neutron and x-ray imaging facility. The new system orients a 90 keV microfocus x-ray source 90° to the neutron beam to allow for simultaneous tomography as shown in Figure 1. This x-ray system will allow for variable energy, brilliance, and magnification while matching the neutron spatial resolution of 15-50 micron. Matching image resolution of the two beams will allow the overlaying of the two tomograms. Simultaneously imaging batteries will allow reaction species, such as lithium, and solid structure to be resolved at identical conditions which can’t be guaranteed for serial imaging (possibly at different facilities). This presentation will describe the new simultaneous imaging system and evaluate its measurement uncertainties while assessing its abilities using an assortment of batteries including alkaline, Ni-Cd, and Li-ion. Figure 1

The water sorption of proton-exchange membranes (PEMs) was measured in situ using high-resolution neutron imaging in small-scale fuel cell test sections. A detailed characterization of the measurement uncertainties and corrections associated with the technique is presented. With high-resolution neutron-imaging detectors, the water distributions across N1140 and N117 Nafion membranes are resolved in vapor-sorption experiments and during fuel cell and hydrogen-pump operation and measurements are in reasonable agreement with a 1-D macrohomogeneous model. The measured in situ water content of a restricted membrane at 80 °C is shown to agree with ex situ gravimetric measurements of free-swelling membranes over a water activity range of 0.5 to 1.0 including at liquid equilibration. Schroeder’s paradox was verified by in situ water-content measurements which go from a high value at supersaturated or liquid conditions to a lower one with fully saturated vapor. At open circuit and during fuel cell operation, the measured water content indicates that the membrane is operating between the vapor- and liquid-equilibrated states.

Neutron imaging technique was performed to visualize the water distribution and movement at flow channels on operating proton exchange membrane fuel cell (PEMFC). According to operation conditions (inlet humidity ratio, temperature, and flow field geometry), the phenomena of water content and movement are investigated and their effects for the fuel cell efficiency are discussed

Publisher’s Note: This paper, originally published on 5/13/2015, was replaced with a corrected/revised version on 7/1/2015. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance. Plants exhibit complex responses to changes in environmental conditions such as radiant heat flux, water quality, airborne pollutants, soil contents. We seek to utilize the natural chemical and electrophysiological response of plants to develop novel plant-based sensor networks. Our present work focuses on plant responses to high-energy radiation – with the goal of monitoring natural plant responses for use as benchmarks for detection and dosimetry. For our study, we selected a plants cactus, Arabidopsis, Dwarf mango (pine), Euymus and Azela. We demonstrated that the ratio of Chlorophyll a to Chlorophyll b of the leaves has changed due to the exposure gradually come back to the normal stage after the radiation die. We used blue laser-induced blue fluorescence-emission spectra to characterize the pigment status of the trees. Upon blue laser excitation (400 nm) leaves show a fluorescence emission in the red spectral region between 650 and 800nm (chlorophyll fluorescence with maxima near 690nm and 735 nm). Sample tree subjects were placed at a distance of 1m from NIST-certified 241AmBe neutron source (30 mCi), capable of producing a neutron field of about 13 mrem/h. This corresponds to an actual absorbed dose of ~ 1 mrad/h. Our results shows that all plants are sensitive to nuclear radiation and some take longer time to recover and take less. We can use their characteristics to do differential detection and extract nuclear activity information out of measurement results avoid false alarms produced environmental changes. Certainly the ultimate verification can be obtained from genetic information, which only need to be done when we have seen noticeable changes on plant optical spectra, mechanical strength and electrical characteristics.

The neutron is electrically neutral, but its substructure consists of charged quarks so it may have an internal charge distribution. In fact it is known to have a negative mean square charge radius (MSCR), the second moment of the radial charge density. In other words the neutron has a positive core and negative skin. In the first Born approximation the neutron MSCR can be simply related to the neutron-electron scattering length b_ne. In the past this important quantity has been extracted from the energy dependence of the total transmission cross-section of neutrons on high-Z targets, a very difficult and complicated process. A few years ago S.A. Werner proposed a novel approach to measuring b_ne from the neutron's dynamical phase shift in a perfect crystal close to the Bragg condition. We are conducting an experiment based on this method at the NIST neutron interferometer which may lead to a five-fold improvement in precision of b_ne and hence the neutron MSCR., Comment: 5 pages, 2 figures

A measurement of the $n\text{\ensuremath{-}}^{3}\mathrm{He}$ coherent scattering length using neutron interferometry is reported. The result, ${b}_{c}=(5.8572\ifmmode\pm\else\textpm\fi{}0.0072)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$, improves the measured precision of any single measurement of ${b}_{c}$ by a factor of eight; the previous world average, ${b}_{c}=(5.74\ifmmode\pm\else\textpm\fi{}0.04)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$, now becomes ${b}_{c}=(5.853\ifmmode\pm\else\textpm\fi{}0.007)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$. Measurements of the $n\text{\ensuremath{-}}p$, $n\text{\ensuremath{-}}d$, and $n\text{\ensuremath{-}}^{3}\mathrm{He}$ coherent scattering lengths have now been performed using the same technique, thus allowing one to extract the scattering length ratios: parameters that minimize systematic errors. We obtain values of ${b}_{n^{3}\mathrm{He}}∕{b}_{np}=(\ensuremath{-}1.5668\ifmmode\pm\else\textpm\fi{}0.0021)$ and ${b}_{nd}∕{b}_{np}=(\ensuremath{-}1.7828\ifmmode\pm\else\textpm\fi{}0.0014)$. Using the new world average value of ${b}_{c}$ and recent high-precision spin-dependent scattering length data also determined by neutron optical techniques, we extract new values for the bound singlet and triple scattering lengths of ${b}_{0}=(9.949\ifmmode\pm\else\textpm\fi{}0.027)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$ and ${b}_{1}=(4.488\ifmmode\pm\else\textpm\fi{}0.017)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$ for the $n\text{\ensuremath{-}}^{3}\mathrm{He}$ system. The free nuclear singlet and triplet scattering lengths are ${a}_{0}=(7.456\ifmmode\pm\else\textpm\fi{}0.020)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$ and ${a}_{1}=(3.363\ifmmode\pm\else\textpm\fi{}0.013)\phantom{\rule{0.3em}{0ex}}\mathrm{fm}$. The coherent scattering cross section is ${\ensuremath{\sigma}}_{c}=(4.305\ifmmode\pm\else\textpm\fi{}0.007)\phantom{\rule{0.3em}{0ex}}\mathrm{b}$ and the total scattering cross section is ${\ensuremath{\sigma}}_{s}=(5.837\ifmmode\pm\else\textpm\fi{}0.014)\phantom{\rule{0.3em}{0ex}}\mathrm{b}$. Comparisons of ${a}_{0}$ and ${a}_{1}$ to the only existing high-precision theoretical predictions for the $n\text{\ensuremath{-}}^{3}\mathrm{He}$ system, calculated using a resonating group technique with nucleon-nucleon potentials incorporating three-nucleon forces, have been performed. Neutron scattering length measurements in few-body systems are now sensitive enough to probe small effects not yet adequately treated in present theoretical models.

In this paper, we will discuss a phase-contrast imaging method that avoids the complications of interferometry to provide phase contrast in weakly absorbing samples. A transversely coherent neutron beam is used with the traditional radiography scheme. Images taken with this scheme show dramatic intensity variations due to sharp changes in the neutron wave refractive index. With some numerical processing these images may be used to reconstruct a quantitative phase radiograph of specimens imaged with this technique.

We have performed high-precision measurements of the coherent neutron scattering lengths of gas phase molecular hydrogen and deuterium using neutron interferometry. After correcting for molecular binding and multiple scattering from the molecule, we find ${b}_{\mathrm{np}}=(\ensuremath{-}3.7384\ifmmode\pm\else\textpm\fi{}0.0020)\mathrm{fm}$ and ${b}_{\mathrm{nd}}=(6.6649\ifmmode\pm\else\textpm\fi{}0.0040)\mathrm{fm}.$ Our results are in agreement with the world average of previous measurements, ${b}_{\mathrm{np}}=(\ensuremath{-}3.7410\ifmmode\pm\else\textpm\fi{}0.0010)\mathrm{fm}$ and ${b}_{\mathrm{nd}}=(6.6727\ifmmode\pm\else\textpm\fi{}0.0045)\mathrm{fm}.$ The new world averages for the $n\ensuremath{-}p$ and $n\ensuremath{-}d$ coherent scattering lengths, including our new results, are ${b}_{\mathrm{np}}=(\ensuremath{-}3.7405\ifmmode\pm\else\textpm\fi{}0.0009)\mathrm{fm}$ and ${b}_{\mathrm{nd}}=(6.6683\ifmmode\pm\else\textpm\fi{}0.0030)\mathrm{fm}.$ We compare ${b}_{\mathrm{nd}}$ with the calculations of the doublet and quartet scattering lengths of several nucleon-nucleon potential models and show that almost all known calculations are in disagreement with the precisely measured linear combination corresponding to the coherent scattering length. Combining the world data on ${b}_{\mathrm{nd}}$ with the modern high-precision theoretical calculations of the quartet $n\ensuremath{-}d$ scattering lengths recently summarized by Friar et al., we deduce a new value for the doublet scattering length of ${}^{2}{a}_{\mathrm{nd}}=[0.645\ifmmode\pm\else\textpm\fi{}0.003(\mathrm{expt})\ifmmode\pm\else\textpm\fi{}0.007(\mathrm{theory})]\mathrm{fm}.$ This value is a factor of 4, more precise than the previously accepted value of ${}^{2}{a}_{\mathrm{nd}}=[0.65\ifmmode\pm\else\textpm\fi{}0.04(\mathrm{expt})]\mathrm{fm}.$ The current state of knowledge of scattering lengths in the related $p\ensuremath{-}d$ system, ideas for improving by a factor of 5 the accuracy of the ${b}_{\mathrm{np}}$ and ${b}_{\mathrm{nd}}$ measurements using neutron interferometry, and possibilities for further improvement of our knowledge of the coherent neutron scattering lengths of ${}^{3}\mathrm{H},$ ${}^{3}\mathrm{He},$ and ${}^{4}\mathrm{He}$ are discussed.

The program in fundamental neutron physics at the National Institute of Standards and Technology (NIST) began nearly two decades ago. The Neutron Interactions and Dosimetry Group currently maintains four neutron beam lines dedicated to studies of fundamental neutron interactions. The neutrons are provided by the NIST Center for Neutron Research, a national user facility for studies that include condensed matter physics, materials science, nuclear chemistry, and biological science. The beam lines for fundamental physics experiments include a high-intensity polychromatic beam, a 0.496 nm monochromatic beam, a 0.89 nm monochromatic beam, and a neutron interferometer and optics facility. This paper discusses some of the parameters of the beam lines along with brief presentations of some of the experiments performed at the facilities.

We have performed the first high precision measurement of the coherent neutron scattering length of deuterium in a pure sample using neutron interferometry. We find b_nd = (6.665 +/- 0.004) fm in agreement with the world average of previous measurements using different techniques, b_nd = (6.6730 +/- 0.0045) fm. We compare the new world average for the nd coherent scattering length b_nd = (6.669 +/- 0.003) fm to calculations of the doublet and quartet scattering lengths from several modern nucleon-nucleon potential models with three-nucleon force (3NF) additions and show that almost all theories are in serious disagreement with experiment. This comparison is a more stringent test of the models than past comparisons with the less precisely-determined nuclear doublet scattering length of a_nd = (0.65 +/- 0.04) fm., Comment: 4 pages, 4 figures

To assess the ability of baseline MRI characteristics to predict the early development of clinically definite MS (CDMS) and combined CDMS/MRI outcomes in 190 patients with a positive MRI at the time of their first demyelinating event.Based on individual and sets of baseline MRI characteristics, the authors evaluated the percentage of patients meeting outcomes of CDMS and various combined CDMS/MRI outcomes by 18 months. They also optimized a cutpoint for dichotomizing each baseline MRI characteristic and evaluated these variables using logistic regression to determine which MRI characteristics best predicted CDMS by 18 months.The presence of two or more gadolinium-enhancing lesions better predicted the development of CDMS and combined CDMS/MRI outcomes by 18 months than any other individual MRI characteristic or set of MRI characteristics. Among patients with two or more gadolinium-enhancing lesions, 52% developed CDMS compared with 24% of patients with fewer than two lesions. For those meeting the criteria of Barkhof et al., 32% of patients developed CDMS compared with 16% of those not meeting these criteria. Irrespective of individual or sets of criteria, however, the majority of patients developed either CDMS or demonstrated disease activity on brain MRI by 18 months.For patients with positive MRI at the time of their initial neurologic event, both gadolinium-enhancing lesions and the Barkhof criteria are predictors for development of CDMS over a short interval. However, these results, based on a combined CDMS/MRI outcome, suggest that the majority of these patients are already in the earliest stages of MS, regardless of whether any further MRI criteria are met.

The interaction of neutrons with matter enables neutron radiography1 to complement X-ray radiography in analysing materials. Here we describe a simple quantitative method that provides a new contrast mechanism for neutron radiography and allows samples to be imaged at low radiation doses. Large phase shifts can be measured accurately from detailed structures not amenable to conventional techniques.

The neutron interferometry technique provides direct access to phase of the neutron wave. If properly applied, the use ofphase information in imaging materials can provide an advantage over normal intensity techniques, which use absorption toprovide image contrast. The National Institute of Standards and Technology Neutron Interferometry and Optics Facility hasbeen working on new methods and devices to exploit the neutron phase information to image materials. We will present herethe latest results ofthis effort and an overview ofthe instruments available at NIST for Neutron Phase Contrast Imaging.Keywords: neutron imaging, phase contrast imaging, neutron interferometry 1. INTRODUCTION The non-destructive imaging methods of neutron radiography and tomography have been applied to numerous problemsinvolving the imaging of materials'. These methods rely on relative changes in the intensity of the scattered beam, due toabsorption and scattering in the imaging material. In some materials this change in intensity is too low to produce significantcontrast between components in the sample which is being imaged. In such cases it might be possible to use the phase of thecoherently scattered wave to greatly enhance the contrast in the sample. Other authors have used both neutrons35, and x-rays9, to image materials with phase contrast techniques.Here we are interested in how the neutron wave is phase shifted by materials. Neutrons passing through matter can bescattered coherently and incoherently as well as absorbed. Generally the macroscopic manifestation of this process may bedescribed by a complex index ofrefraction46 given by,n =

A Reply to the Comment by C. A. Chatzidimitriou-Dreismann et al. (c) 2000 The American Physical Society.

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A method is demonstrated for increasing the effective temporal resolution of neutron imaging, by averaging a consistent time window across multiple PEMFC operational cycles. Rapid water transients are captured during the current density and humidity changes of a cell durability protocol. The response of different cell components is compared by in-plane imaging, while temperature variation and upstream effects are observed by through-plane imaging of larger cells.

The influence of gas diffusion layer (GDL) modifications by laser perforation technique on fuel cell water management is investigated in this study. The liquid water distribution in membrane-electrode-assembly (MEA) and water breakthrough in GDL are visualized using high resolution neutron radiography. The perforated GDL reduces water transport resistance, enhances water breakthrough in GDL, and promotes uniform water distribution in MEA. The observed effects account for the improved fuel cell performance caused by the perforated GDL.

Polarized neutron interferometry has been used to measure the amplitudes for the exchange of up to five photons between the neutron and an oscillating magnetic field. The Rabi spin flip configuration and a configuration in which spin flip is suppressed were investigated, because they raise different questions on the conservation of angular momentum. Because of the linearity of the time-dependent interference signal in the exchange amplitudes, photon exchange probabilities below 1% could be resolved.

The water sorption of proton-exchange membranes (PEMs) was measured in situ using high-resolution neutron imaging in small-scale fuel cell test sections. A detailed characterization of the measurement uncertainties and corrections associated with the technique is presented. An image-processing procedure resolved a previously reported discrepancy between the measured and predicted membrane water content. With high-resolution neutron-imaging detectors, the water distributions across N1140 and N117 Nafion membranes are resolved in vapor-sorption experiments and during fuel cell and hydrogen-pump operation. The measured in situ water content of a restricted membrane at 80 °C is shown to agree with ex situ gravimetric measurements of free-swelling membranes over a water activity range of 0.5 to 1.0 including at liquid equilibration. Schroeder’s paradox was verified by in situ water-content measurements which go from a high value at supersaturated or liquid conditions to a lower one with fully saturated vapor. At open circuit and during fuel cell operation, the measured water content indicates that the membrane is operating between the vapor- and liquid-equilibrated states.

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Most modeling work of the polymer electrolyte membrane fuel cell (PEMFC) assumes an equilibrium relationship between membrane water content and proton conductivity. However, evidence is provided which shows that deviations from the equilibrium relationship exist during the transient hydration and dehydration cycles. Although Nafion® membranes follow a nearly symmetric water uptake and loss, dry membranes gain conductivity much faster than they absorb water while wet membranes lose conductivity at about the same rate they desorb water. This asymmetric behavior should be accounted for when predicting the transitional behavior of PEMFCs.

This paper shows how neutron radiography can be used for in situ quantification of the lithium concentration across battery electrodes, a critical physical system state. The change in lithium concentration between the charged and discharged states of the battery causes a change in number of detected neutrons after passing through the battery. Electrode swelling is also observed during battery charging. The experimental setup and the observations from testing a pouch cell with LFP cathode and graphite anode are reported here. The bulk Li concentration across the electrode and folds of the pouch cell is quantified at various states of charge. To interpret the measurements, the optics of the neutron beam (geometric unsharpness) and detector resolution are considered in order to quantify the lithium concentration from the images due to the thinness of the electrode layers. The experimental methodology provides a basis for comprehensive in situ metrology of bulk lithium concentration.

Evidence of a region of gradual property change between the micro-porous layer and the macroporous layer of bilayer diffusion media is presented, and a mathematical model describing the effects of this gradual interfacial region is developed. The model results in a continuous liquid water saturation distribution across the diffusion media compared to the sharp discontinuity in the liquid phase saturation predicted by the earlier sudden interface models. High-resolution neutron radiography is used to measure the water content profile across the diffusion media, and the results are compared with the model predictions. The effect of the geometric blur, and uncertainty in-neutron radiography, is accounted for by applying the effects of geometric blur to model results. When the blur is considered, the neutron radiography results are found to be very similar to model predictions.

A fundamental experimental and numerical study of the water transport across a perfluorosulfonic acid membrane under a temperature gradient is presented. The water transport phenomenon was experimentally investigated through water flux measurement and neutron radiography. The experimental observations found that water is transported in the direction from the high temperature side to the low temperature side, when both sides of the membrane are sufficiently humidified, and suggest that the transport mechanism is concentration gradient driven. The neutron radiography measurements detected the presence of water content gradient across the membrane and higher water content is seen at a larger thermal gradient. A numerical model was developed to investigate the experimental results. Water transport predictions agreed qualitatively but more accurate material and transport property characterizations are needed for further improvement.

While ionic polymer metal composites (IPMCs) have been studied for more than 10 years, the specific actuation mechanism is still unclear. In this work, neutron imaging, applied potential atomic force microscopy (APAFM) and current sensing atomic force microscopy (CSAFM) methods are employed to fundamentally investigate the actuation mechanism of this electroactive polymer system. Direct neutron imaging allowed a mapping of the water–counterion concentration gradient profile (i.e., a non-flat optical density profile sloping from the cathode to the anode) across an IPMC cross-section. While the neutron imaging method was capable of visualizing inside an operating IPMC, APAFM–CSAFM characterized changes in the nanoscale morphology and local surface properties due to redistribution of water–counterions under electrical stimulation. In APAFM, the darker, more energy dissipative features disappeared as the applied bias was varied from 0 V to 3 V, indicating that the surface became dehydrated. Surface dehydration undoubtedly supports the concept of proton and water migration to the negatively charged substrate. Water–counterion redistribution was further evidenced by CSAFM. With a negatively charged substrate (a 2 V bias), 2.8 pA of the average current were detected over the perfluorosulfonate ionomer (PFSI) surface in contact with AFM tip, which suggests the depletion of positively charged cations on the surface. On the contrary, a positively charged substrate (a −2 V bias) led to the average current of −90 pA over the PFSI surface in contact with the AFM tip, which indicates the formation of a cation-rich fluid on the top surface of the PFSI membranes. The observed water–counterion redistribution upon electrical stimulation directly supports a hydraulic contribution to the overall mechanism of actuation in IPMCs.

The generation and transport of water in both liquid and gas phases during device operation are important areas to understand both steady state and transient performance of proton exchange membrane fuel cells. Localized concentrations of liquid water within the cell can cause flooding, which leads to decreases in cell performance, fuel starvation, degradation, or, in extreme cases, collapse of cell output. A variety of experimental and simulation techniques have been used to elucidate flooding events; yet, a comprehensive understanding of what leads to flooding and the specific details of how flooding affects fuel cell performance, especially during transient operation, have not been completely developed. The work reported here couples direct observations of liquid water flooding, primarily in the gas flow channel, with measurements of cell performance, outlet temperature, and outlet dew point during a step change in current density. Liquid water buildup and water slug dynamics were monitored with the temperature and electrical performance of the cell in real time. The size of the water slugs was connected to the cell performance to illustrate how the liquid water influences cell operation and how the conditions of the cathode gas flow control the liquid water content of the cell.

This study investigated gas and liquid relative permeability, one of the important two-phase transport properties of the porous media used in proton exchange membrane fuel cells. The experiment showed that the gas relative permeability obtained by the gravimetric analysis and neutron imaging agreed at low saturation levels, but differs at high saturation levels. The liquid relative permeability in the saturation level range of 0.3 to 0.8 was measured by neutron imaging. Because of dead air pockets in the porous materials and certain saturation levels are required to establish continuous flow of liquid water through gas diffusion media, liquid permeability at both high and low saturation levels could not be obtained. It was shown that the 3rd-order power correlation of gas permeability generally used in numerical simulation only fitted the porous media of A1 and A3 at low saturation levels.

A surprising observation of the anomalous deep inelastic neutron scattering from liquid H2O–D2O mixtures was reported recently in PRL. It has also been suggested that such anomalous scattering can manifest itself as a deviation of the neutron scattering length density Nb from the conventional theory, based upon the assumption that the molecular volumes of the constituents are additive in the mixture. For H2O–D2O mixtures deviations as much as 5–10% are predicted from the conventionally expected value Nb. This fact would result in serious consequences for the widely used technique of contrast variation in neutron scattering. We have used an advanced neutron interferometry technique in order to verify these predictions and confirmed the experimental practice of isotopic contrast variation with high precision.

The neutron scattering cross section contains significant information about the internal electrical structure of the neutron. Recent achievements in neutron interferometry allow the precision measurements of the neutron scattering length (Δb/b∼10−5) at the crucial point E∼0, where the cross-section measurements are strongly influenced by small-angle scattering at the sample and are not very reliable.

We observed 4π-symmetry of the neutron wave function under rotation in space produced by slowly rotating an adiabatic guide field. In contrast to all previous works, the present experiment is of purely topological nature, i.e. it does not involve the Larmor precession and the result depends only on the topology and not on the value of the magnetic field and the neutron magnetic moment. This is the first experimental observation which cannot be explained by simply applying the common concept of wave optical dynamic phase, consistently with the Stern–Gerlach spin quantization, to each of the “spin-up” and “spin-down” energy eigenstates superposed in the neutron wave function.

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The durability of polymer electrolyte membrane (PEM) fuel cells operated at sub-freezing temperatures has received increasing attention in recent years. 1 The Department of Energy’s PEM fuel cell stack technical targets for the year 2010 include un-assisted startup from –40 o C and startup from -20 o C ambient in as low as 30 seconds with < 5MJ energy consumption. Moreover, the sub-freezing operations should not have any impact on achieving the other technical targets including 5000 hours durability. We have previously characterized the performance of both cloth (E-tek) and paper (SGL) GDLs and reported that the paper GDLs show lower tolerance to sub-freezing temperatures. 2 We have also reported the performance of MEAs at -10 o C where ice formation results in mass transport limitations. 3 Finally we reported on the effect of MEA preparation on durability and also provided low resolution neutron imaging data to identify ice formation in the land/channel areas. 4 In this paper we report the effect of MEA/GDL structure and cell assembly on sub-freezing performance. We also present highresolution neutron radiographs of single cells operated at sub-freezing temperatures to identify the exact location of ice formation and its effect on durability.

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The neutron interferometry and optics facility at the National Institute of Standards and Technology (NIST) has been in full operation since September 1996 as both a NIST research facility and as a national user facility. Unprecedented performance characteristics have been achieved in interferometry. A very busy schedule of experiments in fundamental physics, applied physics, and materials science has developed.

A proton exchange membrane fuel cell (PEMFC) was imaged using neutron radiography under pseudo steady-state operating conditions to determine the total liquid water content of the cell and the liquid water content distribution across the active cell area as a function of cell temperature, current density, and cathode air flow rate. A simple cathode-based model was formulated to rationalize the observed dry inlet regions which were most strongly influenced by temperature and current density. Between temperatures of 40 and 80°C and current densities of 0.5 and 1.5 A cm -2 , the outlet gas temperature was measured to be 1-5°C greater than the cell bulk temperature. This small temperature difference was enough to account for drying of 20-40% of the cell area, depending on the bulk cell temperature. For the cell construction used in this work, the temperature and cathode stoichiometric flow had a marginal effect on the polarization curve performance but had a large effect on the liquid water content and distribution within the cell.

2active area, Nafion 111-IP membrane, and carbon cloth gas diffusion layer. Even though dry hydrogen is supplied to the anode via pressure regulation, accumulation of liquid water in the anode gas distribution channels was observed in most tested conditions. Moreover, the accumulation of liquid water in the anode channels is followed by a significant voltage drop. Anode purges and cathode surges are also used as a diagnostic tool for differentiating between anode and cathode water flooding. The rate of accumulation of liquid water, and its impact on the rate of cell voltage drop is shown for a range of temperature, current density, cathode inlet RH, and air stoichiometric conditions. Operating the fuel cell under dead-ended anode conditions offers the opportunity to observe water dynamics and measured cell voltage during large and repeatable transients.

High-resolution neutron radiography was used to image an operating proton exchange membrane fuel cell in situ. The cross-sectional liquid water profile of the cell was quantified as a function of cell temperature, current density, and anode and cathode gas feed flow rates. Detailed information was obtained on the cross-sectional water content in the membrane electrode assembly and the gas flow channels. At low current densities, liquid water tended to remain on the cathode side of the cell. Significant liquid water in the anode gas flow channel was observed when the heat and water production of the cell were moderate, where both water diffusion from the cathode and thermal gradients play a significant role in determining the water balance of the cell. Within the membrane electrode assembly itself, the cathode side was moderately more hydrated than the anode side of the assembly from 0.1 to 1.25 A cm -2 . The total liquid water content of the membrane electrode assembly was fairly stable between current densities of 0.25 and 1.25 A cm -2 , even though the water in the gas flow channels changed drastically over this current density range. At 60°C, the water content in the center of the gas diffusion layer was depleted compared to the membrane or gas flow channel interfaces. This phenomenon was not observed at 80°C where evaporative water removal is prevalent.

Neutron imaging of a simulated non-isothermal start-up of a fuel cell is performed using the Neutron Imaging Facility at the National Institute of Standards (NIST) Center for Neutron Research. The cell is started from 40 ºC and heated up to operating temperature of 80 ºC using a predefined cell temperature profile. To ensure artifact free imaging, the cell hardware (i.e. end plates, which also contain the flow field and acts as current collector) is made of gold plated aluminum alloy, and multiple (two per plate) cartridge heaters are used to ensure uniformity of cell temperature. In order to ensure a constant inlet relative humidity, the humidifier heaters are also controlled to maintain a desired dew point temperature. It is shown that a relatively constant inlet relative humidity is achieved. Our results show that there is a lag between the total liquid water volume and the temperature profile.

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Here the authors introduce a Fourier based method for phase contrast neutron imaging, report its experimental implementation, and show results for a one-dimensional test phantom. This approach makes use of neutron interferometry to achieve both phase contrast and to spatially code the phase of the neutron with a linear phase grating. The spatial information is recovered from the coherent interference of this phase grating and a spatial phase distribution due to the sample. By moving neutron imaging from real to reciprocal space they avoid the need for position sensitive detectors and improve the potential image resolution.

The National Institute of Standards and Technology (NIST)maintains a sustained focus in the development of advanced neutron imaging facilities and hardware components to enable breakthrough research with vastly improved spatial and temporal resolutions, and to identify and employ research practices important to a wide variety of industrial and scientific applications. NIST's main focus is to enable research with broad appeal and commercial impacts. In this article we will give a brief overview of the NIST facility, select examples of current research, and finally comment on emerging technologies including advance manufacturing where neutron imaging has the potential to play an important role.

High-temperature heat pipes are two-phase, capillary driven heat transfer devices capable of passively providing high thermal fluxes. Such a device using a liquid-metal coolant can be used as a solution for successful thermal management on hypersonic flight vehicles. Imaging of the liquid-metal coolant inside will provide valuable information in characterizing the detailed heat and mass transport. Neutron imaging possesses an inherent advantage from the fact that neutrons penetrate the heat pipe metal walls with very little attenuation, but are significantly attenuated by the liquid metal contained inside. Using the BT-2 beam line at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, preliminary efforts have been conducted on a nickel-sodium heat pipe. The contrast between the attenuated beam and the background is calculated to be approximately 3%. This low contrast requires sacrifice in spatial or temporal resolution so efforts have since been concentrated on lithium (Li) which has a substantially larger neutron attenuation cross section. Using the CG-1D beam line at the High Flux Isotope Reactor (HFIR) of Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, the first neutron images of high-temperature molybdenum (Mo)-Li heat pipes have been achieved. The relatively high neutron cross section of Li allows for the visualization of the Li working fluid inside the heat pipes. The evaporator region of a gravity assisted cylindrical heat pipe prototype 25 cm long was imaged from start-up to steady state operation up to approximately 900 °C. In each corner of the square bore inside, the capillary action raises the Li meniscus above the bulk Li pool in the evaporator region. As the operational temperature changes, the meniscus shapes and the bulk meniscus height also changes. Furthermore, a three-dimensional tomographic image is also reconstructed from the total of 128 projection images taken 1.4o apart in which the Li had already cooled and solidified.

I F the change of government now meditated, can take place, with all our privileges preserved, let it instantly take place: but if they must be consumed in the blaze of royal authority, we shall pay too great a price...." 1 This declaration was the heart of John Dickinson's argument before the Pennsylvania Assembly on May 24, 1764. At issue was a plan, sponsored by such notables as Benjamin Franklin, Joseph Galloway, and John Hughes, to ask George III to replace the colony's proprietary system with a royal government. To his audience Dickinson's words came as an unpleasant surprise. Previously, he had vigorously disagreed with the proprietors. Now he was making his first public statement on the question of royal government. An odd mixture of conservative maxims and radical political doctrines, his speech was long and carefully prepared. When Dickinson finished, Joseph Galloway summarily and haughtily dismissed his ideas. On the following day the Assembly voted overwhelmingly in favor of the plan for royal government. Only in succeeding weeks and months did Pennsylvania's anti-proprietary leaders begin to realize the magnitude of the political whirlwind stirred up by their own program and the counterattacks upon it. Only in later years did some of them come to understand the meaning of Dickinson's warnings about the "blaze of royal authority" and the dangers it would bring to the colonies. John Dickinson's statements of 1764 demonstrated an awareness of the implications of recent British actions, an awareness that was in striking contrast to the shortsightedness of most Pennsylvania politicians. Yet for historians his words