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DOE OSTI2018

Design and operation of a high-heat flux, flush-mounted ‘rail’ Langmuir probe array on Alcator C-Mod.

A poloidal array of toroidally-extended, flush-mounted ‘rail’ Langmuir probes was recently installed on Alcator C-Mod's vertical target plate divertor. The aim was to investigate if a Langmuir probe array could be designed to survive reactor-level heat fluxes and have the ability to make measurements that could be reliably interpreted under reactor-level plasma densities, neutral densities and magnetic fields. Langmuir probes are typically built to have incident field-line angles less than 10 degrees to avoid interpretation issues associated with sheath expansion. However, at the high parallel heat fluxes experienced in reactor-relevant conditions such a probe would quickly overheat and melt. To mitigate both the issues of extreme heat flux and sheath expansion, each probe was designed to be flush with the divertor surface, toroidally-extended and field-aligned, giving it a ‘rail’ geometry. The flush mounted probes have proven to be exceptionally robust surviving the 2015–2016 campaign – a first for a C-Mod probe system. Examination of the probe current-voltage (I-V) characteristics reveals that they are immune to sheath expansion at incident field angles down to approximately 0.5 degrees. Comparison of the flush probes to traditional proud probes shows that both measure the same electron pressure across the divertor plate. However, there are significant and systematic differences in the density, temperature and floating potential. This suggests that there is important physics, perhaps unique to conditions in a vertical-target plate divertor with small field-line attack angles, that affects the I-V characteristics and is not currently included in probe data analyses. Finally, the probe response is examined in the ‘death-ray’ regime, just near detachment. Previous work using proud probes has suggested that the ‘death-ray’ is an artefact of the probe bias. However, on flush mounted probes the ‘death-ray’ manifests itself under different conditions, which may provide insight into this unique phenomenon.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

NASA NTRS1990

Infrared spectroscopy of comets

An observational search for cometary parent molecules using infrared spectroscopy was conducted in the 1 to 5 micron region. The investigation involved two different observing programs, one at moderate spectral resolution and one at fairly high resolution. The lower resolution was used to study cometary spectra in the vicinity of 3.5 micron at wavelength/change in wavelength is approximately or equal to 10(exp 3). Comets P/Brorsen-Metcalf (1989o), Okazaki-Levy-Rudenko (1989r), and Austin (1990c1) were observed with the Cryogenic Spectrometer (CRSP) at Kitt Peak. The detector incorporated an InSb array with 58 spatial elements, each 2.7 min on the sky, and 62 spectral channels per spatial element. An, as yet, unidentified feature was detected at approximately 3.52 micron in Comet Austin (on 1990 May 4, 5, and 6). The feature is possibly present in P/Brorsen-Metcalf (observed on 1989 August 23 and 25), as well. Comet Okazaki-Levy-Rudenko exhibited continuum emission only in this spectral region at the time of the observations (1989 November 14 and 16). The data are presented, and the relationship between the 3.52 micron feature and cometary activity (e.g., water production rate, visibility of the 3.4 micron emission feature) are discussed. The high resolution program probed comet Austin in the 4.8 micron region. These observations were used to search for emission lines comprising the (1-0) vibration-rotation band of the ground electronic state of CO. Retrieval of the lines allows a probe of the population distribution of levels J' = 1 through 4 of the excited (v' = 1) vibrational state within the ground electronic state of CO. Knowledge of this distribution can be used to constrain the rotational temperature. Preliminary analysis suggests the P3 line was present UT May 16 at roughly the 5 sigma level. Results concerning the existence of other lines, and physical conditions inferred therefrom are discussed.

Disanti, Michael A.↗

DOE OSTI2023

Probing individual single atom electrocatalyst sites by advanced analytical scanning transmission electron microscopy

Single atom electrocatalysts (SAEs) are promising next-generation materials for promoting a variety of important reactions, such as the oxygen reduction, nitrogen reduction, and CO 2 reduction reactions. While bulk characterization techniques such as X-ray absorption spectroscopy and Mössbauer spectroscopy have significantly enhanced our understanding of these catalysts, direct probing of individual single metal atom sites at the atomic scale is necessary to understand local variations in the properties of these sites and accelerate design and synthesis of improved SAEs. Aberration-corrected scanning transmission electron microscopy (STEM) has become a powerful tool for providing this type of atomic-scale information about SAE metal sites. These sites are typically unstable under the electron beam, however, which, in combination with conventional acquisition methods and detectors, has limited the type and quantity of information obtainable by spectroscopic STEM techniques. Here, we map multiple individual SAE metal sites in a nitrogen-doped carbon containing atomically dispersed Fe and Re (FeReNC) at the atomic scale by direct electron detection electron energy-loss spectroscopy (EELS). Direct electron detection provides an improved signal-to-noise ratio over conventional scintillator-based detectors and enables detection and real space localization of weak signals. In addition, we demonstrate an automated method for identification of metal atom positions, placement of the probe on these sites, and simultaneous EELS and energy dispersive X-ray spectroscopic (EDS) signal acquisition. This simultaneous acquisition of EELS and EDS provides access to the composition and bonding of a wide range of SAE metal sites. In this study, focusing the probe directly on the metal sites also increases the relevant data acquisition rate by more than an order of magnitude over two-dimensional mapping, enabling improved statistical measurements of site properties. The versatility, sensitivity, and speed that these techniques provide enhances our ability to probe the local elemental and chemical environment of a large number of individual SAE metal site structures at the atomic scale, enabling an improved understanding of the variations in the local properties of these electrocatalysts to be gained. As a result, significantly increased information about individual metal sites will be available to future electrochemical studies through these techniques, accelerating the development of advanced SAEs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Simplicity Beneath Complexity: Counting Molecular Electrons Reveals Transients and Kinetics of Photodissociation Reactions

Abstract Time‐resolved pump–probe gas‐phase X‐ray scattering signals, extrapolated to zero momentum transfer, provide a measure of the number of electrons in a system, an effect that arises from the coherent addition of elastic scattering from the electrons. This allows to identify reactive transients and determine the chemical reaction kinetics without the need for extensive scattering simulations or complicated inversion of scattering data. We examine the photodissociation reaction of trimethylamine and identify two reaction paths upon excitation to the 3p state at 200 nm: a fast dissociation path out of the 3p state to the dimethyl amine radical (16.6±1.2 %) and a slower dissociation via internal conversion to the 3s state (83.4±1.2 %). The time constants for the two reactions are 640±130 fs and 74±6 ps, respectively. Additionally, it is found that the transient dimethyl amine radical has a N−C bond length of 1.45±0.02 Å and a C−N−C bond angle of 118°±4°.

Ruddock, Jennifer M.↗

DOE OSTI2017

Dynamic microstructural evolution of an Al–Zn–Mg–Cu alloy (7075) during continuous heating and the influence on the viscoelastic response

Highlights: • A TEM/HRTEM, FIB-SEM and APT analysis of an Al–Zn–Mg–(Cu) alloy is performed. • Microstructural evolution is analyzed during constant heating rate heat-treatment. • Composition, size, number density and vol. fraction of present phases are reported. • Vol. fraction and kinetic data from rate equations in the literature are validated. • Viscoelastic behavior of the alloy is associated to microstructural transformations. - Abstract: The microstructural evolution of an Al–Zn–Mg–Cu alloy under continuous heating over the temperature range of 298 to 648 K (25 to 375 °C) is characterized by focused ion beam-scanning electron microscopy, transmission electron microscopy, and atom probe tomography. The chemical composition, dimensions, number density, and volume fraction of precipitates are measured comprehensively. Quantitative measurements of Guinier-Preston zones volume fraction and η′/η phase volume fraction are correlated to the viscoelastic response of the Al–Zn–Mg–Cu alloy with temperature in the studied range. This provides valuable evidence indicating that observed variations in the viscoelastic behavior of the Al–Zn–Mg–Cu alloy may be associated with phase transformations involving Guinier-Preston zones and phases η′ and η.

36 MATERIALS SCIENCE↗

DOE OSTI2016

125 Te NMR and Seebeck Effect in Bi 2 Te 3 Synthesized from Stoichiometric and Te-Rich Melts

Bi 2 Te 3 is a well-known thermoelectric material and, as a new form of quantum matter, a topological insulator. Variation of local chemical composition in Bi2Te3 results in formation of several types of atomic defects, including Bi and Te vacancies and Bi and Te antisite defects; these defects can strongly affect material functionality via generation of free electrons and/or holes. Nonuniform distribution of atomic defects produces electronic inhomogeneity, which can be detected by 125 Te nuclear magnetic resonance (NMR). Here we report on 125 Te NMR and Seebeck effect (heat to electrical energy conversion) for two single crystalline samples: (#1) grown from stoichiometric composition by Bridgman technique and (#2) grown out of Te-rich, high temperature flux. The Seebeck coefficients of these samples show p- and n-type conductivity, respectively, arising from different atomic defects. 125 Te NMR spectra and spin–lattice relaxation measurements demonstrate that both Bi 2 Te 3 samples are electronically inhomogeneous at the atomic scale, which can be attributed to a different Te environment due to spatial variation of the Bi/Te ratio and formation of atomic defects. In conclusion, correlations between 125 Te NMR spectra, spin–lattice relaxation times, the Seebeck coefficients, carrier concentrations, and atomic defects are discussed. Our data demonstrate that 125 Te NMR is an effective probe to study antisite defects in Bi 2 Te 3 .

36 MATERIALS SCIENCE↗

DOE OSTI2020

Solvent Mediated Excited State Proton Transfer in Indigo Carmine

Excited state proton transfer (ESPT) is thought to be responsible for the photostability of biological molecules, including DNA and proteins, and natural dyes such as indigo. However, the mechanistic role of the solvent interaction in driving ESPT is not well understood. Here, the electronic excited state deactivation dynamics of indigo carmine (InC) is mapped by visible pump-infrared probe and two-dimensional electronic-vibrational (2DEV) spectroscopy and complemented by electronic structure calculations. The observed dynamics reveal notable differences between InC in a protic solvent, D$_2$O , and an aprotic solvent, deuterated dimethyl sulfoxide (dDMSO). Notably, an acceleration in the excited state decay is observed in D$_2$O (<10 ps) compared to dDMSO (130 ps). Our data reveals clear evidence for ESPT in D$_2$O accompanied by a significant change in dipole moment, which is found not to occur in dDMSO. We conclude that the ability of protic solvents to form intermolecular H-bonds with InC enables ESPT, which facilitates a rapid nonradiative S$_1$ → S$_0$ transition via the monoenol intermediate.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

Laser-induced-fluorescence imaging of a spin-polarized ultracold neutral plasma in a magnetic field

In this work, we report Doppler-sensitive laser-induced-fluorescence (LIF) imaging of an ultracold neutral plasma in a magnetic field. Local values of ion density, hydrodynamic fluid velocity, temperature, and spin polarization are obtained using a fluorescence model based on velocity-resolved rate equations (REs) including the transfer of ions between states due to laser coupling and spontaneous emission. The RE approach captures optical pumping of ions into states that are not driven by the LIF excitation laser, and this is validated with experimental data. Combined molecular-dynamics and quantum-trajectories simulations verify that velocity-changing collisions have a negligible impact on the state population evolution for typical experimental conditions. Relative intensities of Zeeman components of the LIF spectra provide clear evidence that the ions are electron-spin-polarized when created by photoionization of magnetically trapped 88 Sr atoms. This probe opens many possibilities for studying thermal transport and the equilibration of neutral plasmas in overlapping regimes of strong coupling and magnetization.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2023

Charge separation in the photosystem II reaction center resolved by multispectral two-dimensional electronic spectroscopy

The photosystem II reaction center (PSII RC) performs the primary energy conversion steps of oxygenic photosynthesis. While the PSII RC has been studied extensively, the similar time scales of energy transfer and charge separation and the severely overlapping pigment transitions in the Qy region have led to multiple models of its charge separation mechanism and excitonic structure. Here, we combine two-dimensional electronic spectroscopy (2DES) with a continuum probe and two-dimensional electronic vibrational spectroscopy (2DEV) to study the cyt b559-D1D2 PSII RC at 77 K. This multispectral combination correlates the overlapping Qy excitons with distinct anion and pigment-specific Q x and mid-infrared transitions to resolve the charge separation mechanism and excitonic structure. Through extensive simultaneous analysis of the multispectral 2D data, we find that charge separation proceeds on multiple time scales from a delocalized excited state via a single pathway in which Pheo D1 is the primary electron acceptor, while Chl D1 and P D1 act in concert as the primary electron donor.

30 DIRECT ENERGY CONVERSION↗

DOE OSTI2017

Design and Manufacture of Tunable Permanent Magnet Based Quadrupole for Next Generation Electron-Ion Colliders [Slides]

The goal of this project is to design and construct of a prototype of quadrupole permanent magnet for the magnet transport lattice for an electron-ion collider. The technical objectives are not only to achieve the field gradient of 34.42T/m in the region of interest, at a radius 16.0mm, relative to the quadrupole axis (z-axis), but also lower the unwanted field harmonics below one unit out of 10,000, especially for Sextupole, Octupole, and Decapole components. The innovation is 1) modular design for cost saving, and 2) adjusting mechanism of each modular magnet for field tuning. The 300-mm long, quadrupole magnet assembly consists of 12 modules of 100-mm long magnet. The magnetic field tuning was implemented by two ways: first, adjusting mechanically the pole tip position for a majority of field tuning, and second, applying the passive shims for a fine field tuning. The magnetic design and optimization of the quadrupole magnet was performed using finite element analysis. The permanent magnet choice was EEC-N44SH grade with the minimum coercivity of 987kA/m (12.4kOe), which has enhanced resistance to demagnetization due to the radiation. The 3D computer-aided design of quadrupole magnet assembly was created based on the magnetic design results. The overall dimensions are 514x500x300mm and its weight is ~470kg (1,050lbs). The weight and volume were minimized through the optimization. Electron Energy Corporation’s vertically integrated manufacturing capability and over 48-years magnet production experience enabled us to implement a “fast prototyping” process and build two quadrupole magnets. Using wire EDM machines, the pole tip profile was precisely fabricated within ±25µm tolerance. The field gradient 34.22T/m was measured using the real time data acquisition system and a 3D Hall probe mounted on the precision XYZ station with the resolution of 1µm (0.00004”). The magnetic field harmonics were measured using a harmonic coil at the Brookhaven National Laboratory, Upton, NY. Faro® laser tracker was used to track the position of each quadrupole while adjusting the pole location mechanically. The normal harmonics were measured up to 20-poles. Overall the 2 nd magnet test data is better than the 1st magnet because of a better symmetry. The adjusting pole technique was proved effective to lower the Sextupole harmonics from -3.14 to -0.71 unit. The Octupole harmonics was improved from -5.67 to -3.45 unit, still not satisfactory. It is found that the proposed approach is not practical and efficient in terms of the mass production although the modular concept can save the manufacturing cost. Another finding is in reality it is hard to adjust mechanically due to the limit of laser tracker resolution .001” and tweak magnetically the field due to the limited harmonics data. Importantly the Brookhaven National Laboratory’s harmonics data doesn’t indicate where the strong/weak field position, which requires another field map test at each measurement. To resolve these, two alternative solutions are introduced; 1) electromagnet correcting coils and 2) 8-pieces quadrupole magnet.

43 PARTICLE ACCELERATORS↗

NASA NTRS1982

Unveiling Venus with VOIR

The goals and intended capabilities of the Venus Orbiting Imaging Radar (VOIR), scheduled for launch in the mid-1980's, are reviewed. Current Pioneer Venus Orbiter data is considered of unsufficient resolution for actual planetary mapping of the geologic terrain of Venus, while the VOIR possesses an SAR which is able to provide a 600 m line pair resolution. The data would yield information of small-scale roughness and the rms slope of surface features, would serve as an efficient altimeter, and help track the planet's gravity field to within 1/5000 cm/sec resolution. Estimations of wind velocity and turbulence would be made available, and a Langmuir probe, a quadrupole mass analyzer, and an ion analyzer will detect ions, electrons, elemental compositions, concentrations, and densities, as well as velocities.

James, W.↗

NASA NTRS1992

Terrestrial magma ocean and core segregation in the earth

According to the recent theories of formation of the earth, the outer layer of the proto-earth was molten and the terrestrial magma ocean was formed when its radius exceeded 3000 km. Core formation should have started in this magma ocean stage, since segregation of metallic iron occurs effectively by melting of the proto-earth. Therefore, interactions between magma, mantle minerals, and metallic iron in the magma ocean stage controlled the geochemistry of the mantle and core. We have studied the partitioning behaviors of elements into the silicate melt, high pressure minerals, and metallic iron under the deep upper mantle and lower mantle conditions. We employed the multi-anvil apparatus for preparing the equilibrating samples in the ranges from 16 to 27 GPa and 1700-2400 C. Both the electron probe microanalyzer (EPMA) and the Secondary Ion Mass spectrometer (SIMS) were used for analyzing the run products. We obtained the partition coefficients of various trace elements between majorite, Mg-perovskite, and liquid, and magnesiowustite, Mg-perovskite, and metallic iron. The examples of the partition coefficients of some key elements are summarized in figures, together with the previous data. We may be able to assess the origin of the mantle abundances of the elements such as transition metals by using the partitioning data obtained above. The mantle abundances of some transition metals expected by the core-mantle equilibrium under the lower mantle conditions cannot explain the observed abundance of some elements such as Mn and Ge in the mantle. Estimations of the densities of the ultrabasic magma Mg-perovskite at high pressure suggest existence of a density crossover in the deep lower mantle; flotation of Mg-perovskite occurs in the deep magma ocean under the lower mantle conditions. The observed depletion of some transition metals such as V, Cr, Mn, Fe, Co, and Ni in the mantle may be explained by the two stage process, the core-mantle equilibrium under the lower mantle conditions in the first stage, and subsequent downwards separation of the ultrabasic liquid (and magnesiowustite) and flotation of Mg-perovskite in the lower mantle.

Ohtani, Eiji↗

NASA NTRS1994

Observational constraints on black hole accretion disks

We review the empirical constraints on accretion disk models of stellar-mass black holes based on recent multiwavelength observational results. In addition to time-averaged emission spectra, the time evolutions of the intensity and spectrum provide critical information about the structure, stability, and dynamics of the disk. Using the basic thermal Keplerian disk paradigm, we consider in particular generalizations of the standard optically thin disk models needed to accommodate the extremely rich variety of dynamical phenomena exhibited by black hole candidates ranging from flares of electron-positron annihilations and quasiperiodic oscillations in the X-ray intensity to X-ray novae activity. These in turn provide probes of the disk structure and global geometry. The goal is to construct a single unified framework to interpret a large variety of black hole phenomena. This paper will concentrate on the interface between basic theory and observational data modeling.

Liang, Edison P.↗

NASA NTRS1999

Progress of Cometary Science in the Past 100 Years

Enormous strides made by cometary science during the 20th century defy any meaningful comparison of its state 100 years ago and now. The great majority of the subfields enjoying much attention nowadays did not exist in the year 1900. Dramatic developments, especially in the past 30-50 years, have equally affected observational and theoretical studies of comets. The profound diversification of observing techniques has been documented by the ever widening limits on the electromagnetic spectrum covered. While the time around 1900 marked an early period of slow and painful experimentation with photographic methods in cometary studies, observations of comets from the x-ray region to the radio waves have by now become routine. Many of the new techniques, and all those involved with the wavelengths shorter than about 300 nm, were made possible by another major breakthrough of this century - observing from space. Experiments on dedicated Earth-orbiting satellites as well as several deep-space probes have provided fascinating new information on the nature and makeup of comets. In broader terms, much of the progress has been achieved thanks to fundamental discoveries and major advances in electronics, whose applications resulted in qualitatively new instruments (e.g. radiotelescopes) and sensors or detectors (e.g. CCD arrays). The most universal effect on the entire cometary science, from observing to data handling to quantitative interpretations, has been, as in any other branch of science, due to the introduction of electronic computers, with their processing capabilities not only unheard of, but literally unimaginable, in the age of classical desk calculators. As if all this should not be enough, the today's generations of comet scientists have, in addition, been blessed with nature's highly appreciated cooperation. Indeed, in the span of a dozen years, between 1985 and 1997, we were privileged to witness four remarkable cometary events: (i) a return of Halley's celebrated comet; (ii) the impact of an extensively fragmented comet Shoemaker-Levy 9 into Jupiter - a once-in-a-millenium episode; (iii) a closeup, appearance of a small but very active Earth-approaching comet Hyakutake; and (iv) an unforgettable celestial show of comet Hale-Bopp, one of the brightest and the most massive comets ever observed. We have been - and for many years to come will be - harvesting scientifically lucrative findings based on the flood of data accumulated during observational campaigns organized for these and other recent objects. Our understanding of cometary phenomena will continue to grow as observing techniques are further being improved and as more sophisticated theories are being developed. Recent accomplishments achieved in the rapidly expanding field of comets are certain to stimulate the members of this astronomical community in their quest for new discoveries in the 21st century!

Sekanina, Zdenek↗

DOE OSTI2019

Use of Au@Void@TiO 2 yolk-shell nanostructures to probe the influence of oxide crystallinity on catalytic activity for low-temperature oxidations

A series of Au@Void@TiO 2 yolk-shell structures were synthesized and characterized, and their catalytic activity was tested for the oxidation of carbon monoxide. The target of this study was to evaluate the effects of (1) the crystallinity of the TiO 2 shell and (2) the presence of titanate phases. Transmission electron microscopy and X-ray diffraction data show that increasing calcination temperatures, from 850 to 1250 K, leads to the formation of larger titania crystallites. These crystallites are typically in the form of anatase, but some rutile is also made at high temperatures. The general yolk-shell nanostructures retain their basic characteristics upon calcination and upon treatment with HCl (used to remove the titanate phases), but a few structural and chemical changes do take place: (1) the void-space diameter is reduced by approximately 10% when going from T Calc = 850 K to T Calc = 1250 K, a change that is accompanied by a decrease in the surface area (estimated from N 2 adsorption-desorption isotherms); (2) the shell thickness remains unaffected by either calcination or HCl treatment; (3) the pore volume also remains approximately constant with increasing calcination temperature in the catalysts free of titanates but diminishes significantly in the samples with titanate phases; (4) the mesoporosity is minimal in all yolk-shell nanostructures but more noticeable with the pure TiO 2 shells; (5) the titanates have high Na content (measured by X-ray photoelectron spectroscopy), but that Na is fully removed upon treatment with HCl; (6) no Ti 3+ was detectable in any of the samples, but silica, together with C and (minor amounts of) N, was present in all; (7) all Au is in metallic form; (8) two temperature regimes were observed for the catalytic oxidation of CO, as reported for other similar samples in the past, an Arrhenius regime around or above room temperature, and a cryogenic range going to temperatures as low as 120 K; (9) the latter regime is only seen with the samples containing titanate phases; (10) the titanates seem to also aid in the oxidation at higher temperatures; and (11) in general, increasing titania crystallinity leads to a decrease the catalytic activity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Deep learning-enabled probing of irradiation-induced defects in time-series micrographs

Modeling time-series data with convolutional neural networks (CNNs) requires building a model to learn in batches as opposed to training sequentially. Coupling CNNs with in situ or operando techniques opens the possibility of accurately segmenting dynamic reactions and mass transport phenomena to understand how materials behave under the conditions in which they are used. In this article, in situ ion irradiation transmission electron microscopy (TEM) images are used as inputs into the CNN to assess the defect generation rate, defect cluster density, and saturation of defects. We then use the output segmentation maps to correlate with conventional TEM micrographs to assess the model’s ability to detail nanoscale interactions. Next, we discuss the implications of preprocessing and hyperparameters on model variability, accuracy when expanded to other datasets, and the role of regularization when controlling model variance. Ultimately, we eliminate human bias when extrapolating physical metrics, speed up analysis time, decouple reactions that happen at 100 ms intervals, and deploy models that are both accurate and transferable to similar experiments.

Burns, Kory (ORCID:0000000158019909)↗

NASA NTRS2008

Gamma-Ray Burst Afterglows as Probes of Environment and Blast Wave Physics. II. The Distribution of rho and Structure of the Circumburst Medium

We constrain blast wave parameters and the circumburst media ofa subsample of 10 BeppoSAX gamma-ray bursts (GRBs). For this sample we derive the values of the injected electron energy distribution index, p, and the density structure index of the circumburst medium, k, from simultaneous spectral fits to their X-ray, optical, and NIR afterglow data. The spectral fits have been done in count space and include the effects ofmetallicity, and are compared with the previously reported optical and X-ray temporal behavior. Using the blast wave model and some assumptions which include on-axis viewing and standard jet structure, constant blast wave energy, and no evolution of the microphysical parameters, we find a mean value ofp for the sample as a whole of 9.... oa -0.003.0" 2 a_ statistical analysis of the distribution demonstrates that the p-values in this sample are inconsistent with a single universal value forp at the 3 _ level or greater, which has significant implications for particle acceleration models. This approach provides us with a measured distribution ofcircumburst density structures rather than considering only the cases of k ----0 (homogeneous) and k - 2 (windlike). We find five GRBs for which k can be well constrained, and in four of these cases the circumburst medium is clearly windlike. The fifth source has a value of 0 < k < 1, consistent with a homogeneous circumburst medium.

Starling, R. L. C.↗

DOE OSTI2018

Inorganic “Conductive Glass” Approach to Rendering Mesoporous Metal–Organic Frameworks Electronically Conductive and Chemically Responsive

A representative mesoporous metal–organic-framework (MOF) material, NU-1000, has been rendered electronically conductive via a robust inorganic approach that permits retention of MOF crystallinity and porosity. The approach is based on condensed-phase grafting of molecular tin species onto the MOF nodes via irreversible reaction with hydroxyl and aqua ligands presented at the node surface, a self-limiting process termed solvothermal installation (of metal ions) in MOFs (SIM, a solution-phase analog of atomic layer deposition in MOFs). Treatment of the modified MOF with aerated steam at 120 °C converts the grafted tin molecules to tetratin(IV)oxy clusters, with the clusters being sited between insulating pairs of zirconia-like nodes (the zirconium component being key to endowing the parent material with requisite chemical and thermal stability). By introducing new O–H presenting ligands on the modified-MOF node, the high-temperature steam step additionally serves to reset the material to reactive form, thus enabling a second self-limiting tin-grafting step to be run (and after further steam treatment, enabling a third). Difference-envelop-density (DED) analyses of synchrotron-derived X-ray scattering data, with and without installed tin species, show that the clusters formed after one cycle are spatially isolated, but that repetitive SIM cycling adds metal and oxygen ions in a way that enshrouds nodes, links clusters, and yields continuous one-dimensional strands of oxy-tin(IV), oriented exclusively along the c axis of the MOF. Two-probe conductivity measurements show that the parent MOF and the version containing isolated oxy-tin(IV) clusters are electrically insulating, but that the versions featuring continuous strands show an electrical conductivity of 1.8 × 10–7 S/cm after three Sn-SIM cycles. When combined with interdigitated microelectrodes, the solvent-free and conductive-glass-modified material (three Sn-SIM cycles) displays a substantial and persistent increase in electrical conductivity during exposure to 5% H2, indicating a role for dissociated H2 as an electronic dopant. The increase can be repetitively reversed by alternating H2 with air, illustrating the ability of the conductive MOF to function as a resistive sensor for H2 and suggesting further potential applications that may capitalize on the combination of high volumetric surface area, high mesoporosity, high chemical and thermal stability, and significant electrical conductivity.

bio-inspired↗