A break in periodicity occurs in the actinide series between plutonium and americium as the result of the localization of 5f electrons. The subsequent chemistry of later actinides is thought to ...closely parallel lanthanides in that bonding is expected to be ionic and complexation should not substantially alter the electronic structure of the metal ions. Here we demonstrate that ligation of californium(III) by a pyridine derivative results in significant deviations in the properties of the resultant complex with respect to that predicted for the free ion. We expand on this by characterizing the americium and curium analogues for comparison, and show that these pronounced effects result from a second transition in periodicity in the actinide series that occurs, in part, because of the stabilization of the divalent oxidation state. The metastability of californium(II) is responsible for many of the unusual properties of californium including the green photoluminescence.
In situ electrochemical diagnostics designed to probe ionomer interactions with platinum and carbon were applied to relate ionomer coverage and conformation, gleaned from anion adsorption data, with ...O2 transport resistance for low-loaded (0.05 mgPt cm–2) platinum-supported Vulcan carbon (Pt/Vu)-based electrodes in a polymer electrolyte fuel cell. Coupling the in situ diagnostic data with ex situ characterization of catalyst inks and electrode structures, the effect of ink composition is explained by both ink-level interactions that dictate the electrode microstructure during fabrication and the resulting local ionomer distribution near catalyst sites. Electrochemical techniques (CO displacement and ac impedance) show that catalyst inks with higher water content increase ionomer (sulfonate) interactions with Pt sites without significantly affecting ionomer coverage on the carbon support. Surprisingly, the higher anion adsorption is shown to have a minor impact on specific activity, while exhibiting a complex relationship with oxygen transport. Ex situ characterization of ionomer suspensions and catalyst/ionomer inks indicates that the lower ionomer coverage can be correlated with the formation of large ionomer aggregates and weaker ionomer/catalyst interactions in low-water content inks. These larger ionomer aggregates resulted in increased local oxygen transport resistance, namely, through the ionomer film, and reduced performance at high current density. In the water-rich inks, the ionomer aggregate size decreases, while stronger ionomer/Pt interactions are observed. The reduced ionomer aggregation improves transport resistance through the ionomer film, while the increased adsorption leads to the emergence of resistance at the ionomer/Pt interface. Overall, the high current density performance is shown to be a nonmonotonic function of ink water content, scaling with the local gas (H2, O2) transport resistance resulting from pore, thin film, and interfacial phenomena.
Soil nutrients and nitrogen turnover were measured in two soil chronosequences on the Kilauea and Mauna Loa volcanoes, Hawai'i. Organic carbon and nitrogen accumulated rapidly in sites derived from ...volcanic ash and more slowly in sites derived from pahoehoe; a minimum of 22 kg N·ha-1·yr-1were added to tephra sites early in soil development. Substantial nitrogen fixation by an unknown organism or organisms is probably responsible for most of the nitrogen accumulated on ash. Nitrogen turnover increased with soil age in both absolute (mg N/m2) and relative (g N/g of soil N) terms in both chronosequences. In comparison with other localities, net nitrogen mineralization potentials were extremely low in all of the sites except the two oldest ash sites. Nitrogen availability is probably not related to most of the forms of `ohi'a dieback occurring in Hawai'i, but low nitrogen availability is observed in sites susceptible to the dryland form of dieback which occurs early in primary succession.
The Kepler spacecraft has been monitoring the light from 150,000 stars in its primary quest to detect transiting exoplanets. Here, we report on the detection of an eclipsing stellar hierarchical ...triple, identified in the Kepler photometry. KOI-126 A, (B, C), is composed of a low-mass binary masses MB = 0.2413 ± 0.0030 solar mass (Mmiddle dot in circle), MC = 0.2127 ± 0.0026 Mmiddle dot in circle; radii RB = 0.2543 ± 0.0014 solar radius (Rmiddle dot in circle), RC = 0.2318 ± 0.0013 Rmiddle dot in circle; orbital period P₁ = 1.76713 ± 0.00019 days on an eccentric orbit about a third star (mass MA = 1.347 ± 0.032 Mmiddle dot in circle; radius RA = 2.0254 ± 0.0098 Rmiddle dot in circle; period of orbit around the low-mass binary P₂ = 33.9214 ± 0.0013 days; eccentricity of that orbit e₂ = 0.3043 ± 0.0024). The low-mass pair probe the poorly sampled fully convective stellar domain offering a crucial benchmark for theoretical stellar models.
On 2018 February 4.41, the All-Sky Automated Survey for SuperNovae (ASAS-SN) discovered ASASSN-18bt in the K2 Campaign 16 field. With a redshift of z = 0.01098 and a peak apparent magnitude of Bmax = ...14.31, ASASSN-18bt is the nearest and brightest SNe Ia yet observed by the Kepler spacecraft. Here we present the discovery of ASASSN-18bt, the K2 light curve, and prediscovery data from ASAS-SN and the Asteroid Terrestrial-impact Last Alert System. The K2 early-time light curve has an unprecedented 30-minute cadence and photometric precision for an SN Ia light curve, and it unambiguously shows a ∼4 day nearly linear phase followed by a steeper rise. Thus, ASASSN-18bt joins a growing list of SNe Ia whose early light curves are not well described by a single power law. We show that a double-power-law model fits the data reasonably well, hinting that two physical processes must be responsible for the observed rise. However, we find that current models of the interaction with a nondegenerate companion predict an abrupt rise and cannot adequately explain the initial, slower linear phase. Instead, we find that existing published models with shallow 56Ni are able to span the observed behavior and, with tuning, may be able to reproduce the ASASSN-18bt light curve. Regardless, more theoretical work is needed to satisfactorily model this and other early-time SNe Ia light curves. Finally, we use Swift X-ray nondetections to constrain the presence of circumstellar material (CSM) at much larger distances and lower densities than possible with the optical light curve. For a constant-density CSM, these nondetections constrain < 4.5 × 105 cm−3 at a radius of 4 × 1015 cm from the progenitor star. Assuming a wind-like environment, we place mass loss limits of for vw = 100 km s−1, ruling out some symbiotic progenitor systems. This work highlights the power of well-sampled early-time data and the need for immediate multiband, high-cadence follow-up for progress in understanding SNe Ia.
Proton exchange membrane fuel cells (PEMFCs) powered by green hydrogen (H2) have become a promising alternative to conventional hydrocarbon-fueled power generators. Despite technological ...advancements, further improvements in efficiency, durability, and low-cost production are required for the widespread adoption of PEMFCs. Though numerous approaches to improve PEMFC electrodes have been reported, most strategies utilize a single material set (e.g., one combination of catalyst and ionomer) to improve performance. Alternatively, anisotropic (graded) electrode structures with locally tunable properties may yield superior electrode performance due to improved ionic and gas phase transport. In this work, graded cathode catalyst layers (CCLs) incorporating different ionomers (Nafion D2020, Aquivion D79-25BS, and HOPI) were designed and prepared. Performance screening shows that some of these graded electrode structures have comparable performance to optimized single-ionomer electrode structures (D2020) suggesting some synergistic benefit. Additionally, we show that electrodes with lower equivalent weight (EW) D79 ionomer near the membrane and D2020 ionomer near the gas diffusion media outperformed electrodes with the inverted configuration. Finally, EIS analysis shows some graded ionomer structures (e.g. D79/D2020) have better than expected H+ conductivity, generally leading to better electrode performance. However, further optimization of ionomer content and catalyst ink formulations is needed to improve overall PEMFC performance.
•Graded catalysts layers with different ionomers were produced.•Detailed electrochemical studies of electrodes were performed.•Fuel cell performance of electrodes with Nafion was found the highest.•Additional optimization with novel HOPI ionomer required.
Alkaline soils exist under forest vegetation on the Tanana River floodplain of interior Alaska. The objectives of this study were to describe the soil chemical properties and to examine controls on ...these chemical properties along a forest primary-successional sequence. Soil saturation pastes were prepared from duplicate sites along the successional sequence representing bare alluvium (stage I), open willow (stage (III), poplar-alder (stage V), and white spruce (stage VIII). Calcium, Mg, SO4, and ions responsible for alkalinity were the dominant solutes in the saturation extracts. Soil horizons were generally calcareous (CaCO3) and therefore alkaline (pH > 7.0) across the successional sequence. The CaCO3-containing soil horizons were saturated to supersaturated with respect to calcite. Many soil horizons in the plots of stages III and V from one site were saturated with respect to gypsum (CaSO4.2H2O), whereas other stages and sites were generally undersaturated. Phosphate availability in CaCO3-containing soil horizons was apparently constrained by CaCO3 solubility and the solubility products of beta-tricalcium phosphate and hydroxyapatite. There was a strong positive relationship between soluble salt content and silt concentration in the early stages, suggesting that initial salt content is controlled by the texture of the alluvial material; the variation in initial texture largely accounts for the site to site variation in salts. Evaporation apparently plays a role in concentrating soluble salts at the surface during the early stages (III and V), but by the late stages (VIII), the biotic and topographic factors apparently reduce surficial evaporation. This reduction in evaporation, coupled with the production of organic acids, leads to a 42% loss of soluble salts from the soils. This study supports our original hypotheses that physical factors (texture, evaporation) are most important early in the successional sequence and that biotic factors (transpiration, acid leaching) are most important late in the successional sequence in controlling salt distribution in soils on the Tanana River floodplain.
We present the Kepler Object of Interest (KOI) catalog of transiting exoplanets based on searching 4 yr of Kepler time series photometry (Data Release 25, Q1-Q17). The catalog contains 8054 KOIs, of ...which 4034 are planet candidates with periods between 0.25 and 632 days. Of these candidates, 219 are new, including two in multiplanet systems (KOI-82.06 and KOI-2926.05) and 10 high-reliability, terrestrial-size, habitable zone candidates. This catalog was created using a tool called the Robovetter, which automatically vets the DR25 threshold crossing events (TCEs). The Robovetter also vetted simulated data sets and measured how well it was able to separate TCEs caused by noise from those caused by low signal-to-noise transits. We discuss the Robovetter and the metrics it uses to sort TCEs. For orbital periods less than 100 days the Robovetter completeness (the fraction of simulated transits that are determined to be planet candidates) across all observed stars is greater than 85%. For the same period range, the catalog reliability (the fraction of candidates that are not due to instrumental or stellar noise) is greater than 98%. However, for low signal-to-noise candidates between 200 and 500 days around FGK-dwarf stars, the Robovetter is 76.7% complete and the catalog is 50.5% reliable. The KOI catalog, the transit fits, and all of the simulated data used to characterize this catalog are available at the NASA Exoplanet Archive.
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