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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Ecosystem Controls on Methylmercury Production by Periphyton Biofilms in a Contaminated Stream: Implications for Predictive Modeling

Periphyton biofilms produce a substantial fraction of the overall monomethylmercury (MMHg) flux in East Fork Poplar Creek, an industrially contaminated, freshwater creek in Oak Ridge, Tennessee. We examined periphyton MMHg production across seasons, locations, and light conditions using mercury stable isotopes. Methylation and demethylation rate potentials (km, trans av and kd, trans av, respectively) were calculated using a transient availability kinetic model. Light exposure and season were significant predictors of km, trans av, with greater values in full light exposure and in the summer. Season, light exposure, and location were significant predictors of kd, trans av, which was highest in dark conditions, in the spring, and at the upstream location. Light exposure was the controlling factor for net MMHg production, with positive production for periphyton grown under full light exposure and net demethylation for periphyton grown in the dark. Ambient MMHg and km, trans av were significantly correlated. Transient availability rate potentials were 15 times higher for km and 9 times higher for kd compared to full availability rate potentials (km, full av and kd, full av) calculated at 1 d. No significant model for the prediction of km, full av or kd, full av could be constructed using light, season, and location. In addition, there were no significant differences among treatments for the full availability km, full av, kd, full av, or net MMHg calculated using the full availability rate potentials. km, full av was not correlated with ambient MMHg concentrations. The present results underscore the importance of applying transient availability kinetics to MMHg production data when estimating MMHg production potential and flux.

Schwartz, Grace E.↗
DOE OSTI2017

Controlled optical properties via chemical composition tuning in molybdenum-incorporated β-Ga 2 O 3 nanocrystalline films

An approach is presented to design refractory-metal incorporated Ga2O3-based materials with controlled structural and optical properties. The molybdenum (Mo)-content in Ga2O3 was varied from 0 to 11 at% in the sputter-deposited Ga-Mo-O films. Molybdenum was found to significantly affect the structure and optical properties. While low Mo-content (≤4 at%) results in the formation of single-phase (β-Ga2O3), higher Mo-content results in amorphization. Chemically-induced band gap variability (Eg~1 eV) coupled with structure-modification indicates the electronic-structure changes in Ga-Mo-O. The linear relationship between chemical-composition and optical properties suggests that tailoring the optical-quality and performance of Ga-Mo-O films is possible by tuning the Mo-content.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2018

Manipulative experiments demonstrate how long-term soil moisture changes alter controls of plant water use

Tree transpiration depends on biotic and abiotic factors that might change in the future, including precipitation and soil moisture status. Although short-term sap flux responses to soil moisture and evaporative demand have been the subject of attention before, the relative sensitivity of sap flux to these two factors under long-term changes in soil moisture conditions has rarely been determined experimentally. We tested how long-term artificial change in soil moisture affects the sensitivity of tree-level sap flux to daily atmospheric vapor pressure deficit (VPD) and soil moisture variations, and the generality of these effects across forest types and environments using four manipulative sites in mature forests. Exposure to relatively long-term (two to six years) soil moisture reduction decreases tree sap flux sensitivity to daily VPD and relative extractable water (REW) variations, leading to lower sap flux even under high soil moisture and optimal VPD. Inversely, trees subjected to long-term irrigation showed a significant increase in their sensitivity to daily VPD and REW, but only at the most water-limited site. The ratio between the relative change in soil moisture manipulation and the relative change in sap flux sensitivity to VPD and REW variations was similar across sites suggesting common adjustment mechanisms to long-term soil moisture status across environments for evergreen tree species. Overall, our results show that long-term changes in soil water availability, and subsequent adjustments to these novel conditions, could play a critical and increasingly important role in controlling forest water use in the future.

acclimation↗
DOE OSTI2017

Chemical and physical controls on the transformation of amorphous calcium carbonate into crystalline CaCO3 polymorphs

Calcite and other crystalline polymorphs of CaCO3 can form by pathways involving amorphous calcium carbonate (ACC). Apparent inconsistencies in the literature indicate the relationships between ACC composition, local conditions, and the subsequent crystalline polymorphs are not yet established. This experimental study quantifies the control of solution composition on the transformation of ACC into crystalline polymorphs in the presence of magnesium. Using a mixed flow reactor to control solution chemistry, ACC was synthesized with variable Mg contents by tuning input pH, Mg/Ca, and total carbonate concentration. ACC products were allowed to transform within the output suspension under stirred or quiescent conditions while characterizing the evolving solutions and solids. As the ACC transforms into a crystalline phase, the solutions record a polymorph-specific evolution of pH and Mg/Ca.

Blue, Christina R.↗
DOE OSTI2023

Active Suspension Parameters Identification: An Algebraic Approach and its Application to Suspension Travel Control

The parameters of an active suspension need to be identified online, such that the suspension control system can be adapted to mechanical wear and load change. Recursive least squares and observer-based methods are frequently utilized to fulfill this purpose. However, they can yield slow parameter identification due to their asymptotic nature. We propose an algebraic identifier to estimate the parameters of an active suspension online, which does not maintain an asymptotic convergence phase. Simulation results demonstrate the effectiveness of the proposed algebraic approach.

Wang, Zejiang↗
DOE OSTI2012

The Structural Basis for Tight Control of PP2A Methylation and Function by LCMT-1

Proper formation of protein phosphatase 2A (PP2A) holoenzymes is essential for the fitness of all eukaryotic cells. Carboxyl methylation of the PP2A catalytic subunit plays a critical role in regulating holoenzyme assembly; methylation is catalyzed by PP2A-specific methyltransferase LCMT-1, an enzyme required for cell survival. We determined crystal structures of human LCMT-1 in isolation and in complex with PP2A stabilized by a cofactor mimic. The structures show that the LCMT-1 active-site pocket recognizes the carboxyl terminus of PP2A, and, interestingly, the PP2A active site makes extensive contacts to LCMT-1. We demonstrated that activation of the PP2A active site stimulates methylation, suggesting a mechanism for efficient conversion of activated PP2A into substrate-specific holoenzymes, thus minimizing unregulated phosphatase activity or formation of inactive holoenzymes. A dominant-negative LCMT-1 mutant attenuates the cell cycle without causing cell death, likely by inhibiting uncontrolled phosphatase activity. Our studies suggested mechanisms of LCMT-1 in tight control of PP2A function, important for the cell cycle and cell survival.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2023

Fuel performance evaluation of two high burnup PWR core designs during normal operation, control rod withdrawal, and control rod ejection scenarios

There is interest among utilities to extend the current, 18-month operating cycle to 24 months. Economically, this extension would require greater than 5 % enrichment and peak rod average discharge burnup levels above 62 GWd/MTU. A notable challenge of increasing enrichment is the resulting additional excess reactivity encountered during the early stages of fuel life. To accommodate, burnable absorbers beyond soluble boron are introduced into the fuel system. In high burnup fuels, the possibilities of cladding lift-off and fuel melting increase due, in part, to increased rod internal pressures and limited fuel thermal conductivity, respectively. This work collaboratively employs PARCS, RELAP5-3D, and BISON to compare the fuel performance of two high burnup fuel candidates with higher than 5 % enrichment. Here, the fuel performance parameters were compared to current NRC guidance. The results demonstrate an annular fuel design with homogenously blended gadolinium as a burnable absorber operates with greater safety margins during normal operation, allowing for additional operational flexibility. During normal operation, the core design utilizing Integral Fuel Burnable Absorber pins contained fuel pins which reached plenum pressures above 15.5 MPa by the end of the first fuel cycle and fuel pins experienced cladding hoop strains above 1 %. In the Gd core design, only two observed pins experienced plenum pressures above 15.5 MPa and no pins exceeded 1 % cladding hoop strain. During the control rod withdrawal scenario, plenum pressures for pins in both designs marginally exceeded system pressure, however neither experienced excessive hoop strain. The Gd core design experienced a maximum fuel temperature of 2418 K, which is significantly higher than the Integral Fuel Burnable Absorber design at 2157 K, but still within regulatory guidance. We predicted that the fuel in both could return to service after the CRW event. We also predicted that cladding would not fail during the Control Rod Ejection in either core design. Generally, the Integral Fuel Burnable Absorber core design performed with greater safety margin with regards to temperature during normal operation and the transient events. However, the Gd core design performed with greater safety margin regarding plenum pressure and hoop strain limits during normal operation and both transient events.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2018

Control of Calcium Phosphate Nucleation and Transformation through Interactions of Enamelin and Amelogenin Exhibits the “Goldilocks Effect”

Although amelogenin comprises the vast majority of the matrix that templates calcium phosphate nucleation during enamel formation, other proteins, particularly enamelin, are also known to play an important role in the formation of enamel’s intricate architecture. However, there is little understanding of the interplay between amelogenin and enamelin in controlling processes of mineral nucleation and growth. Here we used an in vitro model to investigate the impact of enamelin co-assembly with amelogenin on calcium phosphate nucleation for a range of enamelin-to-amelogenin ratios. We found that amelogenin alone is a weak promoter of nucleation, but addition of enamelin enhanced nucleation rates in a highly non-linear, non-monotonic manner reaching a sharp maximum at a ratio of 1:50. We provide a phenomenological model to explain this effect that assumes only isolated enamelin proteins can act as sites of enhanced nucleation, while enamelin oligomers cannot. Even when co-assembly is random, the model reproduces the observed behavior, suggesting a simple means to tightly control the timing and extent of nucleation and phase transformation by amelogenin and enamelin.

Tao, Jinhui↗
DOE OSTI2016

Dimensionally Controlled Lithiation of Chromium Oxide

Oxide conversion reactions are an alternative approach for high capacity lithium ion batteries but are known to suffer from structural irreversibility associated with the phase separation and reconstitution of reduced metal species and Li2O. In particular, the morphology of the reduced metal species is thought to play a critical role in the electrochemical properties of a conversion material. Here we use a model electrode with alternating layers of chromium and chromium oxide to better understand and control these phase changes in real-time and at molecular length scales. Despite lacking crystallinity at the atomic scale, this superstructure is observed (with X-ray reflectivity, XR) to lithiate and delithiate in a purely one-dimensional manner, preserving the layered structure. The XR data show that the metal layers act as nucleation sites for the reduction of chromium in the conversion reaction. Irreversibility during delithiation is due to the formation of a ternary phase, LiCrO2, which can be further delithiated at higher potentials. The results reveal that the combination of confining lithiation to nanoscale sheets of Li2O and the availability of reaction sites in the metal layers in the layered structure is a strategy for improving the reversibility and mass transport properties that can be used in a wide range of conversion materials.

25 ENERGY STORAGE↗
DOE OSTI2018

Unveiling the Structural Evolution of Ag 1.2 Mn 8 O 16 under Coulombically Controlled (De)Lithiation

MnO 2 materials are considered promising cathode materials for rechargeable lithium, sodium, and magnesium batteries due to their earth abundance and environmental friendliness. One polymorph of MnO 2 , α-MnO 2 , has 2 × 2 tunnels (4.6 Å × 4.6 Å) in its structural framework, which provide facile diffusion pathways for guest ions. In this work, a silver-ion-containing α-MnO 2 (Ag 1.2 Mn 8 O 16 ) is examined as a candidate cathode material for Li based batteries. Electrochemical stability of Ag 1.2 Mn 8 O 16 is investigated through Coulombically controlled reduction under 2 or 4 molar electron equivalents (e.e.). Terminal discharge voltage remains almost constant under 2 e.e. of cycling, whereas it continuously decreases under repetitive reduction by 4 e.e. Thus, detailed structural analyses were utilized to investigate the structural evolution upon lithiation. Significant increases in lattice a (17.7%) and atomic distances (~4.8%) are observed when x in Li x Ag 1.2 Mn 8 O 16 is >4. Ag metal forms at this level of lithiation concomitant with a large structural distortion to the Mn–O framework. In contrast, lattice a only expands by 2.2% and Mn–O/Mn-Mn distances show minor changes (~1.4%) at x < 2. The structural deformation (tunnel breakage) at x > 4 inhibits the recovery of the original structure, leading to poor cycle stability at high lithiation levels. This report establishes the correlation among local structure changes, amorphization processes, formation of Ag 0 , and long-term cycle stability for this silver-containing α-MnO 2 type material at both low and high lithiation levels.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Redox Fluctuations Control the Coupled Cycling of Iron and Carbon in Tropical Forest Soils

In this paper, oscillating redox conditions are a common feature of humid tropical forest soils, driven by an ample supply and dynamics of reductants, high moisture, microbial oxygen consumption, and finely textured clays that limit diffusion. However, the net result of variable soil redox regimes on iron (Fe) mineral dynamics and associated carbon (C) forms and fluxes is poorly understood in tropical soils. Using a 44-day redox incubation experiment with humid tropical forest soils from Puerto Rico, we examined patterns in Fe and C transformations under four redox regimes: static anoxic, "flux 4-day" (4d oxic, 4d anoxic), "flux 8-day" (8d oxic, 4d anoxic) and static oxic. Prolonged anoxia promoted reductive dissolution of Fe-oxides, and led to an increase in soluble Fe(II) and amorphous Fe oxide pools. Preferential dissolution of the less-crystalline Fe pool was evident immediately following a shift in bulk redox status (oxic to anoxic), and coincided with increased dissolved organic C, presumably due to acidification or direct release of organic matter (OM) from dissolving Fe(III) mineral phases. The average nominal oxidation state of water-soluble C was lowest under persistent anoxic conditions, suggesting that more reduced organic compounds were metabolically unavailable for microbial consumption under reducing conditions. Anoxic soil compounds had high H/C values (and were similar to lignin-like compounds) whereas oxic soil compounds had higher O/C values, akin to tannin- and cellulose-like components. Cumulative respiration derived from native soil organic C was highest in static oxic soils. These results show how Fe minerals and Fe-OM interactions in tropical soils are highly sensitive to variable redox effects. Shifting soil oxygen availability rapidly impacted exchanges between mineral-sorbed and aqueous C pools, increased the dissolved organic C pool under anoxic conditions implying that the periodicity of low-redox events may control the fate of C in wet tropical soils.

54 ENVIRONMENTAL SCIENCES↗