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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 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Stand development and other intrinsic factors largely control fine-root dynamics with only subtle modifications from resource availability

Forest productivity depends on resource acquisition by ephemeral roots and leaves. A combination of intrinsic and environmental factors influences ephemeral organs; however, difficulties in studying belowground organs impede mechanistic understanding of fine-root production and turnover. To quantify factors controlling fine-root dynamics, we grew a deciduous hardwood (Populus deltoides Bartr.) and an evergreen conifer (Pinus taeda L.) with distinct soil moisture and nutrient availability treatments. We monitored fine-root dynamics with minirhizotrons for 6 years during early stand development and expressed results on a root length, biomass and mortality-risk basis. Stand development and other intrinsic factors consistently influenced both species in the same direction and by similar magnitude. Live-root length increased to a peak during establishment and slowly declined after roots of neighboring trees overlapped. Root longevity was highest during establishment and decreased thereafter. Root longevity consistently increased with depth of appearance and initial root diameter. Season of appearance affected root longevity in the following order: spring > summer > fall > winter. The influence of soil resource availability on fine-root dynamics was inconsistent between species, and ranked below that of rooting depth, initial diameter, stand development and phenology. Fine-root biomass either increased or was unaffected by greater resource availability. Fine-root production and live root length decreased with irrigation for both species, and increased with fertilization only for poplar. Fine-root mortality risk both increased and decreased depending on species and amendment treatment. Differing responses to soil moisture and nutrient availability between species suggests we should carefully evaluate generalizations about the response of fine-root dynamics to resource availability. Finally, while attempting to describe and explain carbon allocation to fine-root production and turnover, modelers and physiologists should first consider consistent patterns of allocation caused by different depth, diameter, stand development, phenology and species before considering allocation due to soil resource availability.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2014

Weak competing interactions control assembly of strongly bonded TCNQ ionic acceptor molecules on silver surfaces

The energy scales of interactions that control molecular adsorption and assembly on surfaces can vary by several orders of magnitude, yet the importance of each contributing interaction is not apparent a priori. Tetracyanoquinodimethane (TCNQ) is an archetypal electron acceptor molecule and it is a key component of organic metals. On metal surfaces, this molecule also acts as an electron acceptor, producing negatively charged adsorbates. It is therefore rather intriguing to observe attractive molecular interactions in this system that were reported previously for copper and silver surfaces. In this paper, our experiments compared TCNQ adsorption on noble metal surfaces of Ag(100) and Ag(111). In both cases we found net attractive interactions down to the lowest coverage. However, the morphology of the assemblies was strikingly different, with two-dimensional islands on Ag(100) and one-dimensional chains on Ag(111) surfaces. This observation suggests that the registry effect governed by the molecular interaction with the underlying lattice potential is critical in determining the dimensionality of the molecular assembly. Using first-principles density functional calculations with a van der Waals correction scheme, we revealed that the strengths of major interactions (i.e., lattice potential corrugation, intermolecular attraction, and charge-transfer-induced repulsion) are all similar in energy. The van der Waals interactions, in particular, almost double the strength of attractive interactions, making the intermolecular potential comparable in strength to the diffusion potential and promoting self-assembly. However, it is the anisotropy of local intermolecular interactions that is primarily responsible for the difference in the topology of the molecular islands on Ag(100) and Ag(111) surfaces. Finally, we anticipate that the intermolecular potential will become more attractive and dominant over the diffusion potential with increasing molecular size, providing new design strategies for the structure and charge transfer within molecular layers.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Heat-flux control and solid-state cooling by regulating chemical potential of photons in near-field electromagnetic heat transfer

We consider near-field heat transfer with nonzero chemical potential for photons, as can occur between two semiconductor bodies, held at different temperatures with at least one of the bodies under external bias.We show that the dependence of radiative heat flux on chemical potential enables electronic control of both the direction and magnitude of near-field heat transfer between the two bodies. Moreover such a configuration can operate as a solid-state cooling device whose efficiency can approach the Carnot limit in the ideal case. Furthermore, significant cooling can also be achieved in the presence of inherent nonidealities including Auger recombination and parasitic phonon-polariton heat transfer.

42 ENGINEERING↗
DOE OSTI2011

Dynamic Control over Mega-Ampere Electron Currents in Metals Using Ionization-Driven Resistive Magnetic Fields

The possibility of dynamically shaping mega-ampere electron currents generated in solids by ultraintense laser pulses in various conductor materials has been investigated. By tuning the target ionization dynamics, which depends both on the target material properties and on the input electron beam characteristics, we can control the growth of resistive magnetic fields that feedback on the current transport. As a result, collimation, hollowing, or filamentation of the electron beam can all be obtained. These results are beneficial for applications such as the production of secondary particles and radiation sources and fast ignition of inertial confinement fusion.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Detection and Control of Individual Nuclear Spins Using a Weakly Coupled Electron Spin

We experimentally isolate, characterize, and coherently control up to six individual nuclear spins that are weakly coupled to an electron spin in diamond. Our method employs multipulse sequences on the electron spin that resonantly amplify the interaction with a selected nuclear spin and at the same time dynamically suppress decoherence caused by the rest of the spin bath. We are able to address nuclear spins with interaction strengths that are an order of magnitude smaller than the electron spin dephasing rate. Our results provide a route towards tomography with single-nuclear-spin sensitivity and greatly extend the number of available quantum bits for quantum information processing in diamond.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Tuning the Magnetic Exchange via a Control of Orbital Hybridization in Cr 2 ( Te 1 - x W x ) O 6

Here, we report the complex magnetic phase diagram and electronic structure of Cr 2 ( Te 1-x W x ) O 6 systems. While compounds with different x values possess the same crystal structure, they display different magnetic structures below and above x c = 0.7 , where both the transition temperature T N and sublattice magnetization (M s ) reach a minimum. Unlike many known cases where magnetic interactions are controlled either by injection of charge carriers or by structural distortion induced via chemical doping, in the present case it is achieved by tuning the orbital hybridization between Cr 3d and O 2p orbitals through W 5d states. The result is supported by ab initio electronic structure calculations. Through this concept, we introduce a new approach to tune magnetic and electronic properties via chemical doping.

Zhu, M.↗
DOE OSTI2020

Adsorption-controlled growth of MnTe(Bi2Te3)n by molecular beam epitaxy exhibiting stoichiometry-controlled magnetism

We report the growth of the intrinsic magnetic topological system MnTe ( Bi 2 Te 3 ) n by molecular beam epitaxy. By mapping the temperature and the Bi:Mn flux ratio, it is shown that there is a narrow growth window for the n = 1 phase Mn Bi 2 Te 4 with 2.0 < Bi : Mn < 2.6 at 225°C. In this work, the films are stoichiometric and excess Bi and Te is not incorporated. At higher flux ratios (Bi:Mn≥4.5) it is found that the n = 2 Mn Bi 4 Te 7 phase is stabilized. Transport measurements indicate that the Mn Bi 2 Te 4 and Mn Bi 4 Te 7 undergo magnetic transitions around 25 and 10 K, respectively, consistent with antiferromagnetic phases found in the bulk. Further, for Mn-rich conditions (Bi:Mn<2), ferromagnetism emerges that exhibits a clear hysteretic state in the Hall effect, which likely indicates Mn-doped Mn Bi 2 Te 4 . Understanding how to grow ternary chalcogenide phases is the key to synthesizing new materials and to interface magnetism and topology, which together are routes to realize and control exotic quantum phenomena.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Infrastructure Perception and Control Laboratory [ITS Research Lab]

U.S. Department of Energy national laboratories host world-class advanced computing and modeling capabilities. The National Renewable Energy Labowratory (NREL) inaugurated the Infrastructure Perception and Control Laboratory (IPC Lab) to advance mobility reach into real-world applications.

99 GENERAL AND MISCELLANEOUS↗
DOE OSTI2024

Transient Stability Analysis of Direct Droop-Controlled Grid-Forming Inverter with Current Clipping as Current Limiting Control

This paper investigates the transient stability of the direct droop-controlled grid-forming (GFM) inverter which employs the current clipping-based approach for overcurrent limitation. An equivalent circuit model of the direct-droop GFM inverter with current clipping is developed for this purpose. It is shown that during overcurrent conditions, the GFM inverter with current clipping acts as a voltage source with a virtual resistance behind the inverter filters. To compute the parameters of this virtual resistance an analytical expression is given in the paper. Then utilizing the developed circuit, a criteria to exit current limiting during post-fault conditions is given. The developed analytical approaches are then validated through electromagnetic transient (EMT) simulation results in PSCAD software and finally concluding remarks are drawn.

Mohiuddin, Sheik Mohammad↗
DOE OSTI2021

Metabolic Interactions between Brachypodium and Pseudomonas fluorescens under Controlled Iron-Limited Conditions

Plant iron (Fe) nutritional status has a significant impact on rhizosphere microbial communities. While this effect has been attributed to alterations in the composition of root exudates, the underlying mechanisms are not known. Here, we investigated the effect of Fe deficiency on the interactions between the grass Brachypodium distachyon and the common soil bacteria Pseudomonas fluorescens SBW25. Increased phytosiderophore production was the most significant response of Brachypodium root exudation to Fe deficiency. We observed upregulated expression of phytosiderophore biosynthesis genes but lower accumulated exudate concentrations in the presence of Pseudomonas, indicating that the bacteria degrade these phytosiderophores. Pseudomonas did not produce endogenous siderophores under the low Fe conditions. Rather, the bacterial transcriptome revealed upregulation of four genes in response to Fe deficiency that contain motifs associated with cellular uptake of small molecules. Collectively, these results suggest that rhizosphere Pseudomonas fluorescens may take up phytosiderophores produced by grasses in response to Fe deficiency, and we propose target genes that may be involved. Our findings provide insight into the molecular basis for the exchange of C, N, and Fe between plants and bacteria under Fe deficient conditions. These interactions may contribute to differences in the rhizosphere microbial community structure across soil pH regimes.

Boiteau, Rene M.↗