Search NASA⌕ Search

SEARCH · Search NASA

Results for “Structure”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Switch-off slow shock/rotational discontinuity structures in collisionless magnetic reconnection: What to look for in satellite observations: SWITCH-OFF STRUCTURES IN RECONNECTION

In Innocenti et al. (2015) we have observed and characterized for the first time Petschek-like switch-off slow shock/rotational discontinuity (SO-SS/RD) compound structures in a 2-D fully kinetic simulation of collisionless magnetic reconnection. Observing these structures in the solar wind or in the magnetotail would corroborate the possibility that Petschek exhausts develop in collisionless media as a result of single X point collisionless reconnection. Here we highlight their signatures in simulations with the aim of easing their identification in observations. The most notable signatures include a four-peaked ion current profile in the out-of-plane direction, associated ion distribution functions, increased electron and ion anisotropy downstream the SS, and increased electron agyrotropy downstream the RDs.

58 GEOSCIENCES↗
DOE OSTI2017

Structural Basis for Metastability in Amorphous Calcium Barium Carbonate (ACBC)

Metastable amorphous precursors are emerging as valuable intermediates for the synthesis of materials with compositions and structures far from equilibrium. Recently, it was found that amorphous calcium barium carbonate (ACBC) can be converted into highly barium-substituted “balcite,” a metastable high temperature modification of calcite with exceptional hardness. A systematic analysis ACBC (Ca 1-x Ba x CO 3 ·1.2H 2 O) in the range from x = 0–0.5 is presented. Combining techniques that independently probe the local environment from the perspective of calcium, barium, and carbonate ions, with total X-ray scattering and a new molecular dynamics/density functional theory simulations approach, provides a holistic picture of ACBC structure as a function of composition. With increasing barium content, ACBC becomes more ordered at short and medium range, and increasingly similar to crystalline balcite, without developing long-range order. This is not accompanied by a change in the water content and does not carry a significant energy penalty, but is associated with differences in cation coordination resulting from changing carbonate anion orientation. Therefore, the local order imprinted in ACBC may increasingly lower the kinetic barrier to subsequent transformations as it becomes more pronounced. This pathway offers clues to the design of metastable materials by tuning coordination numbers in the amorphous solid state.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Structural Manipulation of Phase Transitions by Self‐Induced Strain in Geometrically Confined Thin Films

Strain engineering is a well-known method often used to tune material properties in thin films. The most studied sources of strain are lattice mismatch and differential thermal contraction between the substrate and film. However, in materials which undergo a structural phase transition (SPT), a third and often overlooked source of strain may play a very significant role. If the substrate confines the area of the film, the SPT may induce stress which changes the evolution of the transition. This is a 2D analog of the isochoric phase transition between water and ice, where the freezing point drops below 0 °C. To illustrate this, the prototypical Mott insulator V 2 O 3 which has an SPT coupled to a metal–insulator transition is used to show how self-induced strain can drastically alter structural and electronic properties. This effect provides an elegant approach for mapping the phase diagram of the SPT and the transitions coupled to it. Moreover, the magnitude of self-straining is tunable by modifying the substrate morphology. Furthermore, this effect may be important for numerous materials which exhibit an SPT and are subjected to geometrical constraints.

Kalcheim, Yoav↗
DOE OSTI2018

Hollow‐Structured Materials for Thermal Insulation

Abstract Heating and cooling represent a significant portion of overall energy consumption of our society. Due to the diffusive nature of thermal energy, thermal insulation is critical for energy management to reduce energy waste and improve energy efficiency. Thermal insulation relies on the reduction of thermal conductivity of appropriate materials that are engineerable in compositions and structures. Hollow‐structured materials (HSMs) show a great promise in thermal insulation since the existence of high‐density gaseous voids breaks the continuity of heat‐transport pathways in the HSMs to lower their thermal conductivities efficiently. Herein, a timely overview of the recent progress in developing HSMs for thermal insulation is presented, with the focus on summarizing the strategies for creating gaseous voids in solid materials and thus synthesizing various HSMs. Systematic analysis of the documented results reveals the relationship of thermal conductivities of the HSMs and the size and density of voids, i.e., reducing the void size below ≈350 nm is more favorable to decrease the thermal conductivity of the HSMs because of the possible confinement effect originated from the nanometer‐sized voids. The challenges and promises of the HSMs faced in future research are also discussed.

Hu, Feng↗
DOE OSTI2020

Impact of Individual Structural Defects in GaAs Solar Cells: A Correlative and In Operando Investigation of Signatures, Structures, and Effects

Defects usually degrade device performance. Thus, many techniques and effort are devoted to studying semiconductor defects. However, it is rarely known: i) how individual defects affect device performance; ii) how the impact depends on the device operating conditions; iii) how the impact varies from one defect to another; and iv) how these variations are correlated to the microscopic-scale defect structure. To address these crucial questions, an array of correlative and spatially resolved techniques, including electroluminescence, photoluminescence, Raman, I–V characteristics, and high-resolution electron microscopy, are used to characterize dislocation defects in GaAs solar cells. Significantly, the study is carried out in a series mode and in operando. This approach provides quantitative and definitive correlation between the atomistic structure of defects and their explicit effects on device performance, thus giving unprecedented insight into defect physics.

dislocation defects↗
DOE OSTI2017

Solvent Effect on the Photoinduced Structural Change of a Phosphorescent Molecular Butterfly

Abstract Photoinduced structural changes (PSC) is one of the fundamental excited‐state dynamic processes, and yet often very challenging to distinguish from competing electronic excited‐state relaxation channels having similar or even comparable timescales. Here, we report a detailed study on the PSC of a pyrazolate bridged platinum(II) binuclear complex, BFPtPZ (C^NPt(μ‐pz′) 2 PtC^N, C^N=2‐(2,4‐difluorophenyl)pyridine, pz′=pyrazolate), a molecular butterfly, using time‐correlated single photon counting measurements at different wavelengths and sample temperatures. Analysis of the results obtained using dichloromethane (DCM) and ethylene carbonate (EC) as solvents enabled us to reveal an unexpected, strong solvent effect on the PSC processes. We show that a rapid PSC process with a characteristic timescale of 323 ps is observed in DCM, which leads to an excitation equilibrium between the ligand center/metal‐to‐ligand charge transfer ( 3 LC/MLCT) and metal‐metal‐to‐ligand charge transfer ( 3 MMLCT) triplet states. The subsequent relaxation from these electronic states to the ground state takes place in several nanoseconds. In contrast, the corresponding PSC process in EC appears slow at all temperatures studied in our experiments and showed no sign of such excitation equilibrium. The observed solvent effect is found to arise from distinct solvent properties including their viscosities and polarities as well as the peculiar electronic excited‐states of the butterfly‐like molecules with charge transfer character.

Ma, Ying‐Zhong↗
DOE OSTI2017

Effect of compensatory immigration on the genetic structure of coyotes

Despite efforts to reduce their impacts on livestock and native ungulates within the southeastern United States, coyotes (Canis latrans) are able to recover from control programs. It is unknown how coyotes compensate for high mortality following trapping, so there is great interest to identify methods that can provide insight into coyote response to intensive trapping. To investigate if population genetic tools can decipher how coyotes recover from intensive trapping, we combined an empirical test of how genetic differentiation, diversity, and familial structure changed following trapping on the Savannah River Site (SRS), South Carolina with spatially explicit genetic simulations.The empirical dataset found that pre- and post-trapping periods had similar genetic diversities and were not genetically differentiated as expected by either compensatory reproduction or immigration from a single genetic source. The post-trapping coyote populations exhibited weaker signatures of philopatry with little evidence for increased dispersal distances of young coyotes, which suggests immigration caused a decrease in familial structure. Our simulations indicated that spatial autocorrelation coefficients and observed heterozygosities change as immigration increases while population differentiation, allelic richness, and displacement distances do not. Collectively, our results suggest that coyotes recover from intensive trapping via both reproduction and immigration, which likely makes preventing compensation difficult. Furthermore, monitoring post-trapping populations may offer more insight into maximizing the effectiveness of control efforts, and based on our simulations, population genetics can provide critical information about the amount of compensatory immigration following trapping.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Optimizing the self-assembly of conjugated polymers and small molecules through structurally programmed non-covalent control

Organic conjugated polymers and oligomers are key electronic materials for applications such as transistors, photovoltaics, and light emitting devices due to their potential for solution processability, mechanical flexibility, and precise structure-based tuning compared to inorganic materials. In dilute environments, the optoelectronic properties of conjugated polymers are largely governed by their constitutional structure and, to a lesser degree, their solution-state intramolecular configuration. In the solid state, intramolecular conformation and intermolecular electronic coupling impact these properties substantially, especially in relation to device performance. Therefore, an increasingly important area of research concerning conjugated materials is developing design strategies aimed at optimizing the solid-state packing for electronic applications. Programming solid-state packing arrangements through discrete non-covalent interactions is an emerging strategy within the context of conjugated polymers. This review focuses on the use of the two most prevalent discrete and directional interactions used to dictate the self-assembly of conjugated polymers and oligomers—hydrogen bonds and chalcogen bonds. Further, we also discuss how these design motifs can imbue conjugated materials with appealing physical properties while simultaneously retaining or improving electronic capabilities.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Fluctuating Hydrodynamic Methods for Fluid-Structure Interactions in Confined Channel Geometries

Computational methods are developed for the study of fluid-structure interactions subject to thermal fluctuations when conned within channels having slit-like geometry. The methods take into account the hydrodynamic coupling and diusivity of microstructures when influenced by their proximity to no-slip walls. We develop stochastic numerical methods subject to no-slip boundary conditions using a staggered nite volume discretization. We show that by imposing an exact fluctuation-dissipation condition to discretize the stochastic driving elds combined with using an exact projection method to enforce incompressibility is sucient to ensure results consistent with statistical mechanics. We demonstrate our methods by investigating how the proximity of ellipsoidal colloids to the channel wall eects their active hydrodynamic responses and passive diusivity. We also study the collective dynamics of a large number of particles by considering the intermediate scattering functions for the relaxation of density fluctuations. We expect our introduced stochastic computational methods to be applicable broadly to applications in which confinement effects play an important role in the dynamics of microstructures subject to hydrodynamic coupling and thermal fluctuations. uctuations.

Wang, Yaohong↗