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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Quantum electron transport in degenerate donor–acceptor systems

In this work, we develop a mathematically rigorous theory for the quantum transfer processes in degenerate donor–acceptor dimers in contact with a thermal environment. We explicitly calculate the transfer rates and the acceptor population efficiency. The latter depends critically on the initial donor state. Additionally, we show that quantum coherence in the initial state enhances the transfer process. If the electron is initially shared coherently by the donor levels, then the efficiency can reach values close to 100%, while an incoherent initial donor state will significantly suppress the efficiency. The results are useful for a better understanding of the quantum electron transport in many chemical, solid state, and biological systems with complex degenerate and quasi-degenerate energy landscapes.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Performance of Scintillation Detectors Based on Quantum Dots in a Semiconductor Matrix (Final Technical Report)

InAs Quantum Dots (QDs) embedded into GaAs semiconductor waveguide have unique scintillation properties, valuable for nuclear security, medical imaging, and high energy physics. In this work, we developed thick (~25um) epitaxial heterostructres with high luminescence efficiency composed of self-assembled nano-engineered InAs QDs grown by molecular beam epitaxy. In this type of detector, the GaAs matrix acts as a stopping material for charged particles or photons generating electrons captured by the QDs acting as luminescence centers. The QD medium is designed to provide fast capture of electrons into QDs (few ps), high QD luminescence efficiency at room temperature (>50%), and strong red-shift of photoluminescence (PL) from the GaAs absorption edge (>250nm). Typical devices consist of a 10-25um thick GaAs layer with embedded sheets of modulation p-type doped InAs QDs and an InGaAs photodetector tuned to the QD emission wavelength. The thick GaAs layer acts also as a waveguide when layer-transferred onto a low-index substrate. Waveguiding and self-absorption (~1cm -1 ) were studied using photoluminescence with scanning laser excitation and modeled with ray optics approximation and geometrical coupling of high-index waveguide to a collection fiber. Scintillating signals from α-particles were analyzed with both an external photodiode (PD) and an integrated PD which provided an improved optical coupling. In the former case, the external InGaAs PD was air-coupled to the scintillator and had the light collection efficiency of about 1% corresponding to limited light extraction through a planar interface with air due to total internal reflection. The mean charge collected by the integrated PD was in the range of (3÷5)×10 4 photoelectrons per 1 MeV of deposited energy, or ~13-20% of the theoretically achievable light yield. Timing of the integrated device was measured by wire-bonding it to the input of an 8 GHz IC. The scintillation response shows an extremely fast 0.3-0.6 ns decay constant and about 40-70 ps time resolution, limited by the system noise. The combined light yield and decay time makes the InAs/GaAs QD heterostructures the fastest high yield scintillation material reported making it valuable for high energy physics and medical imaging applications.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Relaxed Current Matching Requirements in Highly Luminescent Perovskite Tandem Solar Cells and Their Fundamental Efficiency Limits

Perovskite-based tandem solar cells are of increasing interest as they approach commercialization. Here we use experimental parameters from optical spectroscopy measurements to calculate the limiting efficiency of perovskite–silicon and all-perovskite two-terminal tandems, employing currently available bandgap materials, as 42.0% and 40.8%, respectively. We show luminescence coupling between subcells (the optical transfer of photons from the high-bandgap to low-bandgap subcell) relaxes current matching when the high-bandgap subcell is a luminescent perovskite. We calculate that luminescence coupling becomes important at charge trapping rates (=10 6 s -1 ) already being achieved in relevant halide perovskites. Luminescence coupling increases flexibility in subcell thicknesses and tolerance to different spectral conditions. For maximal benefit, the high-bandgap subcell should have the higher short-circuit current under average spectral conditions. This can be achieved by reducing the bandgap of the high-bandgap subcell, allowing wider, unstable bandgap compositions to be avoided. Lastly, we visualize luminescence coupling in an all-perovskite tandem through cross-section luminescence imaging.

14 SOLAR ENERGY↗
DOE OSTI2021

Polarizable Water Potential Derived from a Model Electron Density

A new empirical potential for efficient, large scale molecular dynamics simulation of water is presented. The HIPPO (Hydrogen-like Intermolecular Polarizable POtential) force field is based upon the model electron density of a hydrogen-like atom. This framework is used to derive and parametrize individual terms describing charge penetration damped permanent electrostatics, damped polarization, charge transfer, anisotropic Pauli repulsion, and damped dispersion interactions. Initial parameter values were fit to Symmetry Adapted Perturbation Theory (SAPT) energy components for ten water dimer configurations, as well as the radial and angular dependence of the canonical dimer. The SAPT-based parameters were then systematically refined to extend the treatment to water bulk phases. The final HIPPO water model provides a balanced representation of a wide variety of properties of gas phase clusters, liquid water, and ice polymorphs, across a range of temperatures and pressures. This water potential yields a rationalization of water structure, dynamics, and thermodynamics explicitly correlated with an ab initio energy decomposition, while providing a level of accuracy comparable or superior to previous polarizable atomic multipole force fields. The HIPPO water model serves as a cornerstone around which similarly detailed physics-based models can be developed for additional molecular species.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Electronic Tuning of Photoexcited Dynamics in Heteroleptic Cu(I) Complex Photosensitizers

Photoexcited dynamics of heteroleptic Cu(I) complexes as noble-metal-free photosensitizers are closely intertwined with the nature of their ligands. By utilizing ultrafast optical and X-ray transient absorption spectroscopies, we characterized a new set of heteroleptic Cu(I) complexes [Cu(PPh 3 )( 2 )(BPy R )] + (R = CH 3 , H, Br to COOCH 3 ), with an increase in the electron-withdrawing ability of the functional group (R). Here we found that after the transient photooxidation of Cu(I) to Cu(II), the increasing electron-withdrawing ability of R barely affects the internal conversion (IC) (e.g., Jahn-taller (JT) distortion) between singlet MLCT states. However, it does accelerate the dynamics of intersystem crossing (ISC) between singlet and triplet MLCT states and the subsequent decay from the triplet MLCT state to the ground state. The associated lifetime constants are reduced by up to 300%. Our understanding of the photoexcited dynamics in heteroleptic Cu(I) complexes through ligand electronic tuning provides valuable insight into the rational design of efficient Cu(I) complex photosensitizers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Enhancing Nitrogen Activation in Electrochemical Reduction: The Role of Rare Earth Oxide Surface Configurations

The pursuit of sustainable ammonia synthesis has prompted the exploration of ambient electrochemical nitrogen reduction reaction (e-NRR) as an alternative to the energy-intensive Haber-Bosch process. Here, this study conducted a theoretical investigation into the use of rare earth oxide materials, specifically dysprosium oxide (Dy 2 O 3 ), as potential electrocatalysts for NRR. Utilizing spin-polarized density functional theory calculations, we explored the interaction between Dy 2 O 3 surfaces and nitrogen (N 2 ) molecules, examining the capability of Dy 2 O 3 to adsorb and activate N 2 under ambient conditions. The results indicate that Dy 2 O 3 surfaces exhibit diverse configurations and bonding environments, providing a variety of reactive sites that display different behaviors in N 2 adsorption and activation. The distinctive electronic structure and surface chemistry of a particular Dy 2 O 3 surface configuration were found to significantly enhance the activation of N 2 by promoting charge transfer, which facilitates the NRR process. This research provides deep insights into the mechanistic pathways of N 2 reduction over Dy 2 O 3 , highlighting the surface properties as pivotal in catalysis. These theoretical insights serve as a foundation for the development of novel rare earth-based electrocatalytic materials for efficient ambient e-NRR, potentially transforming ammonia production into a greener and more energy-efficient process.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2021

PCET-Based Ligand Limits Charge Recombination with an Ir(III) Photoredox Catalyst

Upon photoinitiated electron transfer, charge recombination limits the quantum yield of photoredox reactions for which the rates for the forward reaction and back electron transfer are competitive. Taking inspiration from a proton-coupled electron transfer (PCET) process in Photosystem II, here a benzimidazole-phenol (BIP) has been covalently attached to the 2,2'-bipyridyl ligand of [Ir(dF(CF 3 )ppy) 2 (bpy)][PF 6 ] (dF(CF 3 )ppy = 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; bpy = 2,2'-bipyridyl). Excitation of the [Ir(dF(CF 3 )ppy) 2 (BIP-bpy)][PF 6 ] photocatalyst results in intramolecular PCET to form a charge-separated state with oxidized BIP. Subsequent reduction of methyl viologen dication (MV 2+ ), a substrate surrogate, by the reducing moiety of the charge separated species demonstrates that the inclusion of BIP significantly slows the charge recombination rate. The effect of ~24-fold slower charge recombination in a photocatalytic phthalimide ester reduction resulted in a greater than 2-fold increase in reaction quantum efficiency.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2023

Flexoelectricity and New Phenomena

It is easy to miss the scientific implications of our recent work on Triboelectricity. Everyone knows that rubbing and contact can produce static electricity; less appreciated is that the thermodynamic driver has been an open question since static electricity was first observed by Thales of Miletus around 585 BC. People think they understand it, for instance one common explanation that can still be found in the current literature is that differences in the work function drives charge transfer, often called the Volta-Helmholtz hypothesis. As summarized in 1967 by Harper, this fails to explain many experimental observations, for instance that charging can occur when two pieces of the same material are rubbed against each other. We are the first to place triboelectricity on a solid foundation rooted in quantum mechanics – the flexoelectric effect. We were able to explain a significant number of previously unexplained phenomena: 1) Bipolar tribocurrents associated with stick-slip, due to the change in sign of the strain gradients. 2) A one-third power scaling of tribocurrents with indentation force. 3) Tribocharging when two identical materials are used, these being due to local variations in the asperities so there are usually local potential differences. 4) Inhomogeneous charging of insulators, related to the statistical nature of asperities. 5) An experimentally observed reversal in the sign of charge transfer for negative and positive curvature, which is related to a change in the sign of the strain gradient. Exploiting our DOE prior funded work on flexoelectricity, we obtained semi-quantitative matching to existing experimental measurements of the surface charge in triboelectric experiments. The work has been well received in the literature. The work has been the focus of a number of popular science press articles, and also formed the basis for a Podcast for children 6-10 “The Rise and Fall of Static Man” posted in December 2019 by NPR as part of their “Wow in the World” series. I was also briefly interviewed by the Chicago PBS station in January 2020. This work are significant for a wide range of energy applications; to quote from an independent source: "Triboelectric power has plenty of potential, says Wenzhuo Wu, an assistant professor of engineering at Purdue. If the basics of static electricity are better understood, we could maximize the efficiency of wind or wave power generators, Wu says. The body's own movement could be used to power internal medical devices. Imagine being able to create a roof shaped to harness the power of a raindrop — the friction of the rain passing over the surface — to generate triboelectricity, powering the building below it." This is the start of new science, some of which we already partially understand such as the role of band bending in charge transfer. We need to understand charge transfer combining elasticity, quantum mechanics, band bending and defect states. These directly involve several of the DOE Grand Challenges: "How do we control material processes at the level of electrons? How do remarkable properties of matter emerge from complex correlations of the atomic or electronic constituents and how can we control these properties? How do we characterize and control matter away—especially very far away— from equilibrium?" I will argue that this work truly falls into the class of disruptive science; it is not just a simple extension, linear science. Not everyone will accept the approach. Since we explain far more about triboelectricity than anyone before, the preponderance of evidence supports the model. The feedback I have received is that many agree with the work, to quote: "The model makes sense, says Michael McAlpine, a professor of engineering at the University of Minnesota. "It's such a simple explanation, I was surprised I didn't put my finger on that," McAlpine says." The proposal received strong reviews. It was also publicized on the Department of Energy Web Site.

16 TIDAL AND WAVE POWER↗
DOE OSTI2022

Conjugated Polyelectrolyte-Based Complex Fluids as Aqueous Exciton Transport Networks

The ability to assemble artificial systems that mimic aspects of natural light-harvesting functions is fascinating and attractive for materials design. Given the complexity of such a system, a simple design pathway is desirable. Here, we argue that associative phase separation of oppositely charged conjugated polyelectrolytes (CPEs) can provide such a path in an environmentally benign medium: water. We find that complexation between an exciton–donor and acceptor CPE leads to formation of a complex fluid. We interrogate exciton transfer from the donor to the acceptor CPE within the complex fluid and find that transfer is highly efficient. We also find that excess molecular ions can tune the modulus of the inter-CPE complex fluid. Even at high ion concentrations, CPEs remain complexed with significantly delocalized electronic wavefunctions. In conclusion, our work lays the rational foundation for complex, tunable aqueous light-harvesting systems via the intrinsic thermodynamics of associative phase separation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Steric Modulation of Protein‐Mediated Nanoparticle Assembly: Controlling Cluster Size, Polydispersity, and FRET Responses by Rebalancing Short‐ and Long‐Range Interactions

Understanding and manipulating protein-nanoparticle interactions is of broad interest to fields ranging from nanomedicine to the biological fabrication of functional hierarchical materials. This study investigates how steric forces introduced by a pegylated derivative of superfolder green fluorescent protein (sfGFP) that is monofunctional for silica binding modulate the delicate interplay of long-range (electrostatic and van der Waals) and short-range (protein-mediated) interactions in pH-responsive silica nanoparticle (SiNP) assembly by bifunctional silica-binding sfGFP. Increasing the length of the PEG segment and pre-incubating SiNPs with increasing concentrations of pegylated proteins enables precise control over cluster size within the 800–1450 nm range with a sixfold decrease in polydispersity index to a remarkable 0.1 endpoint. Weakening short-range attractive interactions via mutagenesis extends this control to clusters in the 50–250 nm range and reveals that the Förster resonance energy transfer (FRET) efficiency of clusters scales linearly with cluster diameter below 230 nm but increases only by 15% as clusters grow to 1450 nm. Furthermore, these findings enable the development of a system that provides an optical readout to dynamic changes in solution conditions enacted by a combination of pH adjustment and ion charge screening.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1987

Diffraction efficiency of photothermoplastic layers for the recording of discrete holograms

An experimental and theoretical study of the dependence of eta of a digital phase Fourier hologram of a point object on the amount of deformation delta and the discrete-structure parameters representing the hologram is detailed. An expression is given for eta. Experiments were performed on photothermoplastic layers based on polyvinyl carbazole and trinitrofluorenone charge transfer complexes. The maximum eta, 2%, is found at delta = 0.56 micron.

Koreshev, S. N.↗
NASA NTRS1975

Investigation of an inverted meniscus heat pipe wick concept

A wicking concept is described for efficient evaporation of heat pipe working fluids under diverse conditions. It embodies the high heat transfer coefficient of the circumferential groove while retaining the circumferential fluid transport capability of a thick porous wick or screen. Experimental tests are described which substantiate the efficacy of the evaporation technique for a circumferentially-grooved heat pipe charged alternately with ammonia and R-ll (CCl3F). With ammonia, heat transfer coefficients in the range of 2 to 2.7 W/sq cm K were measured at heat flux densities up to 20 W/sq cm while, with R-ll, a heat transfer coefficient of l.0 W/sq cm K was measured with flux densities up to 5 W/sq cm. Heat transfer coefficients and flux densities were unusually high compared to literature data for other nonboiling evaporative surfaces.

Saaski, E. W.↗
NASA NTRS2001

High speed CMOS imager with motion artifact supression and anti-blooming

An image sensor includes pixels formed on a semiconductor substrate. Each pixel includes a photoactive region in the semiconductor substrate, a sense node, and a power supply node. A first electrode is disposed near a surface of the semiconductor substrate. A bias signal on the first electrode sets a potential in a region of the semiconductor substrate between the photoactive region and the sense node. A second electrode is disposed near the surface of the semiconductor substrate. A bias signal on the second electrode sets a potential in a region of the semiconductor substrate between the photoactive region and the power supply node. The image sensor includes a controller that causes bias signals to be provided to the electrodes so that photocharges generated in the photoactive region are accumulated in the photoactive region during a pixel integration period, the accumulated photocharges are transferred to the sense node during a charge transfer period, and photocharges generated in the photoactive region are transferred to the power supply node during a third period without passing through the sense node. The imager can operate at high shutter speeds with simultaneous integration of pixels in the array. High quality images can be produced free from motion artifacts. High quantum efficiency, good blooming control, low dark current, low noise and low image lag can be obtained.

Pain, Bedabrata↗
NASA NTRS2006

Image sensor with motion artifact supression and anti-blooming

An image sensor includes pixels formed on a semiconductor substrate. Each pixel includes a photoactive region in the semiconductor substrate, a sense node, and a power supply node. A first electrode is disposed near a surface of the semiconductor substrate. A bias signal on the first electrode sets a potential in a region of the semiconductor substrate between the photoactive region and the sense node. A second electrode is disposed near the surface of the semiconductor substrate. A bias signal on the second electrode sets a potential in a region of the semiconductor substrate between the photoactive region and the power supply node. The image sensor includes a controller that causes bias signals to be provided to the electrodes so that photocharges generated in the photoactive region are accumulated in the photoactive region during a pixel integration period, the accumulated photocharges are transferred to the sense node during a charge transfer period, and photocharges generated in the photoactive region are transferred to the power supply node during a third period without passing through the sense node. The imager can operate at high shutter speeds with simultaneous integration of pixels in the array. High quality images can be produced free from motion artifacts. High quantum efficiency, good blooming control, low dark current, low noise and low image lag can be obtained.

Pain, Bedabrata↗
DOE OSTI2026

Improvements to Stripping Foils in BTA

To achieve the desired charge state in the Alternating Gradient Synchrotron (AGS), there is a stripping foil in the Booster to AGS (BtA) transfer line. For light ions, such as p, d, and h, they are already fully stripped in the Booster, or ions such as O have few electrons to be stripped off, and a thin foil of aluminum is used. For medium weight ions, such as Zr and Ru, they use what foils are available which have been optimized for heavy and light ions at 100 MeV/u. This worked well in the Zr case which achieved a measured stripping efficiency of 87%, contrary to the Ru case which achieved a measured stripping efficiency of 63%. For the EIC early science, Ag is currently intended to be the first ion. Using our existing foils, this would, at best, yield a helium-like charge state with a theoretical stripping efficiency of 50%. To improve on this, two foils of Ti or Nb have been selected to provide optimal stripping efficiency of Ag, and other energetic medium ions, which can reach 90% efficiency to fully strip the Ag ions. For heavy ions, the Al+C or Al+Ni stripping foils (sandwich foils) are used to produce helium-like charge states. For high intensity operation of Au, the Al foil nears its melting point and, in some cases, has been severely damaged. To maximize foil lifetime, there is a maximum intensity per unit time in BTA dependent on the Booster and AGS timing configurations. To mitigate the risk of melting the Al, replacement foils that use Ti, Nb, or anodized Al have been evaluated for their improved thermal qualities. At the selected thicknesses, each foil has equivalent charge distributions before entering the C foil, and should provide the same stripping efficiency through the entire sandwich.

43 PARTICLE ACCELERATORS↗
DOE OSTI2021

Neural network representation of electronic structure from ab initio molecular dynamics

Despite their rich information content, electronic structure data amassed at high volumes in ab initio molecular dynamics simulations are generally under-utilized. Here we introduce a transferable high-fidelity neural network representation of such data in the form of tight-binding Hamiltonians for crystalline materials. This predictive representation of ab initio electronic structure, combined with machine-learning boosted molecular dynamics, enables efficient and accurate electronic evolution and sampling. When it is applied to a one-dimension charge-density wave material, carbyne, we are able to compute the spectral function and optical conductivity in the canonical ensemble. The spectral functions evaluated during soliton-antisoliton pair annihilation process reveal significant renormalization of low-energy edge modes due to retarded electron-lattice coupling beyond the Born–Oppenheimer limit. The availability of an efficient and reusable surrogate model for the electronic structure dynamical system will enable calculating many interesting physical properties, paving the way to previously inaccessible or challenging avenues in materials modeling.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2023

Stripping Foils at the RHIC Injectors: A History

Stripping foils are used throughout Collider-Accelerator Department to strip ions of unwanted electrons. This is done in stages throughout the complex to optimize the transmission of particles. For polarized protons, they are injected into the AGS Booster as H-, and then stripped to H+. For ions from the tandem source, there are two stripping foils: one at the center and the other at the end of the tandem. The tandem foils will not be covered in this document. For ions out of the EBIS source, there is no use of stripping foils upstream of injection into the Booster. All ions that are not fully stripped in Booster are stripped further in the Booster to AGS (BTA) transfer line stripping foil assembly. Finally, any ions that are not fully stripped in the AGS are either stripped in the AGS to RHIC transfer line stripping foil, or as they are being dumped in the AGS by the plunging stripping foil (PSF) dependent on the particles intensity and charge. The BTA foil is the workhorse of the complex, having to provide the desired charge state of ions as light as O, and as heavy as Uranium. Optimizing the stripping efficiency to the desired charge state, while minimizing the energy lost and emittance growth from transiting the foil, requires careful consideration of the foil materials and thicknesses. This paper serves an overview of stripping foils at C-AD, with a review of foils from the construction of the AGS Booster to now.

43 PARTICLE ACCELERATORS↗
DOE OSTI2022

Regulating Electronic Structure of Single-Atom Catalysts toward Efficient Bifunctional Oxygen Electrocatalysis

We report electronic structure of single-atom catalysts (SACs) is critical for bifunctional oxygen electrocatalysis by adjusting the binding energy in oxygen-containing intermediates. However, the regulation of electronic structure has always been a challenge to improve catalytic reactivity. Herein, by introducing a heterogenous metal, the electronic structure through a direct bonding interaction to the active center atom is effectively adjusted. Partial charge transfer between the two atoms optimizes the binding energy of intermediates and reducing the energy barrier of the catalytic reaction. Theoretical calculations confirm these effects and the uniform distribution of 3d orbitals, leading to the improvement of bifunctional oxygen electrocatalytic reactivity. Benefiting from these attributes, the as-constructed bifunctional catalyst enables outstanding electrocatalytic performances in both oxygen reduction and hydrogen oxidation in various energy storage systems. The generality and expandability of this strategy is demonstrated by further successful development of other dual-metal catalysts systems with various active metals.

36 MATERIALS SCIENCE↗