SEARCH · Engineering Papers

Results for “hole transfer”

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.

SourceResearch connectionFrom yearThrough year

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

At least 559 records · Page 31

All sources NASA NTRS DOE OSTI

DOE OSTI2025

Understanding and controlling the fundamental photochemistry of protonic solar energy conversion (Final Scientific/Technical Report)

The selectivity and activity of most fuel-forming reactions are tunable by the concentration of electrons, holes, protons, and/or hydroxides. That is one reason why traditional solar energy conversion processes are used to drive these reactions, where light directly increases the concentration of electrons and holes. We, instead, took a different approach, using light to increase the concentration of protons and/or hydroxides. For this, we leveraged photophysical and solid-state physics theories, techniques, and design strategies previously developed for the study of traditional solar energy conversion processes. We designed and fabricated several materials platforms based on molecular photoacid and photobase dyes coupled with ion-exchange polymer membranes and assessed their fundamental photoelectrochemistry. We demonstrated control over built-in electric potentials, photovoltages, proton-transfer kinetics, and efficiency limits to our approach. Understanding the basic science of these dye-sensitized protonic membranes and their photochemical mechanisms is of use to the DOE-relevant processes of solar photochemical fuel formation, solar photodialytic desalination, and solar cells. Moreover, functional materials that we developed from this project may be of use to several broad-reaching fields and may provide the foundation for a completely new, inexpensive, and robust solar energy conversion technology.

14 SOLAR ENERGY↗

NASA NTRS1983

Heat transfer characteristics for jet array impingement with initial crossflow

Two-dimensional arrays of circular air jets impinging on a heat transfer surface parallel to the jet orifice plate are considered. The jet flow, after impingement, is constrained to exit in a single direction along the channel formed by the jet orifice plate and the heat transfer surface. In addition to the crossflow which originates from the jets following impingement, an initial crossflow is present which approaches the array through an upstream extension of the channel. The temperature of the initial crossflow air may differ from the jet air temperature. The configurations considered are intended to model the impingement cooled midchord region of gas turbine airfoils in cases where an initial crossflow is also present. Nusselt numbers and dimensionless adiabatic wall temperatures resolved to one streamwise jet hole spacing were experimentally determined for ratios of the initial crossflow rate to the total jet flow rate ranging from zero to unity. These are presented and discussed relative to the flow and geometric parameters.

Florschuetz, L. W.↗

NASA NTRS2003

Experimental Hypersonic Aerodynamic Characteristics of the Space Shuttle Orbiter for a Range of Damage Scenarios

Aerodynamic tests in support of the Columbia accident investigation were conducted in two hypersonic wind tunnels at the NASA Langley Research Center, the 20-Inch Mach 6 Air Tunnel and the 20-Inch Mach 6 CF4 Tunnel. The primary purpose of these tests was to measure the forces and moments generated by a variety of outer mold line alterations (damage scenarios) using 0.0075-scale models of the Space Shuttle Orbiter (approximately 10 inches in length). Simultaneously acquired global heat transfer mappings were obtained for a majority of the configurations tested. Test parameters include angles of attack from 38 to 42 deg, unit Reynolds numbers from 0.26 to 3.0 x10^6 per foot, and normal shock density ratios of 5 (Mach 6 air) and 12 (Mach 6 CF4). The damage scenarios evaluated included asymmetric boundary layer transition, gouges in the windward surface acreage thermal protection system tiles, wing leading edge damage (partially and fully missing reinforced carbon-carbon (RCC) panels), holes through the wing from the windward surface to the leeside, deformation of the wing windward surface, and main landing gear door and/or gear deployment. The aerodynamic data were compared to the magnitudes and directions observed in flight, and the heating images were evaluated in terms of the location of the generated disturbances and how these disturbance might relate to the response of discrete gages on the Columbia Orbiter vehicle during entry. The measured aerodynamic increments were generally small in magnitude, as were the flight-derived values during most of the entry. Asymmetric boundary layer transition (ABLT) results were consistent with the flight-derived Shuttle ABLT model, but not with the observed flight trends for STS-107. The partially missing leading edge panel results best matched both the early aerodynamic and heating trends observed in flight. A progressive damage scenario is presented that qualitatively matches the flight observations for the full entry.

Brauckman, Gregory J.↗

NASA NTRS2012

Processing and Properties of Vacuum Assisted Resin Transfer Molded Phenylethynyl Terminated Imide Composites

Polyimide composites are very attractive for applications that require a high strength to weight ratio and thermal stability. Recent work at NASA Langley Research Center (LaRC) has concentrated on developing new polyimide resin systems that can be processed without the use of an autoclave for advanced aerospace applications. Due to their low melt viscosities and long melt stability, certain phenylethynyl terminated imides (PETI) can be processed into composites using high temperature vacuum assisted resin transfer molding (HT-VARTM). VARTM has shown the potential to reduce the manufacturing cost of composite structures. In the current study, two PETI resins, LARC(Trademark) PETI-330 and LARC(Trademark) PETI-9, were infused into carbon fiber preforms at 260 C and cured at temperatures up to 371 C. Photomicrographs of polished cross sections were taken and void contents, determined by acid digestion, were below 4.5%. Mechanical properties including short block compression (SBC), compression after impact (CAI), and open hole compression (OHC) were determined at room temperature, 177 C, and 288 C. Both PETI-9 and PETI-330 composites demonstrated very good retention of mechanical properties at elevated temperatures. SBC and OHC properties after aging for 1000 hours at temperatures up to 288 C were also determined.

Cano, Roberto J.↗

DOE OSTI2026

Ultrafast Relaxation of MLCT Excited States in Manganese Tricarbonyl Complexes: Insights from Polarization-Resolved Femtosecond X-ray Absorption Spectroscopy at the Mn and Br K-Edges

Optical absorption features that are often described as metal-to-ligand charge transfer (MLCT) bands underlie the utility of many metal coordination complexes by harnessing the energy of light to drive otherwise inaccessible chemical reactions. Excitation of these bands triggers rapid electronic dynamics that can be challenging to understand, due to complicated potential energy landscapes and highly correlated electronic and nuclear degrees of freedom in metal-containing compounds. The lowest-energy absorption bands in Mn complexes containing alpha-diimine, carbonyl, and halide ligands are particularly interesting, due to the delocalized nature of charge transfer upon excitation. Here, in this study, we report experimental ultrafast dynamics measurements for the series of compounds Mn(CO) 3 ( R bpy)Br ( R bpy = 4,4′-disubstituted-2,2′-bipyridine; R = H, CF 3 , or NO 2 ) using polarization-resolved, femtosecond X-ray absorption spectroscopy (XAS) at both the Mn and Br K edges. The appearance of a new absorption feature in the Br pre-edge spectrum upon optical excitation reveals the instantaneous formation of an electronic hole that is partially localized on the Br atom and has a lifetime that depends on the electron withdrawing character of the R bpy ligand. For two of the complexes (R = H or CF 3 ), a large expansion of the Mn–Br distance results in a rapid redistribution of the hole and a corresponding decrease in anisotropy of the absorption feature within 50 fs, after which the absorption into the hole disappears on a time scale shorter than 300 fs. We observe a different result for the NO 2 substituted compound, for which the Mn–Br bond contracts and the absorption into the Br-centered hole persists beyond the 2 ps time scale of our measurement. The Mn pre-edge spectrum also reveals structural changes for these three complexes, but the new Mn absorption features become evident only after the nuclei respond to the initial excitation, which allows mixing of Mn 3d and 4p orbitals. The combined use of ultrafast Mn and Br K-edge spectroscopy provides unique insight into the ways in which bipyridine substitution alters the excited-state dynamics, including a very different structural response for the most strongly electron withdrawing substituent.

Otolski, Christopher J. [Univ. of Kansas, Lawrence↗

NASA NTRS1979

Flow visualization of discrete-hole film cooling with spanwise injection over a cylinder

Insight into the fluid mechanics encountered when film air from a single row of holes is injected over a cylinder in a mainstream at conditions simulating a film cooled, turbulent-vane leading edge was investigated. Smoke was added to the cooling air to visualize its flow path. Film was injected in the spanwise direction at angles of 30 deg and 45 deg to the surface; at angular locations of 15 deg, 30 deg, 45 deg, and 60 deg from the stagnation line; and at various blowing ratios. The observations were related to the measured heat transfer data of others. The results indicate that, in addition to the expected growth in film thickness and the greater penetration of the boundary layer with increasing blowing ration, there was an absence of spanwise spreading and only a small spanwise deflection of the injected film.

Russell, L. M.↗

NASA NTRS1990

Preparation and characteristics of superconducting cuprate thin films: Nd(2-x)Ce(x)CuO4 and substituted Bi-system

Characteristics of the electron-doped-type Nd(2-x)Ce(x)CuO4 systems and substituted Bi2(Sr,Ln)3Cu2Oy system were systematically studied using the high quality thin-film samples. The Nd(2-x)Ce(x)CuO4 thin films with various Ce concentrations, x, were prepared by RF magnetron sputtering on SrTiO3 heated at around 500 C. After subsequent annealing at 1100 C in air, the films showed the c-axis orientation normal to the substrates. By means of the reducing treatment (annealing in a vacuum), superconductivity was induced for the films with 0.14 is less than or equal to x is less than or equal to 0.18. The superconductivity and transport properties of the films were strongly affected by the reducing treatment. The x = 0.15 film exhibited a sharp superconducting transition with zero resistivity at 22 K, in consistent with the diamagnetic properties. The resistivity of the films was fairly low with metallic characteristics, and the sight of the Hall coefficient was negative in the normal state. On the other hand, the normal-state optical measurements showed that the undoped Nd2CuO4 is a semiconductor with a charge transfer gap of 1.3 eV, and that, when Ce ions were doped, a plasma reflection due to the free-carriers came to be seen with the plasma frequency of 1.07 eV for 0.14 is less than or equal to x is less than or equal to 0.18. Moreover, x ray photoemission study revealed that the Cu valence of the film decreased for 2(+) for x = 0 to 1(+) for x = 0.15. These physical properties are in contrast with those of hole-doped-type cuprate superconductors. Bi2(Sr,Ln)3Cu2Oy thin films were also prepared on MgO substrates heated at 600 to 700 C by similar methods. It was found that the growth conditions for Bi-systems with two CuO2 planes were different for each composition and species of lanthanoid in the films. Moreover, preparation of Bi-system with three CuO2 planes was very difficult when lanthanoid atoms were doped in the system. Their electric transport properties and x ray photoemission spectroscopy were investigated. Carrier concentration and Cu valence were discussed with regard to the superconductivity.

Adachi, H.↗

NASA NTRS2013

Multiphysics Modeling of a Single Channel in a Nuclear Thermal Propulsion Grooved Ring Fuel Element

In the past, fuel rods have been used in nuclear propulsion applications. A new fuel element concept that reduces weight and increases efficiency uses a stack of grooved discs. Each fuel element is a flat disc with a hole on the interior and grooves across the top. Many grooved ring fuel elements for use in nuclear thermal propulsion systems have been modeled, and a single flow channel for each design has been analyzed. For increased efficiency, a fuel element with a higher surface-area-to-volume ratio is ideal. When grooves are shallower, i.e., they have a lower surface area, the results show that the exit temperature is higher. By coupling the physics of turbulence with those of heat transfer, the effects on the cooler gas flowing through the grooves of the thermally excited solid can be predicted. Parametric studies were done to show how a pressure drop across the axial length of the channels will affect the exit temperatures of the gas. Geometric optimization was done to show the behaviors that result from the manipulation of various parameters. Temperature profiles of the solid and gas showed that more structural optimization is needed to produce the desired results. Keywords: Nuclear Thermal Propulsion, Fuel Element, Heat Transfer, Computational Fluid Dynamics, Coupled Physics Computations, Finite Element Analysis

Kim, Tony↗

NASA NTRS2024

Time-dependent Photoionization Modeling of Warm Absorbers: High-resolution Spectra and Response to Flaring Light Curves

Time-dependent photoionization modeling of warm absorber (WA) outflows in active galactic nuclei can play an important role in understanding the interaction between WAs and the central black hole. The WA may be out of the equilibrium state because of the variable nature of the central continuum. In this paper, with the help of time dependent photoionization modeling, we study how the WA gas properties change with time and how it reacts to changing radiation fields. Incorporating a flaring incident light curve, we investigate the behavior of WAs using a photoionization code that simultaneously and consistently solves the time-dependent equations of level population, heating and cooling, and radiative transfer. We simulate the physical processes in the gas clouds, such as ionization, recombination, heating, cooling, and the transfer of ionizing radiation through the cloud and present high-resolution time-resolved absorption spectra. We demonstrate that time-dependent radiative transfer is important and the calculations that omit this effect quantitatively and systematically underestimate the absorption. Time-dependent photoionization models provide crucial insights into the characteristics of WAs and can be used to constrain their density and spatial distribution.

Astrophysicists↗

NASA NTRS2024

Time-Dependent Photoionization Modeling of Warm Absorbers: High-Resolution Spectra and Response to Flaring Light Curves

Dev R. Sadaula↗

NASA NTRS2010

Polyimides Based on Asymmetric Dianhydrides (II) (a-BPDA vs a-BTDA) for Resin Transfer Molding (RTM)

A new series of low-melt viscosity imide resins (10-20 poise at 280 C) were formulated from asymmetric 2,3,3',4' -benzophenone dianhydride (a-BTDA) and 4-phenylethynylphthalic endcaps, along with 3,4' -oxydianiline, 3,3' -methylenedianiline and 3,3'- diaminobenzophenone, using a solvent-free melt process. a-BTDA RTM resins exhibited higher glass transition temperatures (Tg's = 330-400 C) compared to those prepared by asymmetric 2,3,3',4' -biphenyl dianhydride, (a-BPDA, Tg's = 320-370 C). These low-melt viscosity imide resins were fabricated into polyimide/T650-35 carbon fiber composites by a RTM process. Composites properties of a-BTDA resins, such as open-hole compression and short-beam shear strength, are compared to those of composites made from a-BPDA based resin at room temperature, 288 C and 315 C. These novel, high temperature RTM imide resins exhibit outstanding properties beyond the performance of conventional RTM resins, such as epoxy and BMI resins which have use-temperatures around 177 C and 232 C for aerospace applications.

Chuang, Kathy C.↗

NASA NTRS2001

Design Evaluation Using Finite Element Analysis of Cooled Silicon Nitride Plates for a Turbine Blade Application

Two- and three-dimensional finite element analyses were performed on uncoated and thermal barrier coated (TBC) silicon nitride plates with and without internal cooling by air. Steady-state heat-transfer analyses were done to optimize the size and the geometry of the cooling channels to reduce thermal stresses, and to evaluate the thermal environment experienced by the plate during burner rig testing. The limited experimental data available were used to model the thermal profile exerted by the flame on the plate. Thermal stress analyses were performed to assess the stress response due to thermal loading. Contours for the temperature and the representative stresses for the plates were generated and presented for different cooling hole sizes and shapes. Analysis indicates that the TBC experienced higher stresses, and the temperature gradient was much reduced when the plate was internally cooled by air. The advantages and disadvantages of several cooling channel layouts were evaluated.

Abdul-Aziz, Ali↗

DOE OSTI2023

Investigation of Electron Extraction and Protection Layers on Cu 2 O Photocathodes

Many semiconductor photoelectrodes used for solar fuel production require the addition of buffer and protection layers to enhance their solar-to-fuel conversion efficiency and long-term stability. For example, Cu 2 O, which is the most efficient oxide-based photocathode but suffers from photocorrosion, has been assembled with various buffer and protection layers to suppress photocorrosion and use more photoexcited electrons for useful reactions such as water reduction to H 2 . However, the abilities of various buffer and protection layers to extract electrons from Cu 2 O have never been directly evaluated. Instead, their abilities were estimated based on the photocurrent for water reduction after adding a hydrogen evolution catalyst on top of them. In these evaluations, as the photocurrent is affected not only by the buffer or protection layer but also by the catalyst, the ability of the buffer or protection layer to extract electrons from Cu 2 O could not be accurately determined or compared. Here, in this study, we demonstrate that 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL), whose reduction rate is faster than the photocorrosion rate of Cu 2 O, can be used as an effective electron scavenger to directly evaluate any change caused by a buffer or protection layer in electron-hole separation in Cu 2 O. In particular, we compared the performances of ZnO and TiO 2 layers on Cu 2 O for extracting electrons and suppressing photocorrosion. We also compared the performances of TiO 2 layers prepared by electrodeposition and atomic layer deposition (ALD) to show that the deposition method can make a striking impact on the performance of the same TiO 2 because it can affect the critical characteristics of the layer (e.g., defect levels, conductivity, interfacial atomic arrangements) that govern interfacial charge transfer in multilayer photoelectrodes.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Balancing Panchromatic Absorption and Multistep Charge Separation in a Compact Molecular Architecture

A panchromatic triad and a charge-separation triad are joined in a crossbar architecture to capture solar energy. The panchromatic triad (T) is comprised of a central free-base porphyrin that is strongly coupled via direct ethyne linkages to two perylene-monoimides. Here, the charge-separation unit incorporates a free-base or zinc chlorin (C or ZnC) as a hole acceptor (or electron donor) and a perylene-diimide (PDI) as an electron acceptor, both attached to the porphyrin via diphenylethyne linkers. The free-base porphyrin is common to both light-harvesting and charge-separation motifs. The chlorin and PDI also function as ancillary light absorbers, complementing direct excitation of the panchromatic triad to produce the discrete lowest excited state of the array (T*). Attainment of full charge separation across the pentad entails two steps: (1) an initial excited-state hole/electron transfer process to oxidize the chlorin (and reduce the triad) or reduce the PDI (and oxidize the triad); and (2) subsequent ground-state electron/hole migration to produce oxidized chlorin and reduced PDI. Full charge separation for pentad ZnC-T-PDI to generate ZnC + -T-PDI¯ occurs with a quantum yield of ~30% and mean lifetime ~1 μs in dimethyl sulfoxide. For C-T-PDI, initial charge separation is followed by rapid charge recombination. The molecular designs and studies reported here reveal the challenges of balancing the demands for charge separation (linker length and composition, excited-state energies, redox potentials, and medium polarity) with the constraints for panchromatic absorption (strong electronic coupling of the porphyrin and two perylene-monoimide units) for integrated function in solar-energy conversion.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2022

π-Extended Ligands in Two-Coordinate Coinage Metal Complexes

Two-coordinate carbene-M I -amide (cMa, M I = Cu, Ag, Au) complexes have emerged as highly efficient luminescent materials for use in a variety of photonic applications, due to their extremely fast radiative rates via thermally activated delayed fluorescence (TADF) from an interligand charge transfer (ICT) process. A series of cMa derivatives were prepared to examine the variables which affect the radiative rate with the goal of understanding the parameters that control the radiative TADF process in these materials. We find that blue emissive complexes with high photoluminescence efficiency (Φ PL > 0.95) and fast radiative rates (k r = 4 x 10 6 s -1 ) can be achieved by selectively extending the π-system of the carbene and amide ligands. Of note is the role played by increasing the separation between the hole and electron in the ICT excited state. Analysis of temperature dependent luminescence data along with theoretical calculations indicate that the hole-electron separation alters the energy gap between the lowest energy singlet and triplet states (ΔE ST ) while keeping the radiative rate for the singlet state unchanged. As a result, this interpretation provides guidelines for the design of new cMa derivatives with even faster radiative rates as well as those with slower radiative rates and thus extended excited state lifetimes.

14 SOLAR ENERGY↗

NASA NTRS2010

Active Galaxy Unification in the Era of X-Ray Polarimetry

Active galactic nuclei (AGNs), Seyfert galaxies, and quasars are powered by luminous accretion and often accompanied by winds that are powerful enough to affect the AGN mass budget, and whose observational appearance bears an imprint of processes that are happening within the central parsec around the black hole (BH). One example of such a wind is the partially ionized gas responsible for X-ray and UV absorption (warm absorbers). Here, we show that such gas will have a distinct signature when viewed in polarized X-rays. Observations of such polarization can test models for the geometry of the flow and the gas responsible for launching and collimating it. We present calculations that show that the polarization depends on the hydrodynamics of the flow, the quantum mechanics of resonance-line scattering, and the transfer of polarized X-ray light in the highly ionized moving gas. The results emphasize the three-dimensional nature of the wind for modeling spectra. We show that the polarization in the 0.1-10 keV energy range is dominated by the effects of resonance lines. We predict a 5%-25% X-ray polarization signature of type-2 objects in this energy range. These results are generalized to flows that originate from a cold torus-like structure, located approximately 1 pc from the BH, which wraps the BH and is ultimately responsible for the apparent dichotomy between type 1 and type 2 AGNs. Such signals will be detectable by future dedicated X-ray polarimetry space missions, such as the NASA Gravity and Extreme Magnetism Small Explorer SMEX, "GEMS" Swank et al. (2008).

Dorodnitsyn, A.↗

DOE OSTI2019

Optical and photoemission investigation of structural and magnetic transitions in the iron-based superconductor Sr 0.67 Na 0.33 Fe 2 As 2

We report the temperature-dependent optical conductivity and angle-resolved photoemission spectroscopy (ARPES) studies of the multiband iron-based superconductor Sr 0.67 Na 0.33 Fe 2 As 2 . Measurements were made in the high-temperature tetragonal paramagnetic phase, below the structural and magnetic transitions at $T_N$ $≃$ 125 K in the orthorhombic spin-density-wave (SDW)-like phase and $T_r$ $≃$42 K in the reentrant tetragonal double-Q magnetic phase where both charge and SDWorder exist, and below the superconducting transition at $T_c$ $≃$ 10 K. The free-carrier component in the optical conductivity is described by two Drude contributions: one strong and broad and the other weak and narrow. The broad Drude component decreases dramatically below TN and Tr , with much of its strength being transferred to a bound excitation in the midinfrared, while the narrow Drude component shows no anomalies at either of the transitions, actually increasing in strength at low temperature while narrowing dramatically. The behavior of an infrared-active mode suggests zone folding below Tr. Below Tc the dramatic decrease in the low-frequency optical conductivity signals the formation of a superconducting energy gap. ARPES reveals holelike bands at the center of the Brillouin zone (BZ), with both electron- and holelike bands at the corners. Below $T_N$, the hole pockets at the center of the BZ decrease in size, consistent with the behavior of the broad Drude component; however, below $T_r$ the electronlike bands shift and split, giving rise to a low-energy excitation in the optical conductivity at $≃$20 meV. The $C_2$ and $C_4$ magnetic states, with resulting spin-density-wave and charge-SDW order, respectively, lead to a significant reconstruction of the Fermi surface that has profound implications for the transport originating from the electron and hole pockets but appears to have relatively little impact on the superconductivity in this material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS1976

Radiative behavior of a gas layer seeded with soot

Gaseous film or transpiration cooling may be used to reduce the heat flux reaching the wall of a container or other structures. Such a protective film, however, is usually not effective for reducing radiative heat transfer as most gases are transparent in the temperature range for which solid walls can exist. Therefore, heat transfer was examined for a gaseous layer seeded with radiation-absorbing carbon particles (soot) and flowing along a surface. The layer was subjected to an external high temperature source of blackbody radiation. The radiative behavior was found to depend on a parameter containing particle concentration, layer thickness and source temperature. Only a very small particle volume concentration, in the range of .0001, was required to obtain high absorption in a 1-cm-thick layer for typical conditions. The results provide the distance along the surface for which the heat transfer to the wall remains within an acceptable limit and the particles remain below a temperature at which they will melt or vaporize. The wall protection by the layer lasts only until the particles vaporize or the layer becomes so hot that it reradiates substantially to the wall. Depending on the layer mass velocity the protection may be effective for a distance along the wall of only a few layer thickness. Hence, to protect greater wall lengths, it will be necessary to introduce the suspension through multiple slots or holes along the wall.

Siegel, R.↗