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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Influence of proton radiation on the minority carrier lifetime in midwave infrared InAs/InAsSb superlattices

Influence of proton radiation on the minority carrier lifetime and on carrier concentrations in InAs/InAsSb superlattices has been studied for radiation doses up to 300 krad. The lifetime decreased from 1.8 μs down to 430 ns as the dose was increased. A variation of the carrier concentration in the range 1–2 × 10{sup 15} with increasing radiation dose was observed. The lifetime drop was however mainly caused by added Shockley-Read-Hall defects in the material. The position of these Shockley-Read-Hall centers was estimated to ∼60 meV below the conduction band edge from comparison between calculated and measured temperature dependencies of the minority carrier lifetime.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Evolution of thermally pulsing asymptotic giant branch stars. IV. Constraining mass loss and lifetimes of low mass, low metallicity AGB stars

The evolution and lifetimes of thermally pulsating asymptotic giant branch (TP-AGB) stars suffer from significant uncertainties. In this work, we analyze the numbers and luminosity functions of TP-AGB stars in six quiescent, low metallicity ([Fe/H] ≲ –0.86) galaxies taken from the ACS Nearby Galaxy Survey Treasury sample, using Hubble Space Telescope (HST) photometry in both optical and near-infrared filters. The galaxies contain over 1000 TP-AGB stars (at least 60 per field). We compare the observed TP-AGB luminosity functions and relative numbers of TP-AGB and red giant branch (RGB) stars, N{sub TP-AGB}/N{sub RGB}, to models generated from different suites of TP-AGB evolutionary tracks after adopting star formation histories derived from the HST deep optical observations. We test various mass-loss prescriptions that differ in their treatments of mass loss before the onset of dust-driven winds (pre-dust). These comparisons confirm that pre-dust mass loss is important, since models that neglect pre-dust mass loss fail to explain the observed N{sub TP-AGB}/N{sub RGB} ratio or the luminosity functions. In contrast, models with more efficient pre-dust mass loss produce results consistent with observations. We find that for [Fe/H] ≲ –0.86, lower mass TP-AGB stars (M ≲ 1 M{sub ☉}) must have lifetimes of ∼0.5 Myr and higher masses (M ≲ 3 M{sub ☉}) must have lifetimes ≲ 1.2 Myr. In addition, assuming our best-fitting mass-loss prescription, we show that the third dredge-up has no significant effect on TP-AGB lifetimes in this mass and metallicity range.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

On sunspot and starspot lifetimes

We consider the lifetimes of spots on the Sun and other stars from the standpoint of magnetic diffusion. While normal magnetic diffusivity predicts lifetimes of sunspots that are too large by at least two orders of magnitude, turbulent magnetic diffusivity accounts for both the functional form of the solar empirical spot-lifetime relation and for the observed sunspot lifetimes, provided that the relevant diffusion length is the supergranule size. Applying this relation to other stars, the value of turbulent diffusivity depends almost entirely on supergranule size, with very weak dependence on other variables such as magnetic field strength and density. Overall, the best observational data for other stars is consistent with the extension of the solar relation, provided that stellar supergranule sizes for some stars are significantly larger than they are on the Sun.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2025

Effect of activation temperature on quantum efficiency and lifetime of NEA truncated nanocone array GaAs photocathode

This study investigates the quantum efficiency (QE) and operational lifetime of a negative electron affinity GaAs truncated nanocone array (TNCA) photocathode benchmarked against a conventional flat GaAs photocathode under varying activation temperatures. The TNCA structure demonstrated a QE of up to 13.6% at 590 nm with room temperature (RT) activation—approximately 1.5 times higher than its flat counterpart. This enhancement is due to Mie resonance effects within the nanostructure, as confirmed by finite-difference time-domain simulations. Moreover, the TNCA photocathode exhibits significantly extended charge lifetime, with enhancement factors of ∼6.1 and ∼19.8 under RT and 50 °C activations, respectively. These gains are primarily attributed to increased effective surface area and optimized dipole layer formation at elevated temperatures. In addition, shorter excitation wavelengths further contribute to lifetime improvements. These findings underscore the TNCA GaAs photocathode’s potential as a high QE, long lifetime electron source for many large-scale electron accelerators.

Cs-NF3 activation↗
DOE OSTI2016

Cosmological constraints on the neutron lifetime

We derive new constraints on the neutron lifetime based on the recent Planck 2015 observations of temperature and polarization anisotropies of the CMB. Under the assumption of standard Big Bang Nucleosynthesis, we show that Planck data constrains the neutron lifetime to τ{sub n} = (907±69) [s] at 68% c.l.. Moreover, by including the direct measurements of primordial Helium abundance of Aver et al. (2015) and Izotov et al. (2014), we show that cosmological data provide the stringent constraints τ{sub n} = (875±19) [s] and τ{sub n} = (921±11) [s] respectively. The latter appears to be in tension with neutron lifetime value quoted by the Particle Data Group (τ{sub n} = (880.3±1.1) [s]). Future CMB surveys as COrE+, in combination with a weak lensing survey as EUCLID, could constrain the neutron lifetime up to a ∼ 6 [s] precision.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Relativistic configuration interaction plus linearized-coupled-cluster calculations of U 2 + energies, g factors, transition rates, and lifetimes

In this study, excitation energies, term designations, g factors, transition rates, and lifetimes of U 2+ are determined using a relativistic configuration interaction (CI) + linearized-coupled-cluster (LCC) approach. The CI-LCC energies are compared with CI + many-body-perturbation-theory (MBPT) and available experimental energies. Close agreement has been found with experiment, within hundreds of cm -1 . In addition, lifetimes of higher levels have been calculated for comparison with three experimentally measured lifetimes, and close agreement has been found within the experimental error. CI-LCC calculations constitute a benchmark test of the CI + all-order method in complex relativistic systems such as actinides and their ions with many valence electrons. The theory yields many energy levels, g factors, transition rates, and lifetimes of U 2+ that are not available from experiment. Lastly, the theory can be applied to other multivalence atoms and ions, which would be of interest to many applications.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Lifetime measurements in the even-even Cd 102 – 108 isotopes

The heaviest T z = 0 doubly-magic nucleus, Sn 100 , and the neighboring nuclei offer unique opportunities to investigate the properties of nuclear interaction. For instance, the structure of light-Sn nuclei has been shown to be affected by the delicate balance between nuclear-interaction components, such as pairing and quadrupole correlations. From Cd to Te, many common features and phenomena have been observed experimentally along the isotopic chains, leading to theoretical studies devoted to a more general and comprehensive study of the region. In this context, having only two proton holes in the Z = 50 shell, the Cd isotopes are expected to present properties similar to those found in the Sn isotopic chain. The aim of this work was to measure lifetimes of excited states in neutron-deficient nuclei in the vicinity of Sn 100 . Here, the neutron-deficient nuclei in the N ≈ Z ≈ 50 region were populated using a multinucleon transfer reaction with a Cd 106 beam and a Mo 92 target. The beamlike products were identified by the VAMOS + + spectrometer, while the γ rays were detected using the AGATA array. Lifetimes of excited states were determined using the recoil distance Doppler-shift method, employing the Cologne differential plunger. Lifetimes of low-lying states were measured in the even-mass Cd 102 – 108 isotopes. In particular, multiple states with excitation energy up to ≈ 3 MeV, belonging to various bands, were populated in Cd 106 via inelastic scattering. The transition strengths corresponding to the measured lifetimes were compared with those resulting from state-of-the-art beyond-mean-field calculations using the symmetry-conserving configuration-mixing approach. Conclusions: Despite the similarities in the electromagnetic properties of the low-lying states, there is a fundamental structural difference between the ground-state bands in the Z = 48 and Z = 50 isotopes. The comparison between experimental and theoretical results revealed a rotational character of the Cd nuclei, which have prolate-deformed ground states with β 2 ≈ 0.2 . At this deformation Z = 48 becomes a closed-shell configuration, which is favored with respect to the spherical one.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Lifetimes of low-lying excited states in Kr 50 36 86

Background: The evolution of nuclear magic numbers at extremes of isospin is a topic at the forefront of contemporary nuclear physics. N = 50 is a prime example, with increasing experimental data coming to light on potentially doubly magic Sn 100 and Ni 78 at the proton-rich and proton-deficient extremes, respectively; however, experimental discrepancies exist in the data for less exotic systems. Purpose: In Kr 86 the B ( E 2 ; 2 1 + → 0 1 + ) value—a key indicator of shell evolution—has been experimentally determined by two different methodologies, with the results deviating by 3 σ . Here, we report on a new high-precision measurement of this value, as well as the first measured lifetimes and hence transition strengths for the 2 2 + and 3 ( 2 ) - states in the nucleus. Methods: The Doppler-shift attenuation method was implemented using the TRIUMF-ISAC γ -ray escape-suppressed spectrometer (TIGRESS) γ -ray spectrometer and the TIGRESS integrated plunger device. High-statistics Monte Carlo simulations were utilized to extract lifetimes in accordance with state-of-the-art methodologies. Results: Lifetimes of τ ( 2 1 + ) = 336 ± 4 (stat.) ± 20 (sys.) fs, τ ( 2 2 + ) = 263 ± 9 (stat.) ± 19 (sys.) fs, and τ ( 3 ( 2 ) - ) = 73 ± 6 (stat.) ± 32 (sys.) fs were extracted. This yields a transition strength for the first-excited state of B ( E 2 ; 2 1 + → 0 1 + ) = 259 ± 3 (stat.) ± 16 (sys.) e 2 fm 4 . Conclusions: Finally, the measured lifetime disagrees with the previous Doppler-shift attenuation method measurement by more than 3 σ , while agreeing well with a previous value extracted from Coulomb excitation. The newly extracted B ( E 2 ; 2 1 + → 0 1 + ) value indicates a more significant reduction in the N = 50 isotones approaching Z = 40 .

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

AYUSH: A Technique for Extending Lifetime of SRAM-NVM Hybrid Caches

Recently, researchers have explored way-based hybrid SRAM-NVM (non-volatile memory) last level caches (LLCs) to bring the best of SRAM and NVM together. However, the limited write endurance of NVMs restricts the lifetime of these hybrid caches. We present AYUSH, a technique to enhance the lifetime of hybrid caches, which works by using data-migration to preferentially use SRAM for storing frequently-reused data. Microarchitectural simulations confirm that AYUSH achieves larger improvement in lifetime than a previous technique and also maintains performance and energy efficiency. For single, dual and quad-core workloads, the average increase in cache lifetime with AYUSH is 6.90X, 24.06X and 47.62X, respectively.

device lifetime↗
DOE OSTI2013

Effect of annealing on the nonequilibrium carrier lifetime in GaAs grown at low temperatures

GaAs samples grown by molecular-beam epitaxy at low (230 Degree-Sign C) temperatures are investigated. One of the samples is subjected to aftergrowth annealing at 600 Degree-Sign C. Using an unconventional pump-probe scheme for measuring the dynamic variation in the light refractive index, the nonequilibrium charge-carrier lifetime (275 {+-} 30 fs before annealing) is determined. Such a short carrier lifetime in the unannealed material is due to the high concentration of point defects, mainly As{sub Ga} antisite defects. According to X-ray diffraction and steady-state optical absorption data, the As{sub Ga} concentration in the samples is 3 Multiplication-Sign 10{sup 19} cm{sup -3}, which corresponds to an arsenic excess of 0.26 at %. Upon annealing at 600 Degree-Sign C, the superstoichiometric As defects self-organize and form As nanoinclusions in the GaAs crystal matrix. It is shown that in this case the nonequilibrium charge-carrier lifetime increases to 452 {+-} 5 fs. This lifetime is apparently ensured by the capture of non-equilibrium charge carriers at metal As nanoinclusions.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Extension of the radiative lifetime of Wannier-Mott excitons in semiconductor nanoclusters

The purpose of the study is to calculate the radiative lifetime of Wannier-Mott excitons in three-dimensional potential wells formed of direct-gap narrow-gap semiconductor nanoclusters in wide-gap semiconductors and assumed to be large compared to the exciton radius. Calculations are carried out for the InAs/GaAs heterosystem. It is shown that, as the nanocluster dimensions are reduced to values on the order of the exciton radius, the exciton radiative lifetime becomes several times longer compared to that in a homogeneous semiconductor. The increase in the radiative lifetime is more pronounced at low temperatures. Thus, it is established that the placement of Wannier-Mott excitons into direct-gap semiconductor nanoclusters, whose dimensions are of the order of the exciton radius, can be used for considerable extension of the exciton radiative lifetime.

36 MATERIALS SCIENCE↗
DOE OSTI2016

CONSTRAINING THE LIFETIME AND OPENING ANGLE OF QUASARS USING FLUORESCENT Ly α EMISSION: THE CASE OF Q0420–388

A toy model is developed to understand how the spatial distribution of fluorescent emitters in the vicinity of bright quasars could be affected by the geometry of the quasar bi-conical radiation field and by its lifetime. The model is then applied to the distribution of high-equivalent-width Ly α emitters (with rest-frame equivalent widths above 100 Å, threshold used in, e.g., Trainor and Steidel) identified in a deep narrow-band 36 × 36 arcmin{sup 2} image centered on the luminous quasar Q0420–388. These emitters are found near the edge of the field and show some evidence of an azimuthal asymmetry on the sky of the type expected if the quasar is radiating in a bipolar cone. If these sources are being fluorescently illuminated by the quasar, the two most distant objects require a lifetime of at least 15 Myr for an opening angle of 60° or more, increasing to more than 40 Myr if the opening angle is reduced to a minimum of 30°. However, some other expected signatures of boosted fluorescence are not seen at the current survey limits, e.g., a fall off in Ly α brightness, or equivalent width, with distance. Furthermore, to have most of the Ly α emission of the two distant sources to be fluorescently boosted would require the quasar to have been significantly brighter in the past. This suggests that these particular sources may not be fluorescent, invalidating the above lifetime constraints. This would cast doubt on the use of this relatively low equivalent width threshold and thus also on the lifetime analysis in Trainor and Steidel.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Effects of Reactive Power on Photovoltaic Inverter Reliability and Lifetime

For predicting the reliability and lifetime of Photovoltaic (PV) inverters, thermal cycling is considered the most important stressors in the inverter system. To realize this, a detailed electrothermal model of the PV inverter was developed along with the controllers. The model was verified by developing an in-house inverter, which was placed inside a thermal chamber to verify the losses and temperature rise within the system at various temperatures. After the verification of the model, a reduced order model of the inverter was implemented which can translate the mission profile of ambient temperature and solar irradiance into load profile of junction temperatures of the switches. The estimated junction temperature data was used to identify the reliability indices and predict the useful lifetime of the inverter system. After developing the models to predict useful lifetime of the system, the impact of reactive power on the overall reliability of the system was studied. Experiments show that lifetime and reliability of the inverter decreases as the operating power factor decreases from unity. The results are presented and discussed here.

14 SOLAR ENERGY↗
DOE OSTI2019

Dominance of Exciton Lifetime in the Stability of Phosphorescent Dyes

Organic light-emitting devices (OLEDs) are widely used for mobile displays, but the relatively short lifetime of blue OLEDs remains a challenge in many applications. Typically, instability is viewed as a material-specific chemical degradation problem. It is known to be alleviated by reducing the operating current or otherwise decreasing the exciton density. Here we have shown that this view is incomplete. For archetypical phosphorescent materials, it is observed that the dependence of photostability on the triplet exciton lifetime follows a cubic power law, steeper than its dependence on exciton density. It is demonstrated that the triplet exciton lifetime not only determines the energy stored within an OLED, it also determines the loss in luminescence by controlling the yield of quenching by defects. The dominant role of the triplet exciton lifetime suggests that the stability of the best OLED materials can be significantly improved via rapid extraction of the energy stored in triplet excitons.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Exceeding 200 ns Lifetimes in Polycrystalline CdTe Solar Cells

CdTe photovoltaics has achieved one of the lowest levelized costs of electricity among all energy sources. However, for decades, carrier lifetimes have been inferior to those of other prevalent solar cell materials. This quality has inhibited common methods to improve solar cell efficiency such as back-surface fields, electron reflectors, or bifacial solar cells. In this work, a significant increase in carrier lifetime to values exceeding 200 ns in fully functional CdTe solar cells is demonstrated. The increased lifetime is achieved by large CdSeTe grains at the absorber/emitter interface, intragrain passivation in the absorber layer, and chemical passivation by forming nanoscale oxidized tellurium species at the transparent conducting oxide interface. The carrier lifetime is correlated to the open-circuit voltage and enables paths for back-surface manipulation and novel cell architectures to further improve CdTe photovoltaic performance.

14 SOLAR ENERGY↗
DOE OSTI2018

Impacts of electrode coating irregularities on polymer electrolyte membrane fuel cell lifetime using quasi in-situ infrared thermography and accelerated stress testing

In-line quality control diagnostics for roll-to-roll (R2R) manufacturing techniques will play a key role in the future commercialization of the polymer electrolyte membrane fuel cell (PEMFC) used in automotive applications. These diagnostics monitor the fabrication of the membrane electrode assembly (MEA), which detect and flag any non-uniformity that may potentially harm PEMFC performance and/or lifetime. This will require quantitative thresholds and a clear distinction between harmful defects and harmless coating irregularities. Thus, novel fuel cell hardware with quasi in-situ infrared (IR) thermography capabilities is utilized to understand how bare spots in the cathode electrode impact MEA lifetime. An accelerated stress test (AST) simulates chemical and mechanical degradation modes seen in vehicular operation. The actual open circuit voltage and rate of change of this voltage are used as in-situ indicators for MEA failure, enabling capture of the progression of failure point development. Bare spot coating irregularities located at the center of the electrode were found to have no impact on MEA lifetime when compared to a pristine MEA. However, MEA lifetime was found to be considerably shortened when these same irregularities are located at the cathode inlet and, especially, the anode inlet regions of the fuel cell.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Sterically Encumbered Heteroleptic Copper(I) β-Diketiminate Complexes with Extended Excited-State Lifetimes

One of the main challenges in developing effective copper(I) photosensitizers is their short excited-state lifetimes, usually attributed to structural distortion upon light excitation. We have previously introduced copper(I) charge-transfer chromophores of the general formula Cu(N^N)(ArNacNac), where N^N is a conjugated diimine ligand and ArNacNac is a substituted β-diketiminate ligand. These chromophores were promising regarding their tunable redox potentials and intense visible absorption but were ineffective as photosensitizers, presumably due to short excited-state lifetimes. Here, we introduce sterically crowded analogues of these heteroleptic chromophores with bulky alkyl substituents on the N^N and/or ArNacNac ligand. Structural analysis was combined with electrochemical and photophysical characterization, including ultrafast transient absorption (UFTA) spectroscopy to investigate the effects of the alkyl groups on the excited-state lifetimes of the complexes. The molecular structures determined by single-crystal X-ray diffraction display more distortion in the ground state as alkyl substituents are introduced into the phenanthroline or the NacNac ligand, showing smaller τ 4 values due to the steric hindrance. UFTA measurements were carried out to determine the excited-state dynamics. Sterically encumbered Cu5 and Cu6 display excited-state lifetimes 15–20 times longer than unsubstituted complex Cu1, likely indicating that the incorporation of bulky alkyl substituents inhibits the pseudo-Jahn–Teller (PJT) flattening distortion in the excited state. Furthermore, this work suggests that the steric properties of these heteroleptic copper(I) charge-transfer chromophores can be readily modified and that the excited-state dynamics are strongly responsive to these modifications.

14 SOLAR ENERGY↗
DOE OSTI2018

Lifetime Enhancement of Visible Light Induced Photocharges in Tungsten and Nitrogen in situ Codoped TiO 2 :WN Thin Films

We report on one-step in situ codoped TiO2 thin films synthesized by cosputtering. The purpose of this acceptor-donor passivated codoping approach is to overcome the optoelectronic limitations that arise for monodoped TiO2 in photocatalytic applications. To evaluate these added benefits, the TiO2:WN thin films were characterized by different techniques. X-ray diffraction patterns and X-ray photoelectron spectral analysis revealed that both N and W dopants are mostly present in the desired substitutional locations. Additionally, the codoping approach was found to reduce the internal strain and defect density of the TiO2:WN films as compared to their monodoped TiO2:N counterparts. This defect reduction is confirmed via photocharge lifetime variation obtained using visible light flash photolysis time-resolved microwave conductivity measurements (FP-TRMC). Photocharge lifetime analysis indicated the presence of three distinct decay processes: charge trapping, recombination, and surface reactions. These characteristic lifetimes of the codoped TiO2:WN films (i.e., 0.08, 0.75, and 11.5 us, respectively) were found to be about double those of their nitrogen monodoped TiO2:N counterparts (i.e., 0.03, 0.35, and 6.8 us), quantitatively confirming the effective passivating outcome of the tungsten-nitrogen codoping approach developed here. The practicality of this method was confirmed by integrating the TiO2:WN films as photoanodes for the electro-photocatalytic, solar light driven degradation of a real pollutant (i.e., atrazine). A significant increase in the degradation kinetics, leading to a 4-fold increase in the pseudo-first-order degradation constant for the optimally doped TiO2:WN photoanodes (0.106 min-1) from the undoped TiO2-x ones (0.026 min-1), is a direct consequence of the increased photocharge lifetimes in tandem with visible light photosensitivity.

14 SOLAR ENERGY↗