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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS2016

Evaluation of Observed and Modelled Aerosol Lifetimes Using Radioactive Tracers of Opportunity and an Ensemble of 19 Global Models

Aerosols have important impacts on air quality and climate, but the processes affecting their removal from the atmosphere are not fully understood and are poorly constrained by observations. This makes modelled aerosol lifetimes uncertain. In this study, we make use of an observational constraint on aerosol lifetimes provided by radionuclide measurements and investigate the causes of differences within a set of global models. During the Fukushima Dai-Ichi nuclear power plant accident of March 2011, the radioactive isotopes cesium-137 (Cs-137) and xenon-133 (Xe-133) were released in large quantities. Cesium attached to particles in the ambient air, approximately according to their available aerosol surface area. Cs-137 size distribution measurements taken close to the power plant suggested that accumulation mode (AM) sulfate aerosols were the main carriers of cesium. Hence, Cs-137 can be used as a proxy tracer for the AM sulfate aerosol's fate in the atmosphere. In contrast, the noble gas Xe-133 behaves almost like a passive transport tracer. Global surface measurements of the two radioactive isotopes taken over several months after the release allow the derivation of a lifetime of the carrier aerosol. We compare this to the lifetimes simulated by 19 different atmospheric transport models initialized with identical emissions of Cs-137that were assigned to an aerosol tracer with each model's default properties of AM sulfate, and Xe-133 emissions that were assigned to a passive tracer. We investigate to what extent the modelled sulfate tracer can reproduce the measurements, especially with respect to the observed loss of aerosol mass with time. Modelled Cs-137and Xe-133 concentrations sampled at the same location and times as station measurements allow a direct comparison between measured and modelled aerosol lifetime. The e-folding lifetime e, calculated from station measurement data taken between 2 and 9 weeks after the start of the emissions, is 14.3 days (95% confidence interval 13.1-15.7 days). The equivalent modelled e lifetimes have a large spread, varying between 4.8 and 26.7 days with a model median of 9.42.3 days, indicating too fast a removal in most models. Because sufficient measurement data were only available from about 2 weeks after the release, the estimated lifetimes apply to aerosols that have undergone long-range transport, i.e. not for freshly emitted aerosol. However, modelled instantaneous lifetimes show that the initial removal in the first 2 weeks was quicker (lifetimes between 1 and 5 days) due to the emissions occurring at low altitudes and co-location of the fresh plume with strong precipitation. Deviations between measured and modelled aerosol lifetimes are largest for the northernmost stations and at later time periods, suggesting that models do not transport enough of the aerosol towards the Arctic. The models underestimate passive tracer (Xe-133) concentrations in the Arctic as well but to a smaller extent than for the aerosol (Cs-137) tracer. This indicates that in addition to too fast an aerosol removal in the models, errors in simulated atmospheric transport towards the Arctic in most models also contribute to the underestimation of the Arctic aerosol concentrations.

Radioactive isotopes↗
DOE OSTI2022

Probing crystallographic orientation-specific carrier lifetimes in epitaxial Ge/AlAs and InGaAs/InP heterostructures

Current silicon (Si) fin transistors rely on (100) and (110) crystallographically oriented surfaces, and the proposed alternate channel transistor technology comprises materials with higher mobility than Si. Crystallographically oriented epitaxial germanium (Ge) and indium–gallium arsenide (InGaAs) have the potential to replace Si in ultra-low power transistor applications. The higher carrier lifetime is an indication of superior material quality, which relates to the leakage current of a fin transistor. To gain insights into the carrier recombination dynamics in these crystallographically oriented epitaxial Ge and InGaAs layers, the contactless microwave photoconductive decay (μ-PCD) technique at an excitation wavelength of 1500 nm was employed to probe the orientation-specific carrier lifetimes. Highly effective carrier lifetimes >200 ns for (100)Ge/AlAs and (110)Ge/AlAs, and ~80 ns for (111)Ge/AlAs heterostructures, were extracted at room temperature. The measured carrier lifetime has a strong dependence on the surface orientation, which could be related to orientation-specific bulk trap states present within the bandgap of Ge. The (111)Ge orientation has 3 times lower carrier lifetime compared to the (100)Ge and (110)Ge surface orientations. On the other hand, the carrier lifetimes of 125 μs and 10 ns were determined from (100)InGaAs/InP and (110)InGaAs/InP heterostructures, respectively. The reduction in carrier lifetimes in both (111)Ge and (110)InGaAs was due to high electrical conductivity or higher bulk trap states present within the bandgap as well as the facet-dependent growth of the (110)InGaAs layer on InP. A surface passivating layer is indispensable for these orientation-specific epitaxial layers to improve the carrier lifetime. Therefore, the higher carrier lifetimes from technologically interesting (100)Ge and (110)Ge surfaces would offer a path for the development of Ge-based ultra-low power electronics, and optoelectronic devices based on the (100)InGaAs layer.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Lifetimes and accretion rates of protoplanetary disks

Protoplanetary disks originate in the collapse of molecular cloud cores. The formation and evolution of disks are influenced by the properties of molecular cloud cores. In this paper we investigate the dependence of disk lifetimes and accretion rates on cloud core properties. We find that the lifetime increases as the angular velocities and the mass of cloud cores increase and that the lifetime decreases as the core temperature increases. We have calculated the distribution of disk lifetimes and disk fractions with stellar age. Our calculations show that the lifetime is in the range of 1–15 Myr and that the typical lifetime is 1–3 Myr. There are a few disks with lifetimes greater than 10 Myr and ∼30% of the disks have lifetimes less than 1 Myr. We also fit the disk fraction by an exponential decay curve with characteristic time ∼3.7 Myr. Our results explain the observations of disk lifetimes. We also find that the accretion rate does not change significantly with ω and generally decreases with T{sub cd}. At the early evolution of the disks, the M-dot --M{sub ∗} relation is about M-dot ∝M{sub ∗}{sup 1.2−2}. Since the effects of the photoevaporation are weak at this stage, this relation is the consequence of the cloud core properties. At the late evolution of the disks, the M-dot --M{sub ∗} relation is about M-dot ∝M{sub ∗}{sup 1.2−1.7}. For low accretion rates at this stage, the M-dot --M{sub ∗} relation results from the effects of X-ray photoevaporation. The calculated M-dot --M{sub ∗} relations are consistent with the observations.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2014

Multimodel Estimates of Atmospheric Lifetimes of Long-lived Ozone-Depleting Substances: Present and Future

We have diagnosed the lifetimes of long-lived source gases emitted at the surface and removed in the stratosphere using six three-dimensional chemistry-climate models and a two-dimensional model. The models all used the same standard photochemical data. We investigate the effect of different definitions of lifetimes, including running the models with both mixing ratio (MBC) and flux (FBC) boundary conditions. Within the same model, the lifetimes diagnosed by different methods agree very well. Using FBCs versus MBCs leads to a different tracer burden as the implied lifetime contained in the MBC value does not necessarily match a model's own calculated lifetime. In general, there are much larger differences in the lifetimes calculated by different models, the main causes of which are variations in the modeled rates of ascent and horizontal mixing in the tropical midlower stratosphere. The model runs have been used to compute instantaneous and steady state lifetimes. For chlorofluorocarbons (CFCs) their atmospheric distribution was far from steady state in their growth phase through to the 1980s, and the diagnosed instantaneous lifetime is accordingly much longer. Following the cessation of emissions, the resulting decay of CFCs is much closer to steady state. For 2100 conditions the model circulation speeds generally increase, but a thicker ozone layer due to recovery and climate change reduces photolysis rates. These effects compensate so the net impact on modeled lifetimes is small. For future assessments of stratospheric ozone, use of FBCs would allow a consistent balance between rate of CFC removal and model circulation rate

CFC↗
NASA NTRS2014

Multimodel Estimates of Atmospheric Lifetimes of Long-Lived Ozone-Depleting Substances: Present and Future

We have diagnosed the lifetimes of long-lived source gases emitted at the surface and removed in the stratosphere using six three-dimensional chemistry-climate models and a two-dimensional model. The models all used the same standard photochemical data. We investigate the effect of different definitions of lifetimes, including running the models with both mixing ratio (MBC) and flux (FBC) boundary conditions. Within the same model, the lifetimes diagnosed by different methods agree very well. Using FBCs versus MBCs leads to a different tracer burden as the implied lifetime contained in theMBC value does not necessarilymatch a model's own calculated lifetime. In general, there are much larger differences in the lifetimes calculated by different models, the main causes of which are variations in the modeled rates of ascent and horizontal mixing in the tropical midlower stratosphere. The model runs have been used to compute instantaneous and steady state lifetimes. For chlorofluorocarbons (CFCs) their atmospheric distribution was far from steady state in their growth phase through to the 1980s, and the diagnosed instantaneous lifetime is accordingly much longer. Following the cessation of emissions, the resulting decay of CFCs is much closer to steady state. For 2100 conditions the model circulation speeds generally increase, but a thicker ozone layer due to recovery and climate change reduces photolysis rates. These effects compensate so the net impact on modeled lifetimes is small. For future assessments of stratospheric ozone, use of FBCs would allow a consistent balance between rate of CFC removal and model circulation rate.

CH3CCl3↗
DOE OSTI2015

Emission lifetimes of a fluorescent dye under shock compression

The emission lifetimes of rhodamine 6G (R6G), were measured under shock compression to 9.1 GPa, with the dual intent of better understanding molecular photophysics in extreme environments and assessing the usefulness of fluorescence lifetime microscopy to measure spatially-dependent pressure distributions in shocked microstructured media. R6G was studied as free dye dissolved in poly-methyl methacrylate (PMMA), or dye encapsulated in silica microparticles suspended in PMMA. Thin layers of these materials in impedance-matched geometries were subjected to planar single-stage shocks created by laser-driven flyer plates. A synchronized femtosecond laser excited the dye at selected times relative to flyer plate arrival and the emission lifetimes were measured with a streak camera. Lifetimes decreased when shocks arrived. The lifetime decrease was attributed to a shock-induced enhancement of R6G nonradiative relaxation. At least part of the relaxation involved shock-enhanced intersystem crossing. For free dye in PMMA, the lifetime decrease during the shock was shown to be a linear function of shock pressure from 0-9 GPa, with a slope of -0.22 ns·GPa -1 . Furthermore, the linear relationship makes it simple to convert lifetimes into pressures. Lifetime measurements in shocked microenvironments may be better than emission intensity measurements, since lifetimes are sensitive to the surrounding environment, but insensitive to intensity variations associated with the motion and optical properties of a dynamically changing structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Tensile Property and Lifetime Prediction for Low-Temperature Aged Uranium-Niobium Alloys

Thermal aging models and lifetime predictions for uranium-niobium (U-Nb) alloys were created using an approach similar to those previously employed. Lifetime estimates for generic U-6Nb components were thus updated; the reported value being 800 years. This update represents a small change in lifetime vs. that of the 2012 assessment (540 years). This lifetime estimate emerged from consideration of several model fits specific to the aging datasets and properties chosen. Aging was quantified using quasi-static tensile properties measured on specimens artificially aged for up to 10 years. The major change relative to the most recent 2012 LANL assessment was that a more comprehensive body of U-Nb literature data was mined, in addition to being augmented by the latest LANL and UK AWE data. The tensile data compilation was published separately (LANL report LA-14493, December 2016). Recognizing the chemical banding of industrially produced U-6Nb, models were developed for the mid-range (6 wt.%) and extrema (4 and 8 wt.%) compositions. Lifetime estimates were calculated for all three alloy classes (4, 6, 8 wt.% nominal) and two measures of total tensile elongation (TE) to failure, namely TE-ext. — extensometer method, and TE-NCD — normalized crosshead displacement method. The conservative assumption was made that whichever composition (4, 6, or 8 wt.% Nb) and property (TE-ext or TE-NCD) was the first to cross the ductility failure threshold would limit the lifetime of the entire component. Tensile strength properties did not figure into the lifetime predictions, but could be useful as age-sensitive diagnostics and were also modeled. Of these, only first yield strength is expected to show a change at 40°C aging vs. time = 0 over the ~100-year timespan of engineering interest. Second yield strength evolves more slowly, and ultimate tensile strength slower still. Among all the models, the apparent activation energies for aging were mostly in the narrow 29– 37 kcal/mol range, which is close to that for diffusion of Nb in gamma-uranium. This agreement may be coincidental. The data from recent long-term aging studies substantially improved the model fit quality and robustness of the lifetimes. Appendices document sensitivity studies of the model fits and lifetimes with respect to using more limited datasets. These results highlight the limitations of relying solely on data from scattered literature studies and smaller datasets more generally.

36 MATERIALS SCIENCE↗
NASA NTRS2013

Systems and methods for circuit lifetime evaluation

Systems and methods for estimating the lifetime of an electrical system in accordance with embodiments of the invention are disclosed. One embodiment of the invention includes iteratively performing Worst Case Analysis (WCA) on a system design with respect to different system lifetimes using a computer to determine the lifetime at which the worst case performance of the system indicates the system will pass with zero margin or fail within a predetermined margin for error given the environment experienced by the system during its lifetime. In addition, performing WCA on a system with respect to a specific system lifetime includes identifying subcircuits within the system, performing Extreme Value Analysis (EVA) with respect to each subcircuit to determine whether the subcircuit fails EVA for the specific system lifetime, when the subcircuit passes EVA, determining that the subcircuit does not fail WCA for the specified system lifetime, when a subcircuit fails EVA performing at least one additional WCA process that provides a tighter bound on the WCA than EVA to determine whether the subcircuit fails WCA for the specified system lifetime, determining that the system passes WCA with respect to the specific system lifetime when all subcircuits pass WCA, and determining that the system fails WCA when at least one subcircuit fails WCA.

Heaps, Timothy L.↗
DOE OSTI2011

ANALYSIS OF NSLS-II TOUSCHEK LIFETIME

As scrapers are adopted for the loss control of NSLS-II storage ring, Touschek lifetime estimations for various cases are required to assure the stable operation. However, to estimate the Touschek lifetime, momentum apertures should be measured all along the ring and, if we want to estimate the lifetime in various situations, it can take extremely long time. Thus, rather than simulating for each case, a semi-analytic methods with the interpolations are used for the measurements of the momentum apertures. In this paper, we described the methods and showed the results. Having enough Touschek lifetime is important for synchrotron light source for the users to perform experiments with stable beams. In NSLS-II, the scrapers will be installed for the loss control. Especially, the horizontal scrapers will be installed where the dispersions are maximum. Therefore, we need to find the proper scraper gap values which do not reduce the lifetime too much for the stable beam operation. To estimate reliable Touschek lifetime, we should measure the momentum apertures at many positions along the ring. For the rough estimation of the Touschek lifetime, the RF momentum acceptance can be used and for a more detailed estimation, the linear approximation of synchrotron oscillation can be used. However, for the strong focussing synchrotrons, like NSLS-II, the linear approximation is not enough to obtain the reliable momentum apertures, and, in general, particle tracking simulations are used. However, for NSLS-II case, we need to track the particle about 400 turns at each point to make it a full synchrotron oscillation period and to obtain the reliable Touschek lifetime we need measure the momentum apertures at several hundred positions at least. Therefore, it can take quite a long time if we want to have the reasonable resolution for the measurements. Furthermore, at the simulation, if we want to measure the aperture inside a element, we should divide the element and this will make the simulation longer. Here, we developed a systematic method to calculate the momentum aperture using interpolation tables. Once the tables are prepared, the precise momentum aperture can be calculated instantly at arbitrary position for various situations.

43 PARTICLE ACCELERATORS↗
DOE OSTI2018

Direct measurements of the lifetime of medium-heavy hypernuclei

The lifetime of a Λ particle embedded in a nucleus (hypernucleus) decreases from that of free Λ decay mainly due to the opening of the Λ N → NN weak decay channel. Furthermore, it is generally believed that the lifetime of a hypernucleus attains a constant value (saturation) for medium to heavy hypernuclear masses, yet this hypothesis has been difficult to verify. This paper presents a direct measurement of the lifetime of medium-heavy hypernuclei that were hyper-fragments produced by fission or break-up from heavy hypernuclei initially produced with a 2.34 GeV photon-beam incident on thin Fe, Cu, Ag, and Bi target foils. For each event, fragments were detected in coincident pairs by a low-pressure multi-wire proportional chamber system. The lifetime was extracted from decay time spectrum formed by the difference of the time zeros between the pairs. The measured lifetime from each target is actually a statistical average over a range of mass with mean about 1/2 of the target mass and appears to be a constant of about 200 ps. Although this result cannot exclude unexpected shorter or longer lifetimes for some specific hypernuclei or hypernuclear states, it shows that a systematic decrease in lifetime as hypernuclear mass increases is not a general feature for hypernuclei with mean mass up to A ≈ 130. On the other hand, the success of this experiment and its technique shows that the time delayed fissions observed and used by all the lifetime measurements done so far on heavy hypernuclei could likely have originated from hyper-fragments lighter than the assumed masses.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

Environment and the Lifetime of Tropical Deep Convection in a Cloud-Permitting Regional Model Simulation

By applying a cloud tracking algorithm to tropical convective systems simulated by a regional high resolution model, the study documents environmental conditions before and after convective systems are initiated over ocean and land by following them during their lifetime. The comparative roles of various environmental fields in affecting the lifetime of convection are also quantified. The statistics of lifetime, maximum area, propagation speed and direction of the simulated deep convection agrees well with geostationary satellite observations. Over ocean, convective systems enhance surface fluxes through the associated wind gusts as well as cooling and drying of the boundary layer. A significant relationship is found between the mean surface fluxes during their lifetime and the longevity of the systems which in turn is related to the initial intensity of the moist updraft and to a lesser extent upper level shear. Over land, on the other hand, convective activity suppresses surface fluxes through cloud cover and the lifetime of convection is related to the upper level shear during their lifetime and strength of the heat fluxes several hours before the initiation of convection. For systems of equal lifetime, those over land are significantly more intense than those over ocean especially during early stages of their lifetime.

convection↗
DOE OSTI2015

Evolution of silicon bulk lifetime during III–V‐on‐Si multijunction solar cell epitaxial growth

Abstract The evolution of Si bulk minority carrier lifetime during the heteroepitaxial growth of III–V on Si multijunction solar cell structures via metal‐organic chemical vapor deposition (MOCVD) has been analyzed. In particular, the impact on Si lifetime resulting from the four distinct phases within the overall MOCVD‐based III–V/Si growth process were studied: (1) the Si homoepitaxial emitter/cap layer; (2) GaP heteroepitaxial nucleation; (3) bulk GaP film growth; and (4) thick GaAs y P 1‐y compositionally graded metamorphic buffer growth. During Phase 1 (Si homoepitaxy), an approximately two order of magnitude reduction in the Si minority carrier lifetime was observed, from about 450 to ≤1 µs. However, following the GaP nucleation (Phase 2) and thicker film (Phase 3) growths, the lifetime was found to increase by about an order of magnitude. The thick GaAs y P 1‐y graded buffer was then found to provide further recovery back to around the initial starting value. The most likely general mechanism behind the observed lifetime evolution is as follows: lifetime degradation during Si homoepitaxy because of the formation of thermally induced defects within the Si bulk, with subsequent lifetime recovery due to passivation by fast‐diffusing atomic hydrogen coming from precursor pyrolysis, especially the group‐V hydrides (PH 3 , AsH 3 ), during the III–V growth. These results indicate that the MOCVD growth methodology used to create these target III–V/Si solar cell structures has a substantial and dynamic impact on the minority carrier lifetime within the Si substrate. Copyright © 2015 John Wiley & Sons, Ltd.

García‐Tabarés, Elisa↗
DOE OSTI2018

Testing Fluorescence Lifetime Standards using Two-Photon Excitation and Time-Domain Instrumentation: Fluorescein, Quinine Sulfate and Green Fluorescent Protein

It is essential for everyone working with experimental science to be certain that their instruments produce reliable results, and for fluorescence lifetime experiments, information about fluorescence lifetime standards is crucial. A large part of the literature on lifetime standards dates back to the 1970s and 1980s, and the use of newer and faster measuring devices may deem these results unreliable. We have tested the three commonly used fluorophores fluorescein, quinine sulfate and green fluorescent protein for their suitability to serve as lifetime standards, especially to be used with two-photon excitation measurements in the time-domain. We measured absorption and emission spectra for the fluorophores to determine optimal wavelengths to use for excitation and detector settings. Fluorescence lifetimes were measured for different concentrations, ranging from 10{sup − 3} − 10{sup − 5} M, as well as for various solvents. Fluorescein was soluble in both ethanol, methanol and sulfuric acid, while quinine sulfate was only soluble in sulfuric acid. Green fluorescent protein was prepared in a commercial Tris-HCl, EDTA solution, and all three fluorophores produced stable lifetime results with low uncertainties. No siginificant variation with concentration was measured for any of the fluorophores, and all showed single-exponential decays. All lifetime measurements were carried out using two-photon excitation and lifetime data was obtained in the time-domain using time-correlated single-photon counting.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

A portable time-domain LED fluorimeter for nanosecond fluorescence lifetime measurements

Fluorescence lifetime measurements are becoming increasingly important in chemical and biological research. Time-domain lifetime measurements offer fluorescence multiplexing and improved handling of interferers compared with the frequency-domain technique. In this paper, an all solid-state, filterless, and highly portable light-emitting-diode based time-domain fluorimeter (LED TDF) is reported for the measurement of nanosecond fluorescence lifetimes. LED based excitation provides more wavelengths options compared to laser diode based excitation, but the excitation is less effective due to the uncollimated beam, less optical power, and longer latency in state transition. Pulse triggering and pre-bias techniques were implemented in our LED TDF to improve the peak optical power to over 100 mW. The proposed pulsing circuit achieved an excitation light fall time of less than 2 ns. Electrical resetting technique realized a time-gated photo-detector to remove the interference of the excitation light with fluorescence. These techniques allow the LED fluorimeter to accurately measure the fluorescence lifetime of fluorescein down to concentration of 0.5 μM. In addition, all filters required in traditional instruments are eliminated for the non-attenuated excitation/emission light power. These achievements make the reported device attractive to biochemical laboratories seeking for highly portable lifetime detection devices for developing sensors based on fluorescence lifetime changes. The device was initially validated by measuring the lifetimes of three commercial fluorophores and comparing them with reported lifetime data. It was subsequently used to characterize a ZnSe quantum dot based DNA sensor.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Relative influence of meteorological conditions and aerosols on the lifetime of mesoscale convective systems

Using collocated measurements from geostationary and polar-orbital satellites over tropical continents, in this paper we provide a large-scale statistical assessment of the relative influence of aerosols and meteorological conditions on the lifetime of mesoscale convective systems (MCSs). Our results show that MCSs’ lifetime increases by 3–24 h when vertical wind shear (VWS) and convective available potential energy (CAPE) are moderate to high and ambient aerosol optical depth (AOD) increases by 1 SD (1σ). However, this influence is not as strong as that of CAPE, relative humidity, and VWS, which increase MCSs’ lifetime by 3–30 h, 3–27 h, and 3–30 h per 1σ of these variables and explain up to 36%, 45%, and 34%, respectively, of the variance of the MCSs’ lifetime. AOD explains up to 24% of the total variance of MCSs’ lifetime during the decay phase. This result is physically consistent with that of the variation of the MCSs’ ice water content (IWC) with aerosols, which accounts for 35% and 27% of the total variance of the IWC in convective cores and anvil, respectively, during the decay phase. The effect of aerosols on MCSs’ lifetime varies between different continents. AOD appears to explain up to 20–22% of the total variance of MCSs’ lifetime over equatorial South America compared with 8% over equatorial Africa. Aerosols over the Indian Ocean can explain 20% of total variance of MCSs’ lifetime over South Asia because such MCSs form and develop over the ocean. Finally, these regional differences of aerosol impacts may be linked to different meteorological conditions.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2024

Increasing Multilayer Ceramic Capacitor Lifetime With Bipolar Voltage Cycling

Enhancing the lifetime of multilayer ceramic capacitors (MLCCs) is critical in many aerospace, naval, or electrical grid applications, where device failure could lead to catastrophic consequences. The migration of oxygen vacancies from the ceramic to the electrode interface under constant bias is known to reduce the lifetime of oxide-based MLCCs. Bias cycling presents an opportunity to enhance MLCC lifetime by reducing oxygen vacancy migration. The ideal frequency range is expected to lie between frequencies low enough to avoid self-heating but high enough to avoid interfacial defect formation. However, the impact of low-frequency bipolar voltage cycling (BVC) on MLCC degradation mechanisms has not been well studied. This work investigates the impact of periodic BVC on the degradation of MLCCs through highly accelerated lifetime testing (HALT) on X7R capacitors. HALT tests were conducted at 255 °C and 60 V using different switching frequencies: 0 (dc), 0.1, 2.5, and 10 Hz. BVC was found to improve the lifetime of MLCCs compared to dc test conditions. MLCCs tested at 10-Hz BVC showed a 311% increase in average time to failure compared to the dc case. Impedance spectroscopy shows that BVC decreases the rate of resistance degradation within MLCCs, indicating that oxygen vacancy migration to the electrodes is mitigated. The impedance spectra taken on BVC samples highlight how grain boundaries play a vital role in trapping oxygen vacancies. Periodic cycling causes oxygen vacancies to become trapped at grain boundaries, resulting in oxygen vacancies taking longer to reach the electrode interface and thus increasing MLCC lifetime. This work highlights not only how BVC can be used to increase MLCC lifetime but also how periodically cycling MLCCs could increase lifetime in extreme environments, such as at elevated temperatures and electric fields.

Chuong, Kayla Y.↗
DOE OSTI2026

Development of high throughput light-sheet fluorescence lifetime imaging microscopy for 3D functional imaging of metabolic pathways in plant and microorganisms (Final Technical Report)

This research program will enable new biochemical contrast in the nanosecond lifetime domain through use of the recently demonstrated electro-optic fluorescence lifetime imaging technique (EO-FLIM) for wide-field lifetime imaging. The Stanford/Stanford Linear Accelerator Center multidisciplinary collaboration -- physics, applied physics, and structural biology -- will develop a light-sheet fluorescence lifetime imaging microscope for functional studies of microbial and plant metabolic pathways and dynamic interactions between plants and microorganisms in the rhizosphere. The proposed approach overcomes the imaging time bottleneck associated with existing fluorescence lifetime imaging methods. Initial demonstrations have shown a factor of 100,000 improvement in photon throughput compared to existing methods. High photon efficiency allowed the first wide-field fluorescence lifetime imaging of single molecules. Recent work has improved the technique’s repetition rate to enable compatibility with mode-locked lasers and demonstrated the combination of wide-field fluorescence lifetime imaging with super-resolution localization microscopy, observations of single molecule dynamics, and observation of donor lifetime quenching in single-molecule imaging. These results were achieved on standard camera sensors and would not have been possible with other wide-field approaches. The throughput and photon economy of the EO-FLIM method enables new BER-relevant imaging opportunities. In particular, scanned single- and two-photon light-sheet excitation will be used to achieve volumetric imaging with time-domain contrast.

47 OTHER INSTRUMENTATION↗
NASA NTRS1993

Collision lifetimes and impact statistics of near-Earth asteroids

We have examined the lifetimes of Near-Earth asteroids (NEA's) by directly computing the collision probabilities with other asteroids and with the terrestrial planets. We compare these to the dynamical lifetimes, and to collisional lifetimes assumed by other workers. We discuss the implications of the differences. The lifetimes of NEA's are important because, along with the statistics of craters on the Earth and Moon, they help us to compute the number of NEA's and the rate at which new NEA's are brought to the vicinity of the Earth. Assuming that the NEA population is in steady-state, the lifetimes determine the flux of new bodies needed to replenish the population. Earlier estimates of the lifetimes ignored (or incompletely accounted for) the differences in the velocities of asteroids as they move in their orbits, so our results differ from (for example) Greenberg and Chapman (1983, Icarus 55, 455) and Wetherill (1988, Icarus 76, 1) by factors of 2 to 10. We have computed the collision rates and relative velocities of NEA's with each other, the main-belt asteroids, and the terrestrial planets, using the corrected method described by Bottke et. al. (1992, GRL, in press). We find that NEA's typically have shorter collisional lifetimes than do main-belt asteroids of the same size, due to their high eccentricities, which typically give them aphelia in the main belt. Consequently, they spend a great deal of time in the main belt, and are moving much slower than the bodies around them, making them 'sitting ducks' for impacts with other asteroids. They cross the paths of many objects, and their typical collision velocities are much higher (10-15 km/s) than the collision velocities (5 km/s) among objects within the main belt. These factors combine to give them substantially shorter lifetimes than had been previously estimated.

Bottke, W. F., Jr.↗