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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 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Rain evaporation and moist patches in tropical boundary layers

Moist patches are areas in the subcloud layer characterized by a positive water vapor anomaly compared to the environment and are considered important in triggering new convective cells. A correct understanding of the origin of the water vapor in these patches is, thus, essential to improving existing convective parameterizations. Recent studies have addressed this problem and have shown that contrary to what was previously thought, the main source of water vapor in moist patches are surface latent heat fluxes, instead of rain evaporation. This paper offers a different perspective to the topic, focusing on the origin only of the water vapor that makes moist patches anomalously moist when compared to the environment. Finally, it is found that near the surface, rain evaporation contributes half as much as latent heat fluxes, implying that a parameterization of the thermodynamic forcing should be more sensitive to environmental variables, like relative humidity, than recently suggested.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

Evaporated Se x Te 1-x Thin Films with Tunable Bandgaps for Short-Wave Infrared Photodetectors

Semiconducting absorbers in high-performance short-wave infrared (SWIR) photodetectors and imaging sensor arrays are dominated by single-crystalline germanium and III-V semiconductors. However, these materials require complex growth and device fabrication procedures. In this work, thermally evaporated Se x Te 1- x alloy thin films with tunable bandgaps for the fabrication of high-performance SWIR photodetectors are reported. From absorption measurements, it is shown that the bandgaps of Se x Te 1- x films can be tuned continuously from 0.31 eV (Te) to 1.87 eV (Se). Owing to their tunable bandgaps, the peak responsivity position and photoresponse edge of Se x Te 1- x film-based photoconductors can be tuned in the SWIR regime. By using an optical cavity substrate consisting of Au/Al 2 O 3 to enhance its absorption near the bandgap edge, the Se 0.32 Te 0.68 film (an optical bandgap of ≈0.8 eV)-based photoconductor exhibits a cut-off wavelength at ≈1.7 μm and gives a responsivity of 1.5 AW -1 and implied detectivity of 6.5 × 10 10 cm Hz 1/2 W -1 at 1.55 μm at room temperature. Importantly, the nature of the thermal evaporation process enables the fabrication of Se 0.32 Te 0.68 -based 42 × 42 focal plane arrays with good pixel uniformity, demonstrating the potential of this unique material system used for infrared imaging sensor systems.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Sodium doping of solution-processed amine-thiol based CIGS solar cells by thermal evaporation of NaCl

Poor crystallinity, high degree of porosity and rough surfaces are the main drawbacks of solution-processed CIGS absorbers resulting in lower power conversion efficiencies when compared to vacuum-based CIGS solar cells. Therefore, promoting absorber grain growth is key to further improve solution-based solar cell performance. The effect of alkali elements such as Na in CIGS absorbers is generally recognised to have beneficial effects not only on the absorber opto-electronic properties but also on the grain growth. In this work, thermal evaporation of a thin layer of NaCl prior to selenisation resulted in absorbers with significantly larger CIGS grains than previously seen with Na diffusing directly from the from soda-lime glass substrate. NaCl is non-toxic, abundant and readily available compound that has not been typically used as an evaporation source, but rather as an additive into CIGS precursor solution. The effect of Na on these solution-processed CIGS devices was primarily observed in the spectacular morphological changes leading to improved carrier collection and minority carrier lifetimes, but less on the absorber doping. Transmission electron microscopy (TEM) revealed voids forming around large CIGS grains upon NaCl addition and these had a negative effect on inter-grain carrier transport. Nonetheless, the resulting device performance doubled from 5% to 10% with addition of Na using this doping approach; however, a compromise between the optimum grain growth and optimum electronic properties had to be made. This study demonstrates a novel, simple and effective Na-doping strategy for CIGS absorbers and reveals the current limitations of the Na-doping in solution-processed atmospherically deposited cells.

14 SOLAR ENERGY↗
DOE OSTI2020

A random unitary circuit model for black hole evaporation

Inspired by the Hayden-Preskill protocol for black hole evaporation, we consider the dynamics of a quantum many-body qudit system coupled to an external environment, where the time evolution is driven by the continuous limit of certain 2-local random unitary circuits. We study both cases where the unitaries are chosen with and without a conserved U(1) charge and focus on two aspects of the dynamics. First, we study analytically and numerically the growth of the entanglement entropy of the system, showing that two different time scales appear: one is intrinsic to the internal dynamics (the scrambling time), while the other depends on the system-environment coupling. In the presence of a U(1) conserved charge, we show that the entanglement follows a Page-like behavior in time: it begins to decrease in the middle stage of the “evaporation”, and decreases monotonically afterwards. Second, we study the time needed to retrieve information initially injected in the system from measurements on the environment qudits. Based on explicit numerical computations, we characterize such time both when the retriever has control over the initial configuration or not, showing that different scales appear in the two cases.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Anti-evaporation of Bardeen de-Sitter black holes

In this paper, we discuss the possibility of the anti-evaporation of degenerate Bardeen de-Sitter black hole. We solve the perturbation equations around the Nariai space–time. The solution of one of the perturbations related to the horizon size demonstrates that horizon of such black hole is constant. The other perturbation is also found to be stable. We further study thermodynamical properties of such black holes. We observe double phase transition at the Nariai limit. - Highlights: • Cosmological constant introduces a cosmological horizon in the Bardeen black hole. • Anti-evaporation is associated with the Nariai space–time. • The perturbations around degenerate horizon are stable. • The size of the degenerate horizon is constant with time. • A double phase transition occurs in the Nariai–Bardeen de-Sitter black hole.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Impact of air distribution on dew point evaporative cooler thermal performance

A new geometry of the dew-point indirect evaporative cooler structure was proposed which enables uniform distribution of water in the working channels and application in traditional air handling units. This arrangement requires a complicated structure of the product channel in which the air is cooled. Further, the airflow direction changes and complicated structure it is associated with local losses affecting the total pressure loss. High-pressure drop limits the use of this dew point evaporative cooler as an alternative source of cooling. Therefore, a verified method based on numerical fluid dynamics (CFD) was used to determine the pressure drop and air distribution in the exchanger. The combination of the CFD method and modified epsilon-NTU allows the thermal performance of the device including the uniformity of air distribution, cooling capacity, and coefficient of performance to be determined. Finally, the appropriate exchanger dimensions, i.e., the channel height, or final distribution regulation may be found depending on the exchanger application.

42 ENGINEERING↗
DOE OSTI2014

Comparison of Cf-252 thin-film sources prepared by evaporation or self-transfer

Californium-252 (Z = 98) is valued as a potent neutron source due to its spontaneous fission decay path. Thin film sources containing Cf-252 were prepared by two techniques: evaporation and self-transfer. The sources were analyzed by alpha and gamma spectroscopy. Results indicate that self-transfer sources exhibit less alpha energy straggling and energy loss than evaporative sources. Fission fragments may also self-transfer, and sources made by self-transfer may need some decay time to reach radioactive equilibrium.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2015

Milagro limits and HAWC sensitivity for the rate-density of evaporating primordial black holes

Primordial Black Holes (PBHs) are gravitationally collapsed objects that may have been created by density fluctuations in the early universe and could have arbitrarily small masses down to the Planck scale. Hawking showed that due to quantum effects, a black hole has a temperature inversely proportional to its mass and will emit all species of fundamental particles thermally. PBHs with initial masses of ~ 5.0 × 10¹⁴ g should be expiring in the present epoch with bursts of high-energy particles, including gamma radiation in the GeV – TeV energy range. The Milagro high energy observatory, which operated from 2000 to 2008, is sensitive to the high end of the PBH evaporation gamma-ray spectrum. Due to its large field-of-view, more than 90% duty cycle and sensitivity up to 100 TeV gamma rays, the Milagro observatory is well suited to perform a search for PBH bursts. Based on a search on the Milagro data, we report new PBH burst rate density upper limits over a range of PBH observation times. In addition, we report the sensitivity of the Milagro successor, the High Altitude Water Cherenkov (HAWC) observatory, to PBH evaporation events.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Primordial Black Holes: Observational characteristics of the final evaporation

For many early universe theories predict the creation of Primordial Black Holes (PBHs). PBHs could have masses ranging from the Planck mass to 105 solar masses or higher depending on the size of the universe at formation. A Black Hole (BH) has a Hawking temperature which is inversely proportional to its mass. Hence a sufficiently small BH will quasi-thermally radiate particles at an ever-increasing rate as emission lowers its mass and raises its temperature. Moreover, the final moments of this evaporation phase should be explosive and its description is dependent on the particle physics model. In this work we investigate the final few seconds of BH evaporation, using the Standard Model and incorporating the most recent Large Hadron Collider (LHC) results, and provide a new parameterization for the instantaneous emission spectrum. We calculate for the first time energy-dependent PBH burst light curves in the GeV/TeV energy range. Moreover, we explore PBH burst search methods and potential observational PBH burst signatures. We have found a unique signature in the PBH burst light curves that may be detectable by GeV/TeV gamma-ray observatories such as the High Altitude Water Cerenkov (HAWC) observatory. Finally, the implications of beyond the Standard Model theories on the PBH burst observational characteristics are also discussed, including potential sensitivity of the instantaneous photon detection rate to a squark threshold in the 5–10 TeV range.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Performance study of a novel dew point evaporative cooler in the climate of central Europe using building simulation tools

This paper presents the performance research of a hybrid air conditioning system (System II) equipped with novel dew point evaporative cooler (DPIEC) for a retail building located in temperate climatic conditions. The new cooler has a dedicated, adaptable structure that maximizes its performance in the climate conditions of central Europe. The unit can operate as a typical regenerative air cooler when outdoor air is drier than indoor air, it can operate as a counter-flow heat recovery unit when indoor conditions are more dry than outdoor and it can operate as a combination of both arrangements. The year-round, hourly-stepped building energy simulations were carried-out using a prognostic tool of energy consumption. The novel dew point evaporative cooler application potential was established using the black-box model based on regression equations was established based on experimental tests on a prototype of novel DPIEC. The hybrid system operation was compared to the typical air handling unit equipped with a standard energy wheel for heat recovery (System I). Presented research shows that DPIEC can cover about 95% of total cooling loads while the energy wheel covers only 9% of total cooling load. System II allows us to save 65% of seasonal electricity consumption comparing to System I. It comes from the fact that during most of the cooling season DPIEC is in operation and it covers most of the cooling loads without the need to switch the chilled water system.

Air conditioning↗
DOE OSTI2013

Evaporation — a key mechanism for the thaumasite form of sulfate attack

Understanding the mechanisms leading to chemical attack on concrete is crucial in order to prevent damage of concrete structures. To date, most studies on sulfate attack and thaumasite formation are based on empirical approaches, as the identification of associated reaction mechanisms and paths is known to be highly complex. In this study, sulfate damaged concrete from Austrian tunnels was investigated by mineralogical, chemical and isotope methods to identify the reactions which caused intense concrete alteration. Major, minor and trace elemental contents as well as isotope ratios of local ground water (GW), drainage water (DW) and interstitial solutions (IS), extracted from damaged concrete material, were analyzed. Locally occurring GW contained 3 to 545 mg L{sup −1} of SO{sub 4} and is thus regarded as slightly aggressive to concrete in accordance to standard specifications (e.g. DIN EN 206-1). The concrete linings and drainage systems of the studied tunnels, however, have partly suffered from intensive sulfate attack. Heavily damaged concrete consisted mainly of thaumasite, secondary calcite, gypsum, and relicts of aggregates. Surprisingly, the concentrations of dissolved ions were extremely enriched in the IS with up to 30,000 and 12,000 mg L{sup −1} of SO{sub 4} and Cl, respectively. Analyses of aqueous ions with a highly conservative behavior, e.g. K, Rb and Li, as well as {sup 2}H/H and {sup 18}O/{sup 16}O isotope ratios of H{sub 2}O of the IS showed an intensive accumulation of ions and discrimination of the light isotopes vs. the GW. These isotope signals of the IS clearly revealed evaporation at distinct relative humidities. From ion accumulation and isotope fractionation individual total and current evaporation degrees were estimated. Our combined elemental and isotopic approach verified wetting–drying cycles within a highly dynamic concrete-solution-atmosphere system. Based on these boundary conditions, key factors controlling thaumasite formation are discussed regarding the development of more sulfate-resistant concrete and concrete structures.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Low temperature perovskite solar cells with an evaporated TiO 2 compact layer for perovskite silicon tandem solar cells

Silicon-based tandem solar cells can overcome the efficiency limit of single junction silicon solar cells. Perovskite solar cells are particularly promising as a top cell in monolithic tandem devices due to their rapid development towards high efficiencies, a tunable band gap with a sharp optical absorption edge and a simple production process. In monolithic tandem devices, the perovskite solar cell is deposited directly on the silicon cell, requiring low-temperature processes (< 200 °C) to maintain functionality of under-lying layers of the silicon cell in case of highly efficient silicon hetero-junction (SHJ) bottom solar cell. In this work, we present a complete low-temperature process for perovskite solar cells including a mesoporous titanium oxide (TiO 2 ) scaffold - a structure yielding the highest efficiencies for single-junction perovskite solar cells. We show that evaporation of the compact TiO 2 hole blocking layer and ultra-violet (UV) curing for the mesoporous TiO 2 layer allows for good performance, comparable to high-temperature (> 500 °C) processes. With both manufacturing routes, we obtain short-circuit current densities (J SC ) of about 20 mA/cm 2 , open-circuit voltages (V OC ) over 1 V, fill factors (FF) between 0.7 and 0.8 and efficiencies (n) of more than 15%. We further show that the evaporated TiO 2 layer is suitable for the application in tandem devices. The series resistance of the layer itself and the contact resistance to an indium doped tin oxide (ITO) interconnection layer between the two sub-cells are low. Additionally, the low parasitic absorption for wavelengths above the perovskite band gap allow a higher absorption in the silicon bottom solar cell, which is essential to achieve high tandem efficiencies.

14 SOLAR ENERGY↗
DOE OSTI2020

Constraining the behavior of gallium isotopes during evaporation at extreme temperatures

Renewed interest in gallium isotope systematics has stemmed from the fact that Ga is moderately volatile and is hypothesized to undergo kinetic fractionation during evaporation. In this work, we present the first Ga isotope data from terrestrial volatile depleted samples including a suite of experimentally heated rhyolitic soils, fallout melt glass, and splash-form tektites from the Australasian strewn field (hereafter termed australite tektites). The Ga in these samples is isotopically heavy compared to Ga in terrestrial basalts and estimates for the composition of the bulk silicate Earth (BSE). For each sample suite the isotopic fractionation of Ga scales with the degree of Ga depletion, consistent with isotopic fractionation caused by evaporation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Structural, optical and gas sensing properties of vertically well-aligned ZnO nanowires grown on graphene/Si substrate by thermal evaporation method

Highlights: • Vertically well-aligned ZnO NW arrays with high aspect ratio and extremely large-surface-volume ratio were synthesized on graphene/Si substrate without the aid of metal catalyst by a simple thermal evaporation. • The gas sensor based on the vertical ZnO NW arrays-graphene structure exhibited high sensitivity and rapid response/recovery characteristics to NO{sub 2} gas at 200oC, and could be used to detect NO{sub 2} gas at very low levels (2, 6, 10 ppm). • The excellent sensing performance is mainly because of the large specific surface area and the presence of oxygen-vacancies at the surface of ZnO NWs, together with the creation of p-n heterojunctions between n-type ZnO NW and p-type graphene. • The common defects and optical properties of the ZnO NW arrays-graphene structure were studied in detail. • The possible growth mechanism of vertically well-aligned ZnO NWs on graphene and the gas-sensing mechanism of sensors were proposed. - Abstract: We report catalyst-free vertical growth of high-density ZnO nanowire (NW) arrays on Si substrate with a graphene buffer layer by thermal evaporation method. Structural, surface morphology, chemical composition and optical properties of ZnO NWs were studied by Field emission scanning electron microscopy (FE-SEM), High-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) and Raman spectroscopy. The results indicate that the grown ZnO NWs are single-crystalline and exhibit the hexagonal wurtzite crystal structure with a preferred orientation along the [0001] direction; all of the ZnO NWs vertically grow on the graphene/Si substrate. The majority of the as-grown NWs have diameters in the range of 250–300 nm and length up to several tens of micrometers, whereas a small portion of NWs have a diameter of approximately of 9–12 nm. Room temperature PL spectrum of the ZnO NWs exhibits a sharp and strong ultraviolet emission at 380 nm and a weak visible emission at around 516 nm. The role of graphene buffer layer in the vertical-aligned ZnO NW arrays growth and its improved optical properties is demonstrated. Importantly, the gas sensor based on these ZnO NWs exhibits high sensitivity and rapid response/recovery characteristics to NO{sub 2} gas at 200 °C, and can detect NO{sub 2} concentration as low as 2, 6 and 10 ppm. The excellent sensing performance is mainly due to the combination of the large specific surface area and the presence of oxygen-vacancies at the surface of ZnO NWs, together with the creation of p-n heterojunctions between n-type ZnO NW and p-type graphene. Such a ZnO NW/graphene structure is very promising for various applications, such as photonic, optoelectronic and sensor devices. In addition, the possible growth mechanism and the NO{sub 2} sensing gas mechanism of the ZnO NWs-graphene structure have been discussed.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Alignment nature of ZnO nanowires grown on polished and nanoscale etched lithium niobate surface through self-seeding thermal evaporation method

Highlights: • ZnO nanowires were grown directly on LiNbO{sub 3} surface for the first time by thermal evaporation. • Self-alignment of the nanowires due to step bunching of LiNbO{sub 3} surface is observed. • Increased roughness in surface defects promoted well-aligned growth of nanowires. • Well-aligned growth was then replicated in 50 nm deep trenches on the surface. • Study opens novel pathway for patterned growth of ZnO nanowires on LiNbO{sub 3} surface. - Abstract: High aspect ratio catalyst-free ZnO nanowires were directly synthesized on lithium niobate substrate for the first time through thermal evaporation method without the use of a buffer layer or the conventional pre-deposited ZnO seed layer. As-grown ZnO nanowires exhibited a crisscross aligned growth pattern due to step bunching of the polished lithium niobate surface during the nanowire growth process. On the contrary, scratches on the surface and edges of the substrate produced well-aligned ZnO nanowires in these defect regions due to high surface roughness. Thus, the crisscross aligned nature of high aspect ratio nanowire growth on the lithium niobate surface can be changed to well-aligned growth through controlled etching of the surface, which is further verified through reactive-ion etching of lithium niobate. The investigations and discussion in the present work will provide novel pathway for self-seeded patterned growth of well-aligned ZnO nanowires on lithium niobate based micro devices.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2013

Evaporation-Induced Buckling and Fission of Microscale Droplet Interface Bilayers

Droplet interface bilayers (DIBs) are a robust platform for studying synthetic cellular membranes; however, to date no DIBs have been produced at cellular length scales. Here, we create microscale droplet interface bilayers ( DIBs) at the interface between aqueous femtoliter-volume droplets within an oil-filled microfluidic channel. The uniquely large area-to-volume ratio of the droplets results in strong evaporation effects, causing the system to transition through three distinct regimes. First, the two adjacent droplets shrink into the shape of a single spherical droplet, where an augmented lipid bilayer partitions two hemi-spherical volumes. In the second regime, the combined effects of the shrinking monolayers and growing bilayer force the confined bilayer to buckle to conserve its mass. Finally, at a bending moment corresponding to a critical shear stress, the buckling bilayer fissions a vesicle to regulate its shape and stress. The DIBs produced here enable evaporation-induced bilayer dynamics reminiscent of endo- and exocytosis in cells.

Boreyko, Jonathan B↗
DOE OSTI2024

Evaporation and Transpiration From Multiple Proximal Forests and Wetlands

Abstract Climate change is intensifying the hydrologic cycle and altering ecosystem function, including water flux to the atmosphere through evapotranspiration (ET). ET is made up of evaporation (E) via non‐stomatal surfaces, and transpiration (T) through plant stomata which are impacted by global changes in different ways. E and T are difficult to measure independently at the ecosystem scale, especially across multiple sites that represent different land use and land management strategies. To address this gap in understanding, we applied flux variance similarity (FVS) to quantify how E and T differ across 13 different ecosystems measured using eddy covariance in a 10 × 10 km area from the CHEESEHEAD19 experiment in northern Wisconsin, USA. The study sites included eight forests with a large deciduous broadleaf component, three evergreen needleleaf forests, and two wetlands. Average T/ET for the study period averaged nearly 52% in forested sites and 45% in wetlands, with larger values after excluding periods following rain events when evaporation from canopy interception may be expected. A dominance analysis revealed that environmental variables explained on average 69% of the variance of half‐hourly T, which decreased from summer to autumn. Deciduous and evergreen forests showed similar E trajectories over time despite differences in vegetation phenology, and vapor pressure deficit explained some 13% of the variance E in wetlands but only 5% or less in forests. Retrieval of E and T within a dense network of flux towers lends confidence that FVS is a promising approach for comparing ecosystem hydrology across multiple sites to improve our process‐based understanding of ecosystem water fluxes.

Environmental Sciences & Ecology↗
DOE OSTI2014

Ultra-High Performance, High-Temperature Superconducting Wires via Cost-effective, Scalable, Co-evaporation Process

Long-length, high-temperature superconducting (HTS) wires capable of carrying high critical current, Ic, are required for a wide range of applications. Here, we report extremely high performance HTS wires based on 5 m thick SmBa2Cu3O7- (SmBCO) single layer films on textured metallic templates. SmBCO layer wires over 20 meters long were deposited by a cost-effective, scalable co-evaporation process using a batch-type drum in a dual chamber. All deposition parameters influencing the composition, phase, and texture of the films were optimized via a unique combinatorial method that is broadly applicable for co-evaporation of other promising complex materials containing several cations. Thick SmBCO layers deposited under optimized conditions exhibit excellent cube-on-cube epitaxy. Such excellent structural epitaxy over the entire thickness results in exceptionally high Ic performance, with average Ic over 1000 A/cm for the entire 22 meter long wire and maximum Ic over 1,500 A/cm for a short 12 cm long tape. The Ic values reported in this work are the highest values ever reported from any lengths of cuprate-based HTS wire or conductor.

Kim, Dr. Hosup↗