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Results for “Temperature Gradient”

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 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Micrometer-sized Water Ice Particles for Planetary Science Experiments: Influence of Surface Structure on Collisional Properties

Models and observations suggest that ice-particle aggregation at and beyond the snowline dominates the earliest stages of planet formation, which therefore is subject to many laboratory studies. However, the pressure–temperature gradients in protoplanetary disks mean that the ices are constantly processed, undergoing phase changes between different solid phases and the gas phase. Open questions remain as to whether the properties of the icy particles themselves dictate collision outcomes and therefore how effectively collision experiments reproduce conditions in protoplanetary environments. Previous experiments often yielded apparently contradictory results on collision outcomes, only agreeing in a temperature dependence setting in above ≈210 K. By exploiting the unique capabilities of the NIMROD neutron scattering instrument, we characterized the bulk and surface structure of icy particles used in collision experiments, and studied how these structures alter as a function of temperature at a constant pressure of around 30 mbar. Our icy grains, formed under liquid nitrogen, undergo changes in the crystalline ice-phase, sublimation, sintering and surface pre-melting as they are heated from 103 to 247 K. An increase in the thickness of the diffuse surface layer from ≈10 to ≈30 Å (≈2.5 to 12 bilayers) proves increased molecular mobility at temperatures above ≈210 K. Because none of the other changes tie-in with the temperature trends in collisional outcomes, we conclude that the surface pre-melting phenomenon plays a key role in collision experiments at these temperatures. Consequently, the pressure–temperature environment, may have a larger influence on collision outcomes than previously thought.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Investigating profile stiffness and critical gradients in shaped TCV discharges using local gyrokinetic simulations of turbulent transport

The experimental observation made on the TCV tokamak of a significant confinement improvement in plasmas with negative triangularity ( δ < 0) compared to those with standard positive triangularity has been interpreted in terms of different degrees of profile stiffness (Sauter et al. Phys. Plasmas 21 055906, 2014) and/or different critical gradients. Employing the Eulerian gyrokinetic code GENE (Jenko et al. Phys. Plasmas 7 1904, 2000), profile stiffness and critical gradients are studied under TCV relevant conditions. For the considered experimental discharges, trapped electron modes (TEMs) and electron temperature gradient (ETG) modes are the dominant microinstabilities, with the latter providing a significant contribution to the non-linear electron heat fluxes near the plasma edge. Two series of simulations with different levels of realism are performed, addressing the question of profile stiffness at various radial locations. Retaining finite collisionality, impurities and electromagnetic effects, as well as the physical electron-to-ion mass ratio are all necessary in order to approach the experimental flux measurements. However, flux-tube simulations are unable to fully reproduce the TCV results, pointing towards the need to carry out radially nonlocal (global) simulations, i.e. retaining finite machine size effects, in a future study. Some conclusions about the effect of triangularity can nevertheless be drawn based on the flux-tube results. In particular, the importance of considering the sensitivity to both temperature and density gradient is shown. The flux tube results show an increase of the critical gradients towards the edge, further enhanced when δ < 0, and they also appear to indicate a reduction of profile stiffness towards plasma edge.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Observation of thermally driven field-like spin torque in magnetic tunnel junctions

We report the thermally driven giant field-like spin-torque in magnetic tunnel junctions (MTJ) on application of heat current from top to bottom. The field-like term is detected by the shift of the magneto-resistance hysteresis loop applying temperature gradient. We observed that the field-like term depends on the magnetic symmetry of the MTJ. In asymmetric structures, with different ferromagnetic materials for free and fixed layers, the field-like term is greatly enhanced. Our results show that a pure spin current density of the order of 10{sup 9 }A/m{sup 2} can be produced by creating a 120 mK temperature difference across 0.9 nm thick MgO tunnelling barrier. Our results will be useful for writing MTJ and domain wall-based memories using thermally driven spin torque.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

A note on convective heat transfer of an MHD Jeffrey fluid over a stretching sheet

This article focuses on the exact solution regarding convective heat transfer of a magnetohydrodynamic (MHD) Jeffrey fluid over a stretching sheet. The effects of joule and viscous dissipation, internal heat source/sink and thermal radiation on the heat transfer characteristics are taken in account in the presence of a transverse magnetic field for two types of boundary heating process namely prescribed power law surface temperature (PST) and prescribed heat flux (PHF). Similarity transformations are used to reduce the governing non-linear momentum and thermal boundary layer equations into a set of ordinary differential equations. The exact solutions of the reduced ordinary differential equations are developed in the form of confluent hypergeometric function. The influence of the pertinent parameters on the temperature profile is examined. In addition the results for the wall temperature gradient are also discussed in detail.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

UNRAVELLING THE COMPONENTS OF A MULTI-THERMAL CORONAL LOOP USING MAGNETOHYDRODYNAMIC SEISMOLOGY

Coronal loops, constituting the basic building blocks of the active Sun, serve as primary targets to help understand the mechanisms responsible for maintaining multi-million Kelvin temperatures in the solar and stellar coronae. Despite significant advances in observations and theory, our knowledge on the fundamental properties of these structures is limited. Here, we present unprecedented observations of accelerating slow magnetoacoustic waves along a coronal loop that show differential propagation speeds in two distinct temperature channels, revealing the multi-stranded and multithermal nature of the loop. Utilizing the observed speeds and employing nonlinear force-free magnetic field extrapolations, we derive the actual temperature variation along the loop in both channels, and thus are able to resolve two individual components of the multithermal loop for the first time. The obtained positive temperature gradients indicate uniform heating along the loop, rather than isolated footpoint heating.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Influence of cyclic thermal treatments on the oxidation behavior of Ti-6Al-2Sn-4Zr-2Mo alloy

Highlights: • The oxidation kinetics of commercially titanium alloys has been extensively investigated following isothermal treatments. • Components in aerospace applications are subjected to temperature gradients which can modify the oxidation kinetics of Ti-6242. • The main goal of the present research is to study the formation of the oxide scale and the alpha case layer under cyclic thermal treatments • Results are with those achieved by isothermal treatments. - Abstract: Ti-6Al-2Sn-4Zr-2Mo is one of the most common titanium alloys for aerospace industry. This alloy experiences oxidation phenomenon at elevated temperatures. In the present study, cyclic thermal treatments were performed in air at 500, 593 and 700 °C, up to 500 cycles, in order to determine the oxidation kinetics and to analyze the oxide scale and alpha-case formation. Moreover, results were compared to those achieved under isothermal conditions to elucidate differences between both thermal conditions. In this sense, metallographic techniques and X-ray diffraction, together with a detailed advanced characterization of the microstructure by Field Emission Scanning Electron Microscopy and Focus Ions Beam, were used to analyze surface oxidation evolution. Results pointed out that cyclic treatments induced a strong increase of the weight gain compared to isothermal treatments. The analysis of the oxide scale revealed the formation of not only rutile, as isothermal treatments, but also anatase. Thickness of the oxide scale was higher for cyclic conditions, while alpha case did not exceed values reached by isothermal treatments and even became lower at 500 °C.

36 MATERIALS SCIENCE↗
DOE OSTI2016

GRAVITY-DARKENED SEASONS: INSOLATION AROUND RAPID ROTATORS

I model the effect of rapid stellar rotation on a planet’s insolation. Fast-rotating stars have induced pole-to-equator temperature gradients (known as gravity darkening) of up to several thousand Kelvin that affect the star’s luminosity and peak emission wavelength as a function of latitude. When orbiting such a star, a planet’s annual insolation can strongly vary depending on its orbital inclination. Specifically, inclined orbits result in temporary exposure to the star’s hotter poles. I find that gravity darkening can drive changes in a planet’s equilibrium temperature of up to ∼15% due to increased irradiance near the stellar poles. This effect can also vary a planet’s exposure to UV radiation by up to ∼80% throughout its orbit as it is exposed to an irradiance spectrum corresponding to different stellar effective temperatures over time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

A New Method of Selective Laser Cladding

A new approach to selective laser cladding using conical laser beams has been elaborated. The initial round laser beam is divided into two or more circular beams with regulated distribution of the laser power between them. The circular beams are converted into conical beams that are focused separately on the surface and on different areas of the deposited material for their efficient heating. The delivery of laser energy into the powder stream is very effective because it provides a complete uniform absorption of laser energy in a dense powder stream (10{sup 4}–10{sup 6} cm{sup –3}). Under wire deposition, substrate heating reduces residual stresses due to temperature gradients and compensates for heat loss from the deposition zone by thermal conduction. The required power density for melt contact formation is significantly reduced.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

A volatile fluid assisted thermo-pneumatic liquid metal energy harvester

A close-cycle self-driving thermal energy harvester using liquid metal as energy carrier fluid has been proposed. The driving force that pushes the liquid metal against flow resistance and gravity is provided by a resistively heated volatile fluid based on thermo-pneumatic principle. The tested harvester prototype demonstrated its capability to extract thermal energy between small temperature gradient, at a scale of 10 °C. During a 5-h operation, it further demonstrated robust liquid metal recirculating performance at a time-average volume flow rate of 14 ml/min with a 12.25 W heating load. The prototype also managed to self-adjust to variable working conditions which indicated the reliability of this method. Advantages of this method include simple-structural design, rigid-motion free operation, and low-temperature actuation. These advantages make it uniquely suited for solar energy and low-grade heat harvesting, high heat flux electronics cooling, as well as autonomous machines actuating.

42 ENGINEERING↗
DOE OSTI2018

Effect of a High Magnetic Field on γ′ Phase for Ni-Based Single Crystal Superalloy During Directional Solidification

The effect of a high magnetic field on the γ′ phase of Ni-based single crystal superalloy during directional solidification is investigated experimentally. The results clearly indicate that the magnetic field significantly reduces the γ′ phase size. Further, the quenching experiment is carried out, and the results found that the length of mushy zone is obviously decreased under a high magnetic field. Based on both experimental results and nucleation mechanism, it is found that the decrease of γ′ phase size should be attributed to the fact that a high magnetic field causes the increase of temperature gradient in front of solid/liquid interface and leads to the increase of undercooling of γ′ phase.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Quantitative evaluations of subtropical westerly jet simulations over East Asia based on multiple CMIP5 and CMIP6 GCMs

As a salient feature of the Asian monsoon system, the East Asian subtropical westerly jet (EASWJ) exerts significant impacts on weather and climate changes in China and even throughout East Asia. In this paper, we applied a new self-adaptive algorithm to detect the EASWJ, identify its boundaries, and then represent its characteristics by defining three indices: the intensity index, meridional displacement index, and width index. Compared to the reanalysis data, we carried out a comprehensive, objective, and quantitative EASWJ evaluation using historical experiments from multiple global climate models (GCMs) in the Coupled Model Intercomparison Project Phases 5 and 6 (CMIP5 and CMIP6). The results show that the multimodel ensemble mean (MME) of both CMIP5 and CMIP6 can simulate the characteristics of winter EASWJ well. While for the other three seasons, the MME of both phase models underestimate the 200-hPa zonal wind (U200) strength in the jet coverage area and overestimate the U200 outside the jet area, such simulation results weaken the meridional shear of the wind field. The EASWJ simulations from the CMIP5 GCMs had no consistent intensity or location characteristic tendencies, and most CMIP5 GCMs tended to simulate relatively wide-coverage jets. In contrast, most CMIP6 GCMs are inclined to simulate significantly weaker, wider, and more-northward jets. Compared to the predecessors in CMIP5, about half of CMIP6 GCMs significantly minimized the jet intensity bias, but remarkable errors were still observed in their jet location and coverage representations. Furthermore, the comparative analysis performed by classifying models based on their evaluated simulation results suggested that the simulated performance of the meridional temperature gradient was important for capturing the EASWJ characteristics. Further in-depth study of the causes of model differences is warranted to improve simulation results.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2019

Thermoelectric Mechanism of Field Emission from Carbon Nanostructures

A model describing peculiarities of field emission from carbon nanomaterials is considered. The model is based on inclusion by drag of electrons by ballistic phonons in the temperature gradient range in the emission center. The model does not require any additional assumptions concerning a special energy structure of the emission center. Quantitative estimates of the thermopower coefficient obtained based on the emission model are in good agreement with experimental results.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Small-Angle Neutron Scattering Measurements of the δ Deuteride Phase in Zircaloy 4

Hydrogen pickup by Zr-based LWR fuel cladding during service leads to the formation of circumferential hydride platelet particles embedded within the Zr matrix. The precipitation of the hydride phase occurs preferentially on the water-side due to the temperature gradient within the clad wall. The non-uniform hydride morphology is known as a rim structure. Significant tensile hoop stresses within the clad wall develop during the fuel lifetime due to fission gas release and pellet-clad mechanical interaction. One potential post-service cladding failure mechanism is delayed hydride cracking (DHC) initiated by hydride reorientation under this tensile hoop stress. As a consequence, hydride reorientation and DHC have been extensively studied over the last few decades to understand the interdependencies of tensile stress, temperature, hydride morphology, and hydrogen concentration. Much of the most recent work in this area has taken advantage of 3. generation synchrotron X-ray sources to perform in situ measurements of hydride phase stability. Neutron scattering based techniques offer unique advantages in the study of hydrogen in metals (typically with deuterium substituted for hydrogen) but are under utilized in the study of hydrides in Zr. We present small-angle neutron scattering (SANS) measurements of the deuteride phase in Zircaloy 4 (Zy4) here. The SANS measurements presented here demonstrate the sensitivity of the technique to deuteride phase particles embedded in a Zy4 matrix. Quantifiable net scattering at 50 wppm effective hydrogen concentration is derived from a subtraction of a reference sample. This demonstrates that the strong χ phase Porod scattering can be effectively eliminated, revealing the response from the deuteride phase. A similar experimental protocol using small-angle X-ray scattering (SAXS) would likely fail due to poor contrast between the hydride phase and the Zr matrix. Stated in different terms, the SAXS response would be dominated by SPP Porod scattering with unresolvable net scattering from hydride particles. (authors)

36 MATERIALS SCIENCE↗
DOE OSTI2015

FAR-INFRARED DUST TEMPERATURES AND COLUMN DENSITIES OF THE MALT90 MOLECULAR CLUMP SAMPLE

We present dust column densities and dust temperatures for ∼3000 young, high-mass molecular clumps from the Millimeter Astronomy Legacy Team 90 GHz survey, derived from adjusting single-temperature dust emission models to the far-infrared intensity maps measured between 160 and 870 μm from the Herschel/Herschel Infrared Galactic Plane Survey (Hi-Gal) and APEX/APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) surveys. We discuss the methodology employed in analyzing the data, calculating physical parameters, and estimating their uncertainties. The population average dust temperature of the clumps are 16.8 ± 0.2 K for the clumps that do not exhibit mid-infrared signatures of star formation (quiescent clumps), 18.6 ± 0.2 K for the clumps that display mid-infrared signatures of ongoing star formation but have not yet developed an H ii region (protostellar clumps), and 23.7 ± 0.2 and 28.1 ± 0.3 K for clumps associated with H ii and photo-dissociation regions, respectively. These four groups exhibit large overlaps in their temperature distributions, with dispersions ranging between 4 and 6 K. The median of the peak column densities of the protostellar clump population is 0.20 ± 0.02 g cm{sup −2}, which is about 50% higher compared to the median of the peak column densities associated with clumps in the other evolutionary stages. We compare the dust temperatures and column densities measured toward the center of the clumps with the mean values of each clump. We find that in the quiescent clumps, the dust temperature increases toward the outer regions and that these clumps are associated with the shallowest column density profiles. In contrast, molecular clumps in the protostellar or H ii region phase have dust temperature gradients more consistent with internal heating and are associated with steeper column density profiles compared with the quiescent clumps.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Deformation Behavior of Pure Copper Castings with As-Cast Surfaces for Electrical Parts

Herein, the tensile deformation behavior of pure copper castings with as-cast surfaces was investigated. It was found that small and sufficiently large uniform elongation types corresponded, respectively, to the presence or absence of a Cu-Cu{sub 2}O eutectic phase in the microstructure. Fissure defects were observed near the as-cast surfaces when the eutectic phase was present, probably leading to stress concentration. On the other hand, surface pinholes were observed when the eutectic phase was absent, and these were not prevented by directional solidification. Although the as-cast specimens (non-directionally solidified) exhibited a slightly reduced uniform elongation compared to the machined specimens in the absence of the eutectic phase, the directionally solidified as-cast samples exhibited a uniform elongation that was similar to that of the machined specimens. In addition, micropore arrays were formed along the grain boundaries inside the castings when the eutectic phase was absent, whereas almost no micropores were observed inside the directionally solidified as-cast specimens because the solidification process involved a faster cooling rate and/or higher temperature gradient. These results do not only suggest the existence of surface pinholes but also indicate that microporosity can reduce the cross-sectional area of the specimens, thus, affecting their uniform elongation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Effect of Bismuth on the Properties of Elastically Stressed AlGaInAsP〈Bi〉/InP Heterostructures

The results of growing elastically stressed AlGaInAsP〈Bi〉 thin epitaxial layers from the liquid phase on indium phosphide substrates in a temperature-gradient field are discussed. The effect of bismuth on the structural perfection, the luminescence properties, and the external quantum yield of AlGaInAsP〈Bi〉/InP heterostructures is investigated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Thermo-mechanical simulations of CO{sub 2} laser–fused silica interactions

CO{sub 2} laser heating of silica glass is used in many scientific and industrial applications. Particularly, localized CO{sub 2} laser heating of silica glass has demonstrated its ability to mitigate surface damage on optics used for high power laser applications. To develop such applications, the control of temperature, heat affected area, and resulting mechanical stresses are critical. Therefore, it is necessary to understand the silica transformation, the material ejection, and the thermo-mechanical stresses induced by the laser heating and subsequent cooling. In this paper, we detail the development of comprehensive thermo-mechanical numerical simulations of these physical processes, based on finite-element method. The approach is developed for 2D or 3D cases to tackle the case of a moving beam at the surface of the sample, and we particularly discuss the choice of the different parameters based on bibliographic inputs. The thermal and mechanical numerical results have been compared to different dedicated experimental studies: infrared thermography measurements at the surface of the irradiated area, optical profilometry measurements of the laser-processed sites, and photo-elastic measurements. Very consistent results are obtained between numerical and experimental results for the description of the temperature gradients, the material ejection, and the residual stresses.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Laser-induced thermoelectric current as a source of generation of THz surface electromagnetic waves

The theory of excitation of a surface electromagnetic wave (typically of a terahertz frequency range) is developed, the source of which is the thermoelectric current that arises when a charged inhomogeneous layer of hot electrons of submicron thickness is generated by an intense laser ultrashort pulse at the surface of a solid target under conditions of crossed electron density and temperature gradients. The efficiency of such a generation mechanism is compared with the efficiency of generation of transient surface radiation caused by a bunch of high-energy electrons leaving the target. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗