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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Elastohydrodynamic lubrication of elliptical contacts for materials of low elastic modulus. I - Fully flooded conjunction

A complete numerical solution is presented to the problem of isothermal elastohydrodynamic lubrication of elliptical contacts for low-elastic-modulus materials operating under fully loaded conditions. No assumption is made for the pressure or film thickness within the contact, and compressibility and viscous effects are taken into account. Because of the dimensionless representation of the coordinates, the actual Hertzian contact ellipse becomes a circle regardless of the value of the ellipticity parameter. A minimum-film-thickness relation and a central-film-thickness relation are derived from examining 17 different cases. Contour plots for detailed illustration of the pressure distribution and film thickness in the conjunction are provided.

Hamrock, B. J.↗
NASA NTRS1989

Nonlinear evolution of oblique waves on compressible shear layers

The effects of critical-layer nonlinearity on spatially growing oblique instability waves on compressible shear layers between two parallel streams are considered. The analysis shows that mean temperature nonuniformities cause nonlinearity to occur at much smaller amplitudes than it does when the flow is isothermal. The nonlinear instability wave growth rate effects are described by an integrodifferential equation which bears some resemblance to the Landau equation, in that it involves a cubic-type nonlinearity. The numerical solutions to this equation are worked out and discussed in some detail. Inviscid solutions always end in a singularity at a finite downstream distance, but viscosity can eliminate this singularity for certain parameter ranges.

Goldstein, M. E.↗
NASA NTRS1993

Protective coatings for high-temperature polymer matrix composites

Plasma-enhanced chemical vapor deposition was used to deposit silicon nitride on graphite-fiber-reinforced polyimide composites to protect against oxidation at elevated temperatures. The adhesion and integrity of the coating were evaluated by isothermal aging (371 C for 500 hr) and thermal cycling. The amorphous silicon nitride (a-SiN:H) coating could withstand stresses ranging from approximately 0.18 GPa (tensile) to -1.6 GPa (compressive) and provided a 30 to 80 percent reduction in oxidation-induced weight loss. The major factor influencing the effectiveness of a-SiN:H as a barrier coating against oxidation is the surface finish of the polymer composite.

Harding, David R.↗
DOE OSTI2020

Isotope effects on the high pressure viscosity of liquid water measured by differential dynamic microscopy

In this paper, differential dynamic microscopy is performed in diamond anvil cells to measure the viscosity of water along the 24 °C isotherm to high-pressure by determination of the tracer diffusion coefficient of monodisperse silica spheres of known diameter and application of the Stokes-Einstein-Sutherland equation. This technique allows liquid samples to be compressed to greater pressure prior to freezing than with other viscometry methods. The highest-pressure measurement was made at 1.67 GPa, considerably deeper into the supercompressed regime than previously reported. The effect of isotopic composition is investigated with samples of normal water, heavy water, and partially deuterated water. When data below 0.25 GPa are excluded a free volume model fits the observed viscosities well yielding a theoretical glass transition density close to that observed in very-high-density amorphous ice. The improved fit above 0.25 GPa coincides with the loss of other anomalous behaviors in liquid water caused by hydrogen bonding and represents a transition to properties closer to those of a simple liquid.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS1987

Excitation of compressional waves and the formation of shocklets in the earth's foreshock

Large-amplitude waves in the earth's foreshock are sometimes observed in highly time-developed form, implying that nonlinearities are sufficiently strong to modify their waveforms before the solar wind carries them out of the foreshock. It is presently suggested that parallel propagating waves grow to finite amplitude in the reflected and intermediate ion zones of the earth's foreshock, and refract as they are carried by the solar wind into the 'diffuse' ion region, thereby becoming increasingly oblique and compressional. The time evolution of oblique, low-frequency compressive waves is simulated by means of a one-dimensional hybrid code in which main ions are treated as superparticles, while diffuse ions are seen as a double-adiabatic fluid and electrons as an isothermal fluid.

Hada, T.↗
DOE OSTI2024

Shock compression of crystalline TeO 2 to the high-pressure fluid regime: Insights from ab initio molecular dynamics simulations

The shock response of fully-dense and porous crystalline tellurium dioxide (TeO 2 ⁠) to the high-pressure and high-temperature fluid regime was investigated within the framework of density functional theory with Mermin’s generalization to finite temperatures. The principal and porous shock Hugoniot curves were predicted from canonical ab initio molecular dynamics (AIMD) simulations, with the phase space sampled along isotherms up to 80 000 K, for densities ranging from ρ = 3 to 17 g/cm 3 . The polymorphs investigated are α-TeO 2 paratellurite (⁠P4 1 2 1 2), TeO 2 cotunnite (⁠Pnma⁠), and TeO 2 post-cotunnite (⁠P2 1 /m⁠). Based on the discontinuity found in the calculated U s – u p slope of TeO 2 post-cotunnite at a shock velocity of U s ≃ 8.35km/s and a particle velocity of u p ≃ 3.64 km/s, the shock melting temperature and pressure are predicted to be ≃ 6500 K and ≃ 170 GPa. Results from the AIMD simulations are in line with the static compression data of TeO 2 paratellurite and cotunnite, and with the recent shock Hugoniot data for single-crystal α- TeO 2 for pressures up to 85 GPa, obtained using the inclined-mirror method and the velocity interferometer system for any reflector combined with powder gun and two-stage light-gas gun.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Significant enhancement of magnetocaloric effect in a NiMnCuGa Heusler alloy through textural modification

Magnetocaloric materials are of increasing interest to bring magnetic refrigeration to everyday households and drastically impact the energy demands for temperature control devices. In this work, a polycrystalline Heusler alloy of composition Ni2Mn0.76Cu0.24Ga with coinciding structural and magnetic transformation temperatures was subjected to compressive stress assisted thermal cycling (SATC) to enhance the magnetic properties by inducing a preferred orientation in the martensite. Isofield magnetization measurements showed a sharpening of the transformation between ferromagnetic martensite and paramagnetic austenite due to SATC. In isothermal magnetization measurements, SATC was seen to increase the magnetostructural coupling. With a 2 T applied magnetic field, the magnetocaloric effect (MCE) increased from ∼10 to ∼25 J/kg K and the refrigeration capacity (RC) almost doubled due to SATC. Heat capacity measurements were largely unaffected by SATC. The change in adiabatic temperature was estimated by using Cp and change in magnetic entropy (ΔSM) calculations. SATC was seen to increase ΔTad from ∼1.2 K to 2 K for an applied magnetic field of 2 T. Neutron diffraction measurements revealed highly textured martensite in the as received state that rotated to a more ideal preferred orientation after SATC that enhanced the magnetostructural transformation; and thus, improving the MCE and ΔTad.

McLeod, M. V. (ORCID:0000000329883516)↗
DOE OSTI2020

Shock compression of vanadium at extremes: Theory and experiment

Here, the equation of state (EOS) and shock compression of bulk vanadium were investigated using canonical ab initio molecular dynamic simulations, with experimental validation to 865 GPa from shock data collected at Sandia's Z Pulsed Power Facility. In simulations the phase space was sampled along isotherms ranging from 3000 K to 50 000 K, for densities between ρ = 3 and 15 g / cm 3 , with a focus on the liquid regime and the body-centered-cubic phase in the vicinity of the melting limit. The principal Hugoniot predicted from first principles is overall consistent with shock data, while it showed that current multiphase SESAME-type EOS for vanadium needed revision in the liquid regime. A more accurate SESAME EOS was developed using constraints from experiments and simulations. This work emphasizes the need to use a combined theoretical and experimental approach to develop high-fidelity EOS models for extreme conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Modeling of Thermal Decomposition of TATB-Based Explosive for Safety Analysis

We investigate and model the cook-off behavior of LX-17 to understand the response of explosive systems in abnormal thermal environments. Decomposition has been explored via conventional ODTX (One-Dimensional Time-to-eXplosion), PODTX (ODTX with pressure-measurement), TGA (Thermo-Gravimetric Analysis), and DSC (Differential Scanning Calorimetry) experiments under isothermal and ramped temperature profiles. The data were used to fit reaction rate parameters for proposed schemes in an ALE3D computational model. This model includes chemical reactions, thermo- and hydro-dynamics, and material properties, including thermal expansion, compressibility, and strength. These parameterizations were carried out utilizing a Python evolutionary optimization method on LLNL’s high-performance computing clusters. Additional experiments are being developed to further characterize and monitor decomposition intermediates to improve the model. Once experimentally validated, this model will be scalable to several applications involving LX-17. Furthermore, the optimization methodology developed herein should be applicable to other high explosive materials.

Chemistry - Chemical explosives↗
DOE OSTI2021

Enhanced work hardening from oxygen-stabilized ω precipitates in an aged metastable β Ti-Nb alloy

High levels of oxygen in solid solution in Ti alloys are considered detrimental to mechanical properties because of embrittlement concerns. In metastable β titanium alloys, the formation of isothermal ω precipitates is also known to cause severe embrittlement and ductility reduction. However, oxygen has been shown to partition to the ω phase during ageing, and this partitioning behavior may potentially impact ω’s mechanical contribution. Using micropillar compression, we compared the deformation behavior of Ti-20Nb (at. %) with oxygen-stabilized ω precipitates to the behavior of oxygen-free specimens. The oxygen-stabilized microstructures showed increased compressive yield strength and enhanced work hardening behavior compared to oxygen-free specimens. In the absence of oxygen, the compressed pillars showed slip band formation and catastrophic failure, and transmission electron microscopy imaging revealed that ω precipitates were sheared within the continuous deformation channels resulting in slip localization. In contrast, oxygen-stabilized ω precipitates were harder to shear and the formation of continuous deformation channels was suppressed during compression, leading to improved work hardening behavior up to 15% strain. Furthermore, this counter-intuitive role of oxygen may offer design strategies to address the significant embrittlement and loss of ductility observed for ω-strengthened β Ti alloys without oxygen and avenues to expand the use of β Ti alloys.

36 MATERIALS SCIENCE↗
NASA NTRS1981

The nature of the solar wind interaction with CO2/CO-dominated comets

The nature of the interaction of the solar wind with the ionospheres (field-free ion cavities) of comets with CO or CO2 as their dominant volatile species is considered and compared with the solar wind interaction with H2O-diominated comets. Calculations of the heliocentric variation of scale lengths defined as the nuclear distance at which the mean free path for ion-neutral collisions becomes equal to the nuclear distance, and the ion and neutral stand-off distances are presented for different modes of energy addition, nuclear radii and ionization time scales. Results indicate that whereas little activity can be expected from H2O-dominated comets at distances beyond 3 AU, COand CO2-dominated comets can be expected to be quite prolific at such distances. In contrast to H2O-dominated comets characterized by weak shocks, CO/CO2-dominated comet solar wind interactions are found to produce strong bow shocks within a specific heliocentric distance for the isothermal case of a CO-dominated comet, and a weak shock outside of this distance. In the general case, two types of cometary ionosphere transitions are found: a transition from strong to weak bow shock and from a compressible (soft) to an incompressible (hard) ionosphere, and a transition from a soft to a hard ionosphere with the bow shock remaining weak at a heliocentric distance dependant on atmospheric optical depth, bolometric albedo, nuclear radius, latent heat of sublimation of nuclear material, ionization time scale and type of heat addition. It is pointed out that the behavior of Comet Humason can be explained by solar wind interactions with a CO- or CO2-dominated comet.

Houpis, H. L. F.↗
NASA NTRS1981

Stellar clustering as induced by a supernova

A possible mechanism for the fragmentation of the expanding shock wave from a supernova to form stellar clusters is considered. A model of supernova shell expansion is constructed in which the ratio of magnetic field intensity to gas density remains constant during the one-dimensional compression of the interstellar medium by the shock, and the gas and field adjust to a quasi-equilibrium within the shell following shock passage. It is shown that the quasi-equilibrium, which may be considered as an isothermal atmosphere, is unstable to a hydromagnetic instability representing a form of the Parker instability, which results in a clumping of gas at intervals on the order of parsecs. The length and time scales of the instability are consistent with the clustering of newly formed stars observed in Canis Major R1, where there is evidence for supernova-induced star formation.

Baierlein, R.↗
NASA NTRS2015

On the Recovery Stress of a Ni50.3Ti29.7Hf20 High Temperature Shape Memory Alloy

Recovery stress in shape memory alloys (SMAs), also known as blocking stress, is an important property generally obtained during heating under a dimensional constraint as the material undergoes the martensitic phase transformation. This property has been instinctively utilized in most SMA shape-setting procedures, and has been used in numerous applications such as fastening and joining, rock splitting, safety release mechanisms, reinforced composites, medical devices, and many other applications. The stress generation is also relevant to actuator applications where jamming loads (e.g., in case the actuator gets stuck and is impeded from moving) need to be determined for proper hardware sizing. Recovery stresses in many SMA systems have been shown to reach stresses in the order of 800 MPa, achieved via thermo-mechanical training such as pre-straining, heat treatments or other factors. With the advent of high strength, high temperature SMAs, recovery stress data has been rarely probed, and there is no information pertinent to the magnitudes of these stresses. Thus, the purpose of this work is to investigate the recovery stress capability of a precipitation strengthened, Ni50.3Ti29.7Hf20 (at.) high temperature SMA in uniaxial tension and compression. This material has been shown to exhibit outstanding strength and stability during constant-stress, thermal cycling, but no data exists on constant-strain thermal cycling. Several training routines were implemented as part of this work including isothermal pre-straining, isobaric thermal cycling, and isothermal cyclic training routines. Regardless of the training method used, the recovery stress was characterized using constant-strain (strain-controlled condition) thermal cycling between the upper and lower cycle temperatures. Preliminary results indicate recovery stresses in excess of 1.5 GPa were obtained after a specific training routine. This stress magnitude is significantly higher than conventional NiTi stress generation capability.

Recovery Stress↗
NASA NTRS1987

Magnetic reconnection and solar flare loops

Reconnection models of the main phase of large solar flares are used to explain the energetics and the motions of the large flare loops that occur during this phase. Correct predictions for the density and temperature of the X-ray emitting loops are obtained by coupling magnetic reconnection with chromospheric ablation. In the reconnection models the ablation is driven by the thermal conduction of heat along magnetic field lines connecting the reconnection shocks in the corona with the flare ribbons in the chromosphere. Combining the compressible reconnection theory of Soward and Priest (1982) with the magnetohydrodynamic (MHD) subshock criteria of Coroniti (1970) shows that the Petschek-type slow-mode shocks in the vicinity of the x-line always dissociate into pairs of isothermal slow-mode subshocks and thermal conduction fronts. The rate of expansion of the loops is a function of the reconnection rate, and loops can be evolving self-similarly in time with their height increasing as sq root t and the reconnection rate decreasing as t to the minus 1.

Forbes, T. G.↗
NASA NTRS1988

Magnetohydrodynamic (MHD) modelling of solar active phenomena via numerical methods

Numerical ideal MHD models for the study of solar active phenomena are summarized. Particular attention is given to the following physical phenomena: (1) local heating of a coronal loop in an isothermal and stratified atmosphere, and (2) the coronal dynamic responses due to magnetic field movement. The results suggest that local heating of a magnetic loop will lead to the enhancement of the density of the neighboring loops through MHD wave compression. It is noted that field lines can be pinched off and may form a self-contained magnetized plasma blob that may move outward into interplanetary space.

Wu, S. T.↗
NASA NTRS1988

Isothermal and bithermal thermomechanical fatigue behavior of a NiCoCrAlY-coated single crystal superalloy

Specimens of single crystal PWA 1480 with group of zone axes (100) orientation, bare, or with NiCoCrAlY coating PWA 276, were tested in low cycle fatigue (LCF) at 650, 870, and 1050 C, and in simplified bithermal thermomechanical fatigue (TMF) tests between these temperatures. These tests were examined as a bridge between isothermal LCF and general TMF. In the bithermal test, an inelastic strain is applied at one temperature, T sub max, and reversed at T sub min. The out-of-phase (OP) test type imposing tension at T sub min and compression at T sub max received most study, since it was more damaging than the in-phase type. Specifically investigated were the effects of: inelastic strain range, the coating, delta T, T sub max, T sub min, and the environment.

Gayda, J.↗
NASA NTRS1988

Isothermal and 'bithermal' thermomechanical fatigue behavior of a NiCoCrAlY-coated single crystal superalloy

Specimens of single crystal PWA 1480 with group of zone axes (100) orientation, bare, or with NiCoCrAlY coating PWA 276, were tested in low cycle fatigue (LCF) at 650, 870, and 1050 C, and in simplified bithermal thermomechanical fatigue (TMF) tests between these temperatures. These tests were examined as a bridge between isothermal LCF and general TMF. In the bithermal test, and inelastic strain is applied at one temperature, T sub max, and reversed at T sub min. The out-of-phase (OP) test type imposing tension at T sub min and compression at T sub max received most study, since it was more damaging than the in-phase type. Specifically investigated were the effects of: inelastic strain range, the coating, delta T, T sub max, T sub min, and the environment.

Gayda, J.↗
DOE OSTI2025

Fluid-like shock compression of dilute polymer nanocomposites

The shock Hugoniot of heterogenous mixtures of discrete particles has been experimentally investigated for porous agglomerates and fluids, while the study of full-density solids has been primarily limited to compressed powders and high-dimensional composites. By dispersing ceria nanoparticles in a polyethylene matrix, we are able to examine the hydrodynamic behavior of a nonporous, heterogenous solid in thermal equilibrium during weak shock compression. Phase-driven discontinuities in the Hugoniot particle velocity–shock velocity (u−D) relationship of pure polyethylene are replicated in the nanocomposites but are shifted to lower velocity and to higher pressure with higher particle concentration. The results are explained using an isothermal, two-velocity fluid model under the hydrodynamic approximation. The model, which assumes a theoretical equation-of-state for ceria and either a low-order or high-order fit to the measured polyethylene Hugoniot, reasonably predicts the Hugoniot for two different polyethylene/ceria nanocomposites. Using the model, the mixture Hugoniot is shown to be insensitive to the Hugoniot of the stiffer constituent when the moduli are sufficiently disparate, while dependence on particle density and volume fraction is preserved through fluid-like motion.

Moore, Nathan W. [Sandia National Laboratories (SN↗