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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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Alloying Effects on the Transport Properties of Refractory High-entropy Alloys

Additive Manufacturing (AM) has opened new frontiers for the design of refractory high-entropy alloys (HEAs) for high-temperature applications. The thermal conductivity of the AM feedstock is among the most important thermo-physical properties that control the melting and solidification process. Despite its significance, there remains a notable gap in both computational and experimental research concerning the thermal conductivity of HEAs. Here, we use density functional theory (DFT) to systematically investigate the alloying effects on the transport properties of Ti-Cr-Mo-W-V-Nb-Ta RHEAs, including electrical and thermal conductivities and Seebeck coefficient. The relaxation time of charge carriers is a key underlying parameter determining thermal conductivity that is exceedingly challenging to predict from first principles alone, and we thus follow the approach by Mukherjee, Satsangi, and Singh [Chem Mater 32, 6507 (2022)] to optimize the relaxation time for RHEAs. Here we validated thermal conductivity predictions on elemental solids, binary and ternary alloys, and RHEAs and compared them against thermodynamic (CALPHAD) predictions and our experiments with good correlations. To understand observed trends in thermal conductivity, we assessed the phase stability, electronic structure, phonon, and intrinsic- and tensile strength of down-selected RHEAs. Our electronic structure and phonon results connect well with the observed compositional trends for thermal transport in RHEAs. Our DFT assessment and CALPHAD predictions provide a unique design guide for RHEAs with tailored thermal conductivity, a critical consideration for AM and thermal-management applications.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Phonon modal analysis of thermal transport in ThO 2 with point defects using equilibrium molecular dynamics

Defects can significantly degrade the thermal conductivity of ThO 2 , an advanced nuclear fuel material as well as a surrogate for other fluorite-structured materials. Here, we investigate how point defects in ThO 2 impact phonon mode-resolved thermal transport. By incorporating phonon modes from lattice dynamics, we decompose the trajectory and heat flux to phonon normal mode space and extract key phonon properties, including phonon relaxation times and their contributions to thermal conductivity. We implement two methods. The first method is based on the Green Kubo formalism to resolve the contribution of each phonon mode to thermal conductivity. The second resolves the lifetime of individual phonon modes and the thermal conductivity is calculated using the Boltzmann transport equation within relaxation time approximation. Notably, a lower contribution of acoustic modes is revealed compared to perturbative approaches considering only three-phonon scattering processes. The effects of four types of point defects are evaluated. The strongest impact on a reduction in thermal conductivity is from Th interstitials, followed by Th vacancies. O interstitials/vacancies have a similar impact, albeit smaller than defects on the thorium sublattice. These observations are consistent with previous studies.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2024

Electrical conductivity of a warm neutron star crust in magnetic fields: Neutron-drip regime

We compute the anisotropic electrical conductivity tensor of the inner crust of a compact star at nonzero temperature by extending a previous work on the conductivity of the outer crust. The physical scenarios, where such crust is formed, involve protoneutron stars born in supernova explosions, binary neutron star mergers, and accreting neutron stars. The temperature-density range studied covers the transition from a semidegenerate to a highly degenerate electron gas and assumes that the nuclei form a liquid, i.e., the temperature is above the melting temperature of the lattice of nuclei. The electronic transition probabilities include (i) the screening of electron-ion interaction in the hard-thermal-loop approximation for the QED plasma, (ii) the correlations of the ionic component in a one-component plasma, and (iii) finite nuclear size effects. The conductivity tensor is obtained from the Boltzmann kinetic equation in relaxation time approximation accounting for the anisotropy introduced by a magnetic field. The sensitivity of the results towards the matter composition of the inner crust is explored by using several compositions of the inner crust, which were obtained using different nuclear interactions and methods of solving the many-body problem. The standard deviations of relaxation time and components of the conductivity tensor from the average are below ≤25% except close to crust-core transition, where nonspherical nuclear structures are expected. Finally, our results can be used in dissipative magnetohydrodynamics simulations of warm compact stars.

Physics↗
DOE OSTI2025

LLRF System Analysis for the Fermilab PIP-II LINAC

Developing long-lived quantum processing units (QPUs) capable of supporting high-fidelity quantum operations is a crucial challenge on the path toward fault-tolerant quantum computing. TESLA-shaped superconducting RF (SRF) cavities, known for photon relaxation times on the order of seconds, provide an excellent foundation for 3D QPUs and quantum memory. This talk presents a novel design that leverages TESLA cavity modes coupled to ancillary transmon qubits, optimized to preserve coherence and control. By carefully engineering the package geometry, optimizing Hamiltonian parameters, and minimizing lossy participation ratios, we achieve photon relaxation times of over 16 ms and 20 ms for the two cavity modes, representing a significant improvement over previous multimode quantum memories. Despite the reduced coupling between the qubit and cavity modes, which is necessary to preserve long lifetimes, the platform supports robust and universal control schemes that are not limited by low coupling strength. We will also discuss how this architecture can lead to scalable, modular quantum computing systems.

Varghese, P. [Fermilab]↗
NASA NTRS1994

Theoretical studies of a molecular beam generator

We consider the problem of modeling a high temperature diatomic gas N2 flowing through a converging-diverging high expansion nozzle. The problem is modeled in two ways. The first model uses a single temperature with variable specific heats as functions of this temperature. For the second model we assume that the various degrees of freedom all have a Boltzmann population distribution which means that each degree of freedom has its own temperature and consequently each system state can be characterized by these temperatures. This suggests the formulation of a second model with a vibrational degree of freedom as having its own temperature along with a rotational-translational degree of freedom with its own temperature. Initially the vibrational degree of freedom is excited by heating the gas to a high temperature. As the high temperature gas expands through the nozzle throat there is a sudden drop in the rotational-translational temperature along with a finite relaxation time for the vibrational degree of freedom to achieve equilibrium with the rotational-translational degree of freedom. That is, the temperature change that occurs when the N2 gas passes through the nozzle throat is so great that the changes in the vibrational degree of freedom lags behind the rotational-translational energy changes. The resulting relaxation time is finite. It is in this context that the term nonequilibrium is used. That is, the term nonequilibrium denotes the fact that the energy content of the various degrees of freedom are characterized by two temperatures. We neglect any chemical reactions resulting from the high temperatures which could also add nonequilibrium effects. We develop the basic equations for the two models in various forms in order to check the derivations with other sources. The final form which is solved numerically consists of the scaled equations in a conservative dimensionless form.

Heinbockel, John H.↗
NASA NTRS1995

Nozzle flow with vibrational nonequilibrium

This research concerns the modeling and numerical solutions of the coupled system of compressible Navier-Stokes equations in cylindrical coordinates under conditions of equilibrium and nonequilibrium thermodynamics. The problem considered was the modeling of a high temperature diatomic gas N2 flowing through a converging-diverging high expansion nozzle. The problem was modeled in two ways. The first model uses a single temperature with variable specific heats as functions of this temperature. For the second model we assume that the various degrees of freedom all have a Boltzmann distribution and that there is a continuous redistribution of energy among the various degrees of freedom as the gas passes through the nozzle. Each degree of freedom is assumed to have its own temperature and, consequently, each system state can be characterized by these temperatures. This suggests that formulation of a second model with a vibrational degree of freedom along with a rotational-translation degree of freedom, each degree of freedom having its own temperature. Initially the vibrational degree of freedom is excited by heating the gas to a high temperature. As the high temperature gas passes through the nozzle throat there is a sudden drop in temperature along with a relaxation time for the vibrational degree of freedom to achieve equilibrium with the rotational-translation degree of freedom. That is, we assume that the temperature change upon passing through the throat is so great that the changes in the vibrational degree of freedom occur at a much slower pace and consequently lags behind the rotational-translational energy changes. This lag results in a finite relaxation time. In this context the term nonequilibrium is used to denote the fact that the energy content of the various degrees of freedom are characterized by two temperatures. We neglect any chemical reactions which could also add nonequilibrium effects. We develop the energy equations for the nonequilibrium model from first principles. The resulting equations, which model the nozzle flow, can be expressed in various forms. In most forms the resulting equations are coupled systems of nonlinear partial differential equations subject to certain boundary conditions. To solve the resulting coupled system of nonlinear partial differential equations, several numerical techniques were investigated: (1) the explicit MacCormack method, (2) the explicit-implicit MacCormack method, (3) the method of operator splitting, (4) factorization schemes, and (5) the Steger-Warming scheme.

Heinbockel, J. H.↗
NASA NTRS2011

Three-Dimensional Modeling of the Solar Wind: From the Coronal Base to the Outer Heliosphere

We have developed a global fully three-dimensional magnetohydrodynamic solar wind model for the region that extends from the coronal base to 100 AU. The simulation domain consists of tree spherical shell subdomains with computational boundaries between them placed at 20 solar radii and 0.3 AU. The location of the first boundary ensures that the flow at the boundary is both supersonic and super-Alfvenic. A steady-state solution in the innermost (coronal) region is obtained by the time-relaxation method. The solution uses a tilted dipole model or solar magnetograms as the boundary condition at the coronal base and includes a flux of Alfven waves in the WKB approximation which provide additional acceleration for the coronal outflow in the open field regions. The intermediate region solution is constructed by the integration of steady-state equations along radius using a marching scheme. The outer region solution (0.3-100 AU) is obtained again by the time relaxation and takes into account turbulence transport and heating as well as heating, flow deceleration, and other effects due to the interstellar pickup protons treated as a separate fluid. We use the model to simulate the global steady-state structure of the solar wind from the coronal base to the heliospheric boundary and compare the results with Ulysses and Voyager observations.

Usmanov, A. V↗
NASA NTRS1985

Correlation of dynamic dielectric measurements with viscosity in polymeric resin systems

Dynamic dielectric analysis has been used to study curing polymer systems and thermoplastics. Measurements have been made over a frequency range of six decades. This wide range of frequencies increases the amount of information which can be obtained. The data are analyzed in terms of the frequency dependence of the specific conductivity sigma and the relaxation time tau, parameters which are characteristic of the cure state of the material and independent of size of the sample. Dynamic dielectric measurements have been correlated with viscosity for the polysulfone thermoplastic UDEL-P1700 and with viscosity and ultrasonic measurements on the DGEBA-type epoxy Epon 828 cured with Agent U. The experimental results suggest that when ionic processes dominate the dielectric response, the intensive property sigma is a good monitor of the resin's viscosity. The results show that the dielectric relaxation time tau can be used to monitor the state of the system and the extent and rate of the reaction.

Kranbuehl, D.↗
NASA NTRS2009

The Structure and Dynamics of Titan's Middle Atmosphere and Troposphere

Titan, after Venus, is the second example in the solar system of an atmosphere with a global cyclostrophic circulation. The origin and maintenance of these superrotating atmospheres is not well understood, but Titan has a strong seasonal modulation in the middle atmosphere, and the seasonal changes in the winds may offer clues. The pole in winter and early spring is characterized by temperatures 20-30 K cooler at 140-170 km than those at low latitudes, and strong circumpolar winds as high as 190 m/s at 200- 250 km. At these levels the polar region is characterized by enhanced concentrations of several organic gases, and also detectable condensates. All this suggests that the polar vortex provides a mixing barrier between winter polar and lower-latitude air masses, analogous to the polar ozone holes on Earth. Because the concentrations of organic gases increase with altitude in the middle atmosphere, the observed enhancements suggest subsidence over the winter pole. Consistent with this are the observed temperatures approximately 200 K at the winter-polar stratopause (280 km), making it the warmest part of the atmosphere. The warm stratopause likely results from adiabatic heating associated with the subsidence. Recent observations in late northern winter and early spring indicate that the warm anomaly at the winter-polar stratopause is weakening;. In contrast to the middle atmosphere, latitude contrasts in tropospheric temperatures are muted. During the northern winter season, they were approximately 5 K at the tropopause and 3 K or less near the surface, being coldest at high northern latitudes. This is understandable in terms of the long radiative relaxation times in the troposphere, compared to times that are much shorter than a season in the upper stratosphere and higher. Curiously, the transition between the small meridional contrast (and presumably seasonal variations) in temperatures observed in the troposphere and the large variations observed at higher altitudes occurs abruptly above 80 km. Here the temperatures in the lower stratosphere, generally increasing with altitude, exhibit a sudden drop with increasing altitude at high northern latitudes, producing the contrast between low and high northern winter latitudes in the upper stratosphere described above. While the radiative relaxation time associated with infrared gaseous coolants decreases with altitude in the stratosphere, the abrupt transition suggests the presence of an optically thick condensate at thermal-infrared wavelengths. Near the surface, temperature lapse rates are adiabatic over the lowest 2 km, with the suggestion of a nocturnal stable inversion over the lowest 200 m in radio-occultation soundings near the morning terminator. At mid and high latitudes in both winter and summer hemispheres, the profiles are more statically stable (i.e., subadiabatic). This is most pronounced in the winter hemisphere.

Flasar, F.M.↗
NASA NTRS2020

Desorption Kinetics of O and CO from Graphitic Carbon Surfaces

The desorption of O/CO from graphitic carbon surfaces is investigated using a one-dimensional model describing the adsorbate interactions with the surface phonon bath. The kinetics of desorption are described through the solution of a master equation for the time-dependent population of the adsorbate in an oscillator state, which is modified through thermal fluctuations at the surface. The interaction of the adsorbate with the surface phonons is explicitly captured by using the computed phonon density Of states (PDOS) of the surface. The coupling of the adsorbate with the phonon bath results in the transition of the adsorbate up and down a vibrational ladder. The adsorbate-surface interaction is represented in the model using a Morse potential, which allows for the desorption process to be directly modeled as a transition from bound to free (continuum) state. The PDOS is a property of the material and the lattice; and is highly sensitive to the presence of defects. The effect of etch pits along with random surface defects on the PDOS is considered in the present work. The presence of defects causes a redshift and broadening of the PDOS, which in turn changes the phonon frequency modes available for adsorbate coupling at the surface. Using the realistic PDOS distributions, the phonon-induced desorption (PID) model was used to compute the transition and desorption rates for both pristine and defective systems. Mathissen’s rule is used to compute the phonon relaxation time for pristine and defective systems based on the phonon scattering times for each of the different scattering processes. First, the desorption rates of the pristine system is fitted against the experimental values to obtain the Morse potential parameters for each of the observed adatoms. These Morse potential parameters are used along with the defective PDOS and phonon relaxation time to compute the desorption rates for the defective system. The defective system rates (both transition and desorption) were consistently lower in comparison with the pristine system. The difference between the transition rates is more significant at lower initial states due to higher energy spacing between the levels. In the case of the desorption rates, the difference between the defective and pristine system is more significant at higher temperatures. The desorption rates for each of the system shows an order of magnitude decrease with the strongly bound systems exhibiting the greatest reduction in the desorption rates.

Swaminathan-Gopalan, Krishnan↗
NASA NTRS1989

Energy loss of fast nonthermal electrons in plasmas

Simple analytical expressions for equilibration times of nonrelativistic monoenergetic electrons in plasmas are evaluated in the 'weak'-beam approximation when the density of the monoenergetic electrons is much smaller than the plasma density. The equilibration time is defined as the time needed by the beam of monenergetic electrons to lose most of its energy as a result of collisions with plasma particles having a Maxwellian energy distribution. The process of energy equilibration is treated as a statistical superposition of both elastic and inelastic collisions in the plasma. The possibility of collisionless equilibration is discussed. Comparison of the equilibration times with the Spitzer relaxation times indicates that the former times are more appropriate for an estimate of the energy loss of the 'weak' electron beams in highly ionized plasmas.

Kunc, Joseph A.↗
DOE OSTI2024

Initial Testing of an In Situ Load Retention Aging Vessel

A thermal aging vessel instrumented with load cells was fabricated. The primary function of the vessel is to continuously monitor the in situ load retention of up to three compressed polymer coupons undergoing thermally accelerated aging under nitrogen. A secondary function is to enable gas sampling of the vessel headspace during thermal aging. Heating of the vessel is achieved using a custom heater jacket. To improve upon our conventional aging study methods which require periodic interruption of aging to perform load testing in an Instron machine at room temperature, this technology aims to automate/facilitate data acquisition/analysis, improve data quality, and enable uninterrupted compression of the polymer which represents the service condition. As an example case to assess functionality of the in situ vessel, the load retention of a siloxane elastomer material additively manufactured by direct-ink-writing (DIW) was measured at three different isothermal aging temperatures for ~1 month. Initial compression of the coupons while near the aging temperature was achieved by temporarily opening the heated vessel to access the interior chamber and manually tightening four nuts to drive the heated compression plate down onto the heated coupons. Initial testing demonstrated achievement of the primary load retention monitoring function. Unfortunately, the vessel leaked which prevented gas sampling; an active purge was used to maintain a nitrogen atmosphere. Welded or otherwise sealed joints, which could be implemented in a future design, would likely eliminate leak paths. To apply time-temperature superposition (TTS), a technique used to provide long-term prediction of the load retention from short-term isothermal data, the load retention needed to be calculated relative to the load at an estimated “equilibrium” time, after most of the transient viscoelastic physical relaxation occurred. The peak load immediately after compression could not be used as the load retention basis for two reasons: (1) age-related changes must be isolated from non-age-related physical relaxation before applying TTS and (2) the manual mechanism used to compress the specimens at the aging temperature was neither smooth nor repeatable which affected the peak load value. To better understand the effect of the mode of initial compression on the measured load, and possibly better estimate “equilibrium” physical relaxation times, systematic stress relaxation experiments were performed using an Instron machine with a thermal chamber. At a given temperature, the DIW polymer was compressed to a fixed strain in either a stepped or continuous manner at two different rates, then held at that strain for 24 hrs. The results indicated that, at a given temperature, the different stress relaxation curves appeared to converge to the same curve at some “equilibrium” time when the non-age-related physical relaxation was mostly complete. Though this observation suggests that the discontinuous manual compression employed by the vessel is feasible, a compression mechanism that is rapid, smooth, and repeatable would enhance its use.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Electrodynamics of photo-carriers in multiferroic Eu 0.75 Y 0.25 MnO 3

Understanding and controlling the antiferromagnetic order in multiferroic materials on an ultrafast time scale is a long standing area of interest, due to their potential applications in spintronics and ultrafast magnetoelectric switching. We present an optical pump-terahertz (THz) probe study on multiferroic Eu 0.75 Y 0.25 MnO 3 . The optical pump predominantly excites the d-d transitions of the Mn 3+ ions, and the temporal evolution of the pump-induced transient conductivity is measured with a subsequent THz pulse. Two distinct, temperature-dependent decay times are revealed. The shorter relaxation time corresponds to spin-lattice thermalization, while the longer one is ascribed to electron-hole recombination. A spin-selection rule in the relaxation process is proposed in the magnetic phase. Slight suppression of the electromagnons was observed after the optical pump pulse within the spin-lattice thermalization time scale. These observed fundamental magnetic processes can shed light on ultrafast control of magnetism and photoinduced phase transitions in multiferroics.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1974

Coherent optical pulse reshaping in a resonant molecular absorber

Investigation of the coherent reshaping of short-duration (2 to 40 nsec) N2O laser pulses in a resonant NH3 absorber for a variable number of absorption lengths. It is found that, for small-area square pulses, the reshaping can produce subnanosecond pulses at the leading and trailing edges. A rapid phase reversal gives rise to amplification for times comparable to the transverse relaxation time.

Hamadani, S. M.↗
NASA NTRS1981

A comparison of rat myosin from fast and slow skeletal muscle and the effect of disuse

Certain enzymatic and structural features of myosin, purified from rat skeletal muscles representative of the fast twitch glycolytic (type IIb), the fast twitch oxidative (type IIa), and the slow twitch oxidative (type I) fiber, were determined and the results were compared with the measured contractile properties. Good correlation was found between the shortening velocities and Ca(2+)-activated ATPase activity for each fiber type. Short term hind limb immobilization caused prolongation of contraction time and one-half relaxation time in the fast twitch muscles and a reduction of these contractile properties in slow twitch soleus. Furthermore, the increased maximum shortening velocity in the immobilized soleus could be correlated with increased Ca(2+)-ATPase, but no change was observed in the enzymatic activity of the fast twitch muscles. No alteration in light chain distribution with disuse was observed in any of the fiber types. The myosin from slow twitch soleus could be distinguished from fast twitch myosins on the basis of the pattern of peptides generated by proteolysis of the heavy chains. Six weeks of hind limb immobilization resulted in both an increased ATPase activity and an altered heavy chain primary structure in the slow twitch soleus muscle.

Unsworth, B. R.↗