Search NASA⌕ Search

SEARCH · Search NASA

Results for “guide stability”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

MPD thruster plasma instability studies

Results of an ongoing experimental/theoretical effort to understand the influence of plasma waves and instabilities upon coaxial plasma thruster performance and longevity are discussed. An experiment which permits measurement of the plasma dispersion relation plus a suite of plasma parameters for each one millisecond quasi-steady discharge is described. This investigation is guided by a linearized Boltzmann-Poisson kinetic numerical model which can accommodate the effects of collisions, inhomogeneities and the magnetic field. Preliminary tests reveal the presence of the low-frequency branch of an ion cyclotron wave. Although the stability of these waves still has to be established through spatial growth rate measurements, it is well known that they are prone to 'current-driven' instabilities. 'Current-driven' instabilities in the collisional tensor conducting plasma of the MPD thruster are analytically discussed and their relevance to the MPD plasma is illustrated by estimating a critical total current at which their onset occurs.

Kelly, A. J.↗
NASA NTRS2011

Postural Stability When Leaning from Perceived Upright

The transition between quiet stance and gait requires the volitional movement of one?s center of mass (COM) toward a limit of stability (LOS). The goal of this study was to measure the effect of leaning from perceived upright on postural stability when voluntarily maintaining fixed stance positions and during perturbations of the support surface. The COM was derived from force plate data in 12 healthy subjects while standing with feet positioned so that lateral base of support was equal to foot length. For all conditions, arms were folded and subjects were instructed to lean without bending at the hips or lifting their feet. The LOS was determined during maximal voluntary leans with eyes open and closed. The COM was then displayed on a monitor located in front of the subject. Subjects were visually guided to lean toward a target position, maintain this position for 10s, return to upright, and then repeat the same targeted lean maneuver with eyes closed. Targets were randomly presented at 2? in 8 directions and between 2-6? in these same directions according to the asymmetric LOS. Subjects were then verbally guided to lean between 2? back and 4? forward prior to a perturbation of the support surface in either a forward or backward direction. The average LOS was 5.8? forward, 2.9? back, and 4.8? in left/right directions, with no significant difference between eyes open and closed. Center of pressure (COP) velocity increased as subjects maintained fixed stance positions farther from upright, with increased variability during eyes closed conditions. The time to stability and COP path length increased as subjects leaned opposite to the direction of the support surface perturbations. We conclude that postural stability is compromised as subjects lean away from perceived upright, except for perturbations that induce sway in the direction opposite the lean. The asymmetric LOS relative to perceived upright favors postural stability for COM movements in the forward direction.

Vanya, Robert D.↗
DOE OSTI2024

Modelling and experiment to stabilize disruptive tearing modes in the ITER baseline scenario in DIII-D

Abstract The achievement of high gain, stationary conditions in a tokamak scenario aimed at producing fusion energy in the ITER Project is crucial to the demonstration that this form of energy can be used in future reactors to provide cheap and clean energy globally. Disruptions are a challenge for the fusion energy field, in particular for the ‘ITER Baseline Scenario’ (IBS), as reproduced in the DIII-D tokamak. This work shows that a solution has been found for the m = 2/ n = 1 tearing modes that have consistently caused disruptions in the IBS: stable operation down to zero input torque was achieved by modifying the current density profile at the beginning of the pressure flattop and the ELM character later in the discharges, guided by previous results showing that the most likely cause of these instabilities is the current density profile. The coupling between sawteeth, n>2 modes and the 2/1 TMs is shown to not be statistically significant, nor the leading origin for the evolution towards instability. Ideal and resistive MHD modeling provide positive verification that a steeper ‘well’ in the region of the q = 2 rational surface leads to worse ideal stability, higher tearing index Δ’ and lower threshold Δ’ c for resistive instabilities, consistent with the experimental results. This provides confidence that the methods used in this work can be extrapolated to other devices and applied to avoid disruptions in ITER and pulsed fusion devices worldwide.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Exploring Subsite Selectivity within Plasmodium vivax N -Myristoyltransferase Using Pyrazole-Derived Inhibitors

N-myristoyltransferase (NMT) is a promising antimalarial drug target. Despite biochemical similarities between Plasmodium vivax and human NMTs, our recent research demonstrated that high selectivity is achievable. Herein, we report PvNMT-inhibiting compounds aimed at identifying novel mechanisms of selectivity. Various functional groups are appended to a pyrazole moiety in the inhibitor to target a pocket formed beneath the peptide binding cleft. The inhibitor core group polarity, lipophilicity, and size are also varied to probe the water structure near a channel. Selectivity index values range from 0.8 to 125.3. Cocrystal structures of two selective compounds, determined at 1.97 and 2.43 Å, show that extensions bind the targeted pocket but with different stabilities. A bulky naphthalene moiety introduced into the core binds next to instead of displacing protein-bound waters, causing a shift in the inhibitor position and expanding the binding site. Our structure-activity data provide a conceptual foundation for guiding future inhibitor optimizations.

60 APPLIED LIFE SCIENCES↗
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 OSTI2025

Discovering Ni/Cu Single-Atom Alloy as a Highly Active and Selective Catalyst for Direct Methane Conversion to Ethylene: A First-Principles Kinetic Study

Direct methane conversion to liquid fuels or value-added chemicals is a promising technology to utilize natural resources without resorting to further petroleum extraction. However, discovering efficient catalysts for this reaction is challenging due to either coke formation or unfavorable C-H bond activation. Herein, we design single-atom alloy (SAA) catalysts to simultaneously eliminate the above two bottlenecks based on mechanism-guided strategies: (1) the active single atom enables favorable C-H bond breaking and (2) the less reactive host metal facilitates C-C coupling and thus avoids strong binding of carbonaceous species. Employing electronic structure theory calculations, we screened the stability of multiple SAAs with 3d-5d transition metals atomically dispersed on a copper surface in terms of avoiding dopant aggregation and segregation. We then evaluated reactivities of the stable SAAs as catalysts for direct methane conversion to C2 products, including methane dehydrogenation and C-C coupling mechanisms. Combining selectivity analysis with kinetic modeling, we predicted that nickel dispersed on copper, i.e., Ni/Cu SAA, is a highly active and selective catalyst that can efficiently transform methane to ethylene. This work designs efficient SAA catalysts for direct methane activation and provides chemical insights into engineering compositions of SAAs to tune their catalytic performances.

Kothakonda, Manish↗
DOE OSTI2024

Down-selection of biomolecules to assemble “reverse micelle” with perovskites

Abstract Biological molecule-semiconductor interfacing has triggered numerous opportunities in applied physics such as bio-assisted data storage and computation, brain-computer interface, and advanced distributed bio-sensing. The introduction of electronics into biological embodiment is being quickly developed as it has great potential in providing adaptivity and improving functionality. Reciprocally, introducing biomaterials into semiconductors to manifest bio-mimetic functionality is impactful in triggering new enhanced mechanisms. In this study, we utilize the vulnerable perovskite semiconductors as a platform to understand if certain types of biomolecules can regulate the lattice and endow a unique mechanism for stabilizing the metastable perovskite lattice. Three tiers of biomolecules have been systematically tested and the results reveal a fundamental mechanism for the formation of a “reverse-micelle” structure. Systematic exploration of a large set of biomolecules led to the discovery of guiding principle for down-selection of biomolecules which extends the classic emulsion theory to this hybrid systems. Results demonstrate that by introducing biomaterials into semiconductors, natural phenomena typically observed in biological systems can also be incorporated into semiconducting crystals, providing a new perspective to engineer existing synthetic materials.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2023

Population-based heteropolymer design to mimic protein mixtures

Biological fluids, the most complex blends, have compositions that constantly vary and cannot be molecularly defined. Despite these uncertainties, proteins fluctuate, fold, function and evolve as programmed. We propose that in addition to the known monomeric sequence requirements, protein sequences encode multi-pair interactions at the segmental level to navigate random encounters; synthetic heteropolymers capable of emulating such interactions can replicate how proteins behave in biological fluids individually and collectively. Here, we extracted the chemical characteristics and sequential arrangement along a protein chain at the segmental level from natural protein libraries and used the information to design heteropolymer ensembles as mixtures of disordered, partially folded and folded proteins. For each heteropolymer ensemble, the level of segmental similarity to that of natural proteins determines its ability to replicate many functions of biological fluids including assisting protein folding during translation, preserving the viability of fetal bovine serum without refrigeration, enhancing the thermal stability of proteins and behaving like synthetic cytosol under biologically relevant conditions. Molecular studies further translated protein sequence information at the segmental level into intermolecular interactions with a defined range, degree of diversity and temporal and spatial availability. This framework provides valuable guiding principles to synthetically realize protein properties, engineer bio/abiotic hybrid materials and, ultimately, realize matter-to-life transformations.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2026

Assessing metal nitrides and metal carbides as supports for thermally stable single-atom catalysts

Single-atom catalysts supported on metal oxides have been demonstrated to exhibit exceptional activity while also maintaining single-atom stability. However, alternative supports such as metal nitrides and carbides have received far less attention. Herein, we use density functional theory to systematically investigate the relative thermal stability of single-atom catalysts over a host of transition metal nitride and carbide supports. By considering the binding and dimerization energies of isolated transition metal atoms across various surface facets, we identify transition metal/support pairs that show the most promise for high-density single-atom catalysts. We find that transition metal atoms can be stabilized on both defect sites and pristine surfaces over transition metal nitrides and carbides. Furthermore, we identify promising metal/support pairings that may be suitable for achieving both stable and high-density single-atom catalysts. Furthermore, these results provide valuable insights to guide synthesis efforts towards achieving stable single-atom transition metal catalysts.

Density functional theory↗
DOE OSTI2023

Emerging investigator series: kinetics of diopside reactivity for carbon mineralization in mafic–ultramafic rocks

The ongoing use of fossil fuels to supply modern energy demands has necessitated research on combating carbon dioxide (CO 2 ) emissions and climate change. Carbon storage via mineral trapping in basalt and related rocks is a promising strategy. However, mineralization rates depend on the variable minerology that makes up these rock formations. Diopside (CaMgSi 2 O 6 ) is a common pyroxene mineral in ultramafic and mafic rocks including basalt, but relatively little work has been done to understand its carbon mineralization kinetics using hydrated supercritical CO 2 , which induces the formation of reactive nanoscale interfacial water films. Here, in situ XRD experiments at 50–110 °C and 90 bar indicate that diopside transforms into a myriad of Mg/Ca carbonates, including huntite [Mg 3 Ca(CO 3 ) 4 ] and very high magnesium calcite (VHMC, i.e., protodolomite). Through ex situ characterization, we were able to constrain reaction pathways for the dissolution–precipitation transformation process including metastable intermediate precipitates. Experiments performed at variable temperatures enabled Avrami-derived rate constants and an apparent activation energy of 97 ± 16 kJ mol –1 , implying the dissolution of diopside is the rate-limiting step. Density functional theory (DFT) calculations, used to gain molecular insight into the surface stability of the diopside during dissolution, suggest that exposed calcium cations are susceptible to dissolution when put in contact with water given their coordination environment. The collective results point to the high CO 2 mineralization potential of diopside in basalts, which could help guide parameterization of reactive transport models needed to design and permit commercial-scale subsurface carbon storage operations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1979

Design Guidelines for the Application of Forebody and Nose Strakes to a Fighter Aircraft Based on F-16 Wind Tunnel Testing Experience

During the YF-16 and F-16 developmental wind tunnel test program, numerous variations in nose and forebody strakes were investigated. These data were reviewed, and the strake aerodynamic characteristics coalesced into design guidelines for the application of strakes to fighter aircraft. The design guides take the form of general equations governing the modification of forebody strakes to obtain a linear pitching moment curve and the calculation of the resulting lift and drag increments. Additionally, qualitative comments are made concerning the effects of strake geometry on lateral/directional stability. It is concluded that the generation of incremental strake lift is primarily dependent upon the area affected by the strake-induced vortex and that strake planform is of secondary importance. Forebody strakes have small beneficial effects on lateral/directional stability if properly designed; however, significant gains are easily attained with nose strakes.

Smith, C. W.↗
DOE OSTI2024

Enhanced Thermal Stability of Conductive Mercury Telluride Colloidal Quantum Dot Thin Films Using Atomic Layer Deposition

Colloidal quantum dots (CQDs) are valuable for their potential applications in optoelectronic devices. However, they are susceptible to thermal degradation during processing and while in use. Mitigating thermally induced sintering, which leads to absorption spectrum broadening and undesirable changes to thin film electrical properties, is necessary for the reliable design and manufacture of CQD-based optoelectronics. Here, low-temperature metal–oxide atomic layer deposition (ALD) was investigated as a method for mitigating sintering while preserving the optoelectronic properties of mercury telluride (HgTe) CQD films. ALD-coated films are subjected to temperatures up to 160 °C for up to 5 h and alumina (Al 2 O 3 ) is found to be most effective at preserving the optical properties, demonstrating the feasibility of metal–oxide in-filling to protect against sintering. HgTe CQD film electrical properties were investigated before and after alumina ALD in-filling, which was found to increase the p-type doping and hole mobility of the films. The magnitude of these effects depended on the conditions used to prepare the HgTe CQDs. With further investigation into the interaction effects of CQD and ALD process factors, these results may be used to guide the design of CQD–ALD materials for their practical integration into useful optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2024

High-Performance Iridium–Molybdenum Oxide Electrocatalysts for Water Oxidation in Acid: Bayesian Optimization Discovery and Experimental Testing

Ir oxides are costly and scarce catalysts for oxygen evolution reaction (OER) in acid. There has been extensive interest in developing alternatives that are either Ir-free or require smaller amounts of Ir to drive the reactions at acceptable rates. One design strategy is to identify Ir-based mixed oxides that achieve similar performance while requiring smaller amounts of Ir. The obstacle to this strategy has been a very large phase space of the Ir-based mixed metal oxides, in terms of the metals combined with Ir and the different crystallographic structures of the mixed oxides, which prevents a thorough exploration of possible materials. In this work, we developed a workflow that uses machine-learning-aided Bayesian optimization in combination with density functional theory to make the exploration of this phase space plausible. This screening identified Mo as a promising dopant for forming acid-tolerant Ir-based oxides for the OER. We synthesized and characterized the Ir–Mo mixed oxides in the form of thin-film electrocatalysts with a known surface area. We show that these mixed oxides exhibited overpotentials ~30 mV lower than a pure Ir control while maintaining 24% lower Ir dissolution rates than the Ir control. Furthermore, these findings suggest that Mo is a promising dopant and highlight the promise of machine learning to guide the experimental exploration and optimization of catalytic materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Chapter 5: Advanced Characterization and Testing of Grid-Forming Resources

In order to compare and define grid-forming (GFM) resources' performance requirements, uniform test procedures are necessary to characterize these resources' capabilities to serve various system needs. Most tests performed on grid-following (GFL) inverters are also required for GFM inverters, with additional tests also needed specifically for GFM inverters. This section describes advanced characterization and testing methods for GFM inverters, with the goal of serving as a guide- line for developing test procedures.

grid-forming resources↗
DOE OSTI2022

Ammonia synthesis on BaTiO 2.5 H 0.5 : computational insights into the role of hydrides

Perovskite oxyhydrides such as BaTiO 2.5 H 0.5 have been found to be able to catalyze NH3 synthesis, but the mechanism and the role of the catalyst's lattice hydrides in the catalytic reaction remain unknown. Here we employ first principles density functional theory to investigate the mechanism of ammonia synthesis and the role of lattice hydrides on a prototypical perovskite oxyhydride, BaTiO 2.5 H 0.5 (BTOH). Two mechanistic hypotheses, the distal and alternating pathways, have been tested on the Ti2O 2 termination of the BTOH (210) surface, previously determined to be the most stable surface termination under the reaction conditions considered. In the distal pathway, H atoms hydrogenate N 2 to form the *N–NH x key intermediates, followed by N–N bond breaking. In the alternating pathway, H atoms hydrogenate N 2 in an alternating fashion to form the *NH x –NH y intermediates before N–N bond breaking and formation of co-adsorbed *NH x /*NH y on the surface. We find that the subsurface hydride vacancy formed after reaction of *N 2 with the lattice hydride is key to the distal pathway, leading to surface nitride formation after breaking the *N–NH 3 bond, while the neighboring surface Ti sites are key to bridging and stabilizing the *NNH intermediate in the alternating pathway. In both pathways, desorption of NH 3 is the most uphill in energy. Our results provide important insights into the role of hydrides and surface vacancies in hydrogenation reactions over BTOH, which will be useful to guide future spectroscopic experiments such as operando IR and inelastic neutron scattering to verify the key intermediates.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Phase equilibria of advanced technology uranium silicide-based nuclear fuel

The phases in uranium-silicide binary system were evaluated in regards to their stabilities, phase boundaries, crystal structures, and phase transitions. The results from this study were used in combination with a well assessed literature to optimize the U-Si phase diagram using the CALPHAD method. A thermodynamic database was developed, which could be used to guide nuclear fuel fabrication, could be incorporated into other nuclear fuel thermodynamic databases, or could be used to generate data required by fuel performance codes to model fuel behavior in normal or off-normal reactor operations. The U 3 Si 2 and U 3 Si 5 phases were modeled using the Compound Energy Formalism model with 3 sublattices to account for the variation in composition. The crystal structure used for the USi phase was the tetragonal with an I4/mmm space. Above 450°C, the U 3 Si 5 phase was modeled. The composition of the USi 2 phase was adjusted to USi 1.84 . The calculated invariant reactions and the enthalpy of formation for the stoichiometric phases were in agreement with experimental data.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Guiding the design of oxidation-resistant Fe-based single atom alloy catalysts with insights from configurational space

The high activity and selectivity of Fe-based heterogeneous catalysts toward a variety of reactions that require the breaking of strong bonds are offset in large part by their considerable instability with respect to oxidative deactivation. While it has been shown that the stability of Fe catalysts is considerably enhanced by alloying them with precious metals (even at the single-atom limit), rational design criteria for choosing such secondary metals are still missing. Since oxidative deactivation occurs due to the strong binding of oxygen to Fe and reduction by adsorbed hydrogen mitigates the deactivation, we propose here to use the binding affinity of oxygen and hydrogen adatoms as the basis for rational design. As it would also be beneficial to use cheaper secondary metals, we have scanned over a large subset of 3d–5d mid-to late transition metal single atoms and computationally determined their effect on the oxygen and hydrogen adlayer binding as a function of chemical potential and adsorbate coverage. We further determine the underlying chemical origins that are responsible for these effects and connect them to experimentally tunable quantities. Our results reveal a reliable periodic trend wherein oxygen binding is weakened greatest as one moves right and down the periodic table. Hydrogen binding shows the same trend only at high (but relevant) coverages and otherwise tends to have its binding slightly increased in all systems. Trends with secondary metal coverage are also uncovered and connected to experimentally tunable parameters.

Hensley, Alyssa↗