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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 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Carbazole-Based Hole-Transport Materials for High-Efficiency and Stable Perovskite Solar Cells

As organic–inorganic halide perovskite solar cells (PSCs) near commercialization, stability challenges during real-world conditions, such as durability at elevated temperatures, still need to be addressed. We have previously reported that doping of triarylamine-based hole-transport layers (HTLs) with a triarylamine-based radical cation salt (EHCz-3EtCz/EH44-ox) leads to enhanced PSC stability at elevated temperatures. While it was shown the radical cation dopant did not need to be identical to the HTL matrix, little was known about dopant exchange to realize the maximum impact on device-level properties (e.g., increase in low intrinsic conductivity, mobility, hydrophobic properties, ease of synthesis, and thermal stability). In this paper, we study the impact of dopant exchange among stable, low-cost, high-glass-transition temperature (Tg), and easily synthesized triarylamine-based HTL and radical triarylamine cation salts as dopants. Using EH44-ox as dopant leads to the improved device-level power conversion efficiency (PCE) for all HTL matrices assessed. Moreover, increasing the number of ethylhexyl chains from one to two per molecule and positioning these chains at the periphery rather than the core resulted in improved hydrophobicity. PSCs based on our HTL formulations have similar power conversion efficiencies (PCE) as those of PSCs based on commercially available HTLs while demonstrating greatly improved device-level stability at elevated temperatures.

14 SOLAR ENERGY↗
DOE OSTI2025

Expanding the homogeneity range of UFe 2 : Formation of Laves phase UFe 2 +Ni

Uranium-based intermetallic Laves phases (UM 2 ) provide valuable insight into the interplay of 5f orbital hybridization and magnetic properties. However, ferromagnetism in these compounds is rare. UFe 2 is a cubic C15 Laves phase with greatest Curie temperature (T C ) of all such compounds, at 162 K. In this study, we report the synthesis and characterization of a novel U-based Laves phase, UFe 2 +Ni, effectively forcing excess Ni in to the UFe 2 structure. UFe 2 +Ni crystallizes in the cubic C15 Laves phase, appearing to deviate from the typical UM 2 formula and exhibiting a significantly extended homogeneity range. Annealing this material leads to an increased ferromagnetic transition temperature, between 35 and 155 K, and decreased lattice parameter with higher annealing temperatures. This variation is attributed to changes in Fe and Ni solubility with annealing temperature. In conclusion, our findings highlight the potential for tuning magnetic properties in U-based Laves phases through compositional and thermal modifications, expanding the understanding of magnetically ordered intermetallics.

Annealing effects↗
DOE OSTI2021

Magnetic properties of the Ising-like rare earth pyrosilicate: D-Er 2 Si 2 O 7

Ising-like spin-1/2 magnetic materials are of interest for their ready connection to theory, particularly in the context of quantum critical behavior. In this work we report detailed studies of the magnetic properties of a member of the rare earth pyrosilicate family, D-Er 2 Si 2 O 7 , which is known to display a highly anisotropic Ising-like g-tensor and effective spin-1/2 magnetic moments. We used powder neutron diffraction, powder inelastic neutron spectroscopy (INS), and single crystal AC susceptibility to characterize its magnetic properties. Neutron diffraction enabled us to determine the magnetic structure below the known transition temperature (T N = 1.9 K) in zero field, confirming that the magnetic state is a four-sublattice antiferromagnetic structure with two non-collinear Ising axes, as was previously hypothesized. Our powder INS data revealed a gapped excitation at zero field, consistent with anisotropic (possibly Ising) exchange. An applied field of 1 T produces a mode softening, which is consistent with a field-induced second order phase transition. To assess the relevance of D-Er 2 Si 2 O 7 to the transverse field Ising model, we performed AC susceptibility measurements on a single crystal with the magnetic field oriented in the direction transverse to the Ising axes. This revealed a transition at 2.65 T at 0.1 K, a field significantly higher than the mode-softening field observed by powder INS, showing that the field-induced phase transitions are highly field-direction dependent as expected. Furthermore, these measurements suggest that D-Er 2 Si 2 O 7 may be a candidate for further exploration related to the transverse field Ising model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Role of Fast Dynamics in Conductivity of Polymerized Ionic Liquids

Polymerized ionic liquids (PolyILs) are promising candidates for a broad range of technologies. However, the relatively low conductivity of PolyILs at room temperature has strongly limited their applications. In this work, we provide new insights into the roles of various microscopic parameters controlling ion transport in these polymers, which are crucial for their rational design and practical applications. Using broadband dielectric spectroscopy and neutron and light scattering techniques, we found a clear connection between the activation energy for conductivity, fast dynamics, and high-frequency shear modulus in PolyILs at their glass transition temperature (T g ). In particular, our analysis reveals a correlation between conductivity and the amplitude of fast picosecond fluctuations at T g , suggesting the possible involvement of fast dynamics in lowering the energy barrier for ion conductivity. We also demonstrate that both the activation energy for ion transport and the amplitude of the fast fluctuations depend on the high-frequency shear moduli of PolyILs, thus identifying a practically important parameter for tuning conductivity. Finally, the parameters recognized in this work and their connection to the ionic conductivity of PolyILs set the stage for a deeper understanding of the mechanism of ion transport in PolyILs in the glassy state.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Comprehensive Data Analysis and Analytic Method Development for PBX9501 Material (FY2019 Annual Report of Aging and Lifetimes Program)

In the polymer bonded explosive (PBX) 9501, a binder consisting of 2.5 wt% nitroplasticizer (NP) and 2.5 wt% Estane R 5703 (Estane) is combined with 94.9 wt% of the high-explosive HMX and 0.1 wt% stabilizer. Because of its flexibility and tensile strength, this binder lowers the sensitivity and improves the manufacturability of PBX 9501. However, like many plasticizers comprised of low molecular weight components, NP has a tendency to diffuse out of the PBX 9501 matrix and can decompose at moderate temperatures into reactive byproducts, such as NO, NO 2 , H 2 O, and HNO x . Through oxidation and hydrolysis, these molecules can further degrade NP and Estane, ultimately degrading the properties of PBX 9501. While gaseous molecules can readily diffuse out of the PBX 9501 charges, the intermediates with lower volatility are more likely trapped in the condensed phase inside the charges and, in a closed system, will co-exist with Estane for an extended period of time creating an ongoing reactive environment. Understanding the rates of formation for these volatiles and intermediates during the NP degradation is therefore a critical prerequisite for understanding the long-term stability of polymeric binders used in munition systems. To work toward this ultimate goal, in the past year, we have conducted comprehensive studies on the physical properties of NP and finished three-year long aging experiment to understand the aging behavior of NP under thermal treatment. Important results are summarized in this annual report. Although NP is widely used in the DOE complex, their physical properties are rather scattered and inconsistent in the open literature. For example, there are at least two widely different values, 14.5°C and -15°C, for the melting point of NP. Although it is known that NP is a eutectic 50:50 mixture of BDNPA and BDNPF, their eutectic phase diagram is not well documented. Furthermore, the effect of temperature on the miscibility between water and eutectic BDNPA/F mixture is rarely reported. To fill these knowledge gaps, a large set of BDNPA/F mixtures with BDNPA concentration ranging from 0 to 100 wt% was analyzed using DSC techniques. In addition to determining the eutectic melt point as -25°C, a phase diagram of the BDNPA/F system was constructed from -30°C to 45°C. With this phase diagram, the phase transition temperatures and composition can be readily found for the BDNPA/F mixtures with various mass ratios.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Melting Point Depression and Phase Identification of Sugar Alcohols Encapsulated in ZIF Nanopores

Sugar alcohols (SAs) have attractive characteristics as phase-change materials, but their relatively high melting temperature limits their application in the real world. Nanoconfinement can be a useful parameter to reduce the melting temperature to pragmatic ranges. Using molecular dynamics simulations, we investigate the phases and behaviors of encapsulated SA in ZIF-8 and ZIF-11, which cannot be experimentally observed. Based on reliable partial charges for the zeolitic imidazolate framework (ZIF) structures calculated by a density functional theory, structural analysis shows that the SA’s attractive interaction with the ZIF structure frustrates the SA crystallization and also elucidates the second-order phase transition between amorphous phases. A methodology is suggested to determine the phase transition temperature of confined materials and used to quantify the melting temperature depression of the ZIF-confined SAs. We also explored the thermal conductivity of SA-in-ZIF composites. Phonon frequency analysis verifies that the presence of SA molecules enhances the heat transfer by adding heat pathways between the nanoporous structure of ZIFs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

High-temperature superconductivity on the verge of a structural instability in lanthanum superhydride

Abstract The possibility of high, room-temperature superconductivity was predicted for metallic hydrogen in the 1960s. However, metallization and superconductivity of hydrogen are yet to be unambiguously demonstrated and may require pressures as high as 5 million atmospheres. Rare earth based “superhydrides”, such as LaH 10 , can be considered as a close approximation of metallic hydrogen even though they form at moderately lower pressures. In superhydrides the predominance of H-H metallic bonds and high superconducting transition temperatures bear the hallmarks of metallic hydrogen. Still, experimental studies revealing the key factors controlling their superconductivity are scarce. Here, we report the pressure and magnetic field dependence of the superconducting order observed in LaH 10 . We determine that the high-symmetry high-temperature superconducting Fm-3m phase of LaH 10 can be stabilized at substantially lower pressures than previously thought. We find a remarkable correlation between superconductivity and a structural instability indicating that lattice vibrations, responsible for the monoclinic structural distortions in LaH 10 , strongly affect the superconducting coupling.

36 MATERIALS SCIENCE↗
NASA NTRS1983

Dynamic Mechanical Properties of N-Phenylnadimide Modified PMR Polyimide Composites

The rheological behavior of newly developd Celion 6000/N-phenylnadimide modified PMR polyimide and PMR-15 composites was investigated. The dynamic mechanical properties were correlated with the structure of N-phenylnadimide modified PMR polyimides. The storage modulus (G'), loss modulus (G''), and loss tangent (tan delta) of four composite systems were measured over a temperature range from -150 to 400 C. Three well defined peaks were seen in the regions of 360, 100, and -120 C, corresponding to the alpha, beta, and gamma relaxations, respectively, of the matrix resins. The activation energies of the alpha, beta, and gamma relaxations were estimated to be 232, 60, and 14 kcal/mole, respectively, for PMR-15. Addition of N-phenylnadimide to the PMR-15 composition lowered the glass transition temperature and the activation energies of PMR-15 polyimide. The dynamic mechanical data appear to be consistent with the formation of a copolymer from N-phenylnadimide and a PMR-15 prepolymer.

Pater, R. H.↗
DOE OSTI2025

Superfluid weight cross-over and critical temperature enhancement in singular flat bands

Nonanalytic Bloch eigenstates at isolated band degeneracy points exhibit singular behavior in the quantum metric. Here, a description of superfluid weight for zero-energy flat bands in proximity to other high-energy bands is presented, where they together form a singular band gap system. When the singular band gap closes, the geometric and conventional contributions to the superfluid weight as a function of the superconducting gap exhibit different cross-over behaviors. The scaling behavior of superfluid weight with the band gap is studied in detail, and the effect on the Berezinskii–Kosterlitz–Thouless transition temperature is explored. It is found that tuning the singular band gap provides a unique mechanism for enhancing the supercurrent and critical temperature of two-dimensional superconductors.

Science & Technology - Other Topics↗
DOE OSTI2022

Impacts of phase state and water content on secondary organic aerosol formation and partitioning (Final Technical Report)

Secondary organic aerosols (SOA) account for a major fraction of particulate matter in the atmosphere, affecting climate, air quality, and public health. SOA formation and evolution are highly complex processes involving both chemical reactions and transport of molecules in air, at particle surfaces, and within particles. The state of the matter, or “phase state,” comprising SOA can vary from a liquid, over an amorphous semi-solid, to a glassy solid, depending on chemical composition, water content, relative humidity (RH), and temperature. The occurrence of glassy and amorphous semi-solid states can pose limitations on the rate of transport of molecules, affecting gas-particle interactions and challenging the treatment of SOA in atmospheric models. The objective of this project was to improve fundamental understanding of the interplay of the phase state of particles and water content on the evolution of SOA formation. We have developed a method to estimate the glass transition temperature (T g ) of organic compounds to predict the phase state and viscosity of SOA. This method has been applied to high resolution mass spectrometry data of various types of SOA, achieving a good agreement with viscosity measurements. In addition, the viscosity estimation method was implemented into a regional model CMAQ to simulate diel and seasonal variations of SOA phase state over the U.S. Applying kinetic multilayer modeling, we analyzed laboratory experiments and ARM field measurements (e.g., HI-SCALE) on kinetic limitations of amine uptake. We have also conducted numerical simulations to estimate equilibration timescales of SOA partitioning. Overall, we have evaluated the impacts of phase state and water content on SOA lifecycle that should contribute to reducing the uncertainty of SOA representation in regional climate and air quality predictions. The project has yielded 17 publications, in which DE-SC0018349 is explicitly acknowledged. Please see the list of publications as below, followed by summary of research activities.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1986

On the location of the maximum homogeneous crystal nucleation temperature

Detailed considerations are given to the location of the temperature of maximum homogeneous nucleation as predicted by classical nucleation theory. It is shown quite generally that this maximum temperature, T-asterisk, must occur above the Kauzmann temperature and that the T-asterisk is such that T-asterisk is greater than T(m)/3, where T(m) is the melting temperature. Also, it is demonstrated tha T-asterisk may be considered to be approximately dependent upon two parameters: gamma, the ratio of the difference in specific heat between the crystal and liquid divided by the entropy of fusion, and E, a reduced activation energy for viscous flow. The variation of T-asterisk with these parameters is described. The relationship of the relative location of T-asterisk to the glass transition temperature, is discussed too. This discussion is couched within the framework of the strong and fragile liquid notion introduced by Angell (1981) and coworkers. Finally, the question of the ultimate limits to the undercooling of liquid metals is considered and its relationhsip to computations of the maximum nucleation temperature in such systems.

Weinberg, Michael C.↗
NASA NTRS1992

Femtosecond resolution of soft mode dynamics in structural phase transitions

The microscopic pathway along which ions or molecules in a crystal move during structural phase transition can often be described in terms of a collective vibrational mode of the lattice. In many cases, this mode, called a 'soft' phonon mode because of its characteristically low frequency near the phase transition temperature, is difficult to characterize through conventional frequency-domain spectroscopies such as light or neutron scattering. A femtosecond time-domain analog of light-scattering spectroscopy called impulsive stimulated Raman scattering (ISRS) has been used to examine the soft modes of two perovskite ferroelectric crystals. The low-frequency lattice dynamics of KNbO3 and BaTiO3 are clarified in a manner that permits critical evaluation of microscopic models for their ferroelectric transitions. The results illustrate the advantages of ISRS over conventional Raman spectroscopy of low-frequency, heavily damped soft modes.

Dougherty, Thomas P.↗
NASA NTRS2024

Martensitic Phase Transition in Complex NiTi-Based Shape Memory Alloys from First-Principles Calculations

Recent rapid progresses in physics theory and computational power have made it possible to accurately predict the phase transitions and martensitic transition temperatures (MTTs) in shape memory alloys (SMAs) from first principles. However, previously theory and calculations [1-4] were applied only to study highly ordered stoichiometric binary alloys such as NiTi, PdTi and NiHf. Here we report on our recent first-principles investigations [5,6] on Ni 0.5 Ti 0.5-x Hf x and Pd x Ni 0.5-x Ti 0.5 ternaries and off-stoichiometric NiTi, and the predicted martensitic phase transitions in these complex SMAs are in good agreement with experimental findings. In particular, the calculated MTTs for all these compositions are within 100 K compared with the corresponding measured data, and our results also reveal the origin of the striking asymmetry in MTT of the off-stoichiometric NiTi near equiatomic compositions. We will address various techniques to overcome the difficulty encountered in studying ternaries and off-stoichiometic binaries associated with disorder and/or much lowered symmetry. Our theoretical approach is expected to be a broadly applicable and predictive theory for designing complex SMAs with desirable properties. References: [1] J. B. Haskins, A. E. Thompson, and J. W. Lawson, Phys. Rev B 94 , 214110 (2016). [2] J. B. Haskins and J. W. Lawson, J. App. Phys. 121 , 205103 (2017). [3] J. B. Haskins, H. Malmir, S. J. Honrao, L. A. Sandoval, and J. W. Lawson, Acta Materialia 212 , 116872 (2017). [4] Z. Wu, J. W. Lawson, and O. Benafan, Phys. Rev. B 106 , L140102 (2022). [5] Z. Wu, H. Malmir, O. Benafan, and J. W. Lawson, Acta Materialia 261 , 119362 (2023). [6] Z. Wu, J. W. Lawson, and O. Benafan, Phys. Rev. B 108 , L140103 (2023).

Zhigang Wu↗
DOE OSTI2023

High-temperature low-humidity proton exchange membrane with “stream-reservoir” ionic channels for high-power-density fuel cells

The perfluorosulfonic acid (PFSA) proton exchange membrane (PEM) is the key component for hydrogen fuel cells (FCs). We used in situ synchrotron scattering to investigate the PEM morphology evolution and found a “stream-reservoir” morphology, which enables efficient proton transport. The short-side-chain (SSC) PFSA PEM is fabricated under the guidance of morphology optimization, which delivered a proton conductivity of 193 milliSiemens per centimeter [95% relativity humidity (RH)] and 40 milliSiemens per centimeter (40% RH) at 80°C. The improved glass transition temperature, water permeability, and mechanical strength enable high-temperature low-humidity FC applications. Performance improvement by 82.3% at 110°C and 25% RH is obtained for SSC-PFSA PEM FCs compared to Nafion polymer PEM devices. The insights in chain conformation, packing mechanism, crystallization, and phase separation of PFSAs build up the structure-property relationship. In addition, SSC-PFSA PEM is ideal for high-temperature low-humidity FCs that are needed urgently for high-power-density and heavy-duty applications.

25 ENERGY STORAGE↗
NASA NTRS1980

The accelerated characterization of viscoelastic composite materials

Necessary fundamentals relative to composite materials and viscoelasticity are reviewed. The accelerated characterization techniques of time temperature superposition and time temperature stress superposition are described. An experimental procedure for applying the latter to composites is given along with results obtained on a particular T300/934 graphite/epoxy. The accelerated characterization predictions are found in good agreement with actual long term tests. A postcuring phenomenon is discussed that necessitates thermal conditioning of the specimen prior to testing. A closely related phenomenon of physical aging is described as well as the effect of each on the glass transition temperature and strength. Creep rupture results are provided for a variety of geometries and temperatures for T300/934 graphite/epoxy. The results are found to compare reasonably with a modified kinetic rate theory.

Griffith, W. I.↗
NASA NTRS2002

Viscoelastic and Mechanical Properties of Thermoset PMR-type Polyimide-Clay Nanocomposites

High temperature thermoset polyimide-clay nanocomposites were prepared by blending 2.5 and 5 wt% of an unmodified Na(+-) montmorillonite (PGV) and two organically modified FGV (PGVCl0COOH, PGVC12) with a methanol solution of PMR-15 precursor. The methanol facilitated the dispersal of the unmodified clay. Dynamic mechanical analysis results showed a significant increase in the thermomechanical properties (E' and E") of 2.5 wt% clay loaded nanocomposites in comparison with the neat polyimide. Higher glass transition temperatures were observed for 2.5 wt% nanocomposites compared to the neat polyimide. Flexural properties measurements for the 2.5 wt% nanocomposites showed a significant improvement in the modulus and strength, with no loss in elongation. This trend was not observed for the 5 wt% nanocomposites. An improvement in the CTE was observed for the PGV/PMR-15 nanocomposites, while a decrease was observed for the organically modified samples. This was attributed to potential variations in the interface caused by modifier degradation.

Abdalla, Mohamed O.↗
DOE OSTI2022

Transport coefficients of the quark-gluon plasma at the critical point and across the first-order line

A bottom-up Einstein-Maxwell-dilaton holographic model is used to compute, for the first time, the behavior of several transport coefficients of the hot and baryon-rich strongly coupled quark-gluon plasma at the critical point and also across the first-order phase transition line in the phase diagram. The observables under study are the shear and bulk viscosities, the baryon and thermal conductivities, the baryon diffusion, the jet quenching parameter $\hat{q}$, as well as the heavy-quark drag force and the Langevin diffusion coefficients. These calculations provide a phenomenologically promising estimate for these coefficients, given that our model quantitatively reproduces lattice QCD thermodynamics results, both at zero and finite baryon density, besides naturally incorporating the nearly perfect fluidity of the quark-gluon plasma. Here we find that the diffusion of baryon charge, and also the shear and bulk viscosities, are suppressed with increasing baryon density, indicating that the medium becomes even closer to perfect fluidity at large densities. On the other hand, the jet quenching parameter and the heavy-quark momentum diffusion are enhanced with increasing density. The observables display a discontinuity gap when crossing the first-order phase transition line, while developing an infinite slope at the critical point. The transition temperatures associated with different transport coefficients differ in the crossover region but are found to converge at the critical point.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Quantum Monte Carlo Simulations of the 2D Su-Schrieffer-Heeger Model

Over the past several years, a new generation of quantum simulations has greatly expanded our understanding of charge density wave phase transitions in Hamiltonians with coupling between local phonon modes and the on-site charge density. A quite different, and interesting, case is one in which the phonons live on the bonds, and hence modulate the electron hopping. This example, described by the Su-Schrieffer-Heeger (SSH) Hamiltonian, has so far only been studied with quantum Monte Carlo in one dimension. Here we present results for the 2D SSH model, show that a bond ordered wave (BOW) insulator is present in the ground state at half filling, and argue that a critical value of the electron-phonon coupling is required for its onset, in contradistinction with the 1D case where BOW exists for any nonzero coupling. We determine the precise nature of the bond ordering pattern, which has hitherto been controversial, and the critical transition temperature, which is associated with a spontaneous breaking of Ζ 4 symmetry.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗