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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 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Quasi-Harmonic Lattice Dynamics of a Prototypical Metal–Organic Framework

Quasi-harmonic lattice-dynamical calculations are performed to investigate the combined effect of temperature and pressure on the structural and mechanical properties of a prototypical metal–organic framework material: MOF-5. The softening upon compression of an A 2 g phonon mode at the Γ point in the high-symmetry F m 3 ¯ m structure is identified, which leads to a symmetry reduction and a group–subgroup phase transition to a low-symmetry F m 3 ¯ phase for compressions larger than 0.8%. The effect of the symmetry reduction on the equation-of-state of MOF-5 is investigated, which provides a static bulk modulus K reducing from 17 to 14 GPa and a corresponding change of K ' (pressure derivative of K ) from positive to negative. The effect of pressure on the negative thermal expansion of the framework and on its mechanical response is analyzed. The evolution of the mechanical anisotropy of MOF-5 as a function of pressure is also determined, which allows identifying the occurrence of a shear-induced mechanical instability at 0.45 GPa.

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

Topology property analysis and application of stable time-delay regions for linear multiple time-delay systems

This study examines the relationship between the topology of stable time-delay regions and the stability analysis of an linear multiple time delay system. To analyze the topology of stable time-delay regions, we construct a function with a value equal to zero for the time-delay points on the boundaries of stable time-delay regions. The function is continuous and differentiable in the whole defining field with a global minimum of zero so that we can locate the boundaries by minimizing the value of the function. Based on the topology analysis, we proposed a performance validation approach for controllers that are designed to stabilize the system using feedback signals with time delays. The method based on the topology analysis is simple and reliable so that can deal with a linear time-invariant system with high order and multiple time delays (more than 3). The example case study shows that the above method is reliable and enables ultra-low latency coordination and control for the future power grid with ubiquitous power electronics.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2018

Optimal Control of the Rigid Inclusion Size in the Problem of Equilibrium of Inhomogeneous Timoshenko Type Plates Containing Cracks

We consider the equilibrium problem for plates with cracks located along rigid inclusions and analyze the dependence of the solution and the derivative of the energy functional on variations of the inclusion size. We establish the solvability of the optimal control problem, where the quality functional is given by the derivative of the energy functional, whereas the control parameter correspond to the inclusion size. A similar analysis is performed for the equilibrium problems for inhomogeneous plates with rigid delaminated inclusions.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2017

Persistent and transient cost efficiency—an application to the Swiss hydropower sector

Electricity prices on the European market have decreased significantly over the past few years, resulting in a deterioration of Swiss hydropower firms’ competitiveness and profitability. One option to improve the sector’s competitiveness is to increase cost efficiency. The goal of this study is to quantify the level of persistent and transient cost efficiency of individual firms by applying the generalized true random effects (GTRE) model introduced by Colombi et al. (Journal of Productivity Analysis 42(2): 123–136, 2014) and Filippini and Greene (Journal of Productivity Analysis 45(2): 187–196, 2016). Applying this newly developed GTRE model to a total cost function, the level of cost efficiency of 65 Swiss hydropower firms is analyzed for the period between 2000 and 2013. A true random effects specification is estimated as a benchmark for the transient level of cost efficiency. The results show the presence of both transient as well as persistent cost inefficiencies. The GTREM predicts the aggregate level of cost inefficiency to amount to 21.8% (8.0% transient, 13.8% persistent) on average between 2000 and 2013. These two components differ in interpretation and implication. From an individual firm’s perspective, the two types of cost inefficiencies might require a firm’s management to respond with different improvement strategies. The existing level of persistent inefficiency could prevent the hydropower firms from adjusting their production processes to new market environments. From a regulatory point of view, the results of this study could be used in the scope and determination of the amount of financial support given to struggling firms.

13 HYDRO ENERGY↗
DOE OSTI2021

Electronic structure of Alq 3 and Liq using soft X-ray spectroscopy and density functional theory calculation

We show the electronic structure of aluminum tris-8-hydroxyquinoline (Alq 3 ) and 8-hydroquinolatolithium (Liq) was investigated using a combination of X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and spectral simulation with density functional theory. The chemical bonding states of Alq 3 and Liq were analyzed using XPS core-level spectra. The band gaps of Alq 3 and Liq were measured by combining the XAS and XES results. Additionally, resonant and non-resonant XES were used to measure the density of states for O sites in the molecules.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Comment on “Solubilities of ammonia in basic imidazolium ionic liquids” [Fluid Phase Equilib. 297 (2010) 34–39]

The enthalpies of solvation of ammonia in imidazolium-based ionic liquids (and other associated thermodynamic properties) are re-analyzed based on the data of [Fluid Phase Equilib. 297 (2010) 34–39], correcting numerical errors in the original analysis. In contrast to the original conclusions, the re-analyzed data demonstrates that the enthalpy of solvation is not a strong function of alkyl chain length. Here, the errors and uncertainties of the analysis procedure are also discussed, suggesting that the strong observed temperature dependence of enthalpy may in fact be an artifact of the systematic neglect of the non-ideality of the ammonia solute.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

The storage performance of automotive cryo-compressed hydrogen vessels

Cryo-compressed hydrogen storage promises to deliver the highest system storage density leading to practical vehicles with range comparable to today's gasoline vehicles and fundamental cost and safety advantages. Yet, cryogenic vessels are complex systems, continuously drifting in thermodynamic space depending on use patterns, insulation performance, vessel characteristics, liquid hydrogen pump performance, and para-H 2 to ortho-H 2 conversion. In this paper, cryogenic vessel fill density results from a previous publication are extended to calculate system storage performance, including volumetric (gH 2 /L), gravimetric (H 2 weight fraction), and vent losses over a broad range of conditions. The results validate previous experiments and models indicating that cryogenic pressure vessels have maximum system density of all available storage technologies while avoiding vent losses in all but the most extreme situations. Design pressures in the range 250–350 bar seem most advantageous due to high system density and low weight and cost, although determining an optimum pressure demands a complete economic and functional analysis. Future insulation, vessel, and liquid hydrogen pump improvements are finally analyzed that, while not experimentally exhibited to date, show promise of being feasible in the future as their level of technical maturity increases, leading to maximum H 2 storage performance for cryo-compressed storage. If proven feasible and incorporated into future cryogenic vessels, these improvements will enable 50 + gH 2 /L system density at 10+% H 2 weight fraction.

08 HYDROGEN↗
DOE OSTI2018

Optical Properties of Two-Dimensional Perovskite Films of (C 6 H 5 C 2 H 4 NH 3 ) 2 [PbI 4 ] and (C 6 H 5 C 2 H 4 NH 3 ) 2 (CH 3 NH 3 ) 2 [Pb 3 I 10 ]

The temperature dependence of absorption and photoluminescence (PL) spectroscopies were used to study the optical properties of 2D perovskite films, including n = 1 and 3 of (PEA) 2 (CH 3 NH 3 ) n-1 [Pb n I 3n+1 ] (PEA = C 6 H 5 (CH 2 ) 2 NH 3 ). In (PEA) 2 [Pb 1 I 4 ] (PEPI), excitons coupling to optical phonons with an average energy of ~30 meV dominate the photophysics of absorption and PL. (PEA) 2 (CH 3 NH 3 ) 2 [Pb 3 I 10 ] (shortened as PMPI 3 ), nominally prepared as n = 3, actually was a mixture of multiple layered perovskites with various n. In absorption, a PMPI 3 film presents respective n materials' excitonic features, coupling to phonons with an average energy of ~30 meV; in analyzing PL peaked singly at ~1.6 eV and its width as a function of temperature, we found that PMPI 3 behaves like PEPI at around 80 K but like 3D perovskite near room temperature, with three times larger electron-phonon interaction strength compared to that in PEPI.

14 SOLAR ENERGY↗
DOE OSTI2020

Reaction of Hydrazine with Solution- and Vacuum-Prepared Selectively Terminated Si(100) Surfaces: Pathways to the Formation of Direct Si–N Bonds

Here, the reactivity of liquid hydrazine (N 2 H 4 ) with respect to H-, Cl-, and Br-terminated Si(100) surfaces was investigated to uncover the principles of nitrogen incorporation into the interface. This process has important implications in a wide variety of applications, including semiconductor surface passivation and functionalization, nitride growth, and many others. The use of hydrazine as a precursor allows for reactions that exclude carbon and oxygen, the primary sources of contamination in processing. In this work, the reactivity of N 2 H 4 with H- and Cl-terminated surfaces prepared by traditional solvent-based methods and with a Br-terminated Si(100) prepared in ultrahigh vacuum was compared. The reactions were studied with X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy, and the observations were supported by computational investigations. The H-terminated surface led to the highest level of nitrogen incorporation; however, the process proceeds with increasing surface roughness, suggesting possible etching or replacement reactions. In the case of Cl-terminated (predominantly dichloride) and Br-terminated (monobromide) surfaces, the amount of nitrogen incorporation on both surfaces after the reaction with hydrazine was very similar despite the differences in preparation, initial structure, and chemical composition. Density functional theory was used to propose the possible surface structures and to analyze surface reactivity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Integrating Density Functional Theory Calculations with Vibrational and Nuclear Magnetic Resonance Spectroscopy

This review focuses on the complementary nature of density functional theory calculations and spectroscopic studies related to cellulose and its interactions within plant cell walls (PCWs). The complexity, structural disorder, and nanoscale nature of PCWs make them difficult to analyze and understand, but advances in sum-frequency generation and nuclear magnetic resonance spectroscopies as well as in density functional theory have made it possible to elucidate structure not observable previously. Combined with other techniques such as wide-angle X-ray scattering and neutron diffraction, details of PCW components (e.g., cellulose, lignin, pectin) and their interactions with one another can be elucidated. Furthermore, these techniques provide new insight into biological and biochemical studies and provide a basis for creating improved models of the PCWs.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Acidities of MgO surface sites: implications for the formation mechanism of Mg(OH) 2

The hydroxylation of periclase (MgO) to brucite (Mg(OH) 2 ) is thought to be an important intermediate step when using MgO to capture CO 2 from the atmosphere. However, the mechanism of hydroxylation of MgO to form Mg(OH) 2 is poorly understood. In this work, we used atomic-scale density functional tight binding simulations coupled with the metadynamics rare event method to analyze the surface chemistry of MgO and the acid dissociation equilibrium constants (pK a ) of its surface sites. The method and parameters were validated by calculating the pK a for hydroxylation of the first shell water bound to aqueous Mg 2+ ion. The pK a value derived using a probabilistic method was 12.3, which is in fair agreement with the accepted value of 11.4, with the difference between them equal to a ∼5 kJ mol −1 error in the calculations. We then extended these pK a calculations to probe the hydroxylation reactions of the surface sites of the MgO(100)–water interface, arriving at pK a s of 5.4 to deprotonate terminal water molecules bound to the surface magnesium sites (η-OH 2 or 〉MgOH 2 ), and 13.9 to deprotonate hydroxylated bridging oxygen sites (μ 5 -oxo or 〉O). Hydroxide (OH − ) adsorption on the surface was also probed and found to be less thermodynamically favorable than deprotonation of the terminal water molecule. The plausibility of the computed pK a s was verified using an activity-based speciation model and compared to pH measurements of water equilibrated with MgO nanoparticles and single crystals. The model predicted a solution pH of 7.1 when surface sites buffered and the pH of 12.0 when MgO dissolution dominated. These are close to the experimental initial solution pHs of 7–7.5 and the long term pHs of ∼10.5. The similarity suggests that the calculated pK a values from the DFTB+/metadynamics simulations are plausible and that these methods can be a useful tool to probe reaction mechanisms involving covalent bonds.

Adapa, Sai Krishna Reddy [Oak Ridge National Labor↗
DOE OSTI2019

Mechanisms underpinning the ultrahigh piezoelectricity in Sm-doped 0.705Pb(Mg 1/3 Nb 2/3 )O 3 -0.295PbTiO 3 : Temperature-induced metastable local structure and field-induced polarization rotation

The solid solution of (100 – x )%Pb(Mg 1/3 Nb 2/3 )O 3 - x %PbTiO 3 (PMN- x PT) exhibits ultrahigh piezoelectric and dielectric properties near the morphotropic phase boundary compositions and, as such, has been extensively studied in recent years. Recently, 2.5 mol. % Sm-doped PMN-29PT polycrystalline ceramics were reported to possess the highest piezoelectric coefficients (~1500 pC/N) among all reported piezoceramics, but the atomic-scale mechanisms for such high piezoelectric properties are not yet clear. Here, in situ X-ray diffraction and X-ray total scattering measurements during the application of an electric field, together with in situ total scattering measurement at different temperatures, were conducted for 2.5 mol. % Sm-doped PMN-29.5PT (2.5Sm-PMN-29.5PT). Both the largest field-induced strain and the piezoelectric response were found in the crystallites oriented with their &#12296 100 &#12297 PC directions parallel to the applied field. The local Pb displacement was analyzed using the reverse Monte Carlo method based on the pair distribution functions at different temperatures, where a temperature-induced directional change of Pb displacement was observed. Based on the experimental observations, a field-induced polarization rotation is suggested to be the dominant mechanism for the ultrahigh piezoelectricity of the 2.5Sm-PMN-29.5PT ceramic, while the ease of polarization rotation is possibly attributed to the temperature-induced metastable local monoclinic symmetries with their polar axes close to &#12296 111 &#12297 PC .

36 MATERIALS SCIENCE↗
DOE OSTI2024

Sound speed measurements in shock compressed cemented tungsten carbide: Evolution of elastic moduli with damage at pressures to 100 GPa

The motivation of the present study is to gain insights into the evolution of elastic properties of cemented tungsten carbides (WC) shock compressed to 100 GPa. Seven plate impact experiments—two front surface impact and five release wave overtake—are conducted to make simultaneous measurements of Hugoniot states and longitudinal sound speeds in shocked WC with 3.7wt.% cobalt binder. The sound speeds along with estimates for bulk sound speeds, obtained using the Birch–Murnaghan EoS, are analyzed to determine the elastic moduli—longitudinal, bulk, and shear—as a function of Hugoniot stress. The longitudinal and bulk sound speeds at Hugoniot states of interest are found to increase linearly with longitudinal stress. Consistent with the increase in sound speeds, the longitudinal and bulk moduli also increase with Hugoniot stress; however, the increase in longitudinal modulus is modest when compared to predictions of theoretical models that account for pressure and temperature dependence of elastic moduli, but with no damage. The shear moduli remain nearly constant at ~318 GPa over the range of Hugoniot states investigated. These values are, however, much lower than those predicted by the Steinberg–Guinan model with no damage. Poisson’s ratio decreases initially from its ambient value of 0.208 to ~0.199 for Hugoniot stress ≤10 GPa indicating consolidation of the WC microstructure with low initial stress; however, with an increase in Hugoniot stress to ~100 GPa, Poisson’s ratio increases to ~0.317, indicating degradation of shear moduli with increasing stress. The product of density and Grüneisen parameter (ρΓ), after an initial spike, remains nearly constant for volumetric strains ≥0.07. The maximum average temperature rise is estimated to be ~286°C at the highest Hugoniot stress employed in the study.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Identification of dislocation-related and point-defects in III-As layers for silicon photonics applications

Abstract The aim of this paper is to identify, analyze and compare the defects present in III-As, as a function of dislocation density, and as a function of the presence/absence of quantum dots (QDs). Such materials are of fundamental importance for the development of lasers and photodiodes for silicon photonics. The study is based on an extensive deep-level transient spectroscopy investigation, carried out on GaAs pin diodes grown on Si and on GaAs (that differ in the dislocation density), with and without embedded QDs. The original results described in this paper demonstrate that: (a) we were able to identify four different defects within the device grown on Si (three electron and one hole traps) and one defect (hole trap) in the device on GaAs, common to both samples; (b) all the majority carrier traps identified are located near midgap, i.e. are efficient non-radiative recombination centers; (c) such defects are absent (or non-detectable) in the sample grown on GaAs substrate, having a very low dislocation density; (d) the presence of QDs does not result in additional defects within the semiconductor material; (e) the analysis of the capture kinetics revealed that two of the identified traps are related to point defects, whereas the other two traps can be associated with point defects located near a dislocation; (f) a comparison with previous reports indicate that the detected traps are related to native III-As defects, or to oxygen-related complexes.

Physics↗
DOE OSTI2019

Charge density wave hampers exciton condensation in 1 T -TiSe 2

The Bose-Einstein condensation of excitons continues to garner immense attention as a prototypical example for observing emergent properties from many-body quantum effects. In particular, titanium diselenide (TiSe 2 ) is a promising candidate for realizing exciton condensation and was experimentally observed only very recently. Surprisingly, the condensate was experimentally characterized by a soft plasmon mode that only exists near the transition temperature $T_c$ of the charge density wave (CDW). Here, we characterize and analyze the experimental spectra using linear-response time-dependent density functional theory and find that the soft mode can be attributed to interband electronic transitions. At the CDW state below $T_c$, the periodic lattice distortions hamper the spontaneous formation of the exciton by introducing a CDW gap. In this work, the band gap raises the soft mode and merges it into the regular plasmon. Our surprising results contradict previous simplistic analytical models commonly used in the scientific literature. In addition, we find that a finite electronic temperature $T_e$ introduces a dissipation channel and prevents the condensation above $T_c$. The combined effect of the CDW and $T_e$ explains the fragile temperature dependence of the exciton condensation. Taken together, our work provides the first ab initio atomic-level framework for rationalizing recent experiments and further manipulating exciton condensates in TiSe 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Spin-orbit coupling effects on predicting defect properties with hybrid functionals: A case study in CdTe

Defect formation energies and transition levels are critical in determining doping behavior and recombination in semiconductor applications. Hybrid functionals are often used to overcome the band gap and delocalization errors of standard density-functional theory, and it is tempting to presume that the defect properties are correctly predicted once the hybrid functional mixing parameter reproduces the experimental band gap. However, pronounced spin-orbit coupling (SOC) effects can have an additional important role, which is clearly shown in this work by analyzing SOC effects originating from the Te-p orbitals in CdTe. In this work, we therefore use a hybrid functional that reproduces the experimental band gap when SOC is included, requiring a larger mixing parameter a=0.33 compared to the conventional choice of a=0.25. This hybrid functional was then used to predict defect properties, e.g., formation energy, transition level, and defect equilibrium. For defect states that do not directly involve the Te-p orbitals, such as the Cd interstitial (Cdi), we find that the effect of SOC on the defect levels can be captured by simply considering the SOC-induced band-edge shift. This is not the case for the A center (ClTe-VCd defect pair), where the localized acceptor state formed by Te-p orbitals is more directly affected by the SOC. For this defect, a mixing parameter as large as a=0.40 is required to reproduce the experimental acceptor level. Regarding the implications for photovoltaics, we suggest that the Cdi, which is the dominant compensating donor, could play an important role as a recombination center. While Cdi is usually thought of as a benign shallow donor, our predicted defect levels in the fully band-gap-corrected calculations are deep enough to raise a concern, and we propose a recombination mechanism for electron capture by Cdi.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Frequency-dependent complex viscosity obtained for a liquid two-dimensional dusty plasma experiment

Strongly coupled plasmas in a liquid phase can be characterized by a complex viscosity η(ω), which is a function of frequency. Data from a single experiment with dusty plasma were analyzed to compare η(ω) obtained by two fundamentally distinct methods. In a nonequilibrium method, a pair of counterpropagating laser beams, separated by a gap, applied a sinusoidal shear to a two-dimensional liquid, and η(ω) was determined using the constitutive relation. In an equilibrium method, there was no externally applied shear, so η(ω) could be calculated with a generalized Green-Kubo relation. The results for these two methods are compared for the real and imaginary parts of η(ω). For both parts, it is confirmed that the two methods yield results that agree qualitatively in their trends with frequency, with the real part diminishing with ω and the imaginary part increasing with ω, as expected for viscoelastic liquids. Quantitatively, the values of η(ω) obtained by the two methods differ slightly. For the experiment that we analyze, values for the real and imaginary parts of η(ω) are substantially greater than those reported in an earlier experiment, which we attribute to shear thinning effects in the earlier experiment. As a result, the experiment we analyze was designed to minimize shear thinning, unlike the earlier experiment.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Full-System Linearization for Floating Offshore Wind Turbines in OpenFAST

The wind engineering community relies on multiphysics engineering software to run nonlinear time-domain simulations (e.g., for design-standards-based loads analysis). Although most physics involved in wind energy are nonlinear, linearization of the underlying nonlinear system equations is often advantageous to understand the system properties and exploit well-established methods and tools for analyzing linear systems. This paper presents the development of the new linearization functionality of the open-source engineering tool OpenFAST for floating offshore wind turbines, as well as the concepts and mathematical background needed to understand and apply it.

17 WIND ENERGY↗