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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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All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Influence of Effective Physical Contact Area on Microstructure and Mechanical Properties of Diffusion-Bonded TC4/1060Al Joints

In the present study, dissimilar TC4 alloy and 1060Al alloy were successfully diffusion-bonded at a very low temperature about 410 °C by improving the effective physical contact area. A sound joint with a crack-free interface and a high shear strength (128 MPa) is obtained with a combination of TC4 surface roughness of 109.90 μm (S{sub a}) and 1060Al surface roughness of 101.43 μm (S{sub a}), and customized parallel surface scratches. Under this condition, the two parent metals share the largest contact area during the physical contact stage of diffusion bonding process, which is consistent with the theoretical calculation. The results show that no Ti-Al binary intermetallic compounds are observed at the interface of the joints. The maximum shear strength of the joint reaches 128 MPa with optimized bonding parameters (410 °C for 120 min with a pressure of 20 MPa), which is higher than the shear strength of 1060Al (80 MPa) base metal. The fractography analysis indicates the joints failed with a ductile fracture at the 1060Al side.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Convection Effects During Bulk Transparent Alloy Solidification in DECLIC-DSI and Phase-Field Simulations in Diffusive Conditions

In order to study the dynamical formation and evolution of cellular and dendritic arrays under diffusive growth conditions, three-dimensional (3D) directional solidification experiments were conducted in microgravity on a model transparent alloy onboard the International Space Station using the Directional Solidification Insert in the DEvice for the study of Critical LIquids and Crystallization. Selected experiments were repeated on Earth under gravity-driven fluid flow to evidence convection effects. Both radial and axial macrosegregation resulting from convection are observed in ground experiments, and primary spacings measured on Earth and microgravity experiments are noticeably different. The microgravity experiments provide unique benchmark data for numerical simulations of spatially extended pattern formation under diffusive growth conditions. The results of 3D phase-field simulations highlight the importance of accurately modeling thermal conditions that strongly influence the front recoil of the interface and the selection of the primary spacing. The modeling predictions are in good quantitative agreements with the microgravity experiments.

36 MATERIALS SCIENCE↗
DOE OSTI2018

A front-fixing finite element method for pricing American options under regime-switching jump-diffusion models

A front-fixing finite element method is applied to solve the partial integro-differential equations (PIDEs) arising in pricing American options under Markov-modulated jump-diffusion models with the free boundaries feature. For this purpose, we used a front-fixing method to transform the pricing problem into a nonlinear parabolic integro-differential equation on a fixed domain. Then the variational form of the resulting problem is solved by a finite element method. Under some appropriate assumptions, we establish the stability of the method and illustrate some numerical results to examine the rate of convergence of the proposed method for the pricing problem and compare its accuracy to some recent works on pricing American options under regime-switching jump-diffusion models.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2017

Microscopic diffusion in hydrated encysted eggs of brine shrimp

We have studied microscopic diffusion of water in fully hydrated encysted eggs of brine shrimp (Artemia). We utilized quasielastic neutron scattering. Dry eggs of brine shrimp were rehydrated using (1) water without additives, (2) eutectic mixture of water and dimethyl sulfoxide, and (3) a concentrated aqueous solution of lithium chloride. Despite the complexity of the hydrated multicellular organism, measurable microscopic diffusivity of water is rather well defined. Pure hydration water in eggs exhibits freezing temperature depression, whereas hydration water in eggs mixed with dimethyl sulfoxide or lithium chloride does not crystallize at all. The characteristic size of the voids occupied by water or aqueous solvents in hydrated brine shrimp eggs is between 2 and 10 nm. Those voids are accessible to co-solvents such as dimethyl sulfoxide and lithium chloride. There is no evidence of intracellular water in the hydrated eggs. The lack of intracellular water in the fully hydrated (but still under arrested development) state must be linked to the unique resilience against adverse environmental factors documented not only for the anhydrous, but also hydrated encysted eggs of brine shrimp.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Modeling diffusion and types I-V sorption of water vapor in heterogeneous systems

Here, a sorption and diffusion model is developed using a mobile-immobile decomposition of the vapor concentration. Henry’s mode represents the mobile species which diffuses through the solid, while Langmuir and pooling modes represent possible immobilization processes that cause nonlinearities in sorption capacity. The model can simulate the five classical sorption types and can incorporate additional dynamics compared to models based on thermodynamic equilibrium. Additionally, the framework allows for discontinuities in material properties encountered in multi-material systems. The resulting non-linear coupled equations are solved by employing a finite element method to discretize in space, and a backward Euler method to discretize in time. The discretized system of equations is solved via a Newton-type iteration scheme at each time-step. Four different materials are parameterized for the model, which are then used in examples to demonstrate the model’s ability to capture complex sorption processes in multi-material systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Dielectric-barrier-discharge plasma-assisted hydrogen diffusion flame. Part 1: Temperature, oxygen, and fuel measurements by one-dimensional fs/ps rotational CARS imaging

One-dimensional hybrid fs/ps CARS imaging provides single-laser-shot measurements of temperature, oxygen, and hydrogen in a plasma-assisted hydrogen diffusion flame. The coaxial dielectric-barrier-discharge burner collapses the Re ~50 hydrogen diffusion flame to within ~5 mm of the burner surface at an applied AC potential of 8.75 kV at 18 kHz, coinciding nicely with the full spatial extent of the 1D CARS measurements. Translating the burner through the measurement volume allowed for measurements at numerous radial locations in increments of 1 mm with a resolution of 140 µm × 30 µm × 600 µm, sufficient to resolve spatial gradients in this unsteady flame. Longer probe delays, required for improved dynamic range in regions of high temperature fluctuations, proved difficult to model as a result of a nontrivial decay in the O 2 Raman coherence arising from complexities associated with the triplet ground electronic state of the O 2 molecule. Oxygen linewidths were treated empirically using the observed O 2 coherence decay in spectra acquired from the product gases of lean, near-adiabatic H 2 /air flames stabilized on a Hencken flat-flame burner. While still leading to errors up to 10% at worst, the empirically determined Raman linewidth factors eliminated any systematic error in the O 2 /N 2 measurements with probe delay. Temperature measurements in the Hencken Burner flames proved to be insensitive to probe pulse delay, providing robust thermometry. Here, demonstration of this technique in both the canonical Hencken burner flames and a new DBD burner validates its effectiveness in producing multiple spatially resolved measurements in combustion environments. Measurements in the DBD burner revealed an unsteady, counterflow flattened flame structure near the fuel orifice which became unsteady as the reaction zone curves towards the surface for larger radial positions. Lastly, fluctuations in the fuel concentration were largest at the source, as the large, plasma-generated, unsteady external toroidal vortex that dominates the transport in this flame provides enhanced ventilation of the flame surface in close proximity to the fuel tube.

42 ENGINEERING↗
DOE OSTI2018

Structure and behavior of water-laden CH 4 /air counterflow diffusion flames

A counterflow configuration was used to measure thermal and species structure in water-vapor diluted nonpremixed methane–air flames. The motivation is to understand the chemical and thermal effects that water has when it is introduced as a diluent into the fuel side. Here, this work is relevant to combustion processes where water is incorporated naturally in the fuel; e.g., methane hydrates, and when water is added intentionally for emission reduction such as in flares and H 2 O/fuel emulsions combustion. Experimental data are compared to 1-D computations. The agreement is generally very good, but the one dimensional counterflow diffusion model overpredicts flame temperature and major radical, OH, concentration very near extinction in highly diluted H 2 O–methane/air diffusion flames. Changes in flame position, flame width, and peak temperature with the addition of water were measured. Flame temperatures were measured with thin filament pyrometry. OH-PLIF is used to characterize the flame reaction zone with water dilution; the OH distribution, flame position and thickness from the OH-PLIF images were measured. The results show that the OH intensity and reaction zone thickness decreases with the increase in water. Predictions and experiments demonstrate that water mainly acts thermally to lower the flame temperature until extinction. The OH maximum intensity shifts towards the air side of the counterflow burner with water addition. OH is also measured with CO 2 dilution of the fuel stream, and the results are compared with H 2 O addition, including comparisons with the OH molar peak predictions obtained using the GRI 3.0 mechanism and the CHEMKIN Pro one dimensional counterflow model. The study indicates that water’s chemical effects are to change the production and depletion of OH, H and O radicals, especially near extinction. Chemical kinetics simulation of the flame demonstrates good agreement in OH and flame temperatures over a wide range of dilution away from extinction, particularly for CO 2 . Lastly, an over prediction of the water carrying capacity near extinction is found for highly water-diluted flames.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Pressure effects on reactivity and extinction of n -dodecane diffusion cool flame

Low-temperature combustion at high pressure has gained increasing interest owing to the growing demand for clean and efficient propulsion and energy systems. In particular, cool flame structure, extinction limits, and fuel reactivities at high pressure are critical in affecting the engine performance at near-limit conditions. Here, this paper investigates the effects of pressure on the cool flame extinction limit, structure, radical index, reactivity, and oxygen concentration dependence by experiments, analysis, and modeling. A large n-alkane, namely n-dodecane (n C 12 H 26 ), is selected to study its diffusion cool flame dynamics and reactivity in a high-pressure counterflow burner up to 10 atm. The experimental results show that higher pressure increases the cool flame extinction strain rates, and that the pressure-weighted extinction strain rate (aP) is proportional to the square of pressure, P 2 . A scaling analysis explains the relationship between the dependence of flame structure, heat release rate, and pressure-weighted cool flame strain rate on pressure. Furthermore, radical indexes at different pressures are measured by isolating the thermal and transport effects from the chemical contribution to diffusion cool flame extinction. The radical index clearly shows that the low-temperature reactivity increases with pressure. In addition, due to the critical role of multiple oxygen addition reactions in low-temperature chemistry, the relationship between the cool flame extinction limit and the oxygen concentration is explored. It is found that the cool flame extinction limits are proportional to the n th power of the oxygen concentration, [O 2 ] n , and increasing pressure leads to stronger extinction limit dependence (larger n) on the oxygen concentration. The present experiment and detailed kinetic analysis show clearly that increasing pressure promotes the low-temperature chemistry including the oxygen addition reactions, while the scaling analysis explains well the experimental results.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2018

Chemically-biased diffusion and segregation impede void growth in irradiated Ni-Fe alloys

Recent irradiations of Ni-Fe concentrated solid solution alloys have demonstrated significant improvement of radiation performance. This improvement is attributed to redistribution of the alloying elements near sinks of point defects (voids, dislocations) due to chemically-biased atomic diffusion, where vacancies have preference to migrate via Fe atoms and interstitials via Ni atoms. In Ni-Fe, all sinks are enriched by Ni atoms, which strongly affects further interactions of radiation-produced mobile defects with voids and dislocations, hence void growth and dislocation climb. Ni-decorated sinks interact stronger with interstitial atoms than vacancies, which enhances dislocation loops growth. At the same time, Ni segregation creates Fe-enriched “channels” for vacancy migration out of the damage region to agglomerate in the outer regions, inaccessible to interstitial atoms. Furthermore, strong effect of chemically-biased diffusion is supported by transmission electron microscope characterization and calls for special attention in designing alloys with desired properties through tuning defect mobilities.

36 MATERIALS SCIENCE↗
DOE OSTI2016

In-pore exchange and diffusion of carbonate solvent mixtures in nanoporous carbon

High resolution magic angle spinning (HRMAS) 1 H NMR spectroscopy has been used to resolve different surface and in-pore solvent environments of ethylene carbonate (EC) and dimethyl carbonate (DMC) mixtures absorbed within nanoporous carbon (NPC). Two dimensional (2D) 1 H HRMAS NMR exchange measurements revealed that the inhomogeneous broadened in-pore resonances have pore-to-pore exchange rates on the millisecond timescale. Pulsed-field gradient (PFG) NMR diffusometry revealed the in-pore self-diffusion constants for both EC and DMC were reduced by up to a factor of five with respect to the diffusion in the non-absorbed solvent mixtures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Investigating Phase-Change-Induced Flow in Gas Diffusion Layers in Fuel Cells with X-ray Computed Tomography

The performance of polymer-electrolyte fuel cells is heavily dependent on proper management of liquid water. One particular reason is that liquid water can collect in the gas diffusion layers (GDLs) blocking the reactant flow to the catalyst layer. This results in increased mass-transport losses. At higher temperatures, evaporation of water becomes a dominant water-removal mechanism and specifically phase-change-induced (PCI) flow is present due to thermal gradients. This study used synchrotron based micro X-ray computed tomography (CT) to visualize and quantify the water distribution within gas diffusion layers subject to a thermal gradient. Plotting saturation as a function of through-plane distance quantitatively shows water redistribution, where water evaporates at hotter locations and condenses in colder locations. The morphology of the 2 GDLs on the micro-scale, as well as evaporating water clusters, are resolved, indicating that the GDL voids are slightly prolate, whereas water clusters are oblate. From the mean radii of water distributions and visual inspection, it is observed that larger water clusters evaporate faster than smaller ones.

25 ENERGY STORAGE↗
DOE OSTI2020

Freestanding vanadium nitride nanowire membrane as an efficient, carbon-free gas diffusion cathode for Li–CO 2 batteries

A freestanding vanadium nitride nanowire (VN-NW) membrane is employed as a carbon-free gas diffusion cathode (GDC) for high-performance Li–CO 2 batteries. Li–CO 2 cells built with the VN-NW GDC exhibit excellent electrochemical performance, achieving 100 discharge/charge cycles with an ultralow round-trip overpotential of <1 V. The VN-NW GDC also delivers vastly improved capacity and rate capability compared to a conventional multiwalled carbon nanotube GDC. The morphology of the discharge products (Li 2 CO 3 + C) is greatly improved with the VN-NW GDC compared to the carbon-based cathode, allowing for facile decomposition on charge. Furthermore, the absence of carbon in the VN-NW GDC allows for easy characterization of the discharge products with a variety of techniques, including transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The impressive performance of the hierarchically structured VN-NW GDC in Li–CO 2 batteries justifies further investigation into noble metal-free and carbon-free materials for gas diffusion cathodes.

25 ENERGY STORAGE↗
DOE OSTI2018

Substitution and diffusion of Cr 2+ and Cr 3+ in synthetic forsterite and natural olivine at 1200–1500 °C and 1 bar

The diffusion and substitution mechanisms of Cr in forsterite were studied as a function of crystallographic orientation and the chemical potentials of all four components in the system MgO-SiO2-Cr-O. Oxygen fugacity (fO2) was varied over 15.4 log units at 1400 °C and was fixed at the iron-wüstite equilibrium for a temperature series (1200–1500 °C). The valence state changes of Cr along some diffusion profiles was also investigated using X-ray absorption near edge structure spectroscopy.

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

Impact of cement composition, brine concentration, diffusion rate, reaction rate and boundary condition on self-sealing predictions for cement-CO 2 systems

Geological CO 2 storage (GCS) plays an important role in curbing CO 2 emissions by reducing the carbon footprint of difficult to decarbonize operations and achieve negative CO 2 emissions through activities like Bioenergy with Carbon Capture and Storage (BECCS) and Direct Air Carbon Capture and Storage (DACCS). Leakage of CO 2 through wells is an important concern when it comes to deployment of large-scale GCS. Existing wells in sites that are otherwise suitable for GCS can act as conduits for stored CO 2 to escape the reservoir. There is broad consensus that the main risk of leakage through wellbores is via fractures/damaged pathways. The results from several studies evaluating the permeability evolution of cement fractures in wells upon leakage of CO 2 agree that smaller fracture apertures, slower brine velocities and higher brine residence times promote self-sealing of fractures by mineral precipitation. Quantitatively, however, the differences in sealing conditions are significant and are typically attributed to differences in experimental conditions or model assumptions. Here we examine the sensitivity of our model, describing CO 2 leakage through wellbores, to cement composition, brine concentration, diffusion rates, and reaction rates. We also evaluate the impact of the boundary condition to allow comparisons between observations from experiments performed at constant flow rate and model predictions made at constant pressure conditions. Our results show that diffusion and reactions rates have the most impact on the self-sealing criteria for cement-CO 2 systems. In addition, conditions associated with self-sealing of fractures at constant flow rate require longer fractures, smaller fracture apertures, and slower velocities than under constant pressure.

58 GEOSCIENCES↗
DOE OSTI2019

Ballistic-diffusive phonon transport in cellulose nanocrystals by ReaxFF molecular dynamics simulations

Cellulose nanocrystals (CNCs), a ubiquitous nano-sized natural biopolymer, have drawn considerable attention in recent years due to their excellent mechanical and chemical properties. However, thermal properties of CNCs, which are critical for their potential applications as structural or functional materials, are less investigated. In this work, their thermal properties were systematically studied by molecular dynamics (MD) simulations in terms of the effects of polymorphs, size, temperature, and strain. It was found that the thermal conductivities depend on the polymorphs of CNCs, but the thermal conductance of unit chain of each polymorph is similar. Moreover, the strong dependence of the thermal conductivities of CNCs on their length and cross-sectional area indicates the existence of remarkable ballistic-diffusive phonon transport. Single chain CNCs can achieve a thermal conductivity of ~6 W/(m∙K), implying their potential as high thermal conductive polymer. The mechanical strain can enhance the alignment of cellulose skeleton, which reduces phonon scattering and thus increases thermal conductivity. Possible underlying mechanism of the varying thermal conductivity was further discussed and attributed to the shift of phonon frequencies and varied degree of orientation. This study reveals the sophisticated ballistic-diffusive phonon transport in CNCs, and pave path to future design of CNC-based materials with desirable thermal properties.

42 ENGINEERING↗
DOE OSTI2021

Visualization, understanding, and mitigation of process-induced-membrane irregularities in gas diffusion electrode-based polymer electrolyte membrane fuel cells

Polymer electrolyte membrane fuel cells (PEMFC) show substantial promise for their application in electric vehicles. For large-scale manufacturing of PEMFCs, roll-to-roll coated gas-diffusion-electrodes (GDE) offer certain advantages over other production pathways. Procedures including hot pressing and coating an ionomer overlayer may be necessary for this manufacturing pathway to enable a suitable catalyst layer/membrane interface. The same procedures may potentially introduce membrane irregularities, especially when thin membranes are used. Limited understanding exists regarding if and to what extent such irregularities impact PEMFC performance and lifetime, and therefore be considered defects. In this study, NREL's customized fuel cell hardware that enables quasi in-situ infrared (IR) thermography studies was utilized to visualize spatial hydrogen crossover and identify membrane irregularities that originated from the GDE-based MEA fabrication process. The structure of these membrane irregularities was investigated by scanning electron microscopy (SEM) and its impact on initial H2/air performance was determined. Accelerated stress testing (AST) revealed that these irregularities develop into failure point locations. These results were validated across many MEAs with identified process-induced membrane irregularities. By selecting specific gas diffusion media properties and by fine tuning the MEA hot pressing parameters, the formation of such membrane irregularities was mitigated.

08 HYDROGEN↗
DOE OSTI2015

The mimetic finite difference method for elliptic and parabolic problems with a staggered discretization of diffusion coefficient

Numerical schemes for nonlinear parabolic equations based on the harmonic averaging of cell-centered diffusion coefficients break down when some of these coefficients go to zero or their ratio grows. To tackle this problem, we propose new mimetic finite difference schemes that use a staggered discretization of the diffusion coefficient. The primary mimetic operator approximates div (k•); the derived (dual) mimetic operator approximates - ∇(•). The new mimetic schemes preserve symmetry and positive-definiteness of the continuum problem which allows us to use algebraic solvers with optimal complexity. We perform detailed numerical analysis of the new schemes for linear elliptic problems and a specially designed linear parabolic problem that has solution dynamics typical for nonlinear problems. We also show that the new schemes are competitive with the state-of-the-art schemes for steady-state problems but provide much more accurate solution dynamics for the transient problem.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2016

Multi-scale diffuse interface modeling of multi-component two-phase flow with partial miscibility

In this paper, we introduce a diffuse interface model to simulate multi-component two-phase flow with partial miscibility based on a realistic equation of state (e.g. Peng–Robinson equation of state). Because of partial miscibility, thermodynamic relations are used to model not only interfacial properties but also bulk properties, including density, composition, pressure, and realistic viscosity. As far as we know, this effort is the first time to use diffuse interface modeling based on equation of state for modeling of multi-component two-phase flow with partial miscibility. In numerical simulation, the key issue is to resolve the high contrast of scales from the microscopic interface composition to macroscale bulk fluid motion since the interface has a nanoscale thickness only. To efficiently solve this challenging problem, we develop a multi-scale simulation method. At the microscopic scale, we deduce a reduced interfacial equation under reasonable assumptions, and then we propose a formulation of capillary pressure, which is consistent with macroscale flow equations. Moreover, we show that Young–Laplace equation is an approximation of this capillarity formulation, and this formulation is also consistent with the concept of Tolman length, which is a correction of Young–Laplace equation. At the macroscopical scale, the interfaces are treated as discontinuous surfaces separating two phases of fluids. Our approach differs from conventional sharp-interface two-phase flow model in that we use the capillary pressure directly instead of a combination of surface tension and Young–Laplace equation because capillarity can be calculated from our proposed capillarity formulation. A compatible condition is also derived for the pressure in flow equations. Furthermore, based on the proposed capillarity formulation, we design an efficient numerical method for directly computing the capillary pressure between two fluids composed of multiple components. Finally, numerical tests are carried out to verify the effectiveness of the proposed multi-scale method.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗