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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 1,567 records · Page 87
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

An adaptive tau-leaping method for stochastic simulations of reaction-diffusion systems

Stochastic modelling is critical for studying many biochemical processes in a cell, in particular when some reacting species have low population numbers. For many such cellular processes the spatial distribution of the molecular species plays a key role. The evolution of spatially heterogeneous biochemical systems with some species in low amounts is accurately described by the mesoscopic model of the Reaction-Diffusion Master Equation. The Inhomogeneous Stochastic Simulation Algorithm provides an exact strategy to numerically solve this model, but it is computationally very expensive on realistic applications. We propose a novel adaptive time-stepping scheme for the tau-leaping method for approximating the solution of the Reaction-Diffusion Master Equation. This technique combines effective strategies for variable time-stepping with path preservation to reduce the computational cost, while maintaining the desired accuracy. The numerical tests on various examples arising in applications show the improved efficiency achieved by the new adaptive method.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

CO DIFFUSION INTO AMORPHOUS H{sub 2}O ICES

The mobility of atoms, molecules, and radicals in icy grain mantles regulates ice restructuring, desorption, and chemistry in astrophysical environments. Interstellar ices are dominated by H{sub 2}O, and diffusion on external and internal (pore) surfaces of H{sub 2}O-rich ices is therefore a key process to constrain. This study aims to quantify the diffusion kinetics and barrier of the abundant ice constituent CO into H{sub 2}O-dominated ices at low temperatures (15–23 K), by measuring the mixing rate of initially layered H{sub 2}O(:CO{sub 2})/CO ices. The mixed fraction of CO as a function of time is determined by monitoring the shape of the infrared CO stretching band. Mixing is observed at all investigated temperatures on minute timescales and can be ascribed to CO diffusion in H{sub 2}O ice pores. The diffusion coefficient and final mixed fraction depend on ice temperature, porosity, thickness, and composition. The experiments are analyzed by applying Fick’s diffusion equation under the assumption that mixing is due to CO diffusion into an immobile H{sub 2}O ice. The extracted energy barrier for CO diffusion into amorphous H{sub 2}O ice is ∼160 K. This is effectively a surface diffusion barrier. The derived barrier is low compared to current surface diffusion barriers in use in astrochemical models. Its adoption may significantly change the expected timescales for different ice processes in interstellar environments.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Comparative experimental investigation on the actuation mechanisms of ionic polymer–metal composites with different backbones and water contents

Water-based ionic polymer–metal composites (IPMCs) exhibit complex deformation properties, especially when the water content changes. To explore the general actuation mechanisms, both Nafion and Flemion membranes are used as the polymer backbones. IPMC deformation includes three stages: fast anode deformation, relaxation deformation, and slow anode deformation, which is mainly dependent on the water content and the backbone. When the water content decreases from 21 to 14 wt. %, Nafion–IPMC exhibits a large negative relaxation deformation, zero deformation, a positive relaxation deformation, and a positive steady deformation without relaxation in sequence. Despite the slow anode deformation, Flemion–IPMC also shows a slight relaxation deformation, which disappears when the water content is less than 13 wt. %. The different water states are investigated at different water contents using nuclear magnetic resonance spectroscopy. The free water, which decreases rapidly at the beginning through evaporation, is proven to be critical for relaxation deformation. For the backbone, indirect evidence from the steady current response is correlated with the slow anode deformation of Flemion-IPMC. The latter is explained by the secondary dissociation of the weak acid group –COOH. Finally, we thoroughly explain not only the three deformations by swelling but also their evolvement with decreasing water content. A fitting model is also presented based on a multi-diffusion equation to reveal the deformation processes more clearly, the results from which are in good agreement with the experimental results.

36 MATERIALS SCIENCE↗
DOE OSTI2014

The exit-time problem for a Markov jump process

The purpose of this paper is to consider the exit-time problem for a finite-range Markov jump process, i.e, the distance the particle can jump is bounded independent of its location. Additionally, such jump diffusions are expedient models for anomalous transport exhibiting super-diffusion or nonstandard normal diffusion. We refer to the associated deterministic equation as a volume-constrained nonlocal diffusion equation. The volume constraint is the nonlocal analogue of a boundary condition necessary to demonstrate that the nonlocal diffusion equation is well-posed and is consistent with the jump process. A critical aspect of the analysis is a variational formulation and a recently developed nonlocal vector calculus. Finally, this calculus allows us to pose nonlocal backward and forward Kolmogorov equations, the former equation granting the various moments of the exit-time distribution.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Electrochemical strain microscopy time spectroscopy: Model and experiment on LiMn{sub 2}O{sub 4}

Electrochemical Strain Microscopy (ESM) can provide useful information on ionic diffusion in solids at the local scale. In this work, a finite element model of ESM measurements was developed and applied to commercial lithium manganese (III,IV) oxide (LiMn{sub 2}O{sub 4}) particles. ESM time spectroscopy was used, where a direct current (DC) voltage pulse locally disturbs the spatial distribution of mobile ions. After the pulse is off, the ions return to equilibrium at a rate which depends on the Li diffusivity in the material. At each stage, Li diffusivity is monitored by measuring the ESM response to a small alternative current (AC) voltage simultaneously applied to the tip. The model separates two different mechanisms, one linked to the response to DC bias and another one related to the AC excitation. It is argued that the second one is not diffusion-driven but is rather a contribution of the sum of several mechanisms with at least one depending on the lithium ion concentration explaining the relaxation process. With proper fitting of this decay, diffusion coefficients of lithium hosts could be extracted. Additionally, the effect of phase transition in LiMn{sub 2}O{sub 4} is taken into account, explaining some experimental observations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Radiation belt electron acceleration during the 17 March 2015 geomagnetic storm: Observations and simulations

Various physical processes are known to cause acceleration, loss, and transport of energetic electrons in the Earth's radiation belts, but their quantitative roles in different time and space need further investigation. During the largest storm over the past decade (17 March 2015), relativistic electrons experienced fairly rapid acceleration up to ~7 MeV within 2 days after an initial substantial dropout, as observed by Van Allen Probes. In the present paper, we evaluate the relative roles of various physical processes during the recovery phase of this large storm using a 3-D diffusion simulation. By quantitatively comparing the observed and simulated electron evolution, we found that chorus plays a critical role in accelerating electrons up to several MeV near the developing peak location and produces characteristic flat-top pitch angle distributions. By only including radial diffusion, the simulation underestimates the observed electron acceleration, while radial diffusion plays an important role in redistributing electrons and potentially accelerates them to even higher energies. Moreover, plasmaspheric hiss is found to provide efficient pitch angle scattering losses for hundreds of keV electrons, while its scattering effect on > 1 MeV electrons is relatively slow. Although an additional loss process is required to fully explain the overestimated electron fluxes at multi-MeV, the combined physical processes of radial diffusion and pitch angle and energy diffusion by chorus and hiss reproduce the observed electron dynamics remarkably well, suggesting that quasi-linear diffusion theory is reasonable to evaluate radiation belt electron dynamics during this big storm.

58 GEOSCIENCES↗
NASA NTRS2001

Microstructure, Mechanical Properties, Hot-Die Forming, and Joining of 47XD Gamma TiAl Rolled Sheets

The microstructure and mechanical properties, along with the hot-die forming and joining of Ti-47Al-2Nb-2Mn-0.8 vol% TiB, sheets (known as 47XD), produced by a low-cost rolling process, were evaluated. A near-gamma microstructure was obtained in the as-rolled condition. The microstructures of heat-treated sheets ranged from a recrystallized equiaxed near-gamma microstructure at 1,200 to 1,310 C, to a duplex microstructure at 1,350 C, to a fully lamellar microstructure at 1,376 C. Tensile behavior was determined for unidirectionally rolled and cross-rolled sheets for room temperature (RT) to 816 C. Yield stress decreased gradually with increasing deformation temperature up to 704 C; above 704 C, it declined rapidly. Ultimate tensile strength exhibited a gradual decrease up to 537 C before peaking at 704 C, followed by a rapid decline at 816 C. The modulus showed a gradual decrease with temperature, reaching approximately 72% of the RT value at 816 C. Strain to failure increased slowly from RT to 537 C; between 537 C and 704 C it exhibited a phenomenal increase, suggesting that the ductile-brittle transition temperature was below 704 C. Fracture mode changed from transgranular fracture at low temperature, to a mixture of transgranular and intergranular fracture at intermediate temperature, to ductile fracture at 816 C, coupled with dynamic recrystallization at large strains. Creep rupture response was evaluated between 649 and 816 C over the stress range of 69 to 276 MPa. Deformation parameters for steady-state creep rate and time-to-rupture were similar: activation energies of approximately 350 kJ/mol and stress exponents of approximately 4.5. Hot-die forming of sheets into corrugations was done at elevated temperatures in vacuum. The process parameters to join sheets by diffusion bonding and brazing with TiCuNi 70 filler alloy were optimized for test coupons and successfully used to fabricate large truss-core and honeycomb structures. Nondestructive evaluation methods, e.g., ultrasonic C-scans and thermography along with metallography, were used to characterize bond quality. Microstructural evaluation during heat treatment, identification of phases at the braze/matrix interface, determination of shear strengths of brazed joints, and deformation mechanisms during tensile and creep processes will be discussed.

Das, G.↗
DOE OSTI2018

Pattern formation during electrochemical and liquid metal dealloying

Dealloying has evolved from a problematic corrosion process to a versatile tool for scalable fabrication of nanostructured metals. While the original, and majority of, work in the area has focused on electrochemical dealloying, a powerful variation of dealloying—liquid metal dealloying—has recently gained popularity. This process relies on a melt to carry out selective dissolution, replacing the traditional electrolyte solution. While electrolytes and molten metals are both suitable dealloying media, they can lead to very different morphologies. In this article, we compare and contrast what is known about the microscale physics and chemistry controlling microstructural evolution in electrochemical and liquid metal dealloying. Here, we conclude that the core phenomenology of porosity evolution—a competition between dissolution and interface diffusion—is similar in both dealloying processes, but that the relative magnitudes of these two processes control interfacial pattern formation.

McCue, Ian↗
DOE OSTI2025

Cathode Upcycling for Direct Recycling of Lithium‐Ion Batteries Using a Precipitation Approach

With the increased production of electric vehicles to reduce carbon emissions, the lithium-ion battery market to supply those vehicles has grown dramatically. To enhance battery sustainability and circularity, direct recycling methods aim to recover intact cathode materials. However, end-of-life cathode materials are typically 15–20 years old and often have lower energy density compared to current cathode materials. To address this challenge, a rapid precipitation process is developed to boost energy density by converting low Ni-compositions, LiNi 0.33 Co 0.33 Mn 0.33 O 2 (NMC111), into higher Ni-compositions (NMC622). This process forms a Ni-rich coating on cathode particles that diffuses into the core upon high-temperature relithiation, increasing compositional homogeneity. The upcycling process leverages existing infrastructure, offering low capital cost and minimal additional chemical input. Through ex situ tomographic transmission X-ray microscopy (TXM), 3D Ni:Co elemental mixing is quantified, confirming that elemental content evens at the secondary particle level with a mean NMC622 composition upon relithiation. However, elemental gradients remain in the single crystalline primary particles. Ex situ high-resolution and in situ wide-angle X-ray diffraction reveals concurrent structural changes during the relithiation process. These findings provide key insights into the structural and chemical mechanisms of elemental diffusion to obtain further improvements of increased capacity and retention through compositional conversion of cathode materials.

Cathode Upcycling↗
NASA NTRS1993

Clouding tracing: Visualization of the mixing of fluid elements in convection-diffusion systems

This paper describes a highly interactive method for computer visualization of the basic physical process of dispersion and mixing of fluid elements in convection-diffusion systems. It is based on transforming the vector field from a traditionally Eulerian reference frame into a Lagrangian reference frame. Fluid elements are traced through the vector field for the mean path as well as the statistical dispersion of the fluid elements about the mean position by using added scalar information about the root mean square value of the vector field and its Lagrangian time scale. In this way, clouds of fluid elements are traced and are not just mean paths. We have used this method to visualize the simulation of an industrial incinerator to help identify mechanisms for poor mixing.

Ma, Kwan-Liu↗
DOE OSTI2018

Microstructure evolution and mechanical properties of Cu/Sn/Ag TLP-bonded joint during thermal aging

Highlights: • The thermal reliability of the Cu/Sn/Ag TLP-bonded joint was investigated through high-temperature storage. • A diffusion reaction occurred at the interface between Cu{sub 3}Sn and Ag{sub 3}Sn during thermal aging. • The mechanism of the Cu{sub 3}Sn/Ag{sub 3}Sn interfacial reaction was investigated from thermodynamics and kinetics. - Abstract: Low-temperature transient liquid phase (TLP) bonding is a promising joining technology for high-temperature electronic devices packaging. However, the effect of thermal aging on the joint microstructure and performance has been rarely studied so far. In this paper, the relationship between microstructural evolution and mechanical property was developed for the Cu/Sn/Ag TLP-bonded joint during 350 °C aging. Experimental results show that an interfacial reaction occurs between the intermetallic compounds (IMCs) of Cu{sub 3}Sn and Ag{sub 3}Sn, which induces that a great amount of Cu{sub 3}Sn particles or wedges embed into the Ag-Sn phase layer. The reaction process is controlled by a volume diffusion of Cu elements from the Cu substrate, and capable of promoting the transformation from Ag{sub 3}Sn into Ag(Sn) solid solution. Cu{sub 41}Sn{sub 11} phase can be formed at the expense of Cu{sub 3}Sn and Cu, of which transformation process is completely achieved after aging for 480 h, and the Cu(Sn) solid solution intimately contacts with the opposite Ag(Sn) solid solution in many areas after aging for 960 h. The joint mechanical property initially declines due to the formation of Cu{sub 41}Sn{sub 11} phase and Cu-Sn IMCs grain coarsening, and then increases because of the complete depletion of Cu{sub 3}Sn and substantial precipitation of Cu(Sn) solid solution. After aging for nearly 1000 h, the joint still maintains a high shear strength of >40 MPa, indicating an excellent thermal reliability.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Improvement of band gap profile in Cu(InGa)Se{sub 2} solar cells through rapid thermal annealing

Highlights: • Proper RTA treatment can effectively optimize band gap profile to more expected level. • Inter-diffusion of atoms account for the improvement of the graded band gap profile. • The variation of the band gap profile created an absolute gain in the efficiency by 1.22%. - Abstract: In the paper, the effect of rapid thermal annealing on non-optimal double-graded band gap profiles was investigated by using X-ray photoelectron spectroscopy and capacitance–voltage measurement techniques. Experimental results revealed that proper rapid thermal annealing treatment can effectively improve band gap profile to more optimal level. The annealing treatment could not only reduce the values of front band gap and minimum band gap, but also shift the position of the minimum band gap toward front electrode and enter into space charge region. In addition, the thickness of Cu(InGa)Se{sub 2} thin film decreased by 25 nm after rapid thermal annealing treatment. All of these modifications were attributed to the inter-diffusion of atoms during thermal treatment process. Simultaneously, the variation of the band gap profile created an absolute gain in the efficiency by 1.22%, short-circuit current density by 2.16 mA/cm{sup 2} and filled factor by 3.57%.

36 MATERIALS SCIENCE↗
DOE OSTI2023

The kinetics of the PuO 2 to Pu 2 O 3 conversion

Here in an oxidizing environment, the oxide formed on plutonium (Pu) metal is composed of a plutonium dioxide (PuO 2 ) top layer and a thin cubic plutonium sesquioxide (Pu 2 O 3 ) middle layer. In a reducing environment, the PuO 2 layer auto-reduces to cubic Pu 2 O 3 . The speed and extent of this conversion depend on the combination of temperature and time. While PuO 2 provides a strong diffusion barrier against unwanted Pu corrosion by gaseous species (like hydrogen), Pu 2 O 3 does not, since its crystal structure has chains of oxygen vacancies. The kinetics of the PuO 2 reduction are, therefore, of fundamental interest and enable researchers to better protect Pu from corrosion. In this report, the oxygen-diffusion-limited kinetics of the dioxide to sesquioxide conversion were obtained by dynamically heating a PuO 2 -covered Pu sample from 294 to 418 K in a high-vacuum vessel equipped with an in situ spectroscopic ellipsometer. The physical/chemical constraints in the conversion process were combined with the ellipsometry method of multi-sample analysis to track the percentage of PuO 2 and to compute the extent of Pu 2 O 3 formation. The resulting diffusion coefficients were compared against and then combined with complementary literature data to produce a comprehensive set of kinetic parameters for reliably modeling oxide conversion over a larger temperature range than spanned by prior studies. The extracted thermal activation energy barrier (43.7 kJ/mol) and pre-exponential factor (5.0 × 10 -10 cm 2 /s) for the oxygen-diffusion-limited process can be used to accurately model the PuO 2 to Pu 2 O 3 transformation in vacuum and/or inert gas applications.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Surface transport processes in charged porous media

Surface transport processes are important in chemistry, colloidal sciences, engineering, biology, and geophysics. Natural or externally produced charges on surfaces create electrical double layers (EDLs) at the solid-liquid interface. The existence of the EDLs produces several complex processes including bulk and surface transport of ions. In this work, a model is presented to simulate bulk and transport processes in homogeneous porous media comprising big pores. It is based on a theory for capacitive charging by ideally polarizable porous electrodes without Faradaic reactions or specific adsorption of ions. A volume averaging technique is used to derive the averaged transport equations in the limit of thin electrical double layers. Description of the EDL between the electrolyte solution and the charged wall is accomplished using the Gouy-Chapman-Stern (GCS) model. The surface transport terms enter into the average equations due to the use of boundary conditions for diffuse interfaces. Two extra surface transports terms appear in the closed average equations. One is a surface diffusion term equivalent to the transport process in non-charged porous media. The second surface transport term is a migration term unique to charged porous media. The effective bulk and transport parameters for isotropic porous media are calculated solving the corresponding closure problems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1980

Array automated assembly task low cost silicon solar array project, phase 2

Analyses of solar cell and module process steps for throughput rate, cost effectiveness, and reproductibility are reported. In addition to the concentration on cell and module processing sequences, an investigation was made into the capability of using microwave energy in the diffusion, sintering, and thick film firing steps of cell processing. Although the entire process sequence was integrated, the steps are treated individually with test and experimental data, conclusions, and recommendations.

Olson, C.↗
DOE OSTI2022

Process and structures for fabrication of solar cells

Contact holes of solar cells are formed by laser ablation to accommodate various solar cell designs. Use of a laser to form the contact holes is facilitated by replacing films formed on the diffusion regions with a film that has substantially uniform thickness. Contact holes may be formed to deep diffusion regions to increase the laser ablation process margins. The laser configuration may be tailored to form contact holes through dielectric films of varying thicknesses.

14 SOLAR ENERGY↗
DOE OSTI2015

Development of a robust modeling tool for radiation-induced segregation in austenitic stainless steels

Irradiation-assisted stress corrosion cracking (IASCC) of austenitic stainless steels in Light Water Reactor (LWR) components has been linked to changes in grain boundary composition due to irradiation induced segregation (RIS). This work developed a robust RIS modeling tool to account for thermodynamics and kinetics of the atom and defect transportation under combined thermal and radiation conditions. The diffusion flux equations were based on the Perks model formulated through the linear theory of the thermodynamics of irreversible processes. Both cross and non-cross phenomenological diffusion coefficients in the flux equations were considered and correlated to tracer diffusion coefficients through Manning’s relation. The preferential atom-vacancy coupling was described by the mobility model, whereas the preferential atom-interstitial coupling was described by the interstitial binding model. The composition dependence of the thermodynamic factor was modeled using the CALPHAD approach. Detailed analysis on the diffusion fluxes near and at grain boundaries of irradiated austenitic stainless steels suggested the dominant diffusion mechanism for chromium and iron is via vacancy, while that for nickel can swing from the vacancy to the interstitial dominant mechanism. The diffusion flux in the vicinity of a grain boundary was found to be greatly influenced by the composition gradient formed from the transient state, leading to the oscillatory behavior of alloy compositions in this region. This work confirms that both vacancy and interstitial diffusion, and segregation itself, have important roles in determining the microchemistry of Fe, Cr, and Ni at irradiated grain boundaries in austenitic stainless steels.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2016

Mesoscopic structural phase progression in photo-excited VO 2 revealed by time-resolved x-ray diffraction microscopy

Dynamical phase separation during a solid-solid phase transition poses a challenge for understanding the fundamental processes in correlated materials. Critical information underlying a phase transition, such as localized phase competition, is difficult to reveal by measurements that are spatially averaged over many phase seperated regions. The ability to simultanousely track the spatial and temporal evolution of such systems is essential to understanding mesoscopic processes during a phase transition. Using state-of- the-art time-resolved hard x-ray diffraction microscopy, we directly visualize the structural phase progression in a VO 2 film upon photoexcitation. Following a homogenous in-plane optical excitation, the phase transformation is initiated at discrete sites and completed by the growth of one lattice structure into the other, instead of a simultaneous isotropic lattice symmetry change. The time-dependent x-ray diffraction spatial maps show that the in-plane phase progression in laser-superheated VO 2 is via a displacive lattice transformation as a result of relaxation from an excited monoclinic phase into a rutile phase. The speed of the phase front progression is quantitatively measured, which is faster than the process driven by in-plane thermal diffusion but slower than the sound speed in VO 2 . Lastly, the direct visualization of localized structural changes in the time domain opens a new avenue to study mesoscopic processes in driven systems.

36 MATERIALS SCIENCE↗