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At least 1,369 records · Page 76
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Precipitation behavior and phase transformation mechanism of super austenitic stainless steel S32654 during isothermal aging

Highlights: • σ phase formed following a sequence: intergranular, cellular and intragranular. • Cellular σ phase was firstly detected in S32654 aged at 800–900 °C. • Cellular σ growth is controlled by both intergranular and bulk Cr, Mo diffusion. • N-rich zone and Cr diffusion near σ phase induced the nucleation of Cr{sub 2}N. • Local needle-like σ transformed into Cr{sub 2}N and R phase through Cr, Mo diffusion. - Abstract: Thermodynamic calculation as well as microstructure characterization was carried out to investigate the precipitation behavior of super austenitic stainless steel S32654 aged at 700–1100 °C. The corresponding precipitation characteristics and transformation mechanisms of secondary phases were proposed. The results demonstrate that σ phases with various morphologies formed during isothermal aging, and followed a sequence of intergranular, cellular and intragranular with aging temperature. The intergranular σ phase nucleated much faster than the intragranular one due to the lower nucleation energy and higher element diffusion rate at grain boundary. Most intragranular σ phase grew into needle-like shape because of the great difference in mobility of the interfaces formed around it. The cellular σ phase was firstly detected in S32654, and its growth process was successively controlled by intergranular diffusion and then by bulk diffusion of Cr and Mo. Both Cr and Mo bulk diffusion and large needle-like σ phase resulted in the incompleteness of cellular precipitation of σ phase. Another new phenomenon was found that σ phase could induce the precipitation of Cr{sub 2}N ahead of cell boundary, between lamellar σ phases and at the σ/γ interfaces. Meanwhile, local parts of needle-like σ phase gradually transformed into Cr{sub 2}N and R phase through Cr and Mo diffusion from the former to the latters.

36 MATERIALS SCIENCE↗
DOE OSTI2024

SEI Formation and Lithium-Ion Electrodeposition Dynamics in Lithium Metal Batteries via First-Principles Kinetic Monte Carlo Modeling

The stabilization and enhanced performance of lithium metal batteries (LMBs) depend on the formation and evolution of the Solid Electrolyte Interphase (SEI) layer as a critical component for regulating the Li metal electrodeposition processes. This study employs a first-principles kinetic Monte Carlo (kMC) model to simulate the SEI formation and Li + electrodeposition processes on a lithium metal anode, integrating both the electrochemical electrolyte reduction reactions and the diffusion events giving place to the SEI aggregation processes during battery charge and discharge processes. The model replicates the competitive interactions between organic and inorganic SEI components, emphasizing the influence of the cycling regime. Results indicate that grain boundaries within the SEI facilitate faster lithium-ion transport compared to crystalline regions, crucial for improving the performance and stability of LMBs. The findings underscore the importance of dynamic SEI modeling for further development of next-generation high-energy-density batteries.

25 ENERGY STORAGE↗
NASA NTRS1975

Study of diffusion coefficients of glasses under zero-G

Diffusion studies of the glass forming ion are examined in zero-g environments and diffusion data obtained from these experiments are unique because of earth based experimental problems. The choice of system for diffusion studies is discussed along with the lab processing. The space processing is described consisting of a heating cycle designed to maximize time exposed to the diffusion temperature without exposing the low viscosity melt to gravitational forces.

Kinser, D. L.↗
DOE OSTI2020

Electrostatic Barriers to Nanoparticle Accessibility of a Porous Matrix

Translocation from one cavity to another through a narrow constriction (i.e., a “hole”) represents the fundamental elementary process underlying hindered mass transport of nanoparticles and macromolecules within many natural and synthetic porous materials, including intracellular environments. This process is complex and may be influenced by long-range (e.g., electrostatic) particle–wall interactions, transient adsorption/desorption, surface diffusion, and hydrodynamic effects. In this study, we used a three-dimensional (3D) tracking method to explicitly visualize the process of nanoparticle diffusion within periodic porous nanostructures, where electrostatic interactions were mediated via ionic strength. The effects of electrostatic interactions on nanoparticle transport were surprisingly large. For example, an increase in the Debye length of only a few nanometers (in a material with a hole diameter of ~100 nm) increased the mean cavity escape time 3-fold. A combination of computational and experimental analyses indicated that this hindered cavity escape was due to an electrostatic energy barrier in the region of the hole, which was quantitatively explained using DLVO theory. These findings explicitly demonstrate that the cavity escape process was barrier-limited and dominated by electrostatic effects.

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

Initial Reduction of the PdO(101) Surface: Role of Oxygen Vacancy Formation Kinetics

Here, we investigated the initial thermal reduction of the PdO(101) surface using density functional theory (DFT) calculations. The defect-free PdO(101) surface consists of parallel rows of undercoordinated Pd and O atoms (Pd cus and O cus , respectively). DFT was used to map out the elementary processes of O cus vacancy formation, oxygen surface atom diffusion along the Pd cus rows, and various O 2 formation pathways. Because oxygen vacancies form during the reduction of the surface, the elementary processes were examined with and without the presence of adjacent O cus vacancies. DFT calculations show that the presence of oxygen vacancies strongly affects the barriers of adjacent surface processes. Barriers for O vacancy formation are reduced by nearly 50% with an adjacent O vacancy. The barrier for O 2 formation along the Pd cus row drops from 1.54 eV on the defect-free PdO(101) surface to 0.81 eV in the presence of an O cus vacancy dimer. Yet, the presence of O cus vacancies also leads to more strongly bound O 2 on the Pd cus row. The strong neighbor effect of O vacancies on the PdO(101) surface will favor the growth of O vacancy chains and increase the rate of thermal reduction of the surface, both features that have been observed in recent ultrahigh vacuum isothermal reduction experiments of the PdO(101) surface.

Defects in solids↗
DOE OSTI2023

Evaluation of Tungsten—Steel Solid-State Bonding: Options and the Role of CALPHAD to Screen Diffusion Bonding Interlayers

Critical aspects of innovative design in engineering disciplines like infrastructure, transportation, and medical applications require the joining of dissimilar materials. This study investigates the literature on solid-state bonding techniques, with a particular focus on diffusion bonding, as an effective method for establishing engineering bonds. Welding and brazing, while widely used, may pose challenges when joining materials with large differences in melting temperature and can lead to mechanical property degradation. In contrast, diffusion bonding offers a lower temperature process that relies on solid-state interactions to develop bond strength. The joining of tungsten and steel, especially for fusion reactors, presents a unique challenge due to the significant disparity in melting temperatures and the propensity to form brittle intermetallics. Here, diffusion characteristics of tungsten–steel interfaces are examined and the influence of bonding parameters on mechanical properties are investigated. Additionally, CALPHAD modeling is employed to explore joining parameters, thermal stability, and diffusion kinetics. The insights from this research can be extended to join numerous dissimilar materials for specific applications such as aerospace, automobile industry, power plants, etc., enabling advanced and robust design with high efficiency.

36 MATERIALS SCIENCE↗
NASA NTRS1976

New advancements in titanium technology and their cost and weight benefits

A process that combines both the superplastic and diffusion bonding properties of metal into one concurrent operation is being developed. Estimates using this technology have indicated that this combined process will result in cost savings up to 70 percent when compared to conventional construction methods, while also saving weight. Many structural forms are possible including sandwich structures made by expanding face sheets and core against die forms. The classic difficulties normally associated with fabricating sandwich structures, such as parts fit-up, close tolerances, adhesive or braze alloy strength, do not exist with this technique. The total potential of Rockwell's patented new processes is limited only by the ingenuity of the designer and is expected to affect significantly future airplane concepts and criteria.

Ascani, L. A.↗
DOE OSTI2018

In situ study of nucleation and growth dynamics of Au nanoparticles on MoS 2 nanoflakes

Two-dimensional (2D) substrates decorated with metal nanoparticles offer new opportunities to achieve high-performance catalytic behavior. However, little is known on how the substrates control the nucleation and growth processes of the nanoparticles. This paper presents the visualization of dynamic nucleation and growth processes of gold nanoparticles on ultrathin MoS 2 nanoflakes by in situ liquid-cell transmission electron microscopy (TEM). The galvanic displacement resulting in Au nuclei formation on MoS 2 was observed in real time inside the liquid cell. We found that the growth mechanism of Au particles on pristine MoS 2 is in between diffusion-limited and reaction-limited, possibly due to the presence of electrochemical Ostwald ripening. A larger size distribution and more orientation variation is observed for the Au particles along the MoS 2 edge than on the interior. Differing from pristine MoS 2 , sulfur vacancies on MoS 2 induce Au particle diffusion and coalescence during the growth process. Density functional theory (DFT) calculations show that the size difference is because the exposed molybdenum atoms at the edge with dangling bonds can strongly interact with Au atoms, whereas sulfur atoms on the MoS 2 interior have no dangling bonds and weakly interact with gold atoms. In addition, S vacancies on MoS 2 generate strong nucleation centers that can promote diffusion and coalescence of Au nanoparticles. Furthermore, the present work provides key insights into the role of 2D materials in controlling the size and orientation of noble metal nanoparticles vital to the design of next generation catalysts.

2D materials↗
DOE OSTI2020

Internally-cooled compressor diaphragm

An internally-cooled diaphragm for an internally-cooled compressor is provided. The internally-cooled diaphragm may include an annular body configured to cool a process fluid flowing through a fluid pathway of the internally-cooled compressor. The annular body may define a return channel of the fluid pathway, and a cooling pathway in thermal communication with the fluid pathway. The return channel may be configured to at least partially diffuse and de-swirl the process fluid flowing therethrough, and the cooling pathway may be configured to receive a coolant to absorb heat from the process fluid flowing through the return channel.

42 ENGINEERING↗
DOE OSTI2021

Drying model of a high salt content cementitious waste form: Effect of capillary forces and salt solution

Highlights: • Drying model for a high salt content cementitious waste form is developed. • Water vapor diffusion and capillary liquid flow are distinguished. • Capillary and salt solution effects are considered in vapor-liquid equilibrium. A water transport model coupling capillary liquid flow with vapor diffusion is developed to describe the drying process for a cementitious waste form with high salinity porewater. Vapor-liquid equilibrium is formulated as the driving force for vapor diffusion and the model accounts for pore capillary and high salinity effects on water thermodynamic activity. Pore filling and porewater surface tension as a function of pore size distribution and water saturation have been quantified for the material. Geochemical speciation modeling is used to simulate porewater activity as a function of composition over the range of saturation. The theoretical relationship between relative humidity and water saturation generally agrees with experimental measurement, and the developed model is capable of predicting drying rates under various external relative humidity conditions. The model was developed to be incorporated into reactive transport models considering the effects of drying such as salt redistribution and efflorescence.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Symmetry and diffusivity of the interstitial hydrogen shallow-donor center in In 2 O 3

Uniaxial stress experiments performed for the 3306 cm -1 vibrational line assigned to the interstitial-hydrogen, shallow-donor center in In 2 O 3 reveal its symmetry and transition- moment direction. The defect alignment that can be produced by a [001] stress applied at 165 K is due to a process that is also a hydrogen- diffusion jump, providing a microscopic determination of the diffusion constant for H in In 2 O 3 and its mechanism. Lastly, our experimental results strongly complement theoretical predictions for the structure and diffusion of the interstitial hydrogen donor center in In 2 O 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1978

Thin silicon solar cell performance characteristics

Refined techniques for surface texturizing, back surface field and back surface reflector formation were evaluated for use with shallow junction, single-crystal silicon solar cells. Each process was characterized individually and collectively as a function of device thickness and bulk resistivity. Among the variables measured and reported are open circuit voltage, short circuit current and spectral response. Substantial improvements were obtained by the utilization of a low cost aluminum paste process to simultaneously remove the unwanted n(+) diffused region, form the back surface field and produce an ohmic contact metallization. The highly effective BSF which results from applying this process has allowed fabrication of cells 0.05 mm thick with initial outputs as high as 79.5 mW/4 sq cm (28 C, AM0) and superior electron radiation tolerance. Cells of 0.02 mm to 0.04 mm thickness have been fabricated with power to mass ratios well in excess of 2 watts per gram.

Gay, C. F.↗
NASA NTRS2003

The Organic Component of Interstellar Dust

The distribution, chemical structure, and formation of organic matter in the interstellar medium are important to our understanding of the overall evolution of dust. The exchange of dust between the dense and diffuse interstellar medium, and the effects of processing on dust within dense clouds will affect the inventory of material available for incorporation into newly forming star and planetary systems. Observational ground-based studies have confirmed the widespread distribution of the 3.4 pm absorption band attributed to aliphatic hydrocarbons in the diffuse interstellar medium of our own galaxy, and in the dusty spectra of a few nearby galaxies, while space based observations from IS0 probed the signatures of corresponding mid-infrared features. Laboratory experiments which utilize both thermal processes and energetic processing by high energy photons and cosmic rays, produce candidate materials which offer close matches to the observed diffuse interstellar medium and extragalactic hydrocarbon absorption features. Through an analysis of the 4000 to 1000 cm (2.5 to 10 micrometers) region of the spectrum of diffuse interstellar medium (DISM) dust compared with the spectra of thirteen chemical entities produced in the laboratory which serve as analogs to the interstellar material, significant constraints have been placed on the applicability of proposed candidate materials to explain the interstellar features. The results indicate that the organic refractory material in the diffuse interstellar medium is predominantly hydrocarbon in nature, possessing little nitrogen or oxygen, with the carbon distributed between the aromatic and aliphatic forms. Long alkane chains H3C-(CH2),- with n much greater than 4 or 5 are not major constituents of this material. Comparisons to laboratory analogs indicate the DISM organic material resembles plasma processed pure hydrocarbon residues much more so than energetically processed ice residues. This result is consistent with a birthsite for the carrier of the 3.4 micrometers band in the outflow region of evolved carbon stars. Comparisons of dust from our own galaxy with that of distant galaxies suggests that the organic component of interstellar dust is widespread and may be an important universal reservoir of prebiotic organic carbon.

Pendleton, Yvonne↗
NASA NTRS1966

Brazing process using'al-Si filler alloy reliably bonds aluminum parts

Brazing process employs an aluminum-silicon filler alloy for diffusion bonding of aluminum parts in a vacuum or inert gas atmosphere. This process is carried out at temperatures substantially below those required in conventional process and produces bonds of greater strength and reliability.

Beuyukian, C. S.↗
NASA NTRS1972

Elevated temperature deformation of TD-nickel base alloys

Sensitivity of the elevated temperature deformation of TD-nickel to grain size and shape was examined in both tension and creep. Elevated temperature strength increased with increasing grain diameter and increasing L/D ratio. Measured activation enthalpies in tension and creep were not the same. In tension, the internal stress was not proportional to the shear modulus. Creep activation enthalpies increased with increasing L/D ratio and increasing grain diameter, to high values compared with that of the self diffusion enthalpy. It has been postulated that two concurrent processes contribute to the elevated temperature deformation of polycrystalline TD-nickel: (1) diffusion controlled grain boundary sliding, and (2) dislocation motion.

Petrovic, J. J.↗
DOE OSTI2016

CMOS-compatible method for doping of buried vertical polysilicon structures by solid phase diffusion

Polysilicon receives attention nowadays as a means to incorporate 3D-structured photonic devices into silicon processes. However, doping of buried layers of a typical 3D structure has been a challenge. We present a method for doping of buried polysilicon layers by solid phase diffusion. Using an underlying silicon oxide layer as a dopant source facilitates diffusion of dopants into the bottom side of the polysilicon layer. The polysilicon is grown on top of the oxide layer, after the latter has been doped by ion implantation. Post-growth heat treatment drives in the dopant from the oxide into the polysilicon. To model the process, we studied the diffusion of the two most common silicon dopants, boron (B) and phosphorus (P), using secondary ion mass spectroscopy profiles. Our results show that shallow concentration profiles can be achieved in a buried polysilicon layer using the proposed technique. We present a quantitative 3D model for the diffusion of B and P in polysilicon, which turns the proposed method into an engineerable technique.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Thermally driven advection for radioxenon transport from an underground nuclear explosion

Abstract Barometric pumping is a ubiquitous process resulting in migration of gases in the subsurface that has been studied as the primary mechanism for noble gas transport from an underground nuclear explosion (UNE). However, at early times following a UNE, advection driven by explosion residual heat is relevant to noble gas transport. A rigorous measure is needed for demonstrating how, when, and where advection is important. In this paper three physical processes of uncertain magnitude (oscillatory advection, matrix diffusion, and thermally driven advection) are parameterized by using boundary conditions, system properties, and source term strength. Sobol' sensitivity analysis is conducted to evaluate the importance of all physical processes influencing the xenon signals. This study indicates that thermally driven advection plays a more important role in producing xenon signals than oscillatory advection and matrix diffusion at early times following a UNE, and xenon isotopic ratios are observed to have both time and spatial dependence.

Sun, Yunwei↗
NASA NTRS2000

The Decomposition of Carbonates and Organics on Mars

The return and analysis of pristine material that is relict of a putative period of chemical evolution is a fumdamental goal of the exobiological exploration of Mars. In order to accomplish this objective, it is desirable to find oxidant-free regions where pristine material can be accessed at the shallowest possible depth (ideally directly from the surface). The objective of our ongoing research is to understand the spatial and temporal distribution of oxidants in the martian regolith and the redox chemistry of the soil; in effect to understand the chemical mechanisms and kinetics relating to the in-situ destruction of organics and the formation of the reactive species responsible for the Viking biology results. In this work, we report on experimental studies of oxidizing processes that may contribute to carbonate and organic degradation on Mars. Organic molecules directly exposed to solar UV may decomposed either directly into CO2, or into more volatile organic fragments. Organic macromolecules not directly exposed to high UV flux are most likely to be affected by atmospheric oxidants which can diffuse to their surfaces. The oxidizing processes examined include: gas-phase oxidants, UV photolysis, and UV-assisted heterogeneous catalysis. For example, assuming a meteroritic infall rate of 4 x 10(exp -4) g/m^2yr (Flynn and McKay 1990) and a flux of organic carbon of 2 x 10(exp -5) g/m^2yr, laboratory measurements of the UV-assisted decomposition of benzenehexacarboxylic acid (mellitic acid, a likely intermediate of kerogen oxidation), indicate its decomposition rate on Mars would exceed the total flux of organic carbon to the planet by over four orders of magnitude. Our measurements indicate that although the decomposition temperature of kerogens in some cases exceeds the temperature limit of the Viking GCMS, it is unlikely kerogens or their decomposition intermediates were present at the Viking landings sites at levels above the GCMS detection limits.

Quinn, Richard C.↗