SEARCH · Search NASA

Results for “grain boundary mobility”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3

All sources NASA NTRS DOE OSTI

DOE OSTI2023

Electrical Analysis of Pulsed Laser Annealed Poly-Si: Ga/SiOx Passivating Contacts

In this contribution, we examine Ga hyper-doped poly-Si/SiOx contacts realized by pulsed laser melting (PLM). Here, we use Ga as a novel p-type dopant and B as a conventional dopant to induce non-equilibrium doping using an excimer laser. We perform simulations to visualize the maximum melt depth profiles within the poly-Si, with a goal of distributing dopants close to the tunneling oxide, but at the same time preserving the passivation. Hall measurements show that sheet resistance for B is lower than Ga due to its higher solid solubility limit in Si. After comparing the Hall active dopant concentration with the chemical concentration obtained by SIMS measurement, we show nearly 100% activation B activation reaching 10^21 cm-3, while only ~20% activation for Ga. Nevertheless, we achieve active doping concentrations of Ga in poly-Si six times higher than its solid solubility limit in Si (~10^19 cm-3). We compare our Hall mobilities with values in the literature for c-Si and show that B mobilities in laser-treated poly-Si are close to that of the literature value for B, while Ga mobilities are lower, possibly due to additional scattering channels within grain boundaries and deformed lattice. We also compare our results on PLM samples with conventional furnace annealed samples, and we show much higher percent activation and mobilities. Previously, we showed a low contact resistivity of 35.5 +/- 2.4 m..omega..cm2. Here, we further confirm this result by scanning spreading resistance microscopy and Kelvin force nanoprobe microscopy. We demonstrate that our poly-Si: Ga/nCz contact exhibits large drift and diffusion currents under normal cell operating voltage, which widens the laser processing window for a good metal/poly-Si/c-Si contact.

high-efficiency solar cells↗

DOE OSTI2026

Cluster Dynamics Simulations of Intra-Granular Fission Gas Bubble Size and Pressure Evolution in UO 2

Fission gases such as xenon (Xe) play a critical role in determining the behavior and response of nuclear fuel. Given that Xe has little solubility in UO 2 , it accumulates and forms bubbles, which significantly impact fuel performance. Intra- and inter-granular bubble nucleation and growth can lead to fuel swelling, and once bubbles interconnect at grain boundaries, fission gas can be released into the plenum. At low temperatures, limited uranium vacancy mobility can restrict swelling, therefore causing the bubbles to become highly pressurized. Consequently, this can induce micro-cracking, promote fission gas release (increasing the likelihood of cladding failure), and even lead to fuel pulverization under accident conditions such as a loss of coolant accident. As bubble evolution is strongly influenced by local temperature and fission rate, markedly different behavior occurs across the radial profile of the fuel pellet. Capturing the mechanisms that underpin bubble evolution is therefore important to predict these behaviors in the fuel. Previous models describing important mechanisms informed by lower length scale simulations have been developed under the NEAMS program. These can describe the evolution of a single bubble type (i.e., single value for radius and pressure) at each position in the pellet, for instance using the Centipede cluster dynamic code. However, in reality, a full distribution in bubble sizes and pressures exists within the microstructure at a given position in the pellet. To address this the cluster dynamics code Xolotl, which can predict Xe and vacancy phase space (i.e., bubble distributions) for intra-granular bubbles, has been used before. Prior work benchmarked the Xolotl code against the Centipede cluster dynamics code to ensure compatibility and to verify that mobile defect properties are adequately transferred between the two codes, along with some physics improvements. In this work, we go further by introducing a physics-based set of improvements that will allow us to accurately predict bubble size distributions and internal bubble pressures under representative UO 2 irradiation conditions. The improvements include (i) coupling bubble-defect reaction energies to a virial equation of state (EOS), (ii) including a bubble surface tension contribution, (iii) incorporating radiation-induced re-solution of Xe and vacancies, (iv) enabling pressure-driven dislocation loop punching through an effective emission of interstitial clusters informed by interstitial loop energetics, (v) accounting for radiation induced athermal diffusion of Xe, and (vi) implementing a Booth-type grain boundary sink representation for all mobile defects and defect clusters. After these modifications, we observe good agreement of Xolotl fission gas bubble size and concentration predictions with legacy experimental measurements. Additionally, it allows the distribution of Xe bubble pressures and radius to also be predicted and compared to data produced through the Advanced Fuels Campaign (AFC) program. Here, we have done this by running simulations under conditions similar to the AFC post-irradiation examination (PIE) samples irradiated at North Anna 2 light water reactor (LWR). Our results shows excellent agreement with these experimental measurements.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2021

Polyferrocenylsilane Semicrystalline Polymer Additive for Solution-Processed p-Channel Organic Thin Film Transistors

In this study, we demonstrated for the first time that a metal-containing semicrystalline polymer was used as an additive to mediate the thin film morphology of solution-grown, small-molecule organic semiconductors. By mixing polyferrocenylsilane (PFS) with an extensively-studied organic semiconductor 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene), PFS as a semicrystalline polymer independently forms nucleation and crystallization while simultaneously ameliorating diffusivity of the blend system and tuning the surface energies as a result of its partially amorphous property. We discovered that the resultant blend film exhibited a 6-fold reduction in crystal misorientation angle and a 3-fold enlargement in average grain width. Enhanced crystal orientation considerably reduces mobility variation, while minimized defects and trap centers located at grain boundaries lessen the adverse impact on the charge transport. Consequently, bottom-gate, top-contact organic thin film transistors (OTFTs) based on the TIPS pentacene/PFS mixture yielded a 40% increase in performance consistency (represented by the ratio of average mobility to the standard deviation of mobility). The PFS semicrystalline polymer-controlled crystallization can be used to regulate the thin film morphology of other high-performance organic semiconductors and shed light on applications in organic electronic devices.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Electrochemical grand potential-based phase-field simulation of electric field-assisted sintering

Here, an electrochemical grand potential functional was proposed to describe the sintering of an ionic ceramic green body. The resultant phase-field description enables simulation of the consolidation of an arbitrary number of granular particles and their interactions with the surrounding void phase. The model includes the effects of charged vacancies and the associated interactions between internal and applied electric fields. Defect segregation to grain boundaries is also accounted for, as well as enhanced interfacial defect mobilities. The model was parameterized for Y 2 O 3 . Simulations of two-particle systems showed that the applied electric field had an increasingly important impact on neck growth as particle size increased. A sudden rapid increase in temperature occurred for larger field strengths, which has been reported to be correlated to the onset of a flash event in flash sintering. Simulations of many particles showed that internal heat generation by Joule heating was localized at particle–particle contacts (grain boundaries), even though their conductivities were lower than nearby internal particle-void interfaces. A percolative path for ionic charge across the green body and the ceramic sintered solid was thus defined, accelerating the Joule heating process as the porosity of the green body is removed.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Assessing the quality of large-area monolayer graphene grown on liquid copper for size-selective ionic/molecular membrane separations

Monolayer graphene growth on liquid copper (Cu) has attracted attention due to advantages of a flat/smooth catalytic growth surface, high synthesis temperature (>1080 °C) as well as the possibility of forming graphene domains that are mobile on the liquid Cu with potential to minimize grain boundary defects and self-assemble into a continuous monolayer film. However, the quality of monolayer graphene grown on liquid copper and its suitability for size-selective ionic/molecular membrane separations has not been evaluated/studied. Here, we probe the quality of monolayer graphene grown on liquid Cu (via a metallurgical process, HSMG ® ) using Scanning Electron Microscope (SEM), High-resolution transmission electron microscope (HR-TEM), Raman spectroscopy and report on a facile approach to assess intrinsic sub-nanometer to nanometer-scale defects over centimeter-scale areas. We demonstrate high transfer yields of monolayer graphene (>93% coverage) from the growth substrate to polyimide track etched membrane (PITEM, pore diameter ~200 nm) supports to form centimeter-scale atomically thin membranes. Next, we use pressure-driven transport of ethanol to probe defects > 60 nm and diffusion-driven transport of analytes (KCl ~0.66 nm, L-Tryptophan ~0.7–0.9 nm, Vitamin B12 ~1–1.5 nm and Lysozyme ~3.8–4 nm) to probe nanoscale and sub-nanometer scale defects. Diffusive transport confirms the presence of intrinsic sub-nanometer to nanometer scale defects in monolayer graphene grown on liquid Cu are no less than that in high-quality graphene synthesized via chemical vapor deposition (CVD) on solid Cu. Our work not only benchmarks quality of graphene grown on liquid copper for membrane applications but also provides fundamental insights into the origin of intrinsic defects in large-area graphene synthesized via bottom-up processes for membrane applications.

36 MATERIALS SCIENCE↗

NASA NTRS1982

Design principles for high efficiency small-grain polysilicon solar cells, with supporting experimental studies

Design principles suggested here aim toward high conversion efficiency (greater than 15 percent) in polysilicon cells. The principles seek to decrease the liabilities of both intragranular and grain-boundary-surface defects. The advantages of a phosphorus atom concentration gradient in a thin (less than 50 microns) base of a p(+)/n(x)/n(+) drift-field solar cell, which produces favorable gradients in chemical potential, minority-carrier mobility and diffusivity, and recombination lifetime (via phosphorus gettering) are suggested. The degrading effects of grain boundaries are reduced by these three gradients and by substituting atoms (P, H, F or Li) for vacancies on the grain-boundary surface. From recent experiments comes support for the benefits of P diffusion down grain boundaries and, for quasi-grain-boundary-free and related structures. New analytic solutions for the n(x)-base include the effect of a power-law dependence between P concentration and lifetime. These provide an upper-bound estimate on the open circuit voltage. Finite-difference numerical solutions of the six Shockley equations furnish complete information about all solar-cell parameters and add insight concerning design.

Lindholm, F. A.↗

DOE OSTI2023

Ab initio calculation of carrier mobility in semiconductors including ionized-impurity scattering

The past decade has seen the emergence of ab initio computational methods for calculating phonon-limited carrier mobilities in semiconductors with predictive accuracy. More realistic calculations ought to take into account additional scattering mechanisms such as, for example, impurity and grain-boundary scattering. Here, in this paper, we investigate the effect of ionized-impurity scattering on the carrier mobility. We model the analytical impurity potential parameterized from first principles by a collection of randomly distributed Coulomb scattering centers, and we include this relaxation channel into the ab initio Boltzmann transport equation, as implemented in the EPW code. We demonstrate this methodology by considering silicon, silicon carbide, and gallium phosphide, for which detailed experimental data are available. Our calculations agree well with experiments over a broad range of temperatures and impurity concentrations. For each compound investigated here, we compare the relative importance of electron-phonon scattering and ionized-impurity scattering, and we critically assess the reliability of Matthiessen's rule. We also show that an accurate description of dielectric screening and carrier effective masses can improve quantitative agreement with experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS2022

A Record of Low-Temperature Asteroidal Processes of Amoeboid Olivine Aggregates from the Kainsaz Co3.2 Chondrite

Amoeboid olivine aggregates (AOAs) from the Kainsaz CO3.2 chondrite were analyzed using transmission electron microscopy in order to gain a more complete understanding of thermal metamorphism on the parent body and the role of fluids during metamorphic heating. The Kainsaz AOAs are dominated by strongly zoned, fine-grained, olivine grains (Fa2-31) with heterogeneous Fe enrichments along the grain boundaries, which are interpreted as the result of 26Fe2+-Mg2+ interdiffusion with the matrix during thermal metamorphism. However, our diffusion calculations show that such AOA olivine zoning and compositions cannot be produced by a simple diffusional exchange during metamorphic heating, unlike chondrule olivine zoning and compositions. In addition, fine-grained ferroan olivine overgrowths occur heterogeneously in crystallographic continuity with olivines on the AOA exteriors. The overgrowths (Fa33-36) are compositionally distinct from the underlying AOA olivines and are not fully equilibrated with the matrix olivines (Fa~20-55).The ferroan olivine overgrowths likely formed by precipitation from fluids in an epitaxial relationship with forsteritic olivine on the edges of AOAs. Texturally and compositionally diverse chromite grains are also observed along olivine grain boundaries, in olivine grains, and in pore spaces between olivine grains. They share a similar crystallographic orientation relationships with adjacent olivine, suggestive of their formation by exsolution and/or epitaxial growth. Collectively, these observations provide evidence for the mobilization of Fe, Mg, Si, Cr, and Al in the presence of fluids along olivine grain boundaries and into olivine grains during thermal metamorphism. We conclude that in Kainsaz AOAs, the strong zonation development in individual olivine grains and the formation of ferroan olivine overgrowths and chromite grains were a fluid-driven process that occurred at relatively low temperatures (<500°C), during the cooling history of the CO3 chondrite parent body, following the peak of thermal metamorphism.

Amoeboid olivine aggregates↗

DOE OSTI2024

Permeation of niobium through grain boundaries in copper

Low mutual solubility is expected to avert interdiffusion in layered composites of phase-separating metals. However, we show that Nb diffuses through polycrystalline Cu—despite the minimal bulk solubility of these elements—due to short circuit transport of Nb along certain grain boundaries in Cu. Atomistic modeling demonstrates that Nb-permeable Cu grain boundaries exhibit negative enthalpy of mixing of Nb, resulting in an enthalpically-stabilized solution of highly mobile Nb atoms that easily diffuse through the boundary. By contrast, Mo, which also has minimal solubility with Cu, has positive enthalpy of mixing at Nb-permeable Cu grain boundaries and does not diffuse through them. Furthermore, our findings suggest material selection and grain boundary engineering as pathways for designing improved diffusion barriers and thermally stable laminate composites.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Permeation of niobium through grain boundaries in copper

36 MATERIALS SCIENCE↗

DOE OSTI2020

ECCI analysis of shear accommodations at grain boundaries in commercially pure alpha titanium

This study uses a novel approach that couples electron channeling contrast imaging (ECCI) with controlled removal of surface material to investigate the slip system/grain boundary interactions by assessing the geometry of slip planes and grain boundaries as a function of depth. The comparison of spacing, relative distributions, and correlations of slip bands across grain boundaries reveals surface biasing on the slip band behavior in the grain boundary region. Furthermore, using analysis of the compatibility factor m’, the Schmid factor of each slip system, and the angles θ between slip planes, a more comprehensive understanding of the details of the mechanisms of strain transfer and heterogeneous deformation is developed. In general, the misalignment of primary slip systems on the grain boundary planes necessitates the activation of secondary accommodating slip systems close to the grain boundary area that are better aligned (higher m’ and lower θ) with respect to the incoming slip system across the grain boundary. Such systems have the same Burgers vector as the primary slip systems within the respective grains, but are typically on low mobility slip planes.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2023

The evolution of grain boundary energy in textured and untextured Ca–doped alumina during grain growth

The role of anisotropic grain boundary energy in grain growth is investigated using textured microstructures that contain a high proportion of special grain boundaries. Textured and untextured Ca–doped alumina was prepared by slip casting inside and outside a high magnetic field, respectively. At 1600°C, the textured microstructure exhibits faster growth than the untextured microstructure and its population of low–angle boundaries increases. Atomic force microscopy (AFM) is employed to measure the geometry of thermal grooves to assess the relative grain boundary energy of these systems before and after growth. In the textured microstructure, the grain boundary energy distribution narrows and shifts to a lower average energy. Conversely, the energy distribution broadens for the untextured microstructure as it grows and exhibits abnormal grain growth. Further analysis of the boundary networks neighboring abnormal grains reveals an energy incentive that facilitates their growth. These results suggest that coarsening is not the only dominant grain growth mechanism and that the system can lower its energy effectively by replacing high energy boundaries with those of low energy. The faster growth of lower energy boundaries suggests that isotropic simulations do not adequately account for anisotropic grain growth mechanisms or anisotropic mobility.

36 MATERIALS SCIENCE↗

DOE OSTI2026

Abnormal grain growth driven by high-temperature proton irradiation in nanocrystalline Ni

In this research we examine a nanocrystalline Ni thin film exposed to high-temperature proton irradiation and compare it with as-deposited and annealed-only counterparts. Despite lacking thermal spikes typical of heavy ions, 400 °C proton irradiation drives pronounced grain growth in select grains, whereas annealing alone yields only modest coarsening. Grain-boundary statistics show fewer low-angle boundaries (10–20°) and more high-angle boundaries (55–60°), consistent with irradiation-enhanced mobility of high-misorientation boundaries. The irradiated films retain a random texture, with no evidence of texture development or sharpening. Mechanisms, such as radiation-enhanced grain boundary diffusion, beam-induced heating, and ion channeling-mediated selective grain growth, are unlikely to be the predominant drivers to explain the resultant microstructure. Instead, we suggest irradiation-induced modifications of grain-boundary structure, including possible complexion transitions, as one plausible explanation for this selective grain growth and retention of random texture. However, additional temperature–dose studies are required to confirm the mechanism.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Modeling of the thermo-mechanical response and texture evolution of WE43 Mg alloy in the dynamic recrystallization regime using a viscoplastic self-consistent formulation

This paper presents a microstructure sensitive model for predicting mechanical response and texture evolution of metals in the dynamic recrystallization regime. A recently proposed viscoplastic self-consistent (VPSC) formulation for the prediction of recrystallization driven by strain energy and intragranular misorientation is extended to hexagonal close-packed (hcp) metals. The model is applied to the dynamic recrystallization of magnesium alloy WE43 at different temperatures and strain rates. Model predictions in terms of stress-strain response and texture evolution are compared to the experimental measurements and acceptable agreement is achieved. According to the model predictions, superplastic behavior of nuclei was found to be the dominant softening mechanism at high temperatures and low strain rates. High concentration of precipitates at the grain boundaries and presence of alloying elements are the likely causes of low boundary mobility, resulting in nucleation dominated dynamic recrystallization. Relatively strong basal compression textures indicate dominant activity of basal slip, which can be achieved only through large difference in slip resistance between soft basal and hard prismatic and pyramidal modes.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Atomic-level mechanisms of short-circuit diffusion in materials

This paper reviews the recent progress in understanding the atomic mechanisms of short-circuit diffusion along materials interfaces, such as grain and interphase boundaries, as well as lattice and interfacial dislocations/disconnections. Recent atomistic computer simulations have shown that short-circuit diffusion is dominated by collective atomic rearrangements in the form of strings and rings of mobile atoms. The process is dynamically heterogeneous in space and time and has many features in common with atomic dynamics in supercooled glass-forming liquids. We discuss examples of grain boundary, interphase boundary, and dislocation diffusion in metals and alloys, including the solute effect on the diffusion rates and mechanisms. Interphase boundaries are exemplified by Al–Si interfaces with diverse orientation relationships and atomic structures. The hierarchy of short-circuit diffusion paths in materials is reviewed by comparing the rates of grain boundary, interphase boundary, and dislocation diffusion. Future directions in the field of short-circuit diffusion in defect core regions are discussed.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Dual phase patterning during a congruent grain boundary phase transition in elemental copper

The phase behavior of grain boundaries can have a strong influence on interfacial properties. Little is known about the emergence of grain boundary phases in elemental metal systems and how they transform. Here, we observe the nanoscale patterning of a grain boundary by two alternating grain boundary phases with distinct atomic structures in elemental copper by atomic resolution imaging. The same grain boundary phases are found by computational grain boundary structure search indicating a first-order transformation. Finite temperature atomistic simulations reveal a congruent, diffusionless transition between these phases under ambient pressure. The patterning of the grain boundary at room temperature is dominated by the grain boundary phase junctions separating the phase segments. Our analysis suggests that the reduced mobility of the phase junctions at low temperatures kinetically limits the transformation, but repulsive elastic interactions between them and disconnections could additionally stabilize the pattern formation.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Understanding the effect of crystal anisotropy on grain growth, texturing and transport via the orthorhombic ?-U system [Slides]

The alpha phase of uranium exhibits the orthorhombic crystal structure, resulting in significant physical property anisotropy. These complex properties make the material challenging to work with in engineering settings but provide a rich arena to investigate the fundamental, multi-scale effects of anisotropy on physical behaviors such as grain growth and microstructure evolution under irradiation. We apply molecular dynamics and phase field modeling to study mass transport and grain growth in alpha-uranium. We characterize the mobilities of crystalline defects and the interfacial energy, and We find that the grain boundary energy is highly variable depending on the interface structure and that the relative rates of defect transport change with temperature. We also find that anisotropic thermal expansion has a significant impact on grain growth kinetics and texture development. Our results are used to explain experimental observations and to provide a basis for further hypotheses into the complex physical behavior of alpha-uranium.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

NASA NTRS1999

Unique Aeolian Transport Mechanisms on Mars: Respective Roles of Percussive and Repercussive Grain Populations in the Sediment Load

Experiments show that when sand-size grains impact a sediment surface with energy levels commensurate for Mars, small craters are formed by the ejection of several hundred grains from the bed. The experiments were conducted with a modified crossbow in which a sand-impelling sabot replaced the bolt-firing mechanism. Individual grains of sand could be fired at loose sand targets to observe ballistic effects unhindered by aerodynamic mobilization of the bed. Impact trajectories simulated the saltation process on dune surfaces. Impact craters were not elongated despite glancing (15 deg.) bed impact; the craters were very close to being circular. High-speed photography showed them to grow in both diameter and depth after the impactor had ricochetted from the crater site. The delayed response of the bed was "explosive" in nature, and created a miniature ejecta curtain spreading upward and outward for many centimeters for impact of 100-300 um-diameter grains into similar material. This behavior is explained by deposition of elastic energy in the bed by the "percussive" grain. Impact creates a subsurface stress regime or "quasi-Boussinesq" compression field. Elastic recovery of the bed occurs by dilatancy; shear stresses suddenly convert the grains to open packing and they consequently become forcefully ejected from the site. Random jostling of the grains causes radial homogenization of stress vectors and a resulting circular crater. A stress model based on repercussive bed dilatancy and interparticle adhesive forces (for smaller grains) predicts, to first order, the observed crater volumes for various impact conditions. On earth, only a few grains are mobilized by a percussive saltating grain; some grains are "knudged" along the ground, and some are partly expelled on short trajectories. These motions constitute reptation transport. On Mars, saltation and reptation become indistinct: secondary or "repercussive" trajectories have sufficient vertical impulse to create a dense saltation population of many tens or hundreds of grains for each single high-speed saltation percussion of the bed. Impact cascading will lead to near-surface distortion of the boundary layer, and choked flow formed by a dense "slurry" of sand, with the majority of grains mobilized by repercussive forces rather than by aerodynamic lift. This proceeds until a fully-matured transport layer imposes self-limitations as grain-population density constrains the free-path motion of individial grains.

Marshall, John R.↗