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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 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Photoluminescence, thermoluminescence and defect centres in Y2O3 and Y2O3:Tb3+ under 100 MeV swift Ni8+ ion beam irradiation

Highlights: • XRD showed decrease of crystallite size after ion irradiated. • Electron spin resonance reveals the presence of F, F{sup +} and O{sub 2}{sup −} centres. • SHI irradiated Y{sub 2}O{sub 3}:Tb{sup 3+} exhibits purity of intense green emission and long life time. • The Tb{sup 3+} doped Y{sub 2}O{sub 3} shows a linear, greater TL efficiency, longer life time and less fading under SHI irradiation. - Abstract: Y{sub 2}O{sub 3} and Y{sub 2}O{sub 3}:Tb{sup 3+} powders, synthesized via the solution combustion method, were studied in order to determine the effect of 100 MeV swift Ni{sup 8+} ions on the photoluminescence (PL), thermoluminescence (TL) and swift ion beam induced defects. Synthesized materials were characterized by X-ray powder diffraction, electron spin resonance, PL intensity, decay measurements and TL. Cubic structures were observed in both the pristine and ion irradiated materials. The ion irradiated materials exhibited smaller crystallite sizes than that of the pristine material. PL peaks in the visible region were identified at 485, 542, 582 and 621 nm due to the {sup 5}D{sub 4}→{sup 7}F{sub 3,4,5,6} transition of Tb{sup 3+}. The PL intensity showed an increase up to 3 × 10{sup 10} ions cm{sup −2} and then a significant decrease with a further increase in the ion fluence. The recorded time resolved PL at an ion irradiated fluence of 3 × 10{sup 10} ions cm{sup −2}, exhibited an increase in the life time of the material. TL glow curves were obtained for the samples irradiated with a swift Ni{sup 8+} ion beam. These glow curves showed a peak at 518 K with a shoulder peak at 473 K and its intensity linearly increased upto a fluence of 1 × 10{sup 11} ions cm{sup −2}. Whereas, the undoped sample showed peaks at 421 and 662 K and its intensity sub-linearly increased upto a fluence of 1 × 10{sup 11} ions {sup −2}. TL efficiency values of the Y{sub 2}O{sub 3}:Tb{sup 3+} sample, irradiated by a 100 MeV Ni{sup 8+} ion beam, have been measured relative to the undoped Y{sub 2}O{sub 3} sample and were found to be 1.34. The Tb{sup 3+} doped sample exhibited a less fading effect under the swift heavy ion beam.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Defect‐Free Sound Insulator Using Single Metal‐Based Friction Stir Process Array

Metals are excellent conductors for phonon transportation such as vibration, sound, and heat. Generally, metal sound insulators require multimaterial structure or defects and unimetal sound insulators are challenging. Therefore, a design of a defect-free sound insulator made by single alloys with multiple friction stir processes (FSPs) is proposed. Periodic friction stir processing can induce superlattice-like local mechanical properties’ modifications. By experimental acoustic characterization, it is observed that FSP can introduce clear acoustic–elastic property contrast on an aluminum plate by the presence of stir zone and heat-affected zones. In numerical simulations, the signature FSP-induced property profile is periodically and parallelly arranged on a long aluminum plate. The transmission gap frequencies are present on the frequency spectrum with the sound propagation direction perpendicular to the FSP paths. Disorder offsets on FSP periodicity are further introduced. Anderson localization is found on a resonance frequency, which provides −11 dB sound reduction by an exponential decay. Due to the finite design length, the slight disorder can also enhance sound insulation in the periodic transmission gap frequency. In conclusion, with analysis and comparison with different configurations, the best performance in the models can achieve −30 dB sound insulation in the 350 mm-long aluminum alloy plate with 14 parallel FSPs.

36 MATERIALS SCIENCE↗
NASA NTRS2004

Tensile Properties of Polymeric Matrix Composites Subjected to Cryogenic Environments

Polymer matrix composites (PMC s) have seen limited use as structural materials in cryogenic environments. One reason for the limited use of PMC s in cryogenic structures is a design philosophy that typically requires a large, validated database of material properties in order to ensure a reliable and defect free structure. It is the intent of this paper to provide an initial set of mechanical properties developed from experimental data of an advanced PMC (IM7/PETI-5) exposed to cryogenic temperatures and mechanical loading. The application of this data is to assist in the materials down-select and design of cryogenic fuel tanks for future reusable space vehicles. The details of the material system, test program, and experimental methods will be outlined. Tension modulus and strength were measured at room temperature, -196 C, and -269 C on five different laminates. These properties were also tested after aging at -186 C with and without loading applied. Microcracking was observed in one laminate.

Whitley, Karen S.↗
DOE OSTI2025

Modeling neutral defects in III-V ternary alloys with a special quasirandom structure: Analysis of As- and III-site point defects in InGaAs

While first-principles density functional theory modeling has become a vital tool to investigate defect properties in semiconductors, the lack of crystalline periodicity in pseudobinary random composition alloys, such as In 1−𝑥 ⁢Ga 𝑥 ⁢As, complicates such analyses. We present a simulation strategy to systematically take into account the variability in the local defect environment in order to predict statistical properties of neutral intrinsic defects in In 1−𝑥⁢ Ga 𝑥 ⁢As. We use a comprehensive sampling from a modest-sized 64-atom special quasirandom structure (SQS) to define a statistically representative set of defects, and use a 512-atom hypercell, a 2 × 2 × 2 supercell of SQS supercells, to achieve cell-size convergence. We articulate an equivalent site principle and describe how it constrains atomic chemical reference energies in computation of defect formation energies in pseudobinary alloys. A simple protocol for estimating reference energies for the Ga and In atoms sharing the III site succeeds in obtaining the equivalence of defects at Ga-sites and In sites in the SQS supercell, (<30 meV differences in average formation energies). For III-site defects, such as the As antisite As III , the statistical variability in formation energies is modest, ≈ 0.1–0.2 eV. The variability in formation energy at As-site defects, such as the As vacancy 𝑣 As , can be much larger, >1 eV. The As antisite is shown to be a low-energy defect and the most likely to be present in as-grown materials, just as in GaAs. All other defects are higher-energy defects unlikely to be important in native material, but potentially important in radiation-damaged material. With a strong variability in defect energies, especially on the As-site, explicit consideration of statistical variability due to compositional randomness will be imperative for meaningful and quantitative comparisons to experiment.

Density functional theory↗
DOE OSTI2019

Deep Learning for Semantic Segmentation of Defects in Advanced STEM Images of Steels

Crystalline materials exhibit long-range ordered lattice unit, within which resides nonperiodic structural features called defects. These crystallographic defects play a vital role in determining the physical and mechanical properties of a wide range of material systems. While computer vision has demonstrated success in recognizing feature patterns in images with well defined contrast, automated identification of nanometer scale crystallographic defects in electron micrographs governed by complex contrast mechanism is still a challenging task. Here, building upon an advanced defect imaging mode that offers high feature clarity, we introduce DefectSegNet - a new convolutional neural network (CNN) architecture that performs semantic segmentation of three common crystallographic defects in structural alloys: dislocation lines, precipitates and voids. Results from supervised training of a small set of high-quality defect images of steels show high pixel-wise accuracy across all three types of defects: 91.60 ± 1.77% on dislocations, 93.39 ± 1.00% on precipitates, and 98.85 ± 0.56% on voids. We discuss the sources of uncertainties in CNN prediction, and the training data in terms of feature density, representation and homogeneity and their effects on deep learning performance. Further defect quantification using DefectSegNet prediction outperforms human expert average, presenting a promising new workflow for fast and statistically meaningful quantification of materials defects.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Persistent Structure and Frustrated Magnetism in High Entropy Rare-Earth Zirconates

The configurational complexity and distinct local atomic environments of high entropy oxides remain largely unexplored, leaving structure-property relationships and the hypothesis that the family offers rich tunability for applications ambiguous. This work investigates the influence of cation size and materials synthesis in determining the resulting structure and magnetic properties of a family of high entropy rare-earth zirconates (HEREZs, nominal composition RE 2 Zr 2 O 7 with RE = rare-earth element combinations including Eu, Gd, Tb, Dy, Ho, La, or Sc). Here, the structural characterization of the series is examined through synchrotron X-ray diffraction and pair distribution function analysis, and electron microscopy, demonstrating average defect-fluorite structures with considerable local disorder, in all samples. The surface morphology and particle sizes are found to vary significantly with preparation method, with irregular micron-sized particles formed by high temperature sintering routes, spherical nanoparticles resulting from chemical co-precipitation methods, and porous nanoparticle agglomerates resulting from polymer steric entrapment synthesis. In agreement with the disordered cation distribution found across all samples, magnetic measurements indicate that all synthesized HEREZs show frustrated magnetic behavior, as seen in a number of single-component RE 2 Zr 2 O 7 pyrochlore oxides. These findings advance the understanding of the local structure of high entropy oxides and demonstrate strategies for designing nanostructured morphologies in the class.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Polarization-dependent photoemission electron microscopy for domain imaging of inorganic and molecular materials

Polarization-dependent photoemission electron microscopy (PD-PEEM) exploits spatial variation in the optical selection rules of materials to image domain formation and material organization on the nanoscale. In this Perspective, we discuss the mechanism of PD-PEEM that results in the observed image contrast in experiments and provide examples of a wide range of material domain structures that PD-PEEM has been able to elucidate, including molecular and polymer domains, local electronic structure and defect symmetry, (anti)ferroelectricity, and ferromagnetism. As a result, we discuss challenges and new directions that are possible with this tool for probing domain structure in materials, including investigating the formation of transient ordered states, multiferroics, and the influence of molecular and polymer order and disorder on excited state dynamics and charge transport.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Electronic shell structures, self-trapped excitons, and defect-bound excitons in Li 2 B 12 H 12

First-principles studies of electronic structures, excitons, and defects in Li 2 B 12 H 12 reveal unique photophysical properties of Li 2 B 12 H 12 and its potential as a neutron scintillator material. We identify the electronic shell structure in (B 12 H 12 ) 2- , which gives rise to strong chemical stability of (B 12 H 12 ) 2- and a large band gap of Li 2 B 12 H 12 . Based on hybrid functional calculations, we show that excitons in Li 2 B 12 H 12 are strongly self-trapped and predict that the exciton excitation and emission are both in the UV range and are not related to the bright blue emission observed in the experiment. The experimentally observed blue emission in Li 2 B 12 H 12 is likely due to the emission of excitons bound to hydrogen vacancies. The calculated strong localization and large exciton trapping energy at hydrogen vacancies indicate enhanced radiative recombination, while the calculated large Stokes shift suggests that the self-absorption and the resonance transfer of the excitation energy may be suppressed. These properties should promote efficient defect emission at room temperature. The efficient defect emission, combined with the large thermal neutron capture cross-section by Li and B and the efficient energy transfer between fast neutrons and H, suggests potential applications of Li 2 B 12 H 12 as a thermal or fast neutron scintillator. Optical characterization of VH may also be used to monitor H and Li concentrations in Li 2 B 12 H 12 -based hydrogen storage materials and solid-state electrolytes in Li batteries.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Intrinsic Defects in MoS 2 Grown by Pulsed Laser Deposition: From Monolayers to Bilayers

Pulsed laser deposition (PLD) can be considered a powerful method for the growth of two-dimensional (2D) transition-metal dichalcogenides (TMDs) into van der Waals heterostructures. However, despite significant progress, the defects in 2D TMDs grown by PLD remain largely unknown and yet to be explored. In this work, we combine atomic resolution images and first-principles calculations to reveal the atomic structure of defects, grains, and grain boundaries in mono- and bilayer MoS 2 grown by PLD. We find that sulfur vacancies and MoS antisites are the predominant point defects in 2D MoS 2 . We predict that the aforementioned point defects are thermodynamically favorable under a Mo-rich/S-poor environment. The MoS 2 monolayers are polycrystalline and feature nanometer size grains connected by a high density of grain boundaries. In particular, the coalescence of nanometer grains results in the formation of 180° mirror twin boundaries consisting of distinct 4- and 8-membered rings. We show that PLD synthesis of bilayer MoS 2 results in various structural symmetries, including AA' and AB, but also turbostratic with characteristic moiré patterns. Moreover, we report on the experimental demonstration of an electron beam-driven transition between the AB and AA' stacking orientations in bilayer MoS 2 . These results provide a detailed insight into the atomic structure of monolayer MoS 2 and the role of the grain boundaries on the growth of bilayer MoS 2 , which has importance for future applications in optoelectronics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Chemical Interpretation of Charged Point Defects in Semiconductors: A Case Study of Mg 2 Si

The electronic structures of charged point defects influence electrical and optical properties of semiconductors. Understanding the orbital interactions responsible for the electronic structures of defects therefore promotes a chemical intuition for defect-driven mechanisms in semiconductors. In this tutorial, we discuss a molecular orbital theory-based framework for understanding defect-induced electronic states based on local chemical interactions between the defect and the atoms surrounding the defect site. By using Mg 2 Si as a case study, we show how both the chemical interactions and molecular orbitals (i. e., wave functions) responsible for the charge state(s) of a defect can be understood from the bonding symmetry of the defect site. We anticipate that a chemistry-based perspective of charged defects will enrich defect engineering efforts for electronic and optical materials.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Local structure and Ca/Si ratio in C-S-H gels from hydration of blends of tricalcium silicate and silica fume

At the microscale, C-S-H gels from alite, or neat Portland cements, has a Ca/Si ratio close to 1.80. At the nanoscale, C-S-H is described by a defective tobermorite structure which allows a maximum Ca/Si ratio close to 1.40. There is no agreement in the location of the extra 0.40 mol of Ca(OH){sub 2} at the nanoscale. Atomistic modelling studies reported Ca(OH){sub 2} species within the tobermorite interlayer space. Other works point toward a fine intermixing of defective tobermorite and nanoportlandite. Here, we have prepared a series of alite blended with silica fume and studied the pastes by several techniques including synchrotron pair distribution function (PDF). In the employed conditions, the C-S-H gel formed by the pozzolanic reaction has nearly the same local structure than the primary C-S-H gel. Furthermore, differential PDF points toward Ca(OH){sub 2} excess having a local structure compatible with few-layer thick nanoparticles stretched along the c-axis.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Understanding of a Novel Irradiation-induced Nanostructuring Process in SiC layer of TRISO fuel particles via Transmission Electron Microscopy

Metallic fission products are found to transport through the SiC layer of the tristructural isotropic (TRISO) nuclear fuel particle and various studies have been undertaken to understand the transport behavior [1, 2]. While the grain boundaries in SiC acts as a primary pathway to dissipate fission products in the form of multi-phased complex compounds, intragranular fission product precipitation has always been observed. Research on neutron irradiation damage tolerance of silicon carbide (SiC) and its composites, used in TRISO fuel, light water reactors (LWR) fuel cladding etc., has traditionally ignored the irradiation-induced “polymorphism” of SiC. In the post-irradiation advanced microscopy effort at Idaho National Laboratory, it has been recently discovered that the intragranular precipitation of fission products in SiC layer of TRISO coated fuels takes place via a dual step nucleation mechanism that involves cubic (?) ?hexagonal (?) SiC polymorphic transition, and subsequent metamorphosis of ?-SiC into fission product precipitates, as shown in Figure 1 [3]. This precipitation behavior cannot be explained by any reported nucleation mechanism. The extent of this precipitation has been observed to be influenced by the nature of the neutron irradiation induced damage structures. These damage structures (e.g., defect morphology and density) are also indicative of the irradiation temperature and neutron fluence levels experimented by the individual TRISO particles. The distinction of neutron irradiation damage structures as a function of the fuel kernel type, burn up level and fabrication histories of TRISO coated particles from the Advanced Gas reactor (AGR-1 and AGR-2) experiments has been examined and compared. This discovery not only adds new fundamental understanding of physical behavior of polymorphic ceramics under irradiation, but also provides the following beneficial traits for high temperature applications. 1) Since the extent of precipitation of the desired phase is directly dependent on the amount of precursor phase, the volume fraction of desired phase can be easily controlled by irradiation parameters. This eliminates the conventional limitation of volume fraction of a desired phase in the system as outlined by the phase diagram of the specific system. 2) Most of the precipitate-strengthened materials for high temperature applications suffer from degradation of size and shape of precipitate upon prolonged thermal exposures. But, precipitates are confined to the shape and size of the morphological template of surrogate phase (e.g. a Pd silicide precipitate is confined by the ?-SiC as shown in Figure 1(c)) in the present nucleation pathway. Hence, there is no possibility of desired phase to undergo any morphological degradation at high temperature that eventually will extend the lifetime of these materials at high temperature.

42 ENGINEERING↗
DOE OSTI2018

IR Thermographic Analysis of 3D Printed CFRP Reference Samples with Back-Drilled and Embedded Defects

Carbon-fiber composite structures may demonstrate a defective behavior due to manufacturing induced anomalies (delamination, dis-bonds) or service related defectives (impact damage, water ingress). Thus, there is a need for a relatively fast and low cost non-intrusive testing schemes such as infrared thermography (IRT). Still, thermography testing requires calibrated samples and coupons to yield best results. The presented research demonstrates the novel use of 3D printing technology to generate IRT calibration samples. In this text, two carbon fiber reinforced polymer samples are 3D printed; the first mimics a 'back-drilled holes' type coupons, while the other is designed to embed air pockets similar to Teflon inserts. The generated samples are then tested using two IRT modalities; namely pulse thermography and lock-in thermography. Furthermore, the resulted thermograms are processed using a principle component analysis, to help highlight the variance of defectives in a consistent manner among the samples. This research findings offer insights on the variation of detectability between embedded and back-printed samples, which might be due to the inserts thickness.

36 MATERIALS SCIENCE↗