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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 937 records · Page 52
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Computational Characterization of Defects in Metal–Organic Frameworks: Spontaneous and Water-Induced Point Defects in ZIF-8

Zeolitic imidazolate frameworks (ZIFs) are an important class of porous crystalline metal–organic framework (MOF) materials that have attracted widespread attention for applications ranging from gas adsorption and separation to catalysis. Although the bulk crystal structures of MOFs are typically well-characterized, comparatively little is known regarding MOF defect structures. Drawing on analogies with conventional silicon-based zeolites, we utilize computational methods to examine the structure and stability of putative point-defect structures (including vacancies, substitutions, and “dangling” linkers) within the prototypical ZIF-8 structure. Considering both postsynthetic (gas-phase) and synthetic (solution-phase) conditions, we find that several of the defect structures lie low in energy relative to the defect-free parent crystal, with barriers to defect formation that are large but surmountable under relevant temperatures. Furthermore, these results are consistent with prior experimental observations of ZIF stability and reactivity and suggest that defects may play an important role in influencing the long-term stability of MOFs under conditions that include exposure to water vapor and trace contaminants such as acid gases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Understanding effects of conductive additives in lithium-sulfur batteries

Since 2009, lithium-sulfur (Li-S) batteries have been attracting much research interest, ascribed to their high energy density and cost-effectiveness. However, their S cathodes are harassed by the poor conductivity, shuttling of lithium polysulfides (LPSs), and large volume change. In improving conductivity of S cathodes, carbon materials are widely used as conductive additives. However, there still lacks a comparative study on their geometric and structural effects in Li-S batteries. To this end, a systematic investigation is conducted on two different typical conductive additives, carbon black (CB) 1 and nitrogen-doped graphene nanosheets (N-GNS) 2 . This work reveals that, compared to the zero-dimensional (0D) CB nanoparticles, the two-dimensional (2D) N-GNS is structurally less defective and has a larger surface area. Furthermore, as a result, the 2D structure of the N-GNS helps achieve higher capacity retention but inhibits Li-ion transportation. In comparison, the 0D structure of the CB facilitates the transportation of Li-ions with a higher initial capacity but exposed to a severer shuttling behavior of LPSs. It is also found that a combination of the CB and N-GNS enables better performance of Li-S batteries, in terms of sustainable capacity, Coulombic efficiency, and rate capability. This study is inspiring for designing better S cathodes for Li-S batteries.

25 ENERGY STORAGE↗
DOE OSTI2016

DEVELOPMENT OF INTERATOMIC POTENTIALS IN TUNGSTEN-RHENIUM SYSTEMS

Reference data are generated using the ab initio method to fit interatomic potentials for the W-Re system. The reference data include single phases of W and Re, strained structures, slabs, systems containing several concentrations of vacancies, systems containing various types of interstitial defects, melt structures, structures in the σ and χ phases, and structures containing several concentrations of solid solutions of Re in bcc W and W in hcp Re. Future work will start the fitting iterations.

Density Functional Theory↗
DOE OSTI2020

Modeling Nondestructive Defect Detection in Additively Manufactured Metallic Structures for Nuclear Applications

The future of quickly, economically produced metallic nuclear reactor parts with minimal supply-chain dependence lies in Laser Powder Bed Fusion (LPBF) Additive Manufacturing (AM): a 3D printing method involving laser melting and net shaping stainless steel and Inconel metallic powder into a solid structure. However, intrinsic features in LPBF frequently leads to the formation of materials defects, such as pores, within 3D printed structures. As safe long-term use in energy applications requires knowledge of all relevant defects before deployment in a reactor, we must develop methods for nondestructive detection of these defects. We are investigating Pulsed Thermal Tomography (PTT), which is a non-contact nondestructive imaging method scalable to arbitrary structure size. Thermal tomography (TT) is a computational method for 3D spatial reconstruction of material thermal effusivity from flash or pulsed thermography temperature data cube. Thermography data cube consists of 2D surface temperature measurements at different times. The objective of the present work is to investigate limits on defect detection in AM metallic structures with PTT. To this effect, we modeled PTT with COMSOL heat transfer computer simulations. We developed a layered media COMSOL simulation consisting of a Stainless Steel 316 (SS316) plate with an internal layer of un-sintered SS316 powder. Thermophysical properties of the powder layer were modeled with equivalent volume mixing model. To account for partial sintering at the boundary of the defect, the transition between solid and powder layers was modeled as a Gaussian. Using data from COMSOL simulations, we reconstructed depth-dependent thermal effusivity, which allowed defect visibility estimation. A series of parametric studies determined that at 1mm depth, 50µm is the smallest detectable defect. In addition, classification of the defects which can lead to early fatigue of the metallic structure in a reactor is briefly discussed.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2023

NanoMechanics: Elasticity and Friction in Nano‐Objects

A large scientific and technological effort is underway to investigate the properties of two‐dimensional (2D) materials to become building blocks in integrated nano‐electronic and photonic circuits, composites, coatings, energy harvesting nano‐ systems, nano‐sensors, and nano‐electro‐mechanical systems (NEMS).While several experiments and calculations have revealed exciting novel phenomena in these nanostructures, many scientific and technological questions remain open. In particular, understanding and controlling the structure and mechanical properties of 2D materials at the interface with a solid surface is of key importance to enable the aforementioned nanotechnologies, as well as to fulfill the potential of strain engineering to tailor their properties. The vision of this DoE research program was to investigate novel mechanical properties and phenomena in 1D and 2D materials with the overarching goal of defining a new basic understanding of mechanical behavior in nano and quantum systems. The group of the PI has developed in the last years several studies on the mechanical properties of Carbon nanotubes and oxide nanobelts, more recently the PI has focused her attention on the properties of two‐dimensional materials, such as graphene and MoS 2 , which are a few‐atomic‐layer thick films and hold a great potential for technological and energy applications. The most studied 2D material is graphene, existing as a single layer of graphite or a few‐layer thick epitaxial graphene film. Graphene possesses a large in‐plane Young’s modulus as well as high intrinsic carrier mobility, and high in‐plane thermal conductivity. Besides graphene, also 2D films of graphene oxide (GO), hexagonal Boron Nitride (h‐BN), and transition metal dichalcogenides such as MoS 2 exhibit unique and excellent properties and hold great promise for nanotechnology applications. This research program was aimed at developing a new basic understanding of the mechanical properties of 1D and 2D materials at the interface with solid substrates, with a focus on the role of defects, materials structure, and substrate interaction. Ultimately, we aimed at developing the basic knowledge and experimental tools for manipulating 1D and 2D materials’ structure, stacking, substrate interaction, defects, and number of layers to realize an entirely new class of ultra‐hard, ultra‐thin, and ultra‐light active materials with ad‐ hoc mechanical properties for a variety of applications.

2D Materials↗
DOE OSTI2018

Structure and Voltage Recovery Driven by Defects Elimination in Li-Rich Layered Oxide Cathode Materials

Lithium-ion batteries (LIBs) are ubiquitous in a wide range of technologies, including cell phones, electric vehicles, and energy grid, and are indispensable in the transition toward sustainable energy technology. The battery performance is inherently connected to the capability of lithium ions to reversibly intercalate with the electrodes. The capacity of classical layered transition metal (TM) oxides, the primary commercial cathode materials, is limited to cationic redox activity. Anionic redox has thus emerged as a new paradigm for designing novel cathodes for next generation LIBs. It is recently confirmed oxygen redox in lithium-rich layered oxides (LRLO) with composition xLi 2 MnO 3 ∙(1-x)LiTMO 2 , which enables this group of materials exhibit reversible capacities exceeding 280 mAh g -1 . Unfortunately, this material undergoes voltage fading and instability that is rooted in irreversible structural transformation in terms of defects formation including lithium and oxygen vacancies, lithium tetrahedron, TM migration, edge dislocations, stacking fault, and local strain. Modification methods including morphology control, composition gradient, surface treatment, foreign elements doping, etc., have been proposed in the literatures to mitigate the defects formation and maintain the structure reversibility. Despite their improvement, the intrinsic voltage fading is still inevitable and remains a challenge that will require considerable effort to overcome. In this work, we demonstrate a path to recover the layer structure and working voltage through high-temperature annealing. The treatment at high temperature recovers the local Li-excess environments around oxygen, oxygen stacking sequence, and eliminates microstrain associated with different defects. As a result, the voltage profile once more shows plateau region during charge and the average discharge voltage is restored of the cycled cathode after heat treatment. Both theoretical calculations and experimental characterizations illustrate the mechanisms underlying the heating induced recovery. The structure metastability and defects removal are decisive in structure and voltage recovery of LRLO cathode material.

Zhang, Minghao↗
DOE OSTI2014

Structure analysis of a Mn-doped willemite-type compound, H{sub 0.12}(Zn{sub 1.89(3)}Mn{sub 0.05(1)}□{sub 0.06})Si{sub 1.00(1)}O{sub 4}

Large single crystals of a well-known green phosphor, Mn-doped willemite (α-Zn{sub 2}SiO{sub 4})-type compound were grown from molten MnF{sub 2}/Li{sub 2}MoO{sub 4}. In this structure Mn{sup 2+} and defects partially replace Zn{sup 2+} at two independent sites, and there are no cation species in the middle of six-membered ring channels. As a consequence of 3% defective sites (□), this structure may have protons for ensuring its electrostatic neutrality. Although the position of H{sup +} could not be clearly found the present study indicates the presence of protons in the vicinity of oxygens to make relative short H⋯O bonds below 1.6 Å. Two unusual, broad IR bands about 1780 and 2360 cm{sup −1} can be explained by disordering of strongly bonded O···H groups. This compound represents a structural unit, H{sub 0.12}(Zn{sub 1.89(3)}Mn{sub 0.05(1)}□{sub 0.06})Si{sub 1.00(1)}O{sub 4}, based on X-ray single crystal diffraction, electron microprobe, and FT-IR spectroscopy. - Graphical abstract: A willemite-type compound, H{sub 0.12}(Zn{sub 1.89(3)}Mn{sub 0.05(1)}□{sub 0.06})Si{sub 1.00(1)}O{sub 4} exhibits protons near to oxygens in the skirt of four- and six-membered ring channels, as a result of defects at two crystallographically different Zn sites. Display Omitted - Highlights: • Structure properties of a 0.05 Mn-doped α-Zn{sub 2}SiO{sub 4} compound using X-ray single crystal diffraction. • Static disorder of Zn, Mn, and defects in the framework of α-Zn{sub 2}SiO{sub 4}. • Sign of strong H⋯O bonds, observed in FT-IR spectroscopy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Crystal structure of HgGa{sub 2}Se{sub 4} under compression

Highlights: ► Single crystals of HgGa{sub 2}Se{sub 4} with defect-chalcopyrite structure were synthesized. ► HgGa{sub 2}Se{sub 4} exhibits a phase transition to a disordered rock salt structure at 17 GPa. ► HgGa{sub 2}Se{sub 4} undergoes a phase transition below 2.1 GPa to a disordered zinc blende. - Abstract: We report on high-pressure x-ray diffraction measurements up to 17.2 GPa in mercury digallium selenide (HgGa{sub 2}Se{sub 4}). The equation of state and the axial compressibilities for the low-pressure tetragonal phase have been determined and compared to related compounds. HgGa{sub 2}Se{sub 4} exhibits a phase transition on upstroke toward a disordered rock-salt structure beyond 17 GPa, while on downstroke it undergoes a phase transition below 2.1 GPa to a phase that could be assigned to a metastable zinc-blende structure with a total cation-vacancy disorder. Thermal annealing at low- and high-pressure shows that kinetics plays an important role on pressure-driven transitions.

36 MATERIALS SCIENCE↗
DOE OSTI2022

3D Bragg Coherent Diffraction Imaging of Extended Nanowires: Defect Formation in Highly Strained InGaAs Quantum Wells

InGaAs quantum wells embedded in GaAs nanowires can serve as compact near-infrared emitters for direct integration onto Si complementary metal oxide semiconductor technology. While the core–shell geometry in principle allows for a greater tuning of composition and emission, especially farther into the infrared, the practical limits of elastic strain accommodation in quantum wells on multifaceted nanowires have not been established. One barrier to progress is the difficulty of directly comparing the emission characteristics and the precise microstructure of a single nanowire. Here, in this study, we report an approach to correlating quantum well morphology, strain, defects, and emission to understand the limits of elastic strain accommodation in nanowire quantum wells specific to their geometry. We realize full 3D Bragg coherent diffraction imaging (BCDI) of intact quantum wells on vertically oriented epitaxial nanowires, which enables direct correlation with single-nanowire photoluminescence. By growing In 0.2 Ga 0.8 As quantum wells of distinct thicknesses on different facets of the same nanowire, we identified the critical thickness at which defects are nucleated. A correlation with a traditional transmission electron microscopy analysis confirms that BCDI can image the extended structure of defects. Finite element simulations of electron and hole states explain the emission characteristics arising from strained and partially relaxed regions. This approach, imaging the 3D strain and microstructure of intact nanowire core–shell structures with application-relevant dimensions, can aid the development of predictive models that enable the design of new compact infrared emitters.

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

Atomistic determination of the surface structure of Cu 2 O(111): experiment and theory

Cuprous oxide (Cu 2 O) is a promising catalyst for several important reactions. However, the atomic structures of defective Cu 2 O surfaces, which critically affect the catalytic properties both thermodynamically and kinetically, are not unambiguously characterized. High-resolution scanning tunneling microscopy (STM), combined with density functional theory (DFT) calculations and STM simulations, has been used to determine the atomic structure of the (111) surface of a Cu 2 O bulk crystal. The single crystal surface, processed by ultrahigh vacuum cleaning and oxygen annealing, shows a (1 × 1) periodicity in the low-energy electron diffraction pattern. The pristine (defect-free) Cu 2 O(111) surface exhibits a lattice of protrusions with hexagonal symmetry under STM, which is attributed to the dangling bonds of the coordinatively unsaturated copper (Cu U ) atoms on the surface. Two types of surface atomic defects are also identified, including the Cu U vacancy and the oxygen-vacancy-induced local surface restructuring. The electronic structure of this surface measured by $dI/dV$ spectroscopy shows an energy band gap of ~1.6–2.1 eV. Consistent with $dI/dV$ measurements, DFT calculations identified surface states within the electronic band gap arising from the Cu ions on the surface. Our results provide a clear picture of the pristine and defective Cu 2 O(111) surface structure in addition to the formation mechanism of the reconstructed surface, paving the way toward studying the site-dependent reactivity of this surface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Entropy-defect synergy for dual luminescence mechanism in spinel: Time-resolved anti-counterfeiting and fingerprint visualization

Multimodal luminescent materials, while promising for anti-counterfeiting, often lack dynamic time-dependent responses and controllable spatial distribution, limiting their encryption capabilities in the spatiotemporal dimension. Here, this work presents a coordinated control strategy based on entropy and defect engineering, and uses a backpropagation (BP) neural network for material screening to successfully prepare spinel Mg 0.8 (Fe 0.04 Co 0.04 Ni 0.04 Cu 0.04 Zn 0.04 )Cr 2 O 4 (MgA 5 CO) phosphors with time-dependent dynamic luminescence behavior. This phosphor simultaneously activated the d-d transition luminescence (∼618 nm) derived from Co 2+ /Cr 3+ and the defect luminescence (∼398 nm) related to zinc vacancies (V Zn ) in a single-phase solid solution. The phosphor exhibits a time-dependent color evolution from pink to purple under fixed-wavelength excitation, due to the different excited-state dynamics and decay lifetimes associated with the d-d transition and defect luminescence. Structural characterization and spectral analysis confirmed the existence of V Zn and its significant role in defect luminescence process. The fluorescent and dynamic luminescent properties of entropy-based spinel oxide enable its use in advanced anti-counterfeiting applications like fingerprint recognition and color-changing dedicated anti-counterfeiting mark, showing promise in high-end and time-dynamic anti-counterfeiting fields. This research not only developed a new type of fluorescent dynamic anti-counterfeiting material, but also provided a new idea for constructing advanced optical functional materials with multiple luminescence mechanisms.

Defect project↗
DOE OSTI2022

Automated Defect Identification for Tri-structural Isotropic Fuels (AUDIT)

During the manufacture of tri-structural isotropic (TRISO)-coated nuclear fuel particles, the potential exists for the formation of internal fissure defects in the uranium oxycarbide (UCO) kernels. These fissures result in a defective fuel particle that can fracture during subsequent fuel processing. Therefore, it is necessary to detect the presence of fissured kernels in a batch to determine if the batch meets specification prior to blending with other batches and upgrading processes. Previous attempts at identifying fissures involved manual inspection of micrographs of UCO fuel kernel cross-sections. This process is tedious, time-consuming and may introduce counting errors making it a good candidate for automation. This work presents a method for the automated detection of fissures in UCO kernels. Image segmentation is used for the extraction of relevant features in the micrographs which then serve as the input to a convolutional neural network used to automatically distinguish between fissured and non-fissured kernels.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2015

Atomically resolved structural determination of graphene and its point defects via extrapolation assisted phase retrieval

Previously reported crystalline structures obtained by an iterative phase retrieval reconstruction of their diffraction patterns seem to be free from displaying any irregularities or defects in the lattice, which appears to be unrealistic. We demonstrate here that the structure of a nanocrystal including its atomic defects can unambiguously be recovered from its diffraction pattern alone by applying a direct phase retrieval procedure not relying on prior information of the object shape. Individual point defects in the atomic lattice are clearly apparent. Conventional phase retrieval routines assume isotropic scattering. We show that when dealing with electrons, the quantitatively correct transmission function of the sample cannot be retrieved due to anisotropic, strong forward scattering specific to electrons. We summarize the conditions for this phase retrieval method and show that the diffraction pattern can be extrapolated beyond the original record to even reveal formerly not visible Bragg peaks. Such extrapolated wave field pattern leads to enhanced spatial resolution in the reconstruction.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

WE-E-18A-01: Large Area Avalanche Amorphous Selenium Sensors for Low Dose X-Ray Imaging

Purpose: A large area indirect flat panel imager (FPI) with avalanche gain is being developed to achieve x-ray quantum noise limited low dose imaging. It uses a thin optical sensing layer of amorphous selenium (a-Se), known as High-Gain Avalanche Rushing Photoconductor (HARP), to detect optical photons generated from a high resolution x-ray scintillator. We will report initial results in the fabrication of a solid-state HARP structure suitable for a large area FPI. Our objective is to establish the blocking layer structures and defect suppression mechanisms that provide stable and uniform avalanche gain. Methods: Samples were fabricated as follows: (1) ITO signal electrode. (2) Electron blocking layer. (3) A 15 micron layer of intrinsic a-Se. (4) Transparent hole blocking layer. (5) Multiple semitransparent bias electrodes to investigate avalanche gain uniformity over a large area. The sample was exposed to 50ps optical excitation pulses through the bias electrode. Transient time of flight (TOF) and integrated charge was measured. A charge transport simulation was developed to investigate the effects of varying blocking layer charge carrier mobility on defect suppression, avalanche gain and temporal performance. Results: Avalanche gain of ∼200 was achieved experimentally with our multi-layer HARP samples. Simulations using the experimental sensor structure produced the same magnitude of gain as a function of electric field. The simulation predicted that the high dark current at a point defect can be reduced by two orders of magnitude by blocking layer optimization which can prevent irreversible damage while normal operation remained unaffected. Conclusion: We presented the first solid state HARP structure directly scalable to a large area FPI. We have shown reproducible and uniform avalanche gain of 200. By reducing mobility of the blocking layers we can suppress defects and maintain stable avalanche. Future work will optimize the blocking layers to prevent lag and ghosting.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2019

Defect-Induced (Dis)Order in Relaxor Ferroelectric Thin Films

The effect of intrinsic point defects on relaxor properties of 0.68 $\mathrm{PbMg_{1/3}Nb_{2/3}O_{3}-0.32 PbTiO_{3}}$ thin films is studied across nearly 2 orders of magnitude of defect concentration via ex post facto ion bombardment. A weakening of the relaxor character is observed with increasing concentration of bombardment-induced point defects, which is hypothesized to be related to strong interactions between defect dipoles and the polarization. Finally, although more defects and structural disorder are introduced in the system as a result of ion bombardment, the special type of defects that are likely to form in these polar materials (i.e., defect dipoles) can stabilize the direction of polarization against thermal fluctuations, and in turn, weaken relaxor behavior.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Interplay between defect transport and cation spin frustration in corundum-structured oxides

Despite the fundamental importance of mass transport, there is still a lack of knowledge regarding the mechanisms in many materials. In particular, how the magnetic spin structure interacts with migrating defects is unclear. In this work, using density functional theory to examine cation interstitial transport in antiferromagnetic corundum-structured oxides, we reveal an interplay between the migration of the interstitial and the magnetic structure of the oxide. The magnetic spin configuration impacts the migration energy of the migrating defect but, critically, the defect modifies the spin configuration. Thus, the two aspects change in concert to modify one another. This has profound implications for mass transport in magnetic materials.

36 MATERIALS SCIENCE↗