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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 OSTI2014

Energy resolved electrochemical impedance spectroscopy for electronic structure mapping in organic semiconductors

We introduce an energy resolved electrochemical impedance spectroscopy method to map the electronic density of states (DOS) in organic semiconductor materials. The method consists in measurement of the charge transfer resistance of a semiconductor/electrolyte interface at a frequency where the redox reactions determine the real component of the impedance. The charge transfer resistance value provides direct information about the electronic DOS at the energy given by the electrochemical potential of the electrolyte, which can be adjusted using an external voltage. A simple theory for experimental data evaluation is proposed, along with an explanation of the corresponding experimental conditions. The method allows mapping over unprecedentedly wide energy and DOS ranges. Also, important DOS parameters can be determined directly from the raw experimental data without the lengthy analysis required in other techniques. The potential of the proposed method is illustrated by tracing weak bond defect states induced by ultraviolet treatment above the highest occupied molecular orbital in a prototypical σ-conjugated polymer, poly[methyl(phenyl)silylene]. The results agree well with those of our previous DOS reconstruction by post-transient space-charge-limited-current spectroscopy, which was, however, limited to a narrow energy range. In addition, good agreement of the DOS values measured on two common π-conjugated organic polymer semiconductors, polyphenylene vinylene and poly(3-hexylthiophene), with the rather rare previously published data demonstrate the accuracy of the proposed method.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

In-situ ETEM study of plasma-facing tungsten nanofuzz oxidation at atmospheric pressure: Microstructure evolution and substrate-free oxidation kinetics

To enable sustainable carbon-free fusion energy, managing reactor structural material degradation during normal operation as well as accident scenarios is vital. Tungsten (W) plasma-facing materials (PFMs) are susceptible to aggressive high-temperature oxidation during air-ingress fusion reactor accidents, yet there's a lack of oxidation kinetic data for irradiated tungsten. Here, in this study, we utilize atmospheric environmental transmission electron microscopy (ETEM) to present the first kinetic data for substrate-free W nanofuzz oxidation at 400 ºC and 500 ºC in 1 bar dry air. Comparison with pristine bulk W during the early parabolic stage suggests an irradiationdecelerated oxidation for W nanofuzz. Our time-resolved in-situ characterization reveals a durable amorphous surface oxide, likely promoted by high-flux He + irradiation-induced surface defects, serving as an effective passivating layer that impedes nanofuzz oxidation onset. This surface oxide layer also interfaces well with newly formed orthorhombic WO 3 , facilitated by stress relief through He bubble shrinkage, providing lasting passivating protection throughout the nanofuzz parabolic oxidation. This new finding challenges conventional notions of irradiation's negative impact on metal oxidation, and calls for advanced characterization to enhance our understanding of fusion energy materials degradation, informed by further accident modeling.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Effects of irradiation on the interface between U-Mo and zirconium diffusion barrier

Irradiated fuel plates were characterized by microscopy that focused on the interface between U–Mo and Zr. Before irradiation, there were three major sub-layers identified in the U–Mo/Zr interface, namely, UZr 2 , Mo 2 Zr, and U with low Mo. The typical total thickness of this U–Mo/Zr interaction is 2–3 microns. The UZr 2 sub-layer formed during fuel fabrication remains stable after irradiation, without large bubbles/porosity accumulation. However, this sub-layer becomes increasingly discontinuous as burnup increases. The low-Mo sub-layer exhibits numerous sub-micron bubbles/porosity at low burnup. Larger, interconnected porosity in this sub-layer was observed in a medium-burnup fuel specimen. However, at higher burnup, regions with the extra-large bubbles/porosity (i.e., larger than 5 ?m) were observed at least 5 microns away from the original location of this sub-layer. The mechanism for the formation of the extra-large bubbles/porosity is still unclear at this time. In general, the U–Mo/Zr interface in monolithic U–Mo fuels is relatively stable after irradiation. As a result, no large detrimental defects, such as large interfacial bubbles or cracks/delamination, were observed in the fuel plates characterized.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2019

MoS 2 –OH Bilayer-Mediated Growth of Inch-Sized Monolayer MoS 2 on Arbitrary Substrates

Due to remarkable electronic property, optical transparency, and mechanical flexibility, monolayer molybdenum disulfide (MoS 2 ) has been demonstrated to be promising for electronic and optoelectronic devices. To date, the growth of high-quality and large-scale monolayer MoS 2 has been one of the main challenges for practical applications. In this paper, we present a MoS 2 –OH bilayer-mediated method that can fabricate inch-sized monolayer MoS 2 on arbitrary substrates. This approach relies on a layer of hydroxide groups (-OH) that are preferentially attached to the (001) surface of MoS 2 to form a MoS 2 –OH bilayer structure for growth of large-area monolayer MoS 2 during the growth process. Specifically, the hydroxide layer impedes vertical growth of MoS 2 layers along the [001] zone axis, promoting the monolayer growth of MoS 2 , constrains growth of the MoS 2 monolayer only in the lateral direction into larger area, and effectively reduces sulfur vacancies and defects according to density functional theory calculations. Finally, the hydroxide groups advantageously prevent the MoS 2 from interface oxidation in air, rendering high-quality MoS 2 monolayers with carrier mobility up to ~30 cm 2 V –1 s –1 . Using this approach, inch-sized uniform monolayer MoS 2 has been fabricated on the sapphire and mica and high-quality monolayer MoS 2 of single-crystalline domains exceeding 200 μm has been grown on various substrates including amorphous SiO 2 and quartz and crystalline Si, SiC, Si 3 N 4 , and graphene Finally, this method provides a new opportunity for the monolayer growth of other two-dimensional transition metal dichalcogenides such as WS 2 and MoSe 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2018

Discrete shear band plasticity through dislocation activities in body-centered cubic tungsten nanowires

Shear band in metallic crystals is localized deformation with high dislocation density, which is often observed in nanopillar deformation experiments. The shear band dynamics coupled with dislocation activities, however, remains unclear. Here, we investigate the dynamic processes of dislocation and shear band in body-centered cubic (BCC) tungsten nanowires via an integrated approach of in situ nanomechanical testing and atomistic simulation. We find a strong effect of surface orientation on dislocation nucleation in tungsten nanowires, in which {111} surfaces act as favorite sites under high strain. While dislocation activities in a localized region give rise to an initially thin shear band, self-catalyzed stress concentration and dislocation nucleation at shear band interfaces cause a discrete thickening of shear band. Our findings not only advance the current understanding of defect activities and deformation morphology of BCC nanowires, but also shed light on the deformation dynamics in other microscopic crystals where jerky motion of deformation band is observed.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Synergy of elastic and inelastic energy loss on ion track formation in SrTiO 3

While the interaction of energetic ions with solids is well known to result in inelastic energy loss to electrons and elastic energy loss to atomic nuclei in the solid, the coupled effects of these energy losses on defect production, nanostructure evolution and phase transformations in ionic and covalently bonded materials are complex and not well understood due to dependencies on electron-electron scattering processes, electron-phonon coupling, localized electronic excitations, diffusivity of charged defects, and solid-state radiolysis. Here we show that a colossal synergy occurs between inelastic energy loss and pre-existing atomic defects created by elastic energy loss in single crystal strontium titanate (SrTiO 3 ), resulting in the formation of nanometer-sized amorphous tracks, but only in the narrow region with pre-existing defects. These defects locally decrease the electronic and atomic thermal conductivities and increase electron-phonon coupling, which locally increase the intensity of the thermal spike for each ion. This work identifies a major gap in understanding on the role of defects in electronic energy dissipation and electron-phonon coupling; it also provides insights for creating novel interfaces and nanostructures to functionalize thin film structures, including tunable electronic, ionic, magnetic and optical properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Quantification of the loss mechanisms in emerging water splitting photoanodes through empirical extraction of the spatial charge collection efficiency

We present that the operando quantification of surface and bulk losses is key to developing strategies for optimizing photoelectrodes and realizing high efficiency photoelectrochemical solar energy conversion systems. This is particularly true for emerging thin film semiconductors, in which photocarrier diffusion lengths, surface and bulk recombination processes, and charge separation and extraction limitations are poorly understood. Insights into mechanisms of efficiency loss can guide strategies for nanostructuring photoelectrodes, engineering interfaces, and incorporating catalysts. However, few experimental methods are available for direct characterization of dominant loss processes under photoelectrochemical operating conditions. In this work, we provide insight into the function and limitations of an emerging semiconductor photoanode, γ-Cu 3 V 2 O 8 , by quantifying the spatial collection efficiency (SCE), which is defined as the fraction of photogenerated charge carriers at each point below the surface that contributes to the measured current. Analyzing SCE profiles at different operating potentials shows that increasing the applied potential primarily acts to reduce surface recombination rather than to increase the thickness of the space charge region under the semiconductor/electrolyte interface. Comparing SCE profiles obtained with and without a sacrificial reagent allows surface losses from electronically active defect states to be distinguished from performance bottlenecks arising from slow reaction kinetics. Combining these insights promotes a complete understanding of the photoanode performance and its potential as a water splitting photoanode. More generally, application of the SCE extraction method can aid in the discovery and evaluation of new materials for solar water splitting devices by providing mechanistic details underlying photocurrent generation and loss.

14 SOLAR ENERGY↗
DOE OSTI2022

Electronic structure of epitaxially grown and regrown GaN pn junctions characterized by scanning Kelvin probe and capacitance microscopy

Epitaxial regrowth of GaN pn junctions is a key technology for realization of a variety of high-performance GaN power electronic devices. However, the regrowth process can introduce impurities and defects that degrade a device’s performance. Here, we show that scanning Kelvin probe force microscopy and scanning capacitance microscopy can be used in a cross-sectional geometry to probe dopant distributions and an electronic structure in epitaxially grown GaN pn junctions. These measurements enable profiling of potential and dopant distributions across GaN pn junctions produced by uninterrupted epitaxial growth and by regrowth on an etched surface. Clear differences are observed in comparisons to the electronic structure of these two types of junctions that can be correlated with results of complementary characterization of dopant distributions reported for similarly grown structures. These measurements also suggest the presence of defects in etch-and-regrow pn junction structures that extend nearly 1 μm below the regrown interface.

Kim, Tae-Hyeon (ORCID:0000000341530617)↗
DOE OSTI2018

Interplay of Structure, Elasticity, and Dynamics in Actin-Based Nematic Materials

Achieving control and tunability of lyotropic materials has been a long-standing goal of liquid crystal research. Here we show that the elasticity of a liquid crystal system consisting of a dense suspension of semiflexible biopolymers can be manipulated over a relatively wide range of elastic moduli. Specifically, thin films of actin filaments are assembled at an oil-water interface. At sufficiently high concentrations, one observes the formation of a nematic phase riddled with +/- 1/2 topological defects, characteristic of a two-dimensional nematic system. As the average filament length increases, the defect morphology transitions from a U shape into a V shape, indicating the relative increase of the material's bend over splay modulus. Furthermore, through the sparse addition of rigid microtubule filaments, one can gain additional control over the liquid crystal's elasticity. We show how the material's bend constant can be raised linearly as a function of microtubule filament density, and present a simple means to extract absolute values of the elastic moduli from purely optical observations. Finally, we demonstrate that it is possible to predict not only the static structure of the material, including its topological defects, but also the evolution of the system into dynamically arrested states. Despite the nonequilibrium nature of the system, our continuum model, which couples structure and hydrodynamics, is able to capture the annihilation and movement of defects over long time scales. Thus, we have experimentally realized a lyotropic liquid crystal system that can be truly engineered, with tunable mechanical properties, and a theoretical framework to capture its structure, mechanics, and dynamics.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Do Flashing and Sealant Materials Need to be Exposed to Extreme Cycling?

In the U.S., over 50% of homes were built before 1980, when minimal building codes existed. The aging of building envelopes can significantly contribute to the air leakage, moisture intrusion, and durability of buildings leading to higher energy demands. Although the durability of building materials and envelope systems has been studied extensively, the durability of specific components like window-wall flashing and sealants remains underexplored. Current standards such as ASTM, NIST, and AAMA primarily focus on durability testing considering the high temperatures only and require sealant testing on anodized aluminum panels, leaving questions about performance in cold or fluctuating temperatures unanswered. Current ASTM and AAMA standards provide guidelines for durability testing but predominantly focus on exposure to higher temperature conditions. It remains unclear how exposure to cold temperatures, or cycles of both hot and cold conditions, affect the durability of these materials. Furthermore, AAMA standards mandate that flashing materials be tested on anodized aluminum panels, which may not reflect real-world applications. Therefore, this initial study aims to explore this concern through testing by constructing two window-wall interface prototypes and applying five different flashing and sealant materials to their joints. These samples were subjected to extreme temperatures, ranging from −34 ℃ (−30 °F) to 50 ℃ (122 °F), over a 28-day period (fourteen cycles of 24 hours), with daily visual inspections to document any signs of deterioration such as wrinkling, distortion, or warping. The study has two main objectives: (1) to determine whether flashing and sealant materials lose their durability and strength when exposed to varying climate conditions, specifically cold climates, and (2) to assess whether the interaction of these materials with different substrates (e.g., lumber, PVC, insulation) affects their durability and strength under varying climate conditions. The findings demonstrate how aging impacts the durability of window-wall systems interfaces, justifying future research that potentially could lead to the development of new testing protocols if significant defects are identified.

Palani, Hevar [ORNL] (ORCID:0000000220211994)↗
DOE OSTI2020

Localization and reduction of superconducting quantum coherent circuit losses

Quantum sensing and computation can be realized with superconducting microwave circuits. Qubits are engineered quantum systems of capacitors and inductors with non-linear Josephson junctions. They operate in the single-excitation quantum regime, photons of $27 \mu$eV at 6.5 GHz. Quantum coherence is fundamentally limited by materials defects, in particular atomic-scale parasitic two-level systems (TLS) in amorphous dielectrics at circuit interfaces.[1] The electric fields driving oscillating charges in quantum circuits resonantly couple to TLS, producing phase noise and dissipation. We use coplanar niobium-on-silicon superconducting resonators to probe decoherence in quantum circuits. By selectively modifying interface dielectrics, we show that most TLS losses come from the silicon surface oxide, and most non-TLS losses are distributed throughout the niobium surface oxide. Through post-fabrication interface modification we reduced TLS losses by 85% and non-TLS losses by 72%, obtaining record single-photon resonator quality factors above 5 million and approaching a regime where non-TLS losses are dominant. [1]M\"uller, C., Cole, J. H. & Lisenfeld, J. Towards understanding two-level-systems in amorphous solids: insights from quantum circuits. Rep. Prog. Phys. 82, 124501 (2019)

Altoé, M Virginia P↗
DOE OSTI2014

Crystal orientation dependence of current-perpendicular-to-plane giant magnetoresistance of pseudo spin-valves with epitaxial Co{sub 2}Fe(Ge{sub 0.5}Ga{sub 0.5}) Heusler alloy layers

The magnetoresistive (MR) properties of Co{sub 2}Fe(Ge{sub 0.5}Ga{sub 0.5}) (CFGG) Heusler alloy-based current-perpendicular-to-plane giant magnetoresistance pseudo-spin-valves (PSVs) are investigated. The PSV films are epitaxially grown on a sapphire (112{sup ¯}0) substrate with an Ag or Cu spacer layer, and their magnetoresistive properties are compared with those of PSV grown on MgO(001) substrates. For substrates with an Ag spacer, the PSV with the (001)[110]{sub CFGG}//(001)[010]{sub Ag} interface grown on MgO(001) exhibits a higher MR output compared with the (110)[001]{sub CFGG}//(111)[11{sup ¯}0]{sub Ag} interface grown on sapphire (112{sup ¯}0). In contrast, a higher MR output is obtained using a Cu spacer with the (110)[001]{sub CFGG}//(111)[11{sup ¯}0]{sub Cu} interface. These results demonstrate that the MR outputs depend upon the crystal orientation at the interface, and that interfaces with a small misfit tend to exhibit a larger MR output. This indicates the influence of crystal orientation as well as lattice mismatch upon the interfacial spin scattering asymmetry.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Tritium Trapping Thermodynamics by Point Defects at Interfacial Fe-Al Phases of the Aluminide Coating

Density functional theory (DFT) simulations have been carried out to evaluate the potential for tritium trapping by metal vacancies in four phases (i.e., (Fe, Cr, Ni), AlFe 3 , NiAl, and AlFe) identified near the interface between the Al coating and 316 stainless steel (316 SS) cladding. In addition, an ab initio thermodynamics approach has been employed to predict the temperature and T 2 -partial pressure dependence on the thermodynamics of tritiated defects. Key results in this work suggest that metal vacancies in the four phases have the potential to favorably trap tritium species. This thermodynamic trend can be correlated to the high energy cost of having an interstitial tritium in the lattice, which has been calculated to be at least 0.48 eV. By combining the results of this study with previous theoretical works investigating tritium behavior in other Fe-Al phases identified in the aluminide coating, suggest that metal vacancies are generally able to trap tritium in various Fe-Al aluminide phases. Especially, it was found that Al vacancies are the most efficient to trap tritium, followed by Fe vacancies, then Ni and Cr vacancies. Strong interactions between tritium and the metal vacancies are occurring by the formation of Fe—T bonds. The formation of Cr—T, Ni—T, or Al—T bonds are found less energetically favorable than Fe—T bonds.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2021

Defect-Driven Oxide Transformations and the Electrochemical Interphase

The redox reaction pathway is crucial to the sustainable production of the fuels and chemicals required for a carbon-neutral society. Our society is becoming increasingly dependent on devices using batteries and electrolyzers, all of which rely on a series of redox reactions. The overall properties of oxide materials make them very well suited for such electrochemical and catalytic applications due to their associated cationic redox properties and the static site-adsorbate interactions. As these technologies have matured, it has become apparent that defect-driven redox reactions, defect-coupled diffusion, and structural transformations that are both time- and rate-dependent are also critical materials processes. This change in focus, considering not only redox properties but also more complex, dynamic behaviors, represents a new research frontier in the molecular sciences as they are strongly linked to device operation and degradation and lie at the heart of various phenomena that take place at electrochemical interfaces. Fundamental studies of the structural, electronic, and chemical transformation mechanisms are key to the advancement of materials and technological innovations that could be implemented in various electrochemical systems. In this Account, we focus on recent studies and advances in characterizing and understanding the dynamic redox evolution and structural transformations that take place in model perovskites and layered oxides under reactive conditions and correlate them with degradation mechanisms and operations in electrolyzers and batteries. We show that the dynamic evolution of oxygen vacancies and cationic migration in the surface or bulk occurs at the solid-liquid interface, using a combination of different synchrotron-based Xray spectroscopies and scattering probes. Detailed redox-structure-reactivity correlation studies show how defects and diffusion processes can be tailored to drive various physical and chemical transformations in electrolyzers and batteries. We also highlight a strong correlation between oxygen redox reactivity and structural reorganization in both model thin films and particles, helping to bridge the gap between fundamental studies of the reaction mechanism and device applications. Finally, on the basis of these findings, we discuss strategies to probe and tune the redox reactivity and structural stability of the redox-active oxide interphase toward devising efficient pathways for energy and chemical harvesting.

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

Free fermion entanglement with a semitransparent interface: the effect of graybody factors on entanglement islands

We study the entanglement entropy of free fermions in 2d in the presence of a partially transmitting interface that splits Minkowski space into two half-spaces. We focus on the case of a single interval that straddles the defect, and compute its entanglement entropy in three limits: Perturbing away from the fully transmitting and fully reflecting cases, and perturbing in the amount of asymmetry of the interval about the defect. Using these results within the setup of the Poincaré patch of AdS _2 2 statically coupled to a zero temperature flat space bath, we calculate the effect of a partially transmitting AdS _2 2 boundary on the location of the entanglement island region. The partially transmitting boundary is a toy model for black hole graybody factors. Our results indicate that the entanglement island region behaves in a monotonic fashion as a function of the transmission/reflection coefficient at the interface.

Kruthoff, Jorrit↗
DOE OSTI2014

Tunable photoluminescence of self-assembled GeSi quantum dots by B{sup +} implantation and rapid thermal annealing

The layered GeSi quantum dots (QDs) are grown on (001) Si substrate by molecular beam epitaxy. The photoluminescence (PL) peak of the as-grown GeSi quantum dots has obvious blue shift and enhancement after processed by ion implantation and rapid thermal annealing. It is indicated that the blue shift is originated from the interdiffusion of Ge and Si at the interface between QDs and the surrounding matrix. The dependence of PL intensity on the excitation power shows that there are the nonradiative centers of shallow local energy levels from the point defects caused by the ion implantation, but not removed by the rapid thermal annealing. The tunable blue shift of the PL position from the 1300 nm to 1500 nm region may have significant application value in the optical communication.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Defects in nonpolar (134{sup ¯}0) ZnO epitaxial film grown on (114) LaAlO{sub 3} substrate

The defects in (134{sup ¯}0)ZnO epitaxial film grown on (114)LaAlO{sub 3} (LAO) have been systematically investigated by using transmission electron microscopy. At the ZnO/LAO interface, the Burgers vectors of misfit dislocations are identified to be 1/3[1{sup ¯}21{sup ¯}0] and 1/2[0001]. Threading dislocations with the Burgers vectors of 1/3〈112{sup ¯}0〉 and 〈0001〉 are distributed on the basal plane. In (134{sup ¯}0)ZnO film, the predominant planar defects are basal stacking faults (BSFs) with 1/6〈202{sup ¯}3〉 displacement vectors. The densities of dislocations and BSFs are about 3.8 × 10{sup 10} cm{sup −2} and 3.1 × 10{sup 5} cm{sup −1}, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗