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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 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Variation of a Defect Structure of Lithium Tetraborate (Li{sub 2}B{sub 4}O{sub 7}) in an External Electric Field

The variation of a defect structure of a lithium tetraborate single crystal under the influence of a high-strength external electric field applied along polar direction [001] has been studied by the X-ray diffraction (XDR) method. The conductivity kinetics has been measured; it is found to agree with changes in the diffraction peak parameters. Application of the electric field with the strength of 300–500 V/mm leads to a sharp broadening of the rocking curve and the increase in the integral intensity by several times, but the curve position and shape are only slightly changed. At higher electric fields from 500 to 1500 V/mm, the process of broadening the curve slows down; however, the shape asymmetry appears and the peak shifts to smaller angles, which is due to an increase in the lattice parameter along axis c. In this case, the changes become irreversible, since the distorted structure is partially recovered with a very low rate (for several months). Two types of the dependences of the rocking curves parameters variation under an external field are interpreted as the manifestation of two mechanisms of the ionic conduction due to mobile lithium (Li{sup +}) ions at low fields and oxygen vacancies (V{sub O}{sup 2+}) at higher fields. The charge carrier migration leads to an increase in the defect concentration and structural changes in a near-surface crystal region. The obtained results have practical importance from the point of view of the controlled change in the defect structure in the crystals with ionic conductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Luminescence of Pr{sup 3+} Impurity Centers and Defects in Sr{sub 9}Sc (PO{sub 4}){sub 7}:Pr{sup 3+}

The spectroscopic characteristics and luminescence decay kinetics of Sr{sub 9}Sc(PO{sub 4}){sub 7}:Pr{sup 3+}(1%) were investigated using synchrotron radiation of ultraviolet (UV), vacuum ultraviolet (VUV) and X-ray ranges, as well as pulsed cathodoluminescence (PCL) methods at temperatures of 10, 90, and 295 K. The luminescence spectra contain three groups of bands and lines. In the UV range, the observed bands correspond to interconfigurational 5d → 4f transitions in Pr{sup 3+} ions. In the visible spectrum, the wide luminescence band of defects dominates, as well as narrow lines associated with intraconfigurational 4f → 4f transitions in Pr{sup 3+} ions. When excited by an electron beam (5 Hz), the main component with a decay time τ ∼ 17 ns dominates in the decay kinetics of the 5d → 4f luminescence. The decay kinetics of 5d → 4f luminescence upon excitation with high-frequency (τ ∼ 8 MHz) synchrotron radiation of the X-ray range contains a fast component (τ ∼ 15 ns) against the background of the slow component of the μs-range. An effective nonradiative energy transfer is observed from impurity centers to defects, as well as from the host to Pr{sup 3+} ions and defects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Defect Mode in Microwave Waveguide Bragg Structures with Metal Pins

A waveguide Bragg structure containing equidistant cylindrical pins that are galvanically coupled to a wide wall of the waveguide is used to implement frequency response functions characterized by the presence of a band gap. Characteristics of a defect mode of the microwave photonic crystal with a pin element as a defect with an n–i–p–i–n structure with controlled conductivity placed in the capacitive gap are experimentally studied and calculated. Controlled reflectance of a microwave signal with a dynamic range of greater than 50 dB is obtained at the frequency of the defect mode.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Defect Structure of GaAs Layers Implanted with Nitrogen Ions

Structural defects formed in epitaxial GaAs layers as a result of 250-keV N{sup +} ion implantation to doses within 5 × 10{sup 14}–5 × 10{sup 16} cm{sup –2} have been studied by the X-ray diffraction (XRD) and transmission electron microscopy techniques. No amorphization of the ion-implanted layer took place in the entire dose range studied. The implantation to doses of 5 × 10{sup 14} and 5 × 10{sup 15} cm{sup –2} led to the appearance of an additional peak on XRD curves, which was related to the formation of a stressed GaAs layer with positive deformation arising due to the formation of point-defect clusters. The implantation to a dose of 5 × 10{sup 16} cm{sup –2} led to the formation of a dense structure of extended defects in the implanted layer, which was accompanied by the relaxation of macrostresses to the initial state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

A Study of the Influence Exerted by Structural Defects on Photoluminescence Spectra in n-3C-SiC

Photoluminescence (PL) spectra have been studied in 3C-SiC/4H-SiC heterostructures and 3C‑SiC single crystals. It was shown that epitaxial 3C-SiC layers grown on 4H-SiC substrates have a markedly poorer crystal perfection than do 3C-SiC single crystals. It was found that doping with aluminum gives rise to a characteristic PL both in epitaxial layers and in 3C-SiC single crystals. At the same time, the electron irradiation of epitaxial layers does not give rise to defect-related PL, in contrast to what is observed for single crystals. An assumption is made that the twin boundaries existing in epitaxial 3C-SiC layers can serve as getters of radiation defects that are components of donor–acceptor pairs responsible for the “defect-related” PL.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Molecular and Structural Probes of Defect States in Quantum Dots for Photoconversion

The objective of this research on quantum confined nanocrystals is to develop a fundamental understanding of the chemical properties of ligand-nanocrystal interfaces that will enable the control of defect-driven charge recombination in these systems. With this control, quantum confined nanocrystals may represent a cost effective means of absorbing light over the full solar spectrum for the production of electricity and fuels from sunlight. New spectroscopic techniques will be developed in this project that allow a direct correlation of molecular structural information about the ligand-nanocrystal interactions with the corresponding density and energetic distribution of surface defects and the influence that these defects have on charge transport and recombination. The insight that is gained will provide a molecular basis for the rational development of novel pathways for the of use quantum dots in systems for solar energy transduction.

14 SOLAR ENERGY↗
DOE OSTI2021

Localized Defect Detection from Spatially Mapped, In-Situ Process Data With Machine Learning

In powder bed fusion additive manufacturing, machines are often equipped with in-situ sensors to monitor the build environment as well as machine actuators and subsystems. The data from these sensors offer rich information about the consistency of the fabrication process within a build and across builds. This information may be used for process monitoring and defect detection; however, little has been done to leverage this data from the machines for more than just coarse-grained process monitoring. In this work we demonstrate how these inherently temporal data may be mapped spatially by leveraging scan path information. We then train a XGBoost machine learning model to predict localized defects—specifically soot–using only the mapped process data of builds from a laser powder bed fusion process as input features. The XGBoost model offers a feature importance metric that will help to elucidate possible relationships between the process data and observed defects. Finally, we analyze the model performance spatially and rationalize areas of greater and lesser performance.

3D printing↗
DOE OSTI2022

Towards Ab-Initio Simulations of Crystalline Defects at the Exascale Using Spectral Quadrature Density Functional Theory

Defects in crystalline solids play a crucial role in determining properties of materials at the nano, meso- and macroscales, such as the coalescence of vacancies at the nanoscale to form voids and prismatic dislocation loops or diffusion and segregation of solutes to nucleate precipitates, phase transitions in magnetic materials via disorder and doping. First principles Density Functional Theory (DFT) simulations can provide a detailed understanding of these phenomena. However, the number of atoms needed to correctly simulate these systems is often beyond the reach of many widely used DFT codes. The aim of this article is to discuss recent advances in first principles modeling of crystal defects using the spectral quadrature method. The spectral quadrature method is linear scaling with respect to the number of atoms, permits spatial coarse-graining, and is capable of simulating non-periodic systems embedded in a bulk environment, which allows the application of appropriate boundary conditions for simulations of crystalline defects. In this article, we discuss the state-of-the-art in ab-initio modeling of large metallic systems of the order of several thousand atoms that are suitable for utilizing exascale computing resourses.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Influence of Superlattice Structure on V-Defect Distribution, External Quantum Efficiency and Electroluminescence for Red InGaN Based µLEDs on Silicon

Achieving high quantum efficiency in long-wavelength LEDs has posed a significant challenge to the solid-state lighting and display industries. In this article, we use V-defect engineering as a technique to achieve higher efficiencies in red InGaN LEDs on (111) Si through lateral injection. We investigate the effects of superlattice structure on the V-defect distribution, the electroluminescence properties, and the external quantum efficiency. Increasing the relative thickness of In in the InGaN/GaN superlattice and the total superlattice thickness correlate with a reduction of active region defects and increased external quantum efficiencies. The highest measured on-chip EQE was 0.15% and based on Monte-Carlo ray tracing simulations for light extraction we project this would correspond to a flip-chip EQE of ~2.5%.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Analysis of Crystalline Defects Caused by Growth on Partially Planarized Spalled (100) GaAs Substrates

We analyze the effect of growth on non-(100) surfaces resulting from incomplete planarization of spalled GaAs wafers on the defect structure of GaAs solar cell layers grown by hydride vapor phase epitaxy (HVPE). Controlled spalling of (100)-oriented GaAs has the potential to reduce substrate costs for III-V epitaxy; however, it creates regularly faceted surfaces that may complicate the growth of high-quality III-V optoelectronic devices. We leverage the anisotropic growth rate of HVPE to planarize these faceted GaAs substrates, reducing the surface roughness and degree of faceting. We observe degraded solar cell performance and material quality in sample areas where facets are not completely removed. We used dark lock-in thermography and photoluminescence to identify recombination in areas that were not fully planarized. We used cathodoluminescence to identify the presence of extended defects in these regions, which are correlated with bandgap fluctuations in the material. We hypothesize that these defects were created by strain from compositional fluctuations in ternary alloys grown on the faceted surfaces. This work elucidates the potential issues of solar cells grown on faceted surfaces and builds understanding toward realizing high performance III-V photovoltaics with the cost-reduction potential of controlled spalling.

14 SOLAR ENERGY↗
NASA NTRS2003

Patterns of software defect data on spacecraft

The identification of patterns of software defect data yields insights into improving the quality of both operational and future spacecraft. This paper describes the results of applying this technique to both post-launch and pre-launch spacecraft. It then describes four key challenges that remain to achieving fuller utilization of defect analysis in future systems.

software defect defect analysis defect data↗
DOE OSTI2018

Breaking the Current‐Retention Dilemma in Cation‐Based Resistive Switching Devices Utilizing Graphene with Controlled Defects

Abstract Cation‐based resistive switching (RS) devices, dominated by conductive filaments (CF) formation/dissolution, are widely considered for the ultrahigh density nonvolatile memory application. However, the current‐retention dilemma that the CF stability deteriorates greatly with decreasing compliance current makes it hard to decrease operating current for memory application and increase driving current for selector application. By centralizing/decentralizing the CF distribution, this current‐retention dilemma of cation‐based RS devices is broken for the first time. Utilizing the graphene impermeability, the cation injecting path to the RS layer can be well modulated by structure‐defective graphene, leading to control of the CF quantity and size. By graphene defect engineering, a low operating current (≈1 µA) memory and a high driving current (≈1 mA) selector are successfully realized in the same material system. Based on systematically materials analysis, the diameter of CF, modulated by graphene defect size, is the major factor for CF stability. Breakthrough in addressing the current‐retention dilemma will instruct the future implementation of high‐density 3D integration of RS memory immune to crosstalk issues.

Zhao, Xiaolong↗
DOE OSTI2018

Metal Oxide Nanocomposites: A Perspective from Strain, Defect, and Interface

Vertically aligned nanocomposite thin films with ordered two phases, grown epitaxially on substrates, have attracted tremendous interest in the past decade. These unique nanostructured composite thin films with large vertical interfacial area, controllable vertical lattice strain, and defects provide an intriguing playground, allowing for the manipulation of a variety of functional properties of the materials via the interplay among strain, defect, and interface. This field has evolved from basic growth and characterization to functionality tuning as well as potential applications in energy conversion and information technology. Lastly, the remarkable progress achieved in vertically aligned nanocomposite thin films from a perspective of tuning functionalities through control of strain, defect, and interface is summarized.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Local p‐ and n‐Type Doping of an Oxide Semiconductor via Electric‐Field‐Driven Defect Migration

Layered oxides exhibit high ionic mobility and chemical flexibility, attracting interest as cathode materials for lithium-ion batteries and the pairing of hydrogen production and carbon capture. Recently, layered oxides emerged as highly tunable semiconductors. For example, by introducing anti-Frenkel defects, the electronic hopping conductance in hexagonal manganites is increased locally by orders of magnitude. Here, local acceptor and donor doping in Er(Mn,Ti)O3 is demonstrated, facilitated by the controlled splitting of anti-Frenkel defects under applied d.c. voltage. By combining density functional theory calculations, scanning probe microscopy, atom probe tomography, and scanning transmission electron microscopy, it is shown that the oxygen defects can readily be moved through the layered crystal structure, leading to nano-sized interstitial-rich (p-type) and vacancy-rich (n-type) regions. The resulting pattern is comparable to dipolar npn-junctions and stable on the timescale of days. These findings reveal the possibility of temporarily functionalizing oxide semiconductors at the nanoscale, giving additional opportunities for the field of oxide electronics and the development of transient electronics in general.

He, Jiali↗
DOE OSTI2018

Pristine or Highly Defective? Understanding the Role of Graphene Structure for Stable Lithium Metal Plating

The role of graphene host structure/chemistry in plating–stripping in lithium metal anodes employed for lithium metal batteries is first examined in this study. Structural and chemical defects are bad, since highly defective graphene promotes unstable solid electrolyte interphase (SEI) growth. This consumes the fluoroethylene carbonate (FEC) additive in the carbonate electrolyte and is correlated with rapid decay in Coulombic efficiency (CE) and formation of filament-like Li dendrites. A unique flow-aided sonication exfoliation method is employed to synthesize “defect-free” graphene (df-G), allowing for a direct performance comparison with conventional reduced graphene oxide (r-GO). At cycle 1, the r-GO is better electrochemically wetted by Li than df-G, indicating that initially it is more lithiophilic. With cycling, the nucleation overpotential with r-GO becomes higher than with df-G, indicating less facile plating reactions. The df-G yields state-of-the-art electrochemical performance, with the post cycled metal surface being relatively smooth and dendrite-free. Conversely, r-GO templates have CE rapidly degrade from the onset, with extensive dendrites after cycling. Severe SEI growth and associated FEC depletion with r-GO are further confirmed by electrochemical impedance analysis and surface science methods. Finally, a new design rule is provided for Li metal templates: An ideal host must be noncatalytic toward SEI formation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Blue Light Emitting Defective Nanocrystals Composed of Earth‐Abundant Elements

Abstract Copper‐based ternary (I–III–VI) chalcogenide nanocrystals (NCs) are compositionally‐flexible semiconductors that do not contain lead (Pb) or cadmium (Cd). Cu‐In‐S NCs are the dominantly studied member of this important materials class and have been reported to contain optically‐active defect states. However, there are minimal reports of In‐free compositions that exhibit efficient photoluminescence (PL). Here, we report a novel solution‐phase synthesis of ≈4 nm defective nanocrystals (DNCs) composed of copper, aluminum, zinc, and sulfur with ≈20 % quantum yield and an attractive PL maximum of 450 nm. Extensive spectroscopic characterization suggests the presence of highly localized electronic states resulting in reasonably fast PL decays (≈1 ns), large vibrational energy spacing, small Stokes shift, and temperature‐independent PL linewidth and PL lifetime (between room temperature and ≈5 K). Furthermore, density functional theory (DFT) calculations suggest PL transitions arise from defects within a CuAl 5 S 8 crystal lattice, which supports the experimental observation of highly‐localized states. The results reported here provide a new material with unique optoelectronic characteristics that is an important analog to well‐explored Cu‐In‐S NCs.

Hansen, Eric C.↗
DOE OSTI2020

Refractive Imaging of Air Shock Above Microscale Defects in Pentaerythritol Tetranitrate (PETN) Films

Physical vapor deposition (PVD) of high explosives can produce energetic samples with unique microstructure and morphology compared to traditional powder processing techniques, but challenges may exist in fabricating explosive films without defects. Deposition conditions and substrate material may promote microcracking and other defects in the explosive films. Here, we investigate effects of engineered microscale defects (gaps) on detonation propagation and failure for pentaerythritol tetranitrate (PETN) films using ultra-high-speed refractive imaging and hydrocode modelling. Observations of the air shock above the gap reveal significant instabilities during gap crossing and re-ignition.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Exact correlators on the Wilson loop in $$ \mathcal{N}=4 $$ SYM: localization, defect CFT, and integrability

We compute a set of correlation functions of operator insertions on the 1/8 BPS Wilson loop in N=4 SYM by employing supersymmetric localization, OPE and the Gram-Schmidt orthogonalization. These correlators exhibit a simple determinant structure, are position-independent and form a topological subsector, but depend nontrivially on the ’t Hooft coupling and the rank of the gauge group. When applied to the 1/2 BPS circular (or straight) Wilson loop, our results provide an infinite family of exact defect CFT data, including the structure constants of protected defect primaries of arbitrary length inserted on the loop. At strong coupling, we show precise agreement with a direct calculation using perturbation theory around the AdS 2 string worldsheet. We also explain the connection of our results to the “generalized Bremsstrahlung functions” previously computed from integrability techniques, reproducing the known results in the planar limit as well as obtaining their finite N generalization. Furthermore, we show that the correlators at large N can be recast as simple integrals of products of polynomials (known as Q-functions) that appear in the Quantum Spectral Curve approach. This suggests an interesting interplay between localization, defect CFT and integrability.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗