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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 1,729 records · Page 96
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Revisiting point defect thermodynamics in group IVB and VB transition metal carbides

We present a comprehensive re-examination of point defect thermodynamics in group IVB and VB transition metal carbides (TMCs) with the rocksalt structure using a combination of density functional theory (DFT) calculations and a statistical mechanical Wagner-Schottky model within the canonical ensemble. The most stable configurations of point defects were discovered using basin-hopping global optimization, driven by either a machine learning interatomic potential (MLIP) or DFT. A key finding is the identification of previously unreported dicarbon antisites—a C–C dimer occupying a metal site—as the structural (constitutional) defects on the carbon-rich side of stoichiometry in all group IVB and VB TMCs except TaC. Furthermore, dicarbon antisite-containing thermal defect complexes, such as quadruple and interbranch defects, can dominate in TMCs under specific stoichiometric and temperature conditions. In conclusion, by incorporating dicarbon antisites into the defect landscape, this work provides a revised understanding of the thermodynamics of point defects in TMCs.

Carbides↗
DOE OSTI2024

The Defects Genome of Janus Transition Metal Dichalcogenides

2D Janus Transition Metal Dichalcogenides (TMDs) have attracted much interest due to their exciting quantum properties arising from their unique two-faced structure, broken-mirror symmetry, and consequent colossal polarization field within the monolayer. While efforts are made to achieve high-quality Janus monolayers, the existing methods rely on highly energetic processes that introduce unwanted grain-boundary and point defects with still unexplored effects on the material's structural and excitonic properties Through high-resolution scanning transmission electron microscopy (HRSTEM), density functional theory (DFT), and optical spectroscopy measurements; this work introduces the most encountered and energetically stable point defects. It establishes their impact on the material's optical properties. HRSTEM studies show that the most energetically stable point defects are single (V S and V Se ) and double chalcogen vacancy (V S −V Se ), interstitial defects (Mi), and metal impurities (MW) and establish their structural characteristics. DFT further establishes their formation energies and related localized bands within the forbidden band. Cryogenic excitonic studies on h-BN-encapsulated Janus monolayers offer a clear correlation between these structural defects and observed emission features, which closely align with the results of the theory. Finally, the overall results introduce the defect genome of Janus TMDs as an essential guideline for assessing their structural quality and device properties.

2D materials↗
DOE OSTI2024

Structural and thermal properties and the origin of the ultralow thermal conductivity in the defect stannite CuIn 2 Se 4

Phase-pure CuIn 2 Se 4 , a ternary metal chalcogenide that forms in a disordered stannite crystal structure, was synthesized to investigate the structure thermal property relationships as well as reveal the origin of the ultralow thermal conductivity this material possesses over a large temperature range. Modeling of the temperature-dependent heat capacity and thermal conductivity revealed distinctive thermal properties and large lattice anharmonicity. Electron localization function calculations highlight the asymmetric bonding inherent to CuIn 2 Se 4 , which together with lattice anharmonicity directly impacts the thermal properties. Our findings reveal the specific atomic arrangement and bonding governing the thermal properties of this ternary metal chalcogenide. Our findings underscore the specific atomic arrangement and bonding governing the thermal properties in this ternary chalcogenide. This study advances the fundamental understanding of stannites, and our findings can be applied to these and other multinary metal chalcogenides of interest for applications where low thermal conductivity is desirable.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Single-Particle Spectroscopy as a Versatile Tool to Explore Lower-Dimensional Structures of Inorganic Perovskites

The remarkable defect tolerant nature of inorganic cesium halide perovskites leading to near unity photoluminescence (PL) quantum yield (QY) and narrow emission linewidth across the entire visible spectrum, have provided a tantalizing platform for a development of a plethora of light-emitting applications. Recently, lower dimensional (2D, 1D, and 0D) perovskites have attracted further attention due to the enhanced thermal, photo and chemical stability as compared to their three-dimensional (3D) analogues. Combination of the external size quantization and internal octahedral organization provides a unique opportunity to study and harness “multidimensional” electronic properties engineered both on atomic scale and the nanoscale. However, crucial research to understand the elementary charge carrier dynamics in lower dimensional perovskites lags far behind the enormous effort to incorporate them into optoelectronic devices. In this Perspective, we provide a review of recent developments that focus on studies of the dynamics of excitonic complexes in Cs-based perovskite nanocrystals using single-particle time-resolved PL spectroscopy and photon correlation measurements. Single photon statistical studies not only offer the unprecedented level of detail to directly assess various recombination pathways but also provide insights into specifics of carrier’s localization. Further, we discuss the underlying physicochemical processes that govern PL emission and draw attention to a number of attributes within this class of the materials, especially lower-dimensional perovskites that may indicate the common origin of the PL emission, as well as provide a route map for the vast unexplored territories where single particle spectroscopy can be a powerful tool to unravel crucial information.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Local electronic descriptors for solute-defect interactions in bcc refractory metals

The interactions between solute atoms and crystalline defects such as vacancies, dislocations, and grain boundaries are essential in determining alloy properties. Here we present a general linear correlation between two descriptors of local electronic structures and the solute-defect interaction energies in binary alloys of body-centered-cubic (bcc) refractory metals (such as W and Ta) with transition-metal substitutional solutes. One electronic descriptor is the bimodality of the d-orbital local density of states for a matrix atom at the substitutional site, and the other is related to the hybridization strength between the valance sp- and d-bands for the same matrix atom. For a particular pair of solute-matrix elements, this linear correlation is valid independent of types of defects and the locations of substitutional sites. These results provide the possibility to apply local electronic descriptors for quantitative and efficient predictions on the solute-defect interactions and defect properties in alloys.

36 MATERIALS SCIENCE↗
NASA NTRS1976

Advanced composites: Fabrication processes for selected resin matrix materials

This design note is based on present state of the art for epoxy and polyimide matrix composite fabrication technology. Boron/epoxy and polyimide and graphite/epoxy and polyimide structural parts can be successfully fabricated. Fabrication cycles for polyimide matrix composites have been shortened to near epoxy cycle times. Nondestructive testing has proven useful in detecting defects and anomalies in composite structure elements. Fabrication methods and tooling materials are discussed along with the advantages and disadvantages of different tooling materials. Types of honeycomb core, material costs and fabrication methods are shown in table form for comparison. Fabrication limits based on tooling size, pressure capabilities and various machining operations are also discussed.

Welhart, E. K.↗
DOE OSTI2021

Atomic Gradient Structure Alters Electronic Structure in 3D across the Bulk and Enhances Photoactivity

Abstract Understanding structure–function relationships enables the design of materials with tailored functionalities. The long‐standing challenge is to design materials with high active volume to improve efficiency. Tailored grain boundaries and lattice defects are traditionally used to tune the electronic structure near interfaces or defects to promote electron and hole separation. However, the active volume of point defect sites or interfaces in these traditional photocatalysts is extremely low. This study reports a structure with continuous atomic positional deformation across the bulk, altering the electronic structure in 3D and creating a significantly higher active volume. Such a structure in anatase is obtained and tuned by phase transformation during the heating process. Transmission electron microscopy and density functional theory results reveal that atomic deformations result in continuous band bending across the particles, facilitating electron–hole separation, inhibiting their recombination, and inducing dramatically enhanced photoactivity. These findings enable a different materials design paradigm that can potentially be harnessed for a broad range of applications.

Ren, Peng↗
DOE OSTI2017

Defect accumulation in swift heavy ion-irradiated CeO 2 and ThO 2

Here, neutron total scattering was used to investigate defect accumulation mechanisms in CeO 2 and ThO 2 irradiated with 2.2 GeV Au ions. Pair distribution function (PDF) analysis was applied to characterize the local structural evolution and irradiation-induced defects as a function of irradiation fluence. CeO 2 exhibits a greater amount of disorder than ThO 2 under the same irradiation conditions. The local structures of the two materials evolve differently as a function of ion fluence, even if similar defects are produced. The PDF analysis indicates that oxygen dimer and/or peroxide defects with distances of ~1.45 Å are formed in CeO 2 , while irradiation-induced defects in ThO 2 result in a change in the mean O–Th–O bond angle and a distortion of local ThO 8 polyhedra. Understanding how bound oxygen defects, such as peroxide, affect bulk oxygen transport in CeO 2 will aid in better predicting and improving properties of fluorite structure materials for fast ion conductor applications.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Defect sink characteristics of specific grain boundary types in 304 stainless steels under high dose neutron environments

Radiation induced segregation (RIS) is a well-studied phenomena which occurs in many structurally relevant nuclear materials including austenitic stainless steels. RIS occurs due to solute atoms preferentially coupling to mobile point defect fluxes that migrate and interact with defect sinks. Here, a 304 stainless steel was neutron irradiated up to 47.1 dpa at 320 °C. Investigations into the RIS response at specific grain boundary types were utilized to determine the sink characteristics of different boundary types as a function of irradiation dose. A rate theory model built on the foundation of the modified inverse Kirkendall (MIK) model is proposed and benchmarked to the experimental results. This model, termed the GiMIK model, includes alterations in the boundary conditions based on grain boundary structure and includes expressions for interstitial binding. This investigation, through experiment and modeling, found specific grain boundary structures exhibit unique defect sink characteristics depending on their local structure. Such interactions were found to be consistent across all doses investigated and had larger global implications including precipitation of Ni-Si clusters near different grain boundary types.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2015

Atomic Oxygen Recombination at Surface Defects on Reconstructed (0001) α-Quartz Exposed to Atomic and Molecular Oxygen

The surface chemistry of silica is strongly affected by the nature of chemically active sites (or defects) occurring on the surface. Here, we employ quantum mechanical electronic structure calculations to study an uncoordinated silicon defect, a non-bridging oxygen defect, and a peroxyl defect on the reconstructed (0001) surface of α-quartz. We characterized the spin states and energies of the defects, and calculated the reaction profiles for atomic oxygen recombination at the defects. We elucidated the diradical character by analyzing the low-lying excited states using multireference wave function methods. We show that the diradical defects consist of weakly coupled doublet radicals, and the atomic oxygen recombination can take place through a barrierless process at defects. We have delineated the recombination mechanism and computed the formation energy of the peroxyl and non-bridging oxygen defects. We found that key recombination reaction paths are barrierless. In addition, we characterize the electronically excited states that may play a role in the chemical and physical processes that occur during recombination on these surface defect sites.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2025

Defect-Mediated Diffusion Pathways in Spodumene Accelerate Lithium Transport

Lithium extraction from naturally occurring α-spodumene is hindered by poor lithium diffusivity, necessitating high-temperature phase transformation to a low-density β polymorph. Although β spodumene exhibits up to 5 orders of magnitude higher lithium-ion diffusivity, both phases have diffusion activation energies between 0.8 and 1 eV, indicating that polymorph density is not the controlling factor over diffusivity. We show that aluminum vacancies facilitate lithium-ion diffusion in α-spodumene by reducing the migration barrier from 2.4 to 0.9 eV. Bond valence site energy and nudged elastic band calculations show a new lithium local minimum site which promotes a one-dimensional percolation network by reducing the lithium intersite distance from 4.5 Å to 2.9 Å. However, aluminum vacancies are energetically unfavorable to percolate through the whole structure, resulting in very low net lithium diffusivity and highlighting the critical role of nonstoichiometric defects in facilitating lithium transport in rigid aluminosilicate structures.

Chemical structure↗
DOE OSTI2021

Atomic Gradient Structures: Atomic Gradient Structure Alters Electronic Structure in 3D across the Bulk and Enhances Photoactivity

Understanding structure–function relationships enables the design of materials with tailored functionalities. The long-standing challenge is to design materials with high active volume to improve the efficiency. Tailored grain boundaries and lattice defects are traditionally used to tune the electronic structure near interfaces or defects to promote electron and hole separation. However, the active volume of point defect sites or interfaces in these traditional photocatalysts is extremely low. In this work, we report a structure with continuous atomic positional deformation across the bulk, altering the electronic structure in three-dimension and creating a significantly high active volume. Such a structure in anatase is obtained and tuned by phase transformation during heating process. Transmission electron microscopy and density functional theory results reveal that atomic deformations result in continuous band bending across the particles, facilitating electron–hole separation, inhibiting their recombination, and inducing dramatically enhanced photoactivity. These findings enable a different materials design paradigm that can potentially be harnessed for a broad range of applications.

TiO2↗
DOE OSTI2021

Solvent Processing and Ionic Liquid-Enabled Long-Range Vertical Ordering in Block Copolymer Films with Enhanced Film Stability

Rapid and reliable processing methods for forming ordered block copolymer (BCP) materials with low defect density in a thin film geometry are required for many nanotechnology applications. Vertically aligned BCP structures, in particular, have applications ranging from nanolithography for electronics and photonics to nanoporous membranes for water remediation and novel batteries for flexible electronics. However, the attainment of nearly complete vertical orientational order of the BCP ordered phase remains challenging. Solvent-based techniques, such as direct immersion annealing (DIA) and solvent vapor annealing (SVA), have immense potential for these applications of BCP films, as it allows for tuning of their thermodynamic, structural, and chain mobility driven kinetic properties. We first demonstrate that DIA, using a judicious choice of binary solvent mixtures along with a relatively hydrophobic ionic liquid (IL), induces the rapid vertical ordering in polystyrene-b-polymethylmethacrylate (PS-PMMA) block copolymer in lamellar films. The IL establishes synergy to the binary solvent mixture of toluene and heptane to 1 create a near-neutral solvent environment for attaining the vertical microstructure of the BCP. Next, we show that IL can suppress the de-wetting of the PS-PMMA films to achieve long-range order using SVA in cylindrical films for long annealing times. Both vertical and horizontal morphology is attained in these films by selecting different solvent environments. Furthermore, attaining enhanced vertical and horizontal BCP structure with long-range defect-free order by tuning solvent quality and using additives like IL can render them useful for many nanotech applications.

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

The Role of Defect Geometry in Localized Emission from Monolayer Tungsten Dichalcogenides

In two-dimensional transition metal dichalcogenides such as tungsten diselenide (WSe 2 ), single photon emission has been broadly attributed to exciton localization from atomic point defects, yet the precise microscopic origins are unclear. This work introduces an empirically grounded computational framework that explains the origins of facile single photon emission in WSe 2 . High-resolution microscopy identifies native defect geometries in monolayer WSe 2 lattices from which the model is built. Here, the qualitative effects of chalcogen type, defect geometry, and mechanical strain on the electronic structure are individually assessed using density functional theory, and a specific divacancy configuration emerges as the candidate for localized single-electron transitions that match observed spectral energies. Spectroscopy and photon correlation measurements further validate this model, establishing a self-consistent link between defect geometry, electronic structure, and quantum emission.

defect emission↗
DOE OSTI2015

An analytical bond-order potential for carbon

Carbon is the most widely studied material today because it exhibits special properties not seen in any other materials when in nano dimensions such as nanotube and graphene. Reduction of material defects created during synthesis has become critical to realize the full potential of carbon structures. Molecular dynamics (MD) simulations, in principle, allow defect formation mechanisms to be studied with high fidelity, and can, therefore, help guide experiments for defect reduction. Such MD simulations must satisfy a set of stringent requirements. First, they must employ an interatomic potential formalism that is transferable to a variety of carbon structures. Second, the potential needs to be appropriately parameterized to capture the property trends of important carbon structures, in particular, diamond, graphite, graphene, and nanotubes. The potential must predict the crystalline growth of the correct phases during direct MD simulations of synthesis to achieve a predictive simulation of defect formation. An unlimited number of structures not included in the potential parameterization are encountered, thus the literature carbon potentials are often not sufficient for growth simulations. We have developed an analytical bond order potential for carbon, and have made it available through the public MD simulation package LAMMPS. We also demonstrate that our potential reasonably captures the property trends of important carbon phases. As a result, stringent MD simulations convincingly show that our potential accounts not only for the crystalline growth of graphene, graphite, and carbon nanotubes but also for the transformation of graphite to diamond at high pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Near-Infrared Emission from Sulfur Heteroatom Defects in Single-Walled Carbon Nanotubes

Quantum defects introduced into single-walled carbon nanotubes (SWCNTs) that support near-infrared emission are promising candidates for many applications. Here, we present a type of quantum defect in which sulfur atoms are incorporated into the crystal lattice of SWCNTs through their reactions with a mild reducing agent, sodium dithionite. When radical anions generated by sodium dithionites react with (6,5)-SWCNTs, two new emission features emerge at 1120 and 1250 nm. Comparing these spectral features with density functional theory calculation results, we attribute them to defects with episulfide or thioether structures. Here, by investigating the influences of reaction conditions including reactant concentration, light irradiation, and solvent type, we identify the reaction mechanism between SWCNTs and sodium dithionite and achieve good control over defect densities. Due to the intrinsic reactivity of sulfur species, the sulfur-containing quantum defects discovered here may facilitate the introduction of more complex and functionally diverse moieties into SWCNTs.

Anions↗