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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 1,207 records · Page 67
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Phonon-Spin Scattering from Unpaired f-electrons in U atoms and It’s Influence on Thermal Transport in Uranium-doped Thorium Dioxide Single Crystals

In this work, impact of low level of uranium (U) atom substitution on thermal conductivity of thorium dioxide (ThO2) is investigated. ThO2 is an electronic insulator with a wide optical band-gap and no unpaired electrons whose thermal transport is governed by phonons. U-substitution introduces unpaired f-electrons resulting in paramagnetic behavior of U-ThO2 at room temperature, which significantly suppresses its thermal conductivity. A single crystal of U-ThO2 with graded composition of U is grown using a hydrothermal synthesis method, and thermal conductivity measurements are performed in regions with uniform composition of U at levels of 0%, 6%, 9% and 16%. Measured thermal conductivity profiles over 77–300 K temperature range are analyzed using an analytical expression for phonon-mediated thermal transport based on Klemens-Callaway model. Temperature dependent thermal conductivity is found to deviate significantly from the Rayleigh scattering trend expected for a simple substitutional point defect with a small perturbation to mass and interatomic forces. With the resonant scattering term, observed large suppression of thermal conductivity at low temperatures can be closely reproduced. Additionally, the extracted phonon-spin coupling constants imply a nonlinear relation of phonon-spin interaction intensity with respect to U doping percentage. Our study reveals how phonon-spin scattering contributed by unpaired f-electrons in U atoms influences thermal transport in the U-ThO2 system.

36 - MATERIALS SCIENCE↗
DOE OSTI2022

Simulation of the Impact of Point Defects and Edge Dislocations on X-Ray Diffraction in Hexagonal (Ni,Co) 1+2 x Ti 1– x O 3 Thin Films

In this work, a computer code for simulating high-resolution X-ray diffraction (XRD) data from disordered crystals with arbitrary spatial composition and local lattice parameters is developed. Simulated patterns are compared with the experimental data collected on a single phase, highly crystalline (Ni 0.42 Co 0.58 ) 2.22 Ti 0.39 O 3 solid-solution thin film exhibiting a large number of subsidiary minima. As a case study, the edge dislocations in hexagonal (Ni,Co) 3 O 3 thin films with Burgers vector parallel and perpendicular to the a-axis and c-axis, respectively, are modeled. No peak profiles are assumed and thus issues related to profile fitting are avoided. Both macroscopic features, such as film thickness, and atomic-scale structure, such as dislocations, are simultaneously modeled. Simulations are run on a desktop machine. Commonly applied database-based phase identification routines can result in wrong phase identification, unless intensities are properly modeled. Modeling tools of diffractometer manufacturers are compared with the present approach. An example of how simulated reciprocal lattice patterns can be used to choose relevant measurement geometries in defected thin films is given.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Evidence for the Band-Edge Exciton of CuInS 2 Nanocrystals Enables Record Efficient Large-Area Luminescent Solar Concentrators

Ternary I-III-VI 2 nanocrystals (NCs), such as CuInS 2 , are receiving attention as heavy-metals-free materials for solar cells, luminescent solar concentrators (LSCs), LEDs, and bio-imaging. The origin of the optical properties of CuInS 2 NCs are however not fully understood. A recent theoretical model suggests that their characteristic Stokes-shifted and long-lived luminescence arises from the structure of the valence band (VB) and predicts distinctive optical behaviours in defect-free NCs: the quadratic dependence of the radiative decay rate and the Stokes shift on the NC radius. If confirmed, this would have crucial implications for LSCs as the solar spectral coverage ensured by low-bandgap NCs would be accompanied by increased re-absorption losses. Here, by studying stoichiometric CuInS 2 NCs, it is revealed for the first time the spectroscopic signatures predicted for the free band-edge exciton, thus supporting the VB-structure model. At very low temperatures, the NCs also show dark-state emission likely originating from enhanced electron-hole spin interaction. In this study, the impact of the observed optical behaviours on LSCs is evaluated by Monte Carlo ray-tracing simulations. Based on the emerging device design guidelines, optical-grade large-area (30×30 cm 2 ) LSCs with optical power efficiency (OPE) as high as 6.8% are fabricated, corresponding to the highest value reported to date for large-area devices.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Automatic MCNP File Generation for Cherenkov Imaging Simulations [Slides]

Overview: Creating physics models to accurately simulate a variety of spent fuel assemblies. These characterized simulations will then be used to create a well documented repository that encompasses a wide variety of different fuel assemblies, burn-up and spent fuel pool conditions, and defects. The final project will use our simulations to train an AI, and for it to be effective it has to have a lot of reliable data; Automating an arbitrary amount of created spent fuel pins with materials, that will be passed into AI models for it to "learn" what a good spent nuclear fuel cell looks like and what a non-spent nuclear fuel cell looks like.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2017

Band Edge Positions and Their Impact on the Simulated Device Performance of ZnSnN 2 -Based Solar Cells

ZnSnN 2 (ZTN) has been proposed as a new earth abundant absorber material for PV applications. While carrier concentration has been reduced to values suitable for device implementation, other properties such as ionization potential, electron affinity and work function are not known. Here, we experimentally determine the value of ionization potential (5.6 eV), electron affinity (4.1 eV) and work function (4.4 eV) for ZTN thin film samples with Zn cation composition Zn/(Zn+Sn) = 0.56 and carrier concentration n = 2x10 19 cm -3 . Using both experimental and theoretical results, we build a model to simulate the device performance of a ZTN/Mg:CuCrO 2 solar cell, showing a potential efficiency of 23% in the limit of no defects present. We also investigate the role of band tails and recombination centers on the cell performance. In particular device simulations show that band tails are highly detrimental to the cell efficiency, and recombination centers are a major limitation if present in concentration comparable to the net carrier density. The effect of the position of the band edges of the p-type junction partner was assessed too. Through this study, we determine the major bottlenecks for the development of ZTN-based solar cell and identify avenues to mitigate them.

14 SOLAR ENERGY↗
DOE OSTI2023

AdS 3 orbifolds, BTZ black holes, and holography

Conical defects of the form (AdS 3 × S 3 )/Z k have an exact orbifold description in worldsheet string theory, which we derive from their known presentation as gauged Wess-Zumino-Witten models. The configuration of strings and fivebranes sourcing this geometry is well-understood, as is the correspondence to states/operators in the dual CFT 2 . One can analytically continue the construction to Euclidean AdS 3 (i.e. the hyperbolic ball $H$$^{+}_{3}$) and consider the orbifold by any infinite discrete (Kleinian) group generated by a set of elliptic elements γi ϵ SL(2, C), $γ$$^{ki}_{i}$ = 1, i = 1, . . . , K. The resulting geometry consists of multiple conical defects traveling along geodesics in $H$$^{+}_{3}$, and provides a semiclassical bulk description of correlation functions in the dual CFT involving the corresponding defect operators, which is nonperturbatively exact in α'. The Lorentzian continuation of these geometries describes a collection of defects colliding to make a BTZ black hole. We comment on a recent proposal to use such correlators to prepare a basis of black hole microstates, and elaborate on a picture of black hole formation and evaporation in terms of the underlying brane dynamics in the bulk.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

HCOOH Decomposition on Sub-Nanometer Pd 6 Cluster Catalysts: The Effect of Defective Boron Nitride Supports Through First Principles

The catalytic properties of a hexagonal boron nitride- (h-BN) supported Pd 6 sub-nanometer cluster in the context of formic acid (HCOOH) decomposition were studied by means of periodic Density Functional Theory (DFT) calculations. The effect of support defectivity – boron (h-B v N) and nitrogen (h-BN v ) monovacancies – on the competition between the formate (HCOO)- and carboxyl (COOH)-mediated decomposition pathways was analyzed. Defects are responsible for charge-transfer leading to a positively or negatively charged cluster, and open new reactive channels in which vacancy-mediated dehydrogenation pathways can occur. Pd 6 cluster reconstructions, induced by the adsorption of reaction intermediates and by the presence of monovacancies in the support, greatly stabilize the formation of CO from COOH, which could drastically decrease the selectivity towards hydrogen production. Here, a simplified descriptor-based analysis, based on selected thermochemical quantities calculated on charged cluster models, suggests that Pd 6 sub-nanometer clusters supported on pristine h-BN and h-BN v can be more selective than Pd 6 supported on defective h-B v N towards HCOOH dehydrogenation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Theory of Defect-Induced Crystal Field Perturbations in Rare-Earth Magnets

We present a theory describing the single-ion anisotropy of rare-earth (RE) magnets in the presence of point defects. Taking the RE-lean 1 ∶ 12 magnet class as a prototype, we use first-principles calculations to show how the introduction of Ti substitutions into SmFe 12 perturbs the crystal field, generating new coefficients due to the lower symmetry of the RE environment. We then demonstrate that these perturbations can be described extremely efficiently using a screened point charge model. We provide analytical expressions for the anisotropy energy that can be straightforwardly implemented in atomistic spin dynamics simulations, meaning that such simulations can be carried out for an arbitrary arrangement of point defects. The significant crystal field perturbations calculated here demonstrate that a sample that is single phase from a structural point of view can nonetheless have a dramatically varying anisotropy profile at the atomistic level if there is compositional disorder, which may influence localized magnetic objects like domain walls or skyrmions. Published by the American Physical Society 2024

25 ENERGY STORAGE↗
DOE OSTI2024

Deuteration removes quantum dipolar defects from KDP crystals

Abstract Dielectric properties of the hydrogen-bonded ferroelectric crystal KH 2 PO 4 (KDP) differ significantly from those of KD 2 PO 4 (DKDP). It is well established that deuteration affects the interplay of hydrogen-bond switches and heavy ion displacements that underlie the emergence of macroscopic polarization, but a detailed microscopic model is missing. We show that all-atom path integral molecular dynamics simulations can predict the isotope effects, revealing the microscopic mechanism that differentiates KDP and DKDP. Proton tunneling generates phosphate configurations that do not contribute to the polarization. At low temperatures, these quantum dipolar defects are substantial in KDP but negligible in DKDP. These intrinsic defects explain why KDP has lower spontaneous polarization and transition entropy than DKDP. The prominent role of quantum fluctuations in KDP is related to the unusual strength of the hydrogen bonds and should be equally important in other crystals of the KDP family, which exhibit similar isotope effects.

Yang, Bingjia (ORCID:0000000340749553)↗
DOE OSTI2018

Imaging, microscopic analysis, and modeling of a CdTe module degraded by heat and light

Photoluminescence (PL), electroluminescence (EL), and dark lock-in thermography are collected during stressing of a CdTe module under one-Sun light at an elevated temperature of 100 degrees C. The PL imaging system is simple and economical. The PL images show differing degrees of degradation across the module and are less sensitive to effects of shunting and resistance that appear on the EL images. Regions of varying degradation are chosen based on avoiding pre-existing shunt defects. These regions are evaluated using time-of-flight secondary ion-mass spectrometry and Kelvin probe force microscopy. Reduced PL intensity correlates to increased Cu concentration at the front interface. Numerical modeling and measurements agree that the increased Cu concentration at the junction also correlates to a reduced space charge region.

14 SOLAR ENERGY↗
DOE OSTI2017

Direct observation of vast off-stoichiometric defects in single crystalline SnSe

Single crystalline tin selenide (SnSe) recently emerged as a very promising thermoelectric material for waste heat harvesting and thermoelectric cooling, due to its record high figure of merit ZT in mediate temperature range. The most striking feature of SnSe lies in its extremely low lattice thermal conductivity as ascribed to the anisotropic and highly distorted Sn-Se bonds as well as the giant bond anharmonicity by previous studies, yet no theoretical models so far can give a quantitative explanation to such low a lattice thermal conductivity. Here, we presented direct observation of an astonishingly vast number of off-stoichiometric Sn vacancies and Se interstitials, using sophisticated aberration corrected scanning transmission electron microscope; and credited the previously reported ultralow thermal conductivity of the SnSe single crystalline samples partly to their off-stoichiometric feature. In order to further validate the conclusion, we also synthesized stoichiometric SnSe single crystalline samples, and illustrated that the lattice thermal conductivity is deed much higher as compared with the off-stoichiometric single crystals. Finally, the scattering efficiency of individual point defect on heat-carrying phonons was then discussed in the state-of-art Debye-Callaway model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

"Source Term Modeling for Advanced Gas Micro-Reactors"

Maintaining the safety of the public, environment, and operating personnel is the most important factor in designing, operating, maintaining, and decommissioning nuclear reactors. In recent years, there has been a growing interest in the development of micro-reactors employing TRi-structural ISOtropic (TRISO)-coated particle fuel. In gas reactors, TRISO fuel plays an important role in the safety case for high temperature reactors because of the fission product retention properties of the fuel. This ability enables the use of a functional containment strategy for the reactor where multiple barriers are used to prevent fission product release to the environment. Part of the safety analysis of these advanced reactors is the assessment of radionuclide releases under normal and accident conditions through the multiple credited safety barriers. Using conservative assumptions, a mechanistic analysis can be performed to quantify these releases that combines the probabilistic assessment of failure with analytic solutions to radionuclide transport equations. Source term modeling for TRISO fuel has been performed for previous reactor designs; however, these models are outdated, in many cases proprietary, and need updates to be applied to the current state of TRISO fuel technology and alternative gas reactor core configurations [1]. Currently, the only publicly available source term assessment for gas reactors is an expert-based Monte Carlo simulation based on the effectiveness of the fuel kernel, coating layers, and graphite block in a modular high temperature gas reactor [2]. Thus, there is a need to develop a simple, versatile, and mechanistic model of fission product release and transport in gas reactor cores that could be applied to a variety of reactors through user inputs and reactor-specific radionuclide inventories. The release is calculated by the diffusion of the key safety important fission products through the kernel, silicon carbide (SiC), graphite for both intact and defective TRISO particles based on fuel and graphite temperatures in the reactor under normal operation. These releases from the fuel enter the coolant where they can plate-out on cooler surfaces. A clean-up model is included for designs with a coolant purification system to remove fission gases. This initial distribution of fission products in the reactor serves as an initial condition for potential releases under postulated accident conditions. The model then can calculate the fission product release for any transient temperature profile and fission product releases can then be used to assess radiological dose to the workers and the public using conventional dose tools. Data on the diffusion of fission products is based on historic German TRISO experiments and the more current Department of Energy (DOE) Advanced Gas Reactor (AGR) TRISO fuel development program. The model is coded in python with inputs and outputs in excel spreadsheets, as well as python plotting utilities to aid in the interpretation of the results. References: [1] INL, NGNP Mechanistic Source Term White Paper, INL-10-17997, July 2010. [2] David A. Petti, Richard R. Hobbins, Peter Lowry, Hans Gougar, “Representative Source Terms and The Influence of Reactor Attributes on Functional Containment in Modular High Temperature Gas-cooled Reactors,” Nuclear Technology, Vol. 184, p. 181-197, Nov. 2013.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2017

The role of Frenkel defect diffusion in dynamic annealing in ion-irradiated Si

The formation of stable radiation damage in crystalline solids often proceeds via complex dynamic annealing processes, involving migration and interaction of ballistically-generated point defects. The dominant dynamic annealing processes, however, remain unknown even for crystalline Si. Here, we use a pulsed ion beam method to study defect dynamics in Si bombarded in the temperature range from -20 to 140 °C with 500 keV Ar ions. Results reveal a defect relaxation time constant of ~10–0.2 ms, which decreases monotonically with increasing temperature. The dynamic annealing rate shows an Arrhenius dependence with two well-defined activation energies of 73 ± 5 meV and 420 ± 10 meV, below and above 60 °C, respectively. Rate theory modeling, bench-marked against this data, suggests a crucial role of both vacancy and interstitial diffusion, with the dynamic annealing rate limited by the migration and interaction of vacancies.

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

Alignment of morphology during high spatial frequency periodic structure formation in GaAs

The interaction among multiple intense ultrashort laser pulses and solids is known to provide a regular nanoscale surface corrugation. A coupled mechanism has been identified that operates in a specific range of fluences in GaAs that exhibits transient loss of the imaginary part of the dielectric function and Χ 2 , which produces a unique corrugation known as high spatial frequency laser induced periodic surface structures (HSFL). The final structures have 180 nm periods, and their alignment perpendicular to the laser polarization is first observed in an intermediate morphology with correlation distances of 150±40 nm. Quantum molecular dynamics simulations suggest that HSFL self-assembly is initiated when the intense laser field softens the interatomic binding potential, which leads to an ultrafast generation of point defects. The morphological evolution begins as self-interstitial diffusion, driven by stress relaxation, to the surface producing 1–2 nm tall islands. An ab initio calculation of excited electron concentration combined with a Drude-Lorentz model of the excited GaAs dielectric function is used to determine that the conditions for SPP coupling at HSFL formation fluences are both satisfied and occur at wavelengths that are imprinted into the observed surface morphologies. The evolution of these morphologies is reported on as the interplay between surface plasmon polaritons that localize defect generation within the structures present on the previous laser exposure and stress relaxation driven defect diffusion.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Ultra-wide bandgap β-Ga 2 O 3 films: Optical, phonon, and temperature response properties

Optical and phonon interactions of Ga 2 O 3 thin films with nanocrystalline morphology were studied at extreme temperatures. The films were grown using a sputtering technique and analyzed via temperature response transmission, Raman scattering, and high-resolution deep-UV photoluminescence (PL). Raman modes indicated that the structure corresponds to the β-phase. The optical-gap at the range of 77–620 K exhibited a redshift of ~200 meV, with a temperature coefficient of ~0.4 meV/K. The optical-gap at room-temperature is 4.85 eV. The electron–phonon interaction model at that temperature range pointed to a low energy phonon, ~31 meV, that is involved in the thermal properties of the optical-gap. Detailed Urbach energy analysis indicated that defects are the dominant mechanism controlling the band-edge characteristics even at an elevated temperature regime where phonon dominance is usually expected. Defects are attributed to the disordered forms of graphite that were detected via Raman scattering and to the granular morphology of the film. A deep-UV laser with an above bandgap exaction line of 5.1 eV was employed to map the PL of the films. The highly resolved spectra, even at room-temperature, show a strong emission of ~3.56 eV attributed to self-trapped holes (STHs). The STH is discussed and modeled in terms of the self-trapped exciton. Moreover, a very distinct but low-intensity emission was found at 4.85 eV that agrees with the value of the optical-gap and is attributed to bandgap recombination. The intensity ratio between the STH and that of the bandgap was found to be 6:1.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Tankyrase regulates apoptosis by activating JNK signaling in Drosophila

Highlights: • Ectopic expression of DTNKS induces wing and eye defects. • DTNKS positively regulates cell apoptosis. • DTNKS regulates apoptosis by controlling JNK signaling activity. Programmed cell death (PCD), or apoptosis, plays essential roles in various cellular and developmental processes, and dysregulation of apoptosis causes many diseases. Thus, regulation of apoptotic process is very important. Drosophila tankyrase (DTNKS) is an evolutionarily conserved protein with poly(ADP-ribose) polymerase activity. In mammalian cells, tankyrases (TNKSs) have been reported to regulate cell death. To determine whether DTNKS plays function in inducing apoptosis in in vivo development, we used Drosophila as a model system and generated transgenic flies expressing DTNKS. We show that ectopic expression of DTNKS promotes caspase-dependent apoptosis and knockdown of DTNKS by RNAi dramatically alleviates apoptotic defect caused by ectopic expression of pro-apoptotic protein hid or rpr in the adult eye. Moreover, our result shows that ectopic expression of DTNKS triggers the activation of c-Jun N-terminal kinase (JNK) signaling, which is required for DTNKS-mediated apoptosis. Taken together, our finding identifies the role of DTNKS in regulating apoptosis by activating JNK signaling in Drosophila.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2025

Phase-field modeling and experiments of dynamic fracture in single crystal quartz

Predicting the onset and characteristics of brittle fracture is important for a wide range of engineering and geological material applications. In this paper, we study important aspects of brittle fracture in α-quartz by phase-field modeling and experiments using a top-down approach. In the modeling framework, the work term in the Griffith energy balance is replaced with internal energy contributions that represent surface energy, thermal energy, and elastic strain energy stored in defects. This allows parametrization of individual energy contributions in terms of internal state variables and keeps track of energy partitioning after the onset of fracture. The path and history dependence of fracture is included in evolution laws for internal state variables, e.g., entropy evolution, while the energy remains a true potential. In the experimental part, dynamic compression experiments coupled with X-ray phase contrast imaging are performed on cube-like samples with a hole. In the top-down analysis, dynamic compression and three point bending experiments from the literature are simulated with the developed phase-field damage model. In conclusion, the fitted model highlights the strain rate, size, and stress state dependence of damage nucleation and evolution in single crystal α-quartz.

36 MATERIALS SCIENCE↗