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At least 1,621 records · Page 90
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Grain boundary widening controls siderite (FeCO3) replacement of limestone (CaCO3)

Abstract The microstructure of minerals and rocks can significantly alter reaction rates. This study focuses on identifying transport paths in low porosity rocks based on the hypothesis that grain boundary widening accelerates reactions in which one mineral is replaced by another (replacement reaction). We conducted a time series of replacement experiments of three limestones (CaCO 3 ) of different microstructures and solid impurity contents using FeCl 2 . Reacted solids were analyzed using chemical imaging, small angle X-ray and neutron scattering and Raman spectroscopy. In high porosity limestones replacement is reaction controlled and complete replacement was observed within 2 days. In low porosity limestones that contain 1–2% dolomite impurities and are dominated by grain boundaries, a reaction rim was observed whose width did not change with reaction time. Siderite (FeCO 3 ) nucleation was observed in all parts of the rock cores indicating the percolation of the solution throughout the complete core. Dolomite impurities were identified to act as nucleation sites leading to growth of crystals that exert force on the CaCO 3 grains. Widening of grain boundaries beyond what is expected based on dissolution and thermal grain expansion was observed in the low porosity marble containing dolomite impurities. This leads to a self-perpetuating cycle of grain boundary widening and reaction acceleration instead of reaction front propagation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Designing Quantum Spin-Orbital Liquids in Artificial Mott Insulators

Quantum spin-orbital liquids are elusive strongly correlated states of matter that emerge from quantum frustration between spin and orbital degrees of freedom. A promising route towards the observation of those states is the creation of artificial Mott insulators where antiferromagnetic correlations between spins and orbitals can be designed. We show that Coulomb impurity lattices on the surface of gapped honeycomb substrates, such as graphene on SiC, can be used to simulate SU(4) symmetric spin-orbital lattice models. We exploit the property that massive Dirac fermions form mid-gap bound states with spin and valley degeneracies in the vicinity of a Coulomb impurity. Due to electronic repulsion, the antiferromagnetic correlations of the impurity lattice are driven by a super-exchange interaction with SU(4) symmetry, which emerges from the bound states degeneracy at quarter filling. We propose that quantum spin-orbital liquids can be engineered in artificially designed solid-state systems at vastly higher temperatures than achievable in optical lattices with cold atoms. Lastly, we discuss the experimental setup and possible scenarios for candidate quantum spin-liquids in Coulomb impurity lattices of various geometries.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Electronic structure of cobalt doped CdSe quantum dots using soft X-ray spectroscopy

Here, the electronic structure and magnetic properties of cobalt doped CdSe quantum dots (QDs) are studied using electron microscopy, soft X-ray spectroscopy, and magnetometry. Magnetometry measurements suggest these QDs are superparamagnetic, contrary to a spin-glass state observed in the bulk analogue. Electron microscopy shows well formed QDs, but with cobalt existing as doped into the QD and as unreacted species not contained in the QD. X-ray absorption measurements at the Co L3-edge suggest that changes in spectra features as a function of particle size can be described considering combination of a cobalt ion in a tetrahedral crystal field and an octahedrally coordinated (impurity) phase. With decreasing particle sizes, the impurity phase increases, suggesting that small QDs can be difficult to dope.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Electronic and protonic conduction in LaFeO 3

Perovskite-type LaFeO3 has promising applications as a negative-electrode material for Ni/metal-hydride batteries. To further improve electrode performance, a thorough understanding of its charge-transport mechanisms is required. We use hybrid density functional theory to investigate the structural and electronic properties of bulk LaFeO3, as well as the effect of point defects and polarons on charge transport. Electronic charge localizes in the form of small hole and electron polarons; this self-trapping occurs in the bulk and also in the vicinity of point defects and impurities. We find that oxygen vacancies act as deep donors, while Fe and La vacancies are acceptors with the induced holes trapped as small polarons. Hydrogen interstitials and Ge and Ti impurities, on the other hand, act as donors that can trap small electron polarons. Hydrogen binds strongly to the La and Fe vacancies and hence these defects are detrimental to protonic conduction. We also elucidate how doping with Sr enhances the electrode performance of LaFeO3:Sr impurities substituting on La sites act as acceptors and enable electronic conductivity through hole polarons, with a hopping barrier of 0.39 eV. In addition, Sr impurities suppress the formation of cation vacancies, thereby enhancing proton conductivity.

Zhu, Zhen↗
DOE OSTI2026

Analysis of RF Sheath-Driven Tungsten Erosion at RF Antenna in the WEST Tokamak

This study applies the newly developed STRIPE (Simulated Transport of RF Impurity Production and Emission) framework to analyze tungsten (W) erosion at RF antenna structures in the WEST tokamak. STRIPE integrates SolEdge3x for edge plasma backgrounds, COMSOL for 3D RF sheath potentials, RustBCA for sputtering yields, and GITR for impurity transport and ion energy–angle distributions. Building on prior work by Kumar et al. (2025) Nuclear Fusion, 65, 076039, which validated STRIPE for WEST ICRH discharge #57877, the present study provides a spatially resolved assessment of gross W erosion at both Q2 antenna limiters under ohmic and ICRH conditions. Simulations using 2D SolEdge3x profiles in COMSOL capture rectified sheath potentials exceeding 300 V, leading to strong upper-limiter localization. Both poloidal and toroidal asymmetries are observed and attributed to RF sheath effects, with modeled erosion patterns deviating from experiment—highlighting sensitivity to sheath geometry and plasma resolution. Erosion is driven primarily by high-charge-state oxygen ions (O6+–O8+), while D+ plays a negligible role. Assuming a plasma composition of 1% oxygen and 98% deuterium, STRIPE predicts a 30-fold increase in gross W erosion from ohmic to ICRH phases, consistent with a >25-fold rise in W-I (400.9 nm) brightness. Quantitative agreement is within 5% in the ohmic phase and 30% under ICRH, demonstrating predictive capability. Importantly, the study shows that the magnitude of ICRH-driven W erosion depends strongly on the concentration of light impurities (O, B, N, C), which drive sputtering through high charge states. Cleaner plasma conditions with reduced impurity content are therefore expected to substantially mitigate antenna W sources in WEST and other toroidal fusion devices. These findings establish STRIPE as a predictive framework for RF-induced plasma–material interactions and support its application to reactor-scale antenna design.

Kumar, Atul [ORNL] (ORCID:0000000277475210)↗
DOE OSTI2014

A white noise approach to the Feynman integrand for electrons in random media

Using the Feynman path integral representation of quantum mechanics it is possible to derive a model of an electron in a random system containing dense and weakly coupled scatterers [see F. Edwards and Y. B. Gulyaev, “The density of states of a highly impure semiconductor,” Proc. Phys. Soc. 83, 495–496 (1964)]. The main goal of this paper is to give a mathematically rigorous realization of the corresponding Feynman integrand in dimension one based on the theory of white noise analysis. We refine and apply a Wick formula for the product of a square-integrable function with Donsker's delta functions and use a method of complex scaling. As an essential part of the proof we also establish the existence of the exponential of the self-intersection local times of a one-dimensional Brownian bridge. As a result we obtain a neat formula for the propagator with identical start and end point. Thus, we obtain a well-defined mathematical object which is used to calculate the density of states [see, e.g., F. Edwards and Y. B. Gulyaev, “The density of states of a highly impure semiconductor,” Proc. Phys. Soc. 83, 495–496 (1964)].

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Epitaxial two-dimensional nitrogen atomic sheet in GaAs

We have grown an epitaxial two-dimensional nitrogen (N) atomic sheet in GaAs by using the site-controlled N δ-doping technique. We observed a change of the electronic states in N δ-doped GaAs from the isolated impurity centers to the delocalized impurity band at 1.49 eV with increasing N-doping density. According to the excitation-power- and temperature-dependent photoluminescence (PL) spectra, the emission related to localized levels below the impurity band edge was dominant at low excitation power and temperature, whereas the effects of the localized levels can be neglected by increasing the excitation power and temperature. Furthermore, a clear Landau shift of the PL-peak energy was observed at several Tesla in the Faraday configuration, in contrast to the case in the impurity limit.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Thermoelectric power factor enhancement with gate-all-around silicon nanowires

The thermoelectric properties of gate-all-around silicon nanowires (Si NWs) are calculated to determine the potential for significant power factor enhancement. The Boltzmann transport equation and relaxation time approximation are employed to develop an electron transport model used to determine the field-effect mobility, electrical conductivity, Seebeck coefficient, and power factor for Si NWs with cross-sectional areas between 4 nm × 4 nm and 12 nm × 12 nm and a range of gate biases. Electrical conductivity for the gated Si NWs was much higher than that of doped Si due to the lack of ionized impurities and correspondingly greater carrier mobility. A significant increase in electrical conductivity with decreasing Si NW cross-sectional area was also observed due to a large increase in the average carrier density. For all Si NWs, the Seebeck coefficient was lower than that of doped bulk Si due to the different energy dependence between ionized impurity and phonon-mediated scattering processes. This decrease was also confirmed with Seebeck coefficient measurements of multigated Si NWs and n-type Si thin-films. Quantum confinement was also found to increase the Seebeck coefficient for <8 nm × 8 nm Si NWs and also at high charge densities. A maximum power factor of 6.8 × 10{sup −3} W m{sup −1} K{sup −2} was calculated for the 6 nm × 6 nm Si NWs with typical Si/SiO{sub 2} interface roughness, which is 2–3 × those obtained experimentally for bulk Si. The power factor was also found to greatly depend on surface roughness, with a root-mean-square roughness of <0.8 nm necessary for power factor enhancement. An increase in ZT may also be possible if a low thermal conductivity can be obtained with minimal surface roughness.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Carrier accumulation near electrodes in ferroelectric films due to polarization boundary conditions

We study the effect of surface polarization on the distribution of free carriers in a wide bandgap semiconductor ferroelectric (FE) film using a thermodynamic approach. We show that free carriers, namely, holes and electrons from ionizable impurities or atomic vacancies can accumulate near the film-electrode interface, if FE polarization profile has a very steep change near the surface that is specified by the extrapolation length. Such an outcome is just the opposite of what happens in a Schottky junction in a partially or fully depleted film. This is also an entirely different effect than what has been often studied in similar structures, where the work function and screening length of the electrode metal determines the electronic character of the interface. Even for low-to-moderate densities of ionizable defects with states within the bandgap close to the band edges, high densities of carriers can localize close to the electrodes in a single domain state FE film when above a critical thickness. For very low densities of such ionizable defects, short extrapolation lengths cause electrical domain formation with minimal carrier accumulation because of the already weak depolarizing fields. This is also true for films below a critical thickness with low-to-moderate densities of ionizable impurities, i.e., electrical domains get stabilized regardless of defect density. The implications of our findings for polarization controlled Schottky to Ohmic-like transition of an interface and experimental results are discussed. It is also found that interfaces of an n-type FE heterostructure can behave like a p-type depending on the barrier heights and impurity density. We conclude that, for low-to-moderate ionizable impurity densities, it is the rate of change of polarization at the interface with position rather than solely its presence that leads to carrier accumulation and that both interfaces can become Ohmic-like with opposite signs of carriers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Defect properties of Sb- and Bi-doped CuInSe{sub 2}: The effect of the deep lone-pair s states

Bi or Sb doping has been used to make better material properties of polycrystalline Cu{sub 2}(In,Ga)Se{sub 2} as solar cell absorbers, including the experimentally observed improved electrical properties. However, the mechanism is still not clear. Using first-principles method, we investigate the stability and electronic structure of Bi- and Sb-related defects in CuInSe{sub 2} and study their effects on the doping efficiency. Contrary to previous thinking that Bi or Sb substituted on the anion site, we find that under anion-rich conditions, the impurities can substitute on cation sites and are isovalent to In because of the formation of the impurity lone pair s states. When the impurities substitute for Cu, the defects act as shallow double donors and help remove the deep In{sub Cu} level, thus resulting in the improved carrier life time. On the other hand, under anion-poor conditions, impurities at the Se site create amphoteric deep levels that are detrimental to the device performance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Dissociative diffusion mechanism in vacancy-rich materials according to mass action kinetics

We conducted two sets of diffusion-reaction numerical simulations using a finite difference method (FDM) in order to investigate fast impurity diffusion via interstitial sites in vacancy-rich materials such as Cu(In,Ga)Se 2 (CIGS) and Cu 2 ZnSn(S, Se) 4 (CZTSSe or CZTS) via the dissociative diffusion mechanism where the interstitial diffuser ultimately reacts with a vacancy to produce a substitutional. The first set of simulations extends the standard interstitial-limited dissociative diffusion theory to vacancy-rich material conditions where vacancies are annihilated in large amounts, introducing non-equilibrium vacancy concentration profiles. The second simulation set explores the vacancy-limited dissociative diffusion where impurity incorporation increases the equilibrium vacancy concentration. In addition to diffusion profiles of varying concentrations and shapes that were obtained in all simulations, some of the profiles can be fitted with the constant- and limited-source solutions of Fick’s second law despite the non-equilibrium condition induced by the interstitial-vacancy reaction. The first set of simulations reveals that the dissociative diffusion coefficient in vacancy-rich materials is inversely proportional to the initial vacancy concentration. In the second set of numerical simulations, impurity-induced changes in the vacancy concentration lead to distinctive diffusion profile shapes. The simulation results are also compared with published data of impurity diffusion in CIGS. And according to the characteristic properties of diffusion profiles from the two set of simulations, experimental detection of the dissociative diffusion mechanism in vacancy-rich materials may be possible.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Runaway electrons and magnetic island confinement

The breakup of magnetic surfaces is a central feature of ITER planning for the avoidance of damage due to runaway electrons. Rapid thermal quenches, which lead to large accelerating voltages, are thought to be due to magnetic surface breakup. Impurity injection to avoid and to mitigate both halo and runaway electron currents utilizes massive gas injection or shattered pellets. The actual deposition is away from the plasma center, and the breakup of magnetic surfaces is thought to spread the effects of the impurities across the plasma cross section. The breakup of magnetic surfaces would prevent runaway electrons from reaching relativistic energies were it not for the persistence of non-intercepting flux tubes. These are tubes of magnetic field lines that do not intercept the walls. In simulations and in magnetic field models, non-intercepting flux tubes are found to persist near the magnetic axis and in the cores of magnetic islands even when a large scale magnetic surface breakup occurs. As long as a few magnetic surfaces reform before all of the non-intercepting flux tubes dissipate, energetic electrons confined and accelerated in these flux tubes can serve as the seed electrons for a transfer of the overall plasma current from thermal to relativistic carriers. The acceleration of electrons is particularly strong because of the sudden changes in the poloidal flux that naturally occur in a rapid magnetic relaxation. Furthermore, the physics of magnetic islands as non-intercepting flux tubes is studied. Expressions are derived for (1) the size of islands required to confine energetic runaway electrons, (2) the accelerating electric field in an island, (3) the increase or reduction in the size of an island by the runaway electron current, (4) the approximate magnitude of the runaway current in an island, and (5) the time scale for the evolution of an island.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Mechanism of Na accumulation at extended defects in Si from first-principles

Sodium (Na) impurities in silicon solar cells are considered to play an important role in potential-induced degradation (PID), a significant cause of solar cell degradation and failure. Shorting due to Na accumulation at extended defects has been suggested as a culprit for PID. However, it is not clear how the extended defects are decorated by Na impurities. Using first-principles density functional theory calculations, we find that Na impurities segregate from the bulk into extended defects such as intrinsic stacking faults and Sigma 3 (111) grain boundaries. The energy barrier required for Na to escape from the extended defects is substantial and similar to the sum of the barrier energy in bulk Si (1.1-1.2 eV) and the segregation energy to the stacking fault (~0.7 eV). Surprisingly, the migration barrier for Na diffusion within the extended defects is even higher than the energy barrier for escaping. Furthermore, the results suggest that the extended defects likely accumulate Na as the impurities segregate to the defects from the bulk, rather than because of migration through the extended defects. Published by AIP Publishing.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Impact of centrifugal drifts on ion turbulent transport

Here, the influence of sonic toroidal rotation on gyrokinetic stability and transport is studied, with important implications for heavy impurity dynamics. When centrifugal drifts and electrostatic trapping corrections are included, significant modifications to the calculated transport of heavy impurities are observed. These high-rotation corrections add to the standard Coriolis drift and toroidal rotation shear drive which are normally included in gyrokinetics. Yet, because of their complexity, centrifugal and electrostatic trapping terms (quadratic in the main ion Mach number) are not generally included in gyrokinetic codes. In this work, we explore the implications of using reduced descriptions of the rotational physics. For heavy impurities such as tungsten, cross terms due to the centrifugal force can dominate the rotation dynamics, and neglecting them is shown to lead to large errors in the impurity particle flux.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

The charge exchange recombination spectroscopy diagnostic on the upgraded Lithium Tokamak eXperiment (LTX-β)

The Lithium Tokamak eXperiment has undergone an upgrade to LTX-β, a major part of which is the addition of neutral beam injection (NBI). NBI has allowed for a new charge exchange recombination spectroscopy (CHERS) system to be installed in order to measure impurity concentrations, ion temperature, and toroidal velocity. Previously on LTX measuring these parameters relied on passive spectroscopy and inversion techniques and had large uncertainty. The CHERS system has 52 total views, split into four groups of 13, half facing toward the beam and half symmetrically facing away from the beam, so the background non-beam related emission can be simultaneously subtracted. Both sets of views sample a major radius of 27-59 cm, with resolution through the beam of 1.5-2.5 cm. LTX-β is expected to have its magnetic axis near 35 cm, with minor radii of 18-23 cm. Three separate spectrometers will be used for the diagnostic, giving the system great flexibility to simultaneously measure emission from multiple impurity lines. The viewing optics are f/1.8, allowing all of the spectrometers to be fully illuminated. Design and calibration of the system as well as the advantages of various configurations of the spectrometers will be highlighted.

Elliott, Drew↗
DOE OSTI2019

Main-ion intrinsic toroidal rotation across the ITG/TEM boundary in DIII-D discharges during ohmic and electron cyclotron heating

Direct measurements of deuterium main-ion toroidal rotation spanning the linear ohmic to saturated ohmic confinement (LOC-SOC) regime and with additional electron cyclotron heating (ECH) are proposed and compared with the more commonly measured impurity (carbon) ion rotation in DIII-D. Main ions carry the bulk of the plasma toroidal momentum, and hence, the shape of the main-ion rotation is more relevant to the study of angular momentum transport in tokamaks. Both in the LOC regime and with ECH, the main-ion toroidal rotation frequency is flat across the profile from the sawtooth region to the plasma separatrix. However, the impurity rotation profile possesses a rotation gradient, with the rotation frequency being lower near the plasma edge, implying a momentum pinch or negative residual stress inferred from the impurity rotation that differs from the main-ion rotation. In the SOC regime, both the main-ion and impurity rotation profiles develop a deeply hollow feature near the midradius while maintaining the offset in the edge rotation, both implying a positive core residual stress. In the radial region where the rotation gradient changes most dramatically, turbulence measurements show that density fluctuations near the trapped electron mode (TEM) scale are higher when the rotation profile is flat and drop significantly when the plasma density is raised and the rotation profile hollows, consistent with instabilities damped by collisions. Linear initial value gyrokinetic simulations with GYRO indicate that the transition from LOC-SOC in DIII-D occurs as TEMs are replaced by ion temperature gradient (ITG) driven modes from the outer radii inwards as the plasma collisionality increases, Z eff decreases, and the power flow through the ion channel progressively increases due to the electron-ion energy exchange. Gyrofluid modeling with trap gyro-Landau fluid (TGLF) successfully reproduces the plasma profiles at key times in the discharge and in time dependent simulations with predictive TRANSP. TGLF indicates that in the LOC and SOC regimes as well as with ECH, subdominant modes are present and that the plasma is not in a pure TEM or ITG binary state, but rather a more subtle mixed state. Predictions of the main-ion rotation profiles are observed with global nonlinear gyrokinetic simulations using GTS and reveal that the flat rotation is due to oscillatory variation of the turbulent residual stress across the profile, whereas the deeply hollow rotation profile is due to a larger-scale, dipole-like stress profile. In these cases, the predicted and observed main-ion rotation profile is consistent with the balance of turbulent residual stress and momentum diffusion.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Microcalorimeter measurement of x-ray spectra from a high-temperature magnetically confined plasma

A NASA-built x-ray microcalorimeter spectrometer has been installed on the MST facility at the Wisconsin Plasma Physics Laboratory and has recorded x-ray photons emitted by impurity ions of aluminum in a majority deuterium plasma. Much of the x-ray microcalorimeter development has been driven by the needs of astrophysics missions, where imaging arrays with few-eV spectral resolution are required. The goal of our project is to adapt these single-photon-counting microcalorimeters for magnetic fusion energy research and demonstrate the value of such measurements for fusion science. Microcalorimeter spectrometers combine the best characteristics of the x-ray instrumentation currently available on fusion devices: high spectral resolution similar to an x-ray crystal spectrometer and the broadband coverage of an x-ray pulse height analysis system. Fusion experiments are increasingly employing high-Z plasma-facing components and require measurement of the concentration of all impurity ion species in the plasma. This diagnostic has the capability to satisfy this need for multi-species impurity ion data and will also contribute to measurements of impurity ion temperature and flow velocity, Zeff, and electron density. Here, we introduce x-ray microcalorimeter detectors and discuss the diagnostic capability for magnetic fusion energy experiments. We describe our experimental setup and spectrometer operation approach at MST, and we present the results from an initial measurement campaign.

Eckart, M. E. (ORCID:0000000338945889)↗
DOE OSTI2023

Geometric Optimization of an Electrochemical Purification Cell to Prevent Corrosion in CSP Plants During Operation

When exposed to moisture or oxygen, molten chloride salts produce corrosive impurities which degrade containment alloys. This can significantly decrease the lifetime and increase costs of molten-salt-based systems. To overcome this barrier, we designed and modeled an electrochemical purification cell to remove the corrosive impurity MgOH+. Various reactor architectures, including continuous stirred tank reactors (CSTRs) and plug flow reactors (PFRs) were investigated. Steady-state thermoelectric properties were evaluated using analytical methods, allowing assessment of the effects of structure and design parameters such as flow rate, cell length, and cross-sectional area of molten salt. The results suggest that our design could most effectively increase reliability and decrease costs of molten-chloride-salt-based systems by protecting them during continuous operation using an annular plug flow reactor.

analytical modeling↗