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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Ab initio investigation of elastic properties of dilute Cu alloys for high-gradient accelerating structures

The elastic properties and solid solution strengthening of several Cu alloys that may be promising for high gradient applications were studied using ab initio calculations based on density functional theory. Specifically, the temperature and solute concentration dependence of elastic properties including bulk and shear moduli, thermal expansion coefficient, and heat capacity of these alloys were calculated. We found that the bulk and shear moduli are linearly dependent on the solute concentration in the dilute regime and change significantly for different Cu alloys. The thermal expansion coefficient and heat capacity are comparatively less sensitive to solute concentrations. Solid solution strengthening is observed to be linearly correlated with the change of lattice parameter of Cu. This work provides an in-depth understanding of the elastic properties of Cu alloys, which is key for predicting their performance at high gradient in normal conducting accelerating structures.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Implementing microwave impedance microscopy in a dilution refrigerator

We report the implementation of a dilution refrigerator-based scanning microwave impedance microscope with a base temperature of ∼100 mK. The vibration noise of our apparatus with tuning-fork feedback control is as low as 1 nm. Using this setup, we have demonstrated the imaging of quantum anomalous Hall states in magnetically (Cr and V) doped (Bi, Sb)2Te3 thin films grown on mica substrates. Both the conductive edge modes and topological phase transitions near the coercive fields of Cr- and V-doped layers are visualized in the field-dependent results. Our study establishes the experimental platform for investigating nanoscale quantum phenomena at ultralow temperatures.

Instruments & Instrumentation↗
DOE OSTI2023

Self-consistent hopping theory of activated relaxation and diffusion of dilute penetrants in dense crosslinked polymer networks

We generalize a microscopic statistical mechanical theory of the activated dynamics of dilute spherical penetrants in glass-forming liquids to study the influence of crosslinking in polymer networks on the penetrant relaxation time and diffusivity over a wide range of temperature and crosslink fraction (f n ). Our calculations are relevant to recent experimental studies of a nm-sized molecule diffusing in poly-(n-butyl methacrylate) networks. The theory predicts the penetrant relaxation time increases exponentially with the glass transition temperature, T g (f n ), which grows roughly linearly with the square root of fn due to the coupling of local hopping to longer-range collective elasticity. Moreover, T g is also found to be proportional to a geometric confinement parameter defined as the ratio of the penetrant diameter to the mean network mesh size. The decoupling ratio of the penetrant and Kuhn segment alpha times displays a complex non-monotonic dependence on f n and temperature that is well collapsed based on the variable T g (f n )/T. A model for the penetrant diffusion constant that combines activated relaxation and entropic mesh confinement is proposed, which results in a significantly stronger suppression of mass transport with degree of effective supercooling than predicted for the penetrant alpha time. Furthermore, this behavior corresponds to a new network-based type of “decoupling” of diffusion and relaxation. In contrast to the diffusion of larger nanoparticles in high temperature rubbery networks, our analysis in the supercooled regime suggests that for the penetrants studied the mesh confinement effects are of secondary importance relative to the consequences of crosslink-induced slowing down of activated hopping of glassy physics origin.

Chemistry↗
DOE OSTI2024

Structural and optical characterization of dilute Bi-doped GaN nanostructures grown by molecular beam epitaxy

We have carried out detailed studies on the epitaxy and characterization of dilute Bi-doped GaN nanostructures. A comprehensive investigation of Bi-doped GaN nanowires and quasi-film epitaxial growth conditions has been performed. Scanning electron microscopy studies show that lowering the GaBiN growth temperature causes gradual changes in top c-plane nanowire morphology due to the incremental incorporation of foreign Bi atoms. This trend is further substantiated by the secondary ion mass spectroscopy analysis of a multi-layer Bi-doped GaN quasi-film. However, it is also found that the amount of Bi incorporation into the GaN lattice is relatively independent of the N2 flow rate variation under the growth conditions investigated. Furthermore, room-temperature micro-Raman spectra show that there are additional peaks near 530, 650, and 729 cm−1 wave numbers in the Bi-doped GaN samples, which can primarily be attributed to Bi local vibrational modes, indicative of a small amount of Bi incorporation in the GaN lattice. Moreover, phonon calculations with density functional theory indicate that Bi replacing the N sites is the likely origin of the experimentally measured Raman modes. X-ray photoelectron spectroscopy measurements have also been obtained to deduce the electronic interaction between the Bi dopant atom and the GaN nanostructure. Such one-dimensional nanowires permit the synthesis of dislocation-free highly mismatched alloys due to strain relaxation, allowing efficient light absorption and charge carrier extraction that is relevant for solar energy harvesting and artificial photosynthesis.

Materials Science↗
DOE OSTI2025

In-plane thermal conductivity and the applicability of the Wiedemann–Franz law in dilute AlCu thin films

The Wiedemann–Franz (WF) law correlates heat and charge transport in metals. However, the validity of this correlation remains an open-ended question, especially in the context of inelastic scattering at room temperature. To address this gap in knowledge, we perform independent measurements of the in-plane thermal and electrical conductivities across four AlCu (0.5% Cu) films [thickness (⁠h⁠) ≈ 174, 98, 53, and 24 nm] using optical pump–probe metrologies and four-point probe techniques, respectively. For in-plane thermal conductivity measurements, we utilize time-domain thermoreflectance, in both concentric and beam-offset configurations, and the time-resolved magneto-optic Kerr effect. Our results show that the WF law overpredicts the thermal conductivity by at least ∼10% in all films, thus demonstrating modest deviations in predicted thermal conductivity when applying the WF law to dilute AlCu films. Using infrared variable angle spectroscopic ellipsometry, we demonstrate increased electron scattering rates in the thinnest film (⁠⁠h⁠ ≈ 24 nm), indicating electron-boundary scattering drives the reduction in in-plane thermal conductivity. Furthermore, this is generally an elastic scattering process, which is supported by our thermal conductivity measurements and analysis.

Electrical conductivity↗
DOE OSTI2025

Large deviations of ionic currents in dilute electrolytes

Here, we evaluate the exponentially rare fluctuations of the ionic current for a dilute electrolyte by means of macroscopic fluctuation theory. We consider the fluctuating hydrodynamics of a fluid electrolyte described by a stochastic Poisson–Nernst–Planck equation. We derive the Euler–Lagrange equations that dictate the optimal concentration profiles of ions conditioned on exhibiting a given current, whose form determines the likelihood of that current in the long-time limit. For a symmetric electrolyte under small applied voltages, number density fluctuations are small, and ionic current fluctuations are Gaussian with a variance determined by the Nernst–Einstein conductivity. Under large applied potentials, the ionic current distribution is generically non-Gaussian. Its structure is constrained thermodynamically by Gallavotti–Cohen symmetry and the thermodynamic uncertainty principle.

Farhadi, Jafar [University of California, Berkeley↗
DOE OSTI2026

Radiation-induced segregation in dilute Fe–Cr: A rate-theory framework for the Cr enrichment–depletion transition at the grain boundary

Radiation-induced segregation (RIS) poses a significant challenge for ferritic Fe–Cr alloys under irradiation, as it can compromise mechanical integrity and increase susceptibility to intergranular corrosion. Yet, the mechanisms governing Cr segregation remain incompletely understood. Here, in this study, we present a physics-based rate-theory model parameterized using self-consistent mean field theory-based Onsager transport coefficients to investigate RIS at the grain boundary in dilute Fe-(0.1 at. %) Cr. Under equal production rates of vacancies and self-interstitial atoms (SIA), and their equal absorption rates by bulk dislocations, the model simulates the experimentally observed transition from Cr enrichment at low temperatures to depletion at higher temperatures. Under these unbiased conditions, systematic investigation reveals that while temperature-dependent transport properties dictate the segregation direction, dose rate, grain size, and dislocation density only influence the magnitude and spatial extent of Cr segregation. However, under more realistic conditions of preferential vacancy production within damage cascade and/or preferential SIA absorption by bulk dislocations, the enrichment-to-depletion transition shifts to lower temperatures. Our findings demonstrate that RIS predictions based solely on transport coefficients are valid only under symmetric point defect flux conditions, and that biases in defect production and absorption must be considered for accurate predictions. This work provides a mechanistic framework for understanding RIS in ferritic alloys and informs alloy design for advanced nuclear systems.

11 - NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Reducing Spontaneous Orientational Polarization via Semiconductor Dilution Improves OLED Efficiency and Lifetime

Spontaneous orientational polarization (SOP) in the electron-transport layer (ETL) of organic light-emitting diodes (OLEDs) is increasingly recognized as a key factor influencing their performance. Here, we show that SOP is dramatically reduced in the common electron-transport material 2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) by coevaporating it together with medium-density polyethylene. Eliminating SOP from the ETL of blue fluorescent OLEDs in this manner reduces their operating voltage by 0.5 V, increases their external quantum efficiency (EQE) by 30%, and leads to a threefold increase in device lifetime. We show that the EQE and lifetime improvements both originate from reduced exciton-polaron annihilation in the emissive layer, and that this leads to a functional relationship between the two quantities that can be used to quantify the rate of annihilation-induced degradation in the device. Furthermore, these results highlight a substantial opportunity to improve OLED performance by controlling SOP through semiconductor dilution and suggest that this capability can be used to systematically isolate and understand exciton-polaron degradation in the pursuit of stable blue OLEDs.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Spin Polarons and Ferromagnetism in Doped Dilute Moiré-Mott Insulators

Moiré heterostructures of transition metal dichalcogenides (TMDs) exhibit Mott-insulating behavior both at half filling as well as at fractional fillings, where electronic degrees of freedom form self-organized Wigner crystal states. An open question concerns magnetic states obtained by lifting the pseudospin-1/2 degeneracy of these states at lowest temperatures. While at half filling virtual hopping is expected to induce (weak) antiferromagnetic exchange interactions, these are strongly suppressed when considering dilute filling fractions. We argue that, instead, a small concentration of doped electrons leads to the formation of spin polarons, inducing ferromagnetic order at experimentally relevant temperatures, consistent with recently observed ferromagnetic states in moiré TMD systems. Here, we predict explicit signatures of polaron formation in the magnetization profile.

Kondo lattice model↗
DOE OSTI2023

Enhanced magnetostriction through dilute Ce doping of Fe-Ga

Doping of magnetostrictive galfenol (Fe 82 Ga 18 , in at.%) with rare-earth elements significantly enhances magnetostriction, with the largest gains achieved in textured melt-spun ribbons. Here, it is demonstrated that even extremely dilute Ce, as little as 65 ppm, can double the magnetostrictive response of galfenol when coupled with an appropriate heat treatment. This improvement is correlated with a compression of the host lattice, both of which reach their maximum extent at the calculated solubility limit of Ce in in body-centered cubic (bcc) galfenol, ~ 50 ppm. Beyond this point, excess Ce segregates into CeGa 2 , which forms an interdendritic network throughout the sample at high Ce levels and cannot be resolutionized through heat treatments. These findings point to the importance of solubility limits (i.e., equilibrium thermodynamics) in determining appropriate doping levels or heat treatment couples to optimize magnetostrictive performance, confirming that overdoping is actively detrimental to both material properties and cost.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Band-gap reduction and band alignments of dilute bismide III–V alloys

Adding a few atomic percent of Bi to III–V semiconductors leads to significant changes in their electronic structure and optical properties. Bismuth substitution on the pnictogen site leads to a large increase in spin-orbit splitting Δ SO at the top of the valence band (Γ 8⁢𝑣 −Γ 7⁢𝑣 ) and a large reduction in the band gap, creating unique opportunities in semiconductor device applications. Quantifying these changes is key to the design and simulation of electronic and optoelectronic devices. Using hybrid functional calculations, we predict the band gap of III–Vs (III = Al, Ga, In and V = As, Sb) with low concentrations of Bi (3.125% and 6.25%), the effects of adding Bi on the valence- and conduction-band edges, and the band offset between these dilute alloys and their III–V parent compounds. As expected, adding Bi raises the valence-band maximum (VBM). However, contrary to previous assumptions, the conduction-band minimum (CBM) is also significantly lowered, and both effects contribute to the sizable band-gap reduction. Changes in band gap and Δ SO are notably larger in the arsenides than in the antimonides. In conclusion, we also predict cases of band-gap inversion (Γ 6⁢𝑐 below Γ 8⁢𝑣 ), and Δ SO larger than the band gap, which are key parameters for designing topological materials and for minimizing losses due to Auger recombination in infrared lasers.

Saboor, Abdul [Univ. of Delaware, Newark, DE (Unit↗
DOE OSTI2022

Stabilization of garnet/Li interphase by diluting the electronic conductor

The high interfacial resistance and lithium (Li) dendrite growth are two major challenges for solid-state Li batteries (SSLBs). The lack of understanding on the correlations between electronic conductivity and Li dendrite formation limits the success of SSLBs. Here, by diluting the electronic conductor from the interphase to bulk Li during annealing of the aluminium nitride (AlN) interlayer, we changed the interphase from mixed ionic/electronic conductive to solely ionic conductive, and from lithiophilic to lithiophobic to fundamentally understand the correlation among electronic conductivity, Li dendrite, and interfacial resistance. During the conversion-alloy reaction between AlN and Li, the lithiophilic and electronic conductive Li x Al diffused into Li, forming a compact lithiophobic and ionic conductive Li 3 N, which achieved an ultrahigh critical current density of 2.6/14.0 mA/cm 2 in the time/capacity-constant mode, respectively. The fundamental understanding on the effect of interphase nature on interfacial resistance and Li dendrite suppression will provide guidelines for designing high-performance SSLBs.

25 ENERGY STORAGE↗
DOE OSTI2020

Learning reference governor for cycle-to-cycle combustion control with misfire avoidance in spark-ignition engines at high exhaust gas recirculation–diluted conditions

Cycle-to-cycle feedback control is employed to achieve optimal combustion phasing while maintaining high levels of exhaust gas recirculation by adjusting the spark advance and the exhaust gas recirculation valve position. The control development is based on a control-oriented model that captures the effects of throttle position, exhaust gas recirculation valve position, and spark timing on the combustion phasing. Under the assumption that in-cylinder pressure information is available, an adaptive extended Kalman filter approach is used to estimate the exhaust gas recirculation rate into the intake manifold based on combustion phasing measurements. The estimation algorithm is adaptive since the cycle-to-cycle combustion variability (output covariance) is not known a priori and changes with operating conditions. A linear quadratic regulator controller is designed to maintain optimal combustion phasing while maximizing exhaust gas recirculation levels during load transients coming from throttle tip-in and tip-out commands from the driver. During throttle tip-outs, however, a combination of a high exhaust gas recirculation rate and an overly advanced spark, product of the dynamic response of the system, generates a sequence of misfire events. In this work, an explicit reference governor is used as an add-on scheme to the closed-loop system in order to avoid the violation of the misfire limit. The reference governor is enhanced with model-free learning which enables it to avoid misfires after a learning phase. Experimental results are reported which illustrate the potential of the proposed control strategy for achieving an optimal combustion process during highly diluted conditions for improving fuel efficiency.

42 ENGINEERING↗
DOE OSTI2024

Mark-18A Loading Capacity and Dilute Acid Rinse of DGA Resin

Savannah River National Laboratory is developing a separations flowsheet for the recovery of valuable elements from irradiated 242 Pu targets. The primary impetus for this effort is to recover the rich amount of 244 Pu contained in the targets, however, the targets also contain other valuable isotopes such as heavy Cm and 241,243 Am. DGA Resin, developed by Eichrom Technologies Inc., was selected to recover these materials from the 7 – 9 M HNO 3 matrix raffinate of the anion exchange process used to separate Pu. The resin has a high affinity for trivalent actinides and rare earth elements (REEs) in moderate to strong nitric acid. After loading the resin, the extracted materials (including Cm and Am) are to be recovered as oxides in a two-step process by thermally drying and reducing the mass and volume of the resin in the process column followed by calcination of the resultant residue. It is desirable to reduce the amount of residual HNO 3 in the resin bed prior to these furnace operations, but the reduced acid concentration is known to lower the distribution coefficients of the components of interest to the extractant. The purpose of this study was to validate the loading capacity of the resin and determine losses to be expected from rinsing the loaded resin with dilute HNO 3 .

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2026

Metallic Phase-Free Zn-Al Mixed Oxide Dual Function Materials Enable High Co Selectivity in Reactive Carbon Capture From Dilute Streams

Scaling conventional carbon capture and utilization methods can be limited by cost and permitting issues associated with transportation of captured CO2. Reactive carbon capture (RCC), in which a single solid-phase dual function material (DFM) is used to both capture CO2 from dilute streams (e.g., flue gas) and catalytically convert the bound species to products in a single unit operation, has the potential to reduce energy and capital costs by over 50% relative to separate capture and conversion. To incentivize adoption, high-value products such as methanol and CO should be targeted. Appealingly, CO can be produced at atmospheric pressure, thereby lowering overall H2 demand; however, high reaction temperatures (> 600 degrees C) and the use of oxidizable transition metals, such as Ni, are often necessary to drive the reverse water-gas shift (RWGS) during reactive desorption of the bound CO2. The sensitivity of these transition metals to oxygen undercuts their utility in point source RCC. To further derisk RCC, it is essential to develop metallic-phase free DFMs that are insensitive to residual oxygen in flue gas and can achieve selective reactive desorption to CO at moderate pressures (< 400 degrees C). To this end, we have developed K-modified Zn-Al mixed oxides (K/ZnAlOx) to convert captured CO2 to CO with > 97% selectivity and yields up to 53% of captured CO2 at 400 degrees C. Complementary in situ spectroscopy studies revealed the role of K-modification in improving RCC performance of unmodified ZnAlOx. The top performing DFM was also subjected to extended RCC cycling with oxygen co-fed with CO2 during the capture test to assess durability under simulated flue gas.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗