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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 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Copper interstitial recombination centers in Cu 3 N

We present a comprehensive study of the earth-abundant semiconductor Cu 3 N as a potential solar energy conversion material, using density functional theory and experimental methods. Density functional theory indicates that among the dominant intrinsic point defects, copper vacancies V Cu have shallow defect levels while copper interstitials Cu i behave as deep potential wells in the conduction band which mediate Shockley-Read-Hall recombination. The existence of Cu i defects has been experimentally verified using photothermal deflection spectroscopy. A Cu 3 N/ZnS heterojunction diode with good current-voltage rectification behavior has been demonstrated experimentally, but no photocurrent is generated under illumination. Finally, the absence of photocurrent can be explained by a large concentration of Cu i recombination centers capturing electrons in p-type Cu 3 N.

14 SOLAR ENERGY↗
DOE OSTI2014

Transport Powered by Bacterial Turbulence

We demonstrate that collective turbulent-like motion in a bacterial bath can power and steer directed transport of mesoscopic carriers through the suspension. In our experiments and simulations, a microwedge-like "bulldozer" draws energy from a bacterial bath of varied density. We obtain that an optimal transport speed is achieved in the turbulent state of the bacterial suspension. This apparent rectification of random motion of bacteria is caused by polar ordered bacteria inside the cusp region of the carrier, which is shielded from the outside turbulent fluctuations.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2024

Stripe and bubble ratchets on asymmetric substrates

We show that various nonmonotonic ratchet effects can arise when mesophase pattern-forming systems, which exhibit anisotropic crystal, stripe, and bubble regimes, are coupled to one-dimensional asymmetric substrates under ac driving. The patterns emerge in the absence of a substrate when the ratio of attraction to repulsion is varied for particles with competing short-range attraction and long-range repulsion potentials. In the presence of the substrate, we find that the ratchet efficiency varies nonmonotonically with increasing attraction, depending upon how well the mesophase morphology matches the substrate spacing and periodicity. For strong repulsion, there is a weak but finite ratchet effect, while at intermediate attraction, there is a robust ratchet effect in which the system forms stripes aligned with the substrate symmetry direction. For strong attraction, large bubbles appear that have weak or no ratchet effects when the bubble width exceeds the substrate lattice spacing, causing the bubble to be only weakly coupled to the substrate. For very strong attraction, small bubbles form and undergo a strong ratchet effect with an efficiency that oscillates as a function of ac drive amplitude. Although the small bubble regime is a strongly correlated regime in which each bubble contains many particles, the system behaves as if it is in a single-particle regime since the bubble width is much smaller than the substrate lattice spacing. We map out the different rectification phases as a function of the pattern morphology, substrate strength, and ac drive amplitude. The pronounced ratchet effects that we observe in some regimes can be exploited for pattern sorting in hard and soft matter systems. Published by the American Physical Society 2024

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Scaling Automatic Vector Data Alignment to Satellite Imagery

Given the tremendous volume of accessible Earth Observation (EO) data, there is a need to develop scalable Geospatial Artificial Intelligence (GeoAI) solutions for time-sensitive applications. Scalability in this context refers to rapidly processing large-scale EO data using high performance computing resources. Accurate mapping of the built environment from remote sensing (RS) imagery has been one of the crucial components in GeoAI workflows for a wide spectrum of humanitarian applications. Derived vector data of built environment is often leveraged for disaster preparedness and response activities. However, factors such as differences in ortho-rectification, atmospheric conditions and human error, results in spatial misalignment between vector data and the timely available RS imagery. Model training for downstream tasks such as object detection, change analysis, etc., is negatively impacted due to such spatial misalignment. Although there has been progress towards automatic alignment of vector data, the lack of scalability remains an open research challenge. This paper proposes to leverage parallel computing to optimize an automatic vector data alignment workflow. It further employs CPU-level multi-core parallelism for improving the performance of the workflow for scalable built environment mapping. We report observations and discuss findings from the preliminary experiments performed on the Summit Supercomputer.

Potnis, Abhishek↗
DOE OSTI2019

Updates on the Progress of the Alternate Topology Modulator (AT-HVCM) to support the Proton Power Upgrade (PPU) at the Spallation Neutron Source

The AT-HVCM, achieved by series-stacking the dual-function resonant filter capacitors following each rectification stage, is the baseline design for the PPU project at the SNS [1]. The project requires three AT-HVCM modulators to power 28 additional 700kW klystrons and associated high beta cavities to accelerate 38mA of beam current to 1.3GeV. Each modulator produces an 82 kV, 120A output pulse of 1.35ms duration at a 60Hz repetition rate. The AT design approach improves upon the existing HVCM topology, reducing stresses on the H bridge IGBTs using zero voltage (ZVS) and current switching (ZCS) techniques and on the resonant filter capacitors by limiting voltage reversal. This paper reviews the prototype development progress, currently focused on evaluating system reliability, and discusses some design techniques to achieve the modulator performance requirements.

Solley, Dennis J.↗
DOE OSTI2024

A Digital Twin Framework for Liquid-cooled Supercomputers as Demonstrated at Exascale

We present ExaDigiT, an open-source framework for developing comprehensive digital twins of liquid-cooled supercomputers. It integrates three main modules: (1) a resource allocator and power simulator, (2) a transient thermo-fluidic cooling model, and (3) an augmented reality model of the supercomputer and central energy plant. The framework enables the study of "what-if" scenarios, system optimizations, and virtual prototyping of future systems. Using Frontier as a case study, we demonstrate the framework's capabilities by replaying six months of system telemetry for systematic verification and validation. Such a comprehensive analysis of a liquid-cooled exascale supercomputer is the first of its kind. ExaDigiT elucidates complex transient cooling system dynamics, runs synthetic or real workloads, and predicts energy losses due to rectification and voltage conversion. Throughout our paper, we present lessons learned to benefit HPC practitioners developing similar digital twins. We envision the digital twin will be a key enabler for sustainable, energy-efficient supercomputing.

Brewer, Wes↗
DOE OSTI2019

Analysis of Diode Reverse Recovery Effect on ZVS Condition for GaN-Based LLC Resonant Converter

LLC resonant converter can achieve zero voltage switching (ZVS) for primary-side devices and zero current switching (ZCS) for secondary-side rectifiers. However, the reverse recovery and junction capacitance (C j ) of secondary-side diode critically affect the ZVS condition of primary-side switches. The effect of C j has been discussed in literature, but not the reverse recovery. In this paper, the reverse recovery charge (Q rr ) is converted to an equivalent capacitance (C rr_eq ) for the study of primary-side ZVS performance. An accurate model during deadtime is derived and further applied to characterize ZVS performance with different reverse recovery charges in different regions. The concept of establishing parameter C total to consider both C j and C rr_eq is proposed to evaluate the effect of the secondary-side rectifiers. This concept provides the guideline for diode and synchronous rectification MOSFET selection to ensure ZVS condition for LLC converters. To verify the concept and the derived model,a 200/400 V 400 W LLC resonant converter prototype operating from 200 to 700 kHz is built and its ZVS performances with different diodes are compared. Two issues caused by Q rr effect, including V ds reverse charging and asymmetrical waveform during deadtime, are explained thoroughly as well.

42 ENGINEERING↗
DOE OSTI2016

All Solution‐Based Fabrication of Copper Oxide Thin Film/Cobalt‐Doped Zinc Oxide Nanowire Heterojunctions

Versatile and intriguing solution‐based processes are utilized to synthesize nanostructured materials for device applications to reduce material production and device fabrication costs. This study presents results on the fabrication and characterization of copper oxide (CuO) coated cobalt‐doped zinc oxide nanowires (Co‐doped ZnO NW s)‐based heterojunction diodes prepared by a two‐step synthesis route through combined hydrothermal growth and sol–gel spin coating. Highly dense, well‐ordered, undoped, and Co‐doped ZnO NW s were successfully grown by hydrothermal method. Complementary CuO thin films were synthesized by sol–gel method and subsequently coated onto both undoped and Co‐doped ZnO NW s through spin‐coating technique. Enhanced diode properties with a rectification ratio of 10 3 at ±2 V and an ideality factor of n = 2.4 (in dark) were obtained for Co‐doped ZnO NW s‐based heterojunction diodes. The obtained results demonstrated that the investigated heterojunction diode structure fabricated by facile and cost‐effective solution‐based processes can be a promising candidate for the next generation optoelectronic devices.

Aksoy Akgul, Funda↗
DOE OSTI2023

A Machine Learning–Based Tire Life Prediction Framework for Increasing Life of Commercial Vehicle Tires

In the commercial freight industry, tire retreading decisions are often conservative due to limited knowledge of a tire’s remaining service life. This practice leads to increased costs and material waste. This paper proposes a machine learning–based approach for estimating tire casing life and retreadability, focusing on usage data rather than wear information. This approach could extend the tire’s lifespan and reduce landfill waste. Data integration from diverse tire casing measurement sources presents challenges, including imbalanced removal data. Our methodology addresses these challenges by using historical inspection, telematics, and finite element modeling (FEM) datasets. We introduce “Tire Casing Energy” as a comprehensive usage input and apply a Variance-Reduction Synthetic Minority Oversampling Technique (VR-SMOTE) for data imbalance rectification. A random forest model is used to estimate the state of the tire casing and the casing removal probability, with Bayesian optimization applied for hyperparameter tuning, enhancing model accuracy. Here, the proposed prediction framework is able to differentiate different truck fleets and tire locations based on their usage parameters. With the aid of this machine learning model, the importance and sensitivity of different tire usage parameters can be obtained, which is beneficial to maximize tire life.

Data balancing↗
DOE OSTI2014

Electrical properties of TiN on gallium nitride grown using different deposition conditions and annealing

This study evaluates the thermal stability of different refractory metal nitrides used as Schottky electrodes on GaN. The results demonstrate that TiN, MoSiN, and MoN possess good rectification and adhesion strength, with barrier heights of 0.56, 0.54, and 0.36 eV, respectively. After thermal treatment at 850 °C for 1 min, the TiN and MoN electrodes still exhibit rectifying characteristics, while the MoSiN degrades to an ohmic-like contact. For further study, several TiN films are deposited using different N{sub 2}/Ar reactive/inert sputtering gas ratios, thereby varying the nitrogen content present in the sputtering gas. Ohmic-like contact is observed with the pure Ti contact film, and Schottky characteristics are observed with the samples possessing nitrogen in the film. The average Schottky barrier height is about 0.5 eV and remains virtually constant with varying nitrogen deposition content. After examining Raman spectra and x-ray photoelectron spectroscopy results, the increase in the film resistivity after thermal treatment is attributed to oxidation and/or nitridation. Films deposited with a medium (40% and 60%) nitrogen content show the best film quality and thermal stability.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Review of recent studies on nanoscale electrical junctions and contacts: Quantum tunneling, current crowding, and interface engineering

The study of charge carrier transport at nanoscale electrical contacts is crucial for the development of next-generation electronics. In this study, we review recent modeling efforts on quantum tunneling, current crowding, and contact resistance across electrical interfaces with nanometer scale dimensions. A generalized self-consistent model for quantum tunneling induced electron transport in metal–insulator–metal (MIM) junctions is summarized. Rectification of a dissimilar MIM junction is reviewed. A modified two-dimensional (2D) transmission line model is used to investigate the effects of spatially varying specific contact resistivity along the contact length. The model is applied to various types of electrical contacts, including ohmic contacts, MIM junction based tunneling contacts, and 2D-material-based Schottky contacts. Roughness engineering is recently proposed to offer a possible paradigm for reducing the contact resistance of 2D-material-based electrical contacts. Contact interface engineering, which can mitigate current crowding near electrical contacts by spatially designing the interface layer thickness or properties, without requiring an additional material or component, is briefly reviewed. Tunneling engineering is suggested to eliminate severe current crowding in highly conductive ohmic contacts by introducing a thin tunneling layer or gap between the contact members. Furthermore, unsolved problems and challenges are also discussed.

2D materials↗
DOE OSTI2023

Ultrawide bandgap vertical β-(Al x Ga1 −x )2O3 Schottky barrier diodes on free-standing β-Ga2O3 substrates

Ultrawide bandgap β-(AlxGa1−x)2O3 vertical Schottky barrier diodes on (010) β-Ga2O3 substrates are demonstrated. The β-(AlxGa1−x)2O3 epilayer has an Al composition of 21% and a nominal Si doping of 2 × 1017 cm−3 grown by molecular beam epitaxy. Pt/Ti/Au has been employed as the top Schottky contact, whereas Ti/Au has been utilized as the bottom Ohmic contact. The fabricated devices show excellent rectification with a high on/off ratio of ∼109, a turn-on voltage of 1.5 V, and an on-resistance of 3.4 mΩ cm2. Temperature-dependent forward current-voltage characteristics show effective Schottky barrier height varied from 0.91 to 1.18 eV while the ideality factor from 1.8 to 1.1 with increasing temperatures, which is ascribed to the inhomogeneity of the metal/semiconductor interface. The Schottky barrier height was considered a Gaussian distribution of potential, where the extracted mean barrier height and a standard deviation at zero bias were 1.81 and 0.18 eV, respectively. A comprehensive analysis of the device leakage was performed to identify possible leakage mechanisms by studying temperature-dependent reverse current-voltage characteristics. At reverse bias, due to the large Schottky barrier height, the contributions from thermionic emission and thermionic field emission are negligible. By fitting reverse leakage currents at different temperatures, it was identified that Poole–Frenkel emission and trap-assisted tunneling are the main leakage mechanisms at high- and low-temperature regimes, respectively. Electrons can tunnel through the Schottky barrier assisted by traps at low temperatures, while they can escape these traps at high temperatures and be transported under high electric fields. This work can serve as an important reference for the future development of ultrawide bandgap β-(AlxGa1−x)2O3 power electronics, RF electronics, and ultraviolet photonics.

Materials Science↗
DOE OSTI2024

Electro-physical properties of surface-barrier diodes on low-resistance n-Cd 0.96 Mn 0.04 Te 0.96 Se 0.04 crystals

The electro-physical properties of Cd 0.96 Mn 0.04 Te 0.96 Se 0.04 n-type conductivity single crystals with relatively low resistivity were studied. The determined resistivity of semiconductor single crystals Cd 0.96 Mn 0.04 Te 0.96 Se 0.04 n-type conductivity is equal to ρ≈2 Ω-cm at 293K. The activation energy of dark conductivity, measured from the temperature dependence of the resistivity of the In/Cd 0.96 Mn 0.04 Te 0.96 Se 0.04 /In structure, is equal to ΔE≈0.11eV. From optical measurements of the absorption coefficient from the energy of photons in the region of large α, the band-gap width of the single crystal was found, which is equal to E g =1.53eV. Diode structures of Ni/Cd 0.96 Mn 0.04 Te 0.96 Se 0.04 /In obtained by vacuum sputtering of Ni on the surface of a single crystal n-Cd 0.96 Mn 0.04 Te 0.96 Se 0.04 were manufactured and studied. Here, the rectification coefficient at a voltage of U=1V is equal to 10 8 . Within the framework of the Sah-Noys-Shockley carrier generation-recombination model, a quantitative match of the experimental measurements of the volt-ampere characteristics with the calculation was achieved.

(Cd,Mn)(Te,Se)↗
DOE OSTI2015

SU-E-T-235: Data Mining for Evaluating Treatment Performances Over a Large Quantity of Data to Monitor and Improve SBRT Workflow

Purpose: To quality assure a large quantity of retrospective treatment cases for treatment performances by randomly sampling is inefficient. Here we provide a method to efficiently monitor and investigate the QA of SBRT workflow over Mosaiq. Methods: The code developed with Microsoft SQL Server Management Studio 2008R2 and VBA was used for retrieving and sorting data from Mosaiq (version 2.3–2.6 during 2012–2015). SBRT patients were filtered by fractional dose over 350cGy and total fraction number less than 6, which SBRT prescriptions were defined. The quality assurance on the SBRT workflow was focused on the treatment deliveries such as patient positioning setup, CBCT indicated offsets and couch shifted corrections. The treatment delivery were done by Varian Truebeam systems and the record/verify by Mosaiq. Results: Total 82 SBRT patients corresponding to 103 courses and 854 CBCT images were found by the retrieval query. Most centers record daily pre-treatment (Pre-Tx: before treatment shift) image-guided shifts along treatment course for inter-fraction motion record, and it is useful to also verify it with post-treatment imaging (Post-Tx: after treatment CBCT verification) to verify intra-fraction motion. Analyzing the details of daily recorded shifts can reveals the information of patient-setup and staff’s record/verify behaviors. 3 examples were provided as solid evidences and on-going rectification for preventing future mistakes. Conclusions: The report gave feasible examples for inspector to verify a large amount of data during site investigation. This program can also be extended to a scheduled data mining with software to periodical analyze the timely records in Mosaiq, for example, a various control charts for different QA purposes. As the current trend of automation in radiation therapy field, the data mining would be a necessary tool in the future, just as the automatic plan quality evaluation has been under development in Eclipse.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2022

Atomic-scale quantum sensing based on the ultrafast coherence of an H 2 molecule in an STM cavity

A scanning tunneling microscope (STM) combined with a pump-probe femtosecond terahertz (THz) laser can enable coherence measurements of single molecules. We report THz pump-probe measurements that demonstrate quantum sensing based on a hydrogen (H 2 ) molecule in the cavity created with an STM tip near a surface. Atomic-scale spatial and femtosecond temporal resolutions were obtained from this quantum coherence. The H 2 acts as a two-level system, with its coherent superposition exhibiting extreme sensitivity to the applied electric field and the underlying atomic composition of the copper nitride (Cu 2 N) monolayer islands grown on a Cu(100) surface. We acquired time-resolved images of THz rectification of H 2 over Cu 2 N islands for variable pump-probe delay times to visualize the heterogeneity of the chemical environment at sub-angstrom scale.

Science & Technology - Other Topics↗
DOE OSTI2017

n-ZnO/p-CuI barrier heterostructure based on zinc-oxide nanoarrays formed by pulsed electrodeposition and SILAR copper-iodide films

A p-CuI/n-ZnO barrier structure is investigated as a promising base diode structure for a semitransparent near-ultraviolet detector. We analyze the crystal structure and electrical and optical properties of zinc-oxide nanoarrays electrodeposited in the pulsed mode and copper-iodide films formed by the successive ionic layer adsorption and reaction (SILAR) method, which were used as the basis for an n-ZnO/p-CuI barrier heterostructure sensitive to ultraviolet radiation in the spectral range of 365–370 nm. Using the I–V characteristics, a shunting resistance of R{sub sh} · S{sub c} = 879 Ω cm{sup 2}, a series resistance of R{sub s} · S{sub c} = 8.5 Ω cm{sup 2}, a diode rectification factor of K = 17.6, a rectifying p–n-junction barrier height of Φ = 1.1 eV, and a diode ideality factor of η = 2.4 are established. It is demonstrated that at low forward biases (0 V < {sup U} < 0.15 V), the effects of charge-carrier recombination and tunneling are equal. As the bias increases above 0.15 V, the tunneling–recombination transport mechanism starts working. The diode saturation current J{sub 0} is found to be 6.4 × 10{sup –6} mA cm{sup –2} for recombination and tunneling charge-carrier transport and 2.7 × 10{sup –3} mA cm{sup –2} for tunneling–recombination charge-carrier transport.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Field Characteristics of Spin-Torque Diode Sensitivity in the Presence of a Bias Current

The use of the spin-torque diode effect, which is generated upon the current-induced transfer of spin angular momentum in magnetic tunnel junctions, opens the prospect of considerably improving microwave sensitivity in the gigahertz frequency band over that of semiconductor Schottky diodes. In this work, the spin-diode effect of microwave signal rectification in a magnetic tunnel junction upon the resonant excitation of spin waves in the free magnetic layer as a result of the current-induced spin-transfer torque effect is analyzed theoretically. Frequency characteristics of the resonant response of a spin-torque diode to a microwave signal are calculated in the linear macrospin approximation as functions of the direction and value of the applied magnetic field and bias current. It is shown that with zero bias current, the maximum rectified voltage across a junction is obtained in the geometry of mutually perpendicular magnetizations of magnetic layers; as the resonant frequency of oscillations in a magnetic field increases, this voltage drops at the retained equilibrium orientation of spins in the layers. After the bias current is switched on, the resonance amplitude of forced oscillations of the free layer spins upon microwave excitation grows sharply at the critical point of the spin-torque diode’s loss of equilibrium state stability. The linewidth at this point is limited only by nonlinear effects. Improving spin-torque diode sensitivity is important for its use in microwave holographic vision systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗