Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “CARRIER MODULATION”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Terrestrial Trunked Radio (TETRA) exposure of neuronal in vitro networks

Highlights: • For the first time, we examined the impact of TETRA-RF on the electrophysiology of neuronal in vitro networks. • We examine whether TETRA short- and long-term exposure could affect the electrophysiology of neuronal in vitro networks. • Analysed parameters were spike rate, burst rate, burst duration and network synchrony. Terrestrial Trunked Radio (TETRA) is a worldwide common mobile communication standard, used by authorities and organizations with security tasks. Previous studies reported on health effects of TETRA, with focus on the specific pulse frequency of 17.64 Hz, which affects calcium efflux in neuronal cells. Likewise among others, it was reported that TETRA affects heart rate variability, neurophysiology and leads to headaches. In contrast, other studies conclude that TETRA does not affect calcium efflux of cells and has no effect on people's health. In the present study we examine whether TETRA short- and long-term exposure could affect the electrophysiology of neuronal in vitro networks. Experiments were performed with a carrier frequency of 395 MHz, a pulse frequency of 17.64 Hz and a differential quaternary phase-shift keying (π/4 DQPSK) modulation. Specific absorption rates (SAR) of 1.17 W/kg and 2.21 W/kg were applied. In conclusion, the present results do not indicate any effect of TETRA exposure on electrophysiology of neuronal in vitro networks, neither for short-term nor long-term exposure. This applies to the examined parameters spike rate, burst rate, burst duration and network synchrony.

63 RADIATION, THERMAL, AND OTHER ENVIRON. POLLUTAN↗
DOE OSTI2021

Gate-tunable plasmons in mixed-dimensional van der Waals heterostructures

Abstract Surface plasmons, collective electromagnetic excitations coupled to conduction electron oscillations, enable the manipulation of light–matter interactions at the nanoscale. Plasmon dispersion of metallic structures depends sensitively on their dimensionality and has been intensively studied for fundamental physics as well as applied technologies. Here, we report possible evidence for gate-tunable hybrid plasmons from the dimensionally mixed coupling between one-dimensional (1D) carbon nanotubes and two-dimensional (2D) graphene. In contrast to the carrier density-independent 1D Luttinger liquid plasmons in bare metallic carbon nanotubes, plasmon wavelengths in the 1D-2D heterostructure are modulated by 75% via electrostatic gating while retaining the high figures of merit of 1D plasmons. We propose a theoretical model to describe the electromagnetic interaction between plasmons in nanotubes and graphene, suggesting plasmon hybridization as a possible origin for the observed large plasmon modulation. The mixed-dimensional plasmonic heterostructures may enable diverse designs of tunable plasmonic nanodevices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Voltage-controlled ferromagnetism and magnetoresistance in LaCoO{sub 3}/SrTiO{sub 3} heterostructures

A LaCoO{sub 3}/SrTiO{sub 3} heterostructure grown on Si (001) is shown to provide electrically switchable ferromagnetism, a large, electrically tunable magnetoresistance, and a vehicle for achieving and probing electrical control over ferromagnetic behavior at submicron dimensions. Fabrication of devices in a field-effect transistor geometry enables application of a gate bias voltage that modulates strain in the heterostructure via the converse piezoelectric effect in SrTiO{sub 3}, leading to an artificial inverse magnetoelectric effect arising from the dependence of ferromagnetism in the LaCoO{sub 3} layer on strain. Below the Curie temperature of the LaCoO{sub 3} layer, this effect leads to modulation of resistance in LaCoO{sub 3} as large as 100%, and magnetoresistance as high as 80%, both of which arise from carrier scattering at ferromagnetic-nonmagnetic interfaces in LaCoO{sub 3}. Finite-element numerical modeling of electric field distributions is used to explain the dependence of carrier transport behavior on gate contact geometry, and a Valet-Fert transport model enables determination of spin polarization in the LaCoO{sub 3} layer. Piezoresponse force microscopy is used to confirm the existence of piezoelectric response in SrTiO{sub 3} grown on Si (001). It is also shown that this structure offers the possibility of achieving exclusive-NOR logic functionality within a single device.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Thermomagnetic responses of semimetals

Solid-state thermomagnetic modules operating based on the Nernst–Ettingshausen effects are an alternative to conventional solid-state thermoelectric modules. These modules are appropriate for low-temperature applications where the thermoelectric modules are not efficient. Here, we briefly discuss the application, performance, similarities, and differences of thermoelectric and thermomagnetic materials and modules. We review thermomagnetic module design, Nernst coefficient measurement techniques, and theoretical advances, emphasizing the Nernst effect and factors influencing its response in semimetals such as carrier compensation, Fermi surface, mobility, phonon drag, and Berry curvature. The main objective is to summarize the materials design criteria to achieve high thermomagnetic performance to accelerate thermomagnetic materials discovery.

42 ENGINEERING↗
NASA NTRS2001

Further Investigations of the Passive Optical Sample Assembly (POSA) - I Flight Experiment

The Passive Optical Sample Assembly-I (POSA-I), part of the Mir Environmental Effects Payload (MEEP), was designed to study the combined effects of contamination, atomic oxygen, ultraviolet radiation, vacuum, then-nal cycling, and other constituents of the space environment on spacecraft materials. The MEEP program is a Phase I International Space Station Risk Mitigation Experiment. Candidate materials for the International Space Station (ISS) were exposed in a specially designed "suitcase" carrier, with identical specimens facing either Mir or space. The payload was attached by EVA to the exterior of the Mir docking module during the STS-76 mission (f'ig. 1). It was removed during the STS-86 mission after an 18-month exposure. During the mission, it received approximately 7 x 1019 atoMS/CM2 atomic oxygen, as calculated by polymer mass loss, and 413 ESH of solar ultraviolet radiation on the Mir-facing side. The side facing away from Mir received significant contaminant deposition, so atomic oxygen fluence has not been reliably determined. The side facing away from Mir received 571 ESH of solar UV. Contamination was observed on both the Mir-facing and space-facing sides of the POSA-I experiment , with a greater amount of deposition on the space facing side than the Mir side. The contamination has been determined to be outgassed silicone photofixed by ultraviolet radiation and converted to silicate by atomic oxygen interaction. Electron spectroscopy for chemical analysis (ESCA) with depth profiling indicated the presence of 26 - 31 nm silicate on the Mir-facing side and 500 - 1000 nm silicate on the space-facing side. The depth profiling also showed that the contaminant layer was uniform, with a small amount of carbon present on the surface and trace amounts of nitrogen, phosphorus, sulfur, and tin. The surface carbon layer is likely due to post-flight exposure in the laboratory and is similar to carbonaceous deposits on control samples. EDAX and FTIR analysis concurred with ESCA for the presence of silicon, oxygen, and carbon. Nearly 400 samples were exposed on POSA-I, which included materials such as thermal control coatings polymeric films, optical materials, and multi-layer insulation blankets. A previous paper discussed the effects of the space environment exposure and contaminant deposition on candidate materials for ISS, including Z93P inorganic thermal control coating, various anodizes, and multi-layer insulation blankets. This paper details the investigation of environmental effects on the remainder of POSA-I samples, particularly the innovative conductive thermal control coatings developed by AZ Technology of Huntsville, AL and HT Research Institute of Chicago, IL. The silicone/silicate contamination had a significant impact on the solar absorptance of white inorganic thermal control coatings on the space-facing side of POSA-I. The effect of contamination on electrical conductivity is discussed. Samples of conductive anodized aluminum developed by Boundary Technologies of Buffalo Grove, IL were also exposed on POSA-I. The effects of the space environment and contaminant deposition on the optical and electrical properties of the conductive anodized aluminum are discussed.

Finckenor, Miria M.↗
NASA NTRS2016

Concept of Operations for a Prospective "Proving Ground" in the Lunar Vicinity

NASA is studying a "Proving Ground" near the Moon to conduct human space exploration missions in preparation for future flights to Mars. This paper describes a concept of operations ("conops") for activities in the Proving Ground, focusing on the construction and use of a mobile Cislunar Transit Habitat capable of months-long excursions within and beyond the Earth-Moon system. Key elements in the conops include the Orion spacecraft (with mission kits for docking and other specialized operations) and the Space Launch System heavy-lift rocket. Potential additions include commercial launch vehicles and logistics carriers, solar electric propulsion stages to move elements between different orbits and eventually take them on excursions to deep space, a node module with multiple docking ports, habitation and life support blocks, and international robotic and piloted lunar landers. The landers might include reusable ascent modules which could remain docked to in-space elements between lunar sorties. The architecture will include infrastructure for launch preparation, communication, mission control, and range safety. The conops describes "case studies" of notional missions chosen to guide the design of the architecture and its elements. One such mission is the delivery of a ~10-ton pressurized element, co-manifested with an Orion on a Block 1B Space Launch System rocket, to the Proving Ground. With a large solar electric propulsion stage, the architecture could enable a year-long mission to land humans on a near-Earth asteroid. In the last case, after returning to near-lunar space, two of the asteroid explorers could join two crewmembers freshly arrived from Earth for a Moon landing, helping to safely quantify the risk of landing deconditioned crews on Mars. The conops also discusses aborts and contingency operations. Early return to Earth may be difficult, especially during later Proving Ground missions. While adding risk, limited-abort conditions provide needed practice for Mars, from which early return is likely to be impossible.

Love, Stanley G.↗
DOE OSTI2019

High performance thermoelectric module through isotype bulk heterojunction engineering of skutterudite materials

We demonstrate filled CoSb 3 skutterudite materials with excellent thermoelectric (TE) performance that results in one of the highest reported single stage module efficiency. Here, the improvement in TE material performance was obtained by creating isotype n/n “bulk heterojunction” structure through assembly of novel skutterudite nanocrystals with different Yb-doping content. Combination of significant increase in carrier transport through heterojunction structure and reduction in long-range acoustic phonon transmission by two-phase mixture resulted in enhanced power factor and reduced lattice thermal conductivity. As a result, the figure-of-merit (zT) of heterojunction TE material is improved by more than 35% compared with pristine single homogeneous material. Using these improved TE materials, a high module conversion efficiency of ~9.15% was obtained when operating between 650 °C and 50 °C. This is one of the highest conversion efficiency among the practically measured single stage modules.

42 ENGINEERING↗
DOE OSTI2016

AgI alloying in SnTe boosts the thermoelectric performance via simultaneous valence band convergence and carrier concentration optimization

SnTe, a Pb-free analogue of PbTe, was earlier assumed to be a poor thermoelectric material due to excess p-type carrier concentration and large energy separation between light and heavy hole valence bands. Here, we report the enhancement of the thermoelectric performance of p-type SnTe by Ag and I co-doping. AgI (1–6 mol%) alloying in SnTe modulates its electronic structure by increasing the band gap of SnTe, which results in decrease in the energy separation between its light and heavy hole valence bands, thereby giving rise to valence band convergence. Additionally, iodine doping in the Te sublattice of SnTe decreases the excess p-type carrier concentration. Due to significant decrease in hole concentration and reduction of the energy separation between light and heavy hole valence bands, significant enhancement in Seebeck coefficient was achieved at the temperature range of 600–900 K for Sn{sub 1−x}Ag{sub x}Te{sub 1−x}I{sub x} samples. A maximum thermoelectric figure of merit, zT, of ~1.05 was achieved at 860 K in high quality crystalline ingot of p-type Sn{sub 0.95}Ag{sub 0.05}Te{sub 0.95}I{sub 0.05}. - Graphical abstract: Significant decrease in hole concentration and reduction of the energy separation between light and heavy hole valence bands resulted in a maximum thermoelectric figure of merit, zT, of ~1.05 at 860 K in high quality crystalline ingot of p-type Sn{sub 0.95}Ag{sub 0.05}Te{sub 0.95}I{sub 0.05}. - Highlights: • AgI alloying in SnTe increases the principle band gap. • Hole concentration reduction and valence band convergence enhances thermopower of SnTe-AgI. • A maximum zT of ~1.05 was achieved at 860 K in p-type Sn{sub 0.95}Ag{sub 0.05}Te{sub 0.95}I{sub 0.05}.

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

Toward Mass-Production of Transition Metal Dichalcogenide Solar Cells: Scalable Growth of Photovoltaic-Grade Multilayer WSe2 by Tungsten Selenization

Semiconducting transition metal dichalcogenides (TMDs) are promising for high-specific-power photovoltaics due to their desirable band gaps, high absorption coefficients, and ideally dangling-bond-free surfaces. Despite their potential, the majority of TMD solar cells to date are fabricated in a nonscalable fashion, with exfoliated materials, due to the lack of high-quality, large-area, multilayer TMDs. Here, we present the scalable, thickness-tunable synthesis of multilayer WSe2 films by selenizing prepatterned tungsten with either solid-source selenium at 900 degrees C or H2Se precursors at 650 degrees C. Both methods yield photovoltaic-grade, wafer-scale WSe2 films with a layered van der Waals structure and superior characteristics, including charge carrier lifetimes up to 144 ns, over 14x higher than those of any other large-area TMD films previously demonstrated. Simulations show that such carrier lifetimes correspond to ~22% power conversion efficiency and ~64 W g-1 specific power in a packaged solar cell, or ~3 W g-1 in a fully packaged solar module. The results of this study could facilitate the mass production of high-efficiency multilayer WSe2 solar cells at low cost.

carrier lifetime↗
DOE OSTI2012

Electroconvection under injection from cathode and heating from above

We study the electroconvection that appears in a nonuniformly heated, poorly conducting liquid in a parallel-plate horizontal capacitor due to the action of an external static electric field on the charge injected from the cathode. It is shown that the heating of the layer from above prevents steady-state convection and that, unlike the isothermal situation, electroconvection can appear in the oscillatory manner as a result of direct Hopf bifurcation. The effect of the heating intensity, the intensity of charge injection from the cathode, and the charge mobility on the thresholds of oscillatory and monotonic electroconvection is analyzed and the characteristic scales and frequencies of critical perturbations are determined. The nonlinear wave and steady-state regimes of the 2D convective structures formed in the poorly conducting liquid under the action of thermogravitational and injection mechanisms of convection are analyzed. The domains of existence of standing, traveling, and modulated waves are determined.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Atomic structure of light-induced efficiency-degrading defects in boron-doped Czochralski silicon solar cells

Boron-doped Czochralski (Cz) Si is the most commonly used semiconductor in the fabrication of solar cells. The minority carrier lifetime of boron-doped Cz Si decreases upon exposure to light due to B–O-related defects, which reduce the performance of ~10 9 solar modules worldwide. Using electron paramagnetic resonance (EPR), we have identified the spin-active paramagnetic signatures of this phenomenon and gained insights into its microscopic mechanism. We found a distinct defect signature, which diminished when the degraded sample was annealed. The second signature, a broad magnetic field spectrum, due to the unionized B acceptors, was present in the annealed state but vanished upon light exposure. These observations show that, on degradation, nearly all the ~10 16 cm –3 B atoms in Cz Si complexed with interstitial O atoms, whereas only ~10 12 cm –3 of these complexes created defects that were recombination-active. The formation rate of these recombination-active defects correlated with the decay of the minority carrier lifetime. Furthermore, the line shape parameters linked these defects to both B and O impurities in Cz Si.

14 SOLAR ENERGY↗
DOE OSTI2023

Holographic Nano‐Imaging of Terahertz Dirac Plasmon Polaritons in Topological Insulator Antenna Resonators

Abstract Excitation of Dirac plasmon polaritons (DPPs) in bi‐dimensional materials have attracted considerable interest in recent years, both from perspectives of understanding their physics and exploring their transformative potential for nanophotonic devices, including ultra‐sensitive plasmonic sensors, ultrafast saturable absorbers, modulators, and switches. Topological insulators (TIs) represent an ideal technological platform in this respect because they can support plasmon polaritons formed by Dirac carriers in the topological surface states. Tracing propagation of DPPs is a very challenging task, particularly at terahertz (THz) frequencies, where the DPP wavelength becomes over one order of magnitude shorter than the free space photon wavelength. Furthermore, severe attenuation hinders the comprehensive analysis of their characteristics. Here, the properties of DPPs in real TI‐based devices are revealed. Bi 2 Se 3 rectangular antennas can efficiently confine the propagation of DPPs to a single dimension and, as a result, enhance the DPPs visibility despite the strong intrinsic attenuation. The plasmon dispersion and loss properties from plasmon profiles are experimentally determined, along the antennas, obtained using holographic near‐field nano‐imaging in a wide range of THz frequencies, from 2.05 to 4.3 THz. The detailed investigation of the unveiled DPP properties can guide the design of novel topological quantum devices exploiting their directional propagation.

Chemistry↗
DOE OSTI2024

Computational Discovery of Intermolecular Singlet Fission Materials Using Many-Body Perturbation Theory

Intermolecular singlet fission (SF) is the conversion of a photogenerated singlet exciton into two triplet excitons residing on different molecules. SF has the potential to enhance the conversion efficiency of solar cells by harvesting two charge carriers from one high-energy photon, whose surplus energy would otherwise be lost to heat. The development of commercial SF-augmented modules is hindered by the limited selection of molecular crystals that exhibit intermolecular SF in the solid state. Computational exploration may accelerate the discovery of new SF materials. The GW approximation and Bethe–Salpeter equation (GW+BSE) within the framework of many-body perturbation theory is the current state-of-the-art method for calculating the excited-state properties of molecular crystals with periodic boundary conditions. In this Review, we discuss the usage of GW+BSE to assess candidate SF materials as well as its combination with low-cost physical or machine learned models in materials discovery workflows. We demonstrate three successful strategies for the discovery of new SF materials: (i) functionalization of known materials to tune their properties, (ii) finding potential polymorphs with improved crystal packing, and (iii) exploring new classes of materials. In addition, three new candidate SF materials are proposed here, which have not been published previously.

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

Signature of Correlated Insulator in Electric Field Controlled Superlattice

On a two-dimensional crystal, a “superlattice” with nanometer-scale periodicity can be imposed to tune the Bloch electron spectrum, enabling novel physical properties inaccessible in the original crystal. While creating 2D superlattices by means of nanopatterned electric gates has been studied for band structure engineering in recent years, evidence of electron correlations-which drive many problems at the forefront of physics research-remains to be uncovered. Here, in this work, we demonstrate signatures of a correlated insulator phase in Bernal-stacked bilayer graphene modulated by a gate-defined superlattice potential, manifested as resistance peaks centered at integer multiples of single electron per superlattice unit cell carrier densities. The observation is consistent with the formation of a stack of flat low-energy bands due to the superlattice potential combined with inversion symmetry breaking. Our work paves the way to custom-designed superlattices for studying band structure engineering and strongly correlated electrons in 2D materials.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Microscopic Origin of Twist-Dependent Electron Transfer Rate in Bilayer Graphene

Using molecular simulation and continuum dielectric theory, we consider how electrochemical kinetics are modulated by the twist angle in bilayer graphene electrodes. By establishing a connection between the twist angle and the screening length of charge carriers within the electrode, we investigate how tunable metallicity modifies the statistics of the electron transfer energy gap. Constant potential molecular simulations show that the activation free energy for electron transfer increases with screening length, leading to a non-monotonic dependence on the twist angle. Here, the twist angle alters the density of states, tuning the number of thermally accessible channels for electron transfer and the reorganization energy by affecting the stability of the vertically excited state through attenuated image charge interactions. Understanding these effects allows us to express the Marcus rate of interfacial electron transfer as a function of the twist angle in a manner consistent with experimental observations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Portable digital lock-in instrument to determine chemical constituents with single-color absorption measurements for Global Health Initiatives

Innovations in international health require the use of state-of-the-art technology to enable clinical chemistry for diagnostics of bodily fluids. We propose the implementation of a portable and affordable lock-in amplifier-based instrument that employs digital technology to perform biochemical diagnostics on blood, urine, and other fluids. The digital instrument is composed of light source and optoelectronic sensor, lock-in detection electronics, microcontroller unit, and user interface components working with either power supply or batteries. The instrument performs lock-in detection provided that three conditions are met. First, the optoelectronic signal of interest needs be encoded in the envelope of an amplitude-modulated waveform. Second, the reference signal required in the demodulation channel has to be frequency and phase locked with respect to the optoelectronic carrier signal. Third, the reference signal should be conditioned appropriately. We present three approaches to condition the signal appropriately: high-pass filtering the reference signal, precise offset tuning the reference level by low-pass filtering, and by using a voltage divider network. We assess the performance of the lock-in instrument by comparing it to a benchmark device and by determining protein concentration with single-color absorption measurements. We validate the concentration values obtained with the proposed instrument using chemical concentration measurements. Finally, we demonstrate that accurate retrieval of phase information can be achieved by using the same instrument.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2019

Bright electroluminescence in ambient conditions from WSe 2 p-n diodes using pulsed injection

Transition metal dichalcogenide (TMDC) monolayers are promising materials for next-generation nanoscale optoelectronics, including high-speed light sources and detectors. However, most past reports on TMDC light-emitting diodes are limited to operation in high vacuum, while most applications require operation under ambient conditions. Here, we study the time-resolved electroluminescence of monolayer WSe 2 p-n junctions under ambient conditions and identify the decay in current over time as the main issue preventing stable device operation. We show that pulsed voltage bias overcomes this issue and results in bright electroluminescence under ambient conditions. This is achieved in a simple single-gate structure, without the use of dual gates, heterostructures, or doping methods. Internal quantum efficiency of electroluminescence reaches ~1%, close to the photoluminescence quantum efficiency, indicating efficient exciton formation with injected carriers. Emission intensity is stable over hours of device operation. Finally, our device exhibits ~15 ns rise and fall times, the fastest direct modulation speed reported for TMDC light-emitting diodes.

2D materials↗
DOE OSTI2022

Light- and Elevated-Temperature-Induced Degradation-Affected Silicon Cells From a Utility-Scale Photovoltaic System Characterized by Deep-Level Transient Spectroscopy

Photovoltaic modules from a utility-scale field experienced power loss by light- and elevated-temperature-induced degradation (LeTID). Samples from the affected monocrystalline silicon cells are cored and extracted from the module packaging and then laser-scribed to form 2-mm diameter isolated areas. Using deep-level transient spectroscopy, a majority-carrier, hole-trap defect with an activation energy of 0.42 eV is detected on degraded and regenerated samples. The LeTID-degraded sample, however, has a larger signal corresponding to a trap density of 1.1 x 10 13 cm -3 , which is about five times larger than the 2.1 x 10 12 cm -3 trap density of the regenerated sample. An increase in filling pulse time from 50 u s to 20 ms shows a slight decrease in activation energy from 0.42 to 0.36 eV suggesting that the defect level may consist of a band of energy states where shallower states continue to fill with long filling times. The capture rate of the defect is directly measured using an increasing series of filling pulsewidths in 15 to 125 ns range. This leads to a measured capture cross section of 5.1 x 10 -17 cm 2 , and using an approximate defect density of 10 13 cm -3 , the majority-carrier-hole lifetime related to this defect is approximately 100 u s when in the LeTID-degraded state.

charge carrier lifetime↗