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Results for “THERMIONIC EMISSION”

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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Surface plasmon enhanced fast electron emission from metallised fibre optic nanotips

Physical mechanisms of electron emission from fibre optic nanotips, namely, tunnelling, multi-photon, and thermionic emission, either prevent fast switching or require intense laser fields. Time-resolved electron emission from nano-sized sources finds applications ranging from material characterisation to fundamental studies of quantum coherence. We present a nano-sized electron source capable of fast-switching (≤1 ns) that can be driven with low-power femtosecond lasers. The physical mechanism that can explain emission at low laser power is surface plasmon enhanced above-threshold photoemission. An electron emission peak is observed and provides support for resonant plasmonic excitation. The electron source is a metal-coated optical fibre tapered into a nano-sized tip. The fibre is flexible and back illuminated facilitating ease of positioning. The source operates with a few nJ per laser pulse, making this a versatile emitter that enables nanometrology, multisource electron-lithography and scanning probe microscopy.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2018

Hollow Cathode Thermal Modelling and Self-Consistent Plasma Solution: Work Function Evaluation for a LaB6 Cathode.

Self-heating hollow cathodes are central components in modern electric thrusters. The plasma discharge inside these devices heats the internal components, thus maintaining the temperatures required for electron emission. Precise knowledge of the physical phenomena governing hollow cathode operation is key to predict their lifetime, specifically, their thermionic emission characteristics. A simulation platform has been built to couple plasma and thermal models of the self-heating hollow cathode to produce a self-consistent solution. A self-consistent solution has been found for a LaB6 hollow cathode operating at 25A and 13 sccm where the work function is assumed to be spatially uniform along the emitter with a value which is allowed to vary as the coupled model iterates to a self-consistent solution. The emitter temperature from the converged solution does not agree with experimental temperature measurements, however. The results of a sensitivity analysis suggest that none of the tolerances in the measurement are responsible for the discrepancy. We hypothesize that either the work function needs to be a function of position along the emitting surfaces or the heat fluxes have been overestimated in the plasma solver.

Meiron, Daniel I.↗
DOE OSTI2024

Evidence of hot carrier extraction in metal halide perovskite solar cells

The presence of hot carriers is presented in the operational properties of an (FA,Cs)Pb(I, Br, Cl) 3 solar cell at ambient temperatures and under practical solar concentration. Albeit, in a device architecture that is not suitably designed as a functional hot carrier solar cell. At 100 K, clear evidence of hot carriers is observed in both the high energy tail of the photoluminescence spectra and from the appearance of a nonequilibrium photocurrent at higher fluence in light J–V measurements. At room temperature, however, the presence of hot carriers in the emission at elevated laser fluence is shown to compete with a gradual red shift in the PL peak energy as photoinduced halide segregation begins to occur at higher lattice temperature. The effects of thermionic emission of hot carriers and the presence of a nonequilibrium carrier distribution are also shown to be distinct from simple lattice heating. In conclusion, this results in large unsaturated photocurrents at high powers as the Fermi distribution exceeds that of the heterointerface controlling carrier transport and rectification.

14 SOLAR ENERGY↗
NASA NTRS1981

A phenomenological model describing orificed, hollow cathode operation

A phenomenological model is presented which describes the emission processes occurring within the orificed, hollow cathode. The model defines a region of ion production inside the cathode and assumes that the cathode emission current can be accounted for by considering the ion and electron currents which cross the boundary of this region. Most of the current (approximately 72%) comes from electrons produced by field-enhanced, thermionic emission from a well-defined emission surface at the downstream end of the insert which circumscribes the ion product region. The electrons produced at the emission surface feed the ionization process in the region of ion production; the rest of the current (approximately 28%) is carried by the ions produced in the process. Moving at the Bohm velocity, these ions leave the production volume; they then complete the current path by being collected at various cathode potential surfaces, including the emission surface. The model establishes the ratio of ion to electron currents at the emission surface on the basis of the energy balance at that surface. Experimental results corroborating the principal assumptions of the model are presented and discussed.

Siegfried, D. E.↗
NASA NTRS2014

Solid-State Thermionic Nuclear Power for Megawatt Propulsion, Planetary Surface and Commercial Power Project

Thermionic (TI) power conversion is a promising technology first investigated for power conversion in the 1960's, and of renewed interest due to modern advances in nanotechnology, MEMS, materials and manufacturing. Benefits include high conversion efficiency (20%), static operation with no moving parts and potential for high reliability, greatly reduced plant complexity, and the potential for reduced development costs. Thermionic emission, credited to Edison in 1880, forms the basis of vacuum tubes and much of 20th century electronics. Heat can be converted into electricity when electrons emitted from a hot surface are collected across a small gap. For example, two "small" (6 kWe) Thermionic Space Reactors were flown by the USSR in 1987-88 for ocean radar reconnaissance. Higher powered Nuclear-Thermionic power systems driving Electric Propulsion (Q-thruster, VASIMR, etc.) may offer the breakthrough necessary for human Mars missions of < 1 yr round trip. Power generation on Earth could benefit from simpler, moe economical nuclear plants, and "topping" of more fuel and emission efficient fossil-fuel plants.

George, Jeffrey↗
DOE OSTI2022

Temperature-Dependent Carrier Extraction and the Effects of Excitons on Emission and Photovoltaic Performance in Cs0.05FA0.79MA0.16Pb(I0.83Br0.17)3 Solar Cells

The photovoltaic parameters of triple cation perovskite [Cs0.05FA0.79MA0.16Pb(I0.83Br0.17)3] solar cells are investigated focusing on the electro-optical properties and differences in performance at low and high temperatures. The signature of a parasitic barrier to carrier extraction is observed at low temperatures, which results in a loss of performance at T < 200 K. Intensity-dependent measurements indicate extraction across this parasitic interface is limited by a combination of the exciton binding energy and thermionic emission. However, the photovoltaic performance of the device is recovered at low intensity - where the photocarrier generation rate threshold is lower than the thermionic extraction rate. Loss of solar cell performance is also observed to be strongly correlated to an increase in photoluminescence intensity, indicating inhibited carrier extraction results in strong radiative recombination and that these systems do not appear to be limited by significant thermally activated non-radiative processes. Evidence of limited carrier extraction due to excitonic effects is also observed with a strong anti-correlation in photoluminescence and carrier extraction observed at lower temperatures.

barrier↗
DOE OSTI2020

Electron emission from particles strongly affects the electron energy distribution in dusty plasmas

The electron energy distribution of a low-temperature dusty plasma has been measured via a Langmuir probe. An unexpected broad peak at energy in the 2–4 V range has been observed. This can be theoretically reproduced for a sufficiently large electron emission rate from the nanoparticles dispersed in the plasma. A careful analysis of the nanoparticle energy balance, using measured values of nanoparticle concentration and plasma density, confirms that particles are sufficiently hot under the conditions of this study to rapidly inject electrons into the plasma via field-assisted thermionic emission. This work suggests that the presence of dust affects the plasma ionization balance more deeply than previously thought.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Electrical and thermal characteristics of LaB6 thermionic hollow cathodes operating in He, D2, Ar, and Xe at 4–200 Pa and 0.25–5 A cm−2

We report characteristics of lanthanum hexaboride (LaB6) cathodes for conditions relevant to a high-voltage, high-current switching device. LaB6 is selected because of its relatively low work function, stability in the presence of many common gas impurities, and a low evaporation rate (long life) at operating temperatures. We have investigated the effect of cathode geometry, gas type, pressure, and current on a LaB6 hollow cathode in a diode configuration with an opposing planar anode. The gas is stagnant at a relatively low pressure of 4–200 Pa (0.03–1.5 Torr) because of device high-voltage standoff requirements. In all cases, the plasma voltage decreases to a plateau value with increasing current where a practical device would operate. The plateau voltage aligns with the fill gas excitation or ionization voltage, depending on conditions. At high current, the cathode temperature exceeds that required for thermionic emission to supply the total current, a behavior that is understood by accounting for back-diffusing bulk electrons.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1984

Evaluation of single crystal LaB6 cathodes for use in a high frequency backward wave oscillator tube

The results of thermionic emission and evaporation studies of single crystal LaB6 cathodes are given. A comparison between the (100), (210) and (310) crystal planes shows the (310) and (210) planes to possess a work function approx 0.2 eV lower than (100). This translates into a significant increase in current density, J, at a specified temperature. Comparison with a state-of-the-art impregnated dispenser cathode shows that LaB6 (310) is a superior cathode in nearly all respects except operating temperature at j 10 A/sq cm. The 1600 K thermionic and room temperature retarding potential work functions for LaB6 (310) are 2.42 and 2.50 respectively.

Swanson, L. W.↗
DOE OSTI2022

Front-End Design for SiPM-Based Monolithic Neutron Double Scatter Imagers

Neutron double scatter imaging exploits the kinematics of neutron elastic scattering to enable emission imaging of neutron sources. Due to the relatively low coincidence detection efficiency of fast neutrons in organic scintillator arrays, imaging efficiency for double scatter cameras can also be low. One method to realize significant gains in neutron coincidence detection efficiency is to develop neutron double scatter detectors which employ monolithic blocks of organic scintillator, instrumented with photosensor arrays on multiple faces to enable 3D position and multi-interaction time pickoff. Silicon photomultipliers (SiPMs) have several advantageous characteristics for this approach, including high photon detection efficiency (PDE), good single photon time resolution (SPTR), high gain that translates to single photon counting capabilities, and ability to be tiled into large arrays with high packing fraction and photosensitive area fill factor. However, they also have a tradeoff in high uncorrelated and correlated noise rates (dark counts from thermionic emissions and optical photon crosstalk generated during avalanche) which may complicate event positioning algorithms. We have evaluated the noise characteristics and SPTR of Hamamatsu S13360-6075 SiPMs with low noise, fast electronic readout for integration into a monolithic neutron scatter camera prototype. The sensors and electronic readout were implemented in a small-scale prototype detector in order to estimate expected noise performance for a monolithic neutron scatter camera and perform proof-of-concept measurements for scintillation photon counting and three-dimensional event positioning.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Reducing the Barrier Height in Organic Transistors

Abstract Reducing the Schottky barrier height and Fermi level de‐pinning in metal‐organic semiconductor contacts are crucial for enhancing the performance of organic transistors. The reduction of the Schottky barrier height in bottom‐contact top‐gate organic transistors is demonstrated by adding 1 nm thick atomic layer deposited Al 2 O 3 on the source and drain contacts. By using two different donor‐acceptor copolymers, both p‐ and n‐ type transistors are investigated. Temperature‐dependent current–voltage measurements from non‐treated, self‐assembled monolayer treated, and Al 2 O 3 treated Au source‐drain contact field‐effect transistors with varying channel lengths are carried out. The drain current versus drain voltage near zero gate voltage, which may be described by the thermionic emission model at temperatures above 150 K, allows the estimation of the Schottky barrier height ( φ B ). The Al 2 O 3 contact‐treated transistors show more than 40% lower φ B compared with the non‐treated contacts in the p ‐type transistor. Similarly, an isoindigo‐based transistor, with n ‐type transport, shows a reduction in φ B with Al 2 O 3 treated contacts suggesting that such ultrathin oxide layers provide a universal method for reducing the barrier height.

Ghobadi, Arash↗
DOE OSTI2025

Influence of Photoemission Geometry on Timing and Efficiency in 4D Ultrafast Electron Microscopy

Broader adoption of 4D ultrafast electron microscopy (UEM) for the study of chemical, materials, and quantum systems is being driven by development of new instruments as well as continuous improvement and characterization of existing technologies. Perhaps owing to the still-high barrier to entry, the full range of capabilities of laser-driven 4D UEM instruments has yet to be established, particularly when operated at extremely low beam currents (~fA). Accordingly, with an eye on beam stability, we have conducted particle tracing simulations of unconventional off-axis photoemission geometries in a UEM equipped with a thermionic-emission gun. Specifically, we have explored the impact of experimentally adjustable parameters on the time-of-flight (TOF), the collection efficiency (CE), and the temporal width of ultrashort photoelectron packets. The adjustable parameters include the Wehnelt aperture diameter (DW), the cathode set-back position (Ztip), and the position of the femtosecond laser on the Wehnelt aperture surface relative to the optic axis (Rphoto). Notable findings include significant sensitivity of TOF to DW and Ztip, as well as non-intuitive responses of CE and temporal width to varying Rphoto. As a means to improve accessibility, practical implications and recommendations are emphasized wherever possible.

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

Influence of the inverse sheath on divertor plasma performance in tokamak edge plasma simulations

Recently, it was shown that strong electron thermionic emission from material walls could result in the formation of an “inverse sheath”, which prevents the flow of cold ions to the wall. Such regimes look very favorably from the point of view of plasma-material interactions at the edge of magnetic fusion devices, where the problem of the erosion of plasma facing components under ion irradiation is one of the key issues for developing of future magnetic fusion reactors. However, it is not clear whether such regimes are compatible with edge plasma parameters and heat removal requirements in fusion reactors.To address the issue of practicality of the “inverse sheath” regime for edge tokamak plasma conditions, we perform a set of numerical simulations with 2D edge plasma transport code UEDGE for a DIII-D-like geometry and magnetic configuration.To describe both “standard” and “inverse sheath” conditions within the framework of the UEDGE code (which does not consider the sheath region per se), at the material surfaces, we apply effective boundary conditions which emulate both “standard” and“inverse sheath” regimes. We demonstrate that for the same input parameters, spatial distributions of edge plasma parameters corresponding to detached divertor and“inverse sheath” regimes are similar, with only a few minor differences. We discuss the compatibility of “inverse sheath” regimes with core plasma parameters.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Anion Resonances and Photoelectron Spectroscopy of the Tetracenyl Anion

The photoelectron spectroscopy of the tetracenyl anion using slow electron velocity-map imaging (SEVI) of cryogenically cooled ions is presented here. The total photodetachment yield as a function of photon energy is used to reveal a rich manifold of anion excited states above the detachment threshold. The lowest energy anionic resonance has a sufficiently long lifetime to yield a vibrationally resolved absorption spectrum that can be directly compared with theoretical predictions. Excitation of this state mostly results in electron detachment via thermionic emission. The total photodetachment yield spectrum is used to select photon wavelengths that minimize the indirect detachment signal to allow acquisition of vibrationally-resolved photoelectron spectra that can inform on the neutral tetracenyl radical. Assignment of spectral features corresponding to the ground and first excited state of the neutral 12-tetracenyl isomer is made with the aid of Franck-Condon simulations. This yields adiabatic electron affinity and term energies that differ significantly from the previously reported values. Weak features corresponding to the ground state of the minor 2-teracenyl and 1-tetracenyl isomers are also identified, which allows for the experimental determination of their electron affinities for the first time.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Next-generation tunnel FETs: exploring material perspectives and areal tunneling configurations

The end of Dennard scaling, which facilitated proportional increases in computing power without added energy costs until the mid-2000s, has underscored the urgent need for innovative semiconductor devices that can enhance energy efficiency. Tunnel field-effect transistors (TFETs) have emerged as promising candidates to surpass the energy efficiency of conventional metal oxide semiconductor field-effect transistors (MOSFETs). Unlike MOSFETs, which rely on thermionic emission to overcome the source-channel potential barrier, TFETs operate through quantum tunneling, potentially enabling sub-60 mV dec −1 subthreshold swing (SS) for low-voltage operation. However, lateral TFETs have faced challenges in achieving adequate on-state current (I ON ) and a broad SS operation window, limiting their practical utility. This review article advocates for areal TFETs, which utilize face-to-face tunnel junctions that ideally offer step-function current turn-on characteristics and allow I ON to scale with device area rather than width. We highlight recent advancements in integrating 2D materials into tunneling structures, which could facilitate efficient band-to-band tunneling through atomically thin layers, while addressing challenges of gate field screening. We then discuss the nearer-term prospects of epitaxial areal TFETs comprising III–V compound semiconductors and group-IV semiconductors based on recent experimental progress. The review examines both quantum mechanical and semiclassical modeling approaches for TFETs, including techniques to reduce the computational complexity. The article delves into ongoing challenges in material synthesis, interface engineering, device fabrication, and integration pathways, concluding with recommendations for future research directions to overcome the fundamental power density limitations of conventional transistor technology.

2D materials↗
DOE OSTI2025

Low leakage current in heteroepitaxial Al 0.7 Sc 0.3 N ferroelectric films on Ga N

Wurtzite ( Al , Sc ) N ferroelectrics are attractive for microelectronics applications due to their chemical and structural compatibility with wurtzite semiconductors, such as Ga N and ( Al , Ga ) N . However, the leakage current in epitaxial stacks reported to date should be reduced for reliable device operation. Here, we demonstrate low leakage current in epitaxial Al 0.7 Sc 0.3 N films on Ga N with well-saturated ferroelectric hysteresis loops that are orders of magnitude lower (i.e., 0.07 A cm − 2 ) than previously reported films (1–19 A cm − 2 ) having similar or better structural characteristics. We also show that, for these high-quality epitaxial ( Al , Sc ) N films, structural quality (edge and screw dislocations), as measured by diffraction techniques, is not the dominant contributor to leakage. Instead, the small leakage in our films is limited by thermionic emission across the interfaces, which is distinct from the large leakage due to trap-mediated bulk transport in the previously reported ( Al , Sc ) N films. To support this conclusion, we show that Al 0.7 Sc 0.3 N on lattice-matched In 0.18 Ga 0.82 N buffers with improved structural characteristics but higher interface roughness exhibit increased leakage characteristics. This demonstration of low leakage current in heteroepitaxial ( Al , Sc ) N films and understanding of the importance of interface barrier and surface roughness can guide further efforts toward improving the reliability of wurtzite ferroelectric devices. Published by the American Physical Society 2025

36 MATERIALS SCIENCE↗
DOE OSTI2020

High Voltage Surge Protection by a Varistor-Filled Air Gap

The electric discharge across a varistor granule filled air gap under a fast-rising voltage pulse was investigated for surge protection applications. The effects of temperature and pressure on the arc and the electrical conduction were analyzed by the characteristic changes in voltage waveforms triggered by a fast-rising high voltage pulse. In addition to the gap size, experimental results show that competing mechanisms among arc conduction, conduction through the varistor granule network, thermionic emission from Joule heating at granule-to-granule contact points, and the magnitude of the switching voltage dictate the maximum surge protection voltage for the filled air gap. Experimental evidence indicated that accumulated degradation was created at small contact points between varistor granules by repetitive assaults from longer duration, high voltage pulses. The uniqueness of using varistor over other dielectric granules in an air gap for surge protection is identified and discussed.

36 MATERIALS SCIENCE↗