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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Photon-enhanced thermionic emission from p-GaAs with nonequilibrium Cs overlayers

Photon-enhanced thermionic emission (PETE), which is promising for increasing the efficiency of solar energy conversion, is studied during cesium deposition on the As- and Ga-rich p-GaAs(001) surfaces and subsequent relaxation in the nonequilibrium Cs overlayer by means of photoemission quantum yield spectroscopy adapted for systems with time-variable parameters. Along with direct photoemission of “hot” electrons excited by light above the vacuum level, the spectra contain PETE contribution of “thermalized” electrons, which are excited below the vacuum level and emit in vacuum due to thermalization up in energy by phonon absorption. Comparing the measured and calculated spectra, the effective electron affinity and escape probabilities of hot and thermalized electrons are obtained as functions of submonolayer Cs coverage. The minima in the affinity and pronounced peaks in the escape probabilities are observed for Cs deposition on both the As- and Ga-rich surfaces. Possible reasons for the low mean values of the electron escape probabilities and for the observed enhancement of the probabilities at certain Cs coverages are discussed, along with the implications for the PETE device realization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Sharp transition between two regimes of operation of dc discharge with two anodes and thermionic emission from cathode

In a dc discharge plasma with two anodes and thermionic emission from cathode, the two anodes are used for plasma control. The main anode is placed between the cathode and the other auxiliary anode has a circular opening for passing electron current from the cathode to the second anode. It is experimentally demonstrated that a plasma may exhibit a sudden transition between two quasi-stable conditions as one increases the cathode-electron current collected by the auxiliary anode through an aperture, i.e., hole, in the main anode. In one regime, a bright glowing “ball-shaped double layer” appears on the plasma side having a potential drop of 10–15 eV and concomitant ionization in the neighboring region attached to the opening. The second regime is characterized by a uniform potential profile in plasma and an absence of the ball-shaped double layer. The transition between these regimes is accompanied by a significant change in plasma properties, such as the electron energy distribution function (EEDF). Controlling the EEDF is a valuable capability in technological applications. Increasing the gas pressure leads to the elimination of the first regime for sufficiently high gas pressure, the threshold being a few Torr. The disappearance of a regime transition can be explained by invoking an EEDF transition, from being nonlocal at low pressure to becoming local at high pressure. Local EEDF is determined by local values of electric field. Nonlocal EEDF is determined by electric field values elsewhere, and the electron can travel without energy loss over a path much longer than the discharge dimension.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Influence of the anode processes on discharges driven by thermionic emission

The influence of electron reflection and secondary electron emission due to electron impact from the anode on discharges driven by thermionic emission is studied by the self-consistent one-dimensional particle-in-cell Monte Carlo collisions model. Two regimes are considered. In the first regime, the two-stream instability is excited and large anode sheath potential is obtained. It is found that in this regime, the reflected electrons play a significant role. In the second regime, the instability is not excited and the anode sheath potential is small. The dominant anode process in this regime is the secondary electron emission. It is shown that in both regimes, the anode processes significantly influence the plasma parameters.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Functionalized carbon nanotubes for thermionic emission and cooling applications

Barium strontium oxide-coated carbon nanotubes (CNTs) were implemented as a work function lowering and field enhancing functional coating on a coiled tungsten filament to create a new thermionic cathode. This cathode resembles conventional oxide cathodes in structure. It has the same coiled tungsten filament as a conventional oxide cathode but uses barium strontium oxide-coated CNTs instead of the traditional barium strontium calcium oxide powder mixture as an emissive coating. The cathode produces a strong thermionic emission. At 1395 K and 2.5 V/μm, the thermionic emission current of 0.87 A or current density of 2.9 A/cm 2 was obtained from this oxide-coated CNT cathode. This level of emission is about three times as large as a conventional oxide cathode operating at similar temperature and field strength. Strong thermionic emissions from the cathode also lead to a large thermionic cooling effect. Temperature reduction as large as 90° was observed from the cathode surface when it was emitting electrons. Strong thermionic emission and a large cooling effect obtained are the result of the combination of the low work function of barium strontium oxide (1.6 eV) and the large field effect induced by the CNTs. Plasma enhanced chemical vapor deposition was used to grow CNTs, and magnetron sputtering deposition was used to deposit the barium strontium oxide functional coating; details of the cathode fabrication are presented to illustrate both the versatility of the processing techniques and the adaptability of barium strontium oxide-coated CNTs as a functional coating. Finally, measurements on thermionic emission and thermionic cooling of the cathode are also presented.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Influence of Xe{sub 2}{sup +} ions on the micro-hollow cathode discharge driven by thermionic emission

The influence of Xe{sub 2}{sup +} dimer ions and excited Xe* atoms on the hollow cathode discharge driven by electron thermionic emission is studied using two-dimensional Particle-in-Cell Monte Carlo Collisions modeling. A comparison with the results of two-component (electrons and Xe{sup +} ions) plasma modeling showed that the presence of the Xe{sub 2}{sup +} dimer ions and excited Xe* atoms in the plasma affects the plasma parameters (density, potential, and ion fluxes toward the cathode). The influence of Xe{sub 2}{sup +} ions and Xe* atoms on the plasma sheath parameters, such as thickness and the ion velocity at the sheath edge, is analyzed.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Thermionic Emission and Negative dI/dV in Photoactive Graphene Heterostructures

Transport in photoactive graphene heterostructures, originating from the dynamics of photogenerated hot carriers, is governed by the processes of thermionic emission, electron–lattice thermal imbalance, and cooling. These processes give rise to interesting photoresponse effects, in particular negative differential resistance (NDR) arising in the hot-carrier regime. The NDR effect stems from a strong dependence of electron–lattice cooling on the carrier density, which results in the carrier temperature dropping precipitously upon increasing bias. Finally, the ON–OFF switching between the NDR regime and the conventional cold emission regime, as well as the gate-controlled closed-circuit current that is present at zero bias voltage, can serve as signatures of hot-carrier dominated transport.

42 ENGINEERING↗
DOE OSTI2020

Laser Powder Bed Fusion Additive Manufacturing In-Process Monitoring and Optimization Using Thermionic Emission Detection

In this project, the emission of electrons during laser powder bed fusion additive manufacturing was identified and investigated to resolve the underlying dynamics driven by laser-material interactions. Laser powder bed fusion involves the selective laser melting of metal powder particles layer by layer which fuse together forming a larger 3D structure. Laser-based additive manufacturing approaches such as laser powder bed fusion hold the potential to revolutionize manufacturing of complex metal components in the aerospace, medical, and automotive industries. However, the difficulty in producing defect-free components by metal 3D printing is a major hurdle for widespread commercial adoption. This project resolved important dynamics in laser powder bed fusion including the identification of plasma formation mechanisms, transitions between laser-induced melting modes and stochastic precursors to defect formation. These findings were realized through implementation of electronic sensing diagnostics into an existing testbed system and harnessing capabilities developed for the time dependent analysis of laser powder bed fusion datasets. Importantly, the results advance our understanding of laser-material interactions and reveal that detection of thermionic emission can also resolve information critical to optimization of the laser powder bed fusion fabrication process. The findings are critical for advancing our fundamental understanding of the laser powder bed fusion process and can help assure the requisite fidelity of fabricated components.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Microfabricated nitrogen-phosphorus detector: chemically mediated thermionic emission

The chemical structures of many chemical warfare agents and toxic industrial chemicals (TICs) contain nitrogen (N) or phosphorus (P) atoms. Detection of these compounds is of great importance to the defense, homeland security, and emergency response communities. Commercially available benchtop thermionic nitrogen phosphorus detectors (NPDs) are capable of phosphorus over carbon molar selectivities of 10 5 which make them particularly attractive for use in GC-based detection systems. In these laboratory instruments thermionic electron emission is provided by a resistively heated glass or ceramic bead suspended above a hydrogen-air flame. Despite nearly 60 years of laboratory use the detailed detection mechanism is poorly understood. A goal of this project was to further our understanding of why these thermionic detectors are highly selective for P and N containing compounds.

Simonson, Robert Joseph↗
DOE OSTI2020

Thermionic emission and energy conversion

This project worked on development of a transformative heat-to-electricity conversion device based on thermionic energy conversion. The research provided a unique opportunity to transform the thermionics technology of the 1950-1990s by applying modern materials and 21st century wafer-scale fabrication methods. Research was conducted to further development of a thermionic energy converter based on modern microfabrication and new low work function anode materials that can surpass all other forms of direct heat-to-electricity conversion. This project completed the following goals: (1) Developed components for use in thermionic energy converters and electron emission devices, including electrode materials for electron emission and electron absorption with ultimate work functions below 5 eV; (2) Designed and fabricated thermionic devices for energy conversion and electron emission based on the components developed in (1); (3) Engineered test systems to characterize electronic, thermal, and mechanical properties of thermionic devices and electron emitters and their components, and test devices and components designed and fabricated in (1) and (2); and (4) Developed theoretical modeling capabilities in support of these technical objectives.

36 MATERIALS SCIENCE↗
DOE OSTI2017

The physical mechanism of the deviation of cathode thermionic emission from the Schottky law

The deviation of the I–V characteristics of cathode materials in microwave devices from the Schottky law is caused by the occurrence of drift and diffusion of oxygen vacancies in barium-oxide crystallites, which are emission-active components of the cathode materials. The processing of experimental dependences using the proposed mathematical tools makes it possible to determine the barium-oxide-crystallite thickness and diffusivity of oxygen vacancies. The known diffusivity allows determining the presence of microimpurities in barium oxide, which affect its emission properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS2004

Thermionic Emission of Single-Wall Carbon Nanotubes Measured

Researchers at the NASA Glenn Research Center, in collaboration with the Rochester Institute of Technology, have investigated the thermionic properties of high-purity, single-wall carbon nanotubes (SWNTs) for use as electron-emitting electrodes. Carbon nanotubes are a recently discovered material made from carbon atoms bonded into nanometer-scale hollow tubes. Such nanotubes have remarkable properties. An extremely high aspect ratio, as well as unique mechanical and electronic properties, make single-wall nanotubes ideal for use in a vast array of applications. Carbon nanotubes typically have diameters on the order of 1 to 2 nm. As a result, the ends have a small radius of curvature. It is these characteristics, therefore, that indicate they might be excellent potential candidates for both thermionic and field emission.

Landis, Geoffrey A.↗