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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Tuning the Electronic Properties of Graphane via Hydroxylation: An Ab Initio Study

The thermodynamic stability of hydroxylated graphane, that is, fully sp 3 graphene derivatives coordinated with -H and -OH groups, has been recently demonstrated by ab initio calculations. Within the density functional theory approach, we investigate the electronic property modifications of graphane by progressive hydroxylation, that is, by progressively substituting -H with -OH groups. When 50% of graphane is hydroxylated, the energy bandgap reaches its largest value of 6.68 eV. The electronic affinity of 0.8 eV for graphane can widely change in the 0.28–1.60 eV range depending on the geometric configuration. Hydroxylated graphane has two interfaces with vacuum, hence its electron affinity can be different on each interface with the formation of an intrinsic dipole perpendicular to the monolayer. We envisage the possibility of using hydroxylated graphane allotropes with tunable electronic affinity to serve as interfacial layers in 2D material-based heterojunctions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1981

Electron bombardment charge-coupled device /CCD/ detectors for the vacuum ultraviolet

Development work is underway at Princeton and NRL on electronic-readout imaging detectors for the VUV which are based on electron-bombardment CCD arrays and opaque alkali-halide photocathodes. These detectors provide single-photoelectron detection capability and wide dynamic range combined with almost the maximum possible quantum efficiency (in excess of 50% in the wavelength range below 1300 A). The Princeton work is based on an oblique-focusing electron optical configuration, while the NRL program is based on a Schmidt optical system with opaque photocathode. Results of preliminary tests at Princeton and overall system concepts for the Princeton and NRL cameras are discussed, and applications to possible space astronomy studies are examined.

Lowrance, J. L.↗
DOE OSTI2020

Stronger field-emission science via coupling novel nanoscale imaging techniques

We implemented a vacuum field emission electron microscope (FEM) using the electron optics of a low-energy /photoemission electron microscope (LEEM/PEEM). Historically, there have been other FEM hardware platforms, and the distinctive feature of our method is that it integrates with the LEEM/PEEM and associated techniques, enabling a powerful multi-capability toolset for studying fundamental materials properties underpinning field emission (FE) and vacuum arc initiation. Typically, LEEM is used to image surface structure, which influences both work function and electric field distribution near a surface, while PEEM is used to map photoelectric work function across a surface. Our FEM adds the capability for spatially-correlated coincident-site measurements of FE currents to go-along with structure and work function. LEEM, PEEM, and our FEM implementation achieve nanoscale spatial resolution relevant for materials studies in nanoscience/engineering. Our approach requires a straightforward calibration of the electron optics to enable focused FEM imaging under intentional electric field variation. We demonstrate the FEM approach by imaging field emitter arrays relevant for vacuum nanoelectronics. We demonstrate submicron spatial resolution and dynamic measurement of FE versus applied electric field. We anticipate this capability will enable fundamental structure-function studies of FE and arc initiation.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1982

Charging and discharging characteristics of dielectric materials exposed to low- and mid-energy electrons

Charging and discharging characteristics of dielectric samples exposed to 1-25 keV and 25-100 keV electrons in a laboratory environment are reported. The materials examined comprised OSR, Mylar, Kapton, perforated Kapton, and Alphaquartz, serving as models for materials employed on spacecraft in geosynchronous orbit. The tests were performed in a vacuum chamber with electron guns whose beams were rastered over the entire surface of the planar samples. The specimens were examined in low-impedance-grounded, high-impedance-grounded, and isolated configurations. The worst-case and average peak discharge currents were observed to be independent of the incident electron energy, the time-dependent changes in the worst case discharge peak current were independent of the energy, and predischarge surface potentials are negligibly dependent on incident monoenergetic electrons.

Coakley, P.↗
NASA NTRS1972

Portable electron beam weld chamber

Development and characteristics of portable vacuum chamber for skate type electron beam welding are discussed. Construction and operational details of equipment are presented. Illustrations of equipment are provided.

Lewis, J. R.↗
DOE OSTI2022

Thermal analysis of RHIC arc dipole magnet cold mass with EIC beam screen

The existing RHIC storage rings – including their superconducting magnet arcs – will be used for the hadron storage ring of the Electron-Ion Collider (EIC). The vacuum chamber of these magnets was not designed for the EIC hadron beams, with shorter bunches and of higher average current than the RHIC beams. With the current stainless steel beam pipe, the resistive-wall (RW) heating will exceed the dynamic heat load budget. Limiting the RW heating is important to prevent the superconducting magnets from quenching and to maintain a low screen temperature necessary to impede the rise of RW heating (higher resistance at higher temperature) as well as to achieve desired ultra-high vacuum. In addition, simulations predict the formation of electron cloud which would further contribute to the dynamic heat load and could compromise the quality and stability of the beam. To reduce the resistive-wall heating and suppress electron cloud, a beam screen will be installed in the vacuum chamber of the RHIC SC magnets. The screen will have a high RRR copper layer at its inner face – useful to reduce the resistive-wall impedance thanks to its high conductivity especially at cryogenic temperatures – and will be coated with a thin layer of amorphous carbon, a material with low secondary electron yield to suppress the formation of electron clouds. The baseline solution envisages a screen that will be cooled by thermal contact to the 4.55 K beam pipe. Detailed thermal analysis have been conducted in ANSYS 2020 for an arc dipole cold mass equipped with a beam screen in order to study the feasibility of a passively-cooled screen and guide its design. Temperature-dependent thermal conductivity properties of all materials in the operating (cryogenic) temperature range are considered. Suitable assumptions and simplifications are made to model the magnet coil and calculate its homogenized thermal conductivity. Sensitivity studies with respect to layer thicknesses and area of contact are carried out and results are presented.

43 PARTICLE ACCELERATORS↗
NASA NTRS1981

Processing of silicon solar cells by ion implantation and laser annealing

Methods to improve the radiation tolerance of silicon cells for spacecraft use are described. The major emphasis of the program was to reduce the process-induced carbon and oxygen impurities in the junction and base regions of the solar cell, and to measure the effect of reduced impurity levels on the radiation tolerance of cells. Substrates of 0.1, 1.0 and 10.0 ohm-cm float-zone material were used as starting material in the process sequence. High-dose, low-energy ion implantation was used to form the junction in n+p structures. Implant annealing was performed by conventional furnace techniques and by pulsed laser and pulsed electron beam annealing. Cells were tested for radiation tolerance at Spire and NASA-LeRC. After irradiation by 1 MeV electrons to a fluence of 10 to the 16th power per sq cm, the cells tested at Spire showed no significant process induced variations in radiation tolerance. However, for cells tested at Lewis to a fluence of 10 to the 15th power per sq cm, ion-implanted cells annealed in vacuum by pulsed electron beam consistently showed the best radiation tolerance for all cell resistivities.

Minnucci, J. A.↗
NASA NTRS1981

Effect of electronic structure of the diamond surface on the strength of the diamond-metal interface

A diamond surface undergoes a transformation in its electronic structure by a vacuum anneal at approximately 900 C. The polished surface has no electronic states in the band gap, whereas the annealed surface has both occupied and unoccupied states in the and gap and exhibits some electrical conductivity. The effect of this transformation on the strength of the diamond metal interface was investigated by measuring the static friction force of an atomically clean meta sphere on a diamond flat in ultrahigh vacuum. It was found that low friction (weak bonding) is associated with the diamond surface devoid of gap states whereas high friction (strong bonding) is associated with the diamond surface with gap states. Exposure of the annealed surface to excited hydrogen also leads to weak bonding. The interfacial bond is discussed in terms of interaction of the metal conduction band electrons with the band gap states on the diamond surface. Effects of surface electrical conductivity on the interfacial bond are also be considered.

Pepper, S. V.↗
NASA NTRS1982

Investigation of degradation mechanisms in composite matrices

Degradation mechanisms were investigated for graphite/polysulfone and graphite/epoxy laminates exposed to ultraviolet and high-energy electron radiations in vacuum up to 960 equivalent sun hours and 10 to the ninth power rads respectively. Based on GC and combined GC/MS analysis of volatile by-products evolved during irradiation, several free radical mechanisms of composite degradation were identified. The radiation resistance of different matrices was compared in terms of G values and quantum yields for gas formation. All the composite materials evaluated show high electron radiation stability and relatively low ultraviolet stability as indicated by low G values and high quantum for gas formation. Mechanical property measurements of irradiated samples did not reveal significant changes, with the possible exception of UV exposed polysulfone laminates. Hydrogen and methane were identified as the main by-products of irradiation, along with unexpectedly high levels of CO and CO2.

Giori, C.↗
DOE OSTI2020

Dephasingless Laser Wakefield Acceleration

Laser wakefield accelerators (LWFAs) produce significantly high gradients enabling compact accelerators and radiation sources, but face design limitations, such as dephasing, occurring when trapped electrons outrun the accelerating phase of the wakefield. In this report we combine spherical aberration with a novel cylindrically symmetric echelon optic to spatiotemporally structure an ultra-short, high-intensity laser pulse that can overcome dephasing by propagating at any velocity over any distance. The ponderomotive force of the spatiotemporally shaped pulse can drive a wakefield with a phase velocity equal to the speed of light in vacuum, preventing trapped electrons from outrunning the wake. Simulations in the linear regime and scaling laws in the bubble regime illustrate that this dephasingless LWFA can accelerate electrons to high energies in much shorter distances than a traditional LWFA—a single 4.5 m stage can accelerate electrons to TeV energies without the need for guiding structures.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1998

X-ray Diffraction Studies of the Structure and Thermochemistry of Alkaline-Earth Oxide-Coated Thermionic Cathodes

NASA LeRC has a broad, active cathode technology development program in which both experimental and theoretical studies are being employed to further development of thermionic cathodes for use as electron sources in vacuum devices for communications and other space applications. One important type of thermionic cathode under development is the alkaline-earth oxide-coated (BaO, SrO, CaO) cathode. Significant improvements in the emission characteristics of this cathode have been obtained through modification of the chemical composition and morphology of the oxide coating, with the best result thus far coming from the addition of In2O3 and Sc2O3. Whereas the In2O3 produces a finer, more uniform particle structure, the exact chemical state and role of the Sc2O3 in the emission enhancement is unknown. The purpose of this cooperative agreement is to combine the studies of the surface chemistry and electron emission at NASA LeRC of chemically modified oxide coatings with a study of the thermochemistry and crystal structure using X-ray diffraction equipment and expertise at Clark Atlanta University (CAU). The study at CAU is intended to provide the description and understanding of the structure and thermochemistry needed for further improvement and optimization of the modified coatings. A description of the experimental procedure, preliminary X-ray diffraction test results, together with the design of an ultrahigh vacuum chamber necessary for high temperature thermochemistry studies will be presented.

Karikari, E. K.↗
NASA NTRS1998

Charging of Single Micron Sized Dust Grains by Secondary Electron Emission: A Laboratory Study

We present the details of a new laboratory study whose objective is to experimentally study the interaction of micron sized particles with plasmas and electromagnetic radiation. Specifically, to investigate under what conditions and to what extent do particles of various compositions and sizes become charged, or discharged, while exposed to an electron beam and ultraviolet radiation environment The emphasis is the study of the two charging mechanisms, secondary emission of electrons and photoelectric effect. The experiment uses a technique known as electrodynamic suspension of particles. With this technique, a single charged particle is electrodynamically levitated and then exposed to a controlled environment. Its charge to mass ratio is directly measured. Viscous drag measurements and the light scattering measurements characterize its size and optical characteristics. The environment to which the particle is expose may consist of room temperature and pressure or a rarefied atmosphere where only one major gaseous constituent is present, or, as in this case, a vacuum environment under electron bombardment or UV radiation . In addition, the environment can be cycled as part of the experiment. Therefore, using this technique, a single particle can be repeatedly exposed to a controlled environment and its response measured, or a single particle can be exposed to similar environments with minor differences and its response measured as a function of only the changed environmental conditions.

Spann, James F., Jr.↗
DOE OSTI2020

Irradiation-induced reactions at the CeO 2 /SiO 2 /Si interface

The influence of high-energy (1.6 MeV) Ar 2+ irradiation on the interfacial interaction between cerium oxide thin films (~15 nm) with a SiO 2 /Si substrate is investigated using transmission electron microscopy, ultrahigh vacuum x-ray photoelectron spectroscopy (XPS), and a carbon monoxide (CO) oxidation catalytic reaction using ambient pressure XPS. The combination of these methods allows probing the dynamics of vacancy generation and its relation to chemical interactions at the CeO 2 /SiO 2 /Si interface. The results suggest that irradiation causes amorphization of some portion of CeO 2 at the CeO 2 /SiO 2 /Si interface and creates oxygen vacancies due to the formation of Ce 2 O 3 at room temperature. The subsequent ultra-high-vacuum annealing of irradiated films increases the concentration of Ce 2 O 3 with the simultaneous growth of the SiO 2 layer. Interactions with CO molecules result in an additional reduction of cerium and promote the transition of Ce 2 O 3 to a silicate compound. Furthermore, thermal annealing of thin films exposed to oxygen or carbon monoxide shows that the silicate phase is highly stabile even at 450 °C.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1983

The diffusivity of hydrogen in Nb stabilized stainless steel

The evolution of hydrogen from 347 stainless steel has been studied by using a real time dynamic technique under ultrahigh vacuum conditions. Auger electron spectroscopy was used to determine the surface composition as a function of time and temperature. The surface film on the electropolished samples was found to be approximately 15 A thick and consisted of a carbon-oxygen complex and a metal oxide (FexOy). Upon heating to 400 C, the carbon-oxygen complex desorbed as CO and the remaining oxygen and carbon began to incorporate. Also at this temperature sulfur began to diffuse out of the bulk to the surface and at approximately 800 C formed a complete monolayer. At 900 C, carbon and oxygen virtually disappeared, leaving the monolayer of sulfur as the only surface contaminant. The hydrogen diffusivity was found to follow closely the equation D = 7.01 x 10 to the -7th exp(-48.0/RT) sq m per second over the entire temperature range studied, thus indicating that hydrogen evolution is not significantly affected by the changing surface composition. The somewhat higher value of the diffusivity obtained in this work compared to past measurements in austenitic stainless steels may indicate the importance of sample preprocessing and ultrahigh vacuum conditions in minimizing the effects of surface layers.

Outlaw, R. A.↗