119 research outputs found
MICROSTRUCTURE AND WORK FUNCTION OF DISPENSER CATHODE COATINGS: EFFECTS ON THERMIONIC EMISSION
Dispenser cathodes emit electrons through thermionic emission and are a critical component of space-based and telecommunication devices. The emission of electrons is enhanced when coated with a refractory metal such as osmium (Os), osmium-ruthenium (Os-Ru), or iridium (Ir). In this work the microstructure, thermionic emission, and work function of thin film Os-Ru coatings were studied in order to relate microstructural properties and thermionic emission.
Os-Ru thin film coatings were prepared through magnetron sputtering and substrate biasing to produce films with an array of preferred orientations, or texture. The effect of texture on thermionic emission was studied in detail through closely-spaced diode testing, SEM imaging, and x-ray diffraction. Results indicated that there was a strong correlation with emission behavior and specific preferred orientations.
An ultra-high vacuum compatible Kelvin Probe was used to measure the work function of W-Os-Ru ternary alloy films to determine the effect W interdiffusion has on work function. The results indicated that a high work function alloy coating corresponded to low work function cathodes, as expected. It was inferred that a high work function alloy coating results in a low work function cathode because it aligns more closely with ionization energy of Ba. The results also proved that this method of evaluating dispenser cathode coatings can distinguish small variations in microstructure and composition and may be a beneficial tool in the development of improved dispenser cathode coatings.
A novel experimental apparatus was constructed to measure the work function of dispenser cathode coatings in-vacuo using the ultra-high vacuum Kelvin Probe. The apparatus is capable of activating cathodes at high temperature and measuring the work function at elevated temperature. The design of this apparatus allows for more rapid evaluation of dispenser cathode coatings
Religion and Fairness in the Ultimatum Game: Examining Mennonite Beliefs and Actions
This study examines religion\u27s impact on ultimatum game outcomes by focusing on Mennonite students, faculty, and staff at Eastern Mennonite University (EMU) in Virginia, USA. Despite doctrinal statements emphasizing community and material concern for the poor, Mennonite offers in the game are not statistically different from other Christian groups. However, there is some indication that the mean offer in this particular sample exceeds those in existing studies. One possible reason for the higher offers could be the EMU campus ethos that emphasizes social justice issues. Also, the higher offer values suggest increased altruism if Proposers correctly foresaw that Responders would reject very few low offers
Correlation Between Microstructure and Thermionic Electron Emission from Os-Ru Thin Films on Dispenser Cathodes
Osmium-ruthenium films with different microstructures were deposited onto dispenser cathodes and subjected to 1000 h of close-spaced diode testing. Tailored microstructures were achieved by applying substrate biasing during deposition, and these were evaluated with scanning electron microscopy, x-ray diffraction, and energy dispersive x-ray spectroscopy before and after close-spaced diode testing. Knee temperatures determined from the close-spaced diode test data were used to evaluate cathode performance. Cathodes with a large {10-11} Os-Ru film texture possessed comparatively low knee temperatures. Furthermore, a low knee temperature correlated with a low effective work function as calculated from the close-spaced diode data. It is proposed that the formation of strong {10-11} texture is responsible for the superior performance of the cathode with a multilayered Os-Ru coating
Composition and Work Function Relationship in OsâRuâW Ternary Alloys
OsâRu thin films with varying concentrations of W were sputter deposited in order to investigate their structureâproperty relationships. The films were analyzed with x-ray diffraction to investigate their crystal structures, and a Kelvin probe to investigate their work functions. An OsâRuâW film with âŒ30 at. % W yielded a work function maximum of approximately 5.38âeV. These results align well with other studies that found work function minima from thermionic emission data on M-type cathodes with varying amounts of W in the coatings. Furthermore, the results are consistent with other work explaining energy-level alignment and charge transfer of molecules on metal oxides. This may shed light on the mechanism behind the âanomalous effectâ first reported by Zalm et al., whereby a high work function coating results in a low work function for emitting cathode surfaces. An important implication of this work is the potential for the Kelvin probe to evaluate the effectiveness of dispenser cathode coatings
STM characterization of the Si-P heterodimer
We use scanning tunneling microscopy (STM) and Auger electron spectroscopy to
study the behavior of adsorbed phosphine (PH) on Si(001), as a function
of annealing temperature, paying particular attention to the formation of the
Si-P heterodimer. Dosing the Si(001) surface with 0.002 Langmuirs of
PH results in the adsorption of PH (x=2,3) onto the surface and
some etching of Si to form individual Si ad-dimers. Annealing to 350C
results in the incorporation of P into the surface layer to form Si-P
heterodimers and the formation of short 1-dimensional Si dimer chains and
monohydrides. In filled state STM images, isolated Si-P heterodimers appear as
zig-zag features on the surface due to the static dimer buckling induced by the
heterodimer. In the presence of a moderate coverage of monohydrides this static
buckling is lifted, rending the Si-P heterodimers invisible in filled state
images. However, we find that we can image the heterodimer at all H coverages
using empty state imaging. The ability to identify single P atoms incorporated
into Si(001) will be invaluable in the development of nanoscale electronic
devices based on controlled atomic-scale doping of Si.Comment: 6 pages, 4 figures (only 72dpi
The Effects of Geometry and Architecture of Polymer Matrix Composites on Acoustic Emissions
The purpose of this project is to study how the variance of the geometry and architecture of polymer matrix composites (PMCs) affect the acoustic emissions measured whilst said structures are subjected to tensile stress. In these tests, a method called Modal AE will be utilized. In Modal AE, some relationships between sources and waves in various materials and geometries are presented in other papers/research. This study will focus on how different thickness and different makeup/ply patterns affect the acoustic emission tests. Some research has been completed and shown how energy attenuation affects the frequency content on PMCs similar to the ones chosen for this project. Using a similar test setup and analysis the goal of this project is to see if the thickness/geometry of these samples will affect energy attenuation and therefore affect frequency content. The reason frequency content is so important in AE is that it is a method of exploring the different damage modes PMCs experience during stress tests. From this data more information can be obtained about frequency content and the effects that energy attenuation/architecture/geometry can have on PMCs. These AE events can also be used to identify the location of a break and to see how internal breaks have propagated through the structure. Polymer matrix composites (PMCs) are widely used in the aerospace industry, particularly in structures securing thrust generating systems to the primary structure. Each test subjects a polymer matrix composite (PMC) of a specific thickness and weave design to determine the damage modes for each sample. The peak frequency of the propagated acoustic emissions with respect to the time domain will also be measured
Structure and Stability of Si(114)-(2x1)
We describe a recently discovered stable planar surface of silicon, Si(114).
This high-index surface, oriented 19.5 degrees away from (001) toward (111),
undergoes a 2x1 reconstruction. We propose a complete model for the
reconstructed surface based on scanning tunneling microscopy images and
first-principles total-energy calculations. The structure and stability of
Si(114)-(2x1) arises from a balance between surface dangling bond reduction and
surface stress relief, and provides a key to understanding the morphology of a
family of surfaces oriented between (001) and (114).Comment: REVTeX, 4 pages + 3 figures. A preprint with high-resolution figures
is available at http://cst-www.nrl.navy.mil/papers/si114.ps . To be published
in Phys. Rev. Let
Unusually strong space-charge-limited current in thin wires
The current-voltage characteristics of thin wires are often observed to be
nonlinear, and this behavior has been ascribed to Schottky barriers at the
contacts. We present electronic transport measurements on GaN nanorods and
demonstrate that the nonlinear behavior originates instead from
space-charge-limited current. A theory of space-charge-limited current in thin
wires corroborates the experiments, and shows that poor screening in high
aspect ratio materials leads to a dramatic enhancement of space-charge limited
current, resulting in new scaling in terms of the aspect ratio.Comment: 4 pages, 3 figures, to appear in Physical Review Letter
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