3,386 research outputs found
Work function and surface stability of tungsten-based thermionic electron emission cathodes
Materials that exhibit a low work function and therefore easily emit
electrons into vacuum form the basis of electronic devices used in applications
ranging from satellite communications to thermionic energy conversion. W-Ba-O
is the canonical materials system that functions as the thermionic electron
emitter used commercially in a range of high power electron devices. However,
the work functions, surface stability, and kinetic characteristics of a
polycrystalline W emitter surface are still not well understood or
characterized. In this study, we examined the work function and surface
stability of the eight lowest index surfaces of the W-Ba-O system using Density
Functional Theory methods. We found that under the typical thermionic cathode
operating conditions of high temperature and low oxygen partial pressure, the
most stable surface adsorbates are Ba-O species with compositions in the range
of Ba0.125O to Ba0.25O per surface W atom, with O passivating all dangling W
bonds and Ba creating work function-lowering surface dipoles. Wulff
construction analysis reveals that the presence of O and Ba significantly
alters the surface energetics and changes the proportions of surface facets
present under equilibrium conditions. Analysis of previously published data on
W sintering kinetics suggests that fine W particles in the size range of
100-500 nm may be at or near equilibrium during cathode synthesis, and thus may
exhibit surface orientation fractions well-described by the calculated Wulff
construction
A Wish for More Archers: Archers at the Battle of Agincourt, 1415
A knight expressed the desire to his king that he would add to their small army ten thousand of the best archers in England. This thesis utilizes archaeology evidence, financial records, iconographic depictions, literary and chronicle sources to understand the men who fought at the battle of Agincourt within the larger context and traditions of archery in England, the interconnections between archers and men-at-arms and the particular actions of those men in the battle itself
Nanoscale Voltage Enhancement at Cathode Interfaces in Li-ion Batteries
Interfaces are ubiquitous in Li-ion battery electrodes, occurring across
compositional gradients, regions of multiphase intergrowths, and between
electrodes and solid electrolyte interphases or protective coatings. However,
the impact of these interfaces on Li energetics remains largely unknown. In
this work, we calculated Li intercalation-site energetics across cathode
interfaces and demonstrated the physics governing these energetics on both
sides of the interface. We studied the olivine/olivine-structured
LixFePO4/LixMPO4 (x=0 and 1, M=Co, Ti, Mn) and layered/layered-structured
LiNiO2/TiO2 interfaces to explore different material structures and transition
metal elements. We found that across an interface from a high- to low-voltage
material the Li voltage remains constant in the high-voltage material and
decays approximately linearly in the low-voltage region, approaching the Li
voltage of the low-voltage material. This effect ranges from 0.5-9nm depending
on the interfacial dipole screening. This effect provides a mechanism for a
high-voltage material at an interface to significantly enhance the Li
intercalation voltage in a low-voltage material over nanometer scale. We showed
that this voltage enhancement is governed by a combination of electron transfer
(from low- to high-voltage regions), strain and interfacial dipole screening.
We explored the implications of this voltage enhancement for a novel
heterostructured-cathode design and redox pseudocapacitors
The Challenges of Nuclear Learning in South Asia
Published as a contributed paper for "Nuclear Learning in South Asia: The Next Decade" (2014).The sustainability of strategic stability in South Asia remains of the highest importance to U.S. national security policy. The South Asian Nuclear Learning project was accordingly conceived to examine the region’s nuclear experiences after the 1998 tests. The end of the first decade (1998-2008) represented a good starting point. Later, the project expanded to further analyze the nuclear learning curve in the next decade, when security environment and technological innovations undertook major shifts. The National Nuclear Security Administration sponsored this research, which involved selected scholars and experts from Pakistan, India and United States. Some of these contributors also helped shape their state’s nuclear decisions in the earlier period following the nuclear tests
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