10,183 research outputs found
The effect of Fe atoms on the adsorption of a W atom on W(100) surface
We report a first-principles calculation that models the effect of iron (Fe)
atoms on the adsorption of a tungsten (W) atom on W(100) surfaces. The
adsorption of a W atom on a clean W(100) surface is compared with that of a W
atom on a W(100) surface covered with a monolayer of Fe atoms. The total energy
of the system is computed as the function of the height of the W adatom. Our
result shows that the W atom first adsorbs on top of the Fe monolayer. Then the
W atom can replace one of the Fe atoms through a path with a moderate energy
barrier and reduce its energy further. This intermediate site makes the
adsorption (and desorption) of W atoms a two-step process in the presence of Fe
atoms and lowers the overall adsorption energy by nearly 2.4 eV. The Fe atoms
also provide a surface for W atoms to adsorb facilitating the diffusion of W
atoms. The combination of these two effects result in a much more efficient
desorption and diffusion of W atoms in the presence of Fe atoms. Our result
provides a fundamental mechanism that can explain the activated sintering of
tungsten by Fe atoms.Comment: 9 pages, 2 figure
High Temperature Partition Function of the Rigid String
We find that the high temperature limit of the free energy per unit length
for the rigid string agrees dimensionally with that of the QCD string (unlike
the Nambu-Goto string). The sign, and in fact the phase, do not agree. While
this may be a clue to a string theory of QCD, we note that the problem of the
fourth derivative action makes it impossible for the rigid string to be a
correct description.Comment: 7 page
Low-Energy Properties of Antiferromagnetic Spin-1/2 Heisenberg Ladders with an Odd Number of Legs
An effective low-energy description for multi-leg spin-1/2 Heisenberg ladders
with an odd number of legs is proposed. Using a newly developed Monte Carlo
loop algorithm and exact diagonalization techniques, the uniform and staggered
magnetic susceptibility and the entropy are calculated for ladders with 1, 3,
and 5 legs. These systems show a low-temperature scaling behavior similar to
spin-1/2 chains with longer ranged unfrustrated exchange interactions. The
spinon velocity does not change as the number of legs increases, but the energy
scale parameter decreases markedly.Comment: 4 pages and 5 figure
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Rapid Steel Tooling Via Solid Freeform Fabrication
With increasing part complexity and requirements for long production runs, tooling has
become an expensive process that requires long lead times to manufacture. This lengthens the
amount oftime from "art to part". Rapid tooling via stereolithography (SLA), filled epoxies, etc.
have been stopgap measures to produce limited prototyping runs from (10 to 500 parts). This
gives poor dimensional analysis and does not allow for limited production runs of 1000+ parts.
The method ofproducing prototype tooling with a powdered metal process has been developed
that produces tooling with a hardness greater than 35 HRC and total shrinkage less than 0.5%.
This tooling process manufactures production ready tooling that will perform extended cycle
runs (100,000+). Manufacturing ofthis tooling takes 1 to 2 weeks and will compare favorably
with production grade steel tooling. Originals drawn in 3D CAD can be used to prototype the
master that will allow for the production ofthe rapid metal tool set.
process starts with a rapid prototyped model made by whatever process is desired or
a machined master. For this paper a Sander's Model Maker II® rapid prototyping machine was
used to fabricate the model. After the model ofthe tool set is made, a silicone rubber negative is
cast around that model. After the silicone rubber model is made, a heated slurry ofmetal
powders and polymers is poured into the mold to create the green tool set. The tool set is left to
cool, and then removed from the silicone rubber mold. The tool set is then debound and sintered
to produce a final tool set with properties approaching hardened tool steel.Mechanical Engineerin
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