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

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