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Wolfer W.G.,Ktech Corporation | Wolfer W.G.,Sandia National Laboratories
Acta Materialia

Lattice parameter changes in nanoparticles can be used to determine the surface stress of solids. In the past a Laplace-Young relationship has been employed to interpret the lattice parameter changes as a function of the particle size. In the meantime, however, atomistic calculations revealed a purely mechanical origin of the surface stress that is consistent with elasticity theory for solid surfaces as developed by Gurtin and Murdoch. In this theory the equilibrium distance for surface atoms may differ from that in the bulk solid, and the elastic properties of the surface layer may also deviate from bulk values. We apply this Gurtin-Murdoch theory to spherical nanoparticles and reanalyze past data as well as results from recent theoretical calculations on lattice parameter changes, thereby enabling us to determine surface properties commensurate with the mechanical interpretation of surface stress. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source

Bulatov V.V.,Lawrence Livermore National Laboratory | Wolfer W.G.,Ktech Corporation | Kumar M.,Lawrence Livermore National Laboratory
Scripta Materialia

Recently it was proposed that voids in crystals could grow by emission of shear dislocation loops [V.A. Lubarda, M.S. Scheider, D.H. Kalantar, B.A. Remington, M.A. Meyers, Acta Materialia 52 (2004) 1397-1408]. Even more recently, this proposal was ostensibly supported by molecular simulations of voids in strained single crystals [S. Traiviratana, E.M. Bringa, D.J. Benson, M.A. Meyers, Acta Materialia 56 (2008) 3874-3886]. The purpose of this comment is to dispute this recent assertion as unfounded. © 2010 Acta Materialia Inc. Source

Mattsson T.R.,Sandia National Laboratories | Lane J.M.D.,Sandia National Laboratories | Cochrane K.R.,Ktech Corporation | Desjarlais M.P.,Sandia National Laboratories | And 4 more authors.
Physical Review B - Condensed Matter and Materials Physics

Density functional theory (DFT) molecular dynamics (MD) and classical MD simulations of the principal shock Hugoniot are presented for two hydrocarbon polymers, polyethylene (PE) and poly(4-methyl-1-pentene) (PMP). DFT results are in excellent agreement with experimental data, which is currently available up to 80 GPa. Further, we predict the PE and PMP Hugoniots up to 350 and 200 GPa, respectively. For comparison, we studied two reactive and two nonreactive interaction potentials. For the latter, the exp-6 interaction of Borodin showed much better agreement with experiment than OPLS. For the reactive force fields, ReaxFF displayed decidedly better agreement than AIREBO. For shocks above 50 GPa, only the DFT results are of high fidelity, establishing DFT as a reliable method for shocked macromolecular systems. © 2010 The American Physical Society. Source

Agency: Department of Defense | Branch: Air Force | Program: STTR | Phase: Phase I | Award Amount: 99.99K | Year: 2009

A non-equilibrium plasma is proposed as a source to generate repetitive high current, electron beams for microwave production in a slow wave structure.  Due to the gas environment inside the hollow cathode electrode, the electron beam will generate a plasma channel minimizing beam space-charge effects allowing self-focusing.  Since the non-equilibrium plasma is effectively the electron gun cathode, the properties of the cathode and hence the generated electron beam may be adjusted by changing the parameters of the plasma.  Due to the plasmas supple structure and pulse shaping properties, the non-equilibrium plasma generated electron source is compact being built around a finite in length, cylindrical anode tube possibly terminated by a diaphragm with orifice housing.  The diaphragm acts as a filter to shape the extracted electron beam and allows for some differential control in vacuum pumping if desired.   External heating units, used in thermionic emission, are not needed and large electron beam accelerating potentials may be relaxed.  The electron beam current and energy will be high enough to inject the beam into a metal plug attached to a needle inserted into the magnetron.  The source will be designed such that the electron beam energy is high enough to yield a low probability of secondary electron emission.  With special attention to capacitive coupling effects, the injected charge collected by the plug redistributes appropriately over the surface of the needle increasing the field for field emission to occur.  The cylindrical symmetry of the needle allows for uniform emission.  Instead of a solid cathode electrode, the cathode is a grid encapsulating the needle providing the appropriate potentials between cathode and anode for magnetron operation.   BENEFIT: At the end of this program (Phases I & II), we will be positioned to advance the development of the technology to produce next generation prototypes of plasma cathode-based electron injectors for use with high-power magnetrons.  It is anticipated that such development will lead directly to commercialization of these devices for military applications. If the Phase I research is successful in the tasks outlined above, the Air Force Office of Scientific Research will have an entirely new, prototypical plasma cathode for use with magnetrons and high-power fast switching applications.  The work will lay a foundation for Phase II research in which the initial development is transitioned into applications of this new technology.)

Agency: Department of Defense | Branch: Missile Defense Agency | Program: SBIR | Phase: Phase I | Award Amount: 99.99K | Year: 2009

The overall objective of this proposal is to develop the necessary components and to design and build a compact pulsed power system for the Missile Defense Agency that will produce little or no debris when activated. In particular, the energy requirements for the HPM payload appear to require an explosive-driven magnetic flux compression generator or one of it derivatives to provide the energy required. This application combined with other requirements of a constrained space, G hardening, and relatively low mass make this a very challenging objective. However, the additional requirement of little and preferably no debris generation make this proposed effort extremely challenging. The primary objective for the Phase I effort is to design, fabricate, and test the explosive generator which will be the key component for the overall power supply. We propose several approaches to enable the design of this explosive generator. First, the metal used in the generator will be minimized. Second, the kinetic energy will be minimized. Third, the quantity of explosive used will be minimized. Finally, the entire generator will be encapsulated in a modern composite to both contain the internal debris and to not spall from the outward shock wave to produce external debris. While extensive theory and computer simulations will be used as tools, the major deliverable for this effort is an explosive generator experimental test that both works as expected and produces little or no external debris.

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