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rss_1.0 Recursos de colección

Caltech Authors (93,037 recursos)
Repository of works by Caltech published authors.

Status = Published

Mostrando recursos 1 - 20 de 41,109

1. An instantaneous inelastic energy loss algorithm for use in molecular dynamics simulations - Shapiro, M. H.; Tombrello, T. A.
We have developed an instantaneous inelastic energy loss algorithm for use in molecular dynamics simulations of sputtering. This algorithm uses distance-of-closest-approach (DCA) criteria to initiate inner-shell electronic excitation, and subsequent removal of the excitation energy following hard atom-atom and ion-atom collisions. The DCA criteria used in the algorithm are based on the electron promotion models of Fano and Lichten [Phys. Rev. Lett. 14 (1965) 627] and Barat and Lichten [Phys. Rev. A 6 (1972) 211]. The amount of energy removed in these hard atom-atom and ion-atom collisions is determined by the energy needed to excite one or more inner-shell electrons...

2. Distribution of damage along an MeV ion track - Tombrello, T. A.
It is well known that at low values or dE/dx the damage along the path or an ion in a dielectric solid is not uniform. This distribution or damaged regions versus ion type and energy provides the most direct path to understanding the mechanism that initiates the formation of the damage track. In this report I extract this distribution from recently published data on tracks in Y_3Fe_5O_(12) and compare it with extant data obtained for mica. It is found that these crystalline materials, for which the greatest amount of data exist, provide a virtually identical picture or the process. For such materials the energy...

3. Comparison of a 1g and centrifuge model dynamic liquefaction test: Preliminary results - Gibson, A. D.; Scott, R. F.
As part of the examination of a recently-derived scaling relation for 1g laboratory tests, a laminar box for a shake table was constructed and used to contain saturated soil in a model scaled to represent the performance of a previous dynamic centrifuge liquefaction test carried out as part of the VELACS project (Arulanandan and Scott, 1993, 1994). A comparison of the results of the 1g and centrifuge tests was made in a preliminary investigation of the efficacy of the method. Liquefaction developed in both centrifuge and 1g tests.

4. On the decay of end-effects due to self-equilibrated loads - Knowles, James K.; Scott, Ronald F.
This note compares the decay of end effects in anti-plane shear for a semi-infinite elastic strip under a self-equilibrated system of concentrated end-loads with the corresponding decay for a smoothly distributed self-equilibrated load.

5. Heat Release Effects on Shear-Layer Growth and Entrainment - Hermanson, J. C.; Mungal, M. G.; Dimotakis, P. E.
The effects of heat release were studied in a planar, gaseous reacting mixing layer formed between two subsonic freestreams; one containing hydrogen in an inert diluent, the other containing fluorine in an inert diluent. Sufficiently high concentrations of hydrogen and fluorine reactants were employed to produce adiabatic flame temperature rises of up to 940 K (adiabatic flame temperature of 1240 K absolute). Although the displacement thickness of the layer for a zero streamwise pressure gradient showed an increase with increasing heat release. the actual thickness of the mixing layer at a given downstream location was not observed to increase and, in fact, was characterized...

6. Duplicate test of Model No 4a: Horizontally layered soil in laminar box - Scott, R. F.; Hushmand, B.; Rashidi, H.
The results of a duplicate test on Model 4a are presented.

7. VELACS model No. 3: Liquefaction in dense and loose sand columns - Hushmand, B.; Scott, R. F.; Rashidi, H.
A dynamic centrifuge test on a saturated specimen of loose and dense sand columns prepared adjacent to each other in a laminar box was performed at Caltech. The test was also duplicated at University of California, Davis (UCD) and at RPI. An earthquake-like dynamic motion was supplied to the base of specimen and the responses of accelerometers, pore pressure and displacement transducers were measured. One of the duplicate tests (UCD) had comparable results to the primary test at Caltech. The input motion of the other duplicate test (RPI) was very different and could not be compared directly to the other two tests.

8. A Simple Model for Finite Chemical Kinetics Analysis of Supersonic Turbulent Shear Layer Combustion - Dimotakis, Paul E.; Hall, Jeffery L.
A simple flow/thermodynamic model is proposed to describe finite chemical kinetic rate combustion in a turbulent supersonic shear layer for the purposes of assessing Damköhler number effects in such flows. Sample calculations and comparisons for the H_2/NO/F_2 chemical system and the H_2/air system are described for a set of initial flow and thermodynamic conditions of the entrained reactants.

9. In-place calibration of USGS pore pressure transducers at Wildlife Liquefaction Site, California, USA - Hushmand, Behnam; Scott, Ronald F.; Crouse, C. B.
Acceleration and pore-pressure data were recorded at the Wildlife Site during the magnitude 6.6 Imperial Valley earthquake of 24 November 1987 which resulted in liquefaction of the site, alone among instrumented sites. Some unusual aspects were observed in the data, including long rise times of the pore pressures resulting in a time lag between the strong ground shaking and maximum pore pressure development. It was not clear from the data obtained whether the liquefaction process at the Wildlife Site was different from that observed in other saturated sand deposits, or if the pore-pressure transducers were not responding correctly. In December 1989, the authors performed an in-situ dynamic...

10. Duplicate test of Model No 2: Sloping loose sand layer - Scott, R. F.; Hushmand, B.; Rashidi, H.
The results of a duplicate test on Model 2 are presented.

11. Model No 3 primary test description and test results - Scott, R. F.; Hushmand, B.; Rashidi, H.
An earthquake-like dynamic motion was supplied to the base of a laminar box filled with saturated sand in a centrifuge at 50g. The two halves of the box, divided by a vertical interface, consisted of loose sand and dense sand, respectively. The test results are reported.

12. Radial consolidation of a phase-change soil - Scott, R. F.
An overconsolidated sensitive clay has been represented by a one-dimensional compressional model in which the clay structure is rigid both below and above the maximum past effective stress and collapses at this stress from a higher void ratio to a lower value. This model is incorporated in several different geometries to describe the behaviour of the sensitive clay under applied stress. These cases include radial compression of a cylinder and a sphere, and expansion of a cylindrical cavity in an infinite region. Solutions are given for displacements and pore pressures.

13. Reply to "Note on isotope sputtering" - Weathers, D. L.; Hutcheon, I. D.; Gnaser, H.; Tombrello, T. A.; Shapiro, M. H.
In his note commenting on the ^(92)Mo-^(100)Mo isotopic fractionation results of Weathers et al., Sigmund raises questions about the experimental procedures used in these measurements. Specifically, he suggests that isotopic fractionation effects may be present in the SIMS analysis of the carbon collector foils; and, based on conventional sputtering theory, he further suggests that these effects may be as large as 50% of the fractionation from the molybdenum targets themselves, if only a small fraction of the molybdenum was removed from the foils. He also raises questions about the possible relationship between changes in the yield and fractionation versus ion fluence.

14. A time-of-flight detector for heavy ion RBS - Döbeli, M.; Haubert, P. C.; Livi, R. P.; Spicklemire, S. J.; Weathers, D. L.; Tombrello, T. A.
We describe the details and performance of a time-of-flight (TOF) spectrometer for application in heavy ion RBS. An energy resolution of better than 1% is achieved for ^(16)O and ^(35)Cl ions in the energy range between 3 and 15 MeV. Using ions with a mass up to 45 amu as projectiles a mass resolution between 1 and 2 amu is obtained over the whole periodic table. At the sample surface a depth resolution of ∼ 1 μg/cm^2 can be achieved. The technique has been used to measure indium profiles in In GaAs/GaAs quantum well samples with a depth resolution of...

15. Predicting latent track dimensions - Tombrello, T. A.
The increased use of columnar damage/latent tracks produced by MeV ions for scientific and technological applications provides ample justification for improving our understanding of the phenomenon. In this paper I give an overview of the systematics of tracks in several materials where the data are sufficiently extensive to make meaningful comparisons. These materials include yttrium iron garnet, crystalline quartz, muscovite mica, polystyrene, mylar, and cellulose triacetate. Although these data can be tied together fairly well by a simple, quantitative model, there are inconsistencies that suggest both that our understanding of the phenomenon and the relationship of data from different experiments...

16. Fractals, Dimensional Analysis and Similarity, and Turbulence - Dimotakis, Paul E.
[no abstract]

17. A molecular dynamics study of collisional excitation/deexcitation at ion-bombarded surfaces - Shiang, Keh-Dong; Cai, Jun; Shapiro, M. H.; Fine, J.; Tombrello, T. A.
On the basis of ab initio potentials, a three-dimensional classical trajectory model of the dynamics of energetic collision events for ion-bombarded alkali halide surfaces is introduced. In this model, a distance-of-closest-approach method is used to determine the occurrence of core excitation and electron capture. The results show that an excited sodium ion may be formed when a sodium 2p electron is promoted into thet 3s state during a hard Ar^+-N^a+ collision. This inner-shell electron excitation is shown to be an initiating process, which can result in the subsequent neutralization and Auger electron emission from Na in the solid. We give...

18. Product Formation in Chemically-Reacting Turbulent Jets - Gilbrech, R. J.; Dimotakis, P. E.
Results from experiments performed in a newly-constructed High Pressure Combustion Facility are presented. The experiments described here are designed to address Reynolds number effects on turbulent mixing and entrainment, and flame length in an axisymmetric, gas-phase turbulent reacting jet, in particular, over a large range of Reynolds numbers.

19. Molecular dynamics simulations of inelastic energy loss effects in sputtering II - Shapiro, M. H.; Tombrello, T. A.
Previous molecular dynamics (MD) simulations of inelastic energy loss effects in sputtering, which have employed pair potentials and velocity-dependent losses, have attributed significant reductions in sputtering yield (up to 40%) to energy losses in atom-atom collisions. Similar simulations in which the average electron density is replaced by the local electron density as calculated from the embedded-atom method (EAM) and Thomas-Fermi local electron densities also yield significant inelastic energy losses in the collision cascade [A. Caro and M. Victoria, Phys. Rev. A 40 (1989) 2287]. Here we report results of EAM MD inelastic energy loss simulations for the Ar^+-Cu system for...

20. Molecular dynamics simulations of inelastic energy loss effects in sputtering - Shapiro, M. H.; Tombrello, T. A.
Previous molecular dynamics (MD) simulations of inelastic energy loss effects in sputtering, which have employed pair potentials and velocity dependent losses, have attributed significant reductions in sputter yield (up to 40%) to energy losses in atom-atom collisions. Similar simulations, in which the average electron density is replaced by the local electron density as calculated from the embedded atom method (EAM) and Thomas-Fermi local densities, also yield significant inelastic energy losses in the collision cascade. We present a preliminary account of EAM MD inelastic energy loss simulations using both a velocity dependent model to account for losses from collisions with valence...

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