1,746 research outputs found
Investigation of the solidification, structure and properties of eutectic alloys including consideration of properties control Progress report
Solidification, structure, and properties of nickel and niobium eutectic alloy
Investigation of the solidification, structure and properties of eutectic alloys including consideration of properties control
Solidification structure and properties of eutetic alloy
Flame zone of a composite propellant expanded by a laser source
Technique scales flame structure linearly with gas kinetic mean free path, which increases two to three orders of magnitude as pressure decreases like amount. Kinetic and transport time scales expand in proportion so that regression rates for laser-induced flames are two to three orders of magnitude slower
Composite solid propellant flame microstructure determination Annual report, 23 Jun. 1967 - 22 Jun. 1968
Composite solid propellant flame microstructure determination
Axion Cosmology and the Energy Scale of Inflation
We survey observational constraints on the parameter space of inflation and
axions and map out two allowed windows: the classic window and the inflationary
anthropic window. The cosmology of the latter is particularly interesting;
inflationary axion cosmology predicts the existence of isocurvature
fluctuations in the CMB, with an amplitude that grows with both the energy
scale of inflation and the fraction of dark matter in axions. Statistical
arguments favor a substantial value for the latter, and so current bounds on
isocurvature fluctuations imply tight constraints on inflation. For example, an
axion Peccei-Quinn scale of 10^16 GeV excludes any inflation model with energy
scale > 3.8*10^14 GeV (r > 2*10^(-9)) at 95% confidence, and so implies
negligible gravitational waves from inflation, but suggests appreciable
isocurvature fluctuations.Comment: 10 PRD pages, 4 figs, V3: updated to match published versio
Elastic Deformation of Polycrystals
We propose a framework to model elastic properties of polycrystals by
coupling crystal orientational degrees of freedom with elastic strains. Our
model encodes crystal symmetries and takes into account explicitly the strain
compatibility induced long-range interaction between grains. The coupling of
crystal orientation and elastic interactions allows for the rotation of
individual grains by an external load. We apply the model to simulate uniaxial
tensile loading of a 2D polycrystal within linear elasticity and a system with
elastic anharmonicities that describe structural phase transformations. We
investigate the constitutive response of the polycrystal and compare it to that
of single crystals with crystallographic orientations that form the
polycrystal.Comment: 4 pages, 4 ps figure
Correlation between stick-slip frictional sliding and charge transfer
A decade ago, Budakian and Putterman (Phys. Rev. Lett., {\bf 85}, 1000
(2000)) ascribed friction to the formation of bonds arising from contact
charging when a gold tip of a surface force apparatus was dragged on
polymethylmethacrylate surface. We propose a stick-slip model that captures the
observed correlation between stick-slip events and charge transfer, and the
lack of dependence of the scale factor connecting the force jumps and charge
transfer on normal load. Here, stick-slip dynamics arises as a competition
between the visco-elastic and plastic deformation time scales and that due to
the pull speed with contact charging playing a minor role. Our model provides
an alternate basis for explaining most experimental results without ascribing
friction to contact charging.Comment: 8 pages, 4 figures, To be appeared in Physical Review
Absolute and convective instabilities of parallel propagating circularly polarized Alfven waves: Beat instability
Ruderman and Simpson [Phys. Plasmas 11, 4178 (2004)] studied the absolute and convective decay instabilities of parallel propagating circularly polarized Alfven waves in plasmas where the sound speed c(S) is smaller than the Alfven speed upsilon(A). We extend their analysis for the beat instability which occurs in plasmas with c(S)>upsilon(A). We assume that the dimensionless amplitude of the circularly polarized Alfven wave (pump wave), a, is small. Applying Briggs' method we study the problem analytically using expansions in power series with respect to a. It is shown that the pump wave is absolutely unstable in a reference frame moving with the velocity U with respect to the rest plasma if U-lU-r, the instability is convective. The signaling problem is studied in a reference frame where the pump wave is convectively unstable. It is shown that the spatially amplifying waves exist only when the signaling frequency is in two narrow symmetric frequency bands with the widths of the order of a(3). These results enable us to extend for the case when c(S)>upsilon(A) the conclusions, previously made for the case when c(S)<upsilon(A), that circularly polarized Alfven waves propagating in the solar wind are convectively unstable in a reference frame of any spacecraft moving with the velocity not exceeding a few tens of km/s in the solar reference frame. The characteristic scale of spatial amplification for these waves exceeds 1 a.u
A high-reflectivity high-Q micromechanical Bragg-mirror
We report on the fabrication and characterization of a micromechanical
oscillator consisting only of a free-standing dielectric Bragg mirror with high
optical reflectivity and high mechanical quality. The fabrication technique is
a hybrid approach involving laser ablation and dry etching. The mirror has a
reflectivity of 99.6%, a mass of 400ng, and a mechanical quality factor Q of
approximately 10^4. Using this micromirror in a Fabry Perot cavity, a finesse
of 500 has been achieved. This is an important step towards designing tunable
high-Q high-finesse cavities on chip.Comment: 3 pages, 2 figure
Warm tachyonic inflation in warped background
We analyze warm tachyonic inflation, proposed in the literature, but from the
viewpoint of four dimensional effective action for tachyon field on a non-BPS
D3-brane. We find that consistency with observational data on density
perturbation and validity of effective action requires warped compactification.
The number of background branes which source the flux is found to be of the
order of 10 in contrast to the order of in the standard cold
inflationary scenario.Comment: 9 pages, RevTe
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