32,533 research outputs found
High temperature circuit breaker
Alternating current circuit breaker is suitable for reliable long-term service at 1000 deg F in the vacuum conditions of outer space. Construction materials are resistant to nuclear radiation and vacuum welding. Service test conditions and results are given
Statistical Mechanics of Vibration-Induced Compaction of Powders
We propose a theory which describes the density relaxation of loosely packed,
cohesionless granular material under mechanical tapping. Using the compactivity
concept we develope a formalism of statistical mechanics which allows us to
calculate the density of a powder as a function of time and compactivity. A
simple fluctuation-dissipation relation which relates compactivity to the
amplitude and frequency of a tapping is proposed. Experimental data of
E.R.Nowak et al. [{\it Powder Technology} 94, 79 (1997) ] show how density of
initially deposited in a fluffy state powder evolves under carefully controlled
tapping towards a random close packing (RCP) density. Ramping the vibration
amplitude repeatedly up and back down again reveals the existence of reversible
and irreversible branches in the response. In the framework of our approach the
reversible branch (along which the RCP density is obtained) corresponds to the
steady state solution of the Fokker-Planck equation whereas the irreversible
one is represented by a superposition of "excited states" eigenfunctions. These
two regimes of response are analyzed theoretically and a qualitative
explanation of the hysteresis curve is offered.Comment: 11 pages, 2 figures, Latex. Revised tex
Dynamic mechanical response of polymer networks
The dynamic-mechanical response of flexible polymer networks is studied in
the framework of tube model, in the limit of small affine deformations, using
the approach based on Rayleighian dissipation function. The dynamic complex
modulus G* is calculated from the analysis of a network strand relaxation to
the new equilibrium conformation around the distorted primitive path. Chain
equilibration is achieved via a sliding motion of polymer segments along the
tube, eliminating the inhomogeneity of the polymer density caused by the
deformation. The characteristic relaxation time of this motion separates
the low-frequency limit of the complex modulus from the high-frequency one,
where the main role is played by chain entanglements, analogous to the rubber
plateau in melts. The dependence of storage and loss moduli, G' and G'', on
crosslink and entanglement densities gives an interpolation between polymer
melts and crosslinked networks. We discuss the experimental implications of the
rather short relaxation time and the slow square-root variation of the moduli
and the loss factor tan at higher frequencies.Comment: Journal of Chemical Physics (Oct-2000); Lates, 4 EPS figures include
Five-year financial analysis of two commercial farms converting to organic: 2000-2005
The financial performance including input and output costs were analysed for two commercial Scottish farms converting to organic production. A comparative analysis for a five year period is provided using data from the Scottish Farm Accounts scheme, and demonstrates that both conversions had positive financial outcomes. Both farms applied strategic changes to their businesses
Dynamical response functions in models of vibrated granular media
In recently introduced schematic lattice gas models for vibrated dry granular
media, we study the dynamical response of the system to small perturbations of
shaking amplitudes and its relations with the characteristic fluctuations.
Strong off equilibrium features appear and a generalized version of the
fluctuation dissipation theorem is introduced. The relations with thermal
glassy systems and the role of Edwards' compactivity are discussed.Comment: 12 pages, 2 postscript figure
Observing the evaporation transition in vibro-fluidized granular matter
By shaking a sand box the grains on the top start to jump giving the picture
of evaporating a sand bulk, and a gaseous transition starts at the surface
granular matter (GM) bed. Moreover the mixture of the grains in the whole bed
starts to move in a cooperative way which is far away from a Brownian
description. In a previous work we have shown that the key element to describe
the statistics of this behavior is the exclusion of volume principle, whereby
the system obeys a Fermi configurational approach. Even though the experiment
involves an archetypal non-equilibrium system, we succeeded in defining a
global temperature, as the quantity associated to the Lagrange parameter in a
maximum entropic statistical description. In fact in order to close our
approach we had to generalize the equipartition theorem for dissipative
systems. Therefore we postulated, found and measured a fundamental dissipative
parameter, written in terms of pumping and gravitational energies, linking the
configurational entropy to the collective response for the expansion of the
centre of mass (c.m.) of the granular bed. Here we present a kinetic approach
to describe the experimental velocity distribution function (VDF) of this
non-Maxwellian gas of macroscopic Fermi-like particles (mFp). The evaporation
transition occurs mainly by jumping balls governed by the excluded volume
principle. Surprisingly in the whole range of low temperatures that we measured
this description reveals a lattice-gas, leading to a packing factor, which is
independent of the external parameters. In addition we measure the mean free
path, as a function of the driving frequency, and corroborate our prediction
from the present kinetic theory.Comment: 6 pages, 4 figures, submitted for publication September 1st, 200
S and D-wave phase shifts in isospin-2 pi pi scattering from lattice QCD
The isospin-2 pi pi system provides a useful testing ground for determining
elastic hadron scattering parameters from finite-volume spectra obtained using
lattice QCD computations. A reliable determination of the excited state
spectrum of two pions in a cubic box follows from variational analysis of
correlator matrices constructed using a large basis of operators. A general
operator construction is presented which respects the symmetries of a
multi-hadron system in flight. This is applied to the case of pi pi and allows
for the determination of the scattering phase-shifts at a large number of
kinematic points, in both S-wave and D-wave, within the elastic region. The
technique is demonstrated with a calculation at a pion mass of 396 MeV, where
the elastic scattering is found to be well described by a scattering length
parameterisation.Comment: Tables of little-group CGCs in ancillary file; v2: minor changes to
reflect published versio
Tapping Spin Glasses
We consider a tapping dynamics, analogous to that in experiments on granular
media, on spin glasses and ferromagnets on random thin graphs. Between taps,
zero temperature single spin flip dynamics takes the system to a metastable
state. Tapping, corresponds to flipping simultaneously any spin with
probability . This dynamics leads to a stationary regime with a steady state
energy . We analytically solve this dynamics for the one dimensional
ferromagnet and spin glass. Numerical simulations for spin glasses and
ferromagnets of higher connectivity are carried out, in particular we find a
novel first order transition for the ferromagnetic systems.Comment: 5 pages, 3 figures, RevTe
Midgap states and charge inhomogeneities in corrugated graphene
We study the changes induced by the effective gauge field due to ripples on
the low energy electronic structure of graphene. We show that zero energy
Landau levels will form, associated to the smooth deformation of the graphene
layer, when the height corrugation, , and the length of the ripple, , are
such that , where is the lattice constant. The
existence of localized levels gives rise to a large compressibility at zero
energy, and to the enhancement of instabilities arising from electron-electron
interactions including electronic phase separation. The combined effect of the
ripples and an external magnetic field breaks the valley symmetry of graphene
leading to the possibility of valley selection
Response properties in a model for granular matter
We investigate the response properties of granular media in the framework of
the so-called {\em Random Tetris Model}. We monitor, for different driving
procedures, several quantities: the evolution of the density and of the density
profiles, the ageing properties through the two-times correlation functions and
the two-times mean-square distance between the potential energies, the response
function defined in terms of the difference in the potential energies of two
replica driven in two slightly different ways. We focus in particular on the
role played by the spatial inhomogeneities (structures) spontaneously emerging
during the compaction process, the history of the sample and the driving
procedure. It turns out that none of these ingredients can be neglected for the
correct interpretation of the experimental or numerical data. We discuss the
problem of the optimization of the compaction process and we comment on the
validity of our results for the description of granular materials in a
thermodynamic framework.Comment: 22 pages, 35 eps files (21 figures
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