5,260 research outputs found
Effect of the chemical state of pyrolysis gases on heat-shield mass
Effect of chemical properties of pyrolysis gases on heat shield mass required for lifting reentry vehicle in typical reentry trajector
Effect of composition and density on the ablative performance of phenolic-nylon
Composition and density effects on ablative performance of phenolic-nylo
Longitudinal muon spin relaxation in high purity aluminum and silver
The time dependence of muon spin relaxation has been measured in high purity
aluminum and silver samples in a longitudinal 2 T magnetic field at room
temperature, using time-differential \musr. For times greater than 10 ns, the
shape fits well to a single exponential with relaxation rates of
\lambda_{\textrm{Al}} = 1.3 \pm 0.2\,(\textrm{stat.}) \pm
0.3\,(\textrm{syst.})\,\pms and \lambda_{\textrm{Ag}} = 1.0 \pm
0.2\,(\textrm{stat.}) \pm 0.2\,(\textrm{syst.})\,\pms
An assessment of the readiness of ablative materials for preflight application to the shuttle orbiter
The shuttle orbiter relies primarily on a reusable surface insulation (RSI) thermal protection system (TPS). The RSI is very efficient in its thermal performance; however, the RSI tile system has shown poor mechanical integrity. The state-of-the-art of the ablative TPS is reviewed, and an assessment made of the ablator's readiness for use on the shuttle orbiter. Unresolved technical issues with regard to the ablative TPS are identified. Short time, highly focused analytical and experimental programs were initiated to: (1) identify candidate ablation materials; (2) assess the data base for these materials; (3) evaluate the need and kind of waterproof coating; (4) calculate thermal and other stresses in an ablator tile; (5) identify an acceptable ablator/RSI tile joint filler; and (6) assess the sensitivity of the ablator to sequential heat pulses. Results from some of these programs are discussed
Dynamics of liquid 4He in Vycor
We have measured the dynamic structure factor of liquid 4He in Vycor using
neutron inelastic scattering. Well-defined phonon-roton (p-r) excitations are
observed in the superfluid phase for all wave vectors 0.3 < Q < 2.15. The p-r
energies and lifetimes at low temperature (T = 0.5 K) and their temperature
dependence are the same as in bulk liquid 4He. However, the weight of the
single p-r component does not scale with the superfluid fraction (SF) as it
does in the bulk. In particular, we observe a p-r excitation between T_c =
1.952 K, where SF = 0, and T_(lambda)=2.172 K of the bulk. This suggests, if
the p-r excitation intensity scales with the Bose condensate, that there is a
separation of the Bose-Einstein condensation temperature and the superfluid
transition temperature T_c of 4He in Vycor. We also observe a two-dimensional
layer mode near the roton wave vector. Its dispersion is consistent with
specific heat and SF measurements and with layer modes observed on graphite
surfaces.Comment: 3 pages, 4 figure
Hidden magnetic transitions in thermoelectric layered cobaltite, [CaCoO][CoO]
A positive muon spin rotation and relaxation (SR) experiment on
[CaCoO][CoO], ({\sl i.e.}, CaCoO, a layered
thermoelectric cobaltite) indicates the existence of two magnetic transitions
at 100 K and 400 - 600 K; the former is a transition from a paramagnetic
state to an incommensurate ({\sf IC}) spin density wave ({\sf SDW}) state. The
anisotropic behavior of zero-field SR spectra at 5 K suggests that the
{\sf IC-SDW} propagates in the - plane, with oscillating moments directed
along the c-axis; also the {\sf IC-SDW} is found to exist not in the
[CaCoO] subsystem but in the [CoO] subsystem. In addition, it is
found that the long-range {\sf IC-SDW} order completes below 30 K,
whereas the short-range order appears below 100 K. The latter transition is
interpreted as a gradual change in the spin state of Co ions %% at temperatures
above 400 K. These two magnetic transitions detected by SR are found to
correlate closely with the transport properties of
[CaCoO][CoO].Comment: 7 pages, 8 figures. to be appeared in Phys. Rev.
Environment-independent decoherence rate in classically chaotic systems
We study the decoherence of a one-particle system, whose classical
correpondent is chaotic, when it evolves coupled to a weak quenched
environment. This is done by analytical evaluation of the Loschmidt Echo, (i.e.
the revival of a localized density excitation upon reversal of its time
evolution), in presence of the perturbation. We predict an exponential decay
for the Loschmidt Echo with a (decoherence) rate which is asymptotically given
by the mean Lyapunov exponent of the classical system, and therefore
independent of the perturbation strength, within a given range of strengths.
Our results are consistent with recent experiments of Polarization Echoes in
nuclear magnetic resonance and preliminary numerical simulations.Comment: No figures. Typos corrected and minor modifications to the text and
references. Published versio
Liquid 4He near the superfluid transition in the presence of a heat current and gravity
The effects of a heat current and gravity in liquid 4He near the superfluid
transition are investigated for temperatures above and below T_lambda. We
present a renormalization-group calculation based on model F for the Green's
function in a self-consistent approximation which in quantum many-particle
theory is known as the Hartree approximation. The approach can handle a zero
average order parameter above and below T_lambda and includes effects of
vortices. We calculate the thermal conductivity and the specific heat for all
temperatures T and heat currents Q in the critical regime. Furthermore, we
calculate the temperature profile. Below T_lambda we find a second correlation
length which describes the dephasing of the order parameter field due to
vortices. We find dissipation and mutual friction of the superfluid-normal
fluid counterflow and calculate the Gorter-Mellink coefficient A. We compare
our theoretical results with recent experiments.Comment: 26 pages, 9 figure
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