579 research outputs found
Metallic liquid hydrogen and likely Al2O3 metallic glass
Dynamic compression has been used to synthesize liquid metallic hydrogen at
140 GPa (1.4 million bar) and experimental data and theory predict Al2O3 might
be a metallic glass at ~300 GPa. The mechanism of metallization in both cases
is probably a Mott-like transition. The strength of sapphire causes shock
dissipation to be split differently in the strong solid and soft fluid. Once
the 4.5-eV H-H and Al-O bonds are broken at sufficiently high pressures in
liquid H2 and in sapphire (single-crystal Al2O3), electrons are delocalized,
which leads to formation of energy bands in fluid H and probably in amorphous
Al2O3. The high strength of sapphire causes shock dissipation to be absorbed
primarily in entropy up to ~400 GPa, which also causes the 300-K isotherm and
Hugoniot to be virtually coincident in this pressure range. Above ~400 GPa
shock dissipation must go primarily into temperature, which is observed
experimentally as a rapid increase in shock pressure above ~400 GPa. The
metallization of glassy Al2O3, if verified, is expected to be general in strong
oxide insulators. Implications for Super Earths are discussed.Comment: 8 pages, 5 figures, 14th Liquid and Amorphous Metals Conference, Rome
201
Coupled Electron Ion Monte Carlo Calculations of Dense Metallic Hydrogen
We present a new Monte Carlo method which couples Path Integral for finite
temperature protons with Quantum Monte Carlo for ground state electrons, and we
apply it to metallic hydrogen for pressures beyond molecular dissociation. We
report data for the equation of state for temperatures across the melting of
the proton crystal. Our data exhibit more structure and higher melting
temperatures of the proton crystal than Car-Parrinello Molecular Dynamics
results. This method fills the gap between high temperature electron-proton
Path Integral and ground state Diffusion Monte Carlo methods
Re(de)fining Narrative Events: Examining Television Narrative Structure
The authors introduce a new analytical instrument, the Scene Function Model, as an expansion of Seymour Chatman\u27s theoretical classification of story events into kernels and satellites. The Scene Function Model is designed to examine the narrative function of television scenes and provide the user with a clearer understanding of the structure of the television narrative
Entropy-Dominated Dissipation in Sapphire Shock-Compressed up to 400 GPa (4 Mbar)
Sapphire (single-crystal Al2O3) is a representative Earth material and is
used as a window and/or anvil in shock experiments. Pressure, for example, at
the core-mantle boundary is about 130 gigapascals (GPa). Defects induced by
100-GPa shock waves cause sapphire to become opaque, which precludes measuring
temperature with thermal radiance. We have measured wave profiles of sapphire
crystals with several crystallographic orientations at shock pressures of 16,
23, and 86 GPa. At 23 GPa plastic-shock rise times are generally quite long
(~100 ns) and their values depend sensitively on the direction of shock
propagation in the crystal lattice. The long rise times are probably caused by
the high strength of inter-atomic interactions in the ordered three-dimensional
sapphire lattice. Our wave profiles and recent theoretical and laser-driven
experimental results imply that sapphire disorders without significant shock
heating up to about 400 GPa, above which Al2O3 is amorphous and must heat. This
picture suggests that the characteristic shape of shock compression curves of
many Earth materials at 100 GPa pressures is caused by a combination of entropy
and temperature.Comment: 12 pages, 4 figure
Properties of planetary fluids at high pressure and temperature
In order to derive models of the interiors of Uranus, Neptune, Jupiter and Saturn, researchers studied equations of state and electrical conductivities of molecules at high dynamic pressures and temperatures. Results are given for shock temperature measurements of N2 and CH4. Temperature data allowed demonstration of shock induced cooling in the the transition region and the existence of crossing isotherms in P-V space
Recommended from our members
Laser-generated metallic hydrogen
Hydrogen reaches the minimum conductivity of a metal at 140 GPa (1.4 Mbar) and 3000 K. These conditions were achieved using a two-stage light-gas gun. The authors have investigated computationally the use of a laser-heated hohlraum to shock compress hydrogen to these conditions in samples sufficiently thin that the metallic fluid might be quenched metastably on release of dynamic pressure. A configuration was found such that the duration of maximum pressure is sufficiently long that the hydrogen film cools by thermal conduction before pressure is released
Case management and Think First completion
“The final, definitive version of this article has been published in the Journal, Probation Journal, Vol 53 Issue 3, 2006, Copyright The Trade Union and Professional Association for Family Court and Probation Staff, by SAGE Publications Ltd at: http://prb.sagepub.com/ " DOI: 10.1177/0264550506066771This article considers the findings of a small-scale study of the practice of case managers supervising offenders required to attend the Think First Group. It explores the interface between one-to-one and group-based work within multi-modal programmes of supervision and seeks to identify those practices that support individuals in completing a group.Peer reviewe
Response of Seven Crystallographic Orientations of Sapphire Crystals to Shock Stresses of 16 to 86 GPa
Shock-wave profiles of sapphire (single-crystal Al2O3) with seven
crystallographic orientations were measured with time-resolved VISAR
interferometry at shock stresses in the range 16 to 86 GPa. Shock propagation
was normal to the surface of each cut. The angle between the c-axis of the
hexagonal crystal structure and the direction of shock propagation varied from
0 for c-cut up to 90 degrees for m-cut in the basal plane. Based on published
shock-induced transparencies, shock-induced optical transparency correlates
with the smoothness of the shock-wave profile. The ultimate goal was to find
the direction of shock propagation in sapphire that is most transparent as a
window. Particle velocity histories were recorded at the interface between a
sapphire crystal and a LiF window. In most cases measured wave profiles are
noisy as a result of heterogeneity of deformation. Measured values of Hugoniot
Elastic Limits (HELs) depend on direction of shock compression and peak shock
stress. The largest HEL values were recorded for shock loading along the c-axis
and perpendicular to c along the m-direction. Shock compression along the m-
and s-directions is accompanied by the smallest heterogeneity of deformation
and the smallest rise time of the plastic shock wave. m- and s-cut sapphire
most closely approach ideal elastic-plastic flow, which suggests that m- and
s-cut sapphire are probably the orientations that remains most transparent to
highest shock pressures. Under purely elastic deformation sapphire has very
high spall strength, which depends on load duration and peak stress. Plastic
deformation of sapphire causes loss of its tensile strength.Comment: 18 pages, 18 figure
Evolution of Ultracold, Neutral Plasmas
We present the first large-scale simulations of an ultracold, neutral plasma,
produced by photoionization of laser-cooled xenon atoms, from creation to
initial expansion, using classical molecular dynamics methods with open
boundary conditions. We reproduce many of the experimental findings such as the
trapping efficiency of electrons with increased ion number, a minimum electron
temperature achieved on approach to the photoionization threshold, and
recombination into Rydberg states of anomalously-low principal quantum number.
In addition, many of these effects establish themselves very early in the
plasma evolution ( ns) before present experimental observations begin.Comment: 4 pages, 3 figures, submitted to PR
- …