54 research outputs found
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The soft-sphere model for metals near the critical point
The soft-sphere model is convenient and accurate for predicting fluid phase thermodynamics. The validity of the model for describing the equation-of-state derivatives, Grueneisen's gamma and sound velocity, is shown. 7 refs., 2 figs
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Raman spectroscopies in shock-compressed materials
Spontaneous Raman spectroscopy, stimulated Raman scattering and coherent anti-Stokes Raman scattering have been used to measure temperatures and changes in molecular vibrational frequencies for detonating and shocked materials. Inverse Raman and Raman induced Kerr effect spectroscopies have been suggested as diagnostic probes for determining and phenomenology of shock-induced chemical reactions. The practicality, advantages, and disadvantages of using Raman scattering techniques as diagnostic probes of microscopic phenomenology through and immediately behind the shock front of shock-compressed molecular systems are discussed
Search for gravitational waves from Scorpius X-1 in the second Advanced LIGO observing run with an improved hidden Markov model
We present results from a semicoherent search for continuous gravitational waves from the low-mass x-ray binary Scorpius X-1, using a hidden Markov model (HMM) to track spin wandering. This search improves on previous HMM-based searches of LIGO data by using an improved frequency domain matched filter, the J-statistic, and by analyzing data from Advanced LIGO's second observing run. In the frequency range searched, from 60 to 650 Hz, we find no evidence of gravitational radiation. At 194.6 Hz, the most sensitive search frequency, we report an upper limit on gravitational wave strain (at 95% confidence) of h095%=3.47×10-25 when marginalizing over source inclination angle. This is the most sensitive search for Scorpius X-1, to date, that is specifically designed to be robust in the presence of spin wandering. © 2019 American Physical Society
Erratum: "A Gravitational-wave Measurement of the Hubble Constant Following the Second Observing Run of Advanced LIGO and Virgo" (2021, ApJ, 909, 218)
[no abstract available
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Issues and future directions in subsecond thermophysics
The primary motivations for trying to measure thermophysical properties of materials on subsecond timescales are to extend measurements to higher temperatures than can be conveniently maintained continuously, or to make measurements on systems out of thermodynamic equilibrium. Since these measurements are difficult, one must keep in mind the needs for the data. These include the ability to use materials cleverly in new high-pressure/high- temperature applications, as well as the development of calibrated models for the response of materials to rapid energy deposition, by laser pulses, for example. Several key areas have been identified in which effort is needed for substantial progress in subsecond measurements, and which have unusual promise for useful new scientific results. These areas include the problems of high-temperature standards, equilibration during rapid heating, measurements at higher temperatures combined with higher pressures, measurements on specific interesting materials, and measurement of microstructural properties at high temperature and their relation to macroscopic response. Each of these areas will be touched upon here
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New ways of looking for phase transitions at multi-megabar dynamic pressures
Shock wave techniques have provided much of our information about very high pressure phase transitions. However, because of the specific thermodynamic region accessible to simple shock wave experiments, phase transitions with very small density changes are difficult to detect. Those phase changes with a density decrease at constant pressure, such as normal melting, are virtually impossible to detect in a shock experiment. Three new techniques have been applied to thermodynamic measurements behind a shock wave, with the result that several previously undetected phase boundaries have been measured. First, optical pyrometry has been used to measure the temperature of shocked SiO/sub 2/. A temperature anomaly was found, indicating a phase change, probably melting, between 0.6 and 1.1 Mbar. Secondly, magnetic probes have been used to measure the velocity of sound in shock compressed metals. The loss of a longitudinal elastic component of the sound wave is indicative of melting. Finally, a novel optical analyzer technique, exploiting the sensitivity of thermal radiation to small temperature changes, has also been used to measure the sound velocity in shock compressed metals. With the extra sensitivity provided by this derivative measurement of an equation of state surface, both solid-solid and melting transitions of iron above 2 Mbar have been detected
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Birch's law and the properties of high temperature fluid metals
By comparing acoustic velocities in fluid metals over a very wide range of densities we have established Birch's Law as an approximate representation over the entire liquid range. For a given liquid metal the acoustic velocity is close to linear in density, with a slope determined by the atomic weight. The measurements include isobaric expansion to less than half normal density, ultrasonics on molten metals at 1 atmosphere, and shock-melted metals to greater than twice normal density. We also find unusual behavior of the Gruneisen gamma, which can be explained in terms of simple fluid models. 15 refs
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Pattern formation by shock processes
Shock waves in condensed media often produce and leave behind periodic patterns and textures. These patterns have been observed both in real time and in postmortem examination. In many cases the patterns can be related to analogous Pattern-forming mechanisms in classical fluid dynamics, such as the Rayleigh-Taylor and Helmholtz instabilities. In other cases, the textures arise from peculiarities in the dynamic stress state immediately behind the leading edge of the shock wave. Periodic waves in the interface between two shock welded metals have a close resemblance to the classical Helmholtz instability. From a practical point of view, these waves are crucial to the formation of a good bond. Impulsive acceleration of an interface can result in the Meshkov instability, which forms patterns qualitatively similar to the Rayleigh-Taylor instability driven by continuous acceleration. However, the patterned stress state left behind after a shock crosses a perturbed interface can result in perturbation growth for shock propagation in either direction across the interface. Even in homogeneous media, the non-hydrostatic component of the stress behind a shock can drive a pattern forming instability. Adiabatic shear banding has been proposed as a mechanism to explain both the patterns observed in shock-compressed and recovered metal samples and the apparent loss of macroscopic shear strength of shocked ceramics. New optical photographs of shocked quartz support this mechanism. 27 references
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High temperature sound speed measurements in expanded liquid tantalum
Cylindrical samples of tantalum are resistively pulse heated to high-temperature states along an isobaric path by capacitive discharge in a high pressure inert gas atmosphere. Using this technique samples may be heated to temperatures up to 10,000/sup 0/K at pressures up to 1.0 GPa. The transient (10/sup -4/ s) heating technique is employed to overcome stability problems. Our measurements are complemented by the additional capability to measure sound speeds in the hot expanded sample. Results of such measurements on tantalum at temperatures up to approx.6000/sup 0/K are presented along with several other quantities calculable from the sound speed data
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