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Postdoc/GRA Info Search LANL
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***Note: the copies of the presentations are available internally only***
Wed. Jan 15, 2-3 pm EOS of Mixtures: (1) water/ice VII and (2) HE reactants/products Shock wave research at the Institute for Shock Physics at Washington
State University has recently involved reverberation experiments in which
liquid water was quasi-isentropically compressed to pressure-temperature
states that should produce ice VII (D. H. Dolan, 2001). As part of the analysis
of these experiments a mixture EOS was developed and used in conjunction
with the WSU finite-difference code COPS. Some of these experiments and interpretation
will be discussed. The same mixture EOS solution method has been used in
high explosive (HE) shock initiation calculations (P. M. Howe, 2002). The
goal here was to provide a simple, reliable EOS option Wed. Jan 8, 2-3 pm Phonon dispersion in Actinides measured
with Inelastic
X-ray Scattering: New opportunities to solve some old problem Michael E. Manley, MST-6
Prior to the development of inelastic neutron scattering
techniques in the 1950's and 1960's there was little data on the dispersion
of phonons in crystals. Since then large advances have been made. However,
despite a large community interested in f-electron elements, neutron results
have remained elusive for many actinides. This has been because many isotopes
have large neutron absorption cross-sections and even in cases where a suitable
isotope exists, crystals large enough for neutron scattering (> 0.1 cm3) were rarely realized. We have begun an effort
to change this by taking advantage of the development of high resolution inelastic
X-ray scattering at the Advanced Photon Source. In our first demonstration
phonon dispersion curves were obtained from inelastic X-ray scattering measurements
on high-purity uranium single crystals at room temperature. Modes displacing
atoms along [00z] and propagating in all three high symmetry directions were
measured. Whereas the acoustic modes agree with the neutron measurements,
the longitudinal optic branch is about 10% higher in energy, but consistent
with higher cutoff energies observed in phonon density-of-states measurements
on polycrystals. The application of this X-ray technique, which requires only
very small samples, opens new possibilities in actinide science.
Wed. Dec 11, 2-3 pm Gruneisen Gamma and Acoustic Velocities for Soft
Sphere Fluids An equation of state surface is defined locally by an EOS point and two orthogonal derivatives. From an experimental standpoint, the most easily measured orthogonal derivatives can be chosen to be Gruneisen's gamma and the sound velocity or isothermal compressibility. Measurements of these quantities for fluid metals and molecular fluids over large density ranges have shown some surprises. Approximate modeling of these parameters over wide ranges of fluid densities has been accomplished with a semi-empirical soft sphere fluid model. This model explicitly shows the limitations of well known rules of thumb relating Gruneisen's gamma to density and Birch's law relating sound velocity to density. I will present a range of experimental and molecular dynamics data and the soft sphere model used to understand the data. Ref.: J. W. Shaner, J. Chem. Phys., 89, 1616 (1988) Wed. Aug 21, 2-3 pm High Explosive Detonation Products EOS Data from
Overdriven Detonation Experiments The chemical reactions in detonating high explosives produce high pressure-temperature reaction products that can then do work on the surroundings as the products expand from the high-pressure state. To model the way in which explosives push on materials, we must know the equations of state of the detonation products that control the P-V paths followed by the products as they expand. A range of diagnostic tools first developed for shock wave experiments in inert materials can be used to obtain EOS data on products in overdriven detonation experiments, where a supported shock wave produces pressures higher than the characteristic detonation pressure (the Chapman-Jouguet pressure). These data have been used by theoreticians at LANL to develop sophisticated new equations of state for detonation products of the explosives PBX 9501 and PBX 9502 Wed. Aug 7, 2-3 pm Ultra-High Pressure Phase Transformations in Rare
Earth Metals The rare earth metals, also known as the lanthanides, are characterized by the filling of the 4f shell. When pressure is employed as the primary variable, the behavior of rare earth metals becomes very interesting. As pressure increases, the inter-atomic distances decrease substantially because of the high compressibility. This has significant effects on their structural and electronic properties. A series of energy dispersive x-ray diffraction experiments have been performed on several of the rare earth metals to multi-megabar pressures with diamond anvil cells and synchrotron radiation. These experiments provide a wealth of structural information, as well as, insights into the electronic behavior. However, there has been considerable debate over some of the issues surrounding static pressure experiments. Some of this debate is warranted and solutions should be considered. One of the main issues deals with non-hydrostatic effects. There are ways to overcome such problems and rare earth metals are excellent candidates to test these possibilities Thurs, Jul 25, 9-10 am Real-Time X-ray Diffraction Measurements to Investigate
Elastic-Plastic Deformation in Shocked LiF X-ray diffraction measurements were used to examine elastic-plastic deformation in Mg-doped (110 ppm) LiF single crystals shocked along [100]. The Mg impurities increase the elastic limit (HEL) significantly as compared to the low 1-2 kbar elastic limit of ultrapure LiF. This high HEL leads to a large amplitude elastic wave (elastic precursor) followed by a slower traveling plastic wave. Three experimental techniques were used to obtain x-ray diffraction data throughout this elastic-plastic wave. Multiple and single x-ray diffraction methods monitoring the (200) and (202) lattice planes were used to determine lattice compression in the peak compressed (plastic) state of shocked LiF. Results from all experiments revealed isotropic compression similar to that observed in previous work on ultrapure LiF. Therefore, we conclude that within experimental error the larger elastic wave amplitudes, arising due to divalent Mg ions, do not affect the lattice compression in shocked LiF crystals. Time-resolved x-ray diffraction measurements, utilizing an x-ray streak camera with 2 nanosecond resolution, were performed to monitor the temporal evolution of the diffraction data from the (200) lattice planes during elastic-plastic deformation. Results from all experiments indicate uniaxial unit cell compression at the elastic-precursor front followed by a rapid transition towards isotropic unit cell compression during stress relaxation. Further results and implications will be discussed. Tues, Jul 23, 2-3 pm Freezing of liquid water in quasi-isentropic loading
At thermodynamic equilibrium, materials tend toward the phase of minimum Gibbs free energy G(T,P). Phase changes do not happen instantaneously; systems remain trapped in a metastable state, unable to access true equilibrium, until some incubation time elapses. Fast transition time scales (1e-10 -- 1e-5 s) in melting and polymorphic transitions have been studied using shock waves; to date, studies of freezing on this scale are inconclusive. Shock freezing is plagued by long incubation times ( > 1e3 s possible in supercooled water) and large temperature rises resulting from shock compression. This talk will describe water freezing research at the Institute for Shock Physics. Shock reverberation was used to produce quasi-isentropic loading, minimizing temperature increase. With this method, freezing is possible in water for pressures above 2 GPa. Freezing was detected on 1e-8 -- 10e-7 s time scales using optical transmission diagnostics. Direct images of the freezing process were also obtained. First order phase changes are inferred from mechanical wave profile measurements. The importance of surface effects will be discussed. Wednesday, July 10, 2002 Phonon Dispersions Extracted from Pair-Density
Functions I explore the possibility of extracting information about lattice dynamics in simple crystal structures from the neutron pair-density function (PDF), obtained from powder neutron diffraction. Contrary to the claims by Dimitrov, Louca and Roeder [PRB 60 (1999) 6204], and in agreement with Reichardt and Pintschovius [PRB 63 (2001) 174302], I find that the PDF alone is not sufficient for constructing accurate phonon dispersions in systems with complex lattice dynamics. However, it is possible to obtain phonon moments within a few percent accuracy. The limitations and problems of this inverse method will be discussed. Wednesday, June 26, 2002 Interpretation of Experiments on Phase Transitions
in Ti and Zr I will discuss the analysis of experiments on high pressure phase transitions in Ti and Zr. This analysis involves constructing accurate free energies for the individual phases, incorporating information from static and dynamic experiments, as well first principles calculations. These free energies are then used in combination with phenomenological transformation rates for direct simulation of time-resolved shock propagation experiments. The data on Zr appears to be consistent with the shock-induced omega -> beta transition occurring close to the equilibrium pressure. This transition is not observed in Ti. For both Ti and Zr, the shock-induced alpha -> omega transition occurs well above the equilibrium phase boundary. Evidence from the literature indicates strong rate effects leading to a non-steady P1 wave. I will present a simple kinetic model that captures many features of the data. Recent data from DX-1 confirms some aspects of this picture, while detailed comparison points up some limitations of the present model. Wednesday, June 12, 2002 Phase Changes in Zirconium and Titanium: Recent
Results We have investigated the alpha to omega phase transformation in two polycrystalline metals - zirconium and titanium - using plate impact experiments and time-resolved wave profile techniques (VISAR). Both high purity and low purity samples - backed by Sapphire windows - were subjected to shock loading using a 50 mm light gas gun above the stress expected to cause the alpha-omega solid-solid transformation to occur. Time-resolved particle velocity profiles were recorded at the sample-Sapphire interface with VISAR. Three-wave structures were observed in both pure Zr and Ti samples and in Zr samples with low levels of impurities present indicating the occurrence of a phase change during shock loading. No transition was observed in Ti-6V-4Al and the lowest purity Zr. This work was done in an attempt to determine the effect of impurities on these relatively low pressure phase transitions. Differences in the transition characteristics of pure and impure Zr will be discussed and initial attempts to model the behavior of pure Zr and Ti samples will be presented. Future work will include investigating the omega to beta phase change in these two metals, as well as extending to Hf metal. Wednesday, May 29, 2002 Equation of State Modeling: Lowering Barriers
to Progress Computer hardware has progressed to the point where any researcher can have a dedicated PC on his desk with more power than a supercomputer of a decade ago. Yet software for equations of state has changed very little in the last 30 years, despite the increased reliance on numerical simulations. Most continuum mechanics codes have `hardwired' the form of equations of state that can be used. The implementations are very inflexible and clumsy by todays standards. There is little compatibility which prevents sharing of source code, leads to much duplication of effort, and slows the spread and usage of improved material models among researchers. Individuals tend to add short lived EOS 'fixes' to their own codes. Some users of continuum mechanics codes go even further and treat equation of state parameters as knobs, to be adjusted until a simulation gives agreement for a particular experiment with little concern for side effects in other applications. Inherent in this methodology are low scientific standards and a serious lack of quality control. This is a significant barrier to developing accurate material models that are needed for a predictive simulation capability. A more systematic and cooperative approach to material modeling is needed. The research community should strive continually to improve equations of state in a database available to everyone. The purpose of this talk is two-fold. First, is to discuss capabilities for an equation of state package that would be of general use to a wide audience. These include a database for material parameters, a library of low level routines for various model equations of state, and software tools to access the database and compute a wide variety of material properties of interest; both thermodynamic properties (such as isentropic and isothermal bulk modulus) and hydrodynamic properties (such as isentropes, shock loci and solutions to Riemann problems). Second, is to raise the issue for the research community of developing a language or protocol to facilitate sharing of information and resources: from experimental data, to source code for material models, to the results of simulations. The internet allows the exchange of information in much more detail than previously possible by publishing in a journal. Progress in material modeling would be much faster if the research community would take advantage of the technology that is now readily available. A coordinated effort is needed and the research community should begin discussions to reach the necessary consensus. (download presentation : LA-UR-02-3437) Wednesday, May 15, 2002 Equation of State for Dense High-Z Plasmas In the limit of low density and high temperature it is possible to develop a complete theoretical description of a plasma by truncating the BBGKY hierarchy. Physically, the relevant expansion parameter is the ratio of potential to kinetic energy and for small values of this parameter the plasma is characterized as weakly coupled. In the opposite limit, where the potential energy exceeds the thermal energy, a perturbative treatment is no longer valid and the plasma is characterized as strongly coupled. In these dense plasmas regimes, it is necessary to self-consistently treat the atomic physics in conjunction with strongly correlated ions. We employ a simplified density functional approach in which the Kohn-Sham equation is solved for both bound and continuum wave functions to construct the electron density around a test ion. The statistical distribution of ions around the test ion is computed with hypernetted-chain theory, a nonperturbative technique which is known to accurately describe strongly correlated Coulomb systems. These two contributions to the charge density cloud are computed iteratively to find a self-consistent potential. Using the resulting solution, the equation of state (EOS) is computed and compared with a simplified model which employs the Thomas-Fermi approximation for the electron contribution. Comparisons are made with the SESAME database over an interesting range of parameters. (download presentation : LA-UR-02-2841)
Measurements of Temperature, Spall and Melt on
Shocked Tin Tin is an interesting material for EOS studies because it has some
solid/solid and solid/liquid phase transitions in the low pressure
range (10's to few hundred kbar). Mabire et al (see references)
measure VISAR signatures that indicated the melt of tin on release,
as well as some temperature measurements above 350 kbar. We will
discuss some recent temperature measures at much lower (~50 kbar)
and intermediate (250 kbar) pressures and how these results fit into
the EOS models of Mabire et al. We also will compare these results
to Dennis Hayes calculations. Recent progress on HE induced spall
measurements of tin using the new Asay Window technique will be compared
to proton radiography at LANSCE. Wednesday, April 17, 2002 Complexity in Phase Stability Modeling The factors that contribute to the complexity of phase stability
calculations are reviewed. Several models of the excess Gibbs free
energy of mixtures are compared and the "physical meaning", if any,
of the model parameters is discussed. To deal with the increasing
complexity generated by the number of components (species) of a system,
a recursive method is proposed. The approach allows for describing
the properties of a n-component system based on the properties of
its (n-1)-component sub-systems. The uncertainty associated with experimental
and calculated thermodynamic data is evaluated and the impact on the
phase diagrams discussed. Wednesday, March 6, 2002 First-Principles-Based Thermodynamic Description
of Solid Copper A tight-binding model is fit to first-principles calculations for
copper that include structures distorted according to elastic constants
and high-symmetry phonon modes. With the resulting model the
first-principles-based phonon dispersion and the free energy are
calculated in the quasi-harmonic approximation. The resulting thermal
expansion, the temperature- and volume-dependence of the elastic
constants, and the Grueneisen parameter are compared with available
experimental data. (download presentation :
LA-UR-02-126) Wednesday, February 20, 2002 Atomistic Studies of Shock Processes Molecular dynamics simulations can be used to directly probe the
response of matter to shock loading and unloading. A variety of
phenomena have been investigated, including plasticity, spallation,
ejecta formation, diffusionless phase transformations (both solid-solid
and melting), and detonations. I will discuss the capabilities and
limitations of such simulations, focusing on two particular systems
which we have recently studied: the bcc-to-hcp transformation of
solid iron, and a model energetic material. A strong crystallographic
orientation dependence on the transformation kinetics is found for
the former system; how the averaging over grains and wave scattering
off of grain boundaries lead to a steady-state in polycrystalline
samples is an important open question which is currently being studied.
For the model "nanodetonics" system, hot spot formation due to void
collapse leads to a marked lowering of the shock-to-detonation threshold
as compared with homogeneous initiation in the perfect crystal. Wednesday, February 6, 2002 Equation of State for Detonation Products An equation of state used for hydrodynamics enters the equations
as adiabatic gamma and Gruneisen gamma, and these functions must
be chosen to represent physical reality. They will be derived and
discussed. For an equation of state to be a function of two variables,
S and v, or p and v, the chemical composition must stay in equilibrium.
Some results showing that the assumption of chemical equilibrium
is not exactly satisfied will be presented. The usual treatment of
the equation of state calibration for detonation products also makes
the assumption that the detonation is a Chapman-Jouguet detonation.
That is, the flow is plane and steady, the tangency condition is met,
the thermicity is positive everywhere, and there is chemical equilibrium
at the CJ point. Some results that test this assumption will be presented.
Even though the underlying assumptions are not quite satisfied, calculations
made with equations of state are usually satisfactory, partly because
the eos is calibrated using explosive parts about the same size as
those n the system calculated. Calibration of the JWL eos will be
discussed. Some experiments to improve the calibration of an eos will
be suggested.
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