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Using laser-interferometry, shock compression and unloading profiles were measured in 1050 aluminum shocked to ~70 GPa. These results were compared to published data on relatively pure (99.99wt% ultra pure, 1050, 1060, and 1100) and precipitate - hardened (2024, 6061) aluminum. Within experimental scatter, Hugoniots and longitudinal sound speeds (in the shocked state) of pure and precipitate-hardened aluminum are quite comparable to 70GPa. This agreement demonstrates that impurity content has minimal influence on the longitudinal stress-volume response and acoustic speeds under shock compression. [ABSTRACT FROM AUTHOR]

The quasi-elastic scattering measurements for 18O+58,62Ni systems have been carried out at Θlab = 150° around Coulomb barrier energies to investigate the effect of nuclear shell closure on the barrier distributions. The 18O+58Ni system leads to N = 40 neutron shell closure and 18O+62Ni system is having N = 44 in the compound system. It is observed that target 2+ and 3-, projectile 2+ inelastic and 2n-transfer couplings are required in coupled-channels fusion model (CCFULL) calculations to get good comparison with the experimental barrier distribution of 18O+62Ni system, whereas projectile 2+ inelastic state coupling is not required for 18O+58Ni system. However, the low energy structure observed in the barrier distribution of 18O+58Ni system is not reproduced by coupled-channels calculations. This suggests, a possible additional effect due to N = 40 shell closure in the compound system not accounted for in coupled-channels calculations. [ABSTRACT FROM AUTHOR]

Sharing similarities with the GEPI pulser which is dedicated to Isentropic Compression Experiments (ICE), VELOCE, an even more compact electrical pulser, has been designed and built in duplicate for SNL and WSU. This type of machine complements gun and laser facilities in the study of material response. In order to achieve a broad loading capability and fast turn around, the design is built around a solid dielectric transmission line to couple current from low inductance capacitors and electrically triggered switches. Peaking capacitors enhanced by a low inductance, multi-channel sharpening switch reduce the quarter period of the pulser to about 470 ns (0–100%). Gas mixtures in the switch cavity and inductances in parallel allow modification of the shape of the induced pressure wave. At 80 kV of charge voltage, the peak current can reach 3.5 MA. Design of the pulser, range of pressures and velocities, as well as potential applications are presented. A consistent numerical tool developed for pulsers design based on a circuit code coupled to a 1D MHD code is also introduced. [ABSTRACT FROM AUTHOR]

A continuum, anisotropic modeling framework has been developed for simulating shock wave propagation in single crystals of arbitrary orientation. Our modeling approach incorporates nonlinear elasticity and crystal plasticity in a thermodynamically consistent tensor formulation. Crystal plasticity was described using models that consider dislocation motion along specified slip planes. Numerical simulations of large amplitude stress wave propagation in single crystals are presented. Using these simulations, issues related to pure mode propagation for nonlinear elastic waves are discussed. Also, the effects of plasticity on wave propagation in single crystals are examined by comparing simulations to wave profile data for copper and for LiF. For copper, a single dislocation model for slip on {111} planes provided good agreement with quartz gauge data for shocks along the [100], [110], and [111] directions. For LiF, slip occurs on {110} planes, and good agreement with data was obtained for shocks along the [100] direction. Using the same dislocation model, simulations were extended to shock wave propagation along the low symmetry [310] direction of LiF, where experimental data are not available. Propagation of quasilongitudinal and quasishear waves for this orientation are examined. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

The vibrational frequencies of the PETN molecular crystal were calculated using the first-principles CRYSTAL03 program which employs an all-electron LCAO approach and calculates analytic first derivatives of the total energy with respect to atomic displacements. Numerical second derivatives were used to enable calculation of the vibrational frequencies at ambient pressure and under various states of compression. Three different density functionals, B3LYP, PW91, and X3LYP were used to examine the effect of the exchange-correlation functional on the vibrational frequencies. The average deviation with experimental results is shown to be on the order of 2–3%, depending on the functional used. The pressure-induced shift of the vibrational frequencies is presented. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Hydrostatic compression of RDX crystal has been investigated using density functional theory (DFT), within the generalized gradient approximation (GGA). Our GGA calculations successfully reproduced the intramolecular geometry but overestimated the lattice constants of RDX crystal. Simulated hydrostatic pressures up to 3.65 GPa were applied to the RDX crystals. The calculated pressure-volume relation is compared with experimental data. Pressure-induced changes of the structural and electronic properties of RDX crystal are discussed. © 2006 American Institute of Physics [ABSTRACT FROM AUTHOR]

Using shock wave reverberation experiments, water samples were quasi-isentropically compressed between silica and sapphire plates to peak pressures of 1–5 GPa on nanosecond time scales. Real time optical transmission measurements were used to examine changes in the compressed samples. Although the ice VII phase is thermodynamically favored above 2 GPa, the liquid state was initially preserved and subsequent freezing occurred over hundreds of nanoseconds only for the silica cells. Images detailing the formation and growth of the solid phase were obtained. These results provide unambiguous evidence of bulk water freezing on such short time scales. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Calculations of the second-order elastic constants of pentaerythritol (PE), and urea were performed using the output of the modeling program CRYSTAL98. CRYSTAL98 performs first principles calculations of periodic systems using Gaussian type basis functions. The elastic constants were obtained by performing a polynomial fit to the calculated total energy of the crystal as a function of elastic strain. When the application of strains resulted in a reduction of crystal symmetry, the crystal unit cells often had to be redefined to use a space group of lower symmetry. To reduce computation times, the new space group was chosen to maximize remaining symmetry. A rigid molecule model was employed, where the internal geometry of the molecule was kept unchanged. An optimization technique was developed for lattice relaxation and tested for both urea and PE. Results obtained using different functionals in the Density Functional Theory (DFT) method were compared to Hartree-Fock (HF) calculations. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The first-principles periodic Hartree-Fock calculations of the unit cell and vibrational frequencies of pentaerythritol crystal are presented. The basis set 6-31G was used to calculate the vibrational frequencies. The calculated OH stretching mode vibrational frequencies using the Crystal program are about 200 wave numbers lower than those calculated using the Gaussian98 program. This is to be expected for crystals of this type having an intermolecular hydrogen bonding interaction. The pressure effect on vibrational frequencies is calculated and compared with experimental data, up to 3 GPa. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

The elastic properties of several molecular crystals (PE, PETN, urea) have been investigated using plane-wave pseudopotential methods based on density functional theory (DFT). The lattice constants, elastic constants and bulk modulus of the molecular crystals were calculated and compared with experiments. Two prevalent density functional methods, LDA and GGA-PBE were tested and compared with each other. We find that LDA typically overestimates the stiffness of these crystals at least by factor of two, while the GGA calculations with the PBE exchange-correlation functional are more reasonable. A large cutoff of the plane-wave basis and a modest sampling of k space are required to describe the molecular crystals. The elastic behavior of urea and PE crystals under uniaxial compressions show interesting anisotropic effects. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Ti-Si systems undergo exothermic reactions, which can be used for self-propagating high temperature synthesis (SHS) of starting powders. Using plate impact loading, titanium silicide (Ti5Si3) samples were synthesized in the present work. Effects of compact density, impact velocity, milling time were studied. Ti and Si powders were first ball-milled in molar ratios of (Ti5Si3) and then compacted. Powders were ball-milled for different time periods, 24h, 100h and 190h, and then compacted for different theoretical densities, 40%, 45%, 50% and 60%. Four rectangular cells were machined in a Cu target and these cells were filled with ball-milled powders. Targets were impacted at different velocities, 250 m/s, 350 m/s and 450 m/s. All samples were recovered after the impact loading and analyzed. Phase analysis, microstructural analysis, microhardness measurements were carried out with recovered samples. Results indicate that lower compact density, higher milling time and higher shock velocity produced higher reactivity in these SHS samples. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]

Shock wave compression experiments are well suited to examine condensed matter changes in real time. A comprehensive understanding of these physical and chemical changes requires timeresolved measurements at a variety of length scales, and theoretical developments to link the experimental observations in a consistent manner. Two representative examples are presented to summarize recent progress in coupling continuum and microscopic results. [ABSTRACT FROM AUTHOR]

Plate impact experiments were carried out to examine the high strain-rate tensile response of alumina-aluminum (A1) composites with tailored microstructures. A novel processing technique was used to fabricate interpenetrating phase alumina-aluminum composites with controlled microstructures. Fused deposition modeling (FDM), a commercially available rapid prototyping technique, was used to produce the controlled porosity mullite ceramic preforms. Alumina-A1 composites were then processed via reactive metal infiltration of porous mullite ceramics. With this approach, both the micro as well as the macro structures can be designed via computer aided design (CAD) to tailor the properties of the composites. Two sets of dynamic tensile experiments were performed. In the first, the metal content was varied between 23 and 39 wt. percent. In the second, the microstructure was varied while holding the metal content nearly constant. Samples with higher metal content, as expected, displayed better spall resistance. For a given metal content, samples with finer metal diameter showed better spall resistance. Relationship of the microstructural parameters on the dynamic tensile response of the structured composites is discussed here. [ABSTRACT FROM AUTHOR]

Experimental methods using both single and multiple x-ray diffraction were developed to determine real time, lattice deformation in directions parallel and perpendicular to shock wave propagation in single crystals subjected to plate impact loading. Initial experiments used single diffraction to monitor the interplanar spacing change, parallel to the shock propagation direction, in LiF crystals shocked along the [111] and [100] directions. These measurements, in combination with the macroscopic volume compression, were used to determine the state of compression of the unit cell. Subsequent development of a multiple diffraction technique permitted simultaneous determination of both the longitudinal and transverse lattice deformations. The present results showed that shock compression, below 4 GPa, along the [111] orientation -- which results in macroscopic elastic deformation -- produced one-dimensional unit cell compression. In contrast, shock compression along the [100] orientation -- which results in macroscopic elastic-plastic deformation -- produced isotropic unit cell compression. The implications of the present results and the ability to make quantitative x-ray diffraction measurements under shock loading are discussed. [ABSTRACT FROM AUTHOR]

Cadmium sulfide crystals were shocked along the c-axis to peak instantaneous stresses ranging from 27 to 53 kbar, above and below the phase transition stress of 32 kbar. Particle velocity measurements, using a VISAR system with nanosecond temporal resolution, were used to determine the time dependent mechanical response at the impact surface at higher time resolutions than those reported by Tang and Gupta, and to complement the recent electronic spectroscopy measurements. For experiments with longitudinal stresses above 32 kbar, but below 53 kbar, the instantaneous impact stress of the initial wurtzite phase is maintained for significant times before relaxing to a metastable state, and then further relaxing to the final rock salt phase. Transition rates and metastable state lifetimes are observed to be stress dependent. At 53.5 kbar impact stress, transition to a metastable state appears to be complete within 3 ns of impact, and the full relaxation to the rock salt phase indicates that the phase transition may proceed through multiple metastable states. Epoxy bonds with VISAR windows degrade the ability to accurately measure fast, stress-relaxing phenomena. [ABSTRACT FROM AUTHOR]

Real time x-ray diffraction measurements were used to examine the polymorphic phase transformation in KCl shocked along the [100] direction. Shock wave continuum data, obtained previously by Hayes, were used to design the experiments and to predict diffraction from KCl shocked to different peak stresses. Here, we present the results obtained below the transition stress: between 1.4 and 2 GPa. Diffraction data obtained were quantitatively related to macroscopic compression. Interplanar spacing measurements revealed isotropic compression of the unit cell in contrast to previously reported results. Above the transition stress, descriptions of the atomic arrangement with respect to shock propagation (not available in the literature) are required for setting up the detection system. Hence, continuum results in combination with various crystallographic considerations were utilized to obtain data above the transition stress. [ABSTRACT FROM AUTHOR]

The pressure-induced phase transition in CdS was investigated using picosecond time-resolved electronic spectroscopy in plate impact shock wave experiments. Real-time changes in the electronic spectra were observed, with 100 ps time resolution, in single crystals of CdS shocked along the c and a axes to peak stresses between 35 and 90 kbar (above the phase transition stress of approximately 30 kbar measured in continuum studies). When shocked to stresses above approximately 50 kbar along the crystal c axis and 60 to 70 kbar along the crystal a axis, the crystals undergo a very rapid change in electronic structure, indicating that significant structural changes occur within the first 100 ps. These results, along with previous ns continuum measurements, make a strong case for a metastable state during the phase transition in shocked CdS. Ab-initio periodic Hartree-Fock calculations (with DFT correlation corrections) were employed to examine the compression of CdS and to determine a possible lattice structure for the proposed metastable structure. These results, along with details of the transformation kinetics and orientational dependence, will be discussed. Work supported by ONR. [ABSTRACT FROM AUTHOR]

Recent experimental developments to examine fundamental processes and mechanisms governing shock wave induced physical and chemical changes are reviewed and summarized. The emphasis is on time-resolved measurements at various length scales and representative examples are provided to discuss achievements in this decade. Directions for future work are indicated. [ABSTRACT FROM AUTHOR]

Transparent, brittle materials are used routinely as optical windows in shock wave experiments. Deformation mechanisms in these materials when shocked above their elastic limit are not well understood. Time-resolved optical measurements (transmission, emission, and scattering) were obtained in shocked c-cut sapphire to examine and understand material failure mechanisms. Emission measurements, in the absence of an external light source, show weak but discernable broadband emission in particular situations. Single pass transmission measurements over 450-650 nm, using a broadband flashlamp, have shown that shorter wavelength light is preferentially extinguished, consistent with Mie scattering. Contributions to light extinction by scattering have been verified. [ABSTRACT FROM AUTHOR]

Extinction spectra of PETN and photodiode transmission records, with 50 and 0.5 ns resolution respectively, were obtained for shock compression along the [110] and [100] directions. Light extinction was found to be due to absorption; the contribution of scattering was measured to be below 0.001%. Results for final stresses up to 10 GPa (stepwise loading) and initial stresses up to 5 GPa are presented. Broad band absorption in the visible appeared in PETN single crystals shocked to as low as 1 GPa. Absorption changes were measured as a function of time, stress, and crystal orientation. The process responsible for absorption occurred on a time scale faster than 0.5 ns. Work is in progress to identify the origin of the absorption. [ABSTRACT FROM AUTHOR]

In previous studies, ruby R-line shifts under shock compression and tension have been measured using the spontaneous luminescence from optically pumped samples. Signal intensities obtained through the use of this method are limited by the short time duration of the experiments (100 ns to several μs) in comparison to the long lifetime of the luminescence (approximately 3 ms). We have investigated the use of stimulated emission as a technique for measuring R-line shifts in shocked ruby crystals. Feasibility experiments were performed both at ambient conditions and under shock compression to 60 kbar using an experimental configuration similar to that used for time resolved ruby luminescence measurements in previous shock wave studies. Signal gain due to stimulated emission was observed, with gains ranging from 1.1 to 3.4, in agreement with calculations performed for the particular experimental configuration used. The present results make a good case for incorporating this technique to measure shock induced R-line shifts in ruby. Work supported by DSWA. [ABSTRACT FROM AUTHOR]

A thermodynamically consistent equation of state (EOS) was developed for unreacted, single-crystal ammonium perchlorate (AP) for shock compression along the [210] and [001] directions. The specific heat, c[SUBv], the pressure-temperature coefficient, (&Part;P/&Part;T)[SUBv], and the isothermal bulk modulus, B[SUBT], were determined from Hugoniot and isothermal compression curves, along with available data at atmospheric pressure. The mechanical response of each orientation of the AP crystal was modeled as an isotropic elastic-plastic solid. Above the HEL, the rate dependent yielding of AP was described with a simple overstress model. Time-resolved Raman spectroscopy experiments were carried out to obtain temperatures in the shocked state. These temperature measurements were used to constrain the values of various EOS parameters. [ABSTRACT FROM AUTHOR]

Fusion barrier distributions have been extracted from the quasi-elastic scattering excitation functions, measured at backward angle θlab = 160° in reactions of 6,7Li+209Bi. The present results have been compared with the barrier distributions obtained from the fusion excitation function measurements for the above mentioned systems. The fusion barrier distributions from the quasi-elastic scattering excitation functions have been analyzed with simplified Coupled Channels calculations using Fresco. Inclusions of resonant states for both 6,7Li projectiles improve the predictions to describe the measured quasi-elastic scattering excitation functions and barrier distributions. For both the reactions peak positions of fusion barrier distributions are shifted towards a lower energy side in comparison to that obtained from the fusion excitation function measurements. The observed discrepancy in peak positions of barrier distributions obtained from quasi-elastic scattering and fusion excitation function measurements has been discussed in terms of total reaction threshold distribution. [ABSTRACT FROM AUTHOR]

Ti-Si systems undergo exothermic reactions, which can be used for self-propagating high temperature synthesis (SHS) of starting powders. Titanium silicide (Ti5Si3) powders were mixed with a ceramic-filler and shock-compressed in plate impact experiments. Effect of ceramic filler concentrations was examined. Mullite was used as an inert ceramic filler in different weight percentage (10, 20, 30) and mixed with ball-milled (pre-alloyed Ti5Si3) powders. Four rectangular cells were machined in a Cu target and these cells were filled with composite powders. Targets were impacted at different velocities, 250 m/s, 350 m/s and 450 m/s. All samples were recovered after impact loading and analyzed. Changes in impact velocity had a significant effect on the reaction kinetics. Increasing filler materials and decreasing milling time reduced the rate of reaction. Increasing mullite content decreased the reaction kinetics, but reduced the residual porosity in the dense compacts. Increase in mullite content also increased the microhardness due to the formation of an in situ intermetallic-ceramic composite. © 2004 American Institute of Physics [ABSTRACT FROM AUTHOR]