Kurt Zimmerman

Experimental developments to obtain real-time x-ray diffraction measurements in plate impact experiments

An experimental facility was developed to obtain real-time, quantitative, x-ray diffraction data in laboratory plate impact experiments. A powder gun, to generate plane wave loading in samples, was designed and built specifically to permit flash x-ray diffraction measurements in shock-compression experiments. Spatial resolution and quality of the diffracted signals were improved significantly over past attempts through partial collimation of the incident beam and the use of two-dimensional detectors to record data from shocked crystals. The experimental configuration and synchronization issues are discussed, and relevant details of the x-ray system and the powder gun are described. Representative results are presented from experiments designed to determine unit cell compression in shock-compressed LiF single crystals subjected to both elastic and elastic-plastic deformation, respectively. The developments described here are expected to be useful for examining lattice deformation and structural changes in ...

Second-order elastic constants of pentaerythritol tetranitrate and cyclotrimethylene trinitramine using impulsive stimulated thermal scattering

Impulsive stimulated thermal scattering (ISTS) was used to determine the complete set of second-order elastic constants for pentaerythritol tetranitrate (PETN) and cyclotrimethylene trinitramine (RDX) single crystals. Despite the weak scattering efficiency of these materials, excellent signal quality was obtained by using an optical heterodyne detection approach. The elastic constants for PETN agree well with previous values obtained from ultrasonic velocity measurements. The elastic constants for RDX are consistent with previous values obtained from ultrasonic velocity measurements and from resonant ultrasound spectroscopy, but show significant differences with values obtained from Brillouin scattering data. The present results demonstrate that the ISTS method, with optical heterodyne detection, provides a useful and accurate approach for determining the elastic constants of energetic crystals.

Real-time microstructure of shocked LiF crystals: Use of synchrotron x-rays

We describe the use of a third generation synchrotron facility to obtain in situ, real-time, x-ray diffraction measurements in plate impact experiments. Subnanosecond duration x-ray pulses were utilized to record diffraction data from pure and magnesium-doped LiF single crystals shocked along the [111] and [100] orientations. The peak stresses were 3.0 GPa for the [111] oriented LiF and between 3.0 and 5.0 GPa for the [100] oriented LiF. For these stresses, shock compression along [111] results in elastic deformation and shock compression along [100] results in elastic-plastic deformation. Because of the quality of the synchrotron x-ray pulses, both shifting and broadening of the diffraction data were obtained simultaneously. As expected, shifts for elastic compression and elastic-plastic compression in shocked LiF were consistent with uniaxial and isotropic lattice compression, respectively. More importantly, diffraction patterns from crystals shocked along [100] exhibited substantial broadening due to e...

Electric‐field‐induced polarization current studies in guest–host polymers

We apply polarization current measurements to study dipole reorientation of nonlinear chromophores that are embedded in a polymer matrix. In particular, we apply a step voltage to a thin film of poly(methyl methacrylate) that is doped with a well‐known dipolar chromophore, Disperse Red 1, or an apparently centrosymmetric squarylium chromophore, and measure the isothermal temporal behavior of the current at several temperatures. By correlating the current with electric‐field‐induced second‐harmonic generation, we find that at very early times, polarization current dominates, while at later times, charge hopping mechanisms probably dominate. For the centrosymmetric squarylium chromophore dopant, we find an early‐time current suggesting polarization decay with no corresponding second‐harmonic signal. This suggests that the squarylium molecule has a dipole moment that is perpendicular to the dominant axis of the polarizability tensor and is consistent with the presence of the cis isomer. A time–temperature–vo...

Real-time, high-resolution x-ray diffraction measurements on shocked crystals at a synchrotron facility

The Advanced Photon Source (APS) at Argonne National Laboratory was used to obtain real-time, high-resolution x-ray diffraction measurements to determine the microscopic response of shock-compressed single crystals. Disk shaped samples were subjected to plane shock wave compression by impacting them with half-inch diameter, flat-faced projectiles. The projectiles were accelerated to velocities ranging between 300 and 1200 m/s using a compact powder gun designed specifically for use at a synchrotron facility. The experiments were designed to keep the sample probed volume under uniaxial strain and constant stress for a duration longer than the 153.4 ns spacing between x-ray bunches. X-rays from a single pulse (<100 ps duration) out of the periodic x-ray pulses emitted by the synchrotron were used for the diffraction measurements. A synchronization and x-ray detection technique was developed to ensure that the measured signal was obtained unambiguously from the desired x-ray pulse incident on the sample while the sample was in a constant uniaxial strain state. The synchronization and x-ray detection techniques described can be used for a variety of x-ray measurements on shock compressed solids and liquids at the APS. Detailed procedures for applying the Bragg-Brentano parafocusing approach to single crystals at the APS are presented. Analytic developments to determine the effects of crystal substructure and non-ideal geometry on the diffraction pattern position and shape are presented. Representative real-time x-ray diffraction data, indicating shock-induced microstructural changes, are presented for a shock-compressed Al(111) sample. The experimental developments presented here provided, in part, the impetus for the Dynamic Compression Sector (DCS) currently under development at the APS. Both the synchronization∕x-ray detection methods and the analysis equations for high-resolution single crystal x-ray diffraction can be used at the DCS.

Shock wave response of ammonium perchlorate single crystals to 6 GPa

Plane shock wave experiments were carried out on ammonium perchlorate single crystals compressed along [210] and [001] orientations to peak stresses ranging from 1.2 to 6.2 GPa. Quartz gauge and velocity interferometer techniques were used to measure the elastic and plastic shock wave velocities, and stress and particle velocity histories in the shocked samples. The measured Hugoniot elastic limit (HEL) was 0.48±0.09 GPa. Above the HEL and up to about 6 GPa, the data show a clear two-wave structure, indicating an elastic-plastic response. Time-dependent elastic precursor decay and plastic wave ramping are discernable and orientation dependent in the low stress data. However, the orientation dependence of the peak state response is small. Hence, data for both orientations were summarized into a single isotropic, elastic-plastic-stress relaxation model. Reasonable agreement was obtained between the numerical simulations using this model and the measured wave profiles. At a shock stress of about 6 GPa and fo...

Picosecond Time-Resolved Electronic Spectroscopy in Plate Impact Shock Experiments: Experimental Development

An experimental method was developed to perform picosecond time-resolved electronic spectroscopy in single-event, plate impact, shock wave experiments. Several experimental difficulties had to be addressed due to the fast time resolution (100 ps) and short time duration (12.7 ns) of such experiments. Procedures are described to address the following experimental issues: (i) synchronization of the light source, detection equipment, and the shock event within the experimental duration, (ii) incorporation of a Nd:YAG laser (operative in a repetitive mode) into the experimental configuration, and (iii) sources of temporal dispersion. Representative results are shown from experiments performed on single crystal CdS. The developments described here are also expected to be useful for shock wave experiments involving Raman, fluorescence, or other types of spectroscopy which require the use of a laser.

High spectral resolution, real-time, Raman spectroscopy in shock compression experiments

The use of Raman measurements to examine molecular changes associated with shock-induced structural and chemical changes in condensed materials often poses two challenging requirements: high spectral resolution and significantly reduced background light. Here, we describe an experimental method that addresses these requirements and provides better quality data than the time resolved approach used previously. Representative measurements are presented for shock compression of two energetic crystals: pentaerythritol tetranitrate and cyclotrimethylene trinitramine. The high spectral resolution data have provided insight into molecular changes that could not be obtained from lower-resolution, time-resolved methods.