Publications

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The behavior of water confined in porous materials influences macroscopic phenomena such as solute and water mobility, ion exchange, and adsorption. While the properties of bulk water are generally understood, that of nanoconfined water remains an active area of research. We used molecular simulation and inelastic neutron scattering (INS) to investigate the effect of local structure on the vibrational behavior of nanoconfined water. We focus specifically on the nanosized pores found in the 2:1 phyllosilicate clay minerals palygorskite and sepiolite. These are charge neutral, Mg-rich trioctahedral clays with idealized formulas Mg{sub 5}Si{sub 8}O{sub 20} (OH){sub 2} {center_dot} 8H{sub 2}O and Mg{sub 8}Si{sub 12}O{sub 30} (OH){sub 2} {center_dot} 12H{sub 2}O for palygorskite and sepiolite, respectively. The regular pattern of inverted phyllosilicate layers results in narrow channels with effective van der Waals dimensions of 3.61 {angstrom} x 8.59 {angstrom} (palygorskite) and 4.67 {angstrom} x 12.29 {angstrom} (sepiolite). These clay minerals represent a unique opportunity to study water adsorbed at 'inner edge' sites of uncoordinated Mg{sup 2+}. INS spectra taken at 90 K reveal a large shift in the water librational edge between palygorskite (358 cm{sup -1}) and sepiolite (536 cm{sup -1}), indicating less restricted water motion in the smaller-pore palygorskite. The librational edge of the reference sample (ice I{sub h}) is similar to sepiolite, which confirms the unique water behavior in palygorskite. We used both classical molecular dynamics (CMD) simulations and more rigorous density functional theory (DFT) calculations to investigate the hydrogen bonding environment and vibrational behavior of structural water, defined as those water molecules coordinated to Mg{sup 2+} along the pore walls. These waters remain coordinated throughout the 1-ns timescale of the CMD simulations, and the resulting vibrational spectra indicate a similar shift in the water librational edges seen in the INS spectra. The DFT-optimized structures indicate differences in hydrogen bonding between palygorskite and sepiolite, which could explain the librational shift. Corner-sharing silicate tetrahedra in palygorskite are tilted with respect to the crystallographic a-axis due to the induced strain of layer inversion. As a result, only two short (1.9 {angstrom}) hydrogen bonds form between each water and the framework. In contrast, the relatively unstrained sepiolite structure, each water forms three hydrogen bonds with the framework, and at greater distances (2.0 {angstrom} - 2.5 {angstrom}).

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The Al Tuwaitha nuclear complex near Baghdad contains a number of facilities from Saddam Hussan's nuclear weapons program. Past military operations, lack of upkeep and looting have created an enormous radioactive waste problem at the Al Tuwaitha complex, which contains various, uncharacterized radioactive wastes, yellow cake, sealed radioactive sources, and contaminated metals that must be constantly guarded. Iraq has never had a radioactive waste disposal facility and the lack of a disposal facility means that ever increasing quantities of radioactive material must be held in guarded storage. The Iraq Nuclear Facility Dismantlement and Disposal Program (the NDs Program) has been initiated by the U.S. Department of State (DOS) to assist the Government of Iraq (GOI) in eliminating the threats from poorly controlled radioactive materials, while building human capacities so that the GOI can manage other environmental cleanups in their country. The DOS is funding the IAEA to provide technical assistance via Technical Cooperation projects. Program coordination will be provided by the DOS, consistent with GOI policies, and Sandia National Laboratories will be responsible for coordination of participants and waste management support. Texas Tech University will continue to provide in-country assistance, including radioactive waste characterization and the stand-up of the Iraq Nuclear Services Company. The GOI owns the problems in Iraq and will be responsible for implementation of the NDs Program.

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Shales and other mudstones are the most abundant rock types in sedimentary basins, yet have received comparatively little attention. Common as hydrocarbon seals, these are increasingly being targeted as unconventional gas reservoirs, caprocks for CO{sub 2} sequestration, and storage repositories for waste. The small pore and grain size, large specific surface areas, and clay mineral structures lend themselves to rapid reaction rates accompanying changes in stress, pressure, temperature and chemical conditions. Under far from equilibrium conditions, mudrocks display a variety of spatio-temporal self-organized phenomena arising from the nonlinear coupling of mechanics with chemistry. Beginning with a detailed examination of nano-scale pore network structures in mudstones, we discuss the dynamics behind such self-organized phenomena as pressure solitons, chemically-induced flow self focusing and permeability transients, localized compaction, time dependent well-bore failure, and oscillatory osmotic fluxes as they occur in clay-bearing sediments. Examples are draw from experiments, numerical simulation, and the field. These phenomena bear on the ability of these rocks to serve as containment barriers.

The characterization of former military munitions ranges is critical in the identification of areas likely to contain residual unexploded ordnance (UXO). Although these ranges are large, often covering tens-of-thousands of acres, the actual target areas represent only a small fraction of the sites. The challenge is that many of these sites do not have records indicating locations of former target areas. The identification of target areas is critical in the characterization and remediation of these sites. The Strategic Environmental Research and Development Program (SERDP) and Environmental Security Technology Certification Program (ESTCP) of the DoD have been developing and implementing techniques for the efficient characterization of large munitions ranges. As part of this process, high-resolution LIDAR terrain data sets have been collected over several former ranges. These data sets have been shown to contain information relating to former munitions usage at these ranges, specifically terrain cratering due to high-explosives detonations. The location and relative intensity of crater features can provide information critical in reconstructing the usage history of a range, and indicate areas most likely to contain UXO. We have developed an automated procedure using an adaptation of the Circular Hough Transform for the identification of crater features in LIDAR terrain data. The Circular Hough Transform is highly adept at finding circular features (craters) in noisy terrain data sets. This technique has the ability to find features of a specific radius providing a means of filtering features based on expected scale and providing additional spatial characterization of the identified feature. This method of automated crater identification has been applied to several former munitions ranges with positive results.

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Deformation and fracture of thin films on compliant substrates are key factors constraining the performance of emerging flexible substrate devices. These systems often contain layers of thin polymer, ceramic and metallic films and stretchable interconnects where differing properties induce high normal and shear stresses. As long as the films remain bonded to the substrates, they may deform far beyond their freestanding form. Once debonded, substrate constraint disappears leading to film failure. Experimentally it is very difficult to measure properties in these systems at sub-micron and nanoscales. Theoretically it is very difficult to determine the contributions from the films, interfaces, and substrates. As a result our understanding of deformation and fracture behavior in compliant substrate systems is limited. This motivated a study of buckle driven delamination of thin hard tungsten films on pure PMMA substrates. The films were sputter deposited to thicknesses of 100 nm, 200 nm, and 400 nm with a residual compressive stress of 1.7 GPa. An aluminum oxide interlayer was added on several samples to alter interfacial composition. Buckles formed spontaneously on the PMMA substrates following film deposition. On films without the aluminum oxide interlayer, an extensive network of small telephone cord buckles formed following deposition, interspersed with regions of larger telephone cord buckles. On films with an aluminum oxide interlayer, telephone cord buckles formed creating a uniform widely spaced pattern. Through-substrate optical observations revealed matching buckle patterns along the film-substrate interface indicating that delamination occurred for large and small buckles with and without an interlayer. The coexistence of large and small buckles on the same substrate led to two distinct behaviors as shown in Figure 2 where normalized buckle heights are plotted against normalized film stress. The behaviors deviate significantly from behavior predicted by rigid elastic solutions. To address this issue we developed a finite element analysis technique that employed a cohesive zone model to simulate interfacial crack growth. Specifying the traction-separation relationship, cohesive strength, and work of separation along with film thickness, film stress, and film and substrate properties, buckle width and height were determined as a function of interfacial toughness. The simulations indicate that an analysis based on rigid substrate solutions significantly underestimate toughness for prescribed buckle widths: a result consistent with an analysis by Yu and Hutchinson that pieced together a solution based on non-linear plate theory with a solution for the linear film on substrate problem. More importantly, the results defined a lower limiting bound to seemingly disparate buckle deflection data. The variance from linear elastic behavior, especially for the small buckles, indicates more than substrate compliance is controlling behavior. Comparison of the experimental results with cohesive zone simulations suggests that the two buckle behaviors are associated with different levels of substrate yielding. In this presentation we will use the results to show how substrate compliance and deformation affect delamination and buckling of films on compliant substrates and provide a means to predict device performance.

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The ability to harvest all available energy from a photovoltaic (PV) array is essential if new system developments are to meet levelized cost of energy targets and achieve grid parity with conventional centralized utility power. Therefore, exercising maximum power point tracking (MPPT) algorithms, dynamic irradiance condition operation and startup and shutdown routines and evaluating inverter performance with various PV module fill-factor characteristics must be performed with a repeatable, reliable PV source. Sandia National Laboratories is collaborating with Ametek Programmable Power to develop and demonstrate a multi-port TerraSAS PV array simulator. The simulator will replicate challenging PV module profiles, enabling the evaluation of inverter performance through analyses of the parameters listed above. Energy harvest algorithms have traditionally implemented methods that successfully utilize available energy. However, the quantification of energy capture has always been difficult to conduct, specifically when characterizing the inverter performance under non-reproducible dynamic irradiance conditions. Theoretical models of the MPPT algorithms can simulate capture effectiveness, but full validation requires a DC source with representative field effects. The DC source being developed by Ametek and validated by Sandia is a fully integrated system that can simulate an IV curve from the Solar Advisor Model (SAM) module data base. The PV simulator allows the user to change the fill factor by programming the maximum power point voltage and current parameters and the open circuit voltage and short circuit current. The integrated PV simulator can incorporate captured irradiance and module temperature data files for playback, and scripted profiles can be generated to validate new emerging hardware embedded with existing and evolving MPPT algorithms. Since the simulator has multiple independent outputs, it also has the flexibility to evaluate an inverter with multiple MPPT DC inputs. The flexibility of the PV simulator enables the validation of the inverter's capability to handle vastly different array configurations.

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It has been experimentally demonstrated that deuterium gas-puff implosions at >15 MA are powerful sources of fusion neutrons. Analysis of these experiments indicates that a substantial fraction of the obtained DD fusion neutron yields {approx} 3 x 10{sup 13}, about 50%, might have been of thermonuclear origin. The goal of our study is to estimate the scaling of the thermonuclear neutron yield from deuterium gas-puff implosions with higher load currents available after the refurbishment of Z, both in the short-pulse ({approx}100 ns) and in the long-pulse ({approx}300 ns) implosion regimes. We report extensive ID and 2D radiation-hydrodynamic simulations of such implosions. The mechanisms of ion heating to the fusion temperatures of 7-10 keV are essentially the same as used in structured gas-puff loads to generate high Ar K-shell yields: shock thermalization of the implosion kinetic energy and subsequent adiabatic heating of the on-axis plasma. We investigate the role of high-atomic-number gas that can be added to the outer shell to improve both energy coupling of the imploded mass to the generator and energy transfer to the inner part of the load, due to radiative losses that make the outer shell thin. We analyze the effect of imposed axial magnetic field {approx}30-100 kG, which can contribute both to stabilization of the implosion and to Joule heating of the imploded plasma. Our estimates indicate that thermonuclear DD neutron yields approaching 10 are within the reach on refurbished Z.

A 0D circuit code is introduced to study the wire array switch concept introduced in. It has been implemented and researched at Imperial College. An exploding wire array, the switch, is in parallel with the load, an imploding wire array. Most of the current flows in the exploding array until it expands and becomes highly resistive. The 0D code contains simple models of Joule energy deposition and plasma expansion for W and Al wires. The purpose of the device is to produce fast Z-pinch implosion, below 100ns on MAGPIE and the Sandia Z machine. Self and mutual inductances are taken into consideration as well as the rocket model for wire ablation. The switch characteristics of the exploding array are prescribed and tuned up to agree with MAGPIE shots. The dependence of the device on the configuration of the arrays is studied and scaling to ZR conditions is explored.

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