Showing papers by "Sandia National Laboratories published in 2001"

Catalysis Research of Relevance to Carbon Management: Progress, Challenges, and Opportunities

Abstract: There is increased recognition by the world’s scientific, industrial, and political communities that the concentrations of greenhouse gases in the earth’s atmosphere, particularly CO_2, are increasing. For example, recent studies of Antarctic ice cores to depths of over 3600 m, spanning over 420 000 years, indicate an 80 ppm increase in atmospheric CO_2 in the past 200 years (with most of this increase occurring in the past 50 years) compared to the previous 80 ppm increase that required 10 000 years.2 The 160 nation Framework Convention for Climate Change (FCCC) in Kyoto focused world attention on possible links between CO2 and future climate change and active discussion of these issues continues.3 In the United States, the PCAST report4 “Federal Energy Research and Development for the Challenges of the Twenty First Century” focused attention on the growing worldwide demand for energy and the need to move away from current fossil fuel utilization. According to the U.S. DOE Energy Information Administration,5 carbon emission from the transportation (air, ground, sea), industrial (heavy manufacturing, agriculture, construction, mining, chemicals, petroleum), buildings (internal heating, cooling, lighting), and electrical (power generation) sectors of the World economy amounted to ca. 1823 million metric tons (MMT) in 1990, with an estimated increase to 2466 MMT in 2008-2012 (Table 1).

Granular flow down an inclined plane: Bagnold scaling and rheology

Abstract: We have performed a systematic, large-scale simulation study of granular media in two and three dimensions, investigating the rheology of cohesionless granular particles in inclined plane geometries, i.e., chute flows. We find that over a wide range of parameter space of interaction coefficients and inclination angles, a steady-state flow regime exists in which the energy input from gravity balances that dissipated from friction and inelastic collisions. In this regime, the bulk packing fraction (away from the top free surface and the bottom plate boundary) remains constant as a function of depth z, of the pile. The velocity profile in the direction of flow vx(z) scales with height of the pile H, according to vx(z) proportional to H(alpha), with alpha=1.52+/-0.05. However, the behavior of the normal stresses indicates that existing simple theories of granular flow do not capture all of the features evidenced in the simulations.

Cadmium zinc telluride and its use as a nuclear radiation detector material

Abstract: We present a comprehensive review of the material properties of cadmium zinc telluride (CZT, Cd1ˇxZnxTe) with zinc content xa 0:1‐0.2. Particular emphasis is placed on those aspects of this material related to room temperature nuclear detectors. A review of the structural properties, charge transport, and contacting issues and how these are related to detector and spectrometer performance is presented. A comprehensive literature survey and bibliography are also included. # 2001 Elsevier Science B.V. All rights reserved.

A Gene Expression Map for Caenorhabditis elegans

Abstract: We have assembled data from Caenorhabditis elegans DNA microarray experiments involving many growth conditions, developmental stages, and varieties of mutants. Co-regulated genes were grouped together and visualized in a three-dimensional expression map that displays correlations of gene expression profiles as distances in two dimensions and gene density in the third dimension. The gene expression map can be used as a gene discovery tool to identify genes that are co-regulated with known sets of genes (such as heat shock, growth control genes, germ line genes, and so forth) or to uncover previously unknown genetic functions (such as genomic instability in males and sperm caused by specific transposons).

High-energy radiation and polymers: A review of commercial processes and emerging applications

Abstract: Ionizing radiation has been found to be widely applicable in modifying the structure and properties of polymers, and can be used to tailor the performance of either bulk materials or surfaces. Fifty years of research in polymer radiation chemistry has led to numerous applications of commercial and economic importance, and work remains active in the application of radiation to practical uses involving polymeric materials. This paper provides a survey of radiation-processing methods of industrial interest, ranging from technologies already commercially well established, through innovations in the active R&D stage which show exceptional promise for future commercial use. Radiation-processing technologies are discussed under the following categories: cross-linking of plastics and rubbers, curing of coatings and inks, heat-shrink products, fiber–matrix composites, chain-scission for processing control, surface modification, grafting, hydrogels, sterilization, natural product enhancement, plastics recycling, ceramic precursors, electronic property materials, ion-track membranes and lithography for microdevice production. In addition to new technological innovations utilizing conventional gamma and e-beam sources, a number of promising new applications make use of novel radiation types which include ion beams (heavy ions, light ions, highly focused microscopic beams and high-intensity pulses), soft X-rays which are focused, coherent X-rays (from a synchrotron) and e-beams which undergo scattering to generate patterns.

Self-assembly of mesoscopically ordered chromatic polydiacetylene/silica nanocomposites

Abstract: Nature abounds with intricate composite architectures composed of hard and soft materials synergistically intertwined to provide both useful functionality and mechanical integrity. Recent synthetic efforts to mimic such natural designs have focused on nanocomposites, prepared mainly by slow procedures like monomer or polymer infiltration of inorganic nanostructures or sequential deposition. Here we report the self-assembly of conjugated polymer/silica nanocomposite films with hexagonal, cubic or lamellar mesoscopic order using polymerizable amphiphilic diacetylene molecules as both structure-directing agents and monomers. The self-assembly procedure is rapid and incorporates the organic monomers uniformly within a highly ordered, inorganic environment. Polymerization results in polydiacetylene/silica nanocomposites that are optically transparent and mechanically robust. Compared to ordered diacetylene-containing films prepared as Langmuir monolayers or by Langmuir-Blodgett deposition, the nanostructured inorganic host alters the diacetylene polymerization behaviour, and the resulting nanocomposite exhibits unusual chromatic changes in response to thermal, mechanical and chemical stimuli. The inorganic framework serves to protect, stabilize, and orient the polymer, and to mediate its function. The nanocomposite architecture also provides sufficient mechanical integrity to enable integration into devices and microsystems.

Development of a Molten-Salt Thermocline Thermal Storage System for Parabolic Trough Plants

Abstract: Thermal storage improves the dispatchability and marketability of parabolic trough power plants allowing them to produce electricity on demand independent of solar collection. One such thermal storage system, a thermocline, uses a single tank containing a fluid with a thermal gradient running vertically through the tank, where hotter fluid (lower density) is at the top of the tank and colder fluid is at the base of the tank. The thermal gradient separates the two temperature potentials. A low-cost filler material provides the bulk of the thermal capacitance of the thermal storage, prevents convective mixing, and reduces the amount of fluid required. In this paper, development of a thermocline system that uses molten-nitrate salt as the heat transfer fluid is described and compared to a two-tank molten salt system. Results of isothermal and thermal cycling tests on candidate materials and salt safety tests are presented as well as results from a small pilot-scale (2.3 MWh) thermocline.

Surface Step Effects on Nanoindentation

Abstract: Atomistic simulation is used to examine nanoindentation of a Au(111) crystal both near and far from a surface step. While the load needed to nucleate dislocations decreases significantly when indenting close to the step, the extent of the step's influence is not as great as seen experimentally. This behavior is explained by measuring the contact area from the simulation data. A new metric, the slip vector, shows material slip coinciding with the directions of a lowest unstable stacking fault barrier. The slip vector is used to calculate an atomic critical resolved shear stress, which is shown to be a good dislocation nucleation criterion.

Method for Computing the Anisotropy of the Solid-Liquid Interfacial Free Energy

Abstract: We present a method to compute accurately the weak anisotropy of the solid-liquid interfacial free energy, a parameter which influences dendritic evolution in materials with atomically rough interfaces. The method is based on monitoring interfacial fluctuations during molecular dynamics simulation and extracting the interfacial stiffness which is an order of magnitude more anisotropic than the interfacial free energy. We present results for pure Ni with interatomic potentials derived from the embedded atom method.

A split Hopkinson pressure bar technique to determine compressive stress-strain data for rock materials

Abstract: This paper presents a split Hopkinson pressure bar technique to obtain compressive stress-strain data for rock materials. This technique modifies the conventional split Hopkinson bar apparatus by placing a thin copper disk on the impact surface of the incident bar. When the striker bar impacts the copper disk, a nondispersive ramp pulse propagates in the incident bar and produces a nearly constant strain rate in a rock sample. Data from experiments with limestone show that the samples are in dynamic stress equilibrium and have constant strain rates over most of the test durations. In addition, the ramp pulse durations can be controlled such that samples are unloaded just prior to failure. Thus, intact samples that experience strains beyond the elastic region and postpeak stresses can be retrieved for microstructural evaluations. The paper also presents analytical models that predict the time durations for sample equilibrium and constant strain rate. Model predictions are in good agreement with measurements.

Computer-attack graph generation tool

Abstract: This paper presents a tool for assessment of security attributes and vulnerabilities in computer networks. The tool generates attack graphs (Phillips and Swiler, 1998). Each node in the attack graph represents a possible attack state. Edges represent a change of state caused by a single action taken by the attacker or unwitting assistant, and are weighted by some metric (such as attacker effort or time to succeed). Generation of the attack graph requires algorithms that match information about attack requirements (specified in attack templates) to information about the network configuration and assumed attacker capabilities (attacker profile). The set of near-optimal shortest paths indicates the most exploitable components of the system configuration. This paper presents the status of the tool and discusses implementation issues, especially focusing on the data input needs and methods for eliminating redundant paths and nodes in the graph.

Crystal and defect chemistry of rare earth cations in BaTiO3

Abstract: This study revisits the issue of rare earth cation substitutions into barium titanate. Analysis based upon crystal chemistry, defect chemistry and metastable states is presented to aid interpretation of experimental data. Recent detailed and highly precise X-ray powder diffraction and Electron Paramagnetic Resonance experiments performed on samples produced with different A/B ratios and fired under different oxygen partial pressure conditions give rise to new insights into the material. Specifically, the site occupancy and the valence states for the rare-earth dopants in barium titanate are considered. Earlier work is also reviewed and compared to the studies performed here. Collectively a classification of the various types of behavior observed for the rare-earth series in barium titanate is presented.

Anomalous exchange interactions in III-V dilute magnetic semiconductors

Abstract: Based on local-density functional calculations, we study the exchange interactions between magnetic dopants Cr, Mn, and Fe in the III-V compounds GaAs, GaN, and AlN. We show the magnetic exchange interactions deviate strongly in behavior expected from simple models, and may explain the observed maximum in critical temperature with impurity concentration. Additionally the magnetism is responsible for a strong, short-range attraction between the magnetic dopants, thus creating an anomalous effective alloy hamiltonian. This suggests that the impurities may aggregate into small nanoclusters of a few magnetic atoms.

Solidification in direct metal deposition by LENS processing

Abstract: Thermal imaging and metallographic analysis were used to study Laser Engineered Net Shaping (LENS™) processing of 316 stainless steel and H13 tool steel. The cooling rates at the solid-liquid interface were measured over a range of conduction conditions. The length scale of the molten zone controls cooling rates during solidification in direct metal deposition. In LENS processing, the molten zone ranges from 0.5 mm in length to 1.5 mm, resulting in cooling rates at the solid-liquid interface ranging from 200–6,000 Ks−1.

The Design and Evaluation of Physical Protection Systems

Abstract: This chapter presents the design and evaluation of physical protection systems (PPS), which integrates people, procedures, and equipment for the protection of assets or facilities against theft, sabotage, or other malevolent human attacks. The design of an effective PPS requires a methodical approach in which the designer weighs the objectives of the PPS against available resources and then evaluates the proposed design to determine how well it meets the objectives. The design of an effective PPS includes the determination of PPS objectives, the initial design or characterization of a PPS, the evaluation of the design, and, in many cases, a redesign or refinement of the system. Characterization of facility operations and conditions requires a thorough description of the facility itself. The PPS must be designed to protect against all of the threats such as criminal outsiders, disgruntled employees, and competitors. It is found that analysis and evaluation of the PPS design begin with a review and thorough understanding of the protection objectives the designed system must meet.

Development of catalytically enhanced sodium aluminum hydride as a hydrogen-storage material

Abstract: The dehydriding of sodium aluminum hydride, NaAlH4, is kinetically enhanced and rendered reversible in the solid state upon doping with selected titanium compounds. Following the initial reports of this catalytic effect, further kinetic improvement and stabilization of the cyclable hydrogen capacity have been achieved upon variation in the method of the introduction of titanium and particle-size reduction. Rapid evolution of 4.0-wt % hydrogen at 100 °C has been consistently achieved for several dehydriding/rehydriding cycles. An improved, 4.8-wt % cyclable capacity has been observed in the material doped with a combination of Ti and Zr alkoxide complexes. Doping the hydride with Ti(OBun)4 and Fe(OEt)2 also produces a synergistic effect, resulting in materials that can be rehydrided to 4 wt % at 104 °C and 87 atm of hydrogen within 17 h. The improved kinetics allowed us to carry out constant-temperature, equilibrium-pressure studies of NaAlH4 that extended to temperatures well below the melting point of the hydride. The 37-kJ/mol value determined for enthalpy of the dehydriding of NaAlH4(s) to Na3AlH6 and Al and the hydrogen plateau pressure of 7 atm at 80 °C are in line with the predictions of earlier studies. The nature of the active catalyst and the mechanism of catalytic action are unknown. The catalytically enhanced hydrides appear to be strong candidates for development as hydrogen carriers for onboard proton exchange membran (PEM) fuel cells. However, further research and development in the areas of rehydriding catalysts, large-scale, long-term cycling, safety and adjustment of the plateau hydrogen pressure associated with dehydriding of AlH6- are required before these materials can be utilized in commercial onboard hydrogen-storage systems.

Shubnikov–de Haas-like oscillations in millimeterwave photoconductivity in a high-mobility two-dimensional electron gas

Abstract: Millimeterwave photoconductivity in a high-mobility $\mathrm{GaAs}\ensuremath{-}{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ two-dimensional electron gas exhibits giant amplitude oscillations in a weak magnetic field. These oscillations resemble the Shubnikov--de Haas effect but their period is determined by $\ensuremath{\omega}/{\ensuremath{\omega}}_{C},$ where $\ensuremath{\omega}$ and ${\ensuremath{\omega}}_{C}$ are the millimeterwave and cyclotron frequencies, respectively. The major observations can be explained in terms of Landau-level transitions between spatially shifted oscillators. A ${2k}_{F}$ momentum transfer is accompanying the transition where ${k}_{F}$ is the electron Fermi wave number.

Mathematical representation of uncertainty

Abstract: As widely done in the risk assessment community, a distinction is made between aleatory (random) and epistemic (subjective) uncertainty in the modeling and simulation process. The nature of epistemic uncertainty is discussed, including (1) occurrence in parameters contained in mathematical models of a system and its environment, (2) limited knowledge or understanding of a physical process or interactions of processes in a system, and (3) limited knowledge for the estimation of the likelihood of event scenarios of a system. To clarify the options available for representation of epistemic uncertainty, an overview is presented of a hierarchy of theories of uncertainty. Modern theories of uncertainty can represent much weaker statements of knowledge and more diverse types of uncertainty than traditional probability theory. A promising new theory, evidence (Dempster-Shafer) theory, is discussed and applied to a simple system given by an algebraic equation with two uncertain parameters. Multiple sources of information are provided for each parameter, but each source only provides an interval value for each parameter. The uncertainty in the system response is estimated using probability theory and evidence theory. The resultant solutions are compared with regard to their assessment of the likelihood that the system response exceeds a specified failure level. In this example, a traditional application of probability theory results in a significantly lower estimate of risk of failure as compared to evidence theory. Strengths and weaknesses of evidence theory are discussed, and several important open issues are identified that must be addressed before evidence theory can be used successfully in engineering applications. * Distinguished Member Technical Staff, Associate Fellow t Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U. S. Department of Energy under contract No. DEAC04-94AL85000. This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Kari Sentz Systems Science and Industrial Engineering State University of New York-Binghamton Binghamton, New York

Equation of state measurements in liquid deuterium to 70 GPa.

Abstract: Using intense magnetic pressure, a method was developed to launch flyer plates to velocities in excess of $20\mathrm{km}/\mathrm{s}$. This technique was used to perform plate-impact, shock wave experiments on cryogenic liquid deuterium ( $L\ensuremath{-}{\mathrm{D}}_{2}$) to examine its high-pressure equation of state. Using an impedance matching method, Hugoniot measurements were obtained in the pressure range of $30--70\mathrm{GPa}$. The results of these experiments disagree with previously reported Hugoniot measurements of $L\ensuremath{-}{\mathrm{D}}_{2}$ in the pressure range above $\ensuremath{\sim}40\mathrm{GPa}$, but are in good agreement with first principles, ab initio models for hydrogen and its isotopes.

Practical Improvements to the Lee‐More Conductivity Near the Metal‐Insulator Transition

Abstract: The wide-range conductivity model of Lee and More [1] is modified to allow better agreement with recent experimental data and theories for dense plasmas in the metal-insulator transition regime. Modifications primarily include a new ionization equilibrium model, consisting of a smooth blend between single ionization Saha (with a pressure ionization correction) and the generic Thomas-Fermi ionization equilibrium, a more accurate treatment of electron-neutral collisions using a polarization potential, and an empirical modification to the minimum allowed collision time. These simple modifications to the Lee-More algorithm permit a more accurate modeling of the physics near the metal-insulator transition, while preserving the generic Lee-More results elsewhere.

Formation of chloropyromorphite in a lead-contaminated soil amended with hydroxyapatite.

Abstract: To confirm conversion of soil Pb to pyromorphite [Pb{sub 5}(PO{sub 4}){sub 3}Cl], a Pb contaminated soil collected adjacent to a historical smelter was reacted with hydroxyapatite in slurries of soil and hydroxyapatite separated by a dialysis membrane and incubated. A crystalline precipitate formed on the dialysis membrane in the slurry systems was identified as chloropyromorphite. Soluble species measured in the soil slurry indicated that dissolution of solid-phase soil Pb was the rate-limiting step for pyromorphite formation. Additionally samples reacted with hydroxyapatite were incubated at field-capacity moisture content. The sequential chemical extraction used to identify species in the field-moist soil incubation experiment showed that hydroxyapatite treatment reduced the first four fractions of extractable Pb and correspondingly increased the recalcitrant extraction residue fraction by 35% of total Pb at 0 d incubation and by 45% after 240 d incubation. the increase in the extraction residue fraction in the 240 d incubation as compared to the 0 d incubation implies that the reaction occurs in the soil but the increase in the hydroxyapatite amended 0 d incubated soil as compared to the control soil illustrates the chemical extraction procedure caused changes in the extractability. Thus, the chemical extraction procedure cannot easily be utilized to confirm changes occurring in the soil as a result of incubation. Extended x-ray absorption fine structure (EXAFS) spectroscopy indicated that the 240 d incubated hydroxyapatite treatment caused a change in the average, local molecular bonding environment of soil Pb. Low-temperature EXAFS spectra (chi data and radial structure functions - RSFs) showed a high degree of similarity between the chemical extraction residue and synthetic pyromorphite. Thus, confirming that the change of soil Pb to pyromorphite is possible by simple amendments of hydroxyapatite to soil.