A model is presented for the linking of helium bubbles growing in aging metal tritides. Stresses created by neighboring bubbles are found to produce bubble growth toward coalescence. This process is interrupted by the fracture of ligaments between bubble arrays. The condition for ligament fracture percolates through the material to reach external surfaces, leading to material micro-cracking and the release of helium within the linked-bubble cluster. A comparison of pure coalescence and pure fracture mechanisms shows the critical HeM concentration for bubble linkage is not strongly dependent on details of the linkage process. The combined stress-directed growth and fracture process produces predictions for the onset of rapid He release and the He emission rate. Transition to this rapid release state is determined from the physical size of the linked-bubble clusters, which is calculated from dimensional invariants in classical percolation theory. The result is a transition that depends on material dimensions. The onset of bubble linkage and rapid He release are found to be quite sensitive to the bubble spacing distribution, which is log-normal for bubbles nucleated by self-trapping.
The immersed-B{sub z} diode is being developed as a high-brightness, flash x-ray radiography source. This diode is a foil-less electron-beam diode with a long, thin, needle-like cathode inserted into the bore of a solenoid. The solenoidal magnetic field guides the electron beam emitted from the cathode to the anode while maintaining a small beam radius. The electron beam strikes a thin, high-atomic-number anode and produces bremsstrahlung. We report on an extensive series of experiments where an immersed-B{sub z} diode was fielded on the RITS-3 pulsed power accelerator, a 3-cell inductive voltage generator that produced peak voltages between 4 and 5 MV, {approx}140 kA of total current, and power pulse widths of {approx}50 ns. The diode is a high impedance device that, for these parameters, nominally conducts {approx}30 kA of electron beam current. Diode operating characteristics are presented and two broadly characterized operating regimes are identified: a nominal operating regime where the total diode current is characterized as classically bipolar and an anomalous impedance collapse regime where the total diode current is in excess of the bipolar limit and up to the full accelerator current. The operating regimes are approximately separated by cathode diameters greater than {approx}3 mm for the nominal regime and less than {approx} 3 mm for the anomalous impedance collapse regime. This report represents a compilation of data taken on RITS-3. Results from key parameter variations are presented in the main body of the report and include cathode diameter, anode-cathode gap, and anode material. Results from supporting parameter variations are presented in the appendices and include magnetic field strength, prepulse, pressure and accelerator variations.
As electronic assemblies become more compact and increase in processing bandwidth, escalating thermal energy has become more difficult to manage. The major limitation has been nonmetallic joining using poor thermal interface materials (TIM). The interfacial, versus bulk, thermal conductivity of an adhesive is the major loss mechanism and normally accounts for an order magnitude loss in conductivity per equivalent thickness. The next generation TIM requires a sophisticated understanding of material and surface sciences, heat transport at submicron scales, and the manufacturing processes used in packaging of microelectronics and other target applications. Only when this relationship between bond line manufacturing processes, structure, and contact resistance is well-understood on a fundamental level will it be possible to advance the development of miniaturized microsystems. This report examines using thermal and squeeze-flow modeling as approaches to formulate TIMs incorporating nanoscience concepts. Understanding the thermal behavior of bond lines allows focus on the interfacial contact region. In addition, careful study of the thermal transport across these interfaces provides greatly augmented heat transfer paths and allows the formulation of very high resistance interfaces for total thermal isolation of circuits. For example, this will allow the integration of systems that exhibit multiple operational temperatures, such as cryogenically cooled detectors.
Sandia National Laboratories, in partnership with the Consumer Product Safety Commission (CPSC), has developed an optical-based sensor for the detection of CO in appliances such as residential furnaces. The device is correlation radiometer based on detection of the difference signal between the transmission spectrum of the sample multiplied by two alternating synthetic spectra (called Eigen spectra). These Eigen spectra are derived from a priori knowledge of the interferents present in the exhaust stream. They may be determined empirically for simple spectra, or using a singular value decomposition algorithm for more complex spectra. Data is presented on the details of the design of the instrument and Eigen spectra along with results from detection of CO in background N{sub 2}, and CO in N{sub 2} with large quantities of interferent CO{sub 2}. Results indicate that using the Eigen spectra technique, CO can be measured at levels well below acceptable limits in the presence of strongly interfering species. In addition, a conceptual design is presented for reducing the complexity and cost of the instrument to a level compatible with consumer products.
The report presents a summary of international perceptions and beliefs about US nuclear policy, focusing on four countries--China, Iran, Pakistan and Germany--chosen because they span the spectrum of states with which the United States has relationships. A paradox is pointed out: that although the goal of US nuclear policy is to make the United States and its allies safer through a policy of deterrence, international perceptions of US nuclear policy may actually be making the US less safe by eroding its soft power and global leadership position. Broadly held perceptions include a pattern of US hypocrisy and double standards--one set for the US and its allies, and another set for all others. Importantly, the US nuclear posture is not seen in a vacuum, but as one piece of the United States behavior on the world stage. Because of this, the potential direct side effects of any negative international perceptions of US nuclear policy can be somewhat mitigated, dependent on other US policies and actions. The more indirect and long term relation of US nuclear policy to US international reputation and soft power, however, matters immensely to successful multilateral and proactive engagement on other pressing global issues.
The results of experiments examining the thermal decomposition of N 2 O and its reactivity with methane in supercritical water at approximately 500°C and 30 MPa are presented. The rate of thermal decomposition is observed to be close to the rate predicted by extrapolating an Arrhenius expression from the literature that has been shown to be valid at 750°C and 1.0 MPa. The observed first-order rate constant at 500°C is 9.4 × 10 -6 s -1 . There is no significant effect on N 2 O stability due to the presence of supercritical water relative to ambient pressure. Measurements exploring the conversion rate of methane in the presence of N 2 O reveal that simple oxidation chemistry competes with polymerization. The data suggest that much of the carbon in the system is converted to (CH 2 ) n oligomers that separates from the supercritical phase. A detailed kinetic mechanism is used to explore characteristics of these competing processes.
Direct numerical simulation (DNS) with complex chemistry was used to study statistics of displacement and consumption speeds in turbulent lean premixed methane-air flames. The main focus of the study is an evaluation of the extent to which a turbulent flame in the thin reaction zones regime can be described by an ensemble of strained laminar flames. Conditional averages with respect to strain for displacement and consumption speeds are presented over a wide range of strain typically encountered in a turbulent flame, compared with previous studies that either made local pointwise comparisons or conditioned the data on small strain and curvature. The conditional averages for positive strains are compared with calculated data from two different canonical strained laminar configurations to determine which is the optimal representation of a laminar flame structure embedded in a turbulent flame: the reactant-to-product (R-to-P) configuration or the symmetric twin flame configuration. Displacement speed statistics are compared for the progress-variable isosurface of maximum reaction rate and an isosurface toward the fresh gases, which are relevant for both modeling and interpretation of experiment results. Displacement speeds in the inner reaction layer are found to agree very well with the laminar R-to-P calculations over a wide range of strain for higher Damköhler number conditions, well beyond the regime in which agreement was expected. For lower Damköhler numbers, a reduced response to strain is observed, consistent with previous studies and theoretical expectations. Compared with the inner layer, broader and shifted probability density functions (PDFs) of displacement speed were observed in the fresh gases, and the agreement with the R-to-P calculations deteriorated. Consumption speeds show a poorer agreement with strained laminar calculations, which is attributed to multidimensional effects and a more attenuated unsteady response to strain fluctuations; however, they also show less departure from the unstrained laminar value, suggesting that detailed modeling of this quantity may not be critical for the conditions considered. For all quantities investigated, including CO production, the R-to-P laminar configuration provides an improved description relative to the twin flame configuration, which predicts qualitatively incorrect trends and overestimates extinction.
Domain decomposed Monte Carlo codes, like other domain-decomposed codes, are difficult to debug. Domain decomposition is prone to error, and interactions between the domain decomposition code and the rest of the algorithm often produces subtle bugs. These bugs are particularly difficult to find in a Monte Carlo algorithm, in which the results have statistical noise. Variations in the results due to statistical noise can mask errors when comparing the results to other simulations or analytic results. If a code can get the same result on one domain as on many, debugging the whole code is easier. This reproducibility property is also desirable when comparing results done on different numbers of processors and domains. We describe how reproducibility, to machine precision, is obtained on different numbers of domains in an Implicit Monte Carlo photonics code.
We investigate the utility of dual-pump coherent anti-Stokes Raman scattering (CARS) for investigations of fuel-rich flames with soot volume loadings up to 2.2 ppm. Our initial characterization of a gas-phase propellant simulating burner is presented. The burner investigated consists of alternating fuel and oxidizer tubes with the option for injection of aluminum particles into the flame. We focus on a C2H2-N2-O2 flame, in which no aluminum particles are yet added. For the required flow rates, this burner provides an array of heavily sooting and highly luminous diffusion flames whose temperature is measured by dual-pump CARS. The nature of the observed soot-induced interference in the CARS spectra for several different pump 2 frequencies is documented and CARS temperatures obtained from data in a spectrally "quiet" region, with the CARS signal beam near 483 nm, are presented. Soot volume fractions are imaged by absorption-calibrated laser-induced incandescence (LII) to quantify the soot loadings at which our dual-pump CARS facility provides reliable measurements.
Three-dimensional direct numerical simulation of a spatially developing slot-burner Bunsen flame has been performed. The simulation is aimed at better understanding the dynamics of turbulent premixed flames in the thin reaction zones regime. A reduced chemical model for methane-air chemistry consisting of 13 resolved species, 4 quasi-steady state species and 73 elementary reactions has been developed specifically for the current simulation. Using the new chemical model a lean premixed methane-air flame at preheated conditions and ambient pressure is simulated. The simulation is performed long enough to achieve statistical stationarity. The data is analyzed to study possible influences of turbulence on the flame thickness. The results show that the average flame thickness increases, in agreement with a few, although not unanimous, experimental results.
We demonstrate use of a Jacobian-Free Newton-Krylov solver to enable strong thermal coupling at the interface between a solid body and an external compressible fluid. Our method requires only information typically used in loose coupling based on successive substitution and is implemented within a multi-physics framework. We present results for two external flows over thermally conducting solid bodies obtained using both loose and strong coupling strategies. Performance of the two strategies is compared to elucidate both advantages and caveats associated with strong coupling.
Sandia National Laboratories has developed a new technique for testing in a combined linear acceleration and vibration environment. Amplified piezo-electric actuator assemblies are used in combination with Sandia's 29-ft centrifuge facility to surpass the load capabilities of previous attempts using traditional mechanical shaker systems. The piezoelectric actuators are lightweight, modular and overcome several limitations presented by a mechanical shaker. They are 'scalable', that is, adding more piezo-electric units in parallel or in series can support larger-weight test articles or wider range of displacement/frequency regimes. In addition, the units could be mounted on the centrifuge arm in various configurations to provide a variety of input directions. The design along with test results will be presented to demonstrate the capabilities of the new piezo-electric Vibrafuge.
The trajectory and entrainment properties of a transverse jet are important to a variety of engineering applications. This study seeks to develop actuation strategies that manipulate the penetration, spread, and vortical structures of the tranverse jet, based on simple vorticity perturbations at the nozzle edge. We use three-dimensional vortex simulations of a transverse jet at high Reynolds number to examine four prototypical actuations, all at a jet-to-crossflow velocity ratio of r = 7. These actuations include a delta-tab on the windward edge of the jet nozzle as well as periodic modulations and inversions of wall-normal vorticity in the shear layer. Small modifications to the vorticity on nascent shear layer are found to have a significant impact on the jet evolution - creating jets that remain confined and penetrate further into the crossflow, or, alternately, jets that quickly spread in the spanwise direction and bend downstream. Vorticity perturbations also hasten or delay the formation of counter-rotating vorticity by modifying the folding of shear-layer segments.
This meeting will continue to cover fundamentals and applications of photoionization and photodetachment, including valence and core-level phenomena and applications to reaction dynamics, ultrashort laser pulses and the study of exotic molecules and anions.
Nanostructured materials are the basis for emerging technologies, such as MEMS, NEMS, sensors, and flexible electronics, that will dominate near term advances in nanotechnology. These technologies are often based on devices containing layers of nanoscale polymer, ceramic and metallic films and stretchable interconnects creating surfaces and interfaces with properties and responses that differ dramatically from bulk counterparts. The differing properties can induce high interlaminar stresses that lead to wrinkling, delamination, and buckling in compression [1,2], and film fracture and decohesion in tension. [3] However, the relationships between composition, structure and properties, and especially adhesion and fracture, are not well-defined at the nanoscale. These relationships are critical to assuring performance and reliability of nanostructured materials and devices. They are also critical for building materials science based predictive models of structure and behavior.