We have found that nanosecond optical parametric oscillators pumped well above threshold by single longitudinal mode pulse produce signal and idler light that is nearly purely phase modulated, even for unseeded operation.
There is a need for an automated system for protecting and monitoring sensitive or classified parts and material. Sandia has developed a real-time personnel and material tracking system (PAMTRAK) that has been installed at selected DOE facilities. It safeguards sensitive parts and material by tracking tags worn by personnel and by monitoring sensors attached to the parts or material. It includes remote control and alarm display capabilities and a complementary program in Keyhole to display measured material attributes remotely. This paper describes the design goals, the system components, current installations, and the benefits a site can expect when using PAMTRAK.
Expert system implementation can take numerous forms ranging form traditional declarative rule-based systems with if-then syntax to imperative programming languages that capture expertise in procedural code. The artificial intelligence community generally thinks of expert systems as rules or rule-bases and an inference engine to process the knowledge. The welding advisor developed at Sandia National Laboratories and described in this paper deviates from this by codifying expertise using object representation and methods. Objects allow computer scientists to model the world as humans perceive it giving us a very natural way to encode expert knowledge. The design of the welding advisor, which generates and evaluates solutions, will be compared and contrasted to a traditional rule- based system.
Traditional definitions of risk partition concern into the probability of occurrence and the consequence of the event. Most safety analyses focus on probabilistic assessment of an occurrence and the amount of some measurable result of the event, but the real meaning of the ``consequence`` partition is usually afforded less attention. In particular, acceptable social consequence (consequence accepted by the public) frequently differs significantly from the metrics commonly proposed by risk analysts. This paper addresses some of the important system development issues associated with consequences, focusing on ``high consequence operations safety.``
Field ion microscopy show a strong correlation between mobility and shape of small clusters on fcc(100) metal surfaces. For self-diffusion on Rh(100) this correlation lead to an oscillatory behavior in the activation energy of surface diffusion as a function of cluster size. Comparison of measured activation energies to theory indicate that the mechanism of cluster diffusion involves individual displacements of edge atoms (ie, perimeter diffusion). Rate-determining step in migration of clusters is partial detachment of one of the perimeter atoms. Relative ease of adatom motion along straight edges of stationary clusters also permits measurements of diffusion barriers at steps, which can be useful in interpretation of fractal vs compact island growth on fcc metal surfaces.
GaAs-based Metal Semiconductor Field Effect transistors (MESFETs) and High Electron Mobility Transistors (HEMTs) have been the focus of research for high-temperature operation due to the 1.42 eV band gap of GaAs that reduces thermal carrier generation as compared to 1.1 eV silicon-based electronics. Although schemes have been proposed to minimize substrate currents at elevated temperatures, high-temperature operation of these devices is ultimately limited by the gate leakage current of the Schottky gate contact. Since a Junction Field Effect Transistor (JFET) has a higher gate barrier to current flow than a Schottky barrier MESFET as a result of the p/n junction gate, JFETs should have superior performance at elevated temperatures. This paper compares the high-temperature performance of a self-aligned GaAs MESFET and JFET. Both devices suffer from substrate leakage at high temperature; however, the JFET has superior gate characteristics and maintains a larger fraction of its room temperature transconductance at 300 C.
We have studied the chemical selectivity and sensitivity of surface acoustic wave (SAW) sensors covered by (COO{sup {minus}}){sub 2}/Cu{sup 2+}-terminated interfaces by examining the response of self-assembled monolayer (SAM) films formed from the solution phase for 36, 84, and 180 h adsorption times. These organomercaptan SAMs were prepared on thin-film Au surfaces having variable, controlled grain size. The SAW response from the carboxylate coordinated Cu{sup 2+}-terminated SAM is compared to that from methyl-terminated SAM, as these films interact with a vapor-phase organophosphonate analyte and the vapors of common organic solvents. Results have implications for designing and reliably fabricating chemical sensors that respond to specific organic analytes.
Ion implantation doping and isolation has played a critical role in realizing high performance photonic and electronic devices in all mature semiconductor materials; this is also expected for binary III-Nitride materials (InN, GaN, AlN) and their alloys as epitaxy improves and more advanced device structures fabricated. This paper reports on recent progress in ion implantation doping of III-Nitride materials that has led to the first demonstration of a GaN JFET (junction field effect transistor). The JFET was fabricated with all ion implantation doping; in particular, p-type doping of GaN with Ca has been demonstrated with an estimated acceptor ionization energy of 169 meV. O-implantation has also been studied and shown to yield n-type conduction with an ionization energy of {similar_to}29 meV. Neither Ca or O display measurable redistribution during a 1125 C, 15 s activation anneal which sets an upper limit on their diffusivity at this temperature of 2.7{times}10{sup {minus}13}cm{sup 2}/s.
Asynchronous Transfer Mode (ATM) is a new data communications technology that promises to integrate voice, video, and data traffic into a common network infrastructure. In order to fully utilize ATM`s ability to transfer real-time data at high rates, applications will start to access the ATM layer directly. As a result of this trend, security mechanisms at the ATM layer will be required. A number of research programs are currently in progress which seek to better understand the unique issues associated with ATM security. This paper describes some of these issues, and the approaches taken by various organizations in the design of ATM layer security mechanisms. Efforts within the ATM Forum to address the user communities need for ATM security are also described.
We present a manipulator placement algorithm for minimizing the length of the manipulator motion performing a visit-point task such as spot welding. Given a set of points for the tool of a manipulator to visit, our algorithm finds the shortest robot motion required to visit the points from each possible base configuration. The base configurations resulting in the shortest motion is selected as the optimal robot placement. The shortest robot motion required for visiting multiple points from a given base configuration is computed using a variant of the traveling salesman algorithm in the robot joint space and a point-to-point path planner that plans collision free robot paths between two configurations. Our robot placement algorithm is expected to reduce the robot cycle time during visit- point tasks, as well as speeding up the robot set-up process when building a manufacturing line.
In recent years, much of the progress in Computer-Aided Manufacturing has emphasized the use of simulation, finite-element analysis, and other science-based techniques to plan and evaluate manufacturing processes. These approaches are all based on the idea that we can build sufficiently faithful models of complex manufacturing processes such as machining, welding, and casting. Although there has been considerable progress in this area, it continues to suffer from difficulties: the first of these is that the kind of highly accurate models that this approach requires may take many person months to construct, and the second is the large amount of computing resources needed to run these simulations. Two design advisors, Near Net-Shape Advisor and Design for Machinability Advisor, are being developed to explore the role of heuristic, knowledge-based systems for manufacturing processes, both as an alternative to more analytical techniques, and also in support of these techniques. Currently the advisors are both in the prototype stage. All indications lead to the conclusion that the advisors will be successful and lay the groundwork for additional systems such as these in the future.
Lithium wall conditioning has been used in a recent campaign evaluating high performance negative central shear (NCS) discharges. During this campaign, the highest values of stored energy (4.4 MJ), neutron rate (2.4 x 10{sup 16}/s), and nT{sub i}{tau} (7 x 10{sup 20} m{sup -3}-keV-s) achieved to date in DIII-D were obtained. High performance NCS discharges were achieved prior to beginning lithium conditioning, but it is clear that shot reproducibility and performance were improved by lithium conditioning. Central and edge oxygen concentrations were reduced after lithium conditioning, Lithium conditioning, consisting of up to four pellets injected at the end of the preceding discharge, allowed the duration of the usual inter-shot helium glow discharge to be reduced and reproducible high auxiliary power discharges, P{sub NBI} {<=} 22 MW, were obtained with plasma currents up to 2.4 MA.
From the beginnings of the U.S. nuclear weapons program, military and civilian dual- agency judgment has been fundamental to achieving nuclear weapon and weapon system safety. This interaction was initiated by the Atomic Energy Act of 1946, which created the Atomic Energy Commission (AEC). The principle of using dual-agency judgment has been perpetuated in the design and assessment of the weapon and weapon system acceptance process since that time. This fundamental approach is still used today in all phases of the weapon life. In this paper, an overview of the history and philosophy of the approach is described.
We have synthesized monolithic particulate gels of periodic mesoporous silica by adding tetramethoxysilane to a homogeneous alkaline micellar precursor solution. The gels exhibit 5 characteristic length scales over 4 orders of magnitude: fractal domains larger than the particle size (>500 nm), particles that are {approximately}150 to 500 nm in diameter, interparticle pores that are on the order of the particle size, a feature in the gas adsorption measurements that indicates pores {approximately}10-50 nm, and periodic hexagonal arrays of {approximately}3 nm channels within each particle. The wet gel monoliths exhibit calculated densities as low as {approximately}0.02 g/cc; the dried and calcined gels have bulk densities that range from {approximately}0.3-0.5 g/cc. The materials possess large interparticle ({approximately}1.0-2.3 cc/g) and intraparticle ({approximately}0.6 cc/g) porosities.
This paper reviews several coupled theoretical and experimental investigations of the effect of microstructure on momentum transport in concentrated suspensions. An expression to predict the apparent suspension viscosity of mixtures of rods and spheres is developed and verified with falling-ball viscometry experiments. The effects of suspension-scale slip (relative to the bulk continuum) are studied with a sensitive spinning-ball rheometer, and the results are explained with a novel theoretical method. The first noninvasive, nuclear magnetic resonance imaging measurements of the evolution of velocity and concentration profiles in pressure-driven entrance flows of initially well mixed suspensions in a circular conduit are described, as well as more complex two-dimensional flows with recirculation, e.g. flow in a journal bearing. These data in nonhomogeneous flows and complementary three-dimensional video imaging of individual tracer particles in homogeneous flows are providing much needed information on the effects of flow on particle interactions and effective theological properties at the macroscale.
Four of the better developed resist schemes that are outgrowths of DUV (248 and 193 nm) resist development are considered as candidates for EUV. They are as follows: trilayer, a thin imaging layer on top of a refractor masking/pattern transfer layer on top of a planarizing and processing layer (PPL); solution developed, organometallic bilayer where the imaging and masking layer have been combined into one material on top of a PPL; and finally silylated resists. They are examined in a very general form without regard to the specifics of chemistry of the variations within each group, but rather to what is common to each group and how that affects their effectiveness as candidates for a near term EUV resist. In particular they are examined with respect to sensitivity, potential resolution, optical density, etching selectivity during pattern transfer, and any issues associated with pattern fidelity such as swelling.
We have deposited ZrO{sub 2}, TiO{sub 2}, and SnO{sub 2} films on ST-cut quartz surface acoustic wave (SAW) devices via sol-gel techniques. The films range from 100 to 300 nm thick and have porosities after calcination at 300{degrees}C that range from 82-88 % for ZrO{sub 2}, 77-81% for TiO{sub 2}, and 57-66% for SnO{sub 2}. In all cases, we have varied the synthesis and processing parameters over a wide range to optimize film properties: metal ion concentration (0.05-1.0 M), the H{sub 2}O:metal ratio (0.3-5.3), the acid concentration in the sol (0.02-0.7 M), the modifier ligand:metal ratio (r = 0.0-1.0), the processing conditions (100-900{degrees}C). The modifier ligand, triethanolamine (TEA), is added to each solution to allow multilayer films to be made crack free. The multilayer films are studied by optical microscopy, ellipsometry, X-ray diffraction, and N{sub 2} sorption. Preliminary high temperature frequency response measurements to target gases, such as, H{sub 2}, NO, NO{sub 2}, and propylene indicate limited sensitivity for the configurations tested.
The hydrolysis and self- and cross-condensation kinetics of the hybrid sol tetraethoxysilane and ethyltriethoxysilane were investigated by high resolution {sup 29}Si NMR spectroscopy. A kinetic model in which hydrolysis is reversible and condensation is irreversible was developed. The authors found excellent agreement between the product distributions measured by {sup 29}Si NMR spectroscopy and calculated by the model. The cross-condensation rates for each of the sols were intermediate to the condensation rates of the individual components. Calculations show that for these sols, the concentration of cross-condensed species is a weak function of the relative rates of self-condensation.
The Sr-Bi-Ta-O system is of interest for thin-film non-volatile ferroelectric memories. A better understanding of the process by which the perovskite phase forms can provide insight for improved processing of this ferroelectric compound. The authors have prepared thin-films by a chemical method using Sr-acetate, Bi-acetate and Ta-ethoxide; cation ratios were {approximately} 1:2:2 for Sr, Bi, and Ta, respectively. Results of in-situ crystallization studies using High-Temperature Grazing-Incidence X-ray Diffraction (HTGIXRD) have demonstrated that a fluorite structure, forming in the {approximately}600--700 C range, acts as an intermediate phase prior to the crystallization of the perovskite. Additional samples with cation ratios of {approximately} 1:0.8:2 were also investigated. Results for samples prepared with the 0.8 Bi content indicated that a pyrochlor phase forms which contains a substantial deficiency in Bi compared to the composition of the perovskite phase. The structures of the pyrochlore and fluorite phases and their relation to the formation of the perovskite ferroelectric are discussed.
Silicon nitride powders with an average size as low as 7 nm are synthesized in a pulsed radio frequency glow discharge. The as-synthesized silicon nitride powder from a silane/ammonia plasma has a high hydrogen content and is sensitive to oxidation in air. Post-plasma heating of the powder in a vacuum results in nitrogen loss, giving silicon-rich powder. In contrast, heat treatment at 800 C for 20 minutes in an ammonia atmosphere (200 Torr pressure) yields a hydrogen-free powder which is stable with respect to atmospheric oxidation. Several approaches to synthesizing silicon carbide nano-size powders are presented. Experiments using silane/hydrocarbon plasmas produce particles with a high hydrogen content as demonstrated by Fourier transform infrared analysis. The hydrogen is present as both CH and SiH functionality. These powders are extremely air-sensitive. A second approach uses a gas mixture of methyltrichlorosilane and hydrogen. The particles have a low hydrogen content and resist oxidation. Particle morphology of the silicon carbide is more spherical and there is less agglomeration than is observed in the silicon nitride powder.
Development of capillary stress in porous xerogels, although ubiquitous, has not been systematically studied. The authors have used the beam bending technique to measure stress isotherms of microporous thin films prepared by a sol-gel route. The thin films were prepared on deformable silicon substrates which were then placed in a vacuum system. The automated measurement was carried out by monitoring the deflection of a laser reflected off the substrate while changing the overlying relative pressure of various solvents. The magnitude of the macroscopic bending stress was found to reach a value of 180 MPa at a relative pressure of methanol, P/Po = 0.001. The observed stress is determined by the pore size distribution and is an order of magnitude smaller in mesoporous thin films. Density Functional Theory (DFT) indicates that for the microporous materials, the stress at saturation is compressive and drops as the relative pressure is reduced.
Using infrared light scattering microscopy, the authors have directly observed the inhibition of photon propagation in a 2-dimensional photonic lattice fabricated as a hexagonal array of AlGaAs posts. The lattice was formed by reactive ion etching of {approximately}400 nm diameter posts defined by electron beam lithography. The lattice design parameters correspond to a photonic bandgap near 1.5 {micro}m as calculated by Meade et al. This hexagonal array of posts is an improvement over early honeycomb lattices because it is easier to fabricate. The photonic lattice of 1.4 {micro}m high posts was incorporated into waveguide designed for single mode at 1.5 {micro}m. Several waveguide/lattice combinations were fabricated, including M-bar and K-bar lattice orientations aligned parallel to the waveguide and different numbers of lattice periods. The waveguide/lattice structures were fabricated on GaAs substrates that were subsequently thinned and cleaved to couple light into the waveguide facets. Using a specially designed triple infrared microscope system, they simultaneously imaged the input and output facets and the top surface of the waveguide as laser light was focused onto the input facet. Because of internal scattering in the waveguide, light is scattered upward outward and can be imaged with an infrared camera. Images for reflected input, waveguide scattered light, and transmitted output light for the waveguide with (left images) and without the photonic lattice (right images) are shown. The lefthand image shows how the lattice interrupts the transport of light through the waveguide.
The purpose of this contribution is to propose an ``Authentication Information Element`` that can be used to carry authentication information within the ATM signaling protocols. This information may be used by either signaling entity to validate the claimed identity of the other, and to verify the integrity of a portion of a message`s contents. By specifying a generic authentication IE, authentication information can be generated by any signature algorithm, and can be appended to any ATM signaling message. Procedures for the use of this information element are also provided.
Software process improvement has become a popular pastime, for a variety of reasons. The Software Engineering Institute`s summary of experimental data, which resulted in the Capability Maturity Model, has now had considerable corroboration. There are nearly as many software processes as there are combinations of developers, users, and products. Similarly, there are probably as many software process improvement approaches. However, the meta-process for performing process improvement is quite straightforward. Processes can be represented by a small number of abstractions, with variety supplied through implementation details. The scheme for improvement is almost self-evident: figure out where you are now, use a software process maturity guide to identify shortcomings, plot a change in a direction to eliminate a shortcoming, and go for it. This paper won`t dwell on the meta process and its enactment; the authors simply assume one is in place. Rather, they consider some ways to improve the testing aspects of your software process. These may be changes in what you do for testing as well as in how you do it.
GaN is an attractive material for use in high-temperature or high-power electronic devices due to its high bandgap (3.39 eV), high breakdown field ({approximately}5 {times} 10{sup 6} V/cm), high saturation drift velocity (2.7 {times} 10{sup 7} cm/s), and chemical inertness. To this end, Metal Semiconductor FETs (MESFETs), High Electron Mobility Transistors (HEMTs), Heterostructure FETs (HFETs), and Metal Insulator Semiconductor FETs (MISFETs) have all been reported based on epitaxial AlN/GaN structures (Khan 1993a,b; Binari 1994 and 1995). GaN Junction Field Effect Transistors (JFETs), however, had not been reported until recently (Zolper 1996b). JFETs are attractive for high-temperature operation due to the inherently higher thermal stability of the p/n junction gate of a JFET as compared to the Schottky barrier gate of a MESFET or HFET. In this paper the authors present the first results for elevated temperature performance of a GaN JFET. Although the forward gate properties are well behaved at higher temperatures, the reverse characteristics show increased leakage at elevated temperature. However, the increased date leakage alone does not explain the observed increase in drain current with temperature. Therefore, they believe this first device is limited by temperature activated substrate conduction.