Publications

We present calculations of the specific contact resistance for metals to GaN. The calculations include a correct determination of the Fermi level taking into account the effect of the degenerate doping levels, required in creating tunneling ohmic contacts. Using a recently reported improved WKB approximation suitable in representing the depletion width at the metal-semiconductor interface, and a two band k-p model for the effective masses, specific contact resistance was determined as a function of doping concentration. The specific contact resistance was calculated using the best data available for barrier heights, effective masses and dielectric coefficients for GaN. Because the barrier height at the metal-semiconductor interface has a very large effect on the contact resistance and the available data is sketchy or uncertain, the effect of varying the barrier height on the calculated specific contact resistance was investigated. Further, since the III-V nitrides are being considered for high temperature device applications, the specific contact resistance was also determined as a function of temperature.

No abstract available.

Lost circulation is a persistent problem in geothermal drilling and often accounts for a significant fraction of the cost of drilling a typical geothermal well. The US Department of Energy sponsors work at Sandia National Laboratories to develop technology for reducing lost circulation costs. This paper describes a downhole tool that has been developed at Sandia for improving the effectiveness and reducing the cost of cementing operations used to treat lost circulation zones. This tool, known as the drillable straddle packer, is a low-cost, disposable assembly used for isolating a loss zone and directing the flow of cement into the zone. This paper describes the tool concept, hardware design, deployment procedure, laboratory testing, and technical issues addressed during the development process.

Variation of model size as determined by grid density is studied for both model refinement and damage detection. In model refinement 3 it is found that a large model with a fine grid is preferable in order to achieve a reasonable correlation between the experimental response and the finite element model. A smaller model falls victim to the inaccuracies of the finite element method. As the grid become increasing finer, the FE method approaches an accurate representation. In damage detection the FE method is only a starting point. The model is refined with a matrix method which doesn`t retain the FE approximation, therefore a smaller model that captures most of the dynamics of the structure can be used and is preferable.

This paper presents a high bandwidth fiber-optic communication system intended for post accident recovery of weapons. The system provides bi-directional multichannel, and multi-media communications. Two smaller systems that were developed as direct spin-offs of the larger system are also briefly discussed.

To model the shock-induced behavior of porous or damaged energetic materials, a nonequilibrium mixture theory has been developed and incorporated into the shock physics code, CTH. Foundation for this multiphase model is based on a continuum mixture formulation given by Baer and Nunziato. In this nonequilibrium approach, multiple thermodynamic and mechanics fields are resolved including the effects of material relative motion, rate-dependent compaction, drag and heat transfer interphase effects and multiple-step combustion. Benchmark calculations are presented which simulate low-velocity piston impact on a propellant porous bed and experimentally-measured wave features are well replicated with this model. This mixture model introduces micromechanical models for the initiation and growth of reactive multicomponent flow which are key features to describe shock initiation and self-accelerated deflagration-to-detonation combustion behavior. To complement one-dimensional simulation, two dimensional numerical simulations are presented which indicate wave curvature effects due to the loss of wall confinement.

This contribution presents some lessons learned in the development of cooperation and knowledge transfer across the numerous interfaces involved in managing a corporate research laboratory.

This contribution addresses requirements for ATM signaling channel authentication. Signaling channel authentication is an ATM security service that binds an ATM signaling message to its source. By creating this binding, the message recipient, and even a third party, can confidently verify that the message originated from its claimed source. This provides a useful mechanism to mitigate a number of threats. For example, a denial of service attack which attempts to tear-down an active connection by surreptitiously injecting RELEASE or DROP PARTY messages could be easily thwarted when authenticity assurances are in place for the signaling channel. Signaling channel authentication could also be used to provide the required auditing information for accurate billing which is impervious to repudiation. Finally, depending on the signaling channel authentication mechanism, end-to-end integrity of the message (or at least part of it) can be provided. None of these capabilities exist in the current specifications.

Crystal lattices are infinite periodic graphs that occur naturally in a variety of geometries and which are of fundamental importance in polymer science. Discrete models of protein folding use crystal lattices to define the space of protein conformations. Because various crystal lattices provide discretizations of the same physical phenomenon, it is reasonable to expect that there will exist ``invariants`` across lattices that define fundamental properties of protein folding process; an invariant defines a property that transcends particular lattice formulations. This paper identifies two classes of invariants, defined in terms of sublattices that are related to the design of algorithms for the structure prediction problem. The first class of invariants is, used to define a master approximation algorithm for which provable performance guarantees exist. This algorithm can be applied to generalizations of the hydrophobic-hydrophilic model that have lattices other than the cubic lattice, including most of the crystal lattices commonly used in protein folding lattice models. The second class of invariants applies to a related lattice model. Using these invariants, we show that for this model the structure prediction problem is intractable across a variety of three-dimensional lattices. It`` turns out that these two classes of invariants are respectively sublattices of the two- and three-dimensional square lattice. As the square lattices are the standard lattices used in empirical protein folding` studies, our results provide a rigorous confirmation of the ability of these lattices to provide insight into biological phenomenon. Our results are the first in the literature that identify algorithmic paradigms for the protein structure prediction problem which transcend particular lattice formulations.

Approximately 13% by volume of the US Department of Energy (DOE) current backlog of radioactive waste is characterized as high-level waste. Transportation of these wastes requires that the waste package have adequate shielding against gamma radiation. This project investigates the radiation shielding performance of titanium and depleted uranium, which have been proposed for use as gamma shielding materials in DOE transportation packages, by experimentally determining their buildup factors. Buildup factors are important in shield heating and radiation damage calculations. A point-isotropic-source type of buildup factor is the most useful for application in the point-kernal approach utilized in many simple shielding codes. The point-kernal method provides reasonable results for cases in which the shield is made of one solid material and the source can be approximated as one homogeneous material. The point-kernal method has been incorporated into a large number of shielding codes treating three-dimensional geometry using buildup factor data in some form. Buildup factors vary with a number of parameters such as the distance of penetration through the attenuating medium; the geometric configuration of the attenuating medium, source and detector position; the composition of the medium; the detector response function; and the energy and direction of emission of the source photons, ideally taken to be monoenergetic and isotropic.

It is likely that the ongoing process to produce the 1996 version of the IAEA Regulation for the Safe Transport of Radioactive Materials, IAEA Safety Series 6(SS 6) will result in a more stringent package qualification standard for air transport of large quantities of radioactive materials (RAM) than is included in the 1990 version. During the process to define the scope of the new requirements there was extensive discussion of their impact on, and application to, fissile material package qualification criteria. Since fissile materials are shipped in a variety of packaging ranging from exempt to Type B, each packaging of each type must be evaluated for its ability to maintain subcriticality both alone and in arrays and in both damaged and undamaged condition. In the 1990 version of SS 6 "damaged" means the condition of a package after it had undergone the "tests for demonstrating the ability to withstand accident conditions in transport," i.e., Type B qualification tests. These tests conditions are typical of severe accidents in surface modes but are less severe than air mode qualification test environments to be applied to Type C packages. As a result, questions arose about the need for a corresponding change in the 1996 SS 6 to define "damaged" to include the Type C test regime for criticality evaluations of fissile packages in air transport.

Medical radiotherapy has traditionally relied upon the use of external photon beams and internally implanted radioisotopes as the chief means of irradiating tumors. However, advances in accelerator technology and the exploitation of novel means of producing radiation may provide useful alternatives to some current modes of medical radiation delivery - with reduced total dose to surrounding healthy tissue, reduced expense, or increased treatment accessibility. This paper will briefly overview currently established modes of radiation therapy, techniques still considered experimental but in clinical use and innovative concepts under study that may enable new forms of treatment or enhance existing ones. The potential role of low energy, ion-induced nuclear reactions in radiotherapy applications is examined specifically for the 650 keV d(3He,p)4 He nuclear reaction. This examination will describe the basic physics associated with this reaction's production of 17.4 MeV protons and the processes used to fabricate the necessary materials used in the technique. Calculations of the delivered radiation dose, heat generation, and required exposure times are presented. Experimental data is also presented validating the dose calculations. The design of small, lower cost ion accelerators, as embodied in "nested"-tandem and radio frequency quadrupole accelerators is examined, as is the potential use of high-output 3He and deuterium ion sources. Finally, potential clinical applications are discussed in terms of the advantages and disadvantages of this technique with respect to current radiotherapy methods and equipment. © 1995.

Intense, pulsed ion beams were used to melt and rapidly resolidify Types 316F, 316L and sensitized 304 stainless steel surfaces to eliminate the negative effects of microstructural heterogeneity on localized corrosion resistance. Anodic polarization curves determined for 316F and 316L showed that passive current densities were reduced and pitting potentials were increased due to ion beam treatment. Type 304 samples sensitized at 600°C for 100 h showed no evidence of grain boundary attack when surfaces were ion beam treated. Equivalent ion beam treatments were conducted with a 6061-T6 aluminum alloy. Electrochemical impedance experiments conducted with this alloy exposed to an aerated chloride solution showed that the onset of pitting was delayed compared to untreated control samples.

Inorganic polycrystalline hydrotalcite, Li2[Al2(OH)6]2·CO3·3H2O, coatings can be formed on aluminum and aluminum alloys by exposure to alkaline lithium carbonate solutions. This process is conducted using methods similar to traditional chromate conversion coating procedures, but does not use or produce toxic chemicals. The coating provides anodic protection and delays the onset of pitting during anodic polarization. Cathodic reactions are also inhibited which may also contribute to corrosion protection. Recent studies have shown that corrosion resistance can be increased by sealing hydrotalcite coated surfaces to transition metal salt solutions including Ce(NO3)3, KMnO4 and Li2MoO4. Results from these studies are also reported.

The Department of Energy's (DOE) non-nuclear facilities generally require only a qualitative accident analysis to assess facility risks in accordance with DOE Order 5481.1B, Safety Analysis and Review System [DOE, 1986]. Achieving a meaningful qualitative assessment of risk necessarily requires the use of suitable non-numerical assessment criteria. Typically, the methods and criteria for assigning facility specific accident scenarios to the qualitative severity and likelihood classification system in the DOE order requires significant judgement in many applications. Systematic methods for more consistently assigning the total accident scenario frequency and associated consequences are required to substantiate and enhance future risk ranking between various activities at Sandia National Laboratories (SNL). Currently, Sandia National Laboratories' Risk Management and National Environmental Policy Act (NEPA) department has initiated a project to develop improved criteria for performing qualitative risk assessments in accordance with the DOE order requirements. Products of this effort are an improved set of qualitative descriptions that permit: (1) definition of the severity for both technical and programmatic consequences that may result from a variety of accident scenarios and, (2) qualitative representation of the likelihood of occurrence. These sets of descriptions are intended to provide, in a qualitative manner, definitions that can be compared with the DOE criteria for assessing facility risks.

Water determination in semiconductor process gases is desirable in order to extend the life of gas delivery systems and improve wafer yields. We review our work in applying Fourier transform infrared spectroscopy to this problem, where a 10 ppb detection limit has been demonstrated for water in N2, HCl, and HBr. The potential for optical determination of other contaminants in these gases is discussed. Also, alternative optical spectroscopic approaches are briefly described. Finally, we discuss methods for dealing with interference arising from water in the instrument beam path, yet outside the sample cell.

Liquid properties are measured from the changes they induce in the resonant frequency and damping of thickness-shear mode quartz resonators. A smooth-surfaced resonator viscously entrains the contacting fluid and responds to the density-viscosity product. Separation of density and viscosity is accomplished using two devices: one with a smooth surface and one with a corrugated surface that traps fluid. By observing the difference in stored and dissipated energies in the contacting fluid, its non-Newtonian characteristics can also be determined.

The use of passive safety concepts to achieve very high levels of assured safety is described. Some background on safety implementations and safety assessment methodology are reviewed, the salient features of passive safety implementations are demonstrated, and some applications for which passive safety is particularly appropriate are suggested. Included are requirements on components and processes, and design principles. Critical agents (humans, stress, equipment, and timing) are some of the pertinent factors considered.

This paper reviews the evolution of polymer electrolytes from the conventional PEO-LiX salt complexes to the more conducting polyphosphazene and copolymers, gelled electrolytes etc. It also reviews the various chemical approaches including modifying PEO to synthesizing complicated polymer architecture. In addition, it discusses the effect of various lithium salts on the conductivity of PEO-based polymers. Charge/discharge and cycle life data of polymer cells containing oxide and chalcogenide cathodes and lithium (Li) anode will be reviewed. Finally, future research directions to improve the electrolyte properties will be presented.

Structure based on aluminum-oxide layers have led to dramatic improvements in VCSELs such as power conversion efficiencies in excess of 50% and threshold currents below 10μA. The low index, insulating aluminum-oxide, formed by selective wet thermal oxidation of AlGaAs, serves as an effective index guide as well as a current injection aperture. This paper presents data on devices with either two aligned apertures above and below the active region or with a single effective aperture above the active region leading to slope efficiencies of up to 1W/A.

A surface micromachined pressure sensor array has been designed and fabricated. The sensors are based upon deformable, silicon nitride diaphragms with polysilicon piezoresistors. Absolute pressure is detected by virtue of reference pressure cavities underneath the diaphragms. For this type of sensor, design tradeoffs must be made among allowable diaphragm deflection, diaphragm size, and desirable pressure ranges. Several fabrication issues were observed and addressed. Offset voltage, sensitivity, and nonlinearity of 100 μm diameter sensors were measured.

The effect of solvent addition on the phase separation, mechanical properties and thermal stability of silica/siloxane composite materials prepared by in situ reinforcement was examined. The addition of a solvent enhances the miscibility of the reinforcement precursor, a partial hydrolyzate of tetraethoxysilane (TEOS-PH), with the polydimethylsiloxane (PDMS) polymer. As a result, the phase separation at the micron level, termed the large-scale structure, diminished in size. This decrease in particle size resulting from the addition of moderate amounts of solvent was accompanied by an improvement in the mechanical properties. However, solvent addition in the excess of 50 weight percent led to a decrease in mechanical properties even though the large-scale structure continued to diminish in size. Small Angle X-Ray Scattering (SAXS) was used to examine the angstrom level or small-scale structure. This small-scale structure was only affected by the presence of solvent, not the amount. The silica/siloxane composite materials showed the same thermal transition temperatures as the original PDMS material.

The preparation of light emitting diodes employing a new class of materials, 5,10-dihetera-5,10-dihydro-indeno[3,2b]indenes, as hole transport agents is described. These materials have been found to be more resistant to degradation by singlet oxygen than a poly(p-phenylene vinylene) (PPV) derivative.

Extensions of the German LIGA process have brought about fabrication capability suitable for cost effective production of precision engineered components. The process attributes allow fabrication of mechanical components which are not capable of being made via conventional subtractive machining methods. Two process improvements have been responsible for this extended capability which involve the areas of thick photoresist application and planarization via precision lapping. Application of low-stress x-ray photoresist has been achieved using room temperature solvent bonding of a preformed photoresist sheet. Precision diamond lapping and polishing has provided a flexible process for the planarization of a wide variety of electroplated metals in the presence of photoresist. Exposure results from the 2.5 GeV National Synchrotron Light Source storage ring at Brookhaven National Laboratory have shown that structural heights of several millimeter and above are possible. The process capabilities are also well suited for microactuator fabrication. Linear and rotational magnetic microactuators have been constructed which use coil winding technology with LIGA fabricated coil forms. Actuator output forces of 1 milliNewton have been obtained with power dissipation on the order of milliWatts. A rotational microdynamometer system which is capable of measuring torque-speed data is also discussed.

Plasma processes for etching and desmear of electronic components and printed wiring boards (PWB) are difficult to predict and control. Non-uniformity of most plasma processes and sensitivity to environmental changes make it difficult to maintain process stability from day to day. To assure plasma process performance, weight loss coupons or post-plasma destructive testing must be used. The problem with these techniques is that they are not real-time methods and do not allow for immediate diagnosis and process correction. These tests often require scrapping some fraction of a batch to insure the integrity of the rest. Since these tests verify a successful cycle with post-plasma diagnostics, poor test results often determine that a batch is substandard and the resulting parts unusable. These tests are a costly part of the overall fabrication cost. A more efficient method of testing would allow for constant monitoring of plasma conditions and process control. Process anomalies should be detected and corrected before the parts being treated are damaged. Real time monitoring would allow for instantaneous corrections. Multiple site monitoring would allow for process mapping within one system or simultaneous monitoring of multiple systems. Optical emission spectroscopy conducted external to the plasma apparatus would allow for this sort of multifunctional analysis without perturbing the glow discharge. In this paper, optical emission spectroscopy for non-intrusive, in situ process control will be explored along with applications of this technique towards process control, failure analysis and endpoint determination.