Monte Carlo simulations of phosphate tetrahedron connectivity distributions in alkali and alkaline earth phosphate glasses are reported. By utilizing a discrete bond model, the distribution of next-nearest neighbor connectivities between phosphate polyhedron for random, alternating and clustering bonding scenarios was evaluated as a function of the relative bond energy difference. The simulated distributions are compared to experimentally observed connectivities reported for solid-state two-dimensional exchange and double-quantum NMR experiments of phosphate glasses. These Monte Carlo simulations demonstrate that the polyhedron connectivity is best described by a random distribution in lithium phosphate and calcium phosphate glasses.

This paper discusses the results obtained in an ongoing study geared towards developing advanced mixed-metal oxide sorbents for desulfurization of coal-derived fuel gases in the temperature range of 350 to 550{degrees}C. The paper focuses on the study related to the development of durable sorbents suitable for fluidized-bed application and addresses thermodynamic considerations, sulfidation kinetics, regenerability, and the physical and chemical characteristics of a number of novel sorbents.

Los Alamos National Laboratory is establishing a research and processing capability for beryllium. The unique properties of beryllium, including light weight, rigidity, thermal conductivity, heat capacity, and nuclear properties make it critical to a number of US defense and aerospace programs. Concomitant with the unique engineering properties are the health hazards associated with processing beryllium in a particulate form and the potential for worker inhalation of aerosolized beryllium. Beryllium has the lowest airborne standard for worker protection compared to all other nonradioactive metals by more than an order of magnitude. This paper describes the design characteristics of the new beryllium facility at Los Alamos as they relate to protection of the workforce. Design characteristics to be reviewed include; facility layout, support systems to minimize aerosol exposure and spread, and detailed review of the ventilation system design for general room air cleanliness and extraction of particulate at the source.

The light ion impurities C, 0 and H have been implanted or diffused into GaN and related compounds and their effect on the electrical properties of these materials measured by Hall, C-V and SIMS as a function of annealing temperatures from 300--11OO{degree}C. While C in as-grown GaN appears to create an acceptor under MOMBE conditions, implanted C shows no measurable activity. Similarly, implanted 0 does not show any shallow donor activity after annealing at {le}700{degree}C, but can create high resistivity regions (10{sup 6} {Omega}/{open_square}) in GaN, AlInN and InGaN for device isolation when annealed at 500--70O{degree}C. Finally, hydrogen is found to passivate shallow donor and acceptor states in GaN, InN. InAlN and InGaN, with dissociation of the neutral complexes at >450{degree}C. The liberated hydrogen does not leave the nitride films until much higher annealing temperatures (>800{degree}C). Typical reactivation energies are {approximately}2.0 eV for impurity-hydrogen complexes.

BC13, with addition of Nz, Ar or Hz, is found to provide smooth anisotropic pattern transfer in GaAs, GaN, GaP, GaSb and AIGriAs under Inductively Coupled Plasma conditions, Maxima in the etch rates for these materials are observed at 33% N2 or 87$'40 Hz (by flow) addition to BC13, whereas Ar addition does not show this behavior. Maximum etch rates are typically much higher for GaAs, Gap, GaSb and AIGaAs (-1,2 @rein) than for GaN (-0.3 ymu'min) due to the higher bond energies of the iatter. The rates decrease at higher pressure, saturate with source power (ion flux) and tend to show maxima with chuck power (ion energy). The etched surfaces remain stoichiometric over abroad range of plasma conditions.

A parametric study of the etch characteristics of GaN, AIN and InN has been earned out with IC1/Ar and IBr/Ar chemistries in an Inductively Coupled Plasma discharge. The etch rates of InN and AIN were relatively independent of plasma composition, while GaN showed increased etch rates with interhalogen concentration. Etch rates for all materials increased with increasing rf chuck power, indicating that higher ion bombardment energies are more efficient in enhancing sputter resorption of etch products. The etch rates increased for source powers up to 500 W and remained relatively thereafter for all materials, while GaN and InN showed maximum etch rates with increasing pressure. The etched GaN showed extremely smooth surfaces, which were somewhat better with IBr/Ar than with IC1/Ar. Maximum selectivities of- 14 for InN over GaN and >25 for InN over AIN were obtained with both chemistries.

The role of the inert gas additive (He, Ar, Xe) to C12 Inductively Coupled Plasmas for dry etching of GaAs and GaSb was examined through the effect on etch rate, surface roughness and near-surface stoichiometry. The etch rates for both materials go through a maximum with Clz 0/0 in each type of discharge (C12/'He, C12/Ar, C12/Xc), reflecting the need to have efficient ion-assisted resorption of the etch products. Etch yields initially increase strongly with source power as the chlorine neutral density increases, but decrease again at high powers as the etching becomes reactant-limited. The etched surfaces are generally smoother with Ax or Xe addition, and maintain their stoichiometry.

The effects of the additive noble gases He, Ar and Xe on chlorine-based Inductively Coupled Plasma etching of InP, InSb, InGaP and InGaAs were studied as a function of source power, chuck power and discharge composition. The etch rates of all materials with C12/He and C12/Xe are greater than with C12/Ar. Etch rates in excess of 4.8 pndmin for InP and InSb with C12/He or C12/Xe, 0.9 pndmin for InGaP with C12/Xe, and 3.8 prdmin for InGaAs with Clz/Xe were obtained at 750 W ICP power, 250 W rf power, - 1570 C12 and 5 mTorr. All three plasma chemistries produced smooth morphologies for the etched InGaP surfaces, while the etched surface of InP showed rough morphology under all conditions.

A parametric study of etch rates and surface morphologies of In-containing compound semiconductors (InP, InGaAs, InGaAsP, InAs and AlInAs) obtained by BClj-based Inductively Coupled Plasmas is reported. Etch rates in the range 1,500-3,000 &min. are obtained for all the materials at moderate source powers (500 W), with the rates being a strong function of discharge composition, rf chuck power and pressure. Typical root-mean-square surface roughness of-5 nm were obtained for InP, which is worse than the values obtained for Ga-based materials under the same conditions (-1 run). The near surface of etched samples is typically slightly deficient in the group V element, but the depth of this deficiency is small (a few tens of angstroms).

Patterning the group-IH nitrides has been challenging due to their strong bond energies and relatively inert chemical nature as compared to other compound semiconductors. Plasma etch processes have been used almost exclusively to pattern these films. The use of high-density plasma etch systems, including inductively coupled plasmas (ICP), has resulted in relatively high etch rates (often greater than 1.0 pmhnin) with anisotropic profiles and smooth etch morphologies. However, the etch mechanism is often dominated by high ion bombardment energies which can minimize etch selectivity. The use of an ICP-generated BCl~/C12 pkyma has yielded a highly versatile GaN etch process with rates ranging from 100 to 8000 A/rnin making this plasma chemistry a prime candidate for optimization of etch selectivity. In this study, we will report ICP etch rates and selectivities for GaN, AIN, and InN as a function of BCl~/Clz flow ratios, cathode rf-power, and ICP-source power. GaN:InN and GaN:AIN etch selectivities were typically less than 7:1 and showed the strongest dependence on flow ratio. This trend maybe attributed to faster GaN etch rates observed at higher concentrations of atomic Cl which was monitored using optical emission spectroscopy (OES). ~E~~~~f:~ INTRODUCTION DEC j 4898 Etch selectivi

Proton implantation in GaN is found to reduce the free carrier density through two mechanisms - first, by creating electron and hole traps at around Ec-0.8eV and Ev+0.9eV that lead to compensation in both n- and p-type material, and second, by leading to formation of (AH)O complexes, where A is any acceptor (Mg, Ca, Zn, Be, Cd). The former mechanism is usefid in creating high resistivity regions for device isolation, whereas the latter produces unintentional acceptor passivation that is detrimental to device performance. The strong affinity of hydrogen for acceptors leads to markedly different redistribution behavior for implanted in n- and p-GaN due to the chemical reaction to form neutral complexes in the latter. The acceptors may be reactivated by simple annealing at 2600{degrees}C, or by electron injection at 25-150{degrees}C that produces debonding of the (AH) centers. Implanted hydrogen is also strongly attracted to regions of strain in heterostructure samples during annealing, leading to pile-up at epi-epi and epi-substrate interfaces. II? spectroscopy shows that implanted hydrogen also decorates VG, defects in undoped and n-GaN.

Stainless steel-zirconium (SS-Zr) alloys are being considered as waste forms for the disposition of metallic waste generated during the electrometallurgical treatment of spent nuclear fuel. The waste forms contain irradiated cladding hulls, components of the alloy fuel, noble metal fission products, and actinide elements. The baseline waste form is a stainless steel-15 wt% zirconium (SS-15Zr) alloy. This article presents microstructure and some of the corrosion studies being conducted on the waste form alloys. Electrochemical corrosion, immersion corrosion, and vapor hydration tests have been performed on various alloy compositions to evaluate corrosion behavior and resistance to selective leaching of simulated fission products. The SS-Zr waste forms are successful at the immobilization and retention of fission products and show potential for acceptance as high-level nuclear waste forms.

Electrochemical corrosion tests have been conducted on simulated stainless steel-zirconium (SS-Zr) metal waste form (MWF) samples. The uniform aqueous corrosion behavior of the samples in various test solutions was measured by the polarization resistance technique. The data show that the MWF corrosion rates are very low in groundwaters representative of the proposed Yucca Mountain repository. Galvanic corrosion measurements were also conducted on MWF samples that were coupled to an alloy that has been proposed for the inner lining of the high-level nuclear waste container. The experiments show that the steady-state galvanic corrosion currents are small. Galvanic corrosion will, hence, not be an important mechanism of radionuclide release from the MWF alloys.

On June 12, 1992, the U. S. Department of Energy Oak Ridge Operations Office and the U. S. Environmental Protection Agency (EPA) Region IV signed a Federal Facility Compliance Agreement (FFCA) to regulate the treatment of wastes governed by the Land Disposal Restrictions (LDR) of the Resource Conservation and Recovery Act (RCRA). Compliance Requirement 5 of the agreement states that `. . . DOE shall submit to EPA for review and approval a plan for the treatment of the LDR prohibited wastes identified in Appendices 1B, 2B, and 3B. This plan must identify the treatment strategy for such wastes to meet LDR treatment standards and must include a schedule, not to exceed two (2) years after the submittal of this plan (i.e., March 1995), for the evaluation and prioritization of treatment method options, treatability studies, if required, and technology development. The FFCA divided the mixed wastes currently stored on the Oak Ridge Reservation (ORR) into two categories. Appendix A listed those wastes for which existing treatment methods and facilities exist. Appendix B listed wastes for which no identified treatment methods or facilities exist on the ORR.

Rectangular trench profiles are modeled with analytic etch rates determined from measured ion distribution functions. The pattern transfer step for this plasma etch is for trilayer lithography. Argon and chlorine angular ion energy distribution functions measured by a spherical collector ring analyzer are fit to a sum of drifting Maxwellian velocity distribution functions with anisotropic temperatures. The fit of the model ion distribution functions by a simulated annealing optimization procedure converges adequately for only two drifting Maxwellians. The etch rates are proportional to analytic expressions for the ion energy flux. Numerical computation of the etch profiles by integration of the characteristic equations for profile points and connection of the profiles points is efficient.

The coordinated use of geographically distributed computers, or metacomputing, can in principle provide more accessible and cost- effective supercomputing than conventional high-performance systems. However, we lack evidence that metacomputing systems can be made easily usable, or that there exist large numbers of applications able to exploit metacomputing resources. In this paper, we present work that addresses both these concerns. The basis for this work is a system called Nimrod that provides a desktop problem-solving environment for parametric experiments. We describe how Nimrod has been extended to support the scheduling of computational resources located in a wide-area environment, and report on an experiment in which Nimrod was used to schedule a large parametric study across the Australian Internet. The experiment provided both new scientific results and insights into Nimrod capabilities. We relate the results of this experiment to lessons learned from the I-WAY distributed computing experiment, and draw conclusions as to how Nimrod and I-WAY- like computing environments should be developed to support desktop metacomputing.

A model of superconductivity in layered high-temperature superconducting cuprates is proposed, based on the extended saddle point singularities in the electron spectrum, weak screening of the Coulomb interaction and phonon-mediated interaction between electrons plus a small short-range repulsion of Hund's, or spin-fluctuation, origin. This permits to explain the large values of {Tc}, features of the isotope effect on oxygen and copper, the existence of two types of the order parameter, the peak in the inelastic neutron scattering, the positive curvature of the upper critical field, as function of temperature etc.

This paper describes the approach and result of an assessment of health risks associated with long-term exposure to ozone. The health endpoint of interest is the probability of formation of mild lesions in the centriacinar region of the lung among children living in New York City. The risk model incorporates an exposure model and a health model. The exposure model is preliminary results of the probabilistic NAAQS Exposure Model (P-NEM) for ozone, and the health model is the judgments of active researchers about the likelihood of formation of ozone-induced lesions in the human lung. Children and New York City were chosen as the population and city of interest because it is believed that children are more sensitive to ozone than any other group of people, and New York City is more representative of other urban areas than Los Angeles, the other city of which P-NEM exposure results are available. Risk results are presented for ten exposure distributions generated by P-NEM, two air quality scenarios (``as-is`` and ``attainment``), and two exposure periods (1 and 10 ozone seasons). The results vary across experts, are not very sensitive to variations in P-NEM exposure distributions, are lower for attainment conditions than as-is conditions, and …

This paper deals with background and practical experience with preconditioned gradient methods for sparse linear systems for `very large` structural problems. The conjugate gradient method with diagonal preconditioning (CG/D) is demonstrated to substantially increase the size of structural problems that can be analyzed, significantly reduce computer storage requirements, and cut computing cost; thus allowing for much more detailed modeling and increased engineering efficiency. For one case for a structural system with 396,087 unknowns, the conjugate gradient method with diagonal preconditioning is demonstrated to be a factor of sixty faster than the direct method. For certain problems, however, the number of iterations required by the CG/D method is excessive and improved methods are needed. A stand-alone iterative solver research computer program was developed to evaluate the merits of various matrix preconditioners. A matrix preconditoner based on a shifted incomplete Cholesky factorization algorithm was demonstrated to be superior to other choices. The stand-alone program incorporates an effective data management strategy which utilizes disk and solid state auxiliary computer storage devices to make it possible to efficiently solve excessively large structural problems on state-of-the-art vector and parallel computers. The background of gradient methods, algorithms for their implementation, and practical experience in their applications …

In this paper, a time-dependent engine model is used for predicting hydrogen engine efficiency and emissions. The model uses basic thermodynamic equations for the compression and expansion processes, along with an empirical correlation for heat transfer, to predict engine indicated efficiency. A friction correlation and a supercharger/turbocharger model are then used to calculate brake thermal efficiency. The model is validated with many experimental points obtained in a recent evaluation of a hydrogen research engine. A The validated engine model is then used to calculate fuel economy and emissions for three hydrogen-fueled vehicles: a conventional, a parallel hybrid, and a series hybrid. All vehicles use liquid hydrogen as a fuel. The hybrid vehicles use a flywheel for energy storage. Comparable ultra capacitor or battery energy storage performance would give similar results. This paper analyzes the engine and flywheel sizing requirements for obtaining a desired level of performance. The results indicate that hydrogen lean-burn spark-ignited engines can provide a high fuel economy and Equivalent Zero Emission Vehicle (EZEV) levels in the three vehicle configurations being analyzed.

This paper describes a hybrid vehicle simulation model which can be applied to many of the vehicles currently being considered for low pollution and high fuel economy. The code operates in batch mode with all the vehicle information stored in data files. The code calculates power train dimensions, fuel economy for three driving schedules, time for 0-96 km/h at maximum acceleration, hill climbing performance, and pollution generation rates. This paper also documents the application of the code to a hybrid vehicle that utilizes a hydrogen internal combustion engine. The simulation model is used for parametric studies of the vehicle. The results show the fuel economy of the vehicle as a function of vehicle mass, aerodynamic drag, engine efficiency, accessory load, and flywheel efficiency. The code also calculates the minimum flywheel energy and power to obtain a desired performance. The hydrogen hybrid vehicle analyzed in the paper has a range of 480 km (300 miles), with a predicted gasoline equivalent fuel efficiency of 33.7 km/liter (79.3 mpg).

Flywheels have the possibility of providing high turnaround efficiency and high specific power output. These characteristics are very important for the successful manufacture of parallel and series hybrid vehicles, which have the potential for providing high fuel economy and very low emissions with range and performance comparable to today`s light-duty vehicles. Flywheels have a high specific power output, but relatively low specific energy output. Therefore, it is of importance to determine energy and power requirements for flywheels applied to light-duty vehicles. Vehicle applications that require an energy storage system with high power and low energy are likely to benefit from a flywheel. In this paper, a vehicle simulation code and a flywheel model are applied to the calculation of optimum flywheel energy storage capacity for a parallel and a series hybrid vehicle. A conventional vehicle is also evaluated as a base-case, to provide an indication of the fuel economy gains that can be obtained with flywheel hybrid vehicles. The results of the analysis indicate that the optimum flywheel energy storage capacity is relatively small. This results in a low weight unit that has a significant power output and high efficiency. Emissions generated by the hybrid vehicles are not calculated, but …

Enclosures for confinement of special nuclear materials (SNM) have evolved into the design of gloveboxes. During the early stages of glovebox technology, established practices and process operation requirements defined design criteria. Proven boxes that performed and met or exceeded process requirements in one group or area, often could not be duplicated in other areas or processes, and till achieve the same success. Changes in materials, fabrication and installation methods often only met immediate design criteria. Standardization of design criteria took a big step during creation of ``Special-Nuclear Materials R and D Laboratory Project, Glovebox standards``. The standards defined design criteria for every type of process equipment in its most general form. Los Alamos National Laboratory (LANL) then and now has had great success with Pu-238 processing. However with ever changing Environment Safety and Health (ES and H) requirements and Ta-55 Facility Configuration Management, current design criteria are forced to explore alternative methods of glovebox design fabrication and installation. Pu-238 fuel processing operations in the Power Source Technologies Group have pushed the limitations of current design criteria. More than half of Pu-238 gloveboxes are being retrofitted or replaced to perform the specific fuel process operations. Pu-238 glovebox design criteria are headed …

This article provides comments on "Fluorescent Probe Studies on the Microstructure of Polystyrene-Poly (vinylpyridine) Diblock Copolymer Film," published in 'Macromolecules,' 1992.