Showing papers by "Oak Ridge National Laboratory published in 1991"

Factorial sampling plans for preliminary computational experiments

Abstract: A computational model is a representation of some physical or other system of interest, first expressed mathematically and then implemented in the form of a computer program; it may be viewed as a function of inputs that, when evaluated, produces outputs. Motivation for this article comes from computational models that are deterministic, complicated enough to make classical mathematical analysis impractical and that have a moderate-to-large number of inputs. The problem of designing computational experiments to determine which inputs have important effects on an output is considered. The proposed experimental plans are composed of individually randomized one-factor-at-a-time designs, and data analysis is based on the resulting random sample of observed elementary effects, those changes in an output due solely to changes in a particular input. Advantages of this approach include a lack of reliance on assumptions of relative sparsity of important inputs, monotonicity of outputs with respect to inputs, or ad...

Microstructural design of toughened ceramics

Abstract: On discute des mecanismes de renforcement des ceramiques par pontage des fissures a l'aide de trichites et d'autres types de renforcants discontinus. On montre que le renforcement par pontage des fissures peut etre combine avec d'autres mecaniques de renforcement (par exemple par transformation de phase) et que l'on peut obtenir des effets de synergie. La conception des materiaux renforces repose fortement sur le controle des proprietes des materiaux et des constituants microstructuraux

Fuel ethanol from cellulosic biomass.

Abstract: Ethanol produced from cellulosic biomass is examined as a large-scale transportation fuel. Desirable features include ethanol's fuel properties as well as benefits with respect to urban air quality, global climate change, balance of trade, and energy security. Energy balance, feedstock supply, and environmental impact considerations are not seen as significant barriers to the widespread use of fuel ethanol derived from cellulosic biomass. Conversion economics is the key obstacle to be overcome. In light of past progress and future prospects for research-driven improvements, a cost-competitive process appears possible in a decade.

Bayesian Prediction of Deterministic Functions, with Applications to the Design and Analysis of Computer Experiments

Abstract: This article is concerned with prediction of a function y(t) over a (multidimensional) domain T, given the function values at a set of “sites” {t (1), t (2), …, t (n)} in T, and with the design, that is, with the selection of those sites. The motivating application is the design and analysis of computer experiments, where t determines the input to a computer model of a physical or behavioral system, and y(t) is a response that is part of the output or is calculated from it. Following a Bayesian formulation, prior uncertainty about the function y is expressed by means of a random function Y, which is taken here to be a Gaussian stochastic process. The mean of the posterior process can be used as the prediction function ŷ(t), and the variance can be used as a measure of uncertainty. This kind of approach has been used previously in Bayesian interpolation and is strongly related to the kriging methods used in geostatistics. Here emphasis is placed on product linear and product cubic correlation func...

Gelcasting of Alumina

Abstract: Gelcasting is a novel method for molding ceramic powder based on a synthesis of concepts derived from traditional ceramics and polymer chemistry. Gelcasting of alumina is described in this paper. The process is based on the in situ polymerization of acrylamide monomers as the setting mechanism for forming the green body. It has the following features: slurries with high solids loading and low viscosity (1.8 Pa {center dot} s at 62 vol% solids), dried bodies containing less than 4 wt% binder, and the ability to fabricate complex-shaped bodies.

Locating protein-coding regions in human DNA sequences by a multiple sensor-neural network approach.

Abstract: Genes in higher eukaryotes may span tens or hundreds of kilobases with the protein-coding regions accounting for only a few percent of the total sequence. Identifying genes within large regions of uncharacterized DNA is a difficult undertaking and is currently the focus of many research efforts. We describe a reliable computational approach for locating protein-coding portions of genes in anonymous DNA sequence. Using a concept suggested by robotic environmental sensing, our method combines a set of sensor algorithms and a neural network to localize the coding regions. Several algorithms that report local characteristics of the DNA sequence, and therefore act as sensors, are also described. In its current configuration the "coding recognition module" identifies 90% of coding exons of length 100 bases or greater with less than one false positive coding exon indicated per five coding exons indicated. This is a significantly lower false positive rate than any method of which we are aware. This module demonstrates a method with general applicability to sequence-pattern recognition problems and is available for current research efforts.

A review of recent developments in Fe3Al-based alloys

Abstract: Fe3Al-based iron aluminides have been of interest for many years because of their excellent oxidation and sulfidation resistance. However, limited room temperature ductility (<5%) and a sharp drop in strength above 600 °C have limited their consideration for use as structural materials. Recent improvements in tensile properties, especially improvements in ductility produced through control of composition and microstructure, and advances in the understanding of environmental embrittlement in intermetallics, including iron aluminides, have resulted in renewed interest in this system for structural applications. The purpose of this paper is to summarize recent developments concerning Fe3Al-based aluminides, including alloy development efforts and environmental embrittlement studies. This report will concentrate on literature published since about 1980, and will review studies of fabrication, mechanical properties, and corrosion resistance that have been conducted since that time.

A review of arsenic (III) in groundwater

Abstract: Recent improvements in sample collection and analytical techniques have suggested that As(III) is more prevalent in groundwater than previously believed. Indeed, reducing conditions in alluvial aquifers supplying single families may result in significant exposures to naturally occurring As(III). These results are noteworthy because As(III) is both more toxic and more mobile in the environment than As(V). The literature contains contradictory information concerning the appropriate preservation and analytical techniques for determining As(III). It appears that several previously reported occurrences of As(V) may have been predominantly As(III), but the samples were either not preserved or analyzed properly. For example, separation of arsenic species by ion exchange is apparently necessary to obtain reliable analytical results for certain environmental samples. The problems encountered with investigating As(III) in the environment are due to the complex series of geochemical reactions undergone by a...

Alpha-decay event damage in zircon

Abstract: Based on density measurements, X-ray diffraction analysis, and high-resolution trans-mission electron microscopy of a suite of natural zircon samples from Sri Lanka (0.06 xl0r5 to 6.3 x l0rs a-decay events/mg), three stages of damage accumulation may bedelineated. Stage I ( 8 x 10'5 a-decayevents/mg) consists of material that is entirely aperiodic as far can be determined byX-ray or electron diffraction. There was no evidence for the formation of ZtO, or SiO, asfinal products during the last stage of metamictization. Based on rnodeled density changes,aperiodic regions continue to experience a change in structure as they are redamaged.During stage II of the process, the modeled density of aperiodic regions changes from 4.5g/crn3 lo 4.1 g/cm3. Fission fragment damage does not contribute to the process of meta-mictization. The amorphization process is consistent with a model for the multiple overlapof displacement cascades, suggesting amorphization occurs as a result of defect accumu-lation rather than directly within a single displacement cascade.Comparison of results for natural zircon with those for Pu-doped zircon showed thatdose-rate variations (even as great as a factor of 108) had no substantial effect on thedamage accumulation process. Unit-cell parameters increased and density decreased morefor the Pu-doped zircon than for natural zircon in early stages of damage accumulation(<3 x 10,5 a-decay events,/mg), suggesting that annealing of point defects in the earlystages of the damage accumulation process occurs in natural zircon under ambient con-ditions. This accounts for the distinct sigmoidal shape of the damage cnryes for naturalzircon and the apparent incubation period before onset ofamorphization.IxrnonucrroN ppm U and2to 2000 ppm Th (Ahrens etal.,1967;Gorz,Zircon (ZrSiOo, I4r/amd, Z : 4) is a common acces- 1974), it is one of the most important phases used insory mineral in crustal igneous, metamorphic, and sedi- U-Th-Pb dating techniques (Faure, 1977;JiLger and Hun-mentary rocks, as well as in lunar materials, meteorites, z1ke4 1979). Of particular interest are processes by whichand tektites. Because zircon typically contains 5 to 4000 the U-Th-Pb systematics are disturbed (e.g., Silver and0003-004x/9 1/09 l 0- l 5 I 0$02.00 l 5 r0

Hierarchical fuzzy control

Abstract: In a conventional rule based fuzzy control system, the rules are of the following form: if (condition) then (action), and all rules are essentially in a random order. The number of rules increases exponentially as the number of the system variables, on which the fuzzy rules are based, is increased. In this paper, the rules are structured in a hierarchical way so that the total number of rules will be a linear function of the system variables. The hierarchical fuzzy control algorithm developed in this paper is applied to control the feedwater flow to a steam generator of a power plant. The simulation results show that the hierarchical fuzzy controller yields superior performance over the conventional PID controller.

Above-surface neutralization of highly charged ions: The classical over-the-barrier model

Abstract: The neutralization of highly charged ions during interaction with metallic surfaces is accompanied by the ejection of a large number of secondary electrons Recent experiments demonstrate two main contributions to this electron ejection process: one from the region below the surface and the second from the above-surface portion of the trajectory We present a theoretical analysis of the neutralization dynamics above the surface, prior to impact, based on the classical over-the-barrier model The theory incorporates resonant multielectron capture of conduction electrons, resonant loss into unoccupied states of the conduction band, and intra-atomic Auger deexcitation The effective barrier potential includes quantum corrections to the classical image potential The effect of below-barrier (``tunneling'') transfer is investigated The solution of a coupled system of rate equations allows the approximate determination of the n-shell populations, the projectile charge state, and the total number of Auger electrons The calculation describes the transient formation of ``hollow'' atoms We find satisfactory agreement with recent data for K Auger yields by Meyer et al [Phys Rev Lett 67, 723 (1991)]

X-linked lymphoreticular disease in the scurfy (sf) mutant mouse.

Abstract: Scurfy (sf) is a spontaneous, sex-linked, recessive mutation that maps to the extreme proximal portion of the X chromosome, about 2 centimorgans from sparse fur (spf). Hemizygotes for sf manifest several clinical disorders, evident at 14 days of age, including scaliness and crusting of the eyelids, ears, and tail, runting, reddening and swelling of the genital papilla, anemia, cachexia, and early death (average, 24 days). Our studies indicate that the phenotype of hemizygous scurfy is not, as has been suggested, a model for human X-linked ichthyosis, but appears to be a disease primarily affecting the lymphoreticular, and possibly the hematopoietic, systems. Gross lesions include marked splenomegaly, hepatomegaly, enlarged lymph nodes, and variable thickening of the ears. The characteristic histologic lesion is a lymphohistiocytic proliferation and infiltration of peripheral lymph nodes, spleen, liver, and skin. In routine hematoxylin and eosin-stained sections, these lesions efface lymph node architecture, thicken the dermis, and form nodular portal infiltrates in the liver. Scurfy lesions characteristically contain a population of large blastlike cells with round to oval nuclei, a vesicular chromatin pattern, and prominent single nucleoli. Mixed perivascular infiltrates of lymphocytes, macrophages, and granulocytes sometimes are found in kidney, heart, pancreas, lung, and mesenteries. There is excessive hematopoiesis in the liver and spleen. Cells expressing B220 or Thy-1 antigens localize to appropriate areas in the lymph nodes and spleen, but are rare in the portal infiltrates and are absent from the skin. There is a marked, polyclonal increase in serum IgG, severe Coombs'-positive anemia, and leukocytosis with atypical mononuclear cells. Scurfy mice are negative for antinuclear antibodies. Despite their morphologically aberrant lymphoreticular system, scurfy mice can exist in a conventional environment without evidence of opportunistic infection. Raising scurfy mice in a specific-pathogen-free environment does not alter disease expression. Thus, while our findings indicate that scurfy disease may be the result of immune dysfunction, it is not a classic immunodeficiency.

Dynamic traffic assignment for urban road networks

Abstract: This paper presents a nonlinear programming formulation of the dynamic user-equilibrium assignment problem (DUE) for urban road networks with multiple trip origins and destinations. DUE is a temporal generalization of the static user-equilibrium assignment problem (SUE) with additional constraints to insure temporally continuous paths of flow. In DUE, the full assignment period of several hours is discretized into shorter time intervals of 10–15 minutes each for which trip departure matrices are assumed to be known. This formulation of DUE includes SUE as a special case in which there is only one time interval for the full assignment period. The assumption of steady-state flows allows SUE to have all linear constraints, but DUE requires nonlinear flow continuity constraints. Whereas SUE is typically solved by methods of linear combinations, these methods create temporally discontinuous flows if applied to DUE. A dynamic traffic assignment heuristic (DTA) is presented that generates approximate solutions to DUE in an efficient manner for large networks. DTA is not a convergent solution algorithm for DUE, but was designed instead to produce assignments that approximate the DUE optimality conditions. An overview of alternative dynamic assignment approaches is given, including the limitations of other optimization and simulation approaches. Test results presented in this paper show that DTA generates both static and dynamic assignments that approximately satisfy the user-equilibrium conditions of these problems.

Fish size and habitat depth relationships in headwater streams

Abstract: Surveys of 262 pools in 3 small streams in eastern Tennessee demonstrated a strong positive relationship between pool depth and the size of the largest fish within a pool (P 10.8 d in deep pools. In the striped shiner experiment in shallow artificial streamside troughs, no individuals 75–100 mm TL survived as long as 13 d, where-as smaller (20–25 mm) fish had 100% survival over 13 d. The results of the experiments show that predation risk from wading/diving animals (e.g., herons and raccoons) is much higher for larger fishes in shallow water than for these fishes in deeper water or for smaller fish in shallow water. We discuss the role of predation risk from two sources (piscivorous fish, which are more effective in deeper habitats, and diving/wading predators, which are more effective in shallow habitats) in contributing to the bigger fish — deeper habitat pattern in streams.

Vapor-phase fabrication and properties of continuous-filament ceramic composites.

Abstract: The continuous-filament ceramic composite is becoming recognized as necessary for new, high-temperature structural applications. Yet because of the susceptibility of the filaments to damage from traditional methods for the preparation of ceramics, vapor-phase infiltration has become the fabrication method of choice. The chemical vapor infiltration methods for producing these composites are now being studied in earnest, with the complexity of filament weaves and deposition chemistry being merged with standard heat and mass-transport relationships. Two of the most influential effects on the mechanical properties of these materials are the adhesion and frictional force between the fibers and the matrix, which can be controlled by a tailored interface coating. A variety of materials are available for producing these composites including carbide, nitride, boride, and oxide filaments and matrices. Silicon carbide-based materials are by far the most advanced and are already being used in aerospace applications.

Deformation twinning in h.c.p. metals and alloys

Abstract: The important role of deformation twinning in plastic deformation of h.c.p. metals and alloys is emphasized and the twin nucleation mechanisms are examined. A bifurcated homogeneous nucleation model is presented on the basis of the classical nucleation theory, the shape bifurcation theory and the elastic inclusion model. The activation energy for twin formation depends most sensitively on the twin-boundary energy. The structures and energies of (1122) and (1011) coherent twin boundaries are obtained by atomistic simulations using the Lennard-Jones potential for a model h.c.p. metal. The anisotropic coupling effect of normal stresses on twin disolocation mobility is determined over a wide temperature range (0-1200 K). The available experimental data on titanium and zirconium are consistent with the prediction from the proposed nucleation model and the temperature-dependent mobility of twin dislocations. Alloying effects on twinning and future research areas are discussed.

Ion Trap Mass Spectrometry

Abstract: This paper reports on the quadrupole ion trap which is a mass spectrometer whose essential components can be held in one hand. But is has a mass range of about 10{sup 5} daltons per charge, provides molecular weight and structural information on biopolymers, and has the greatest sensitivity of all mass spectrometers. These features, however, has become available only within the past few years. They stem from an almost neglected 1958 invention, one in which interest was maintained by only a few research groups, notably those of John Todd at the University of Kent in England and Ray March at Trent University in Canada. Development of a new scanning method by George Stafford and his coworkers of Finnigan Corp. provided the impetus that led Finnigan to introduce a commercial ion trap in 1983. Since then, the device has been transformed from a simple gas chromatography detector to a high-performance mass spectrometer.

Small Mammals as Monitors of Environmental Contaminants

Abstract: Many potentially harmful chemicals, both natural and anthropogenic, are released to the environment every year. Although chemical analyses of soil, air, and water can provide information on the concentrations of specific compounds present, these analyses alone are inadequate to assess the availability and potential toxicity of contaminants to humans and wildlife. Animals in these environments, however, integrate contaminant exposure spatially, temporally, and across media. Therefore, mammalian body burdens and responses are uniquely realistic indicators of mammalian exposure to chemicals.

Benzo-a-Pyrene: Environmental Partitioning and Human Exposure:

Abstract: A multimedia transport model was used to evaluate the environmental partitioning of benzo-a-pyrene (BaP). Measured and predicted environmental concentrations were used to estimate the accumulation of BaP in the food chain and the subsequent extent of human exposure from inhalation and ingestion. Results show that BaP partitions mainly into soil (82%) and sediment (17%) and that the food chain is the dominant pathway of human exposure, accounting for about 97% of the total daily intake of BaP. Inhalation and consumption of contaminated water are only minor pathways of human exposure. The long-term average daily intake of BaP by the general population of the U.S. is estimated to be 2.2 micrograms (micrograms) per day. Cigarette smoking and indoor activities do not substantially increase human exposure to BaP relative to exposures to background levels of BaP present in the environment. Since the increased lifetime risk associated with human exposure to background levels of BaP is 3.5 {times} 10(-4), the authors conclude that ingestion of food items contaminated with BaP may pose a serious health threat to the U.S. population.72 references.

New evidence for a pressure-induced phase transformation during the indentation of silicon

Abstract: Scanning electron micrographs of indents in (111) silicon reveal that a thin layer of material immediately adjacent to the indenter is plastically extruded. The fact that the material can be deformed in this way indicates that it has metallic-like mechanical properties. This is presented as new evidence that a pressure-induced phase transformation to the metallic state occurs during the indentation of silicon.

Structure and phases of the Au(111) surface: X-ray-scattering measurements

Abstract: The results of a comprehensive synchrotron x-ray-scattering study of the clean, reconstructed Au(111) surface between 300 and 1250 K are presented. Two surface phases are identified and characterized. For T865 K, the reconstruction consists of a series of discommensurations, separating surface regions with the correct face-centered-cubic (fcc) ABC stacking sequence from regions with a faulted ABA stacking sequence. A key feature of this phase is the existence of an equilibrium density of kinks between rotationally equivalent domains. At 865 K, the reconstruction starts to lose long-range order. The first-order transformation to a disordered phase is complete by 880 K. In this phase, the topmost layer has hexagonal symmetry and is on the average isotropically compressed compared with bulk (111) planes. Between 880 and 1250 K the translational correlation length and the average compression of the overlayer evolve only slightly. Interpretation of the x-ray reflectivity, both specular and nonspecular, establishes, however, that the occupation of fcc C sites and faulted A sites is approximately constant throughout the temperature range studied. In addition, the separation between the top two layers is expanded 3% relative to the bulk (111) interlayer spacing and the surface-normal vibrational amplitude in the surface layers is enhanced with respect to the bulk value.

Low voltage Ohmic and electron cyclotron heating assisted startup in DIII-D

Abstract: There is considerable interest in the development of low voltage startup scenarios for large tokamaks since it is proposed that in ITER the electric field which will be applied for ionization and plasma current ramp-up will be limited to values of E ≤ 0.3 V/m. Studies of low voltage startup have been carried out in DIII-D with and without electron cyclotron preionization and preheating. Successful Ohmic startup has been achieved with E ~ 0.25 V/m by paying careful attention to error fields and prefill pressure, while electron cyclotron heating (ECH) assisted startup with E ~ 0.15 V/m has been demonstrated. ECH assisted startup gives improved reliability at such low electric fields and permits operation over an extended range of prefill pressures and error magnetic fields. Using ECH, startup at E = 0.3 V/m with |B⊥| > 50 G over most of the vessel cross-section has been demonstrated. Such an error field represents an increase by more than a factor of two over the highest value for which Ohmic startup was achieved at the same electric field. During low voltage Ohmic startup with extreme values of prefill pressure and/or error magnetic fields, excessive breakdown delays are observed. The experimental data agree well with theoretical predictions based on the Townsend avalanche theory. ECH assisted startup is always prompt. The primary effect of ECH during the plasma current ramp-up is a decrease of the resistive component of the loop voltage Vrcs. A significant reduction (~30%) in Vres is achieved for low ECH powers (PRF ~ 300-400 kW), but a further large increase in PRF results in only a modest additional decrease in Vres. ECH was not applied over the whole ramp-up phase in these experiments and produced a reduction in volt-second consumption up to the current flat-top (Ip ~ 1 MA) of 10%. These experiments confirm that the low electric fields specified in the ITER design are acceptable and demonstrate the substantial benefits which accrue from the use of ECH assisted startup.

Structural origin of reduced critical currents at YBa2Cu3O7–δ grain boundaries

Abstract: THE critical current density across individual grain boundaries in thin films of the high-Tc superconductor YBa2Cu3O7–δ (YBCO) has been found1–4 to be inversely proportional to lattice misorientation for tilts up to ∼10°. Reports of impurity segregation5,6 at grain boundaries, and variations in the chemical stoichiometry7,8, have led to the view that deviations from the ideal composition are responsible for the depressed superconducting order parameter at the boundary. Here we present images of YBCO grain boundaries obtained by a scanning transmission electron microscope in Z-contrast mode9,10, which show that chemical segregation does not necessarily occur at these boundaries. A simple model of the strain associated with the grain-boundary dislocations provides a reasonable physical explanation of the suppressed superconductivity. The surprisingly large effect of strain implied by our model has implications beyond critical currents, for the physics and applications of any thin-film YBCO structures involving strained epitaxial layers.

Carbon–Nitrogen Pyrolyzates: Attempted Preparation of Carbon Nitride

Abstract: Precursors for the preparation of carbon nitride were selected from a number of relatively high nitrogen content organic materials. The precursors were pyrolyzed in a closed system at 700°C under 225 MPa. The resulting residues showed little nitrogen loss, but were amorphous with relatively low densities. Raman spectroscopy and 13C NMR indicate the presence of trigonal carbon.