Showing papers by "Oak Ridge National Laboratory published in 1988"

Indices of landscape pattern

Abstract: Landscape ecology deals with the patterning of ecosystems in space. Methods are needed to quantify aspects of spatial pattern that can be correlated with ecological processes. The present paper develops three indices of pattern derived from information theory and fractal geometry. Using digitized maps, the indices are calculated for 94 quadrangles covering most of the eastern United States. The indices are shown to be reasonably independent of each other and to capture major features of landscape pattern. One of the indices, the fractal dimension, is shown to be correlated with the degree of human manipulation of the landscape.

Bayesian Variable Selection in Linear Regression

Abstract: This article is concerned with the selection of subsets of predictor variables in a linear regression model for the prediction of a dependent variable. It is based on a Bayesian approach, intended to be as objective as possible. A probability distribution is first assigned to the dependent variable through the specification of a family of prior distributions for the unknown parameters in the regression model. The method is not fully Bayesian, however, because the ultimate choice of prior distribution from this family is affected by the data. It is assumed that the predictors represent distinct observables; the corresponding regression coefficients are assigned independent prior distributions. For each regression coefficient subject to deletion from the model, the prior distribution is a mixture of a point mass at 0 and a diffuse uniform distribution elsewhere, that is, a “spike and slab” distribution. The random error component is assigned a normal distribution with mean 0 and standard deviation ...

Band-gap tailoring of ZnO by means of heavy Al doping

Abstract: Films of ZnO:Al were produced by weakly reactive dual-target magnetron sputtering. Optical band gaps, evaluated from spectrophotometric data, were widened in proportion to the Al doping. The widening could be quantitatively reconciled with an effective-mass model for n-doped semiconductors, provided the polar character of ZnO was accounted for.

A new look at density limits in tokamaks

Abstract: While the results of early work on the density limit in tokamaks from the ORMAK and DITE groups have been useful over the years, results from recent experiments and the requirements for extrapolation to future experiments have prompted a new look at this subject. There are many physical processes which limit the attainable densities in tokamak plasmas. These processes include: (1) radiation from low Z impurities, convection, charge exchange and other losses at the plasma edge; (2) radiation from low or high Z impurities in the plasma core; (3) deterioration of particle confinement in the plasma core; and (4) inadequate fuelling, often exacerbated by strong pumping by walls, limiters or divertors. Depending upon the circumstances, any of these processes may dominate and determine a density limit. In general, these mechanisms do not show the same dependence on plasma parameters. The multiplicity of processes leading to density limits with a variety of scaling has led to some confusion when comparing density limits for different machines. The authors attempt to sort out the various limits and to extend the scaling law for one of them to include the important effects of plasma shaping, i.e. ;e = k, where ne is the line average electron density (1020 m−3), κ is the plasma elongation and (MAm−2) is the average plasma current density, defined as the total current divided by the plasma cross-sectional area. In a sense, this is the most important density limit since, together with the q-limit, it yields the maximum operating density for a tokamak plasma. It is shown that this limit may be caused by a dramatic deterioration in core particle confinement occurring as the density limit boundary is approached. This mechanism can help explain the disruptions and Marfes that are associated with the density limit.

Chemically sensitive structure-imaging with a scanning transmission electron microscope

Abstract: Conventional high-resolution electron microscopy uses the phase-contrast method1, in which the diffracted beams emerging from the sample are recombined on the viewing screen of the microscope. The resultant contrast depends on the relative phases of the diffracted beams, which is sensitive to microscope and sample parameters, so that images must be interpreted by means of simulation, and defect models are somewhat empirical. By using a high-angle detector in a scanning transmission electron microscope, these problems may be avoided and high atomic-number contrast may be obtained. Here we present results of this technique applied to single crystals of the high-transition-temperature superconductors YBa2Cu3O7–x and ErBa2Cu3O7–x. The heavy-atom planes are directly imaged as bright lines, and the probable structure of an observed defect is directly inferred from its image.

Response of northern forests to CO2-induced climate change

Abstract: Climate changes resulting from increases in atmospheric CO2 are expected to alter forest productivity and species distributions. But forest response to climate change depends in part on changes in soil water and nitrogen availability which limit tree growth. Here we report an investigation into the possible responses of northeastern North American forests to a warmer and generally drier climate by driving a linked forest productivity/soil process model with climate model predictions corresponding to a doubling of CO2. The greatest changes occurred at the current boreal/cool temperate forest border. Simulated productivity and biomass increased on soils that retained adequate water for tree growth and decreased on soils with inadequate water. Simulated changes in vegetation composition altered soil nitrogen availability, which in turn amplified the vegetation changes. The simulated responses of the forests were results of a positive feedback between carbon and nitrogen cycles, bounded by negative constraints of soil moisture availability and temperature.

Stability of Aqueous α-Al2O3 Suspensions with Poly(methacrylic acid) Polyelectrolyte

Abstract: Stability of aqueous α-Al2O3 suspensions with Na+ salt of poly(methacrylic acid) (PMAA-Na) polyelectrolyte was studied as a function of pH. At a given pH, the transition from the flocculated to the dispersed state corresponded to the adsorption saturation limit of the powders by the PMAA. As the pH was decreased, the adsorption saturation limit increased until insolubility and charge neutralization of the PMAA was approached. The critical amount of PMAA required to achieve stability is outlined in a stability map.

Toughening Behavior in Whisker-Reinforced Ceramic Matrix Composites

Abstract: The toughening behavior of whisker-reinforced ceramics is analyzed in terms of a whisker bridging zone immediately behind the crack tip. This approach is consistent with microscopy observations which reveal that intact bridging whiskers exist behind the crack tip as a result of debonding of the whisker-matrix interface. The theoretical results based on both the stress intensity and the energy change introduced by bridging whiskers reveal the dependence of toughening upon the composition and matrix, interface, and whisker properties. Furthermore, the analytical models of whisker bridging accompanied by very limited pullout accurately describe experimental observations of the toughening behavior in several SiC-whisker-reinforced ceramics. Such analytical descriptions also indicate that increases in whisker size and strength and modification of interface properties will result in further increases in toughness by whisker reinforcement.

Changes in landscape patterns in Georgia, USA

Abstract: The objectives of this study were to determine how landscape patterns in Georgia, USA have changed through time and whether the spatial patterns varied by physiographic region. Historical aerial photography was used to analyze spatial patterns of land use from the 1930's to the 1980's. Land use patterns were quantified by: (1) mean number and size of patches; (2) fractal dimension of patches; (3) amount of edge between land uses; and (4) indices of diversity, dominance, and contagion. Forest cover increased in aerial extent and in mean patch size. The mean size of agricultural patches increased in the coastal plain and decreased in the mountains and piedmont. Edges between land uses decreased through time, indicating less dissection of the landscape. Fractal dimensions also decreased, indicating simpler patch shapes. Indices of diversity and dominance differed through time but not among regions; the contagion index differed among regions but not through time. A geographic trend of decreasing diversity and increasing dominance and contagion was observed from the mountains to the lower coastal plain. Landscape patterns exhibited the greatest changes in the piedmont region. Overall, the Georgia landscape has become less fragmented and more connected during the past 50 years. Changing patterns in the landscape may have implications for many ecological processes and resources.

Current status of cytogenetic procedures to detect and quantify previous exposures to radiation.

Abstract: The estimation of the magnitude of a dose of ionizing radiation to which an individual has been exposed (or of the plausibility of an alleged exposure) from chromosomal aberration frequencies determined in peripheral blood lymphocyte cultures is a well-established methodology, having first been employed over 25 years ago. The cytogenetics working group has reviewed the accumulated data and the possible applicability of the technique to the determination of radiation doses to which American veterans might have been exposed as participants in nuclear weapons tests in the continental U.S.A. or the Pacific Atolls during the late 1940s and 1950s or as members of the Occupation Forces entering Hiroshima or Nagasaki shortly after the nuclear detonations there. The working group believes that with prompt peripheral blood sampling, external doses to individuals of the order of about 10 rad (or less if the exposure was to high-LET radiation) can accurately be detected and measured. It also believes that exposures of populations to doses of the order of maximum permissible occupational exposures can also be detected (but only in populations; not in an individual). Large exposures of populations can also be detected even several decades after their exposure, but only in the case of populations, and of large doses (of the order of 100 to several hundred rad). The working group does not believe that cytogenetic measurements can detect internal doses from fallout radionuclides in individuals unless these are very large. The working group has approached the problem of detection of small doses (≤; 10 or so rad) sampled decades after the exposure of individuals by using a Bayesian statistical approach. Only a preliminary evaluation of this approach was possible, but it is clear that it could provide a formal statement of the likelihood that any given observation of a particular number of chromosomal aberrations in a sample of any particular number of lymphocytes actually indicates an exposure to any given dose of radiation. It is also clear that aberration frequencies (and consequently doses) would have to be quite high before much confidence could be given to either exposure or dose estimation by this method, given the approximately 3 decades of elapsed time between the exposures and any future blood sampling. Additional research on the problem is clearly needed, but at the moment it appears unlikely that determination of chromosomal aberration frequencies in peripheral blood lymphocytes will prove a useful method of determining ionizing radiation doses to individual veterans (though it might prove useful in showing that doses to veterans as a population were not greatly in excess of those presently estimated).

Current- and spin-density-functional theory for inhomogeneous electronic systems in strong magnetic fields.

Abstract: We formulate the current- and spin-density-functional theory for electronic systems in arbitrarily strong magnetic fields. A set of single-particle self-consistent equations which determine, in addition to the ground-state energy, the density, the spin density, the current density, and the spin-current density, is derived and is proved to be gauge invariant and to satisfy various physical requirements, including the continuity equation. For a magnetic field of constant direction in space, we prove that the exchange-correlation energy functional ${E}_{\mathrm{xc}}$[${n}_{\ensuremath{\uparrow}}$,${n}_{\ensuremath{\downarrow}}$,${j}_{p\ensuremath{\uparrow}}$,${j}_{p\ensuremath{\downarrow}}$] ${n}_{m}$T (\ensuremath{\downarrow})r) is the \ensuremath{\uparrow} (\ensuremath{\downarrow}) component of the density and ${\mathrm{j}}_{\mathit{p}\mathrm{\ensuremath{\uparrow}}\phantom{\rule{0ex}{0ex}}(\mathrm{\ensuremath{\downarrow}})}$(r) is the \ensuremath{\uparrow} (\ensuremath{\downarrow}) component of the ``paramagnetic'' current density] is actually a functional of ${n}_{\ensuremath{\uparrow}}$(r), ${n}_{\ensuremath{\downarrow}}$(r), ${\ensuremath{ u}}_{\mathrm{\ensuremath{\uparrow}}}$(r)\ensuremath{\equiv}\ensuremath{ abla}\ifmmode\times\else\texttimes\fi{}${\mathrm{j}}_{\mathrm{p}\mathrm{\ensuremath{\uparrow}}}$(r)/${\mathit{n}}_{\mathrm{\ensuremath{\uparrow}}}$(r), and ${\ensuremath{ u}}_{\mathrm{\ensuremath{\downarrow}}}$(r)\ensuremath{\equiv}\ensuremath{ abla}\ifmmode\times\else\texttimes\fi{}${\mathrm{j}}_{\mathrm{p}\mathrm{\ensuremath{\downarrow}}}$(r)/${\mathit{n}}_{\mathrm{\ensuremath{\downarrow}}}$(r). An explicit form of ${E}_{\mathrm{xc}}$, which is local in ${\ensuremath{ u}}_{\ensuremath{\uparrow}}$(r) and ${\ensuremath{ u}}_{\ensuremath{\downarrow}}$(r), is derived from linear-response theory. The generalizations to finite-temperature ensembles and to magnetic fields of arbitrarily varying directions are presented.

Resource utilization scales and landscape pattern

Abstract: The spatial patterning of resources constrains the movement of consumers on the landscape. Percolation theory predicts that an organism can move freely if its critical resource or habitat occupies 59.28% of the landscape. Sparse resources require an organism to operate on larger resource utilization scales. Multiple critical resources necessitate larger scales, while substitutable resources ease the scale requirements. Contagious spatial patterns require larger scales to permit movement between resource clusters. The study indicates a strong link between spatial pattern and ecological processes on a landscape.

Infiltration, macroporosity, and mesoporosity distributions on two forested watersheds

Abstract: Macro- and mesopore processes substantially control the subsurface flow in forested watersheds. Limited field scale information is available on spatial variability of macropore infiltration and associated porosity of the hydrologically active macro- and mesopores. The double-ring and tension infiltrometer methods were employed at 37 and 39 locations, respectively, on two contrasting forested watesheds. The spatial variability of infiltration under ponded-flow (macropore-flow) and under 2-, 5-, and 14-cm water tension (mesopore-flow) conditions was determined. The frequency distributions were tested for lognormality with the Shapiro-Wilk ω-statistic and with isopleth probability analysis, and the spatial dependence of these data was tested with semivariogram analysis. The infiltration rates were found to be lognormally distributed, with the mesopore infiltration rates as variable (coefficients of variation of 102-184%) as the macropore infiltration rates (coefficient of variation of 107%). Macropore flow constitued 85% of the ponded flux; however, the mesopore fluxes were large (~ 2 × 10-5 m s-1) and were considered sufficient to infiltrate rainfall without macropores filling and contributing to the flow. The large measured infiltration rates were associated with exceedingly small macroporosities of 0.0003 m³ m-3 and 0.0002 m³ m-3 for the two watersheds. The reduction in infiltration rate with increased tension was described as an exponential function of the diameter of the largest effective pore. Semivariograms revealed no spatial dependence for separation distances >4 m at one watershed and weak spatial dependence for separation distances <15 m at the other. Infiltration into these forested watersheds can be considered a stochastic process for hydrologic modelling

Effect of oxygen pressure on the orthorhombic-tetragonal transition in the high-temperature superconductor YBa 2 Cu 3 O x

Abstract: High-resolution in situ x-ray-diffraction measurements were made of the lattice constants of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathrm{x}}$ as it was heated and cooled through the orthorhombic-tetragonal transition at a series of ${\mathrm{O}}_{2}$ pressures The transition is sharp, continuous, and reversible in temperature and pressure As ${\mathrm{O}}_{2}$ pressure is reduced from 10 to 0005 atm, the transition temperature falls from 676\ifmmode\pm\else\textpm\fi{}5 \ifmmode^\circ\else\textdegree\fi{}C to 521\ifmmode\pm\else\textpm\fi{}10 \ifmmode^\circ\else\textdegree\fi{}C while oxygen content at the transition falls from 666\ifmmode\pm\else\textpm\fi{}001 to 659\ifmmode\pm\else\textpm\fi{}002 Dilation along the c axis is observed as oxygen is removed Under helium, an irreversible but time-dependent orthorhombic-to-tetragonal transition occurs during heating

Turbulence structure in a deciduous forest

Abstract: Three-dimensional wind velocity components were measured at two levels above and at six levels within a fully-leafed deciduous forest. Greatest shear occurs in the upper 20% of the canopy, where over 70% of the foliage is concentrated. The turbulence structure inside the canopy is characterized as non-Gaussian, intermittant and highly turbulent. This feature is supported by large turbulence intensities, skewness and kurtosis values and by the large infrequent sweeps and ejections that dominate tangential momentum transfer. Considerable day/night differences were observed in the vertical profiles of the mean streamwise wind velocity and turbulence intensities since the stability of the nocturnal boundary layer dampens turbulence above and within the canopy.

Thermodynamics of aqueous association and ionization reactions at high temperatures and pressures

Abstract: Electrochemical and electrical conductance cells have been widely used at ORNL over the years to quantitatively determine equilibrium constants and their salt effects to 300°C (EMF) and 800°C (conductance) at the saturation pressure of water (EMF) and to 4000 bars (conductance). The most precise results to 300°C for a large number of weak acids and bases show very similar thermodynamic behavior, which will be discussed. Results for the ionization constants of water, NH3(aq), HCl(aq), and NaCl(aq), which extend well into the supercritical region, have been fitted in terms of a model with dependence on density and temperature. The entropy change is found to be the driving force for ion-association reactions and this tendency increases (as it must) with increasing temperature at a given pressure. Also, the variation of all thermodynamic properties is greatly reduced at high fixed densities. Considerable variation occurs at low densities. From this analysis, the dependence of the reaction thermodynamics on the P-V-T properties of the solvent is shown, and the implication of large changes in hydration for solutes in the vicinity of the critical temperature will be discussed. Finally, the change in the molar compressibility coefficient for all reactions in water is shown to be the same and dependent only on the compressibility of the solvent.

Adaptive transmission relaying concepts for improved performance

Abstract: Concepts for adaptive protective relaying of transmission lines are presented. These include changes in online relay settings, relay characteristics, or logic in response to power system or environmental changes, or as a result of operating experience. Such changes as line-out or generator-out contingencies, which affect fault current distributions and degrade the system's security level, initiate adaptive protection system responses. Adaptive relaying is shown to be capable of improving relaying reliability and power system security and of achieving better utilization of transmission facilities. Most of the concepts require a hierarchical computer system, involving front-line parallel processors, a substation host, and remote central processors, all linked by channels that transmit data or relaying changes prior to or after a disturbance. It is suggested that maximum value from adaptive relaying will result from integration with existing or planned substation control and data acquisition functions and interfacing with the central energy management system (EMS). Emphasis is placed on use of interim hardware approaches to allow evolutionary development. >

Thermal analysis of poly(butylene terephthalate) for heat capacity, rigid-amorphous content, and transition behavior

Abstract: Thermal analysis was performed on poly(butylene terephthalate), PBT, between 210 and 560 K. By combination of experimental heat capacities with computations with an approximate frequency spectrum of 65 group and 19 skeletal vibrations, preliminary recommended ATHAS (1988) heat capacities are proposed for the solid state from 0 to 600 K. The Tarasov parameters used for the computation of the skeletal vibrations were θ1 = 542 K and θ3 = 80 K for crystalline PBT and θ3 = 40 K for amorphous PBT. The glass transition temperature of amorphous PBT was found on efficiently quenched samples to be 248 K, much lower than for semicrystalline PBT where a 310–325 K glass transition temperature is typical. The increase in heat capacity calculated for 100% amorphous samples is 107 J/(K · mol) at 248 K and 77 J/(K · mol) at 320 K. The equilibrium melting temperature is estimated to be 518 K. The unique existence of rigid-amorphous fractions of the semicrystalline polymers is discussed with quantitative data for samples crystallized from the glass and from the melt between 275 and 490 K. The rigidamorphous fraction varies between above 0,9 for cold-crystallized samples to 0,3 for samples crystallized at 490 K. The crystallinity varied from below 0,1 to 0,5. The crystallinity could be separated into four parts, melting at high, medium, and low temperatures, and a part crystallized on cooling after isothermal crystallization. The sequence of crystallization of differently melting crystals was established.

Scale and resolution of forest structural pattern

Abstract: An individual tree-based forest succession model was modified to simulate a forest stand as a grid of contiguous 0.01-ha cells. We simulated a 9 ha stand for 750 years and sampled the stand at 50 yr intervals, outputting structural variables for each grid cell. Principal components analysis was used to depict temporal patterns in forest structure as observed in 0.01 ha samples (individual grid cells). We then resampled the grid using square aggregates of 4 to 100 grid cells as quadrats. Principal component scores recalculated for the aggregates, using the original (0.01 ha scale) scoring matrix, depict the effects of obervational scale on perceived patterns in forest structure. Larger quadrats reduce the apparent variation in forest structure and decrease the apparent rate of structural dynamics. Results support a scale-dependent conceptualization of forest systems by illustrating the qualitative difference in forest dynamics as viewed at the scale of individual gap elements as compared to the larger scale steady state mosaic. The aggregation exercise emphasizes the relationship between these two observational scales and serves as a general framework for understanding scaling relationships in ecological phenomena.

Atmospheric Sampling Glow Discharge Ionization Source for the Determination of Trace Organic Compounds in Ambient Air

Abstract: A new atmospheric sampling ion source, based on the establishment of a glow discharge in ambient air drawn into a region of reduced pressure, is described. The source is simple, rugged, and relatively maintenance-free, exhibits a very short memory, and is extremely sensitive for compounds with high proton affinities, high electron affinities, high gas-phase acidities, and/or low ionization potentials. The effects of discharge voltage and ion source pressure on the nature of the mass spectra observed are described. These operating parameters affect the absolute number of ions observed and, particularly for positive ions, affect the distribution of the reagent ions and the degree of fragmentation. For illustrative purposes, the limit of detection of dynamic range of the ion source coupled with a mass spectrometer are discussed for 2,4,6-trinitrotoluene. For the present system, a detection limit of 1-2 parts per trillion and a linear dynamic range of at least 6 orders of magnitude are observed.

A melting and solidification study of alloy 625

Abstract: The melting and solidification behavior of Alloy 625 has been investigated with differential thermal analysis (DTA) and electron microscopy. A two-level full-factorial set of chemistries involving the elements Nb, C, and Si was studied. DTA results revealed that all alloying additions decreased the liquidus and solidus temperatures and also increased the melting temperature range. Terminal solidification reactions were observed in the Nb-bearing alloys. Solidification microstructures in gastungsten-arc welds were characterized with transmission electron microscopy (TEM) techniques. All alloys solidified to an austenitic (γ) matrix. The Nb-bearing alloys terminated solidification by forming various combinations of γ/MC(NbC), γ/Laves, and γ/M6C eutectic-like constituents. Carbon additions (0.035 wt pct) promoted the formation of the γ/MC(NbC) constituent at the expense of the γ/Laves constituent. Silicon (0.4 wt pct) increased the formation of the yJLaves constituent and promoted formation of the γ/M6C carbide constituent at low levels (<0.01 wt pct) of carbon. When both Si (0.4 wt pct) and C (0.035 wt pct) were present, the γ/MC(NbC) and γ/Laves constituents were observed. Regression analysis was used to develop equations for the liquidus and solidus temperatures as functions of alloy composition. Partial derivatives of these equations taken with respect to the alloying variables (Nb, C, Si) yielded the liquidus and solidus slopes t(m L , m S ) for these elements in the multicomponent system. Ratios of these liquidus to solidus slopes gave estimates of the distribution coefficients (k) for these same elements in Alloy 625.

The DORT Two-Dimensional Discrete Ordinates Transport Code

Abstract: (1988). The DORT Two-Dimensional Discrete Ordinates Transport Code. Nuclear Science and Engineering: Vol. 99, No. 1, pp. 88-89.

Sources of sulphur in forest canopy throughfall

Abstract: Atmospheric deposition of sulphate is the primary link between the atmosphere and acidification of soils and aquatic ecosystems1. The flux of sulphate to forest soils can be measured in the water that drips from trees following the interception of rain to form stemflow and throughfall (the sum of which is designated here as TF). Enrichment of sulphur in TF over that found in rain is widely reported2–4; sulphur sources include the wash-off of previously dry-deposited sulphate particles and SO2 and the leaching of internal plant sulphur from foliage (termed foliar leaching)5. To quantify foliar leaching, we labelled mature trees in the field with radiosulphur and measured atmospheric sulphur concentrations and fluxes. Here we report that dry deposition provides >85% of the enrichment of sulphate in the TF flux to soils below three different tree species at low-elevation sites in the southeastern United States. This supports evidence from several forests that total atmospheric deposition of sulphate is strongly reflected in the measured flux in TF.

Mechanical Properties of Nanometre Volumes of Material: use of the Elastic Response of Small Area Indentations

Abstract: A new, differential method for determining the stiffness of a sub-micron indentation contact area is presented. This allows measurement of elastic modulus as well as plastic hardness, continuously during a single indentation, and without the need for discrete unloading cycles. Some of the new experiments that become possible with this technique, especially at the nanometre scale, are described. We show quantitatively that electropolished tungsten reproducibly exhibits the ideal theoretical lattice strength at small indentation loads.