Showing papers by "Sandia National Laboratories published in 1982"

A distribution-free approach to inducing rank correlation among input variables

Abstract: A method for inducing a desired rank correlation matrix on a multivariate input random variable for use in a simulation study is introduced in this paper. This method is simple to use, is distribution free, preserves the exact form of the marginal distributions on the input variables, and may be used with any type of sampling scheme for which correlation of input variables is a meaningful concept. A Monte Carlo study provides an estimate of the bias and variability associated with the method. Input variables used in a model for study of geologic disposal of radioactive waste provide an example of the usefulness of this procedure. A textbook example shows how the output may be affected by the method presented in this paper.

Sol-gel transition in simple silicates II☆

Abstract: Silicate gels were prepared under a range of conditions in which the rate of hydrolysis was varied from fast to slow with respect to the rate of condensation. When hydrolysis was fast, larger, more highly condensed polymers were formed during gelation. Conversely, for slow hydrolysis, smaller, less highly condensed polymers were formed. These gels dried to low density coarse textured and high density fine textured gels, respectively. High temperatures, (>800°C) were required to densify the coarse gels by viscous sintering. Lower temperatures were sufficient to densify fine gels by a process which was postulated to consist of polymer relaxation followed by condensation and pore collapse.

A molecular interpretation of stress corrosion in silica

Abstract: The mechanical strength of many glasses and ceramic materials decreases with time under static loading and ambient environments. This strength loss is associated with slow growth of pre-existing surface flaws due to a stress-corrosion process. To make long-term strength predictions for ceramic components, it is important to understand the stress-corrosion mechanism. We have studied stress corrosion in vitreous silica exposed to water and several non-aqueous environments and report here that environments which enhance stress corrosion are composed of molecular groups with electron donor sites on one end and proton donor sites at the other. These results suggest a detailed chemical model for the interaction of the environment with mechanically strained bonds in the solid at the tip of a crack. The proposed model also has implications for the long-term strength behaviour of a wide variety of brittle materials.

In-Situ Stresses: The Predominant Influence on Hydraulic Fracture Containment

Abstract: In-situ experiments, which are accessible by mineback, have been conducted to determine the parameters that control hydraulic fracture containment. These experiments demonstrate that a stress contrast between the pay zone and a bounding layer is the most important factor controlling fracture height. Material property interfaces are shown to have little effect. 19 refs.

Toward a comprehensive chemical kinetic mechanism for the oxidation of acetylene: Comparison of model predictions with results from flame and shock tube experiments

Abstract: A chemical kinetic model for the oxidation of acetylene is presented. The model is usedto calculate flame speeds for adiabatic, freely propagating flames, composition profiles for lean and moderately rich burner-stabilized flames, and induction times and exponential growth constants for shock induced ignition. These results are then compared to the corresponding experimental results currently available. Rate coefficient information is derived predominantly from reaction-specific experiments. Since the comparisons made here are generally favorable, the kinetic model is consistent with a very large body of experimental, and in some cases theoretical, information. Important aspects of the reaction mechanism are discussed.

Application of New Least-squares Methods for the Quantitative Infrared Analysis of Multicomponent Samples

Abstract: Improvements have been made in previous least-squares regression analyses of infrared spectra for the quantitative estimation of concentrations of multicomponent mixtures. Spec- tral baselines are fitted by least-squares methods, and overlap- ping spectral features are accounted for in the fitting procedure. Selection of peaks above a threshold value reduces computation time and data storage requirements. Four weighted least- squares methods incorporating different baseline assumptions were investigated using FT-IR spectra of the three pure xylene isomers and their mixtures. By fitting only regions of the spectra that follow Beer's Law, accurate results can be obtained using three of the fitting methods even when baselines are not cor- rected to zero. Accurate results can also be obtained using one of the fits even in the presence of Beer's Law deviations. This is a consequence of pooling the weighted results for each spectral peak such that the greatest weighting is automatically given to those peaks that adhere to Beer's Law. It has been shown with the xylene spectra that semiquantitative results can be obtained even when all the major components are not known or when expected components are not present. This improvement over previous methods greatly expands the utility of quantitative least-squares analyses.

Measurement of the Velocity of the Crystal-Liquid Interface in Pulsed Laser Annealing of Si

Abstract: The conductance of Au-doped Si single crystals was measured during irradiation with 30-nsec $Q$-switched ruby-laser pulses at energy densities above the Si melting threshold (\ensuremath{\sim}0.9 J/${\mathrm{cm}}^{2}$). The sample conductance is determined primarily by the thickness of the molten layer so that the solid-liquid interface velocity can be found from the current transient. The interface velocity during crystallization was found to be 2.7\ifmmode\pm\else\textpm\fi{}0.1 m/sec, in close agreement with calculated values based on a heat-flow model.

Effect of bias on radiation-induced paramagnetic defects at the silicon-silicon dioxide interface

Abstract: Electron spin resonance measurements have been made on gamma‐irradiated (111) Si/SiO2 structures as a function of bias across the oxide. We observe a large change in the density of radiation‐induced paramagnetic Pb centers with bais. We conclude that Pb defects (trivalent silicons at the Si/SiO2 interface) account for a very large portion of the radiation‐induced interface states.

Laboratory Investigation on the -Effect of In-Situ Stresses on Hydraulic Fracture Containment

Abstract: Laboratory experiments have been conducted to determine the effect of in situ stress variations on hydraulic fracture containment. Fractures were initiated in layered rock samples with prescribed stress variations, and fracture growth characteristics were determined as a function of stress levels. Stress contrasts of 2-3 MPa were found to be sufficient to restrict fracture growth in laboratory samples of Nevada tuff and Tennessee and Nugget sandstones. The required stress level was found not to depend on mechanical rock properties. However, permeability and the resultant pore pressure effects were found to be important. Tests conducted at bimaterial interfaces between Nugget and Tennessee sandstone show that the resultant stresses set up near the interface due to the applied overburden stress affect the fracture behavior in the same way as the applied confining stresses. These results provide a guideline for determining the in situ stress contrast necessary to contain a fracture in a field treatment.

Quantum chemical studies of a model for peptide bond formation: formation of formamide and water from ammonia and formic acid

Abstract: The S/sub N/2 reaction between ammonia and formic acid has been studied as a model reaction for peptide bond formation using the semiempirical MNDO and ab initio molecular orbital methods. Two reaction mechanisms have been examined, i.e., a stepwise and a concerted reaction. The stationary points of each reaction including intermediate and transition states have been identified and free energies have been calculated for all geometry optimized reaction species to determine the thermodynamics and kinetics of each reaction. The stepwise mechanism was found to be more favorable than the concerted one by both MNDO and ab initio calculations. However, the ab initio method predicts both mechanisms to be fairly competitive with free energies of activation of about 50 kcal/mol. Despite excellent agreement between both methods in the calculated entropies and thermal energies, the minimum basis set character of MNDO leads to values of free energy of activation much higher than those obtained by the ab initio method. The basis set dependence and effect of correlation of the computed ab initio results and the relative effects of polarization and correlation were also investigated by using a number of basis sets up to 6-31G** and estimates of correlation energy by Moller-Plesset perturbationmore » theory up to fourth order. Correlation energy was found to ba a significant factor in the stabilization of transition states.« less

Measurements of Enhanced Stopping of 1-MeV Deuterons in Target-Ablation Plasmas

Abstract: Enhancement of the energy loss of 1-MeV deuterons in target-ablation plasmas over that in cold targets has been observed when signficant ionization is present in the plasma. Scaling of enhanced stopping with target ionization is consistent with stopping by free electrons and the remaining bound electrons. Measured energy losses for Mylar and aluminum targets are also in agreement with hydrocode calculations.

Deep-level nitrogen centers in laser-annealed ion-implanted silicon

Abstract: An electron-paramagnetic-resonance (EPR) study dealing with the means for introducing substitutional N into silicon and the structure of N centers is presented in this paper. Nitrogen can be introduced into crystalline silicon by ${\mathrm{N}}^{+}$ implantation and subsequent pulsed-ruby-laser annealing. The primary criteria for introducing substitutional N in silicon are that enough energy be supplied during pulsed-ruby-laser annealing to melt the implanted region and that crystalline silicon regrowth occurs. The fraction of the implanted N observed by EPR to be incorporated into substitutional sites (labeled SL5) was measured as a function of ${\mathrm{N}}^{+}$ implant energy, ${\mathrm{N}}^{+}$ fluence, and laser annealing energy, and was found to be \ensuremath{\lesssim} 10%. Residual implantation lattice damage is tentatively believed to enhance the formation of other N-defect centers (SL6, SL7) upon thermal annealing between 400 and 450\ifmmode^\circ\else\textdegree\fi{}C. Unlike the other group-V impurities (P, As, Sb, Bi), substitutional N is observed to be a deep-level impurity with axial symmetry about a $〈111〉$ (${C}_{3v}$ symmetry) direction. The characteristic reorientation time of the SL5 center is reasonably well described by the Arrhenius expression $\ensuremath{\tau}={\ensuremath{\tau}}_{0}\mathrm{exp}(\frac{E}{{k}_{B}T})$ over the measured temperature region $35\ensuremath{\lesssim}T\ensuremath{\lesssim}200$ K with $E=0.107\ifmmode\pm\else\textpm\fi{}0.02$ eV and ${\ensuremath{\tau}}_{0}\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12}$ sec. The reorientation of the SL5 center is enhanced under illumination at low temperatures by (near-) band-gap incandescent light. Contrary to our prior assertion, the position of the N-donor level in the band gap remains unknown. The axial symmetry of the substitutional N center is attributed to a distortion in the positions of the substitutional N and neighboring silicon atoms. Possible explanations for this distortion are reviewed.

Trapping and surface permeation of deuterium in He-implanted Fe

Abstract: Iron was ion implanted with He and deuterium (D) and then heated at a constant rate of 2 K/min. The evolving depth distribution of the D was monitored using the nuclear reaction 2D(3He,p)4He, and resulting data were analyzed by applying the diffusion equation with appropriate trapping terms. Two defect traps for D were identified, the respective binding enthalpies being 0.53±0.07 and 0.71±0.07 eV when referenced to a solution site. The weaker of these traps is believed to be a monovacancy, while the stronger may be a vacancy cluster. A third type of trap, with strength 0.78±0.08 eV, was found to be associated with He. It is proposed that the responsible entities are ∼1 nm He bubbles which were observed by transmission electron microscopy, and that the D is bound to the walls of these bubbles by a mechanism similar to chemisorption. The analysis also yielded an estimate of the D recombination coefficient at the electropolished and air‐exposed Fe surface, ∼5×10−19 cm4/s at a temperature of 500 K.

Comparison between CARS and corrected thermocouple temperature measurements in a diffusion flame.

Abstract: Coherent anti-Stokes Raman spectroscopy (CARS) has been used to obtain radial temperature profiles in an axisymmetric methane diffusion flame. Temperatures were obtained from analysis of background-free nitrogen Q- and O-branch spectra. The spectra were analyzed with a nonlinear least-squares CARS fitting program and compared to measurements from radiation- and conduction-corrected thermocouples. Excellent agreement was obtained in regions of relatively constant temperature, whereas improved CARS spatial resolution was required to obtain agreement near steep temperature gradients.

Static team problems--Part I: Sufficient conditions and the exponential cost criterion

Abstract: The stationary conditions of Radner are shown under relaxed hypotheses to be sufficient to establish the global optimality of candidate control laws for static team problems with convex cost. This extension of Radner's theorem establishes the optimality of affine laws for the exponential of a quadratic performance index with jointly Gaussian state and observation variables. The class of performance indexes of team problems for which the optimal solution is known is thereby enlarged.

Implementation of Rosenbrock Methods

Abstract: Rosenbrock formulas have shown promise in research codes for the solution of initial-value problems for stiff systems of ordinary differential equations (ODEs). To help assess their practical value, the author wrote an item of mathematical software based on such a formula. This required a variety of algorithmic and software developments. Those of general interest are reported in this paper. Among them is a way to select automatically, at every step, an explicit Runge-Kutta formula or a Rosenbrock formula according to the stiffness of the problem. Solving linear systems is important to methods for stiff ODEs, and is rather special for Rosenbrock methods. A cheap, effective estimate of the condition of the linear systems is derived. Some numerical results are presented to illustrate the developments.

Error Analysis and Prevention of Cosmic Ion-Induced Soft Errors in Static CMOS RAMs

Abstract: Cosmic ray interactions with memory cells are known to cause temporary, random, bit errors in some designs. The sensitivity of polysilicon gate CMOS static RAM designs to logic upset by impinging ions has been studied using computer simulations and experimental heavy ion bombardment. Results of the simulations are confirmed by experimental upset cross-section data. Analytical models have been extended to determine and evaluate design modifications which reduce memory cell sensitivity to cosmic ions. A simple design modification, the addition of decoupling resistance in the feedback path, is shown to produce static RAMs immune to cosmic ray-induced bit errors.

The effect of defects on the fatigue crack initiation process in two p/m superalloys: part i. fatigue origins

Abstract: Two high strength P/M nickel-base superalloys, AF-115 and AF2-1DA, with different defect populations, were tested to determine the effect of preexisting defects on the fatigue crack initiation process. Strain controlled continuous cycle fatigue tests were performed at room and at elevated temperature; these were followed by fractographic examination to characterize both the location and character of the fatigue origins. In most cases, particularly at elevated temperature, the initiation process was associated with a large pre-existing defect, either a pore or a nonmetallic inclusion. There was also a change in the location of the crack that caused failure as the strain range varied: at high strain ranges initiation occurred at or near the specimen’s surface, while at the lower strain ranges the failure originated in the specimen’s interior. The initiation mode for both alloys at room temperature was different than at elevated temperature. At room temperature, Stage I crystallographic cracking at or near the surface dominated the process in all strain range regimes. This difference was attributed, in part, to the differences in deformation mode for nickel-base superalloys at room and elevated temperature.

Improved Methodology for Using Embedded Markov Chains to Describe Cyclical Sediments

Abstract: Embedded Markov chains may be used to describe rock sequences in which a lithology is not or cannot be observed following itself. Such chains lead to a transition matrix with zeros on the main diagonal. To test the hypothesis of randomness in an embedded Markov chain, we apply Goodman's (1968) model of quasi-independence and compare it to previously used methods (which we now believe are invalid) in the geological literature. Data in the literature show quite different results (depending on the original method) when reanalyzed in this way. We present tests for homogeneity, a spatial analogue to stationarity, of multiple embedded chains and for symmetry and Markov chain order. Matrices from vertical or laterally spaced sequences can be tested for homogeneity and conclusions drawn regar ing variations of processes over space. A normalized difference is proposed as an aid in interpreting the difference between observed transition frequencies and transition probabilities estimated with a model of independence or quasi-independence.

Defects and impurities in thermal oxides on silicon

Abstract: Oxides grown at 1100 °C in dry oxygen for 60 min to a thickness of 1350 A on silicon with and without subsequent forming gas anneals were 60Co γ irradiated at 4 K with doses up to 106 rad (Si). In situ electron paramagnetic resonance measurements at 10 K revealed ≊3×1017 atomic hydrogen/cm3 and ∼1017 oxygen‐hole centers/cm3 in the oxide. The paramagnetic dangling bond on the silicon side of the Si/SiO2 interface (Pb center) was also observed. The (relative) concentration of these centers was measured as a function of isochronal annealing between 10 and 300 K.

Rank correlation plots for use with correlated input variables

Abstract: A method for inducing a desired rank correlation matrix on multivariate input vectors for simulation studies has recently been developed by Iman and Conover (1982). The primary intention of this procedure is to produce correlated input variables for use with computer models. Since this procedure is distribution free and allows the exact marginal distributions to remain intact it can be used with any marginal distributions for which it is reasonable to think in terms of correlation. In this paper we present a series of rank correlation plots based on this procedure when the marginal distributions are normal, lognormal, uniform and loguniform. These plots provide a convenient tool both for aiding the modeler in determining the degree of dependence among input variables (rather than guessing) and for communicating with the modeler the effect of different correlation assumptions. In addition this procedure can be used with sample multivariate data by sampling directly from the respective marginal empirical di...

Channel-collector transistors

Abstract: By merging the channel regions of two-junction field-effect transistors with the collector region of a bipolar junction transistor (BJT), one achieves a quasi-cascode configuration called a Channel-Collector Transistor (CCT). Its terminal properties are those of a modified, improved bipolar transistor. Prototype devices have been fabricated with common-emitter current gain (β F ) in excess of 1200 while still maintaining the common-emitter, open-base breakdown voltage (BV CE0 ) greater than 140 V and the output resistance r 0 typically greater than 200 kΩ. The present brief describes an efficient, qualitative equivalent circuit for the structure and also presents an experimental device graphically illustrating the CCT's advantages and disadvantages when compared to a conventional BJT.

Hydrogen-air detonations

Abstract: The Three Mile Island nuclear plant accident has triggered renewed interest in fundamental combustion studies in hydrogen-air mixtures. The present study is concerned with the problem of detonation of atmospheric, hydrogen-air mixtures and reports new experimental results on cell sizes lambda and critical tube diameters d/sub c/. The results confirm the empirical correlation d/sub c/ = 13lambda. Comparison of the critical tube diameter of stoichiometric hydrogen-air mixtures (d/sub c/ = 20 cm) with that of stoichiometric acetylene-air mixtures (d/sub c/ = 12 cm) indicates that hydrogen is slightly less sensitive than acetylene. The relatively slow increase in d/sub c/ for H/sub 2/-rich mixtures (as compared to lean mixtures) indicates that fuel-rich mixtures are more hazardous from the detonation point of view than fuel-lean mixtures. Regarding the detonability limits, the present paper formally defines limit criteria for different boundary conditions based on the cell size of the mixture. For fully confined detonations in tubes, limits are set by the onset of single-headed spin.