Showing papers by "Sandia National Laboratories published in 1984"

Embedded-atom method: Derivation and application to impurities, surfaces, and other defects in metals

Abstract: We develop the embedded-atom method [Phys. Rev. Lett. 50, 1285 (1983)], based on density-functional theory, as a new means of calculating ground-state properties of realistic metal systems. We derive an expression for the total energy of a metal using the embedding energy from which we obtain several ground-state properties, such as the lattice constant, elastic constants, sublimation energy, and vacancy-formation energy. We obtain the embedding energy and accompanying pair potentials semiempirically for Ni and Pd, and use these to treat several problems: surface energy and relaxation of the (100), (110), and (111) faces; properties of H in bulk metal (H migration, binding of H to vacancies, and lattice expansion in the hydride phase); binding site and adsorption energy of hydrogen on (100), (110), and (111) surfaces; and lastly, fracture of Ni and the effects of hydrogen on the fracture. We emphasize problems with hydrogen and with surfaces because none of these can be treated with pair potentials. The agreement with experiment, the applicability to practical problems, and the simplicity of the technique make it an effective tool for atomistic studies of defects in metals.

The Prisoners’ Problem and the Subliminal Channel

Abstract: Two accomplices in a crime have been arrested and are about to be locked in widely separated cells. Their only means of communication after they are locked up will he by way of messages conveyed for them by trustees -- who are known to be agents of the warden. The warden is willing to allow the prisoners to exchange messages in the hope that he can deceive at least one of them into accepting as a genuine communication from the other either a fraudulent message created by the warden himself or else a modification by him of a genuine message. However, since he has every reason to suspect that the prisoners want to coordinate an escape plan, the warden will only permit the exchanges to occur if the information contained in the messages is completely open to him -- and presumably innocuous. The prisoners, on the other hand, are willing to accept these conditions, i.e., to accept some risk of deception in order to be able to communicate at all, since they need to coordinate their plans. To do this they will have to deceive the warden by finding a way of communicating secretly in the exchanges, i.e., of establishing a “subliminal channel” between them in full view of the warden, even though the messages themselves contain no secret (to the warden) information‡. Since they anticipate that the warden will try to deceive them by introducing fraudulent messages, they will only exchange messages if they are permitted to authenticate them.

Hole traps and trivalent silicon centers in metal/oxide/silicon devices

Abstract: We report electron spin resonance (ESR) measurements of E′‐center (a ‘‘trivalent silicon’’ center in SiO2) density as well as capacitance versus voltage (C‐V) measurements on γ‐irradiated metal/oxide/silicon (MOS) structures. We also report a considerable refinement of earlier ESR measurements of the dependence of radiation‐induced Pb ‐center (a ‘‘trivalent silicon’’ center at the Si/SiO2 interface) occupation as a function of the Fermi level at the Si/SiO2 interface. These measurements indicate that the Pb centers are neutral when the Fermi level is at mid‐gap. Since the Pb centers are largely responsible for the radiation‐induced interface states, one may take ΔVmg Cox/e (where ΔVmg is the ‘‘mid‐gap’’ C‐V shift, Cox is the oxide capacitance, and e is the electronic charge) as the density of holes trapped in the oxide. We find that radiation‐induced E′ density equals ΔVmg Cox/e in oxides grown in both stream and dry oxygen. Etch‐back experiments demonstrate that the E′ centers are concentrated very near the Si/SiO2 interface (as are the trapped holes). Furthermore, we have subjected irradiated oxide structures to a sequence of isochronal anneals and find that the E′ density and ΔVmg annealing characteristics are virtually identical. We conclude that the E′ centers are largely responsible for the deep hole traps in thermal SiO2 on silicon. This observation coupled with observations regarding the Pb center indicates that two intrinsic centers, both involving silicon atoms lacking one bond to an oxygen atom, are largely responsible for the two electrically significant aspects of radiation damage in MOS devices: charge buildup in the oxide and interface‐state creation at the Si/SiO2 interface.

Fractal geometry of colloidal aggregates

Abstract: Measurement of the fractal dimension, $D$, of colloidal aggregates of small silica particles is reported. We observe power-law decay of the structure factor $[S(k)\ensuremath{\sim}{k}^{\ensuremath{-}D}]$ by both light and x-ray scattering showing that the aggregates are fractal. $D$ is found to be 2.12\ifmmode\pm\else\textpm\fi{}0.05, which is intermediate between recent numerical results for the kinetic models of diffusion-limited aggregation ($D=2.5$) and cluster aggregation ($D=1.75$), but is rather close to the value for lattice animals ($D=2.0$), which are equilibrium structures.

Melting temperature and explosive crystallization of amorphous silicon during pulsed laser irradiation

Abstract: Measurements during pulsed laser irradiation indicate that amorphous Si melts at a temperature 200 \ifmmode\pm\else\textpm\fi{} 50 K below the crystalline value. Below energy densities required to melt the amorphous layer fully, the data are interpreted in terms of an explosive crystallization. The initial liquid layer solidifies to form coarse-grained polycrystalline Si. A thin, self-propagating liquid layer travels through the remaining amorphous Si at a velocity of 10-20 m/s, producing fine-grained polycrystalline Si.

Phosphate glass dissolution in aqueous solutions

Abstract: Rates of aqueous dissolution are reported for a series of phosphate glasses having the composition (50 − X)M2O·(X)CaO·50P2O5. Dissolution reactions involving the consumption of H+ and OH− were monitored using pH stat titration techniques, solution analyses using inductively coupled plasma emission, and depth profiling of corroded glass surfaces for H and other elements using elastic recoil detection analysis and Rutherford backscattering. These analytical results indicate that the phosphate glasses dissolve uniformly due to acid- or base-catalyzed hydration of the polymeric phosphate network. The rate and nature of this hydration appears to be controlled by the surface pH and/or charge which develops at the glass/ solution interface. The implication of these results on dissolution models for both the phosphate and silicate glasses is discussed.

Correlating the Radiation Response of MOS Capacitors and Transistors

Abstract: A new technique is presented for separating the threshold-voltage shift of an MOS transistor into shifts due to interface states and trapped-oxide charge. Using this technique, the radiation responses of MOS capacitors and transistors fabricated on the same wafer are compared. A good correlation is observed between p-substrate capacitors and n-channel transistors irradiated at 10 V, as well as between n-substrate capacitors and p-channel transistors irradiated at 0 V. These correlations were verified for samples having large variations in the amount of radiation-induced trapped holes and interface states. An excellent correlation is also observed between n-channel capacitors and n-substrate transistors irradiated under positive bias. The use of capacitors separately fabricated on control wafers for potential use in process development or monitoring is clearly demonstrated.

Physical Mechanisms Contributing to Device "Rebound"

Abstract: The physical mechanisms that produce rebound have been identified. The positive increase in threshold voltage during a bias anneal is due to annealing of oxide trapped charge. Rebound can be predicted by measuring the contribution to the threshold voltage from radiation-induced interface states immediately after irradiation.

Conduction mechanism in boron carbide

Abstract: Electrical conductivity, Seebeck-coefficient, and Hall-effect measurements have been made on single-phase boron carbides, B(1-x)C(x), in the compositional range from 0.1 to 0.2 X, and between room temperature and 1273 K. The results indicate that the predominant conduction mechanism is small-polaron hopping between carbon atoms at geometrically inequivalent sites.

Fractal Geometry of Silica Condensation Polymers

Abstract: The fractal dimension, $D$, of branched silica polymers is measured by small-angle x-ray scattering. $D$ is independent of branching ratio and does not change during the course of polymerization to a gel.

Electronic Structure of a "Poisoned" Transition-Metal Surface

Abstract: The authors report self-consistent linearized-augmented-plane-wave calculations of the electronic structure perturbations induced by a catalytic ''poison,'' S, on a Rh(001) surface, focusing particularly on their distance dependence. The S-induced charge density vanishes beyond the immediately adjacent Rh atoms. However, the Fermi-level density of states, which is not screened, and which governs the ability of the surface to respond to the presence of other species, is substantially reduced by the S even at nonadjacent sites.

Data Compendium for Plasma-Surface Interactions

Abstract: A review of particle-solid processes pertinent to modelling plasma-wall interactions is presented, and sets of recommended data are given. Analytic formulas are used where possible; otherwise, data are presented in the form of tables and graphs. The incident particles considered are e−, H, D, T, He, C, O, and selfions. The materials include the metals aluminum, beryllium, copper, molybdenum, stainless steel, titanium, and tungsten and the nonmetals carbon and TiC. The processes covered are light ion reflection, hydrogen and helium trapping and detrapping, desorption, evaporation, sputtering, chemical effects in sputtering, blistering caused by implantation of helium and hydrogen, secondary electron emission by electrons and particles, and arcing.

Platinum-group minerals in alpine chromitites from southwestern Oregon

Abstract: Electron microprobe and scanning electron microscopy. The platinum-group minerals found in the chromitites are laurite, porous Ru-rich alloys, Os-Ir- and Pt-rich alloys with highly variable compositions, an Ir-Cu-rich sulfide, a Pt arsenide, and three relatively rare platinum-group sulfides. The platinum-group minerals are 1 to 30 mu m in diameter and occur as inclusions in chromite grains; some of these inclusions also contain silicates and millerite. Base metal alloys (largely Ni-Fe) occur in cracks in the chromitites, along with magnetite, ferritchromite, chromian chlorite, and serpentine. The primary platinum-group mineral assemblage is interpreted to have formed before or during crystallization of the chromite grains and not as a result of exsolution from the chromite.--Modified journal abstract.

The Thermal and Structural Properties of a Hanna Basin Coal

Abstract: In an effort to understand the cavity growth mechanisms occurring during an Underground Coal Gasification (UCG) test, a study of the thermomechanical effects has been initiated at Sandia National Laboratories. The first phase of this study has been the determination of the intrinsic thermal and structural properties of the Hanna Basin Coal that was utilized in a series of four UCG tests near the town of Hanna, Wyoming. The result of this study is a consistent set of thermal and structural properties of a Hanna Basin coal. This set has been used in a model that successfully simulated the growth of the cavity observed during the Hanna II UCG test.

Critical Properties of the Void Percolation Problem for Spheres

Abstract: The method outlined by Kerstein has been used to simulate percolation of the void region between overlapping, randomly located spheres. The computed threshold is in agreement with the previous result of Kert\'esz. In addition, three critical exponents are computed and are found to be in agreement with the universality hypothesis. This constitutes the first such evaluation for a three-dimensional nonlattice problem and the first test of universality for a percolation problem with no underlying network defined a priori.

Low-energy x-ray response of photographic films. II. Experimental characterization

Abstract: Optical density versus exposure data have been obtained at nine photon energies in the 100–2000-eV x-ray region for five spectroscopic films (Kodak films 101-07, SB-392, RAR 2492, RAR 2495, and RAR 2497). These data were determined operationally by a direct comparison of the peak absolute intensities of spectral lines, which were measured with a calibrated proportional counter, with the microdensitometer tracings of the corresponding photographically recorded spectral lines. Film-resolution limits were deduced from an analysis of contact microradiograms of linear zone plates constructed of gold bars. The relationship between the specular densities as measured here and the diffuse densities have been experimentally determined for the five films. Finally, experimental measurements of the optical density versus the angle of incidence of exposing radiation of constant intensity were obtained. These data, relating density to the x-ray intensity, its photon energy, and its angle of incidence, are shown to be fitted satisfactorily in the 100–10 000-eV region by the semiempirical mathematical model relations that were derived in Part I of this research [ J. Opt. Soc. Am. B1, 818– 827 ( 1984)].

First and second derivatives of two electron integrals over Cartesian Gaussians using Rys polynomials

Abstract: Formulas are developed for the first and second derivatives of two electron integrals over Cartesian Gaussians Integrals and integral derivatives are evaluated by the Rys polynomial method Higher angular momentum functions are not used to calculate the integral derivatives; instead the integral formulas are differentiated directly to produce compact and efficient expressions for the integral derivatives The use of this algorithm in the ab initio molecular orbital programs gaussian 80 and gaussian 82 is discussed Representative timings for some small molecules with several basis sets are presented This method is compared with previously published algorithms and its computational merits are discussed

Free volume and the kinetics of aging of polymer glasses

Abstract: Application du modele stochastique decrit par Robertson a la recouvrance du volume de l'acetate polyvinylique. Comparaison avec les donnees de Kovacs

Crystallization of a Glass‐Ceramic by Epitaxial Growth

Abstract: The crystallization mechanism in a modified Li/sub 2/O-Al/sub 2/O/sub 3/-SiO/sub 2/ glass containing P/sub 2/O/sub 5/ nucleating agent was investigated by transmission electron microscopy, X-ray diffraction, and differential thermal analysis. During an initial 1000/sup 0/C treatment P/sub 2/O/sub 5/ and Li/sub 2/O react to precipitate Li/sub 3/PO/sub 4/ crystallites. At lower temperatures cristobalite, lithium metasilicate, and lithium disilicate crystallize by epitaxial growth on those Li/sub 3/PO/sub 4/ crystallites. Crystallographic orientation relations for epitaxy were determined by electron diffraction, and lattice misfits were found to be in the range -5.3 to +3.8%. These results provide the first direct proof that glass-ceramics can crystallize by epitaxial growth on heterogeneous nuclei formed from intentionally added nucleating agents.

Semiconductor grain-boundary admittance: Theory

Abstract: The electrical admittance due to majority carriers in semiconductor bicrystals is calculated as a function of frequency and applied dc voltage. At low frequencies the admittance is controlled by charge trapping at the grain boundary which modulates the thermionic emission current across the boundary. The admittance does not generally obey a simple Debye-type form, although in most cases it is similar. Examples for the frequency and voltage dependence of the admittance are given for two energy distributions of the grain-boundary trap states.

Solving Low Density Knapsacks

Abstract: Let al,,an and s be a set of integers The knapsack (or subset sum) problem is to find a 0–1 vector (el,,en) such that Σ eiai = s or to show that such a vector does not exist The integers al,,an are sometimes referred to as weights The general knapsack problem is known to be NP complete [5,6] Several cryptosystems based on the knapsack problem have been designed [9,12,16] In April, 1982, Adi Shamir [14] announced a method for breaking the Merkle-Hellman cryptosystem Since that time there has been a flurry of activity to extend his results to include all of the proposed knapsack based cryptosystems [1,2,3,7,13]

A numerical simulation of a plastic fluid in a parallel-plate plastometer

Abstract: The parallel-plate plastometer provides a simple experimental method for determining the rheological properties of plastic fluids. Previous theoretical analyses of flow in the plastometer have lead to conflicting results. In this paper, a numerical method is used to solve the plastometer flow problem and resolve some questions regarding the theoretical analysis. The plastometer problem is also used to demonstrate the accuracy of the numerical method in obtaining solution to non-trivial problems involving fluids with a yield stress.

Comparison of Asymptotically Distribution-Free Procedures for the Analysis of Complete Blocks

Abstract: The Friedman test (or sign test when k = 2) depends entirely on within-block rankings. In a recent paper, Quade (1979) attempted to provide a test with more power than the Friedman test by considering a k-sample extension of the Wilcoxon signed ranks test. This is done by taking advantage of the between-block information. A third way to approach the problem and still retain both the within- and between-block information is first to transform all the observations to ranks from 1 to bk (b blocks and k treatments) and then to apply the parametric F test to the ranks. This approach is shown to be asymptotically distribution-free under suitable conditions. Computer simulation results indicate that this procedure is both robust and powerful for small sample sizes.

Precision and accuracy of the calcite strain-gage technique

Abstract: The twinned-calcite strain-gage technique allows the determination of strain magnitude and direction in mildly deformed sedimentary rocks. In that the measurements required for the calculation are rather time-consuming, it is important to know the minimum number required to obtain a precision that is comparable with the accuracy of the result. This paper documents a series of tests on three different limestones having different textures and porosities in order to determine the precision and accuracy of the technique as a function of the number of measurements and the number of perpendicular thin sections. The tests show that, for most purposes, the best measurement strategy will be to make 25 measurements on each of two perpendicular thin sections. The best-determined principal strain axis is the one having a magnitude that is opposite in sign from the other two. Over the range of zero to 11% shortening, the magnitude of the odd-signed strain axis will than have a precision and accuracy of about 11% in magnitude and a precision and accuracy of about 6° to 8° in orientation. Differences in texture and porosity do not appear to have significant effects upon precision and accuracy.

A coolability model for postaccident nuclear reactor debris

Abstract: A one-dimensional model is developed for boiling heat removal and dryout in particulate debris. The model can be used for predicting the coolability of postaccident debris from a nuclear reactor (either light water or liquid-metal fast breeder). The model includes the effects of both laminar and turbulent flow, two-phase friction, gravity, capillary force, and channels at the top of the debris. The model is applicable to debris on permeable supports with liquid entering the debris bottom or to debris on impermeable plates. In the latter case, the plate can be either adiabatic or cooled on the bottom.The model predicts channel length, the liquid fraction within the debris as a function of elevation, the incipient dryout power, the dry zone thickness as a function of power, and the existence of downward heat removal by boiling (in bottom-cooled debris), all for both uniform and stratified debris.

Paramagnetic trivalent silicon centers in gamma irradiated metal‐oxide‐silicon structures

Abstract: We find that two paramagnetic ‘‘trivalent silicon’’ centers appear to be primarily responsible for radiation damage in metal‐oxide‐silicon structures.

The effect of quench rate on the microstructure, mechanical properties, and corrosion behavior of U-6 Wt Pct Nb

Abstract: The effect of cooling rate on the microstructure, mechanical behavior, corrosion resistance, and subsequent age hardenability of U-6 wt pct Nb is described and discussed. Cooling rates in excess of 20 Ks-1 cause the parent γ-phase to transform martensitically to a niobium supersaturated variant of the α-phase. This martensitic phase exhibits low hardness and strength, high ductility, good corrosion resistance, and substantial age hardenability. As cooling rate decreases from 10 Ks-1 to 0.2 Ks-1, fine scale microstructural changes (consistent with spinodal decomposition) occur to an increasing extent. These changes produce large increases in hardness and strength and large decreases in ductility, slight decreases in corrosion resistance, and slight changes in age hardenability. At cooling rates less than 0.2 Ks-1 the parent phase undergoes cellular decomposition to a coarse two-phase lamellar microstructure. This lamellar microstructure exhibits intermediate strength and ductility, substantially reduced corrosion resistance, and no age hardenability. An analysis of the cooling rates at the centers of water quenched plates indicates that fully martensitic microstructures can be obtained in plates as thick as 50 mm.

Repetitively pulsed metal ion beams for ion implantation

Abstract: Recently developed techniques for producing high current, pulsed ion beams have suggested the possibility of a pulsed implanter technology. Potential advantages of such an approach include lower capital costs, and higher average beam currents for applications requiring metal ions which are difficult to produce in conventional dc sources. We report here the development of a prototype 80 kV pulsed implanter. This implanter runs at 32 pulses per second, 2 A (titanium) peak ion current, a 6 μs accelerating pulse length, and ~100 μA titanium particle current. Significant currents have been demonstrated for titanium, copper and carbon; ions from any conducting solid can be used in the ion source. The characteristics of the source are discussed along with implanter operating data. Our results suggest that the accelerating gap runs in a primarily source dominated mode. The absolute beam currents and charge state are characterized by surface analysis of implanted samples. Techniques for scaling this technology to high (> 20 mA) average currents are discussed.

Eigenvalues and Stable Time Steps for the Uniform Strain Hexahedron and Quadrilateral

Abstract: On obtient des formules simples pour limiter les valeurs limites maximums ou pour les calculer exactement pour les elements de deformation hexaedrique a 8 nœuds et en quadrilatere a 4 nœuds. L'analyse de valeurs propres est limitee aux materiaux lineaires isotropes