Showing papers by "Lawrence Berkeley National Laboratory published in 1990"

Advances in electrochemical science and engineering

Abstract: Microelectrodes in solid-state ionics, Jurgen Fleig nonlinear dynamics in electrochemical systems, Katharina Krischer the electrochemistry of diamond, Yuri V. Pleskov passivity of metals, Hans-Henning Strehblow.

Structural properties and electronic structure of low-compressibility materials: β- Si 3 N 4 and hypothetical β- C 3 N 4

Abstract: We present a first-principles pseudopotential study of the structural and electronic properties of \ensuremath{\beta}-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ and the hypothetical compound \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$. \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$, which is ${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$ in the \ensuremath{\beta}-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ structure, with C substituted for Si, is used as a prototype for investigating the properties of possible covalent C-N solids. The calculated lattice constant, bulk modulus, and electronic band structure of \ensuremath{\beta}-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ are in good agreement with experimental results. This gives support for the predicted properties of \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$. The bulk modulus of \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$ is found to be comparable to diamond, and its moderately large cohesive energy suggests that the prototype structure may be metastable. Although the crystal structure and the valencies of the constituent atoms are similar in \ensuremath{\beta}-${\mathrm{Si}}_{3}$${\mathrm{N}}_{4}$ and \ensuremath{\beta}-${\mathrm{C}}_{3}$${\mathrm{N}}_{4}$, the electronic bonding properties in these two solids are found to differ. The large core size and repulsive p pseudopotential of the second-row element, Si, results in a more ionic Si-N bond compared with a covalent C-N bond.

Particle Size Effects for Oxygen Reduction on Highly Dispersed Platinum in Acid Electrolytes

Abstract: The particle size effect for oxygen reduction kinetics on highly dispersed Pt particles in acid electrolytes are discussed. It is suggested that the change in the fraction of surface atoms on the (100) and (111) crystal faces of Pt particles, which are assumed to be cubo‐octahedral structures, can be correlated to the mass activity (A/g Pt) and specific activity (μA/cm2 Pt) of highly dispersed Pt electrocatalysts. The maximum in mass activity that is observed at ~3.5 nm in several studies is attributed to the maximum in the surface fraction of Pt atoms on the (100) and (111) crystal faces, which results from the change in surface coordination number with a change in the average particle size. The reduction of oxygen on supported Pt particles in acid electrolytes is classified as a demanding or structure‐sensitive reaction; the specific activity increases with an increase in particle size.

Too Many Rodent Carcinogens: Mitogenesis Increases Mutagenesis

Abstract: In the Perspective "Too many rodent carcinogens: Mitogenesis increases mutagenesis" by Bruce N. Ames and Lois Swirsky Gold (31 Aug., p. 970), the last paragraph on page 970 (continuing on page 971) was incorrectly printed. It should have read, "One major group of natural chemicals in the human diet are the chemicals that plants produce to defend themselves, the natural pesticides ( 4 ). We calculate that 99.9% (by weight) of the pesticides in our diet are natural. Few natural pesticides have been tested in at least one rodent species, and again about half (27/52 ) are rodent carcinogens. These 27 occur commonly in plant foods ( 10 ). The human diet contains thousands of natural pesticides, and we estimate that the average intake is about 1500 mg per person per day ( 4 ). This compares to a total of 0.09 mg per person per day of residues of about 100 synthetic pesticides ( 4 ). In addition, of the mold toxins tested at the MTD (induding aflatoxin), 11 out of 16 are rodent carcinogens." Also, in paragraph 3 on page 970, "47,000 8-hydroxydeoxyguanosines per cell" should have been "90,000" per cell.

Weak scale effective supersymmetry

Abstract: The idea of supersymmetry at the weak scale should be tested without regard to the Planck-scale origin of any specific model. A class of low-energy effective supersymmetric theories is derived from four assumptions: minimal field content, {ital R}-parity conservation, absence of quadratic divergences, and naturalness of near-flavor conservation. Current experiments are testing and constraining this wide class of supersymmetric models, and not just a specific {ital N}=1 supergravity model.

Expression of extracellular matrix components is regulated by substratum

Abstract: Reconstituted basement membranes and extracellular matrices have been demonstrated to affect, positively and dramatically, the production of milk proteins in cultured mammary epithelial cells. Here we show that both the expression and the deposition of extracellular matrix components themselves are regulated by substratum. The steady-state levels of the laminin, type IV collagen, and fibronectin mRNAs in mammary epithelial cells cultured on plastic dishes and on type I collagen gels have been examined, as has the ability of these cells to synthesize, secrete, and deposit laminin and other, extracellular matrix proteins. We demonstrate de novo synthesis of a basement membrane by cells cultured on type I collagen gels which have been floated into the medium. Expression of the mRNA and proteins of basement membranes, however, are quite low in these cultures. In contrast, the levels of laminin, type IV collagen, and fibronectin mRNAs are highest in cells cultured on plastic surfaces, where no basement membrane is deposited. It is suggested that the interaction between epithelial cells and both basement membrane and stromally derived matrices exerts a negative influence on the expression of mRNA for extracellular matrix components. In addition, we show that the capacity for lactational differentiation correlates with conditions that favor the deposition of a continuous basement membrane, and argue that the interaction between specialized epithelial cells and stroma enables them to create their own microenvironment for accurate signal transduction and phenotypic function.

Nanometre-sized diamonds are more stable than graphite

Abstract: DIAMONDS just 3–5 nm in diameter have recently been recovered from carbonaceous residues of detonations1. They have also been found in meteorites2, nucleate homogeneously in the gas phase3, and diamond-like films can be grown in low-pressure hydrogen4. Conditions are very different in these four cases, but the nearly equal sizes in both meteorites and detonations, and the necessity of nucleation centres for growth of synthetic diamonds, indicate the existence of a common underlying factor. Ultra-small diamonds, too small to be detected readily, may in fact be far more prevalent than presently realized. We arrive at this conclusion by comparing calculated heats of formation of small tetrahedral (diamond) and hexagonal (graphitic) clusters. In agreement with Nuth's discussions on diamonds in interstellar media5,6, we conclude that surface energies are an important aspect in the stabilization of microcrystalline diamonds. For surface bonds terminated with hydrogen atoms, we find that diamonds smaller than ∼3 nm in diameter are energetically favoured over polycyclic aromatics (the precursors to graphite), withou/equiring the high pressures or extreme kinetic conditions usually associated with diamonds.

Changes in lipoprotein subfractions during diet-induced and exercise-induced weight loss in moderately overweight men.

Abstract: We studied separately the effects of weight loss by calorie restriction (dieting) and by calorie expenditure (primarily, running) on lipoprotein subfraction concentrations in sedentary, moderately overweight men assigned at random into three groups as follows: exercise without calorie restriction (n = 46), calorie restriction without exercise (n = 42), and control (n = 42). Plasma lipoprotein mass concentrations were measured by analytic ultracentrifugation for flotation rates (F0(1.20), S0f) within high density lipoprotein (HDL) (F0(1.20) 0-9), low density lipoprotein (LDL) (S0f 0-12), intermediate density lipoprotein (IDL) (S0f 12-20), and very low density lipoprotein (VLDL) (S0f 20-400) particle distributions. Particle diameter and flotation rate of the most abundant LDL species were determined by nondenaturing polyacrylamide gradient gel electrophoresis and analytic ultracentrifugation, respectively. During the 1-year trial, the exercisers ran (mean +/- SD) 15.6 +/- 9.1 km/wk, and the dieters ate 340 +/- 71 fewer kilocalories per day than at baseline. Total body weight was reduced significantly more in dieters (-7.2 +/- 4.1 kg) and exercisers (-4.0 +/- 3.9 kg) than controls (0.6 +/- 3.7 kg). As compared with mean changes in controls, the exercisers and dieters significantly increased HDL2 mass (48.6% and 47.1%, respectively), decreased VLDL mass (-23.9% and -25.5%), and increased LDL peak particle diameter (2.4 and 3.2 A). When adjusted to an equivalent change in body mass index by analysis of covariance, 1) exercise-induced and diet-induced weight loss produced comparable mean changes in the mass of small LDL and VLDL, and in LDL peak particle diameter; 2) the exercisers versus control group difference in HDL2 was attributed to the exercisers' reduced body mass index; and 3) HDL2 increased significantly less in dieters than in exercisers. In dieters, low calorie intake might mitigate the effects of weight loss on HDL2.

Chemical structure and physical properties of diamond-like amorphous carbon films prepared by magnetron sputtering

Abstract: Thin films of amorphous carbon (–C) and amorphous hydrogenated carbon (a–C: H) were prepared using magnetron sputtering of a graphite target. The chemical structures of the films were characterized using electron energy loss spectroscopy (EELS) and Raman spectroscopy. The mass density, hardness, residual stress, optical band gap, and electrical resistivity were determined, and their relation to the film’s chemical structure are discussed. It was found that the graphitic component increases with increasing sputtering power density. This is accompanied by a decrease in the electrical resistivity, optical band gap, mass density, and hardness. Increasing the hydrogen content in the sputtering gas mixture results in decreasing hardness (14 GPa to 3 GPa) and mass density, and increasing optical band gap and electrical resistivity. The variation in the physical properties and chemical structures of these films can be explained in terms of the changes in the volume of sp2-bonded clusters in the a–C films and changes in the termination of the graphitic clusters and sp3-bonded networks by hydrogen in the a–C: H films.

Identifying extended entity-relationship object structures in relational schemas

Abstract: Relational schemas consisting of relation-schemes, key dependencies and key-based inclusion dependencies (referential integrity constraints) are considered. Schemas of this form are said to be entity-relationship (EER)-convertible if they can be associated with an EER schema. A procedure that determines whether a relational schema is EER-convertible is developed. A normal form is proposed for relational schemas representing EER object structures. For EER-convertible relational schemas, the corresponding normalization procedure is presented. The procedures can be used for analyzing the semantics of existing relational databases and for converting relational database schemas into object-oriented database schemas. >

Seawater strontium isotopic variations from 2.5 million years ago to the present.

Abstract: Measurements of marine carbonate samples indicate that during the past 2.5 million years the 87Sr/86Sr ratio of seawater has increased by 14 x 10-5. The high average rate of increase of 87Sr/86Sr indicates that continental weathering rates were exceptionally high. Nonuniformity in the rate of increase suggests that weathering rates fluctuated by as much as ±30 percent of present-day values. Some of the observed shifts in weathering rates are contemporaneous with climatic changes inferred from records of oxygen isotopes and carbonate preservation in deep sea sediments.

Iodine intercalation of a high-temperature superconducting oxide

Abstract: INTERCALATION compounds are formed by inserting guest atomic or molecular species between weakly bound (usually by van der Waals forces) slabs of host materials without changing the inner crystal structure of the individual slabs. The best-known examples of host materials are graphite and the transition metal dichalcogenides1. These compounds can be made with different stage index n, where n denotes the number of slabs between adjacent intercalated layers. Intercalation provides a unique, well controlled approach to changing the physical and electronic properties of host materials over a wide range1. If intercalation can be adopted in the layered high-transition-temperature (high-Tc) superconductors, it could lead to the ability to engineer their properties, with a view to investigating the mechanism responsible for high-Tc superconductivity, improving the superconducting properties of the pristine materials, and developing new high-Tc superconductors and superconducting devices. We report here the successful synthesis and preliminary physical characterization of a stage-1 iodine-intercalated high- Tc superconductor, IBi2Sr2CaCu2Oy.

Pseudorapidity distributions of charged particles produced in $\bar{p}p$ interactions at $\sqrt{s} = 630$ GeV and 1800 GeV

Abstract: We present measurements of the pseudo-rapidity (η) distribution of charged particles (dNch/dη) produced within |η|≤ 3.5 in proton-antiproton collisions at s= of 630 and 1800 GeV. We measure dNch/dη at η = 0 to be 3.18±0.05(stat)±0.10(sys) at 630 GeV, and 3.95±0.02(stat)±0.13(sys) at 1800 GeV. Many systematic errors in the ratio of dNch dη at the two energies cancel, and we measure 1.26±0.01±0.04 for the ratio of dNch/dη at 1800 GeV to that at 630 GeV within |η|≤ 3. Comparing to lower energy data, we observe an increase faster than In(s) in dNch dη at η=0

Expression of the Transforming Growth Factor-α/Epidermal Growth Factor Receptor Pathway in Normal Human Breast Epithelial Cells

Abstract: To better understand the possible roles and interactions of transforming growth factor-alpha (TGF alpha) and its receptor, the epidermal growth factor (EGF) receptor in human breast epithelium, we have studied the expression of TGF alpha and the EGF receptor in a series of normal human mammary epithelial cells derived from reduction mammoplasty before in vitro propagation, during short term proliferation in vitro, and after immortalization. Increased TGF alpha mRNA expression coincided with conversion of the cells to a proliferative state in vitro. After establishment, propagation, and proliferation in vitro, the cells expressed high levels of both TGF alpha and EGF receptor mRNAs. Addition of diverse growth inhibitory agents, including 12-O-tetradecanoylphorbol-13-acetate (TPA), TGF beta, and sodium butyrate, to one of these rapidly proliferating cell populations (no. 184) failed to reduce the expression of either TGF alpha or the EGF receptor. Likewise, cessation of growth associated with both senescence and confluence of the 184 cells did not result in reduced expression. However, regulation of TGF alpha mRNA could be demonstrated by withdrawal of EGF from the medium or by antibody-mediated blockade of the EGF receptor in 184 cells. Antibody-mediated EGF receptor blockade also results in inhibition of growth and [3H]thymidine labeling. An autoregulatory autocrine loop appears operant in proliferating breast epithelial cells. Both growth and levels of TGF alpha mRNA expression are controlled by binding of ligand to the EGF receptor. These studies suggest a role for the TGF alpha/EGF receptor pathway in normal breast cell physiology.

Low-temperature long-range oxygen order in YBa 2 Cu 3 O z

Abstract: OXYGEN ordering in YBa2Cu3Oz, is responsible for the tetragonal-to-orthorhombic phase transition at high temperature, and gives rise to additional ordered superstructures at low temperature1–3. We suggest here that such ordered superstructures are intimately related to the 'plateaus' in the plot of transition temperature versus oxygen content that have been observed by various investigators4,5. We present a consistent theoretical model for oxygen ordering in YBa2Cu3Oz, featuring a parameter-free phase diagram for this system and an algorithm for determining further ordered phases, the predicted structures and diffraction patterns of which agree closely with recent experimental findings2,3.

Pseudospin symmetry and quantized alignment in nuclei.

Abstract: Nine superdeformed bands in several nuclei of the mass-190 region all have transitions of the same or equivalent energies to within an average of about 1 keV--almost identical. Furthermore, it is found that transitions of the same energy do not always connect levels of the same spin, indicating noncollective angular momentum alignments that are all very nearly integers (0, 1\ensuremath{\Elzxh}, or 2\ensuremath{\Elzxh}). Possible explanations involving pseudospin are discussed.

Phase measurement for surface infrared-visible sum-frequency generation

Abstract: We present what are to our knowledge the first measurements of the phase of the infrared-visible sum-frequency generation signal. We introduce two techniques and apply the measurements to the determination of the polar orientation of a selected group of atoms within an adsorbed molecule.

Anisotropy of Grain Growth in Alumina

Abstract: Grain growth in theoretically dense undoped and MgO-doped polycrystalline Al2O3 was studied, and average grain-boundary migration rates were compared with those of a-plane and c-plane sapphire during migration into the same undoped and MgO-doped materials. The results are discussed in terms of a grain-size-dependent grain-boundary mobility-grain-boundary energy product, Mbγb. The grainsize dependencies of the Mbγb products for seed and matrix grains differ. Seed orientation appears to affect the nature of solute-boundary interactions. The importance of grain-boundary structure on migration characteristics is also indicated by a demonstration of twin-formation-enhanced grain growth.

High-resolution 27 AI NMR spectroscopy of the aluminophosphate molecular sieve VPI-5

Abstract: ALUMINIUM plays an important part in determining the properties of many materials, such as the catalytic behaviour of zeolites. Aluminophosphate molecular sieves, in particular, have useful applications as superlattice hosts in the fabrication of quantum-effect devices1. Although nuclear magnetic resonance (NMR) spectroscopy is often a sensitive probe of solids, the use of 27Al NMR to investigate the structure of aluminosilicates and alumino-phosphates has been severely limited because anisotropic second-order quadrupolar interactions, responsible for spectral broadening, cannot be eliminated by conventional magic-angle-spinning or multiple-pulse techniques. Here we report the first high-resolution NMR spectra of 27A1 in a solid using double rotation2–4 and demonstrate its usefulness for probing subtle structural perturbations in the aluminophosphate molecular sieve VPI-55,6. From our results, we conclude that high-resolution 27A1 NMR is capable of resolving discrete framework aluminium sites, permitting quantitative investigation of site-specific adsorbate interactions with the VPI-5 host.