Showing papers by "Paul Scherrer Institute published in 1998"

Insertion Electrode Materials for Rechargeable Lithium Batteries

Abstract: The performance and safety of rechargeable batteries depend strongly on the materials used. Lithium insertion materials suitable for negative and positive insertion electrodes are reviewed. Future trends, such as alternative materials for achieving higher specific charges are discussed. (orig.) 1041 refs.

Kondo Scattering Observed at a Single Magnetic Impurity

Abstract: Electron scattering of isolated Ce adatoms on Ag(111) surfaces was studied with a low-temperature scanning tunneling microscope. Tunneling spectra obtained on and near Ce reveal a characteristic dip around the Fermi energy which is absent for nonmagnetic Ag adatoms. This feature is detected over a few atomic diameters around Ce atoms at the surface. The transition matrix element from the localized $f$ electron to the tunnel-current carrying continuum states bears a strong resemblance to discrete autoionized states. We interpret the dip spectrum as a Fano interference for the limit where the $f$ orbital has a very small matrix element.

Heterogeneous production of nitrous acid on soot in polluted air masses

Abstract: Polluted air masses are characterized by high concentrations of oxidized nitrogen compounds which are involved in photochemical smog and ozone formation. The OH radical is a key species in these oxidation processes. The photolysis of nitrous acid (HNO2), in the morning, leads to the direct formation of the OH radical and may therefore contribute significantly to the initiation of the daytime photochemistry in the polluted planetary boundary layer. But the formation of nitrous acid remains poorly understood: experimental studies imply that a suggested heterogeneous formation process involving NO2 is not efficient enough to explain the observed night-time build-up of HNO2 in polluted air masses1. Here we describe kinetic investigations which indicate that the heterogeneous production of HNO2 from NO2 on suspended soot particles proceeds 105 to 107 times faster than on previously studied surfaces. We therefore propose that the interaction between NO2 and soot particles may account for the high concentrations of HNO2 in air masses where combustion sources contribute to air pollution by soot and NOx emissions. We believe that the observed HNO2 formation results from the reduction of NO2 in the presence of water by C–O and C–H groups in the soot. Although prolonged exposure to oxidizing agents in the atmosphere is likely to affect the chemical activity of these groups, our observations nevertheless suggest that fresh soot may have a considerable effect on the chemical reactions occurring in polluted air.

The precision of proton range calculations in proton radiotherapy treatment planning: experimental verification of the relation between CT-HU and proton stopping power

Abstract: The precision in proton radiotherapy treatment planning depends on the accuracy of the information used to calculate the stopping power properties of the tissues in the patient's body. This information is obtained from computed tomography (CT) images using a calibration curve to convert CT Hounsfield units into relative proton stopping power values. The validity of a stoichiometric method to create the calibration curve has been verified by measuring pairs of Hounsfield units and stopping power values for animal tissue samples. It was found that the agreement between measurement and calibration curve is better than 1% if beam hardening effects in the acquisition of the CT images can be neglected. The influence of beam hardening effects on the quantitative reading of the CT measurements is discussed and an estimation for the overall range precision of proton beams is given. It is expected that the range of protons in the human body can be controlled to better than +/-1.1% of the water equivalent range in soft tissue and +/-1.8% in bone, which translates into a range precision of about 1-3 mm in typical treatment situations.

Transition from Quantum Hall to Compressible States in the Second Landau Level: New Light on the ν = 5 / 2 Enigma

Abstract: Quantum Hall states at filling fraction $\ensuremath{ u}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5/2$ are examined by numerical diagonalization. Spin-polarized and spin-unpolarized states of systems with $N\ensuremath{\le}18$ electrons are studied, neglecting effects of Landau level mixing. We find that the ground state is spin polarized. It is incompressible and has a large overlap with paired states like the Pfaffian. For a given sample, the energy gap is about 11 times smaller than at $\ensuremath{ u}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/3$. Evidence is presented of phase transitions to compressible states, driven by the interaction strength at short distance. A reinterpretation of experiments is suggested.

Spin asymmetries A1 and structure functions g1 of the proton and the deuteron from polarized high energy muon scattering

Abstract: We present the final results of the spin asymmetries A1 and the spin structure functions g1 of the proton and the deuteron in the kinematic range 0.0008

A scattering theoretic approach to scalar relativistic corrections on bonding

Abstract: The atomic characteristics, which govern changes in bonding properties due to relativistic effects in heavy atoms, are identified from a scattering theoretic standpoint. It is shown that within an all-electron calculation scalar relativistic corrections to valence orbitals relevant to atomic bonding properties can be made via a local pseudopotential for all elements. The present approach reproduces molecular geometries and vibrational frequencies excellently for a test set of relatively simple molecules, where good experimental data are available. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 423–433, 1998

Antigen binding forces of individually addressed single-chain Fv antibody molecules

Abstract: Antibody single-chain Fv fragment (scFv) molecules that are specific for fluorescein have been engineered with a C-terminal cysteine for a directed immobilization on a flat gold surface. Individual scFv molecules can be identified by atomic force microscopy. For selected molecules the antigen binding forces are then determined by using a tip modified with covalently immobilized antigen. An scFv mutant of 12% lower free energy for ligand binding exhibits a statistically significant 20% lower binding force. This strategy of covalent immobilization and measuring well separated single molecules allows the characterization of ligand binding forces in molecular repertoires at the single molecule level and will provide a deeper insight into biorecognition processes.

A chemical sensor based on a micromechanical cantilever array for the identification of gases and vapors

Abstract: We have built and operated a novel setup for the characterization and identification of gases or vapors based on sequential position readout via a beam-deflection technique from a microfabricated array of eight cantilever-type sensors. Each of the cantilevers can be coated on one side with a dif- ferent sensor material to detect specific chemical interactions. We demonstrate that disturbances from vibrations and turbu- lent gas flow can be effectively removed in array sensors by taking difference signals with reference cantilevers. For ex- ample, H2 can be detected by its adsorption on a Pt-coated sensor because a change in surface stress causes a static bending of the sensor. The diffusion of various alcohols into polymethylmethacrylate induces resonance frequency shifts in a dynamic measuring mode and bending in the static mode, which allows one to distinguish between the various alcohols. Sensor devices for detection of gases and vapors via spe- cific coatings are gradually gaining importance in chemistry, materials science, and biochemistry owing to the increas- ing demand for detection of analytes at monolayer coverage. A field of increasing interest is the construction of so-called "electronic noses" capable of discerning different odors via a typical response pattern of the receptor layers to an analyte. Most devices currently applied involve square centimeter- sized detection areas and comparatively large gas volumes (typically 50- 1000 cm 3 ) resulting in relatively long response

How does the human brain deal with a spinal cord injury

Abstract: The primary sensorimotor cortex of the adult brain is capable of significant reorganization of topographic maps after deafferentation and de-efferentation. Here we show that patients with spinal cord injury exhibit extensive changes in the activation of cortical and subcortical brain areas during hand movements, irrespective of normal (paraplegic) or impaired (tetraplegic patients) hand function. Positron emission tomography ([O-15]-H2O-PET) revealed not only an expansion of the cortical 'hand area' towards the cortical 'leg area', but also an enhanced bilateral activation of the thalamus and cerebellum, The areas of the brain which were activated were qualitatively the same in both paraplegic and tetraplegic patients, but differed quantitatively as a function of the level of their spinal cord injury. We postulate that the changes in brain activation following spinal cord injury may reflect an adaptation of hand movement to a new body reference scheme secondary to a reduced and altered spino-thalamic and spine-cerebellar input.

Replication errors: cha(lle)nging the genome

Abstract: Since the discovery of a link between the malfunction of post-replicative mismatch correction and hereditary non-polyposis colon cancer, the study of this complex repair pathway has received a great deal of attention. Our understanding of the mammalian system was facilitated by conservation of the main protagonists of this process from microbes to humans. Thus, biochemical experiments carried out with Escherichia coli extracts helped us to identify functional human homologues of the bacterial mismatch repair proteins, while the genetics of Saccharomyces cerevisiae aided our understanding of the phenotypes of human cells deficient in mismatch correction. Today, mismatch repair is no longer thought of solely as the mechanism responsible for the correction of replication errors, whose failure demonstrates itself in the form of a mutator phenotype and microsatellite instability. Malfunction of this process has been implicated also in mitotic and meiotic recombination, drug and ionizing radiation resistance, transcription-coupled repair and apoptosis. Elucidation of the roles of mismatch repair proteins in these transduction pathways is key to our understanding of the role of mismatch correction in human cancer. However, in order to unravel all the complexities involved in post-replicative mismatch correction, we need to know the cast and the roles of the individual players. This brief treatise provides an overview of our current knowledge of the biochemistry of this process.

Characterization of Layered Lithium Nickel Manganese Oxides Synthesized by a Novel Oxidative Coprecipitation Method and Their Electrochemical Performance as Lithium Insertion Electrode Materials

Abstract: Lithium nickel manganese oxides, LiNi 1-y Mn y O 2+δ , (0 ≤ y ≤ 05) were prepared via a new solution technique The corresponding mixed nickel manganese hydroxide precursors were synthesized in an oxidative coprecipitation method Subsequent calcination in the presence of LiOH leads to crystalline products with a partially disordered layered-type α-NaFeO 2 structure X-ray photoelectron spectroscopic analysis has indicated a strong enrichment of lithium at the surface The electrochemical performance of these materials as positive electrodes in lithium-ion batteries was evaluated as a function of the calcination temperature and manganese content A calcination temperature of 700°C leads to the best cycling stability At this temperature, a sufficiently high degree of crystallinity was achieved, having a strong influence on the cycling stability of these 4 V materials The specific charge and cycling stability obtained for the solution-prepared pure lithium nickel oxide, LiNiO 2 , was low, but was significantly enhanced by replacing some nickel with manganese With increasing manganese content, the specific charge increased to about 170 mAh g -1 for materials with a Ni:Mn ratio of about 1:1 Ex situ magnetic susceptibility measurements proved that during lithium deinsertion, the trivalent manganese is preferentially oxidized, and seems to be the more reactive redox center in these oxides

Rare earth elements in soil and in soil-grown plants

Abstract: Concentrations of the rare earth elements (REEs) La, Ce, Nd, Sm, Eu, Gd, Tb, Yb and Lu were determined in leaves of 6 plant species (Norway spruce, silver fir, maple, ivy, blackberry, and wood fern), and in pertinent soils and soil extracts, also taken from the same site. The distribution of the individual REEs in plants showed little or no agreement with that in the soil or the soil extracts. Ce had a negative anomaly with respect to the soil in all plants. The REE distribution patterns of fir and spruce were almost identical, but differed profoundly from that of the other species. In most cases, concentration ratios between species were a smooth function of the atomic number of the REE. Very similar results were obtained at 2 additional sites.

Plastic behavior of nanophase metals studied by molecular dynamics

Abstract: We report molecular-dynamics simulations of plastic deformation of Ni nanophase samples with different grain structures, temperature, and applied stress. We analyze the mechanical and thermal activation of the elementary process contributing to plastic deformation at the grain boundaries and provide a quantitative interpretation in terms of a general nonlinear viscous behavior whose temperature, stress, and grain-size dependence is determined.

Random and exchange anisotropy in consolidated nanostructured Fe and Ni: Role of grain size and trace oxides on the magnetic properties

Abstract: Results of magnetization measurements on nanocrystalline Fe and Ni produced by inert-gas condensation are presented. The grain size, which is about 10 to 20 nm in the as-prepared state, is increased by annealing the samples incrementally from 100 \ifmmode^\circ\else\textdegree\fi{}C to 1000 \ifmmode^\circ\else\textdegree\fi{}C. The coercive field shows a pronounced variation with grain size, with a maximum at around 30 nm and a steep decrease for smaller grain sizes. The coercivity is discussed on the basis of the random-anisotropy model that predicts that the effective anisotropy constant is reduced by averaging over magnetically coupled grains. This behavior is observed as long as the grain size is smaller than the effective bulk domain-wall width. The model also accounts for the approach to saturation in nanostructured Fe yielding values for the ferromagnetic correlation length and the anisotropy constant of the grains. The latter is about four times higher than the bulk value of Fe. Hysteresis measurements at 5 K after field cooling show a shift and broadening of the hysteresis loops for both Fe and Ni, which is attributed to an exchange coupling between the ferromagnetic grains and antiferromagnetic or ferrimagnetic (oxide) interfacial phases. The hysteresis shift decreases and finally vanishes with increasing grain size. This is indicative of a restructuring of the oxides, which is confirmed by the coercive field of the Fe samples showing a step at about 120 K caused by a phase transition of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}.$ The step vanishes again with further increasing grain size. The saturation magnetization of the Ni samples increases with increasing annealing temperature, a fact that is attributed to the evolution of the oxides also.

Spin waves and the critical behaviour of the magnetization in

Abstract: Inelastic neutron scattering from a single crystal of the quasi-two-dimensional antiferromagnet has been used to measure the spin wave dispersion curve at 4 K. The exchange integrals were subsequently calculated from linear spin wave theory. The values meV, meV, meV and meV are within stability conditions calculated from mean-field theory. In addition, the critical behaviour of the gap in the spin wave energy at the Brillouin zone centre has been measured, and compared to the critical behaviour of the magnetization from neutron scattering data of the magnetic (020) Bragg peak. The gap varies with magnetization for , and with the square of the magnetization for . Two possible explanations are proposed: a competition between single-ion and dipolar anisotropies; or a crossover to XY-like excitations.

Aerosol transport to the high Alpine sites Jungfraujoch (3454 m asl) and Colle Gnifetti (4452 m asl)

Abstract: Atmospheric transport processes, relevant to high Alpine sites, were deduced from 2 sets of aerosol records: a 9-year record from the Jungfraujoch (3454 m) on the northern side of the Swiss Alps and a 2.5-year record from Colle Gnifetti (4452 m) on the southern side. A classification scheme for synoptic weather types was applied to separate the aerosol data into groups corresponding to different atmospheric transport conditions. For both sites, vertical aerosol transport by thermally driven convection, acting between late spring and late summer, was found to be the dominant transport process. In summer, the thermally driven aerosol transport to both sites caused an increase of the seasonally averaged aerosol concentration between 0800 and 1800 local standard time by a factor of two. Under anticyclonic conditions, when subsidence on a synoptic scale is present, the thermally driven aerosol transport is most pronounced. Therefore, the aerosol determining thermal transport takes place within a synoptic scale vertical motion of opposite direction. Under cyclonic conditions, when lifting on a synoptic scale is present, the thermally driven aerosol transport is nearly absent. In winter, thermally driven convection does not contribute to the aerosol concentrations at both sites. Nevertheless, also in winter statistically significant differences in aerosol concentration were found between cyclonic and anticyclonic weather conditions, which can be attributed to the vertical transport acting on the synoptic scale. These differences in aerosol concentration were small compared to the corresponding differences in summer. Within the weather types, which are dominated by horizontal advection in the Alpine region, the aerosol concentrations are more diffcult to interpret with respect to the effective transport process. DOI: 10.1034/j.1600-0889.1998.00006.x

PEM water electrolysers: evidence for membrane failure in 100kW demonstration plants

Abstract: The long term behaviour of two 100kW proton exchange membrane (PEM) water electrolyser plants is analysed. The systems had to be shut down due to problems with excessive levels of hydrogen in the oxygen product stream. The time to breakdown was different by a factor of nearly 10 from plant to plant. Post mortem analysis of the cell stacks revealed that the Nafion® 117 membrane is the weakest part in a PEM electrolyser regarding long term performance. Substantial thinning of the membranes in the stacks was detected. The degradation process was found to depend on the position within an individual cell, as well as of the position of the cell in the electrolyser stack. The dissolution process proceeds from the interface between the cathode and the membrane, is not specific with respect to the ion exchange groups, and is most likely triggered and/or enhanced by local stress on the membrane.