Table of contents

We report a switching of conducting channel and magnetoresistance (MR) in very thin metallic films deposited on Si substrate with native SiO₂ surface. The resistance of the metal-oxide-semiconductor (MOS) structure significantly increases when the temperature is lowered to a threshold value T_c of about 250 K. At room temperature the samples exhibit a high conductivity, and a positive MR of about 18%. Below T_c, the resistivity is increased by ~50% and the positive MR disappears. This effect can be explained by the conducting channel switching from the Si electron inversion layer to the upper metallic films when the temperature is decreased. It has also been found that the MR of Cu₈₀Co₂₀-SiO₂-Si structure changes sign with the change of temperature, which is correlated to the switching of the conducting channel.

The present review outlines, on the basis of specific examples taken from our laboratory, the most important steps in (i) preparing supported nanoassembled model catalysts and (ii) investigating their size-dependent catalytic properties. We describe the cluster generation and present evidence for softlanding of the clusters onto solid surfaces. Subsequently, the growth and the characterization of the cluster support material, thin magnesium oxide films, are discussed, including the role of surface defects. Then the thermal stability of Cu clusters and the individual electronic structure of small Ag and Cu clusters on MgO are addressed. Finally, two examples of cluster size-dependent heterogeneous catalytic reactions are presented, (i) the cyclotrimerization of acetylene on nanoassembled Pd catalysts, and (ii) the CO oxidation on nanoassembled Au catalysts. We show experimentally and together with first-principle calculations that the interaction of such small metal clusters with the oxide surface strongly changes their catalytic properties. In contrast to large particles an additional electron in localized valence states of small clusters represents a major change. It is this charge transfer which turns inactive gold and palladium clusters into active model catalysts opening new perspectives to tune catalytic processes on the nanoscale.

Results of time-dependent modelling of electrohydrodynamic effects on the surface of a liquid metallic conductor are reported for a regime where no electron, ion or particle emission occurs. The Navier-Stokes equations, with free liquid boundaries subject to Maxwell field stress, surface-tension stress and viscous action, have been solved by a method that uses transformation of the interfaces into a rectangle; this overcomes a problem of surface oscillations that appeared using the marker-and-cell technique. The situation geometry is a deep unbounded surface with axial symmetry. With time, an almost flat surface evolves into a cone-like shape, with the angle of the cone depending on the initial shape of the surface. We describe this structure as a dynamic Taylor cone. The time-dependent profiles of the surface shape are in good agreement with experimental observations of this process. The calculations have also shown that, when the protrusion is formed, the time dependences of the surface radius of curvature, the electric field value at the protrusion apex and the axial velocity of the liquid metal, exhibit a run-away behaviour: the physical values become very large for a short time. As a cusp evolves on a surface, the Maxwell stress acting outwards becomes very large and overtakes the growth of both the surface tension and viscous stress acting inwards. Analysis of the time dependences of physical values can strongly assist the development of analytical treatments of such phenomena, and give insight into the problem of the dynamic description of operating liquid metal ion source atomisers. The development of numerical methods using transformation of the interfaces appears very useful for the treatment of problems in which the cathode or the anode significantly change shape. This situation occurs, for example, when a liquid surface is covered by a metal plasma and the evolution of the surface occurs in the context of a Langmuir shield.

The paper reports a study on the kinetic parameters of some thermoluminescent tissue equivalent materials recently prepared in Vinca Institute: Li₂B₄O₇:Mn,Si, Li₂B₄O₇:Cu, Li₂B₄O₇:Cu,In and MgB₄O₇:Dy,Na. The kinetics parameters have been determined using several methods. The temperature lag effect, which could produce large errors in the parameters' determination when they are determined using high heating rates, has also been taken into consideration.

Y₂O₃:Eu³⁺ thin films were prepared by pulsed laser ablation on silicon (100) substrate. The relative photoluminescence intensity dependence on annealing time was studied. The microstructures of annealing thin films were studied by atomic force microscopy. It was observed that the photoluminescence intensity at 611 nm from thin film annealed at 1000 °C for 4 h tended to saturate, the average crystal size of the thin film was estimated to be 71 nm by x-ray diffraction measurements.

The role of a spin-dependent electronic structure in the giant magnetoresistance effect (GMR) of metallic magnetic multilayers is considered. The general transmission amplitude (coefficient) for an electron Bloch wave passing from one magnetic layer to another one through a nonmagnetic spacer in the ballistic regime is found for the case of perfect interfaces. The Boltzmann equation, which incorporates this transmission coefficient and spin dependent scattering (SDS) in a bulk of magnetic layers, is solved for a magnetic multilayer in the current-in-plane (CIP) geometry. It is shown that, if there is no spin asymmetry in an electron scattering by imperfections, an electron refraction (a difference in the electron velocities in different layers/different sheets of the Fermi surface) itself results in a negative magnetoresistance and thus can lead to an enhancement of the GMR. The interference of electronic waves leads to the oscillations of the magnetoresistance as a function of the spacer thickness. These oscillations can survive an integration over the Fermi surface and their periods are determined by the stationary vectors of the spacer Fermi surface. The oscillating quantum corrections are calculated explicitly. A new approach to accounting for imperfections of the interfaces is proposed. The general formula for the magnetoresistance, which accounts for both the coherent and diffuse electron scattering at the interfaces, is obtained.

This paper deals with the propagation of electromagnetic waves along a wire coil surrounded by a highly conductive screen with electron plasma present in the coil interior. A dispersion relation in finite form is obtained. This relation passes correctly into the dispersion relation of electron plasma surface waves when the coil is removed and into that of coil waves when the plasma is non-existent. The wave on the plasma-coil combination is always faster than the plasma or coil wave taken separately. An approximate addition rule is proved valid for low plasma densities. According to this rule the phase velocity of the wave on the plasma-coil combination is compounded from the phase velocities of the plasma and coil waves. The characteristic impedance of the helical waveguide is discussed.

Sealed-off CO₂ lasers excited by a dielectric barrier discharge are studied. The laser output power is measured at excitation frequencies between 0.3 and 0.6 MHz; discharge gaps are in the range 1.6-6.0 mm. The excitation efficiency of the all-solid-state generator reaches 90%. Due to the scaling laws of radio frequency excitation no laser activity should be possible for the laser under investigation. The use of high-frequency excitation (0.3-0.6 MHz) combined with a dielectric barrier discharge gives the possibility of creating a homogeneous stable CO₂ laser gas discharge with high laser activity. The specific laser power follows the 1/d scaling law due to gas cooling by diffusion. Even the optimal gas pressure can be increased by high-frequency excitation compared to the radio frequency excitation.

The ability to generate acoustic longitudinal and shear bulk waves in solid materials using a single transducer is a suitable feature required in several devices and systems involving ultrasonic waves. The problem addressed here is that of the, possibly wideband, multimode buffer delay line, usable, for example, in non-destructive evaluation applications. The solution considered for this purpose is to use rotated, Y-cut, single-layer or two-layer transducers. The theory pertinent to this kind of transducer is briefly recalled and the results of the numerical analysis using two different delay line materials and three different piezoelectric materials are reported in detail. One experimental realization is also described.

We present results of Monte Carlo simulations for the components of electron diffusion tensor in time-varying E×B fields. In particular we follow the development of anomalous behaviour due to the change of the sign of the electric field which was found for longitudinal diffusion in purely electric rf fields (K Maeda et al 1997 Phys. Rev. E 55 5901). Similar behaviour was found to be present in crossed fields for the longitudinal ND_E component and absent for one perpendicular ND_B component. It turns out that anomalous behaviour for the ND_E×B component is induced even for a relatively small magnetic field. Reid's ramp model was used for calculations but the conclusions are quite general as these effects occur for most real gases. The transient kinetic phenomena are usually not taken into account in continuum models of rf plasmas. Our results show that it is important to include such processes in modelling and to verify the electron kinetic codes for their ability to represent the transient phenomena.

This work is devoted to the study of atomic oxygen recombination on a glass surface, mainly in connection with atomic sources development. In this paper we present a non-stationary model for atomic oxygen recombination on a fused silica surface. Kinetics equations for oxygen atoms, taking into account heterogeneous reactions between gaseous atoms and the surface (Eley-Rideal mechanisms), as well as homogeneous processes involving surface migration of adsorbed species (Langmuir-Hinshelwood mechanisms), are solved. Surface reaction coefficients are calculated, and the choice of numerical values for surface parameters is discussed. The solution to the equations is compared to our previous experiments concerning the influence of the surface state on atomic recombination. An estimation is made of surface reaction coefficient values.

The laser-induced fluorescence technique is used to study NO removal in N₂/NO mixtures, after excitation by a homogeneous photo-triggered discharge. Time-resolved experiments are performed in the decaying post-discharge in order to measure the NO concentration 200 µs after a 75 ns duration current pulse. For the first time the NO removal is measured for a single current pulse excitation of the polluted mixture near atmospheric pressure. We discuss the influence of both the electrical energy deposited in the discharge and the initially applied reduced electric field on the NO removal efficiency. It is shown that near 100% of the initial NO molecules can be removed with a single shot, depending on experimental conditions. A detailed kinetic analysis is performed, which leads to straightforward analytical approach used to estimate the N atom density produced by the discharge as a function of the electrical parameters values. It strengthens that, with regard to known kinetic processes, the ideal case for 100% NO removal is to create the exact number of N atoms corresponding to the number of NO molecules, at the lowest energetic cost.

Langmuir probes and a quadrupole mass spectrometer were used to determine the plasma parameters of an oxygen plasma in a planar inductive discharge. The electron density, effective electron temperature, the dc plasma potential and the electron energy probability function (EEPF) in the discharge centre plane were investigated as functions of power, gas pressure and radial position. The ion energy distribution and relative density of positive ions at the radial sheath edge were investigated as functions of power and pressure. A volume-averaged global model of the electronegative oxygen discharge is developed. The model uses a power balance equation to account for energy deposited into the plasma and lost via collisions and particle flux. The particle densities are modelled via rate equations estimated from collision cross sections assuming Maxwellian electron energy distribution functions. The volume-averaged model is shown to predict the experimental trends over a range of process conditions.

The present work concerns the experimental and theoretical analysis of the electrical behaviour and N₂O/SiH₄ dissociation of a classical RF discharge used for SiO₂ thin-film deposition. Electric and deposition rate measurements are undertaken at 0.5 and 1 Torr gas pressures. The reactor modelling involves electrical, hydrodynamic and mass transfer models. The electrical model enables the calculation of the electron impact dissociation rates required for the mass transfer model, while the gas velocities are determined by the hydrodynamic model. Only an electrical discharge model accounting for the negative-ion conversion reactions O^-/SiH₄ and NO^-/SiH₄ allows good agreement between the measured and calculated power densities particularly at 1 Torr. Furthermore, a simplified chemical scheme which includes 16 species (N₂O, N₂, O₂, NO, NO₂, N, O(³P), O(¹D), SiH₄, SiH₃, SiH₃O, H₂SiO, H, H₂, OH and H₂O) is used in the mass transfer model. The corresponding results (deposition rates) are quite consistent with the measurements.

The influence of the Joule heating on the performance of thoriated tungsten cathodes working at current densities of 10³-10⁴ A cm^-2 is investigated. The variation of the degree of coverage of the cathode surface by thorium atoms is considered, together with the heat conduction and the diffusion of thorium in the cathode bulk. The nonlinear system of equations governing these processes is solved numerically and the time evolution of the spot temperature is obtained. The effects of different working conditions on the cathode lifetime are discussed. It is found that a critical current density exists for which the service time of the cathode has a minimum. It is shown that a small increment in the cathode cooling temperature significantly increases the cathode lifetime.

The chemical stability of 10% SF₆-90% N₂ mixtures (100 kPa pressure) has been studied under sparking in the presence or absence of an organic insulator (teflon, kel'F, polyethylene, polypropylene, nylon, megelit), and/or added impurities (0.2% O₂ or H₂O). The study was carried out experimentally and numerically.

The production rates of the main decomposition products of SF₆: (SF₄ + SOF₂), S₂F₁₀, etc, and of products containing nitrogen, NF₃, N₂O, were evaluated. The results from the model led to trends similar to those observed experimentally concerning the effects on gas phase composition, of the presence of atoms such as O, H, C, etc, from the vaporized insulator and/or the added impurities.

A comparison of the chemical stability of the SF₆-N₂ mixture with that of pure SF₆ has also been made. The results can be summarized as follows:

• in all cases, the major byproducts formed were (SF₄ + SOF₂) and S₂F₁₀;

• the use of the SF₆-N₂ mixture instead of SF₆ led to the decreased formation of degradation products;

• vaporization of an organic insulator caused an increase in the production of degradation products which was less intense for the mixture than for pure SF₆;

• the study carried out with nitrogen-diluted SF₆ showed the importance of foreign atoms (C, H, O, etc) on the proportion of SF₆ that is not regenerated. Owing to their tendency to trap fluorine atoms, the impurity atoms prevent SF₆ from recombining and can thus lead to the deterioration of the dielectric qualities of the gas mixture.

This paper presents an experimental study on the influence of an aromatic additive (pyrene) on the propagation of positive filamentary streamers and breakdown in liquid cyclohexane. Experiments are carried out in point-plane gaps up to 5 cm over a wide voltage range. With pyrene, the propagation is facilitated, and thus the breakdown voltage is lowered in point-plane geometry. Streamers also become very branched, and a much larger number of filaments propagate than in pure cyclohexane. Correlated to this, a large increase of the inception voltage of fast streamers (acceleration voltage) is observed at high voltage. This fact can be explained by macroscopic electrostatic properties of streamers, which depend on their geometrical structure. When more and more filaments propagate at high voltage they shield each other, which in turn limits their tip field (and thus velocity) at a nearly constant value over a large voltage range. The propagation velocity is thus not only determined by microscopic processes at the filament heads. Consequently, it does not always constitute an adequate parameter to directly appreciate and compare microscopic propagation processes. A discussion about streamer mechanisms is also presented.

The transition of a high-current glow to an arc and the energy dissipation during the glow phase have been investigated in dry air by static breakdown. The measurements have been made for uniform-field gaps up to 2 cm at gas pressures of 1-20 Torr. A low-inductance capacitor of 1.89 µF and a discharge circuit with a coaxial system were used to produce a transient glow discharge with a current in excess of 400 A. The experimental results showed that the high-current glow discharge occurred at a cathode current density of 3.2 A cm^-2, which is almost two orders larger than the value deduced from the formula for Cu-air normal glow; j_n = 240×10^-6p², where j_n and p are the current density and gas pressure, respectively. The transition from the glow to an arc starts to develop in the discharge region near the cathode. The energies dissipated in the discharge or in the cathode fall region during the glow phase depend on the gas pressure and/or the electrode separation. However, the dissipated energy density in the cathode fall region during the glow phase is independent of the gas pressure and the electrode separation, and is approximately 0.035 J cm^-3 in the present experiment.

We investigated the positive charge carrier transport in poly(4,4'-biphenylenevinylene), PBPV, thin films using Au and Al electrodes. It was observed that Au/PBPV/Al devices present space-charge limited transport, whereas in Al/PBPV/Al devices the charge transport is injection limited. The positive charge carrier mobility was found to be approximately 6×10^-9 cm² V^-1 s^-1. Applying the Richardson-Schottky thermionic emission theory to Al/PBPV/Al devices we determined the Al/PBPV barrier height for positive charge carrier injection.

The absorption of a 4 M NaCl solution in calcium-silicate brick was investigated by nuclear magnetic resonance scanning. This method has the advantage that quasi-simultaneously both the moisture and the Na profile can be measured during absorption. It was found that during the absorption process the Na ions clearly stay behind and hardly any Na is present near the wetting front. This is caused by binding of the Na ions to the pore surface. It is shown that both the moisture and the Na profiles during absorption can be scaled using the Boltzmann-Matano transformation.

The simulation studies of free jets are required in order to understand the physical processes occurring during expansion of an atomic beam from a high-intensity source. The atomic beam undergoes transition from the continuum regime to the non-equilibrium transition regime of flow. Various models based on continuum equations of change and the simplified Boltzmann equation are inadequate to handle the flow in the transition regime, particularly for the effect of various geometrical boundary conditions. The numerical simulation of the free jet is more suitable for these studies. In this paper we have described the development of a direct simulation Monte Carlo (DSMC) code, which was implemented on parallel processors. The model calculations have been verified with published results for simpler conditions of free jet and have also been validated with experimental results. Studies of some of the parametric studies with various source as well as substrate geometries are discussed in this paper.

The control of exposure to welding fume is necessary to meet health and safety obligations. The work reported here examines the fundamentals of welding-fume formation. A physical chemistry model of the metal vapour mechanism for fume formation has been developed for non short-circuiting transfer gas metal arc welding (GMAW). The model includes the important contribution made by direct condensation of metal vapour onto the weldpool and workpiece, in removing a substantial fraction of the fume. The model shows that droplet size and wire feed speed control the fine fume formation rate. The understanding developed so far, indicates that the smaller the detached droplet size, the lower the total fume formation rate. The physics behind this is explained. The model gives an insight into how process modification might be used to control fume at source. Control at source is believed to be the most cost-effective and energy-efficient technique for dealing with welding fume. It is anticipated that the understanding gained from this project will be applied to determine the practical limits for the control of welding fume at its source.

The concept of the efficiency of a process is used to analyse various thermodynamic power cycles with ideal gases. The Otto cycle is treated by considering irreversibilities coming exclusively from expansion and compression processes. For this cycle, the maximum irreversible work and the maximum efficiency are obtained in terms of the isentropic efficiencies and of the maximum and minimum temperatures of the reversible cycle. The results obtained are easily applicable to the Brayton cycle and have some similarities with those obtained from finite-time thermodynamics. The expression found for the efficiency of the Otto cycle for irreversible maximum work is similar to that obtained by maximizing the irreversible work in the Curzon-Ahlborn-Nokivov engine.

A self-closing crowbar switch has been designed and tested up to a voltage of 500 kV. The crowbar utilizes the consistent nature of electrical breakdown in water under highly non-uniform electric fields as the timing mechanism for closure and has no requirement for external triggering. The device can be operated with closure times which are controllable over the range 200-500 ns by altering the gap spacing of the switch electrodes. Measurements of the crowbar performance are presented, as well as the important aspects associated with the electrostatic design.

The photoconductor amorphous selenium (a-Se) has been used in medical imaging since the 1950s, and is currently under study for use in digital radiography. The quantity of interest for radiographic applications is the energy required to create a detectable electron-hole pair, W_±, which is governed by the recombination of electron-hole pairs. Both geminate and columnar recombination theories have been invoked to describe recombination in a-Se, without success. In this work, we develop a Monte Carlo code which follows individual collisions along the x-ray/electron track structure in order to determine the initial positions of the electron-hole pairs within a-Se. We subsequently simulate the transport of the pairs within an electric field to calculate the amount of recombination. We use these simulations to calculate W_± as a function of applied electric field and incident x-ray energy. We find excellent agreement between our simulations and experimental results. This indicates that the recombination occurs not only between geminate pairs but also between other pairs created along the track. This inter-track recombination leads to an energy dependent recombination probability which could not be explained with previous recombination theories.