Table of contents

In the present work, the effect of longitudinal and transverse magnetoimpedances connected with the transverse permeability and longitudinal permeability for amorphous ribbons of CoFeNiSiB has been investigated in detail. Our results strongly show that there exists a correlation between the magnetic field dependences of the magnetoimpedance and the magnetic permeability.

Continuous and plain CrSi₂layers were directly formed on Si by high-current Cr ion implantation into Si wafers, which were heated simultaneously by the ion beam itself and maintained at relatively low temperatures down to 150 °C. Interestingly, the CrSi₂layers so obtained were of n-type and their Hall mobility was as high as 1085 cm²V^-1s^-1 .

A new white-emitting organic diode was realized simply by inserting a doped hole-blocking layer between the hole-transporting layer (HTL) and the electron-transporting layer (ETL). The structure of this device is ITO/CuPc/NPB/blocking layer:rubrene/Alq/MgAg. Copper phthalocyanine (CuPc) was used as a buffer layer; N,N´-bis-(1-naphthyl)-N,N´-diphenyl-1.1´-biphenyl-4-4´-diamine (NPB) was the HTL; the tris(8-quinolinolato)aluminium complex (Alq) was the ETL; and the trimer of N-arylbenzimidazole (TPBi), 2-(4-biphenylyl-5-(4-tertbutylphenyl)-1,2,3-oxadiazole (PBD) or the 1,2,4-triazole derivative (TAZ) were used as the blocking layers, in which rubrene is doped. The emission spectrum of this device covers a wide range of the visible region and can be sensitively adjusted by the concentration of rubrene. The white emission with CIE (Commission International de l'Eclairage) coordinates x= 0.31, y= 0.32, a maximum luminance of 8635 cd m^-2and maximum luminous efficiency 1.39 lm w^-1(4.9 V) were obtained in the device with a concentration of 1.5% rubrene in TPBi.

Thermoluminescence (TL) and radio-thermoluminescence spectral analysis techniques have been applied to doped calcium sulphate samples designed for radiation measurements at elevated temperatures. CaSO₄ :Dy, when co-doped with Ag, provides a TL dosimetric peak near 350 °C which is useful for radiation measurements at high temperatures. Dopants of Ce, Mn and Dy variously move the peak temperature from 400 °C to 200 °C. Each dopant ion gives a characteristic emission spectra, which for CaSO₄ :Ce, Mn samples indicate that there is a systematic temperature difference of ~7 °C between the glow peaks from the Ce and Mn sites. The CaSO₄ :Dy samples show a discontinuity in the emission wavelength from the Dy ions near T= 200 °C and a decrease in the radioluminescence fluorescence in the same temperature region. In each case it is proposed that the dopants form part of large, complex defects, instead of isolated trapping and recombination centres. The data offer further evidence for a localized phase transition of the defect complex at 200 °C.

Low-temperature data, from 20 K, show similar differences in the peak temperature from the various dopants and additionally indicate reproducible discontinuities in the wavelength positions and intensities, for all samples, at T= 230 K. This again suggests structural phase adjustments of the defect sites.

The effect of Mn substitution for Fe in Sm₂ Fe_17-x Mn_xcompounds on the structure and magnetocrystalline anisotropy was studied by x-ray diffraction and magnetization measurements. The compounds prepared have a rhombohedral Th₂ Zn₁₇ -type structure single phase except for a small amount of -Fe for x= 1-4, while x= 5 has Nd₃ (Fe,Ti)₂₉ -type monoclinic structure. The Curie temperature T_Cis found to increase to 430 K for x= 1, then decrease to 255 K for x= 5. The saturation magnetization at 5 K decreases with increasing Mn concentration. Room temperature anisotropy changes from easy planar type to cone type and to easy c -axis type with increasing Mn concentration for x= 1, 2 and 3 respectively. The anisotropy field for x= 3 is about 1 T at 100 K and decreases to 0.4 T at room temperature.

When a high-power laser beam is irradiated on the surface of material, it is well known that a cavity, called a keyhole, is generated in the molten material. To examine the effect of laser beam pulse shape on the keyhole behaviour, we observed it directly with an in situmicrofocused x-ray transmission imaging system. As a result, it was found that the keyhole began to be generated after about 0.6-0.7 ms and became deepest after about 1.5 ms from the laser irradiation initiation and collapsed about 1 ms after laser power declination when a pulse modulated laser beam of 1.1 ms rise time and 4.6 kW peak power was used. It was also understood that the generation and the collapse of the keyhole was repeated, synchronizing with laser beam irradiation of 100 Hz. The transition of the keyhole depth and the cross sectional area had good agreement with the transition of the irradiated beam shape. Moreover, it was observed that the porosity was frequently generated at the beam shape in which the laser power was decreased rapidly from a high peak power.

With the objective to envisage short pulses for laser-shock hardening of materials, this paper reports experiments where laser-shock amplitudes Pgenerated with 0.6-3 ns laser pulses at = 1.06 µm in a confined regime with water have been compared with those achieved with the usual 10-30 ns configuration. First, the experimental characterization of shock waves with polyvinylidene fluoride (PVDF) and electromagnetic (EMV) gauges shows that the short durations allow the generation of higher shock amplitudes than longer duration pulses (10 GPa versus 5 GPa) because of an increase of the pressure saturation intensity threshold I_thwith short pulses (up to 100 GW cm^-1at 0.6 ns). Above I_th , a pressure pulse shortening accompanies the saturation. The P= f (I ) curves have been confirmed by surface deformation measurements induced on a Al12Si alloy. Second, the use of 10 µm aluminium coatings on 316L steel targets impacted at 40 GW cm^-2irradiance was shown to provoke a 25% maximum increase of the peak pressures by type mismatch acoustic impedance effects. Lastly, the PVDF technique is shown to be an accurate method to measure laser shock wave profiles in the 0-200 GW cm^-2regime, whereas the EMV technique is limited to intensity values of less than 20 GW cm^-2 .

This paper presents the first step in developing a tool which would be able to predict the breakdown voltage of air insulation under a positive switching impulse. The discharge model uses a map of the Laplacian potential and the electric field computed by an electric field software. The model is based on the previous work of different authors who have studied the positive discharge theoretically as well as experimentally. After a presentation of the different aspects of the model some results are analysed. These results concern classical rod-plane configurations for which valuable and accurate measurements are available. The analysis points out some weaknesses of the model that will have to be corrected to treat realistic configurations. Nevertheless, as the modification of only one parameter with the gap distance permits us to compute U₅₀to an acceptable accuracy, the model could already be used to compare two insulating structures of the same type.

A self-consistent one-dimensional model of the anode region of inert gas discharges at low pressures and currents is developed on the basis of non-local electron kinetics in spatially inhomogeneous fields. The model is based on the kinetic equation for electrons, the momentum transfer equation for ions and Poisson's equation. These equations are solved in the extended, non-uniform, quasi-neutral plasma disturbed by the anode and in the thin collisional sheath of space charge. The input parameters are the discharge current, neutral gas pressure and the radius of the tube. The axial profiles of the field, potential, radially-averaged densities of charged and excited particles, rates of excitation and ionizations and the ion current densities are obtained from the anode to the positive column. Two different cases are modelled which correspond to the self-consistent formation of either monotonous or non-monotonous electric field profiles. These cases correspond to different mechanisms of ion production in the sheath and the quasi-neutral plasma in the anode region. Particular calculations are carried out for neon discharges.

A model is reported of the cathode region of a dc glow discharge at high current density. The distributions of most important plasma species (such as fast electrons, fast neutrals and ions, and energetic sputtered metal atoms) are determined either analytically or using simple semi-analytic methods, taking into account possible strong gas-density/temperature gradients. A heat-transfer equation is used to calculate the gas temperature, and the power-deposition profiles due to fast neutrals and energetic metal atoms are determined at the kinetic level. For the conditions of interest, the power-deposition profiles are shown to be strongly non-local and the assumption of Joule (local) heating to be generally inappropriate. Fast neutrals reflected from the cathode, as well as fast recoil neutrals, are shown to play a key role in determining the rate and spatial distribution of gas heating. Spectroscopic measurements of the gas temperature in a hollow-cathode discharge are carried out over a wide range of discharge conditions. The experimental and predicted gas temperatures are compared and acceptable agreement is found. A strong sensitivity of the model predictions to the assumed particle-surface interaction data is observed. The model can be readily adapted to other types of glow discharge and other geometries. It is computationally efficient and can be useful for fast self-consistent modelling of glow-discharge plasmas.

The growth of soot-like carbonaceous materials were studied by shadowgraphy during 20 ms CO₂laser pulse irradiation of a graphite-Co/Ni composite target in an Ar atmosphere at room temperature. Carbonaceous deposits were characterized by Raman scattering and transmission electron microscopy. We discuss the formation process of single-wall carbon nanotubes in a high-temperature viscous carbon flow.

Research into medical x-ray detectors based on amorphous selenium (a-Se) has seen a resurgence lately due to the advent of flat-panel active matrix technology. There is presently no quantitative theory, however, to explain the signal formation due to the interaction of ionizing radiation with a-Se. One of the main obstacles in the characterization of this process is the lack of suitable interaction cross-sections. In this work we develop an expression for the calculation of inelastic electron cross-sections from a dipole oscillator strength distribution, which includes relativistic and exchange effects. We construct the oscillator strength distribution from known experimental results and calculate the total and cumulative differential cross-sections for incident electron energies ranging from a few eV to 23 MeV. The total cross-section agrees with that of isolated Se atoms at high energies, as expected, and provides the low energy cross-sections which depend on the dielectric properties of the amorphous state. From these cross-sections, Monte Carlo codes can be developed which simulate the creation of individual electron-hole pairs, which will lead to a better understanding of the signal-formation process in a-Se based x-ray detectors.

The correlation between cluster decomposition and the structure of pulsed supersonic jets of argon clusters is analysed. The longitudinal and transverse space structures of short supersonic pulses ( 0.5 ms) of argon clusters have been studied far from the skimmer, by a movable mass spectrometer and near the skimmer, by an electron impact ionizer, depending on the stagnation pressure. It has been found that the pulses consist of two distinct fractions of clusters, light and heavy, which are also differently localized in space. The heavy fraction is located inside the pulses and the light fraction is mainly found in the outlying area of the pulses. It has been shown that neutral clusters reveal instability for a long period of time (~0.6 ms) after condensation. This instability results in additional divergence of the cluster beam. Therefore, the change of the cluster beam cross section as a function of the pressure correlates with the corresponding change in the cluster instability.

The transient microwave photoconductivity of AgCl microcrystals homogeneously doped with [Ru(CN)₆ ]^4-was investigated as a function of the exposure intensity and doping level at 120 K. Models involving deep electron and/or hole trapping cannot explain the experimentally observed strong increase of the signal response time and the decrease of the signal height with the doping level. A new model including both intrinsic centres and [Ru(CN)₆ ]^4-associated shallow electron traps was proposed and studied intensively with the aid of a newly developed computer simulation program. General conclusions from the model study have been summarized. With this model the effect of [Ru(CN)₆ ]^4-on both the signal height and response time can be successfully simulated.

Both intrinsic centres and [Ru(CN)₆ ]^4-associated centres were studied at 120 K in AgCl microcrystals with and without [Ru(CN)₆ ]^4-dopant, by the transient microwave photoconductivity technique. The densities, capture cross sections and lower limits of the trap depths of the dominant intrinsic centres in AgCl microcrystals can be estimated by a careful comparison between the experimental and simulated transient microwave photoconductivity in the reference emulsion. It has been shown that, by studying the transient microwave photoconductivity of a silver halide emulsion system with long response times (e.g. AgCl microcrystals homogeneously doped with [Ru(CN)₆ ]^4- ), not only the capture cross section and trap depth of the [Ru(CN)₆ ]^4-related centres, but also the trap depth of the intrinsic centres can be more accurately determined. In this way, a new possible method to determine the trap depths of intrinsic centres in silver halides was suggested.

The scattering of electromagnetic waves by a sphere with highly absorbing randomly distributed inclusions has been considered. An approximate formula based on the geometrical optics approximation has been examined. Numerical comparisons demonstrate it to be an extremely useful approximation in predicting the absorption efficiency, asymmetry parameter and albedo of the sphere.