Table of contents

The Jahn-Teller effect is used semi-theoretically to analyse UV visible spectra of the diamond-like carbon films on Si substrates. By deconvolution of UV visible absorption spectra of the typical films, different absorption lines are found. For each sample, six lines are distinguished, which were related to V° by the Jahn-Teller effect. Our theoretical approach, based on the distortion of the V° as the consequence of the stress in the films, is in agreement with experimental results. The shift of the Jahn-Teller lines toward the zero phonon line of V° were found to be in accordance with stress decrease in the films. The difference between room temperature and low temperature spectra is discussed. We found that the splitting of the excited state of V° in the films under stress is twice as large as that of the ground state.

A novel, pulsed, supersonic, cluster-beam source based on microplasma ablation has been realized and tested. Its intensity and stability allows one to overcome the limitations encountered in laser vaporization and pulsed arc cluster sources. Supersonic carbon cluster beams have been obtained and characterized. Target ablation processes and cluster growth have been investigated, showing the presence of mechanisms substantially different from those observed in other plasma-based sources. In particular, we show that the precise confinement of the ablation plasma in the source is of fundamental importance for the production of cluster beams with high intensity and stability. This opens new opportunities for the study of free and supported clusters and for the synthesis of nanostructured materials.

This article addresses the basic physics of the magnetic recording process with special emphasis on thin-film recording media for high-density magnetic recording. Magnetization reversal, recording theory as well as noise theory is reviewed. The onset of super-paramagnetism, which potentially limits the storage density, is discussed in detail.

Time- and space-resolved emission spectroscopic measurements have been performed to investigate the formation of molecular species in the plasma plume generated by pulsed-laser ablation of Al, C and Ti targets in either N₂ or O₂ low-pressure atmospheres. The recorded spectra show that the interaction between the vapour plume and the ambient gas gives rise to the partial dissociation and ionization of the ambient gas and that the formation of nitride or oxide diatomics depends strongly on the binding energies of these species with respect to dissociation potential of the gas molecules. Thus, nitride formation in the gas phase occurred only during carbon ablation while oxide molecules were detected in the plasma plume originating from every target material. The influence of the atomic mass of the target material on the gas-phase reactions is also discussed. The results contribute to a better understanding of the mechanisms involved in the thin-film synthesis by reactive pulsed laser deposition.

The magnetic properties of micrometre-sized thin film elements fabricated by electron beam lithography have been investigated for films evaporated under different conditions. Using the well established technique of varying the angle of incidence of evaporation we have produced elements with an additional contribution to the anisotropy. Lorentz microscopy has been used to study the effect of this anisotropy on the domain structures and switching behaviour for acicular elements which already possess a strong shape anisotropy. Our observations for elements with the induced anisotropy either parallel or perpendicular to the shape anisotropy indicate that the demagnetizing effects are still dominant. However the hysteresis loops and magnetization reversal of the elements are changed considerably with the additional constraint of induced anisotropy.

The low-temperature heat capacities of the two ternary intermetallic compounds GdNiGa and GdNiIn, which order ferromagnetically at 30.5 and 93.5 K respectively, were measured as a function of the magnetic field up to 9 tesla. From the experimental data the isothermal magnetic entropy change (S_magn) and the adiabatic temperature raise (T_ad) were obtained at 5 and 9 T. The refrigerant capacity, calculated at 5 T, is 530 J kg^-1 for GdNiGa, while for GdNiIn a lower value of 290 J kg^-1 was obtained.

A self-consistent one-dimensional modelling of a Xe gas discharge between electrodes covered with dielectric barrier is presented for power frequencies from 50 kHz to 1 MHz and gas pressures from 10 to 400 Torr. Spatiotemporal profiles of the concentration of electrons, ions, excited atoms and excimers are obtained. Excimers are mainly produced in the sheath regions. The efficiencies of spontaneous emission from excimers and resonance-state atoms increase with an increase in the input powers for gas pressures higher than 50 Torr. A characteristic period during which the barrier wall charge significantly influences the electric field in bulk region was found, and discharge properties in the period are discussed.

This paper deals with electron flux densities and electron energy flux densities in positive column discharges. Recent kinetic calculations by Uhrlandt and Winkler have revealed the interesting physical phenomenon of radially inward directed energy flux densities in positive column plasmas. We have used a self-consistent positive column model, based on an accurate and highly detailed Monte Carlo code, to study this effect in more depth. The results of this study show a rather complex physical picture of electron particle and energy flux densities. Electrons with low energies usually exhibit radially outward directed particle and energy flux densities. At energies above the threshold for electronic excitation particle and energy flux densities are usually inward directed. Only close to the wall, at total energies above the wall potential energy, do these flux densities point towards the wall. The thickness of this `wall loss region' scales with the electron mean-free-path for momentum transfer, i.e. with the inverse of the neutral gas density. The results presented can be qualitatively interpreted in terms of the total-energy picture for the electrons.

Measurements of the ratio of the transverse diffusion coefficient to the mobility D_T/µ (or the characteristic energy eD_T/µ) were performed for the reduced electric field (E/N) from 12 Td to 300 Td (1 Td = 10^-21 V m²). The ratio of longitudinal diffusion coefficient to mobility (D_L/µ) is determined from the fits of the appropriate current ratios obtained from a Townsend-Huxley experiment as a function of pressure and geometry for a fixed E/N. The results cover a range of moderate E/N and bridge the gap between the two sets of data available in the literature, while covering the range of energies corresponding to the vibrational excitation.

We report on an investigation in the magnetized alkaline plasma of a single-ended Q-machine where the current-voltage characteristic of the cold end plate showed some new nonlinear features. The most interesting phenomena are sudden jumps of the electron saturation current which are partly accompanied by hysteresis. These new phenomena are best visible for low densities. The current jumps are explained as an indication that the well known oscillating nonlinear potential structures, which appear in the plasma column when an electron current is drawn through it, and which are known as the potential relaxation instabilities, originate from collective effects of excitation and ionization interactions produced in front of the cold plate where accelerated electrons suffer inelastic collisions with the alkaline vapour present in the plasma column.

In a single-ended Q-machine the current-voltage characteristic of a small collector shows a sudden jump of the current at the onset of the electrostatic ion-cyclotron instability. This jump was also accompanied by hysteresis. This is explained as an indication that the well known nonlinear potential structures that appear periodically in front of the collector during one cycle of a high-amplitude electrostatic ion-cyclotron instability, originate from collective effects of excitation and ionization interactions produced in front of the cold plate where accelerated electrons suffer inelastic collisions with the alkaline vapour present in the plasma column.

A numerical model for an electric arc stabilized by a water vortex has been proposed. The two-dimensional axisymmetric model includes the discharge area between the cathode and the orifice of the arc chamber. The production of water plasma, i.e. the rate of evaporation of a water wall, is taken either from experiments or is determined numerically by fitting of the outlet plasma parameters to the experimental ones. The computer results concern thermal, fluid dynamic and electrical characteristics of such arcs for the currents 300, 400, 500 and 600 A. It is found, for example, that the role of thermal diffusion within the discharge increases with current. The power losses from the arc due to radial conduction and radiation represent around 50% of the input power. Rotation of the plasma column due to the induced tangential velocity component has negligible effect on the overall arc performance. The calculated velocities, pressure drops and electrical potentials are in good agreement with experiments carried out on the water plasma torch PAL-160 operating at our Institute.

The paper deals with a numerical model of an electric arc stabilized by a water vortex. The axisymmetric model involves the area between the cathode and the output nozzle of the arc. The rate of evaporation of water (production of water plasma) is determined from radial conduction and radiation heat fluxes near the water-water-vapour phase transition. The influence of non-uniform evaporation rate along the discharge coordinate on the outlet arc parameters is studied for the currents 300 and 600 A. It is found from calculations that part of the power spent on evaporation is in the range 1.4-3.1% of the total input power. The dominant source of power losses from the arc is plasma radiation, which exceeds conduction losses by a factor of two to four. Since the majority of the arc discharge is nearly thermally fully-developed, the effect of non-uniformity of evaporation on the overall arc performance is minor. The calculated arc outlet characteristics are in good agreement with our data published previously, as well as with experiments carried out on the water plasma torch operating at our Institute.

This paper presents some of the results obtained from a systematic series of laboratory investigations into the corona emission characteristics of air terminals. Two particular aspects are considered, namely the effect on corona emission of changing the distance between the tip of the air terminal and the overhead energized electrode, and the relationship between the two fundamental corona parameters (`amplitude coefficient' and onset electric field) and the height of the air terminal above a ground plane. The implications of the results for lightning protection designs are discussed.

The elastic properties of the vitreous system Bi₂O₃-B₂O₃ have been measured by ultrasonic pulse echo overlap technique at a frequency of 10 MH_z and at room temperature. For the purpose of measuring the ultrasonic velocity with high accuracy, the McSkimin criterion for determining the correct cyclic overlap between echoes is used. The elastic properties of the present glass system as a function of composition are discussed on the basis of the elastic internal energy due to the deformation. Also presented is a full discussion of the Poisson's ratio; which is found to be rather sensitive to the glass composition.

Results are presented of a study on the effect of moisture contents and temperature on the thermal conductivity of wood-aggregate-clay-cement composites, energy saving concretes. The thermal conductivity has been measured in the range of 2-60 °C, which covers the majority of thermal conditions in which the material is used. The unit cell of the parallel model, already validated for dry wood concretes, i.e. two-phase system, has been extended in order to calculate the thermal conductivity of wood concretes for various moisture contents. Experimental results show a significant dependence of thermal conductivity on moisture contents and temperature. The theoretical analysis of these phenomena has been conducted in two stages using the classical De Vries approach. The first stage consists of estimating the thermal conductivity at low temperatures for the three-phase system (i.e. unsaturated materials) using a modified unit-cell parallel model. The validity of the expression developed is then tested by comparing experimental results with computed values of effective thermal conductivity. The second part consists of determining the `f' factor that characterizes the effects of temperature on thermal conductivity. The experimental results of thermal conductivity at various moisture contents and temperatures were compared with the results obtained from De Vries's approach and a very high level of agreement was observed.

A study of the synthesized Se₈₀Ge₂₀ glassy system has been carried out. X-ray diffraction patterns and differential thermal analysis thermograms of the system studied were used to obtain an insight into the structural information. The dependence of the electrical conductivity of amorphous thin films of vacuum evaporated Se₄Ge, measured either in darkness or after different durations of light exposure on temperature and exposure time have been studied for different thicknesses. The conductivity showed an initial gradual decrease with time, ending with saturation. The activation energy was found to increase by increasing the period of light soaking at any thickness. The results were explained on the basis that the conduction takes place in the localized state in the band tails.

In this paper we have introduced the following improvement on our previous method for auto-separating thermally stimulated current (TSC), namely, both the general-order kinetic equation of a thermally stimulated process (TSP) and the real peak found by calculating the activation energy varying with the increment of peak temperature are used to replace the first-order kinetic equation and the peak point obtained from the experimental TSC curve, respectively. Using this method we have separated successfully the experimental TSC curve of polyimide film, and found that this complex curve is composed of six sub-curves in the temperature range 250-500 K, one of which obeys second-order kinetics. Moreover, by comparing the parameters obtained with the cleaning peak method and our improved method it can be confirmed that the latter is very effective. As a result we believe that this improved method is more reasonable, and describes the real physical processes in a polymer much better than before.

We describe a new technique that realizes the quantitative estimation of the biological cell concentration in an aqueous medium. The proposed dielectrophoretic impedance measurement method utilizes the positive dielectrophoretic force to capture suspended biological particles onto an interdigitated micro-electrode array in pearl-chain formation. Higher cell concentration results in faster development of the pearl chains, which are electrically connected in parallel within the electrode gap to increase the conductance and capacitance between the electrodes. By monitoring temporal variation of the electrical impedance, it is possible to quantitatively evaluate the cell population according to a theoretical model of the cell collection process. It has been demonstrated that a suspension liquid of Escherichia coli could be accurately assayed in about 10 min at 10⁵ cm^-3 concentration.

The microwave dielectric properties of Ba(Mg_1/3,Ta_2/3)O₃ (BMT) ceramic resonators are studied by the Whispering Gallery mode and conventional methods in the frequency range 6-18 GHz. The BMT samples show a maximum quality factor of 25 000 at 13.024 GHz (Q × f = 325 000). The temperature variation of the quality factor was measured in the range -14 to 47 °C. The temperature variation of resonant frequencies for different modes have also been measured accurately using a peltier circuit.

The present work represents investigations of angular emission distributions in laser-produced particle beams resolved for the different ion groups (up to q=4) and the neutral particle component. The measurements are for a spectrum of target masses: ¹² C, ²⁷Al, ⁴⁸ Ti, ⁵⁹Ni, ⁹⁶Mo and ¹⁸¹Ta. The plasma was produced by obliquely incident Q-switched pulses (TAU=5~ns and LAMBDA=1.06~MU m) of a Nd-YAG laser focused to energy densities ranging from about 20 to 180~J~cm^-2. For the first time the results reveal in detail that the emission distributions systematically depend on the degree of ionization of the particles in the cloud. While for the neutral particles the angular emission is always dominated by a broad background with an additional, but less pronounced, peaked component, the background component rapidly but continuously diminishes with the increasing charge state of the ions. If, in the usual way, the emission distribution is approximated by the superposition of a cosine and a cosⁿ fit function, the distribution of ions with qgeq 2 can already be well fitted by a cosⁿ function alone. It seems highly probable, that this behaviour essentially is a result of the recombination dynamics during the expansion. It was found that the effect holds for all atomic masses investigated, whereby the mass dependence of the exponent n for all species behaves alike, approximately following a A^3/4 law.

In this article, figure 7 was wrongly replaced by figure 5. The correct figure 7 is shown here.

Figure 7 Accelerating voltage dependence of the 2.7 eV CL peak build-up at room temperature. The sample is a 100 nm SiO₂ film on Si substrate. The beam current is 1nA and the accelerating voltages are 1.5, 3, 5 and 10 kV, respectively. This gives a dose rate of ~7.1 × 10⁸, ~9 × 10⁸, ~5.9 × 10⁸ and ~1.9 × 10⁸ rad s^-1 in the oxide layer respectively.}