Table of contents

In the limit of a small Debye length ( lambda _D to 0) the analysis of the plasma boundary layer leads to a two-scale problem of a collision free sheath and of a quasi-neutral presheath. Bohm's criterion expresses a necessary condition for the formation of a stationary sheath in front of a negative absorbing wall. The basic features of the plasma-sheath transition and their relation to the Bohm criterion are discussed and illustrated from a simple cold-ion fluid model. A rigorous kinetic analysis of the vicinity of the sheath edge allows one to generalize Bohm's criterion accounting not only for arbitrary ion and electron distributions, but also for general boundary conditions at the wall. It is shown that the generalized sheath condition is (apart from special exceptions) marginally fulfilled and related to a sheath edge field singularity. Due to this singularity, smooth matching of the presheath and sheath solutions requires an additional transition layer. Previous investigations concerning particular problems of the plasma-sheath transition are reviewed in the light of the general relations.

The keyhole formed when a laser is used as the power source for a weld between two pieces of metal contains partially ionized metal vapour as well as other material. This flow is linked with the fluid motion and thermal conduction taking place in the liquid and solid regions surrounding it. A simple model is presented which describes the flow parallel to the axis of the keyhole when the welding speed is in the medium speed range. It is shown that shock waves may be present at its ends. The formulation is time dependent. Investigation of blind keyholes shows that there may be no solutions consistent with the assumptions of the model, but that when there are, solutions exist both with and without shocks present at the keyhole entrance. The character of the solutions can vary considerably. As welding speed increases the process becomes successively more dominated by direct absorption at the keyhole walls.

Deals with the problem of the determination of scalar and vector potentials of electromagnetic fields providing time focusing of strongly diverging packs of charged particles (specific values of relative energy spread being about 100%, angular spread attaining tens of degrees). The problem is solved within the framework of static non-relativistic corpuscle optics. A theory of ideal time and space-time focusing encompassing and systematizing all known particular cases of such focusing is developed. Necessary and sufficient conditions of isochronous motion and requirements for inner symmetry of motion (the latter increasing with the growth of the quality of focusing) are determined. It is shown that the obtained results make it possible to realize an adequate choice of harmonic fields for constructing the time-of-flight apparatus. They can also be used to determine the accuracy of field sustaining according to the known time-focusing properties.

The dynamic response of an infinite plate under an arbitrarily distributed transverse-moving load is treated within the framework of three-dimensional elasticity theory. A closed-form solution in terms of elementary functions is obtained for each harmonic component of the load by the Fourier transform method in conjunction with contour integration. Dispersion relations are presented and frequencies for modes of free vibration which are symmetric and antisymmetric are discussed in detail.

Considers the motion of a fluid film squeezed between two rotating parallel plane surfaces in the presence of a magnetic field applied perpendicular to the surfaces. Attention is given to the case where a similarity solution can be obtained. Approximate analytical solutions are given, and a numerical solution to the resulting nonlinear ordinary differential equations is presented. The combined effects of the magnetic forces and the centrifugal inertial forces on the velocity profiles, the load capacity and the torques that the fluid exerts on the surfaces are studied. In general, the results show that these two forces have opposite effects.

The adaptation of the perfect linear model takes into account the thickness of the space-charge sheath, which is set up between the plasma and the plasma electrode in such reflex ion sources, for the particular case of the two-stage Accel-Decel optics. The authors then compare the behaviour of hydrogen or argon ion beams extracted from discharges either with or without a space charge sheath. Experimental data show that the 'adapted perfect linear model' provides convenient predictions about the envelope of ion beams extracted from reflex plasma sources.

The behaviour of microdischarges in air-fed ozonizers is investigated experimentally and theoretically. High-speed streak and single-picture photographs as well as precise current measurements reveal details in the discharge development, which are modelled by a quasi two-dimensional numerical simulation. The modelling is based on the solution of a system of continuity equations in connection with the Poisson equation. Space charge effects have been taken into account. The results of the modelling are compared with those of experiments. Fundamental features of microdischarges, which influence the properties of air-fed ozonizers are described.

An accurate and efficient method for solving the Boltzmann equation is presented. Using a multi-term expansion technique, the time-of-flight (TOF) electron swarm parameters (such as the centre-of-mass drift velocity W_r, the longitudinal D_L and transverse D_T diffusion coefficients) can be evaluated. The method utilizes an amalgamation of the conventional two-term expansion and the Galerkin method. The first two terms of the Legendre polynomial expansion of the electron energy distribution are obtained by the two-term method, while the third-order and higher-order terms are deduced by the Galerkin method. The present technique is applied to determine the electron energy distribution using Fourier components to solve the Boltzmann equation and TOF swarm parameters in methane gas. Good agreement with Monte Carlo simulation is obtained and the method is also applied to a previously described model gas.

The electron kinetics in weakly ionized helium under the action of DC and HF fields of angular frequency omega is investigated by solving the homogeneous electron Boltzmann equation using the classical two-term expansion approximation. The analysis is based on a consistent set of electron cross sections which is derived by adjusting experimental cross section data in such a way that calculated and measured electron swarm parameters are in agreement. In the case of HF fields the analysis is based on the DC effective field approximation which is valid for omega > tau _e^-1, where tau _e is the characteristic time for electron energy relaxation by collisions with the atoms. The influence of omega on the electron energy distribution function, transport parameters, rate coefficients and fractional power transfer is investigated and a detailed comparison of the DC and HF situations is made. It is shown that for reduced effective fields in the range 10^-16-10^-15 V cm², as typically found in low-pressure discharges, the mean electron kinetic properties are nearly the same in the whole range of omega > tau _e^-1 as for the DC case.

A parallel plate discharge, operated in argon in the pressure range p=0.001 to 1 mbar, was investigated with special interest on the mechanism of power deposition and on the electron energy distribution function (EEDF). Various diagnostic methods were used in combination in order to obtain a sufficient characterization of the discharge. The predominant channel of power flux was found to change with increasing pressure from Joule-type heating in the plasma bulk to heating by beam-like secondary electrons emitted by the powered electrode. Measured EEDFS differ from Maxwellians and change their shape along the discharge axis.

Fractal dimensions of different parts of a fracture surface formed by slow stable crack propagation, induced by the combined effect of hydrogen and static bending moment, were determined using the method of fracture profile analysis. The material used for this investigation was 30CrMnSiNi₂A steel having the chemical composition (wt.%): 0.26% C, 1.02% Cr, 1.23% Mn, 1.08% Si, 1.56% Ni, 0.029% P and 0.007% S. The results showed that the fractal dimension increases with increasing transgranular fracture which increases as the crack propagates. This means that the increase of the fractal dimension responds to the increase of the energy needed to form the fracture surface, and in the authors' case the process of forming a fracture surface by slow stable crack propagation is one of increasing fractal dimension of the fracture surface; when the fractal dimension reaches a critical value, the crack propagation becomes unstable.

Threshold switching in bulk amorphous samples of (Se_0.7Te_0.3)_100-xZn_x has been studied experimentally. It is observed that above a certain voltage across the sample the current through the material rises to a very high value without any increase in the potential across the sample. This threshold value of the voltage depends upon the zinc concentration in the material and the temperature of the sample. The switching behaviour is completely reversible in nature.

Results are reported on the solid state reaction of the system BaO-CaO-Fe₂O₃ at 1200 degrees C. Magnetometry, X-ray diffractometry, and the analysis of chemical equilibrium equations have been used to study how Ca²⁺ enters the hexaferrite cell. For nominal compositions given by (1.01 BaO+6Fe₂O₃+xCaO) evidence is obtained of the occupation by Ca²⁺ of Fe³⁺ sites. The solubility limit in this type of substitution is established at 0.26 Ca atoms per formula unit. The limiting phase has a saturation magnetization of 68.0 EMU g^-1, as corresponds to a collinear spin structure. For nominal compositions (1.01 CaO+6Fe₂O₃+zBaO) there is also substitution of Ca for Ba, with a substitution ratio (CaO/BaO) of 8/5. The saturation magnetization is low (64.9 EMU g^-1) and would correspond to a non-collinear spin structure.

Lead titanate (PbTiO₃) thin films have been deposited on silicon, platinum-coated silicon and titanium silicide by RF-magnetron sputtering from pressed powder targets. The film structure was investigated by X-ray diffraction and was found to be strongly dependent on substrate temperature. Dielectric properties of the films are reported.

The absorption and luminescence of titanium-doped LiF single crystals has been studied for crystals grown under different experimental conditions (method, environment and starting products) in order to separate the optical spectra of Ti³⁺ and Ti⁴⁺ centres. The results show two charge transfer bands at 200 and 220 nm, the former mainly associated with Ti⁴⁺ centres and the second due to Ti³⁺ centres. Photoluminescence results show a visible luminescence band that may be due to centres involving Ti⁴⁺ and oxygen ions.

The photostimulated luminescence (PSL) and thermally stimulated luminescence (TSL) properties of two new X-ray storage phosphors, Ba₂B₅O₉Br-Eu²⁺ and Ba₅GeO₄Br₆-Eu²⁺, are reported and discussed. The PSL measurements show that bromine vacancies act as electron traps and that Eu²⁺ is involved in the hole trapping. A direct recombination mechanism, suggested by PSL results, is confirmed by TSL measurements. Analysis of the glow curves shows first-order kinetics in combination with low-frequency factors which is characteristic for centre-to-centre recombination.

The responses of several tin oxide based sensors to contaminant gases, such as methane, carbon monoxide and hydrogen, are shown to be greatly affected by the oxygen concentration in the ambient atmosphere. A model is described which explains the observed results. At low oxygen concentration (<or=2% V/V) changes in resistance of factors up to 10⁴ are observed.

The authors have developed a model for explaining the thermally stimulated depolarization current obtained for a ferroelectric 60:40 mol.% VDF/TrFe (vinylidene fluoride-trifluoroethylene) copolymer, combining a slow process with a fast one. Initially the polarization decays slowly due to either the thermal depolarization of dipolar units originating in the beta -phase crystallites or the variation of the pyroelectric current. Subsequently the decay is accelerated and eventually brought to an end by the ferroelectric to paraelectric transition. Results of the measurements could be fitted by a theoretical curve.