Table of contents

A simple expression is derived for the velocity of motion of a `head-on boundary` in a thin magnetic layer subjected to a uniform applied magnetic field. Its relevance to magnetization reversal in thin films and transition motion in magnetic recording media is discussed. The expression is modified to apply to non-uniform fields produced by recording heads to define the limiting velocities of recorded transitions before distortions occur to their profiles.

Non-conventional heating treatments, such as joule heating, can produce materials with attractive physical properties. Although extensively explored in amorphous wires and ribbons, one of the major drawbacks of the technique is the intrinsic difficulty in determining the annealing temperature. The temperature behaviour of two selected wires submitted to a DC electrical current of increasing amplitude was calibrated by comparing the temperature dependence of the material`s saturation magnetization obtained during joule heating to that measured during conventional annealing in a furnace. As a result, a correlation between the average temperature and current density was obtained. A theory describing the thermal effects of a continuous current flow in amorphous wires is proposed, showing that the thermal treatment is nearly uniform over the entire wire volume, under typical experimental conditions. The effect of fluctuations of the wire diameter on the steady-state temperature is also discussed.

Magnetoimpedance in as-cast, non-magnetostrictive CoFeBSi amorphous ferromagnetic wires, submitted to AC electric current, i_rms in the 0.1-20 mA range and frequencies between 100 Hz and 100 kHz, is analysed in terms of equivalent circuits. The effects of the bias longitudinal field, H_dc, up to 3600 A m^-1 are also investigated. It is shown that the equivalent circuit representing the wire frequency behaviour can be approximated by a series R_sL_s arrangement, in series with a parallel L_pR_p arm. L_s and L_p inductor elements are associated with the rotational and domain wall contributions to circumferential permeability, respectively. R_p is related to wall damping and R_s accounts for all the resistances in the circuit (the wire itself, contacts and so on). The circumferential permeability associated with domain wall movements exhibits a maximum for i_rms=5 mA (that is, a circumferential field H_rms^phi =12 A m^-1), similar to the classical behaviour of wall permeability. The increase in bias field has the effect of strongly decreasing the L_p value; for H_dc=3600 A m^-1, the series circuit along accounts for the frequency response of the wire. The association of the circuit elements with basic magnetization processes is discussed. Results are interpreted in terms of the influence of both fields (DC bias, H_dcr,and AC circumferential, H_rms^phi fields) on the inner-core-outer-shell magnetic structure of the wire.

The nucleation fields of isotropic nanocrystalline single-phase Sm-Fe-N magnets prepared by mechanical alloying have been determined experimentally. Single-phase Sm-Fe-N magnets possess a distinct angular dependence of the nucleation field, giving rise to a nucleation-controlled magnetic reversal process. The influence of the substitution of Co and Ti for Fe is discussed. Additionally, two-phase (Sm₂Fe₁₇N_x+ alpha -Fe) alloys have been investigated by measuring the perpendicular and the parallel components of the magnetization and their remanence. Remanence enhancement has been observed; that is, the magnetic reversal in these two-phase alloys is strongly affected by intergrain exchange interaction. This may give rise to cascades of magnetization reversal due to collective processes. A phenomenological explanation of the magnetic reversal in these systems is suggested.

A new model is proposed for a mechanism dominating the charge generated on a particle due to impact or contact with metal plate. In this model, the impact/contact charging of a particle is determined as the remaining charge by the charge relaxation process due to gaseous discharge in atmospheric conditions; here, some difficulties of the simple condenser model are avoided. The model prediction is in good agreement with the `equilibrium charges` given as the result of impact charging experiments. Furthermore, the impact charging experiments conducted in Ar gas indicated that the impact charging characteristics changed corresponding to the breakdown limit potential of the environmental gas; this was also predicted quantitatively by the model. As a result of the proposal of this new model, hereafter in the case of treating charge generating phenomena due to impact or contact, in atmospheric conditions at least, consideration of the charge relaxation process due to gaseous discharge should be essential.

Electrostatic stresses at the boundary of a dielectric give rise to a spurious effect that influences measurements of the quadratic electro-optic effect and quadratic electrostriction. In set-ups designed for the measurement of small strains induced by high electric fields, their influence on components like sample holders can be slightly reduced by an appropriate choice of material. A straightforward way to derive the dependence of these so-called Maxwell stresses at the surface of a triclinic crystals on dielectric constants and sample orientation is demonstrated. The method allows one to predict Maxwell stresses at the boundary of crystalline samples of any symmetry and is similar to Mohr`s circle construction that is often applied in mechanical engineering.

This paper deals with the absorption and defocusing of a CO₂ laser beam by the laser-induced plasma plume in deep penetration welding. To derive the `effective` intensity distribution in the focal plane theoretically, the laser beam propagation through the plasma plume is calculated by solving the paraxial wave equation with a finite-difference scheme. Corresponding to experimental results, documented in the literature, the properties of the plasma plume (spatial temperature distribution and shielding gas content) are pre-set within the calculation. Parametric studies demonstrate that the intensity at the focus is reduced due to the defocusing effect of the plasma plume, mainly, and only to a minor extent due to absorption within the plume. Because of refraction within the plume, the intensity distribution in the focal plane is dependent on the plasma`s size, position and temperature. On studying the dependency of the optical properties on plasma temperature and shielding gas composition, it is found that, by applying a shielding gas mixture of He and Ar in the ratio 3:1, the variation of the focal diameter with plasma temperature can be significantly reduced. This shielding gas mixture, therefore, is recommended for enhancing process stability when welding with high-power CO₂ lasers.

The dynamics of the melt pool and keyhole was investigated during CO₂ laser welding using high-speed video photography and the laser reflectometer technique. A low-power argon laser beam, focused on the weld pool, provided illumination to obtain a direct image of the weld pool surface. The near-surface plasma emission background was decreased by using a narrow-bandwidth interference filter centred at the argon laser wavelength (514 nm). A variation in the shape of the keyhole opening with a characteristic frequency higher than 1 kHz was observed both during spot welding and during welding with a moving beam. For the case of spot welding with a 20 ms laser pulse, long-wavelength (about 1 mm) oscillations of the weld pool were observed with a frequency during the laser pulse and the first 5 ms after the laser pulse in the range 200-500 Hz. In the time interval starting at 25 ms and ending at approximately 40 ms from the beginning of the laser pulse, the long-wave oscillation frequency increased up to 1.3 kHz. The solidification time was determined to be approximately equal to the pulse duration for the spot welding. Surface deformation during cooling was also observed. This information is used to develop a model illustrating the dynamics of the post-pulse weld pool.

We consider a specific model of a heat pump, where the irreversibilities come from finite thermal conductance between the working fluid and the reservoirs, heat leak between the reservoirs and internal irreversibility inside the pump. A steady-flow approach of finite-time thermodynamics is applied to calculate the maximum coefficient of performance and the corresponding heating capacity. We also determine the bounds for the upper and lower temperatures of the working fluid.

This paper concerns the effect of Hall current on unsteady laminar hydromagnetic flow which arises in fluids due to the interaction of gravity and density difference caused by the simultaneous diffusion of thermal energy and chemical species near an infinite vertical porous plate subjected to suction or injection velocity in the presence of a constant magnetic field. The results are discussed with respect to hydromagnetic parameter (Mt), Hall parameter m, the buoyancy ratio parameter N, suction or injection parameter a and Schmidt number Sc for Pr=0.71 with respect to air.

A theoretical model of a large-area planar plasma producer based on surface wave (SW) propagation in a plasma-metal structure with a dielectric sheath is presented. The SW which produces and sustains the microwave gas discharge in the planar structure propagates along an external magnetic field and possesses an eigenfrequency within the range between electron cyclotron and electron plasma frequencies. The spatial distributions of the produced plasma density, electromagnetic fields, energy flow density, phase velocity and reverse skin depth of the SW are obtained analytically and numerically.

The feasibility of detecting the silylene radical SiH₂ in a silane plasma directly by laser-induced fluorescence is demonstrated. Transitions in the (020-000) vibrational band were excited by a pulsed dye laser and the fluorescence was detected at the (010-000) band. In order to observe the fluorescence signal in the presence of scattered laser radiation and emission from the plasma, both wavelength-selective and fast gated detection were applied. The results suggest volume recombination to be the predominant loss mechanism.

The decrease in the fume formation rate at the transition from globular to spray mode during welding has been investigated. A physical model describing this phenomenon has been developed. The essence of the consideration is that the model takes into account the different heating of droplets of the liquid metal by the are plasma in the two welding modes. New experimental results have been obtained for welding of stainless steel in a shielding gas containing oxygen. The decrease of the fume formation rate estimated on the basis of the model is in agreement with experimental data obtained and in the literature.

The present paper reports the ionization coefficient in tetraethoxysilane determined experimentally by the steady state Townsend method for E/N in the range 450-2000 Td. Low-temperature plasmas of tetraethoxysilane mixtures may be used to perform plasma-enhanced chemical vapour deposition of silicon dioxide thin films for microelectronics at low substrate temperatures. Since tetraethoxysilane is a low-vapour-pressure liquid at room temperature, heating coils are attached to the ionization chamber to maintain the temperature of the chamber higher than that of the tetraethoxysilane source so that the required vapour pressure is obtained yet no condensation of tetraethoxysilane occurs on the chamber walls. Measurements indicate that the ionization coefficient increases very rapidly at about E/N of 450 Td, at which the value is 7.7*10^-18 cm², then the increase tends to saturate and reaches a value of 4.9*10^-16 cm² at E/N of 2000 Td. Furthermore, it is found that the logarithm of the current has a linear dependence on electrode separation. This suggests that electron attachment is very small or practically absent in this vapour.

We report results of a two-dimensional simulation (three-dimensional, cylindrically symmetric) of positive streamer propagation in atmospheric pressure nitrogen between parallel-plate electrodes. The streamer is initiated at the anode. The most important features of the streamer dynamics are (i) high radial field around the streamer head (about 100 kV cm^-1) which leads to significant expansion of the head in the radial direction and (ii) the formation of a secondary wave which propagates along the streamer channel towards the cathode and increases channel conductivity.

Measurements of normalized plasma emission coefficients for the 696.55 nm Ar I line for free burning argon arcs at 1 atm are presented for various arc parameters. These coefficients show variations in the cathode region of the arc, depending upon the arc current, the cathode composition and the mode of operation of the arc, and suggest departures from local thermodynamic equilibrium in the plasma near the cathode tip. In this paper, these results are discussed and an explanation is proposed based on the overpopulation of the ground state level of the neutral atoms. This overpopulation is due to the injection of a large mass flow of cold gas into the arc core under the influence of pressure gradients resulting from the magnetic pinch force in the cathode region.

The stability of arc motion and plasma parameters such as temperature and copper vapour density of electric arcs in rotation between two copper electrodes correlate with the intensity of the noise in the arc voltage, the time distribution of rotation frequency and fluctuations of Cu and Ar emission line intensities. The more stable arc produced by 1 vol% of CO contamination added to argon yields white voltage noise compared to a 1/f voltage noise above the rotational frequency for less stable arcs. Very stable rotating arcs showing extremely small erosion rates are compared to less stable ones by using noise analysis of the measured parameters. Less stable arcs produced by 1 vol% N₂ in argon show a time distribution of the rotation frequency with a long tail at large residence times and significant noise in the neutral copper emission line intensity. These correlate with an increase in the erosion rate of the cathode.

A relation between the mean average energy of accelerated ions epsilon _i and the maximum rate of current rise in a vacuum discharge is established. The relationship takes the power form epsilon _i varies as i^1.3 for different ion species over a wide range of discharge parameters for i>i₀=3*10⁸ A s^-1. For smaller values of i the corresponding value of epsilon _i is almost independent of the discharge parameters. The ion mass and charge-dependence of epsilon _i are also discussed.

Electron heating up to temperatures T_e>1 keV is revealed in the cathode current-carrying plasma jet of a vacuum discharge at the current rise rate dl/dt>10¹³ A s^-1 and the pulse length tau <1 ns. It was found that the jet consists of a set of separate filaments expanding into small solid angles, which can explain the measured value of the cathode plasma temperature T exceeding those obtained earlier by more than two orders of magnitude.

A method is presented for the computation of X-ray grazing-incidence diffraction in multilayers with lateral lattice mismatch. This method is based on the dynamical diffraction theory and on a matrix form of boundary conditions. Numerical examples are given to prove the validity of the theory. In present form the algorithm can be used for computing grazing-incidence diffraction by completely relaxed superlattices. The possibilities of extending this method to partially relaxed multilayers are discussed.

Photostimulable phosphors are used in digital X-ray photography. We report a new phosphor material (Ba,Sr)F_1+xBr_1-x:Eu²⁺ which exhibits very good photostimulated luminescence (PSL) properties. Using X-ray diffraction measurements, it is established that the F excess and the Sr fraction are incorporated into the BaFBr lattice. The combination of a F excess and the partial replacement of Ba²⁺ by Sr²⁺ yields, unexpectedly, improved PSL performance.

A STM study of ultra-small cadmium sulphide particles formed in a bilayer of cadmium arachidate exposed to a hydrogen sulphide atmosphere is reported. Particles of nanometre size were identified as negative features inside the film after the reaction. Voltage-current characteristics of particles of different apparent sizes were measured by means of the microscope probe. The behaviour of the tunnelling current indicates the occurrence of a Coulomb staircase at room temperature and reveals a new phenomenon which shows the appearance of regions with negative resistance.

For pt I see ibid., vol. 27, p. 1063-9 (1994). A numerical finite difference approach is applied to the problem of defect sizing. The influence of size, depth, severity and material properties are investigated. Some agreement is found between the numerical results presented here, and an earlier analytical approach. Deviations from the are found, and explained in terms of simplifying assumptions model.

A comprehensive model has been developed for the determination of the strain, stress and microhardness of polycrystalline thin films from the below-band-gap optical absorption data which reflect the defect states at the grain boundary regions. The model has been applied to describe the absorption data of AlN and diamond films and consequent interpretation of the mechanical properties of the same is presented.

A detailed investigation both of the DC and of the AC electrical properties of the Schottky barrier formed between aluminium and electrodeposited poly(3-methylthiophene) is reported. The devices show rectification ratios up to 2*10⁴ which can be increased further after post-metal annealing. The reverse characteristics of the devices follow predictions based on the image-force lowering of the Schottky barrier, from which the doping density can be estimated. As the forward voltage increases, the device current is limited by the bulk resistance of the polymer with some evidence for injection limitation at the gold counter-electrode at high bias. In the bulk-limited regime, the device current is thermally activated near room temperature with an activation energy in the range 0.2-0.3 eV. Below about 150 K the device current is almost independent of temperature. Capacitance voltage plots obtained at frequencies well below the device relaxation frequency indicate the presence of two distinct acceptor states. A set of shallow acceptor states are active in forward bias and are believed to determine the bulk conductivity of the polymer. A set of deeper acceptors are active only for very small forward voltages and for all reverse voltages, namely when band banding causes the Fermi energy to cross these states. The density of these deeper states is approximately an order of magnitude greater than that of the shallow states. Evidence is presented also for the influence of fabrication conditions on the formation of an insulating interfacial layer at the rectifying interface. The presence of such a layer leads to inversion at the polymer surface and a modification of the I-V characteristics.

A phenomenon of bipolar thermally stimulated conductivity in semi-insulating material has been analysed assuming that there are only two types of levels in a bandgap, which act as traps as well as recombination centres. This model is applied to compute the thermally stimulated conductivity observed in single crystals of gallium arsenide. Experimentally observed double peaks may be explained in terms of a contribution of electrons and holes due to the bipolar character of the conductivity.

Using an evaporation chamber equipped with a ZnSe compound source, we have grown ZnSe on GaAs(001) and characterized the surface using low-energy electron diffraction (LEED), Raman scattering spectroscopy and reflectance anisotropy spectroscopy (RAS). High-quality layers of ZnSe were obtained and revealed by LEED to have a c(2*2) surface reconstruction and the RAS spectra were measured. The RAS spectra proved to be reproducible when a similar surface was obtained by de-capping a Se-capped ZnSe(001) sample.

Transmittance in inhomogeneous composite structures in which the arrangement or shape of an absorbing component is anisotropic, depends on the incident direction in relation to the axes that define the anisotropy. There are easily distinguished preferred transmittance directions in a given plane of incidence. However a complete reversal in the preferred direction, which is at first sight counter intuitive, has been observed in several anisotropic thin films containing small quantities of silver inclusions in partially ordered arrays. This switching occurs at a particular wavelength and is studied in terms of transmittance at equal angles of incidence either side of the normal. A filter which is low pass or high pass depending on direction of incidence may be possible based on this phenomenon. A theoretical explanation is given in terms of a two-component effective dielectric response of the composite. In the samples studied the splitting of the resonant absorption spectra for the two principal directions is seen to be the underlying cause of switching in the preferred transmittance direction. The experimentally observed optical data, and the observed microstructures, demand the use of advanced effective medium models which incorporate strong interaction effects.

This study focuses on the development of an in situ method which makes use of inverse scattering, namely the utilization of scattering measurements to infer morphological features and optical properties of the scatterer itself. Specifically, use is made of an exact formulation, developed recently by the authors, for the internal field of agglomerated structures composed of Rayleigh-size primary particles. All possible combinations of the scattering/extinction quantities with respect to the incident/scattered polarization state of the radiation field as well as the scattering angle are considered. Closed form (exact) expressions for the partial derivatives are presented for the selected quantities and the most suitable sets for data inversion are selected using the criteria for maximum sensitivity and maximum accuracy. The depolarization ratio in the vertical polarization state and the extinction normalized differential scattering intensities were identified as the most suitable set for inverting the corresponding measured quantities for the real and imaginary parts of the refractive index. Specific inversion examples are presented for the real and imaginary parts of the refractive index of primary particles agglomerated in the forms of straight chains, clusters and randomly branched chains. It is demonstrated that, for refractive indices typical of carbonaceous materials, such as flame soot, the present methodology yields reliable values. In addition it provides the prototype for extending the inversion scheme to higher than two-dimensional problems.

The weight loss due to laser-induced vaporization of alumina and zirconia is investigated both theoretically and experimentally. To calculate the weight loss of the target under laser irradiation a model is used describing the laser-induced vaporization and the dynamics of the vapour/plasma state including the absorption of laser radiation. The weight loss is not an increasing function of the fluence over the whole range but has a local maximum due to absorption of laser radiation in the plasma state. The energy deposited in the plasma state due to the absorption of laser radiation is calculated and compared to the weight loss data. Different pulse shapes of laser radiation influence the maximum value of the weight loss but leave the principal dependence on the fluence unaltered. Furthermore, it is shown that increasing the back pressure from 0.06 to 100 mbar leads to considerably smaller weight losses.

In a high-power (30 kW), inductively coupled argon-oxygen plasma used as a tool to deposit high-temperature superconducting YBa₂Cu₃O_7-x films rotational bands of the Schumann-Runge system B³ Sigma (u^-)-X³ Sigma (g^-) oxygen were detected in the spectral regions around 340 and 360 nm. After derivation of the line strength factors for the triplet system, rotational temperatures can be determined by fitting calculated relative intensities to those observed experimentally, and the contributions of R and P branches to the lines observed can be identified. The rotational temperature 7 at a distance of 34.5 cm from the exit of the plasma torch where the deposition of superconducting films takes place lies in the range 1850-2050 K depending on the RF power input which was varied in the range 8-27 kW. Near the torch exit, temperatures in the range 2600-3400 K are observed. They depend strongly on gas composition and gas flow rate, in contrast to positions far away from the torch exit. Excitation temperatures near the torch exit which were derived from Ar lines are considerably higher than the rotational temperatures, indicating that the latter are typical of the colder outer parts of the plasma.

A method to determine the temperature of metallic amorphous alloys when heated with a current pulse (`flash annealing`) is proposed. With some energetical considerations we calculate the temperature of the sample during the fast heat treatment. Theoretical curves calculated in that way are in good agreement with experimental results. The temperature of the sample during the pulse is experimentally determined by measuring the spontaneous magnetization during the current heating and comparing this with the magnetization-temperature curve obtained when heating in a conventional furnace. The influence of the different parameters appearing in the mathematical expressions on the heating process, such as the characteristics of the material, the geometry of the ribbon or the annealing conditions, is also studied.

The anodic oxidation of amorphous Al-32 at% Zr alloy films at high current efficiency results in amorphous anodic films, comprising a mixture of Al₂O₃ and ZrO₂ units, of uniform composition and thickness. The use of an immobile xenon marker reveals that the films develop, under the high field, at the alloy/film and film/electrolyte interfaces by the inward transport of O^2- and OH^- ions and the outward transport of cations respectively. This mechanism is typical of growth of amorphous anodic films involving the cooperative transport of cations and anions. The total transport number of cations is about 0.35. Zirconium and aluminium are incorporated into the anodic films, in their alloy proportions, without the development of significant enrichments or depletions of zirconium at the alloy/film interface. Zr⁴⁺ ions, Which are immobile in the usual crystalline anodic zirconia, migrate outwards at about the same rate as Al³⁺ ions. Boron species, incorporated into the films from the electrolyte, are immobile and are distributed uniformly in the film material formed at the film/electrolyte interface by outward migration of cations.

This paper describes exploding foil opening and closing switch techniques for use in capacitor bank and flux compressor output conditioning circuits. A simple approach was evolved during an extensive experimental programme, aimed at producing a current pulse with a rise time of a few nanoseconds from the output of a flux compressor (or flux compression generator). Capacitor-based experiments provide data for an empirical model for copper foils, used subsequently in a computer program to predict the results of experiments in which exploding foils are used as both opening and closing switches. The switches operate automatically, thus avoiding the problems of triggering between stages of a conditioning circuit. Data from a 1 MJ flux-compressor exploding-foil opening-switch experiment with a run time of 160 mu s are presented and analysed using the theoretical model.