Table of contents

Ferromagnetic resonance equations in a single-crystal film with a (111) plane have been newly derived using the energy method. The derivations were performed when an applied static field is rotated in both (i) the vertical (110) plane containing the film normal and a (112) axis and (ii) the film plane or the (111) plane. The angular variations of the resonance field at 34 GHz are numerically calculated for a FCC Co film with an anisotropy parameter of K₁/M=-0.5 kOe, saturation magnetization of 4 pi M=17.6 kG and magnetogyric factor of gamma /2 pi =3.10 GHz kOe^-1.

The alternating current conductivity of Li_0.5+0.5xGe_xFe₂O₄ (where x=0.0, 0.2, 0.3 and 0.5) has been studied as a function of frequency, whereby the conductivity showed a dispersion with respect to frequency. In the range of dispersion it was found that sigma ( omega ) alpha omega ^S, where S is a constant. At f=10⁵ Hz, S is equal 0.45, 0.8, 0.7 and 0.6 for x=0.0, 0.2, 0.3 and 0.5 respectively. The variation of loss tangent (tan delta ) with frequency was studied in the range from 100 Hz to 1 MHz. There were peaks in tan S at f=5.5*10⁴, 5*10⁴ and 1.5*10⁵ Hz for the samples of x=0.2, 0.3 and 0.5 respectively. However, for x=0.0, tan delta gave a maximum in the range of frequency (2-6)*10⁴ Hz. At the same time, a minimum value of epsilon ' (the real part of the complex dielectric) was found at the corresponding frequency value.

The transient electromagnetic field produced by a current pulse moving along a straight line is considered. The algorithm for constructing the components of the electromagnetic field is given. This algorithm enables us to obtain directly the space-time representation of the electromagnetic pulse and to reveal the peculiarities of its dependence on the source-current characteristics.

A boundary element code is used to study the electromagnetic field scattered by a helix at microwave frequencies. The far-field forward and backward scattering amplitudes are computed for an incident linearly polarized wave with various polarization directions, and a number of orientations of the helix. These computations allow us to study the magnitude of the field scattered in the forward and backward directions as a function of frequency and orientation. They are also the starting point for computing the effective wavenumbers of left and right circularly polarized waves in a chiral composite made of a dispersion of helices in a host medium. Several methods to compute the effective impedance of chiral composites in order to obtain all three constitutive parameters of such media are proposed. Numerical results are given and compared with existing experimental and theoretical data.

The average polarizabilities of a helix are computed using the far-field forward and backward scattering amplitudes, and subsequently used for deriving the constitutive parameters of helix-loaded composites via a Maxwell Garnett treatment. Computational results are first compared with some results obtained earlier with other effective-medium theories. Then, Maxwell Garnett estimates for the effective properties of several helix-loaded materials (chiral and transversely bianisotropic uniaxial composites) are compared with experimental data measured by the means of a free space bench in the 6-18 GHz range. Numerical and experimental determinations of the rotation angle, ellipticity, chirality parameter, permittivity, and permeability are presented and discussed in detail. Results for the reflection and transmission coefficients of helix-loaded slabs and the reflection coefficients of metal-backed slabs are also presented.

Results are presented from a study of the field-assisted diffusion of silver ions from silver films into soda-lime glass. These indicate that it is necessary to take into consideration the concentration dependence of the self-diffusion coefficient of both the silver and the sodium ions in a pseudo-mixed-alkali effect in order to predict the silver diffusion profiles accurately. A field assisted diffusion model that incorporates this effect is developed and model coefficients are deduced. The model is applied to post-bake diffusions and deep field-assisted diffusion profiles. The model is applicable to the design and manufacture of optical waveguides and lenses in glass by field-assisted diffusion.

A simple theoretical approach that was developed previously to calculate shock Hugoniot of simple dense fluids is used to calculate shocked states of liquid nitric oxide and liquid nitrogen-oxygen mixtures as well as the detonation wave structure of liquid nitric oxide. The theoretical calculations provide an adequate description of the shock compression and the detonation performance.

Previously we had suggested a new approach to finding the criteria of partial similarity. Our method was applied by ourselves and later by other authors to diverse problems in which the transfer of radiation plays an important role. In the present paper the generalized form of our method is given. The criterion of similarity of hydraulic resistance of diverse ducts under non-isothermal flow of viscous liquids is derived. Thus our approach was successfully applied to diverse problems in different fields.

The water absorption of fired-clay brick has been investigated by nuclear magnetic resonance scanning of one-dimensional moisture concentration profiles. Because of the large amount of paramagnetic ions present in these materials, the hydrogen nuclei have a very short transverse relaxation time and a broad resonance line width. Therefore standard imaging techniques cannot be applied. This paper presents the first quantitative study on these materials using nuclear magnetic resonance. It is found that the overall behaviour of the moisture diffusivity for absorption of all investigated fired-clay bricks can be approximated by an exponential function of the moisture content.

The motion of electrons in the cathode region of a magnetron glow discharge in low-pressure argon has been studied by Monte Carlo simulation. The influences of the transverse magnetic field on energy distribution of electron flux and ionization have been investigated for normal and abnormal discharges. It has been shown that the magnetic field increases collisions, and consequently decreases the mean electron energy. In normal discharges, under the magnetic field, fewer electrons enter the negative glow region with high energy, while ionization is remarkably enhanced in the cathode region. In abnormal discharges, it is found that the magnetic field required to influence the discharge has to be stronger than that in normal discharges. Under such a magnetic field, for example 0.3 T, though there are still a great number of electrons entering the negative glow region with energy corresponding to the cathode fall potential, the mean electron energy is decreased, and the electrons entering the negative glow region are less beam-like. Analyses for the simulation results and consideration of the practical use of the magnetic field have been given.

The collisional regime of the low-voltage beam discharge in helium is investigated experimentally. The spatial distribution of basic plasma parameters along the gap is determined. The conditions under consideration correspond to the case in which the free path of cathode beam electrons is substantially less than the inter-electrode gap, and their energy relaxation length conditioned by binary collisions is substantially greater than the inter-electrode gap. In this case, the distribution function of fast electrons is weakly asymmetric in velocity space. It is shown that the collisional mechanism of beam energy relaxation is realized only for small discharge currents (j_s<or approximately=0.1 A cm^-2). At sufficiently large discharge currents (j_s>or approximately= 0.8 A cm^-2) a more effective wave mechanism of beam energy relaxation switches on in a threshold-determined way.

For pt.I see ibid., vol.28, p.689-93 (1995). The collisional regime of the low-voltage beam discharge in helium is investigated theoretically. The conditions under consideration correspond to the case in which the distribution function of fast beam electrons is weakly asymmetric in velocity space. It is shown that the beam energy relaxation can take place either due to binary collisions or by excitation of Langmuir waves, depending on the beam current magnitude. The theoretical value of the beam threshold current, above which the change of relaxation mechanism occurs, is determined. In the case of collisional relaxation the distribution function of fast electrons is found, and the concentration of metastable excited atoms is determined. Good agreement between theoretical and experimental results is obtained.

Low pressure (0.13<P<26.7 mbar) breakdown conditions were studied in a direct current point-plane gap discharge in air. Influences of the gas pressure P and geometrical factors (gap distance, electrode configuration with various tip and rod radii for the anode) on the breakdown voltage V_s are presented. Curves of V_s versus P were drawn in a Paschen-like manner, and a roughly 550 V minimum value V_s(min) was obtained. This minimum always corresponds to the transition between the Townsend-like regime and the subnormal glow regime of the discharge, with a development in the lateral direction between the rod (anode) and the vessel walls. As the pressure increases, the discharge direction development is modified. For pressures higher than about 6.7 mbar, depending on the experimental conditions, an axial breakdown (between point and plane) is observed. This is highly dependent on the effective geometry of the discharge vessel (rod radius, rod-to-walls distance, point tip radius and point-plane gap) in the <3 cm inter-electrode gap distance considered. To explain this phenomenon, a simple mathematical model was established. It permits one, despite simplifying assumptions, to show why the V_s(min) value is obtained with a lateral development of the discharge when at higher pressure breakdown it occurs axially. This model points out the relative role of the various parts of the anode (rod and tip curvature radii) in the breakdown potential as a function of pressure.

An analysis of the ion charge distribution measurements in a vacuum arc plasma has been carried out. It has been demonstrated that, by using the Saha equation, one can determine the electron temperature T_e and density N_e in the cathode spot plasma. It has also been shown that it is possible to estimate a microspot diameter d and a current density j in the microspot by applying a theoretical model. The values obtained for T_e approximately=4-10 eV, N_e approximately=10²⁶-10²⁸ m^-3, d approximately=0.2-0.5 mu m and j approximately=(1-30)*10¹² A m^-2 are in agreement with recent measurements.

A one-dimensional steady state three-gas model for compressible media is used to formulate a boundary value problem for the axial neutral gas transport in the low-pressure positive column. A by-pass tube configuration is considered. The electron temperature and the axial neutral gas drift velocity, both the values on the axis, and the axial component of electric field occur as eigenvalues. The radial profiles of the number densities, of the radial and axial drift velocities of electrons, ions and neutral atoms, of the electric potential and of the electron temperature are calculated. It is shown that the axial gas flow is caused by the difference in the axial momentum transfer between elastic and inelastic collisions from ions to atoms and from electrons to atoms. The direction of the axial neutral gas drift depends on the density of charge carriers. The ion collisions dominate and drive the neutral gas to the cathode if the density of charged particles is very low and, therefore, the positive boundary layer with ion excess is thick. The electron collisions dominate and drive the neutral gas to the anode if the density of charged particles exceeds a threshold depending on the gas density. The electron temperature varies only slightly across the column.

A particle-in cell/dynamic Monte Carlo simulation technique was developed to study the spatiotemporal electron dynamics in radio-frequency glow discharges. The electric field profile in a parallel plate one-dimensional geometry was obtained from a self-consistent fluid simulation. Using this profile, the particle-in-cell/dynamic Monte Carlo simulation yielded the spatiotemporal electron velocity distribution function between the electrodes. A strongly electronegative chlorine discharge and an electropositive argon discharge were considered. At a pressure of 100 mTorr, the distribution function was non-Maxwellian except at the midgap of the chlorine discharge. The tail of the distribution was strongly modulated inside the sheath. Electron 'pile-ups' near the walls, charge double layers and negative plasma potential during part of the cycle were observed in the electronegative discharge at 13.56 MHz. Time-average results from the particle-in-cell/dynamic Monte Carlo simulation matched the fluid simulation favourably, even at a local Knudsen number Kn approximately=1, with differences confined mainly to the volume around the plasma/sheath interface.

A study has been performed to investigate the kinetics of production of N atoms and NO molecules in DC N₂-O₂ flowing glow discharges at pressure rho =2.0 Torr for discharge currents I=15, 30, 80 mA. Measurements of the plasma parameters, reduced electric field E/N, electronic density n_e, as well as of the gas temperature T_g, the vibrational temperature T_nu of N₂ molecules and the concentration of N atoms and NO molecules have been carried out over the whole range of oxygen percentages (0-100%). This set of measurements is compared to the results of a one-dimensional model including a detailed vibrational kinetics, a large number of chemical reactions and the thermal balance of the discharge.

The effects of thermal activation on the dislocation emission from an atomistic crack tip are discussed. Molecular dynamics simulations at different constant temperatures are carried out to investigate the thermal effects. The simulated results show that the processes of the partial dislocation generation and emission are temperature dependent. As the temperature increases, the incipient duration of the partial dislocation nucleation becomes longer, the critical stress intensity factor for partial dislocation emission is reduced and, at the same loading level, more dislocations are emitted. The dislocation velocity moving away from the crack tip and the separations of partial dislocations are apparently not temperature dependent. The simulated results also show that, as the temperature increases, the stress distribution along the crack increases slightly. Therefore stress softening at the crack tip induced by thermal activation does not exist in the present simulation. A simple model is proposed to evaluate the relation of the critical stress intensity factor versus temperature. The obtained relation is in good agreement with our molecular dynamics results.

Thermograms of Ge_xSe_1-x amorphous samples of different compositions (0.05<or=x<or=0.30) were obtained at four different heating rates. It was possible to estimate the activation energy of crystallization of two compositions only (x=0.15 and 0.30). The cohesive energy and related parameters such as coordination number (m) and number of constraints N_infinity were calculated. A trail was made to correlate our results with available published experimental data. The high-frequency dielectric constant epsilon _infinity was determined for different compositions using published data. Obtained values of epsilon _infinity were correlated with obtained results.

Thin polycrystalline SiC films grown on Si(111) at 830 degrees C using solid state evaporation were characterized by X-ray diffraction, transmission electron microscopy and atomic force microscopy. The thin-film crystallites have a preferred growth orientation azimuthal and nearly perpendicular to the substrate surface. The orientation distribution function was characterized to be Gaussian with full width at half maximum of 2.5 degrees . The size of crystallites normal to the surface (the length of columns) is of the order of the thin film thickness, whilst the size parallel to the surface is in the range 20-70 nm.

Structural studies on AgGaSe₂ films prepared in vacuum of 10^-4 Pa onto glass or quartz substrates by a flash evaporation technique and annealed at 573 K for 2 h show that these films were polycrystalline and exhibited tetragonal chalcopyrite structure with a strong (112) orientation with a=0.599 nm, c=1.089 nm and (c/a)=1.816. The optical constants (the refractive index n, the absorption index k and the absorption coefficient varies as ) of polycrystalline AgGaSe₂ films were determined from spectrophotometric measurements of the transmittance and reflectance carried out at normal incidence in the wavelength range 500-2000 nm. Graphical representations of surface and volume energy loss functions as well as the real optical conductivity as a function of photon energy show the existence of three possible optical transitions corresponding to 1.79, 1.95 and 2.2 eV respectively.

The structural and electrical properties in the temperature range 76-473 K of Sb_x,Se_1-x (0<or=x<or=0.9) thin films grown on glass and quartz substrates and annealed in vacuum at 473 K were studied. The films were investigated by X-ray diffraction and electron microscope techniques. The results showed that Sb_xSe_1-x alloys and annealed films with x=0, 0.1, 0.7 and 0.9 have hexagonal structure and orthorhombic structure for x=0.3, 0.4, and 0.5. From the variation of the DC electrical conductivity with temperature (76-473 K), two processes of conduction mechanism were observed (band mechanism and hopping conduction in localized states). The DC electrical conductivity was found to be of the order of 10^-12 Omega ^-1 cm^-1 to 10^-2 Omega ^-1 cm^-1 for selenium rich compositions and from 10^-5 Omega ^-1 cm^-1 to 10 Omega ^-1 cm^-1 for the antimony rich compositions. The value of the thermoelectric power was found to be 120 mu V per degree for pure selenium and is always positive for x=0, 0.1, 0.3, 0.4 and 0.9 showing that structures are of p-type conduction, while for x=0.5 the thermoelectric power is negative and equals -440 mu V per degree, i.e. of n-type conduction.

The role of principal additives is investigated in an attempt to obtain high-threshold-field ZnO-based varistors in the traditional way. In the order of effect level, these are Sb₂O₃, Co₃O₄, Cr₂O₃ and MnO₂. In the range of 0.5-5 mol%, the increase of additive content decreases the average ZnO grain size. As a result, the number of grains per unit thickness increases, which gives rise to the threshold field. The value of barrier voltage v_b calculated from experimental data is about 1.4-2 V. The potential barrier height determined from the temperature dependence of the current-voltage characteristics is 0.3-0.5 eV. The abnormally large v_b of a sample doped with 8 mol% Sb₂O₃ is quantitatively interpreted by the influence of the spinel phase through the energy-band-bending effect caused by the Bi₂O₃-rich phase and the hypothesis of a 'preferential path' of the current.

Effect of air annealing on the electrical resistivity of Cu_1.9Se thin films grown by chemical bath deposition method has been studied in the temperature range 100-500 degrees C. It was found that resistivity initially increases and reaches a maximum value of 1.6 Omega cm at around 250 degrees C, beyond which it decreases. The mechanism responsible for the observed resistivity behaviour has been explained by a combination of electron paramagnetic resonance absorption and transmission electron diffraction measurements.

The tip shape of a Au-Si liquid-metal ion source in terms of its dependence on the total ion emission current has been investigated in situ in the Halle 1 MeV electron microscope. Within the emission current range 7-90 mu A there is a linear dependence of the cone half-angle a and the length I of the jet-like protrusion on the increased emission current. The jet-like protrusion at the Taylor cone vertex was not so pronounced as has been observed in the cases of elementary liquid-metal ion sources. No spatial shifts of the Taylor cone are seen during the experiments. In contrast to elementary liquid-metal ion sources, there is no indication of 'Faraday droplets' being emitted from the surroundings of the Taylor cone.

Ablation of materials (Cu is presented in this report) in air at an ambient gas pressure of 1 bar with a KrF excimer laser (3-47 J cm^-2) leads to gasdynamic processes above the target surface which affect the processing result, the efficiency of the treatment and the debris in the environment of the irradiated area. These laser-induced processes have been diagnosed using fast schlieren photography and shadowgraphy. Five discontinuities have been discerned and their propagation mechanisms have been detected. A physical interpretation of the discontinuities is given along the lines of existing theories and plausible reasoning. The locally most advanced discontinuity can be explained by the classical Sedov-Taylor blast wave theory, and conclusions on the energy content in the shock wave, the pressure distribution and the surface pressure evolution will be presented. The results show that, at high energy densities (3-47 J cm^-2), about 80% of the available laser pulse energy is deposited in the shock wave. A reduction in the energy density leads to a decrease in the fraction of the energy that is deposited in the shock wave. Close behind the first discontinuity follows a second one that is interpreted as the ionization front. The contact front, which separates shocked air and ablated material vapour, has been observed within the laser pulse duration. The complex structure of the contact front is interpreted in terms of gas flow phenomena inside the two outer discontinuities.

This paper describes the development of a very efficient computer model for the design and performance prediction of explosive-driven helical generators. The model is based on simple theoretical considerations. Validation of the model is achieved by comparing the theoretical and measured performances of existing both high- and low-energy generators. It is shown that, although the basic model predicts accurately the load current history of high-energy generators, a somewhat more elaborate model is needed for low-energy devices. The model has been used in the design of a simple 1 MJ generator with an eight-section stator coil, intended for use as a current source in an investigation of high-current conditioning systems. A description is given of the construction and testing of this device. Experimental results are in accordance with predictions from the design code and establish that, when primed with 40 kJ at 50 kA from a capacitor bank and using 15 kg of high explosive, the generator is capable of delivering an output of 1 MJ at 7 MA to a coaxial load.