Table of contents

The behaviour of bubbles rising in AC isomotive electric fields of 50 Hertz frequency was investigated in several dielectric liquids. Isomotive electrodes were immersed in the sample liquids and aligned so that the motion of single bubbles of nitrogen rising in the interelectrode space was opposed by the dielectrophoretic force. The transit time of each bubble was timed over two set distances as a function of the voltage applied to the electrodes. The set distances were defined by three laser beams so that their interruption by a bubble could be detected optoelectronically and timed using a microcomputer. Analysis of the data presented indicates that the dielectrophoretic force on the bubbles is only position independent in the interior of the isomotive electric field.

The keyhole formed during penetration welding with a laser is modelled so far as its thermal absorption characteristics are concerned by the combination of a point and a line source. In this way a representation of the broader surface section of the weld is obtained in addition to the lower section. A simple analytical form for the temperature distribution is obtained and possible weld profiles are found numerically from this. These are compared with examples of actual welds; it is possible in this way to estimate the proportion of the laser power absorbed in each section of the weld. Graphs and some simple approximations are given which allow it to be done easily for a given weld.

Calculations have been made for the electron swarm parameters in mixtures of argon and metal vapour for E/N varying from 2.82 to 282 Td. Three metal vapours, namely sodium, potassium and caesium, have been investigated for conditions in which the concentrations of metal vapour varied from 0.001% to 20%. The results have shown that for a small percentage of metal vapour both the ionisation coefficient and the drift velocity have higher values than in either pure argon or pure metal vapour. The mean energy and the ratio of radial diffusion coefficient to mobility have both shown monotonic variations at a fixed E/N from the higher values in pure argon to the lower values in pure metal vapour. The radiation emission has also been computed. It is observed that for a range of E/N determined by the percentage of metal vapour, that most of the swarm energy is emitted as metal vapour radiation.

Electron swarm characteristic energies (D_r/ mu , where D_r is the lateral diffusion coefficient and mu the mobility) have been measured in pure CF₄ at 293 K, using the Townsend-Huxley method. For E/N values (the ratio of applied electric field to gas number density) less than 23 Td (1 Td=10^-21 Vm²), dissociative attachment does not interfere with the measurements. For E/N values between 23 and 44 Td lower and upper limits to D_r/ mu have been evaluated using present measurements and published attachment coefficients. A Monte Carlo simulation of electron transport through CF₄ has provided low-energy elastic and vibrationally inelastic cross sections which are consistent with measured swarm transport parameters.

The authors present an experimental study of the evolution of electron density during the formation process of a plasma column produced by a surface wave. The experiments have been performed in argon gas, using pulsed microwave power at 2.45 GHz, in the 0.3-30 Torr pressure range. The time elapsed between the arrival of the ionisation front at a given axial position and the moment the electron density reaches 90% of its stationary value (stabilisation time) has been determined. This stabilisation time increases exponentially with axial position towards the end of the plasma column, as the power of the surface wave decreases.

In the design of a white light laser, the control of colour is an important problem which has to be solved. It is usually supposed that if the ion energy can be controlled continuously, an adjustment of the ratio of the elementary colour might be achieved. For the purpose of developing an ion source with energy control, Matsumoto and co-workers (1983) inserted a grid electrode into the cathode dark space of a glow discharge, and its electrical characteristics were measured. In this paper, a theoretical analysis is given to clarify the control mechanism of this type of ion source.

The author has examined the leakage current characteristics of a stacked SiO₂/Si₃N₄/SiO₂ interpoly dielectric layer in EPROM devices. The electrical characteristics were measured by applying either a positive or negative voltage on the top polysilicon gate (control gate). In both cases, for small applied voltages, the author observed enhanced electrical conduction apparently as a result of 'weak' regions in the insulators. After these 'weak' regions were removed by passing excessive currents through them, the conduction mechanism followed Fowler-Nordheim behaviour for electron injection from the bottom polysilicon layer (floating gate) and Poole-Frenkel behaviour for electron injection from the top polysilicon layer (control gate).

The initial susceptibility of a textured fine-particle system has been calculated for a log-normal and Gaussian particle-size distribution. The texture considered would result from cooling a fine-particle system such as a ferrofluid below the melting point T_f in the presence of magnetic fields of different values. Both strong coupling cases (a_f=a_m>1) and weak coupling cases (a_f=a_m<1) are considered. It is concluded that the susceptibility depends appreciably on the value of the standard deviation sigma for the strong coupling case but is much less sensitive to variations in sigma for weak coupling. The susceptibility is essentially determined only by the distribution of superparamagnetic particles in the system and since this varies with sigma some change in susceptibility is to be expected. The effect of terminating the distribution at a specific value of y_u=D/D_v is also considered.

Gas temperatures along the axis of a hollow-cathode He-Cd laser have been measured. The effect of gas temperature variations on calculations of both the excited-state number densities and the heavy-body Penning collision rates are assessed. Implications on the gas kinetics of these discharge types, in particular, on the role of Penning collisions in populating the 5s²²D_5/2 level of the cadmium ion are discussed.

Phase-matched detection of CO₂ laser radiation by non-linear upconversion in AgGaS₂ is reported for both type-I and type-II interactions. The crystal is pumped by a Nd:YAG laser while the signal is supplied from a CO₂ laser tunable from 9.2 to 10.8 mu m.

Elastic moduli of polycrystalline, silicon and germanium are derived, using different approximations, for both an isotropic distribution of crystallites and a small number of textures. A simple application to the piezoresistance in polysilicon is outlined.

The authors have used a vibrating sample magnetometer to measure induced persistent currents in a ring of high-T_c superconductor. They have also examined the variation of currents induced in the ring by cooling in a range of applied fields, and correlated these results with the magnetisation curve for the sample. They believe that this technique enables the critical current l_c to be measured without making electrical contacts on the sample.

The high-T_c superconductor YBa₂Cu₃O_7-x has been found to become more 'metallic' as the maximum sintering temperature is increased from 1187 to 1234 K. The superconducting transition width also becomes narrower and smoother with a slight increase in the transition temperature. These improved electrical characteristics have been found to be associated with larger particle size and better long-range ordering of the Cu-O linear structure. On the basis of the effective-medium approximation, the authors also argue that the observed behaviour of the logarithmic temperature derivative of normal state resistivity is not controlled by densification or preferential orientation of the c axis perpendicular to the disc surface of the sintered pellets.

Expressions are derived which estimate the systematic error involved in the determination of the activation energy by the initial rise method. It has been shown that the general belief that the initial rise method does not depend on the order of kinetics is rather a misconception if one takes the value of E thus determined in a quantitative sense. The applicability of these expressions has not only been discussed but also tested for some experimental peaks.