Table of contents

When the potential of a small loop of wire is raised by a few volts with respect to a flat counter electrode it is possible to trap a dielectric particle in a position of stable equilibrium. 50 mu m diameter particles of divinyl benzene suspended in water have been studied. The dielectrophoretic response of such particles as a function of the frequency of the applied voltage should enable relaxation processes at the particle-liquid interface to be determined.

A technique for obtaining pressure-concentration isotherms of the palladium hydrogen system is described. A quartz crystal thickness monitor is used to directly determine the quantity of hydrogen absorbed by an evaporated palladium film. The measured phase diagrams agree with previous measurements on bulk samples. The method can be applied to any metal film-gas system.

The vaporisation of Hg_0.8Cd_0.2Te crystals under Knudsen effusion conditions has been investigated by the technique of modulated-beam mass spectrometry. Hg is the only dissociation vapour species detected at temperatures below 200 degrees C. Between 200 degrees C and 450 degrees C both Hg and Te₃ was present but Cd was detected only above 450 degrees C. This grossly incongruent vaporisation leads to considerable concentration gradients within the charge crystals which have been observed directly using SEM, EDEX and X-ray diffraction techniques. Crystals heated to 420 degrees C for several hours under Knudsen effusion conditions lose HgTe via the vapour species Hg and Te₂ and develop a surface layer of CdTe which, though porous, remains epitaxial. The development of concentration gradients within the charge at lower temperature leads to a diffusion rate limitation of the mercury vapour pressure developed within the Knudsen oven.

The iteration technique is used to find the relation between the linear functional and Pade approximants. Two examples are solved as applications: (i) the neutron escape probability and (ii) the reflection and transmission function in radiative transfer and in turn the emergent and transmitted intensities for a finite slab and the emergent intensity for a semi-infinite medium. Numerical calculations are done and compared with the exact results obtained from other techniques. It is found that the Pade approximants converge to the exact results.

A description is given of measurements made at 35 GHz of the complex transmission coefficient for an isotropic and an anisotropic material using a free space microwave bridge. The medium-loss isotropic material exhibited the effects of multiple internal reflections which have been fully accounted for. In the case of the high-loss anisotropic material the complex reflection coefficient was also measured. The measurements were made in a microwave bridge with the sample placed in free space between horns, with a waveguide reference arm. Separate circuits for transmission and reflection are described. The anisotropic material consisted of coated, conducting parallel glass fibres moulded in an epoxy resin disc. Measurements are reported for the electric vector parallel and perpendicular to the fibre direction. Values of the refractive index n and the absorption index K where calculated from the measured complex transmission and reflection coefficients.

The effects of iron on the uniformity of the field produced by a current carrying axisymmetric conductor are considered. Using a perturbation analysis a simple analytic expression is obtained which describes the field close to the axis of symmetry. The accuracy is estimated by comparing the results with those obtained by using finite-difference methods.

Scattering and absorption efficiencies are calculated for chains of spheres in the Rayleigh approximation, and compared with individual particles. For aligned chains significant differences are found for the scattering case which arise due to multiple scattering and coherent phase effects. The absorption efficiency is not influenced by the latter, but the changes are found to be of the same proportion as those in the scattering efficiency due to multiple scattering alone. For randomly oriented chains the effects of multiple scattering are small within the range considered.

An acoustical theory of the photoacoustic effect is presented in such a way that expected signal strengths from microphones in gaseous media and from hydrophones in liquid media can be compared. For liquid media it is shown that the sample acoustic impedance is often quite important. Numerical comparisons of hydrophones and microphones indicate that photoacoustic signals from the former can be stronger than from the latter.

The instability of a thin sheet of liquid moving in the same direction as an air stream is studied theoretically. It is found that instability occurs if the Weber number is less than a critical value related to the ratio of the air and liquid stream velocities. Also, the wavelength for maximum instability is found to be inversely proportional to this critical value.

The transient electrical properties of a planar probe which is immersed in a flowing continuum plasma and in which the flow velocity is parallel to the probe surface are studied theoretically. When the probe is pulsed negatively, the resultant ion current overshoots its final equilibrium value. The time constant for recovery from this overshoot is shown to yield the information about the plasma flow over the probe. When the plasma sheath edge is inside the hydrodynamic boundary layer it is able to directly probe the velocity structure of the hydrodynamic boundary layer. The theory used to describe the probe's electrical properties differs from other thick sheath theories in that both gas compressibility and ion/electron recombination effects are considered.

Time delays of electrical breakdown in a gas-filled diode (t_d) were measured as a function of the time between two successive measurements ( tau ) at cathode temperatures of 300K, 523K and 673K. The diode was filled with nitrogen at 4 mbar with an oxygen impurity of about 10 ppm. It is shown that the breakdown time delays at 673K are one order of magnitude smaller than those at 300K.

Eddying gas motion due to j*B forces in the current diverging from an anode spot is normally unlikely to have any significant effect on a glow discharge, but may be more important in some discharges intermediate in character between glows and heavy-current arcs.

Measurements are reported of the electrical properties of wall-stabilised arcs subject to an imposed axial flow of nitrogen under steady current conditions. The current was in the range 10-60 A DC and the gas flow rate was 0-0.6 g s^-1 with stabilising bore radii of 2.5 mm and 5 mm. The results of the investigation confirm the existence of two axial zones corresponding to a flow entrainment region and a fully developed column region. The electric field strength along the fully developed region increases with mass flow rate as turbulent effects become more pronounced. An analytical arc model, based upon boundary layer approximations and taking account of molecular and turbulent radial diffusion and axial convection, is presented. Results of calculations using this model predict the trends observed experimentally.

Glow discharges operated in narrow capillary tubes filled with helium, neon, or a mixture of both, feature strongly negative current-voltage characteristics at discharge currents of several mA and total pressures of a few Torr. These characteristics are computed assuming that the positive column, by virtue of wall-charge effects in the narrow tube, is not quasi-neutral, and hence the ion velocity directed towards the wall is greater than the mean thermal velocity. Measured and computed characteristics coincide remarkably well. The current and pressure range mentioned above should prove to be of substantial interest for He-Ne laser applications.

The electrical properties, viz resistivity and I-V characteristics, of evaporated CdSe films sandwiched between two Al electrodes have been studied in situ at 30 degrees C and at 8*10^-6 Torr within the thickness range of 2400-2500 AA as function of substrate temperature (50-150 degrees C). The observed change in resistivity in I-V characteristics could be explained by stoichiometric changes of the film material and film disorder of thermally evaporated films. Space-charge-limited current conduction was observed on films prepared at a substrate temperature of 150 degrees C.

A method is presented for determining the optical constants of films which have a slightly rough upper surface and which are somewhat optically inhomogeneous along the normal to the film surface. It is shown how the RMS height of surface irregularities, the value of introduced parameter a describing the optical and inhomogeneity and film thickness can be calculated on the basis of measured transmittance and reflectance in the region of interference fringes.

An examination has been carried out of the way in which the incident electron beam angle affects the cathodoluminescence output versus beam voltage characteristics of thin luminescent films of Zn₂SiO₄:Mn. This has enabled the change in form of the electron depth-dose curves with beam angle to be deduced. The observed trend is in agreement with previous results using much higher beam voltages.

Temperature drops down field electron emitter needles from their supporting (heated) filaments to their submicroscopic hemispherical tips were measured by a new technique. Emitter needles of hafnium, which undergo an allotropic solid phase transformation at a known high temperature, were heated until the alpha to beta transformation which occurred at the tip could be seen through the field electron emission pattern. An optical pyrometer was used to determine the filament temperature. Using published optical pyrometer was used to determine the filament temperature. Using published values of emissivity and thermal conductivity for bulk hafnium, computed results, using classical heat conduction and radiation theory, gave temperature drops that were much too small. Calculated temperature drops for Hf below the transformation temperature and for W needles are presented for comparison.

An analysis is presented of the structural changes which occur on cathode surfaces arced in vacuum. Cadmium, copper, molybdenum and tungsten cathodes carrying currents up to a few hundred amps showed a variety of erosion patterns which have been interpreted as representing a number of stages with different degrees of erosion. Cathode spots moving either randomly or under the influence of an external magnetic field produce basically the same erosion pattern. This pattern consists of craters varying in size from less than one micrometre up to several micrometres. A three-dimensional analysis of a crater-structure is given and quantitative information is obtained on crater size distributions and current densities for different metals. The influence of surface contamination on cathode tracking is pronounced.

The effect of the real rate of deposition on the superconducting properties and structure of tin films deposited in vacuum on room-temperature amorphous substrates has been observed and studied for rates in the 5-200 AA s^-1 range. It is suggested that the oxygen contamination coefficient K, and thus film purity, changes with the rate of deposition. At higher rates, above 100 AA s^-1, pure smooth films were obtained, suitable for the fabrication of superconducting tunnel junctions.