Table of contents

It is shown that LeBlanc's hopping electron model, despite being originally based on faulty data, still gives good agreement with apparent charge mobilities derived from nanosecond breakdown formative time-lags. Thus this model has not been proved invalid, as has been assumed, although direct supporting evidence is still required.

This review deals mainly with direct current electroluminescence (dcel) in manganese and copper activated ZnS. The preparative techniques and dcel observations on powders, thin films and single crystals are described. The basic mechanisms are outlined and possible future developments are discussed.

Experiments have been carried out in which two sharp diamond wedges were pressed together orthogonally under a known load inside a scanning electron microscope. The object of the experiment was to ascertain whether plastic deformation is possible in diamond at room temperature when it is subjected to shear stresses approaching the theoretical strength of a perfect diamond crystal. Calculation shows that it should be possible to achieve shear stresses of this magnitude using a crossed-wedge geometry in which the stresses are predominantly compressive. The wedges were made from type IIb diamond by cleavage. The results showed that there was a threshold load below which no damage of any sort could be detected on the edges of the diamonds. Above this value, indentation occurred suddenly. Subsequent examination of the indentation showed it to have a very jagged appearance as if it had been formed by a fracture process. It was concluded that plastic deformation is not possible in diamond at room temperature except possibly in very small amounts not resolvable in the scanning electron microscope.

The results of an X-ray line-broadening investigation of the effects of progressive deformation by rolling on two niobium-titanium alloys are reported and compared with similar measurements made on filed powders. The critical-current-critical-field superconducting properties of the same alloys are also reported as a function of deformation and heat treatment. Correlation of the X-ray diffraction and superconducting results suggests that some improvement in superconducting properties after low-temperature post-deformation heat treatment can be associated with the initiation of a recovery process in the deformed alloy. The peak effect is observed in one of the alloys investigated.

A direct electrical heating method to determine the thermal conductivity of metals at high temperatures has been developed by using a particular geometry of the specimen such that the non-linearity due to radiation losses in the differential equation of heat flow can be eliminated. The geometrical form of a specimen is deduced theoretically such that direct electrical heating produces a uniform temperature gradient along its length. This optimum form can be approximated to a sector of a circle with radial flow of current. The differential equation of heat flow then does not have to be integrated and the thermal conductivity at any temperature can be calculated irrespective of the temperature variation of the resistivity and emissivity of the material. The method has been verified by determining the thermal conductivity of nickel at high temperatures.

Employing an appropriate model of the potential energy of an electron outside the surface of a metal, which takes into account the image force, the Fermi energy of electrons has been calculated for thin films and tiny spherical particles. It is seen that the thermionic currents, emitted by a thin tungsten film of thickness 50 Å deposited on an insulator substrate of dielectric constant equal to 10, or by a spherical tungsten particle of radius 100 Å at a temperature 1000 °K, are about 79% and 70% of the corresponding values obtained by using the value of work function for bulk metal.

The effect of image force on field emission of electrons from negatively charged solid particles has been analytically investigated and some numerical results have been presented in the form of a graph between ln (n_em/F²) and 1/F, where n_em is the emission rate per unit area and F is the surface field strength. The results have been compared with those obtained for a corresponding plane surface using the WKB approximation. It is seen that the variation for a spherical surface has a significant departure from the corresponding approximate straight line for a plane surface.

The theory of space-charge-limited current controlled by a single set of shallow traps is extended to allow for the reduction of the effective trap depth caused by high electric fields. This reduction, the Frenkel effect, may become appreciable in fields of a few kilovolts per centimetre. A current-voltage characteristic is given which, though following the Mott-Gurney Law at low fields, predicts much greater currents than given by this law at high fields.

Calculations are presented of the components of conductance and capacitance of a condenser made of material of continuously varying conductivity. The model is a plane parallel slab with electrodes on the parallel faces, the permittivity being uniform and the conductivity being a function of distance through the slab perpendicular to the faces. Both conductivity and permittivity are taken as independent of frequency. Two different conductivity profiles are considered in detail, one in which the conductivity is a linear function of depth near the surface, and the other in which the resistivity is a similar linear function. Conductivity profiles resembling these might arise from impurity diffusion from the surface, or during crystal growth.

When the ratio of interior to surface conductivity is of the order of ten or less, these models both give impedances which vary with frequency in a manner extremely close to that predicted by a simple Debye model. It may thus happen that experimental curves can be fitted to any of these models equally well, and the number of adjustable constants is the same. The question of which model, if any, is apt must then be decided on other grounds; one set of experimental results is cited to which the layered conductivity model is believed to apply.

A phenomenological theory has been developed for non-linear self-modulation of an amplitude modulated electromagnetic wave, propagating in a magnetoplasma along the direction of the magnetic field, taking both the self interaction and mutual interaction of the two modes into account; unlike presently available theories this theory is not restricted to small values of the modulation frequency. It is shown that the ordinary mode of propagation becomes demodulated and the extraordinary mode of propagation becomes over-modulated during its propagation through the medium. Further, the change in modulation depth with the frequency of the wave is gradual for the ordinary mode and resonant for the extraordinary mode (around the gyrofrequency).

The effect on plasma measurements at high pressure of the thermal sheaths in front of plane cooled probes is discussed both for stationary and flowing plasmas. In stationary plasmas, the measured electron energy would be dependent on the probe temperature owing to the losses in the thermal sheath. A critical review of some of the earlier experimental work with cooled probes shows the limitations of the stationary analysis since most high pressure and high temperature plasmas are flowing plasmas. It is suggested that in flowing plasmas the cooled probes give reliable results for electron energy owing to the dependence of the thermal boundary layer thickness on the distance from the leading edge of the probe along the flow direction. An order of magnitude estimation is made for the thermal layer thickness of a low gas flow argon arc at atmospheric pressure. The normal use of highly negative cooled-probe data for charge density measurements is reasonably accurate both for stationary plasmas and for flowing plasmas where the probe surface is aligned along the flow direction. However, in flame and alkali-seeded inert gas plasmas, the probe current would become dependent on the cooled probe temperature owing to the influence on the thermal ionization process in the vicinity of the probe.

Pressure differences resulting from current flow in low-pressure arcs have been reported in the literature. The subject has attracted relatively little attention, however, and has remained somewhat obscure. The present paper contains a description of two effects which have been observed in a low-pressure mercury arc, namely an increase in the gas pressure and a flow of neutral gas from cathode to anode. A theory is formulated on the basis of the momentum equations for the axial flow of ions, electrons and neutral atoms, together with the similarity laws which govern the positive column. The two effects are satisfactorily accounted for by the theory.

In a low-pressure discharge with a gas anode, under certain conditions, periodical phenomena can occur. These are maintained by superficial processes caused by fast electrons which collide with the walls of the tube in the negative glow region.

In a d.c. gas discharge with metallic anode the same phenomena can occur; it is possible to give a unified explanation of these two phenomena.

On the presumption that a sharp edge may be represented by a hyperbola, a conformal transformation method is used to derive electric field equations for a sharp edge suspended above a flat plate. A further transformation is then introduced to give electric field components for a sharp edge suspended above a thin slit. Expressions are deduced for the field strength at the vertex of the edge in both arrangements. The calculated electric field components are used to compute ion trajectories in the simple edge/flat-plate case. The results are considered in relation to future study of ion focusing and unimolecular decomposition of ions in field ionization mass spectrometers.

The stability of the shock front under free-flow down the expansion tube of an electromagnetically driven shock tube is investigated. The results of a wide range of experiments covering different electrode geometries, ambient gas pressures and front velocities in hydrogen, helium and argon are analysed. It is found that a condition for stability can be found in terms of a critical hydrodynamic Reynold's number of about 9000 for the flow. The implications of this result are discussed.

An analysis is given of the effects of absorption on the reflected and transmitted components of the x-ray beam diffracted from bulk specimens irradiated at glancing angles such that the edge of the specimen is in the centre of the incident beam. It is shown that this unconventional geometry leads to a situation in which the transmitted component arises mainly from the surface layers of the specimens, whereas the reflected component arises mainly from the bulk of the specimen. It is further shown that the reflected component has a linewidth not much different from that of the incident beam, whilst the transmitted component has a width at least an order of magnitude smaller than that of the incident beam. The relevance of this analysis to the identification of thin oxide layers present on bulk specimens is briefly discussed.

A simple dielectric motor has been developed in which the driving torque is derived by interaction of the interelectrode field with charge deposited on the rotor surface from the semi-insulating liquid ambient. Optimization of the torque has been effected by using a multi-electrode structure, and by shaping of the electric field distribution using a high-permittivity shell on the rotor. Useful output powers of 50 mw have been obtained at an overall efficiency of approximately 1%; it is not obvious how larger or more efficient motors might be constructed.

The possibility of utilizing dilute polymer latex dispersions for calibration of light-scattering photometers has been explored. For this purpose, Mie angular scattering functions for spherical particles of known size distribution and refractive index must be computed. It has been demonstrated that the computed intensities are too sensitive to the small, but finite, polydispersity of so-called monodisperse latex dispersions, and to slight variations of the mean particle size. Because of this sensitivity, the calibration method based on the use of Mie scatterers is impractical and unreliable.

Layers of carbon or graphite have been grown by pyrolysis of acetylene brought into contact with a platinum surface heated to 1000°C (1273 K) in ultra-high vacuum. The carbon surface has been imaged by field-emission microscopy, and field-evaporation and oxidation techniques have been used in a preliminary study of two types of structure. Work function values of the smoother type of surface obtained range from 3·6 to 3·9 ev.

Observations by scanning electron microscopy of an alloy/oxide interface exposed after oxidation have shown a repeating pitting structure within the grooves at the alloy grain boundaries. The possibility that these observations illustrate a defect macrostructure within the alloy grain boundaries is discussed.

Silicon carbide films having thicknesses ranging from 300 to 8000 Å have been deposited on a variety of substrates by electron beam evaporation at rates of up to 250 Å min⁻¹. The crystal structure of the films was investigated by transmission and reflection electron diffraction, and was usually amorphous. The optical absorption edge was at about 5200 Å which corresponds to an energy gap of about 2·4 ev. The films had resistivities ranging from 100 to 6700 Ω cm and exhibited activation energies of approximately 0·16 ev over the temperature range 290-380°K.

The thermal diffusivity k_d of silicon between - 50°C and 100°C has been obtained by measuring the phase-shift of a thermal wave between two points on a small sample. The relation between the thermal diffusivity and phase-shift is obtained by solving the heat flow equations. The present results agree closely with those obtained by conventional methods.

Electroluminescent behaviour of ZnO-CdS alloy systems incorporating copper as activator and bromine or aluminium separately as the donor impurity, has been investigated. Binary solid solutions, with different proportional composition by weight of the two compounds ZnO and CdS, have been formed in an atmosphere of pure nitrogen through heat treatment.

The peaks of electroluminescent emission have been found to shift progressively from blue to orange, with stepwise rise in the proportional contents of CdS in the phosphor matrix.

Numerical curve fitting of glow peaks, using a computer, is shown to be more objective and accurate than the existing graphical curve fitting method in evaluating trap activation energies. Besides using the numerical curve fitting method for glow peaks of first-order kinetics, the method is extended to the case of second-order thermoluminescence or thermally stimulated current peaks. The method was checked both for numerically generated peaks and experimental thermoluminescence curves of NaCl and ZnS: Er³⁺ samples. The results are in good agreement with the given values of activation energies in the former case and values calculated by other means in the latter.