Table of contents

The behaviours of the wall relaxation and the diffusant relaxation of a high purity polycrystalline YIG, slightly deficient, are investigated from 77K to T_c (550K) by measuring the complex initial permeability over a broad frequency range (up to 1000 MHz). A critical temperature T_cr ( approximately 310K) is defined when the two relaxation processes merge together. Below T_cr one finds a large effect of the 'relaxed' diffusants upon the wall mobility (or damping); above T_cr, where the diffusants can follow the wall motion, no appreciable effect is observed. This later fact is in contradiction with the existing theories of the 'diffusion damping' of the domain walls.

The effect of copper sulphide species on the photoactivity of cadmium chalcogenide photoelectrodes is discussed. This species, believed to be CuS, is applied using two methods. In the first, polished CdSe electrodes are dipped into a solution containing cupric ions, immersed in polysulphide solution and subsequently operated as anodes. In the latter technique, copper species migrates from a sulphided, brass counter electrode to an etched CdSe or CdSe_xTe_1-x photoelectrode. In the former case, the main effect of the copper treatment is an increase in the short-circuit photocurrent while in the latter case, the dark cathodic current decreases and the open-circuit voltage increases. The effect of these treatments is explained by preferential deposition of copper (or some copper-containing species) at surface defect sites, resulting in the partial passivation or blocking of these sites.

Rutherford backscattering (RBS)/channelling and X-ray photoelectron spectroscopy (XPS) measurements have been performed to elucidate surface states of cubic-structure ZnS crystals doped with iodine. The XPS studies combined with an Ar⁺-ion sputtering technique revealed that adsorption of oxygen and chemical oxidation at the cleaved (110) faces resulted in significant changes of the surface properties, and that a large oxygen concentration, above 10¹⁹ cm^-3, was detectable at the surface and in the bulk of the crystal prepared under various oxidation conditions. It is tentatively suggested that the chemically oxidised layer consists of ZnO and ZnS mixed alloys. The formation of oxide layers was applied to low-resistive n-ZnS and allowed the fabrication of an efficient blue light-emitting diode (LED) with the M pi S structure. It is demonstrated that the blue LED, with an effective interfacial layer of about 300 AA, exhibit a high-efficiency forward-bias injection blue luminescence around 2.76 eV at room temperature (an external quantum efficiency of about 8*10^-4 photons/electron) as a result of the involvement of two kinds of donor-acceptor pair recombination centres.

The thermal etching and the evaporation of ZnS single crystals were studied in vacuum. The Langmuir and the Knudsen evaporation processes on (111), (0001), (1010) and (1120) faces were investigated. In the Langmuir case on cationic polar faces etch pits are formed, whilst on anionic close packaged planes hillocks are observed. Rapid spiral decomposition on the A-surfaces and growth spirals on the B-surfaces are described. In these etch-features symmetrical stress fields are pointed out. From the elimination of different faces and temperature dependent etch pattern anisotropies the relation of the different evaporation rates can be approximated. For Knudsen evaporation the resulting near-equilibrium morphologies are also given. On the basis of the interferometric examination of the morphology it is supposed that the rate determining step for Langmuir evaporation is the generation of kinks.

Thin films of rare earth doped zinc sulphide have been made by co-evaporation and shown to given bright DC electroluminescence at low applied voltages. Thermal annealing of the films caused copper migration and led to degradation in performance which manifested itself in lower brightnesses and higher operating voltages.

A method is described of improving the high frequency components of the spectrum derived by the fast Fourier transform. It is particularly applicable to the analysis of time series which decay with time such as in stress relaxation. The method involves the addition of components at the same frequency derived from fast Fourier transforms taken over a succession of increasing time intervals. A numerical example is given.

The equations for radiative transfer require scattering parameters in the form of the extinction, absorption and scattering efficiencies and the phase function. Averaged efficiencies for clouds of spherical particles of complex refractive index m=1.5-i0.012 illuminated by black body radiation are presented. These are seen to depend only on the product of the modal particle size and the radiation temperature.

The laser emission of YAlO₃:Er at 1.663 mu m was investigated in order to find the best conditions with respect to crystal orientation, Er concentration, and output coupling. Using ten different laser rods the following optimised data have been obtained: b axis laser rod (Pnma notation), 1.25% Er doping, and 85% output mirror reflectivity. The temperature dependence of laser threshold and efficiency was measured between -50 degrees C and 50 degrees C. At room temperature the authors obtained 34 J threshold energy and 0.07% slope efficiency. In addition laser emission of LiYF₄:Ho, Er, Tm at 2.065 mu m was investigated. The slope efficiency was 0.22%. Laser emission of YAlO₃:Ho, Er, Tm at 2.12 mu m is briefly mentioned.

Traces the flow of power in both the core and the cladding of a step-index fibre. Particular attention is focused on behaviour as cut-off is approached, where the power in the cladding rises very rapidly, extending outwards over the whole cross-section of the fibre to the surface of the cladding. Cut-off must be accompanied by collapse of the field of the mode concerned and release of the energy stored in that field, to give radiation and/or conversion to another guided wave.

The nonequilibrium behaviour of electrons in a drift tube in SF₆ is investigated using a Monte Carlo simulation. It is shown that, in the case of a steady-state experiment, the mean properties of the electrons (drift velocity, number density, mean energy) present some strong spatial oscillations and that attachment occurs in some very localised regions of the gap. The question of the validity of the hydrodynamic description in that case is considered. In spite of these spatial oscillations of the macroscopic parameters, the first transport coefficients (deduced from the simulation of a time-of-flight experiment) seem to quickly reach their equilibrium values. Results from steady-state, pulsed Townsend and time-of-flight experiments are discussed.

Pulse currents of 10 ns rise time and 20 ns half-width were obtained with a gas discharge phototube driven by a nitrogen pulsed laser. The current pulse time characteristics are discussed as well as their variation with phototube voltage. The possibility of obtaining high and fast current pulses in gas discharge devices is analysed.

The ionisation coefficient in Kr-Hg mixtures has been measured for E/n values between 2*10^-20 V m² and 2*10^-18 V m² and for a mercury atom fraction varying from 8*10^-4 to 8*10^-2. The experiments were performed with a parallel-plate apparatus in which the current is measured as a function of the plate separation at constant electric field. The ionisation coefficient was found to be a function of E/n and the mercury atom fraction only. The increase in the ionisation coefficient for small admixtures of mercury is mainly due to associative ionisation. From the experimental results the effective rate coefficient for excitation of the appropriate krypton states is obtained as a function of E/n, as well as the ratio of the rate coefficients for depopulation of these excited krypton states by nonionising collisions with krypton and ionising collisions with mercury atoms respectively.

It has been shown that the chromatic dispersion in the boundary zone between the negative glow and cathode fall space of a cold cathode discharge is in many instances correctly predicted qualitatively by the rules that the maximum of intensity of a spectral feature is the nearer the negative glow the lower its excitation potential, and that the width of the intensity versus position curve increases with the excitation potential. The effective excitation potential is however not necessarily the least potential for which the spectrum can be excited by electron impact, but may be a higher one which allows for the excitation cross-section being comparable at higher voltages with its value near threshold. Spectra with excitation potentials approaching the cathode fall must be excluded from the rules because they can only appear relatively close to the negative glow. When more information is required than is furnished by the rules, and it is impracticable to obtain this accurately using a Boltzmann transport theorem, Monte Carlo or equivalent analysis of the discharge, it can sometimes be obtained semi-quantitatively from a simpler approximate model of the fall space. In some discharges allowance may have to be made for changes in number density of the atoms or molecules due to heating of the gas and electrodes, and for the effects of metastable atoms and molecules and resonance radiation.

A theoretical analysis of the characteristics of high-frequency and direct-current argon discharges at low pressures is carried out and a comparison of the basic properties of these discharges is presented. This analysis uses recent calculations of electron energy distributions and collision rate coefficients in argon under the action of uniform AC and DC electric fields together with a steady-state discharge model expressing the balance between collisional ionisation of the gas and the loss of electrons to the wall. The electric field strength, E, required for the steady-state operation of planar and cylindrical discharges is calculated as a function of the gas density N, the angular frequency omega , and the diffusion length for the discharge tube Lambda , assuming predominant direct ionisation.

Spectroscopic techniques have been used to measure the temperature distribution in free-burning arcs in argon at 1 atm pressure. The experiment provides evidence for departures from LTE in the arc, and demonstrates deficiencies in the theory describing continuum emission from high temperature plasmas. This paper describes the apparatus and data reduction procedures and compares the measurements with recent theoretical calculations.

The properties of ablation-stabilised arcs confined in uniform cylinders, open at both ends, are calculated. It is assumed that the arc temperature is uniform with radius. The axial variations of the arc temperature, pressure, radius and plasma velocity are calculated as functions of current by solving the coupled equations of conservation of mass, momentum and energy. It is assumed that a specified fraction of the arc radiation is absorbed at the ablating cylinder wall and that the remainder is absorbed in the ablated vapour from the wall. Radiation losses from the arc are treated by assuming either (i) that the effective emission coefficients of the arc are proportional to pressure and that the arc radiation losses can be characterised by a net emission coefficient, or (ii) that the arc emits black-body radiation appropriate to the central temperature of the arc. For both approximations, detailed numerical solutions show that the arc temperature and radius are almost uniform with axial position. By assuming that the arc temperature and radius are indeed uniform, simple analytic formulae for arc temperature, pressure and Mach number are obtained. Comparisons between theoretical and experimental voltage-current characteristics show good agreement.

A compact sputtering device uses a supersonic flow of argon to produce a collimated beam (semi-divergence angle 3 degrees , diameter approximately 10 mm, beam length 200 mm) of metal vapour with a ground state concentration of the order of 10¹⁴ cm^-3. The concentration increases as the square of the discharge current density over the range 0 to 600 mA cm^-2. The gas flow decouples the processes responsible for the sputtering and subsequent excitation of the metal atoms, allowing separate discharges and independent control of the metal atom concentration and of the excitation discharge current. The proposed model describes the influence of the gas flow on an abnormal glow discharge in terms of an effective pressure. This allows the existing theory of the cathode fall to be applied to a glow discharge with a gas flow.

Steaming (droplet formation) above the surface of liquid water that is being heated can first be observed at about 50 degrees C under normal atmospheric conditions. The droplets have been presumed by previous observers to form on condensation nuclei that are always present due to impurities in atmospheric air. But simple experiments, reported here, show that droplet nucleation apparently can occur spontaneously with warming, beginning at about 50 degrees C, even in purified saturated air at atmosphere pressure that is free of nuclei other than water clusters. The observations suggest new ideas about structure in water vapour. A critical cluster size of about 45 can explain observations over a wide range of temperatures and water vapour partial pressures.

Space-charge-limited current behaviour is reported for the temperature range 200K to 350K in undoped p-GaSe, parallel to the c axis, for samples showing the most frequently observed activation energy. The conduction is found to be extrinsic, (N_A-N_D)=3*10¹⁶ m^-3 and the dominant hole level is a single discrete trap with E_t=0.195 eV and N_t=4*10²² m^-3.

The temperature dependence in the range 215-343K of the high-field electrical conduction in evaporated thin potassium iodide films has been studied for 1 V applied voltage, covering a range from microseconds to several hours. The results show that the universal dielectric response is valid separately in two or three regions for temperatures lower or higher than 269K respectively. Change in the temperature of the films has a different effect in each region on the transition probability, which is associated with the hopping mechanism.

This paper presents an experimental study of carrier drift in polycrystalline pentacene layers under small-signal conditions. The shape of the transient current, caused by pulsed bulk carried generation, and the transit time field-dependence are analysed in detail for various temperatures. The data obtained indicate a dispersive transport regime. A theory of dispersive transport with bulk generation is formulated in order to explain the experimental results. The theory is based on the concept of transport controlled by carrier trapping on localised levels distributed over a wide energy region. Particular attention is given to the exponential and Gaussian models of trap energy distribution. The Gaussian model seems more realistic for interpretation of experimental data on polycrystalline pentacene layers.

A method to find the depth of penetration of charge into an insulator contacted by a metal is described. Results are given for the polymer polymethylmethacrylate (PMMA) and these are discussed in terms of current models of contact electrification.

A theory of the magnetisation process in ferromagnets, based on existing ideas of domain rotation and domain wall motion is presented. This has been developed via a consideration of the various energy terms into a mathematical description of the process leading to an equation of state for a ferromagnet. The differential equation has been solved and a solution containing terms up to the second order presented, showing the essential features of ferromagnetic hysteresis. The theory has then been used to explain the effects of stress on magnetisation. It has been found that the magnetisation approaches the anhysteretic curve when a ferromagnet is subjected to stress and this is the underlying principle behind such changes in magnetisation. The change of magnetisation with stress can not be predicted solely on the basis of the magnetostriction coefficient except in special cases when the initial (zero stress) conditions of magnetisation lie on the anhysteretic. This condition is also approximately satisfied at higher fields.

Dielectric measurements on metastable solid MBBA were performed in the frequency range from 0.5 to 100 kHz under pressures up to 6 kbar. The application of pressure shifts the dielectric relaxation curves towards higher temperatures. With increasing pressure, the dielectric relaxation strength increases in the low pressure region up to about 3.5 kbar and after that tends to saturate in the high pressure region. The activation parameters, such as the activation enthalpy, entropy and free energy obtained from the linear relation between the logarithmic relaxation frequency and the reciprocal absolute temperature, increase with increasing pressure. The inflection point p_t was found to be at about 3.5 kbar on the curves of activation parameters versus pressure. These facts suggest that different relaxation mechanisms should be applied to the low pressure region below p_t and the high pressure region above p_t. The pressure dependence of the glass transition temperature was inferred from the pressure dependence of the loss maximum temperature.

Current-voltage curves, using blade-plane electrodes in various liquids, whatever the polarity of the blade exhibit a sharp bend. At low fields, there is a linear relationship while at high fields, the current varies as V^alpha (3< alpha <6), V being the applied voltage. Stationary field plots (using the Kerr technique) and transient current measurements show there is a homocharge injection by the blade. The injected charges, most likely ions, induce a liquid motion. This electrohydrodynamic (EHD) motion is thought to be the dominant factor in charge transport at high fields.

The anomalous enhanced backscattering of fast light ions from heavy amorphous solid targets is studied within the formalism of transport theory. The angle distribution of emerging ions is obtained by a variational procedure, where the correlation in the collisions on the incoming and outgoing paths is treated as a perturbation. When using a power potential for the ion-target interaction this distribution can be expressed in terms of reduced variables for the emerging angle and backscattering depth, and the dependence on the collision energy and atom species appears only on the scale factors. The observation of the enhancement in the distribution of backscattered ions will depend on the value of these scale factors as compared with the experimental resolutions.

This paper outlines an analysis correlating new experimental data and accounting for the characteristics of aluminium heating in vacuum, up to the production of surface plasma and further, under microsecond pulsed CO₂ laser radiation in the intensity range 10⁷-10¹⁰ W cm^-2.