Table of contents

Monte Carlo simulations of electron scattering in silver have been performed at 6 and 20 keV to investigate the relation between electron transport, backscattering and depth distribution of characteristic X-ray production of tracer elements. The simulation models used combine the use of elastic cross sections, either of Rutherford or of Mott type, with approaches to inelastic scattering given by either the Bethe stopping power or by the energy loss- and momentum transfer-dependent dielectric functions derived from optical constants. A comparison of the simulation results shows that the model with Mott cross section and dielectric function results in better agreement between calculated and experimental energy spectra of backscattered electrons. However, the dominant term in models of Phi ( rho z) curves is elastic scattering and the Mott cross section yields better agreement with experimental results.

An analysis of the electromagnetic response of a cylindrical conductor of infinite length which has a uniform coating of bi-isotropic medium is rigorously formulated by constructing cylindrical vector wavefunctions. Subject to the radius and thickness of the coating, an anisotropic impedance boundary condition in the spectral domain is developed which holds in a general context.

The relativistic fifth-order geometrical aberrations of the combined focusing-deflection system and their coefficients have been derived. The variational functions and some of their gradients are given as are all aberrations and the fundamental method of derivation with software REDUCE.

The dissociation of carbon dioxide and the reaction of carbon monoxide with oxygen caused by a high-energy (approx. 100 keV) electron beam in a typical carbon dioxide laser gas mixture has been observed. The variation of reaction rates with electron energy and current, interelectrode spacing, and gas composition has been studied. The rates of both processes suggest that the reactions are caused by unthermalized secondary electrons. The conditions were investigated under which the carbon monoxide oxidation reaction could be used to offset dissociation in a laser and thus prolong its sealed life. For a secondary to primary current ratio of one hundred this condition should be satisfied for any practical device. A sealed run was carried out which demonstrated that dissociation by secondary electrons could be offset by the oxidation of carbon monoxide by the primary electrons.

A fast flow CO₂ laser is investigated numerically including the effects of turbulence and convection. The model developed was applied to lasers with various discharge tube lengths. Gas flow and active medium characteristics such as temperature, velocity, small-signal gain and quantum densities were obtained along the discharge tube. The influence of the gas flow velocity and the pressure on the laser characteristics was considered. The output laser power was estimated and its increase due to turbulence and azimuthal velocity component was confirmed. The results obtained are in very good agreement with experimental data measured for a real CO₂ laser.

A complete integration of the one-dimensional (1D) equations of a CW CO₂ laser plasma with fast turbulent motion is presented. The electronic excitation rates in CO₂-N₂-He-CO-O₂-Xe gas mixtures have been taken from the solution of the homogeneous, steady state Boltzmann equation. The model predicts the power characteristics of laser operation under various conditions on the basis of technical input data only. It can be used for the optimization of different discharge structures (DC, RF, longitudinal, transversal) as well as for parametric studies of multiple discharge lasers.

The thermal diffusivity of gadolinium in the temperature range 25-900 degrees C has been measured with application of the laser flash method. The thermal conductivity has been deduced by using published specific heat and density data. Despite the debated values at room temperature, the thermal conductivity of gadolinium is very low compared with that of almost any other pure metal.

The steady-state heat conduction equation is solved in a prolate spheroidal coordinate system. Some simple solutions are derived in the limiting cases of both low and high Peclet numbers. The analysis was carried out in such a way as to avoid the physical details of conditions inside the weld pool, so that the solutions are restricted to the solid region outside the weld pool. This procedure was specifically adopted because these conditions are difficult to gain access to experimentally, as is the precise detailed shape of the pool; the solutions obtained can be verified experimentally. The high-Peclet-number approximation is likely to be particularly useful in the case of laser welding where large translation speeds of the weld piece are of interest. The solution of the problem is given in the form of a series as well as in an asymptotic form. The asymptotic method of solution presented can be adapted to any smooth shape of weld pool with only minor alterations, since the method involves integration in the tangent plane to the weld pool and the results of such an integration are independent of the global form of the weld pool. The asymptotic result is compared with the exact solution in a number of special geometric configurations. These are prolate spheroidal weld pool geometries with various aspect ratios and cylindrical weld pool geometries with elliptical or circular cross sections. The results of these comparisons were found to be satisfactory.

An exergy and anergy balance in the process of spray evaporation in hot gas surroundings has been made to determine the second law efficiency of the process. The law of entropy generation in droplet evaporation (from the authors' earlier work) has been taken as the fundamental information for evaluating the entropy generation rate due to evaporation of the dispersed phase in the spray. The entropy generation rate due to transport processes in the carrier phase has been evaluated from a two-phase separated flow model of the spray. A theoretical model for exergy analysis of the process of spray evaporation has been developed to predict the second law efficiency of the process in terms of pertinent controlling parameters like free stream temperature, initial Reynolds number and initial drop size distribution of the spray.

Plasma analysis using emission spectroscopy has been carried out in the cathode fall and the negative glow of N₂-H₂ abnormal glow discharges for steel surface nitriding. The spatial distributions of the selected excited states of N₂, N, N₂⁺, N⁺, NH, H₂, H and Fe have been measured in terms of their dependence on the main process parameters, such as the cathode temperature (400-550 degrees C), H₂ percentage in the gas mixture (10-70%) and total pressure in the nitriding reactor (400 and 800 Pa), corresponding to the given values of the discharge current (115-260 mA) and applied voltage (465-635 V). The experimental data presented yield reliable information on the occurrence of excited states in the cathode region and on the effect of the process parameters on the cathode fall thickness, which determines the energy distributions of ions bombarding the cathode and is an important input parameter in plasma process modelling.

In Brock's (1964) analysis concerning the thermophoretic force on a small particle suspended in a rarefied plasma with a temperature gradient, the effect of the presence of an external magnetic field on the ion and electron components of the thermophoretic force was demonstrated. The particle suspended in the plasma was assumed to be electrically neutral, which is obviously not realistic. Consequently, Brock obtained similar expressions for the thermophoretic force components due to, respectively, atoms, ions and electrons for the case without the external magnetic field. The present paper improves Brock's analysis by taking the charging of the suspended particle into account and obtains somewhat different expressions from his results for the thermophoretic force components due to ions and electrons.

Side-wall profiles have been calculated for sputtering of amorphous Si substrates using an Ar plasma, under conditions typically used for reactive ion etching of semiconductor materials. The authors combine ion energy and angular distributions, calculated for ions travelling through the oscillating RF sheath, with a description of the interaction of these ions with an amorphous Si surface. Resultant side-wall profiles are presented for a range of plasma parameters, including reactor pressure, applied RF voltage and frequency, resist erosion rates and chemical etching component. Calculated profiles show the generic forms of profiles seen in experimental etch processes. Low-pressure plasmas give rise to vertical, anisotropic side-walls with rough surfaces, whilst higher pressure plasmas produce smoother surfaces and exhibit a degree of mask undercut. Resist erosion during the etch process gives rise to sloping side-walls, whilst chemical etching effects produce isotropic profiles characteristic of wet etching.

Current growth waveforms corresponding to initial phases of negative corona current pulses were measured in nitrogenous gas mixtures at pressures of 6.67-40 kPa. The measurements showed that the steep pulse rise is not dependent on cathode secondary emission processes for pressures above 30 kPa. This, together with a discontinuous transition in the current waveforms observed with increasing gap voltage in N₂ at pressures of 20-26.6 kPa, implies that the feedback-to-cathode Townsend ionization mechanism was supplanted by a faster streamer-like mechanism. At pressures below 20 kPa a positive-streamer-like wave results in complex Trichel pulse shapes with a step on the pulse leading edge and a hump on the pulse trailing part.

It has been demonstrated that the general topological characteristics of branched electrical discharges can be efficiently described by the concept of fractal geometry. However, researches carried out on this subject hitherto have been restricted to computer simulation of discharge patterns without reference to experimental data and physical discharge mechanisms. In the work, therefore, a specific discharge type is studied, namely a Lichtenberg figure in SF₆, for which experimental results are presented together with a discussion of the underlying physical processes and numerical simulations. Good agreement is found between experiment and simulation and it is shown that the approximate fractal scaling of the discharge figure is a consequence of specific underlying physical mechanisms.

The plate current for a negative corona in a corona triode is observed to become positive and to saturate at a value I_s, when the grid is biased positively and the point-to-grid distance is less than 15 mm for corona currents of up to 60 mu A. The saturation current I_s decreases exponentially with increasing point-to-grid distance and this strong dependence appears to indicate that the current is linked to those high-energy (hot) electrons which survive attachment, and are capable of ionizing molecules below and on their way to the grid. Simple calculations based on a theoretical model for hot electrons show that their energy is sufficient for causing such ionization. This inverted polarity current can also explain surface charge decay measurements presented in the literature for polymer samples treated in a corona triode when the grid was counterbiased with respect to the corona source.

A non-equilibrium one-dimensional model is proposed for the plasma sheaths near the electrodes of electric arcs where the cathode is a thermionic emitter. Three coupled equations are solved: (a) the charge continuity equation, which accounts for ambipolar diffusion, recombination and ionization, (b) a form of Ohm's law relating the effective electrical conductivity with a local electric field and (c) the energy balance equation, accounting for thermal conduction, Ohmic heating, plasma radiation and the effects which occur at the electrode surface, i.e. thermionic cooling, ion heating, radiative heating from the plasma and radiative cooling by thermal emission. The three equations give distributions of temperature, electric field and charge density within the sheath. Results are given for the sheath at the thoriated-tungsten cathode and anode for a 200 A arc in argon. Calculations have also been made of sheath properties for various electrode materials to determine the critical current density sustainable by a cathode without cathode melting. Critical current densities for anode melting are less than for cathode melting because of the cooling effect at the cathode of thermionic emission. The authors find that the effect of ion heating tends to keep the cathode temperature near to the temperature for which the current density from thermionic emission equals the imposed current density.

The decrease of the normal current density at the cathode of a glow discharge raises the threshold of contraction. To achieve this the addition of small numbers of atoms with low ionization potential to the near-cathode plasma is proposed. Adsorbing at the cathode surface, these atoms decrease the cathode work function and increase the ion-electron emission coefficient gamma . The degree of coverage of the surface by these atoms, the work function and gamma depend on the intensity of bombardment of surface by ions, i.e. on the current density. As a result, gamma falls as a function of current density. It is shown that this leads to the decrease of normal current density at the cathode of a glow discharge.

An experimental and theoretical study of the positive column of the CF₄ DC glow discharge is presented. It includes the experimental determination of the reduced electric field E_z /N and the electron impact excitation rate constant K_Ar of the line ( lambda =750.3 nm, epsilon _th=13.5 eV) of the actinometric addition (Ar) in CF₄ plasma for a wide range of parameters (pressure p=0.2-2.3 Torr, current density j=4-35 mA cm^-2). All experimental values of the reduced electric field can be described within the limits of experimental error by the universal dependence E_z/N on r₀N (r₀₌0.9 cm, the discharge tube radius). Also the experimental dependence of k_Ar on E_z/N has been obtained. These results have been used to test different theoretical models of the CF₄ discharge positive column. Comparison of the experimental and calculated values of E_z /N and K_Ar has shown the validity of the theoretical model developed in this work. This model takes into account the large negative ion concentration in the positive column and predicts its division into regions of ion-ion plasma (in the centre) and electro-ion plasma (near the tube wall). The model also accounts for the effect of non-locality of the electron energy distribution function. It has been shown that this effect can be accounted for by solving the Boltzmann equation for the EEDF in the spatially non-uniform case. The incorrectness of the notion of ambipolar diffusion in the plasma of a strongly electronegative gas with high negative ion concentrations has been reaffirmed.

The non-ideal continuity, momentum and energy equations describing plasma characteristics in confined high-pressure discharges in cylindrical ablative capillaries were solved numerically. The model equations used included (a) viscous and radiative and thermal conduction effects, (b) space dependent ionization and correction to Spitzer's electrical conductivity, and (c) non-ideal equation of state, as provided by SESAME library. For illustration, the case of polyethylene capillaries and discharge parameters appropriate for electrothermal launchers were considered. Analysis of the results indicated the consideration of the non-ideal equation of state to have significant consequences on the plasma characteristics. The sensitivity of these effects to variations in the discharge parameters was then studied and the results are also presented here. In the last part of the work the authors present the results of the implementation of a method aimed to determine self-consistent boundary conditions at the ends of the capillary as well as of the investigation of their effect on the solutions obtained.

The authors have extended the Green function approach for calculation of effective elastic constants of isotropic polycrystalline material to anisotropic textured polycrystals which have an axis of symmetry. The Green function for this symmetry has been calculated and effective elastic constants are evaluated for transversely isotropic aggregate with perturbation correction up to second order over uniform strain or stress field. The closeness of averagings of Voigt or Reuss type with experimental results and the relation between perturbation calculation and bounds are discussed. This method would find important application to anisotropic materials like short fibre composites and drawn polymers.

The DC resistivity and Seebeck coefficient have been measured as a function of temperature in the system Ca_1-xY_xTi_1-xCo_xO₃ for the samples with x<or=0.15 in the range 300-800 K. AC conductivity has been measured as a function of temperature and frequency. Results of these measurements show that electrical transport occurs mainly by a Debye-type loss process at temperatures less than 375 K and by excitation of charge carriers at localized states close to band edges and hopping among the localized sites.

A new class of organic conductors, mixed valence DIPS Phi ₄ polyiodides, have been used as conductive additives in reticulate doping of polymers. Conductive films have been prepared using three modifications of the reticulate doping technique: a standard one-step method (direct casting), a two-step method consisting of swelling the film containing molecularly dispersed DIPS Phi ₄ polyiodides, and a recently developed method in which oxidation of molecularly dispersed DIPS Phi ₄ and simultaneous crystallization of its polyiodides in situ is induced by treatment of the pristine film containing the donor with I₂/CH₂CI₂ vapours. The first and the third methods have been found to be the most advantageous. Conductive films with additive concentrations as low as 0.5 wt.% and conductivities of the order of 10^-3 S cm^-1 were obtained. In all three methods, the best results have been obtained when the conductive network consisted of the most conducting DIPS Phi ₄, 2.281 salt.

Langmuir-Blodgett films built of a mesomorphic side chain polymethacrylate are investigated from the point of view of their phase changes against temperature. With heating, a strong 'out of plane' molecular reorganization towards a smectic phase takes place giving rise to an increase of the order parameter.

Studies of short-circuit and open-circuit thermally stimulated discharge currents (TSDC) for electron-beam- and corona-charged polypropylene (PP) samples are reported. The experimental results indicate that open-circuit TSDC peaks at about 70 degrees C and 100 degrees C are caused by the detrapping of charges from surface and bulk traps respectively, while the origin of a peak at 150 degrees C is presently not clear. The short-circuit TSDC shows the existence of a positive-current peak between two negative peaks when the sample has been irradiated with a large dose. This current-reversal phenomenon verifies the fact that the charge layer in electron-beam-charged PP broadens towards the rear electrode when the injected charge density is increasing. A qualitative model to describe the charge dynamics of PP is suggested.

A critical appraisal of the peak shape method for the determination of activation energy in thermally stimulated luminescence (TSL) and thermally stimulated conductivity (TSC) has been made.

A method of drawing glass micropoint emitters having an internal conducting coat is described. The field emission characteristics of emitters with a silverdag internal coat show a switch-on effect to a high (1 to 10 mu A), stable saturation current. On lowering the voltage, the emission current follows a Fowler-Nordheim emission equation until a threshold voltage is attained. A model of the emission is presented in which a hot-electron emission process is in series with a large ohmic resistor. The performance of these emitters is superior to that of previously described glass-coated tungsten emitters.

CdS_xTe_1-x films (0<x<1) were deposited by simultaneous evaporation of CdS and CdTe powders from a vertical two-zone hot wall evaporation jig onto glass substrates. Electrical conductivity, thermoelectric and optical properties along with microstructural properties were studied in order to derive information on the electron transport processes in the films. Raman scattering was also studied for the films.

The effect of gas pressure and re-emission of sputtered species from the electrodes on the deposition rate of these species is considered theoretically. lt is shown that increase of gas pressure makes the deposition rate comparatively insensitive to re-emission of sputtered material from both electrodes. This provides the principal advantage for multi-component thin-film preparation.

Density measurements of PECVD hydrogenated silicon nitride films, Si_xN_yH_z, are reported. Two methods are investigated: the critical angle of reflection of X-rays and direct calculations with atomic composition deduced from infrared chemical bond analysis. Two kinds of sample are studied: as-deposited films with different atomic compositions and films with given composition post-annealed up to 1000 degrees C. On the first kind of sample both methods confirm the increase of density of the films when they are prepared in nitrogen-rich plasma. Moreover, from X-ray measurements, the authors deduce a maximum density of 3+or-0.1, close to the high-temperature CVD silicon nitride value. In the second kind of sample, no significant change is observed in the specific mass, and this agrees with stability properties of the low-hydrogen-content almost stoichiometric silicon nitride reported recently.

At a lithium liquid metal ion source two concentric emission bundles with sharp outlines are observed, which may indicate two different kinds of emitted ions and/or emission mechanisms. The angular intensity of the narrow bundle is the highest yet reported for liquid metal ion sources.