Table of contents

The improved finite-element flux-corrected transport method (FE-FCT), developed by the authors, has been applied in its full two-dimensional form to the numerical solution of streamer development and propagation in air at atmospheric pressure. The numerical algorithm used employs the FE-FCT method for the solution of transport equations of charged species under the action of space-charge electric field, with the field obtained from the solution of Poisson's equation in cylindrical co-ordinates.

Results of two-dimensional simulations of cylindrically symmetric streamers in air between parallel plate electrodes are presented. The breakdown voltages predicted by the model for different gap lengths are in agreement with the experimental measurements, which demonstrates the validity of the model. Furthermore, the avalanche-to-streamer transition - as described in Raether H 1964 Electron avalanches and breakdown in gases(London: Butterworths) - and the formation of anode and cathode directed streamers are numerically observed.

A nonlinear stress-strain behaviour at room temperature was observed in the ferroelectric Pb(Zr_0.58 Fe_0.20 Nb_0.2 Ti_0.02 )_0.994 U_0.006 O₃(PZFNTU) ceramic beams in the three-point bending test. This nonlinear phenomenon is attributed to the ferroelectric domain reorientation during bending. A stress relaxation behaviour which obeys perfectly a logarithmic time law has also been found. This relation is similar to the ageing of the dielectric constant. A Weibull analysis showed that the batch of the sintered PZT ceramic samples tested had a high Weibull modulus and a high flexural strength.

Aluminium thin films were deposited by a laser ablation technique from solid cylindrical targets rotated on a lead screw. Both smooth-surfaced targets and targets with screw threads cut into the area to be irradiated were used. The targets were irradiated with a focused circular spot size of 2.5 mm in diameter, at a power density equal to 5.4±0.2 × 10⁸W cm^-2 . The polar distribution of the ablated atomic material was found to vary as a function of the target screw thread pitch, with the exponent n , in the polar distribution f ( ) = cosⁿ , varying from 13.5±1.3 for no pitch to a minimum of 5±0.7 for a screw thread pitch of 0.2 mm. The use of such novel target geometries forms a possible basis for increasing thickness uniformity during the pulsed laser deposition of thin films.

Epilayers of nitrogen doped ZnSe have been grown on GaAs(100) substrates by pulsed laser ablation deposition assisted by a nitrogen beam produced by a supersonic nozzle beam source. X-ray photoelectron spectroscopy (XPS) shows that the epilayers are composed of 83.8% Zn and Se, 10.4% N and 5.8% O, and other kinds of impurity are rare. XPS spectra of Zn(2p_3/2 ) and N(1s) core levels indicate that doped nitrogen with an estimated concentration of over 10²¹cm^-3exists in the as-grown epilayer. Atomic force microscopy indicates that the surface of an as-grown ZnSe epilayer on GaAs(100) is flat with roughness of about 1.2 nm, which is less than that of some good-quality metalorganic vapour phase epitaxy grown layers. X-ray diffraction results show that these ZnSe epilayers are a characteristic of single crystallinity.

Fe/Hf multilayers were prepared by alternate deposition of pure hafnium and iron. As-deposited samples were then annealed at 380 °C and 550 °C, respectively, to investigate the microstructure and magnetic property changes of the films. It was found that all as-deposited Fe/Hf films exhibited the soft ferromagnetic property where the coercive force was lower than 30 Oe. The coercive force and the magnetic moment per Fe atom changed significantly with the annealing temperature and the Fe layer thickness (t_Fe ). For t_Fe= 2.2-4.2 nm films, their soft ferromagnetic properties were stable upon thermal annealing at 380 and 550 °C, while for t_Fe 9.5 nm and t_Hf= 13.2 nm films, after annealing at 550 °C for 20 min, as-deposited films transformed from having a soft ferromagnetic property to having a hard ferromagnetic one. The residual magnetization and maximum magnetic energy product of the films could come up to about 0.81 µ_Band 8.8 kJ m^-3(1.1 MGOe), for the Fe(9.5 nm)/Hf(13.2 nm) films annealed at 550 °C. It was also found that there were two metastable phases with fcc structure in the annealed Fe-Hf films. The magnetic change mechanism was discussed in terms of interdiffusion in the Fe/Hf interface and phase transformation in the films upon thermal annealing.

A new approach, the multipole theory (MT) method, is presented for the computation of two-dimensional (2D) unbounded eddy current problems. The essential concept is to represent the solution of the governing partial differential equation by the generalized MT formulae of the 2D Helmholtz equation and the 2D Laplace equation. The least squares method reduces the problem to the solution of a set of linear equations. Ampere's law, as an additional constraint, is used to guarantee that the total net current is not changed due to the induced fields, and the radiation boundary conditions are satisfied. The eddy current problems of elliptic conductor, square conductor and a pair of circular conductors are considered as examples. The results obtained by the MT method are compared with experimental results and previously published work. It is shown that the MT method is an effective approach for computation of 2D unbounded eddy current problems.

Mathematical formulization of the thermal response in a two-layer photoacoustic cell, where the interference of thermal waves in a sample is compounded by the inter-layer interference effect, has been carried out by employing the thermal-wave interference theory. The physical meanings of the obtained formulae are then discussed in terms of the thermal-wave interference to explain the origin of the thermal phase lag for various cases. In addition, it is found that the inter-layer thermal-wave interference results in an extra phase lag.

In this paper we employ second-order perturbation and the technique of nonlinear reflection of acoustic waves at an interface to analyse the physical process of cumulative second-harmonic generation of generalized Lamb-wave (GLW) propagation in a solid waveguide consisting of a solid plate and a solid half-space. As in the case of second-harmonic generation of Lamb-wave propagation in a solid plate, in general, cumulative second-harmonic generation of GLW propagation does not occur. However, the present paper shows that, under certain conditions, the GLW second harmonic arising from the nonlinear interaction of the partial bulk acoustic waves and the restriction of the two boundaries of the solid waveguide does retain a cumulative growth effect. Through a second-order boundary condition, the existence condition of second-harmonic generation of GLW propagation has been determined, and through the initial condition of excitation the analytical solution of the cumulative second harmonic of GLW propagation formally obtained. Numerical results show the cumulative effect of GLW second-harmonic field patterns. The technique of analysis in this paper yields a physical insight into the process of cumulative second-harmonic generation of GLW propagation not previously available.

A mathematical expression for the electric force acting on a sphere placed in a non-uniform electrostatic field is derived taking into account the conductivity and permittivity of both the sphere and the medium, as well as the initial velocity and charge of the spherical particle. The expression demonstrates a time-dependent variation of the force in a non-uniform field. New formulae are given for calculating the particle charge and the electric field strength inside and outside the sphere. An example is given of particle motion in a field with constant gradient in a direction normal to the field vector. It is demonstrated that oscillatory motion of the sphere in a non-uniform field is possible at certain sphere parameters and field strength gradients as a result of instability of the equilibrium state of the system.

The composition variations occurring in decaying SF₆arc plasmas in the presence of atoms released from the vaporization of organic insulators (e.g. Teflon, polyethylene, polypropylene, megelit, nylon), copper, oxygen and water were studied between 12 000 K and 300 K by means of a chemical kinetics model. From the results obtained at 300 K and a pressure of 101.3 kPa: (i) the role of the impurities on the formation of the SF₆decomposition products: SF₄ , SOF₂ , SO₂ F₂and S₂ F₁₀ , was determined; (ii) it was confirmed that the vaporization of an organic insulator leads to the appearance of CF₄and an increase in the generation of the major byproduct (SF₄ +SOF₂ ) which is correlated to the production of CF₄ ; (iii) it was seen that, for a given amount of vaporized insulator, insulators that contain fluorine atoms brought about less SF₆decomposition than those that did not.

A free-flowing plasma in a railgun refers to a plasma which is not impeded by a projectile during a firing. One advantage in performing experiments with such plasmas is that spectroscopic measurements can be made when they are ejected. In this paper we analyse absorption and emission spectra of a free-flowing plasma for wavelengths between 300 and 625 nm in several firings. Calculations of the degree of ionization for the various species identified on the spectra are used to produce an estimate for the temperature of a free-flowing plasma in a RAPID railgun that lies between 11 × 10³and 25 × 10³K. This temperature range is reduced to 11 × 10³K by using a special computer code that predicts the thermochemical functions and transport coefficients of partially- and fully-ionized plasmas. The code is then used to develop temperature estimates of the plasma armature in railgun firings with projectiles. For these plasmas, which are expected to be denser than free-flowing plasmas, a temperature estimate of 14 × 10³K is obtained for a RAPID railgun firing at shot-out.

The kinetics of argon dimer (Ar₂ *) are studied in detail by model calculations. The model is applicable to discharge as well as e-beam excitation of high-pressure argon gas. The time-transient behaviour of the temperature and the vibration-translational relaxation of the excited state dimer have been considered in the model calculations. The model validity is verified by comparing the model predictions with discharge and e-beam experiments. Excellent agreement has been reached for the e-beam experiments. The predicted discharge voltage and current waveforms are in good agreement with the experiments of Ninomiya and Nakamura. The predicted net gain of 0.002 cm^-1at 3.5 atm is about three times smaller.

A recently developed analytic formula for radiation trapping in cylindrical geometry with partial frequency redistribution and with a spectral lineshape dominated by almost any combination of natural (radiative) broadening, resonance collisional broadening, and Doppler broadening is generalized to include foreign or buffer gas broadening. The effect of foreign gas broadening is incorporated by rescaling the radius and absorbing gas density. In some cases rescaling results in an unphysically small radius. The analytic formula reported here, which is accurate to zero radius, is an improved version of the earlier formula. The accuracy of the improved formula is verified with 198 new simulations, in addition to the original 294 simulations, all using a highly realistic Monte Carlo code.

A computer simulation of a thermal plasma that utilizes a detailed line-by-line radiative analysis coupled to a flow and temperature-field analysis has been written. The radiative transport portion of the model uses the S-N discrete ordinates method and the required spectral radiative properties are calculated from basic atomic data. The flow and temperature-field portion of the calculation is handled using the finite-volume method. The thermal plasma configuration studied using this coupled model is a two-dimensional, axisymmetric, 200 A, 1 cm long, free-burning arc located in a one-atmosphere argon environment. Results from this analysis are compared with those from an uncoupled, line-by-line radiative analysis of this same arc and from a flow and temperature-field analysis that utilizes net emission coefficients to model the radiative source term. As thought, noticeable differences are found between results from the coupled and uncoupled analyses. This work proves that it is feasible to conduct two-dimensional, coupled, line-by-line calculations in a thermal plasma, but the computational times are large.

The Fast Fourier Transform method has been applied to light and voltage fluctuations in a low power (<5 kW) dc plasma torch burning in argon. The influence of the gas flow on the development of turbulence in the plasma jet has been studied. The temporal and spatial non-homogeneities observed in the plasma flow have been linked to the dynamic processes in a dc plasma torch.

The fluctuations generated by arc root motion have been used to determine the velocity in a plasma jet. The phase angle relations resulting from time variations of light intensity profiles taken simultaneously across different sections of the plasma have been used to determine the velocity for different operating conditions of the dc torch. The experimental results have been compared with the theoretical predictions of a 2D MHD model of the dc plasma torch. The experimental and theoretical values appear to agree with a maximum deviation in the 5-10% range.

Thermally stimulated depolarization current (TSDC) curves were obtained by modelling global and thermal sampling experiments on polar systems, considering that the observed relaxation processes are formally interpreted in terms of a distribution of uncoupled activated Debye-like processes with an Arrhenius temperature dependence. Relaxational processes occurring in the solid glassy state and near the glass transition in some polymeric systems were analysed according to this model. Some previously reported experimental results were compared with the simulated data. Three scenarios with different types of characteristic distribution of time were investigated: (i) the distribution arises from a distribution of activation energies, (ii) the distribution arises from a distribution of pre-exponential factors or (iii) the system follows a compensation behaviour, in which a linear relationship between the two thermokinetic parameters is observed.

Technological applications of semiconductors depend critically on the ability to dope them. Single crystals of CuGaSe₂were doped during crystal growth either by a post-growth diffusion step or by ion-implantation, in order to study the limits of extrinsic doping. The electrical and optical properties of the doped samples are analysed by Hall effect and photoluminescence (PL) measurements. The carrier concentration at room temperature can be adjusted between 2 × 10¹⁹cm^-3(p-type) and 10¹⁷cm^-3(n-type). Various donor and acceptor levels are identified and ascribed to dopant-induced point defects taking into account the dopant concentration and/or the post-growth treatment of the single crystals.

The atom superposition and electron delocalization molecular orbital (ASED-MO) semiempirical method is used to analyse the atomic hydrogen-Fe interaction. The face centred cubic (fcc) Fe model contains a stacking fault and as a comparison the H-fcc Fe (normal) system is also studied. The solid is represented by a cluster of 180 metallic atoms distributed in five layers. The interstitial atoms localized in different geometric positions found an energetic minimum in a zone close to octahedral interstitial holes in the stacking fault. The electronic structure shows that the H-Fe bond involves mainly the Fe 4s and 4p orbitals and the 1s H orbital. The Fe-Fe bond near H is destabilized, to approximately 27% of its original value.

Thermally stimulated current measurements in the temperature range 10-20 K have been performed on a set of silicon p⁺ /n/n⁺junction detectors. The samples were irradiated with fluences from 6.4 × 10¹¹cm^-2up to 1.1 × 10¹⁴cm^-2(1 MeV neutron equivalent) in order to investigate the changes in the shallow level concentrations induced by irradiation. The measurements showed a broadening of the peak shape and a decrease in the height of the spectral components as the fluence increases. A phosphorus removal of approximately one order of magnitude is observed for the most irradiated sample, while the boron component is almost independent of the fluence. At the highest level of irradiation the phosphorus and boron levels show comparable concentrations.

On the basis of the Ashcroft and Sturm theory on the optical properties of metals, a theoretical model is proposed for the prediction of frequency- and temperature-dependent interaction properties of pure metals subjected to laser irradiation in laser processing applications. This model is directly suitable for on-line coupling to numerical process modelling without any recursion to external experimental data fitting. The model results show a general good agreement both with other theoretical and experimental results for the case of aluminium, and are considered to represent an important step forward in view of the practical development of laser processing applications, for which a lack of reliable laser-matter interaction constants is noticeable.

Computed tomography is a useful tool for gas and plasma diagnostics. Unfortunately, viewing access in many tomographic experiments, such as in plasma physics, is strongly limited, which leads to a highly underdetermined inversion problem. A promising approach to this problem is the maximum entropy (ME) method. An application of the ME concept to the problem of two-view tomography is considered in this paper. It is shown that if additional prior information of the source is available it can be effectively incorporated into the maximum entropy algorithm. The crucial role of prior information is illustrated by several numerical examples.