Table of contents

The ablative mechanisms associated with radiation produced by a free-running Er:YAG laser have been studied using pellethane as a target medium. Probe beam techniques have been used and provide evidence that each individual relaxation oscillation within the free-running pulse can lead to ablation. A simple model, which takes into account the presence of the ablation plume, has been applied to the data and fits well. A threshold fluence for ablation of was obtained with an absorption coefficient of . The studies may have relevance to medical applications of the laser.

A kind of nano-amorphous thin film was prepared by RF sputtering. The results of XRD and HREM analysis showed that the film was mainly composed of nano-amorphous particles sized about 10 nm on average. The electrical conductivity was measured using an AC impedance technique in the temperature range 283 - 373 K. It was found that the film was a lithium ion conductor with a conductivity of at room temperature and the ionic conductivity of the film varied with the temperature according to the Arrhénius equation from which an activation energy of was calculated. Compared with that of other conductors reported previously, the value is fairly low. A hypothesis based on ion migration through the highly disordered interface of nano-particles has been put forward to explain this anomalous result.

Secondary-ion mass spectrometry (SIMS) and x-ray photo-electron spectroscopy (XPS) studies have been performed on -irradiated (Bi-2212) superconductor. Results have been explained in terms of various phenomena such as changes in the oxidation states of the individual elements of the compound or radiation-induced defects, occurring due to -irradiation.

We demonstrate that by using thin films of metal oxides, such as vanadium oxide (VOx), molybdenum oxide (MoOx) and ruthenium oxide (RuOx), as a hole-injecting layer for an organic electroluminescent (EL) device consisting of N,N'-diphenyl-N,N-bis(3-methylphenyl1)1,1'-biphenyl-4,4' diamine (TPD) and tris-(8-quinolinolato) aluminium (Alq), the EL device performance can be significantly improved. The `operating voltage' of the device is reduced with respect to a device with a well known indium - tin-oxide (ITO) electrode for hole injection. We consider that the improvement of the operating voltage is attributable to the lower energy barrier for hole injection at the metal oxide/TPD interface.

Effects of hydrogen dilution on properties of Cu films deposited using the metal - organic plasma chemical vapour deposition method were examined. On increasing the dilution from 70 to 88%, the Cu concentration and grain size of the films increase from about 75 to 99% and from about 10 nm to above 100 nm respectively; also their resistivity decreases concomitantly to about . Films of high quality are obtained in the high dilution range above 80% , for which the emission intensity of H is high, suggesting that hydrogen atoms may play an important role in eliminating impurities from the films. The density of Cu atoms in the plasma measured by a light-absorption method is of the order of , which is lower by two orders of magnitude than the density necessary to explain the deposition rate of typical in our experiments.

In situ optical emission spectroscopy has been used to measure the direct current biased hot-filament diamond deposition processes for gas mixtures of methane, argon and hydrogen. The effects of bias on diamond growth and optical emission were investigated. The increase of bias voltage slightly raises the atomic hydrogen concentration which is estimated by using the Ar (750 nm) line as actinometry. The bias has little influence on hydrocarbon species and electron temperature. The enhancement of diamond nucleation and smoothing of the film surface by positive bias are mainly caused by electron bombardment on the surface of the substrate and growing film rather than on the change of gaseous environment.

This review deals with the definition and development of hard magnetic materials with special emphasis on more recent developments in this field. After discussing the fundamental properties of all hard magnetic materials, based on the intrinsic and extrinsic properties, some remarks on the history of hard magnetic materials and permanent magnets are given. The important hard ferrites are only briefly treated. Emphasis, however, is laid on the rare earth (RE) intermetallics and the hard magnetic materials based on them. Experimental techniques for the measurement of the basic properties of hard magnetic materials, e.g. the ordering temperature, the magnetization, the anisotropy field, the physical and technical hysteresis loop and the properties, derived from this loop are discussed.

Examples of modern permanent magnets, based on RE - cobalt alloys, on RE - Fe - B alloys and RE - Fe - nitrides, etc are explained. The principles of the production of sintered and bonded magnets are given. Finally some typical applications of these modern permanent magnets are discussed with respect to the different properties of the materials.

Future developments and the possibility of superconducting permanent magnets are briefly discussed. Concluding remarks are concerned with the possibilty and necessity of still larger energy products at elevated temperatures and the benefits of high-tech permanent magnets for modern and energy saving devices.

We develop a phenomenological, computation model based on a system of interacting spin-pairs to examine the thermodynamics of magnetization kinetics in permanent magnets. In particular, we examine the evolution of the system across the magnetic Gibbs free energy surface, at constant field and temperature (viscosity conditions), from different non-equilibrium starting configurations. Under viscosity conditions, the system will converge to either the (single) local free energy minimum, if it exists, or the global minimum. The Gibbs free energy will decrease monotonically along a convergence path, but the model shows clearly that there is no such constraint on the internal energy, the entropy or the magnetization.

Reptation describes the change in magnetization that occurs when a magnet is subjected to repetitive applications of a constant reverse field. Magnetic viscosity describes the change in magnetization at a constant reverse field due to thermal fluctuation. In highly disordered permanent magnet materials, such as remanence-enhanced , which are subjected to cyclic applications of a reverse field, the two effects are concurrent. We present experimental results which clearly demonstrate the coexistence of these two distinct effects in a permanent magnet material and we calculate and compare the magnitude of the reptation field and the viscosity parameter . is shown to be of the order of , highlighting the contribution to demagnetization processes made by reptation.

This paper studies the focusing characteristics of the relativistic e-beam in a radially polarized axisymmetric laser field. A root mean square (rms) envelope equation, which describes the evolution of the relativistic e-beam rms envelope has been derived. Together with analytical calculations and simulations of electron trajectories, the e-beam transport properties in the laser fields are studied and the possible application of this laser configuration as a laser-based lens is analysed.

A new kind of power analysis has recently been presented which is based on the maximization of the power density and predicts smaller and more efficient non-regenerative Joule - Brayton engines than those designed at maximum power. In this paper we apply the power density maximization method to regenerative gas turbines using a theoretical framework where the optimal operating conditions of the heat engine are expressed in terms of the isentropic efficiencies of the compressor and turbine and of the heat exchanger efficiency. It is shown that, unlike non-regenerative results, real regenerative gas turbines are less efficient at maximum power density conditions than at maximum power conditions.

The influence of the applied voltage pulse polarity on the performance of a HF laser pumped by a sliding surface discharge is investigated. Details are presented for the polarity dependence of the discharge properties as well as for their relation to the physical processes that affect the sliding discharge evolution. The output energy and efficiency were higher when the initiating or stressed electrode was in relative positive potential, irrespective of which electrode was grounded. The experimental results indicate that for positive polarity the discharge was more uniform, self terminating in type, with reduced electron density and temperature. The increase in output energy and efficiency is attributed to the improvement of the spatial uniformity of laser pumping, to a more efficient utilization of discharge input energy due to an increase in the steady state discharge time phase, as well as to the reduced electron temperature and to the increased steady state voltage which may also result in more efficient laser pumping.

The theoretical basis for the numerical simulation of stationary penetration welding with a laser beam is presented. The characteristics of the self-consistent model are the following. Vapour channel, weld pool and solid are considered as a nonlinear thermodynamic continuum. The laser-induced channel formation, multiple reflection of the laser beam in the channel, and plasma generation are taken into account. The absorption coefficient of the plasma is assumed as dependent on the degree of ionization. The geometry of the channel is determined on the basis of the pressure equilibrium at the channel surface taking the enthalpy of the molten mass into account. The input parameters for the simulation are the geometry of the workpiece, the material characteristic values dependent on temperature and the technological parameters such as laser beam power, beam diameter at focus, focus position, divergence angle of the beam, type of working gas and welding velocity. The results comprise the distribution of enthalpy and temperature, the shape and dimensions of the vapour channel of the weld pool and of the completed weld, and the energy losses by reflection, vaporization, thermal radiation and plasma shielding. The model was verified with welding experiments on steel and aluminium alloy. Data taken from the literature were also used for verification.

An irreversible combined cycle is used to analyse the performance of a two-stage combined heat engine system. The specific power output of the system is adopted as an objective function for optimization. The maximum specific power output and the corresponding efficiency are derived. The optimally combined conditions of two irreversible cycles are determined. Several special cases are discussed. The results obtained are compared with those of a single-stage irreversible heat engine. Moreover, it is expounded that the performance of a two-stage irreversible combined cycle may be described by an equivalent simplified cycle system and the optimal performance of an n-stage irreversible combined heat engine system is given. The aim of this research is to provide some new significant conclusions and redress some errors existing in a related investigation.

An irreversible three-heat-source heat pump may be treated as a combined cycle of a finite-size irreversible Carnot heat engine driving an irreversible Carnot heat pump. The influence of thermal resistance and working fluid internal dissipation on the optimal performance of a three-heat-source heat pump is investigated by using a finite-time thermodynamic approach. The results obtained here are more realistic than those of classical thermodynamics.

The AC and impulse breakdown voltages of (HT90) vapour - mist suspended in atmospheric air are measured for a sphere - sphere and a needle - plane gap. A significant enhancement of by the vapour - mist suspension is observed. The mechanism of the enhancement is discussed in terms of data of the mist droplet diameter distribution, electron impact ionization and attachment coefficient of the vapour, the measurements of which we also report. The enhancement by the vapour suspension can be explained by the fact that the vapour has a high limiting value at which ( is the gas pressure at ); . The effect of the mist on values is found to be most pronounced for the impulse voltage case.

A new theoretical model for the potential distribution in the surroundings of a cylindrical conductor placed inside a neutral plasma permitted us to analyse the effect of the positive ion thermal motion on the ion current collected by a cylindrical Langmuir probe. The new theoretical model includes the ABR (Allen - Boyd - Reynolds) theory as a limiting case, which is that of a negligible ion temperature compared to the electron temperature.

CCD camera images of the near-cathode region of an electric arc, working at atmospheric pressure in argon, permitted observation of some interesting phenomena that take place in this region, e.g. a `dark zone' in the cathode sheath region, low-frequency (f<0.1 Hz) fluctuations of discharge parameters and emitted light, the existence of several hot spots on the cathode surface, different kinds of movements of the arc spot on the cathode surface, and modification of the cathode surface structure.

Emission spectra of the 516.611 nm Swan band of the molecule were employed for evaluation of the rotational temperature in different plasma sources. This diagnostic method may be applied in the temperature range from 300 - 6000 K and is especially useful for noisy and spectrally not well-resolved spectra when the apparatus function of a recording system is unknown. The described method is based on a comparison of experimental data with the theoretically calculated spectrum. The numerical minimization procedure is started with the temperature value obtained from the Boltzmann plot or the intensity ratio of two selected spectrum components. An analysis of a theoretically calculated spectrum is presented. Two plasma sources were examined, a plasma torch recombining jet and high-voltage-triggered dielectric barrier discharge (DBD).

A two-dimensional fluid simulation of the plasma column present in a vacuum arc centrifuge after the anode grid has been developed. The simulation accounts for the effect of collisions on the plasma column, which is assumed to consist of multiply charged ions and electrons. It is found that the radial outwards plasma drift associated with collisions produces a spreading of the column density profile. With the plasma assumed to be in electrical contact with the anode grid, this in turn leads to a flow of current directed radially inwards, which increases the angular velocity of the plasma.

The 1.5D simulation is used to study the positive-streamer properties in air in a 20 cm sphere - plane gap versus gas temperature and density. It is shown that an increase in temperature up to 900 K at atmospheric pressure and a decrease in density by a factor of three at room temperature strongly affect the average electric field required for bridging the gap and the charge transferred through the streamer to the cathode; the temperature effect is much more pronounced than is the density effect. The calculated results qualitatively conform to available experimental data. The density effect is associated with a decrease in the rates of three-body processes such as three-body electron attachment to or conversion of ions into ions. The temperature effect is due to the decomposition of positive cluster ions which decreases the rate of dissociative electron - ion recombination. Electron attachment and detachment processes become important only when the electron density greatly decreases (by a factor of ten and more).

The radial temperature distribution is of decisive importance for the understanding of a high-pressure gas discharge. A new method is proposed to determine the radial temperature distribution in high-pressure mercury discharges by means of spectroscopic and electric measurements. An attempt is made to approximate the radial temperature profile upon the wall, apart from the radiating core, by considering a trial function. The coefficients of the function are calculated using the electrical conductivity profile of the arc.

This study into radiative transfer in a arc plasma is devoted first to measure the relative absorption of the arc radiation by cold gas; second, to calculate this absorption; and third, to obtain a simplified expression for this absorption useful in arc modelling. In general, more than 50% of the radiation emitted by a arc is absorbed in the surrounding gas.

A recently developed electro-optical Pockels effect technique, which allows the dynamic measurement of surface charge distributions, is employed to observe the dynamic behaviour of a partial discharge in a point - dielectric gap. One period of an 8 kV ac voltage is applied to a needle electrode with gap spacings ranging from 0 to 7 mm. Different discharge behaviour and different deposited surface charge distribution patterns are observed for different point - dielectric gap spacings. The electrode gap spacings can be roughly divided into three regions, namely the surface discharge region, the transition region and the space discharge region. In the transition region, both surface discharges and space discharges which consist of a spark discharge and corona discharge are observed. The memory effect associated with the accumulated surface charges plays an important role in controlling the discharge behaviour. The surface charge field component may either suppress or enhance the local field near the point electrode depending on the phase of the applied voltage. `Back-discharges' are the typical result of the memory effect. Negative surface charge deposited from previous discharges can have a considerable influence on the propagation of the subsequent positive surface streamer channels depending on whether they develop in the surface discharge region or in the transition region.

It is proposed to use a great body of vacuum arc plasma parameters measured in combination with a theoretical model of a current-carrying plasma jet for the determination of cathodic microjet parameters. The most probable values of microspot diameter and microjet current thus obtained lie in the range and I = 1 - 5 A, respectively. It is also shown that the mean ion charge variations observed can be essentially explained by a periodical variation of the ionization energy set with the atomic number of the cathode material.

The aim of this study was to demonstrate the correlation between the light emitted by electric arc and its different parameters such as the current, voltage and power. The correlation is determined by two method of analysis: temporal and statistical correlation.

A theoretical model of a low-current electric arc at the anode is presented. This method is based on the numerical resolution of a closed system of equations which makes it possible to determine the magnitude of a few anode parameters. The input parameters required are the arc current and physical constants of the anode material. We restrict ourselves to three anode parameters: power lost by conduction, spot radius and current density. The results from our calculations are compared with experimental measurements based on a calorimetric method for an arc burning in argon and arc current between 2 - 6 A. A comparison of our results and those reported in the literature is also made.

We have investigated the partial discharge (PD) due to electrical treeing degradation in low-density polyethylene (LDPE), ethylene - vinyl acetate copolymer (EVA) and ethylene - acrylic acid copolymer (EAA) by a computer-aided partial discharge measurement system which allowed us to obtain phase-resolved PD pulse data. The experimental results revealed that the PD magnitude was strongly affected by the instantaneous applied voltage and that the occurrence of a PD was determined by the time derivative of the applied voltage (). The PD pulse-sequence analysis revealed the following: (i) a PD occurs in a discharge path which consists of a tree trunk and branches extending from the trunk; (ii) in each discharge path at most one PD occurs per half cycle. Based on these facts, a model of PDs due to electrical treeing was proposed. The influences of applied voltage and frequency were investigated by applying a triangular voltage. The number and average magnitude of PDs increased linearly with applied voltage whereas the PD charge per cycle increased quadratically. These results are in good agreement with the model.

Diamond nanoparticles synthesized by the detonation of explosives were used as an additive in paraffin oil. The tribological properties of the two-phase lubricant of paraffin oil and diamond nanoparticles were investigated. The results show that, under boundary lubricating conditions, this kind of two-phase lubricant possesses excellent load-carrying capacity, anti-wear and friction-reduction properties. The ball-bearing effect of diamond nanoparticles existed between the rubbing faces, the surface polishing and the increase in surface hardness effects of the diamond nanoparticles are the main reasons for the reduction in wear and friction.

The variation in the ultrasonic attenuation versus time t is measured during the action of tensive and compressive stress both for cold-worked and for annealed polycrystalline 99.999 wt% Al at room temperature. The results are analysed in the light of the theory of the interaction between dislocation and point defects. decreases when the dislocation moves into the atmosphere of point defects, whereas increases when the dislocation moves away from the atmosphere of point defects.