Table of contents

A practical review of the use of laser ablation for the deposition of thin films of oxide superconductors and metals is presented. Details of the design and operation of a laser ablation deposition system are discussed, together with the influence of operating parameters such as laser wavelength and power density on the film growth, with the aim of assisting the researcher beginning to use and/or study laser ablation deposition.

In this paper we present results on the dependence of the longitudinal sound velocity of amorphous magnetostrictive ribbons and wires, tested either in the as-cast stage or after stress relief on the magnetic field along the anisotropy axis. Results show a variation of the sound velocity that is in good agreement with results concerning the triangle E/E effect. Results are also given concerning the change in sound velocity for magnetostrictive ribbons and wires before and after annealing. These results can be used to decide the kind of treatment and biasing field of such materials when they are to be used as magnetostrictive delay lines.

The magnetic viscosity, and the reversible and irreversible susceptibilities of a sample of anisotropic sintered Nd-Fe-B have been measured starting from various initial magnetized states. It is shown that the heights of energy barriers preventing the reversal of the magnetization in the grains depend on the initial magnetic state of the material. The change in energy barrier distribution is shown to be accompanied by a change in the nature of the magnetization reversal process, which cannot be described solely in terms of simple model pinning mechanisms.

The static image theory of Kelvin for a conducting sphere and that of Neumann for a material sphere, rediscovered recently, is augmented with a theory for a sphere with impedance boundary conditions. The image of a point source is seen to consist of a point source and line source consisting of two components. The theory is tested by showing that the image in a planar impedance surface is obtained as a special case.

Many physical processes can be described in terms of nonlinear dynamics, i.e. chaos. It is thus of interest to the experimentalist to discover whether an observed time series can be modelled as a chaotic process. Techniques for recognizing chaos exist which rely on measuring the fractal dimension of the underlying strange attractor but may be disrupted by the presence of noise on the experimental data. This paper firstly examines claims that the fluctuating radar cross section of the sea surface can be recognized as a low-dimensional chaotic process and then goes on to consider whether identification of such a process is possible in the presence of additive or multiplicative noise. The relevance of these results is discussed with regard to the problem of recognizing chaos in high-resolution radar images which are corrupted by multiplicative speckle noise.

An integrated mathematical model for laser welding of thin metal sheets under a variety of laser material processing conditions has been developed and tested against the results of experiments. Full account is taken in the model of the interaction of the laser-generated keyhole with the weld pool. Results calculated from the model are found to agree well with experiment for appropriate values of the keyhole radius. The analysis yields values for power absorption in the metal. In a complementary calculation the total absorption of the laser energy is determined from detailed consideration of the inverse Bremsstrahlung absorption in the plasma and Fresnel absorption at the keyhole walls. To test these results, experiments were performed on 1 mm mild steel using a high-speed video camera, which measured the surface dimensions of the melt pool. Processing parameters were varied to study the effect on the melt pool; parameters considered included traverse speed, laser power and shroud gas species. The general shape of the weld pool was found to depend on whether penetration was full, partial or blind; only the results for full penetration were compared with the theory, which is for complete penetration only.

Fine tuning of narrow-bandwidth interference filters based on volume holographic gratings fixed in photorefractive LiNbO₃ has been demonstrated. A filter with a peak wavelength at 518.45 nm, peak reflectivity 32.5% and bandwidth of 50 pm was used. Filter tuning has been achieved both by varying the filter temperature and by applying an electric field. By varying the filter temperature between 35 and 60 degrees C a temperature tuning coefficient of 5.1 pm K^-1 was obtained. The effect is induced by the temperature-dependent change of refractive index and by the lattice thermal expansion. By applying a direct current electric field, an electrical tuning coefficient of 5.9 pm kV^-1 has been measured too. In this case the tuning mechanisms are the electro-optic and piezoelectric effects. On the other hand, with an applied alternating current electric field, a beam intensity modulation from 90% to 20% has been demonstrated.

Formative time lags of electrical breakdown in a torus have been measured at 0.01-2.0 Torr in hydrogen, helium, nitrogen and air. The torus is a toroidal Pyrex discharge tube of 6.5 cm major radius and 1.5 cm minor radius. Single-turn voltage is applied to the torus by electromagnetic induction in the range 3200-5120 V. The results show that the formative time lag in hydrogen is smallest among the gases tested. Consideration has been given to effects of photo-ionization as a fast breakdown mechanism in hydrogen.

Analytical results are presented concerning heat transfer to a metallic spherical particle immersed in a plasma flow for the extreme case of free-molecule regime and thin plasma sheath. The results show that as the surface temperature of the heated particle is comparatively high, additional heat transfer mechanisms due to radiation heat loss and thermionic emission from the particle surface should be all taken into account in the calculation of heat transfer to the particle. Radiation heat loss becomes appreciable at wall temperatures lower than those at which significant thermionic emission occurs. As the plasma temperature is comparatively low, radiation heat loss even restricts the surface temperature of the particle to such a low value that the effect of thermionic emission on heat transfer can be ignored. The heat flux to the particle increases with increasing gas pressure or speed ratio, and decreases with increasing electron/heavy-particle temperature ratio for a fixed electron temperature of two-temperature plasmas.

A visualization and a quantification technique for a latent electric charge image on a dielectric material has been developed by using the Pockels effect and a computer imaging technique. An optical phase modulation technique and a computer image lock-in amplifier technique were used to improve the signal-to-noise ratio. The measurement system has several improved performances; sensitivity of surface change of 1 nC cm^-2, spatial resolution of 200 mu m and discrimination of charge polarity. The basic principle of advanced electro-optic surface charge measurement is described and the typical surface charge distribution of positive discharge streamer produced by a high impulse voltage (5 kV_peak) is demonstrated. Also, comparison is made between the modulation method and the non-modulation method.

A PIC/MC simulation is performed on a cylindrical low-pressure gas discharge between two coaxial electrodes in an axial magnetic field. This case study employs an asymmetry of 1:7 between inner and outer electrodes. It is based on natural mass argon gas with experimentally given cross sections for elastic, excitation and ionization electron collisions, as well as elastic and charge exchange ion collisions. The simulation shows that ionization occurs mostly at the inner sheath edge. Negative space charge regions build up in front of the momentary anode sheath. These lower the plasma potential with respect to the electrode and induce electron leakage currents to the anode. The ionization degree is 3*10^-4, and 10% of the dissipated power goes into ionization in this particular case. The improvement of these features for the design of an ion source is discussed.

Ultraviolet absorption spectroscopy using a ring dye laser and a hollow cathode lamp was applied to measurement of Si(3p², ¹D₂) and Si(3p², ³P₂) atom density decay in the afterglow of a radio frequency SiH₄-Ar plasma. The dependence of the Si density decay rate on SiH₄ and Ar partial pressure and also on the radio frequency input power was investigated, from which Si(3p²,¹D₂)-SiH₄ reaction rate constant was determined to be (7.4+or-0.4)*10^-10 cm³ molecule^-1 s^-1, the Si(3p², ³P₂)-SiH₄ reaction rate constant (3.5+or-1.0)*10^-10 cm³ molecule^-1 s^-1 and the diffusion coefficient for Si(3p², ¹D₂) in Ar (at 320 K) (4.0+or-0.8)*10⁴ cm² Pa s^-1.

Ar-N₂ mixture positive columns of direct current glow discharges are experimentally and theoretically investigated. The electron energy distribution function of the column, which has a great deviation from the Maxwellian, is affected by the amount of N₂. The N₂ and Ar metastable species densities estimated from the rate equations are inversely proportional to N₂ content. There is a large discrepancy between experimental and theoretical electron energy distribution functions, especially for N₂ positive columns. The value of pR obtained experimentally is predicted from the measured electron energy distribution function, where p is the gas pressure and R the tube radius.

Ion implantation of copper, gallium and silver ions into quartz substrates has been performed. The implantation parameters were identical for each series of implants: 55 keV ion energy and an implanted dose of 6.4*10¹⁶ ions cm^-2. However, the temperature of the substrate was varied with implants being done at room temperature, 373 K, 498 K, 623 K and 873 K. Rutherford backscattering was used to provide depth profiles for the implanted ions and the substrate elements; the ion profiles were compared with simulations from the TRIM code. The ion profiles were significantly affected by temperatures in all three cases: in particular, bimodal distributions formed at 498 K for the copper and gallium implants and at 623 K for the silver. The ions amorphized the substrate to depths commensurate with the tail of the implant except in the 873 K silver implant. In this case none of the silver was retained and the substrate completely recrystallized. Evidence of recrystallization was also evident in the gallium 873 K implants, which had a smaller amorphization depth than the lower temperature implants.

A method known in the general theory of nonlinear waves is extended to problems dealing with shock-loaded materials obeying elastic-plastic deformation kinetics. As a result a set of approximate independent nonlinear equations belonging to different families of longitudinal characteristics is obtained. It is shown that the interaction of these waves may be described implicitly by a heterogeneous shift of the phase variable in the solution constructed without regard for the interaction. The self-action structure of an incident wave reaching a free surface of a metal plate has been considered on the basis of the equations obtained. This effect is important for the correct interpretation of experimental data.

Hydrogen permeation through a martensitic stainless steel, coated with amorphous (a)-SiC film has been studied in the temperature range 480-550 K for an inlet pressure of 2*10⁵ Pa. It is found that: (i) a-SiC thin coating acts as an efficient barrier to the H₂ permeation flux; (ii) chemical reactions occur between migrating hydrogen and free C atoms in the a-SiC layers with the formation of hydrocarbon compounds; and (iii) N⁺ implantation of the a-SiC improves the barrier performances in terms of both hydrogen permeation and reduction in release of hydrocarbons.

In this paper, the relationships between phenomenological and thermodynamic models used for characterization of insulating materials aged under multiple thermal-electrical stresses are derived. Phenomenological models fitting life lines for insulating materials that show or do not show a tendency to threshold are considered and compared with appropriate thermodynamic models. The thermodynamic models account for the electrical field action with an activation-energy-lowering factor that differs for threshold and non-threshold materials. The dependence of this factor on applied stresses is thoroughly explained. The models are applied to results of life tests performed on XLPE cables. The comparison shows very good agreement of test data with the threshold model when a suitable temperature-dependence of model parameters is assumed.

The effects of interface roughness on the resonant tunnelling hot electron transistor current are studied by calculating an average effective transmission coefficient for the quantum well structure. Our model for the transmission coefficient includes both quantum interference and bulk scattering. The interface roughness is represented by a rectangular distribution having the width of one atomic layer. The results indicate that the interface roughness will lower and broaden the resonant peak. The emitter peak tunnelling current, base current and collector current will be reduced by 50% at room temperature. The effect of interface roughness on the gain is relatively weak.

Beam instability in a metal-insulator-semiconductor microstructure with periodical field electrodes is considered. The instability appears as a consequence of the inversion of electron distribution function in gallium arsenide in the strong electric field. Space charge wave amplification in the two-dimensional electron gas of a GaAs(AlGa)As heterostructure is also considered. The amplification of a space charge wave in such microstructures seems to be a basis for new microwave amplifiers.

Electron movement in a spatially periodic high-frequency electric field is considered. If the spatial period is small enough and the frequency is sufficiently high, the effect of this field on the charged particle is in a sense equivalent to the effect of a time-independent effective potential that is a sequence of deep 'dynamic' quantum wells in which the electrons are confined. Also a physical model is proposed, which could be described in terms of such a Hamiltonian.

The potentialities of the material combinations InP/GaInAs and GaAs/GaInAs for single-drift-region heterojunction impact avalanche transit time diodes are investigated by computer simulation. The results indicate better device performance for the GaAs/GaInAs diodes than for the corresponding InP/GaInAs diodes. The reason for this is observed to be the large phase distortion caused by tunnelling of charge carriers in the InP/GaInAs diodes. The large phase distortion disturbs the phase relation between the radiofrequency voltage and current considerably for normal impact avalanche transit time mode operation, causing performance deterioration in the InP/GaInAs single-drift-region diodes.

The influence of monomer concentration and electrode conductivity on the morphology of electrochemically deposited poly (o-anisidine) films has been investigated using scanning electron microscopy. The films are found to exhibit a fibrillar morphology, except on lower conductivity electrodes and at lower concentrations of the monomer. The monomer concentration predominantly governs the rate of reaction and deposition of films; the effect of which is observed in terms of various types of patterning of fibres, namely anastomistic, reticulate, stillate, flabellate and so on. The number density of fibres is seen to be affected by the electrode conductivity.

The effects of ultraviolet light on alpha -Al₂O₃:C crystals were studied. Thermoluminescence, X-ray-induced luminescence and photostimulated thermoluminescence were investigated. Emission as well as stimulation spectra were measured. The main thermoluminescence and photostimulated thermoluminescence peaks appeared in both types of crystals at the same temperature of 180 degrees C and with the same thermal activation energy of 1.12 eV. The 410 nm band was dominant in C-doped crystals, while the 325 nm band was the main one in pure samples. Al₂O₃:C crystals which had previously been exposed to ionizing radiation and heated only to 500 degrees C, were found to be sensitive to extremely low doses of monochromatic near-ultraviolet light. Possible applications of photostimulated thermoluminescence in these crystals for ultraviolet dosimetry are discussed.

Preheating is the simplest energetically justifiable approach taken to ensure that the charges yielding luminescence have the same activation energies in natural and irradiated aliquots. Luminescence depletion due to preheating of Holocene and Quaternary sediments with natural and artificial doses was examined and found to behave as expected. Preheating was found to have the following undesirable effects. In aliquots with very high doses, preheating lowered the onset of saturation by 20 to 40%. This effect is thought to be due to thermal quenching of recombination centres. In optically zeroed quartz and feldspar aliquots, preheating caused the luminescence signal to increase. In aliquots with geologically very low adsorbed doses, preheating to 230 degrees C caused dose response growth curves to become nonlinear. This effect can be explained in terms of second-order kinetic behaviour. A procedure is suggested which corrects for the undesirable thermal effects in young samples. When verified against thermoluminescence measurements, this procedure yielded correct optical dose estimates for sediments <or=12000 years in age.

A model for the heat transfer between a heating element and a thermoluminescent sample in the case of contact heating is used to estimate the temperature lag of the sample with respect to the heating element during linear heating. According to the model the temperature lag is linearly dependent on the heating rate beta and the temperature difference between the heating element T₁ and the surroundings T_g. For a LiF:Mg, Ti(TLD-100) sample with dimensions 3.2*3.2*0.9 mm³ the temperature lag Delta T during linear heating can be described by Delta T=1.7 beta +0.037 (T₁-T_g). Compared to Delta T the temperature gradient across the TLD-100 sample can be neglected. The consequences of neglecting phenomena such as temperature lag and gradient, which always occur to some extent in TL measurements, on trapping parameters, (activation energy and frequency factor) derived by glow curve analysis are discussed.

Cathodoluminescence (CL) spectra have been recorded from float glass samples and the data used to determine the depth profile of tin introduced from the float process. The luminescence data were interpreted in terms of intrinsic emission bands at 380 and 410 nm and well resolved extrinsic emission bands resulting from the tin impurities at 575 and 630 nm. The presence of the tin produces an optical waveguide in the glass surface as a result of the float production process. It was therefore possible to depth profile the tin by measurement of the waveguide modes. Overall, the depth of tin diffusion was up to 8-9 mu m in the samples studied. The lifetimes and intensity of the signals are temperature dependent. Thermal treatments modify the CL spectra and such changes are considered the result of valence of aggregation states of the tin impurities.

The ultraviolet induced thermoluminescence in sintered ZrO₂ doped by Er₂O₃ and the origins of the luminescence centre and trapping centre of sintered ZrO₂ have been studied. The intensity of the 85 degrees C or the 90 degrees C thermoluminescence peak for ZrO₂ doped with Er₂O₃ was 1.7 times higher than an undoped sample. We assumed that the electron trapping centre for the 90 degrees C thermoluminescence peak of ZrO₂ is formed from a Zr ion and an oxygen vacancy. For the 210 degrees C thermoluminescence peak, we assumed that the electron trapping centre is formed from a Zr ion and a defect complex which is formed from an oxygen vacancy and a Zr ion with an electron attached to it. The origin of the luminescence centre is suggested to be associated with the oxygen vacancy and the Ti ion in an asymmetrically sevenfold coordination of ZrO₂ in the monoclinic phase. To estimate the trap depths we have proposed a HHe²⁺ like model which describes the trap depth as an electron trapped by an effective positive electronic charge localized on an oxygen site and perturbed by two effective positive electronic charges localized on a cation site.

Backscattered electrons are more efficient at producing secondary rather than primary electrons. Using a thin layer of oxidized aluminium on five different substrates (C, Al, Cu, In and Au), we have measured how the efficiency ratio ( beta ) varies as a function of primary electron energy, thereby characterizing quantitatively this well-known effect. We also carried out Monte Carlo simulations to calculate beta for thin Al films on Be, Cu and Au substrates. Interestingly, these metallic films behave in a similar manner to the insulating films investigated experimentally.

Fast sweep experiments on transient absorption and emission that allow for broad band observation of microwave rotational transitions are described. A theoretical and experimental investigation of the phenomenon has been carried out. The performance of a digital receiver unit used for signal processing is demonstrated by experiments showing resolution and sensitivity for the single sweep mode.

It has been predicted that the floating potential of particles in plasma may become positive when the particle surface temperature is high enough, but, to our knowledge, no positive floating potential has been obtained yet. In the present paper the floating potential theory of high-temperature particles in plasma is developed to cover the positive potential range for the first time, and a general approximate analytical formula for the positive floating potential with a thin plasma sheath and subsonic plasma flow is derived from the new model recently proposed by the authors. The results show that when the floating potential is positive, the net flux of charge incident on the particle approaches a constant similar to the 'electron saturation' phenomena in the case of the electric probes.

CdS:Sb thin film photoelectrodes with antimony doping concentration in the range from 0.005-0.5 wt.% have been prepared in an aqueous alkaline medium. Photo-electrochemical cells are then fabricated employing these films as an active photoelectrode and the sulphide/polysulphide redox couple as electrolyte. The electrical and optical properties of these cells have been examined under suitable conditions and evaluated in terms of various cell parameters such as i_sc, V_oc, eta , ff, R_s, R_sh, n_d Phi _B and V_fb. It has been found that the cell performance is improved significantly after doping and it is optimum at 0.075 wt.% Sb doping level in CdS. The incremental enhancement in the various properties of a cell is explained on the basis of the modified electrical, optical and structural properties of the thin film compositions.

The particle flux produced by an obliquely incident Nd Q-switched pulse (20 ns) on a Ta target has been analysed with regard to its angular distribution resolved for both its neutral and ion components. The laser intensity has been varied in the range between about 10¹⁰-10¹¹ W cm^-2, which is appropriate for many low-irradiance applications. It is observed that, at all emission angles and for the whole range of laser intensities, the number of neutral species clearly dominates the composition of the particles. While for relatively low laser energies the angular distribution shows the usual smooth cos-behavior, an additional strong directive emission cone, superimposed upon the cos-distribution, develops if the laser energy is enhanced. Both the strength and the width strongly depend on the laser intensity.