Table of contents

The time post discharge of a low-pressure pulsed dc discharge in pure oxygen is used to investigate the atomic oxygen recombination on fused silica surface. With the intention of studying this recombination for different surface states, we perform before each pulsed experiment a wall treatment by means of dc discharges under different experimental conditions. Then, we monitor the decrease of the atomic oxygen in time post discharge by time resolved VUV resonant absorption spectroscopy. We have shown that it is possible to obtain for a given wall treatment, a pulse after pulse variation of this decrease. We have attributed this variation to a filling of the chemisorption sites. Finally, we have determined the surface reaction probability of atomic oxygen on fused silica surface and we have compared it to published values.

Epitaxial La_0.7Sr_0.3MnO₃ thin films have been grown on LaAlO₃(001) substrates by pulsed laser deposition under different target-plume-substrate configurations. Outgrowths of various shape and size from some of the manganite films are observed. When the substrate is placed within the visible laser-produced energetic plasma plume, the deposited films are outgrowth-free and exhibit good crystallinity and high electrical conductivity. The formation mechanisms and control measures of the outgrowths in pulsed laser deposited manganite films are discussed.

A novel approach to simulating the dominant dynamic processes present during concentrated energy beam welding of metals is presented. A model for the transient behaviour of the front keyhole wall is developed. It is assumed that keyhole propagation is dominated by evaporation-recoil-driven melt expulsion from the beam interaction zone. Results from the model show keyhole instabilities consistent with experimental observations of metal welding, metal cutting and ice welding.

The authors found that the electroluminescence (EL) efficiency of organic light-emitting-diodes (LEDs) consisting of triphenyl diamine derivative (TPD) and aluminium quinoline (Alq3) is enhanced by the introduction of a co-evaporated layer of TPD and Alq3 at the interface between the TPD and Alq3 layers. The organic LED with a best co-evaporated layer has two to four times higher EL efficiency (lm W^-1) than that without. The highest enhancement was obtained by the introduction of a 20 nm thick co-evaporated layer prepared using the doping ratio of TPD:Alq3 = 1:10. Considering that the photoluminescence efficiency of the co-evaporated layer did not increase, it was concluded that the increase of active sites for carrier transport and recombination due to the enlargement of the effective contact area between TPD and Alq3 causes the improvement of the EL efficiency.

We review the topic of scanning tunnelling microscopy (STM) studies of silicides, with an emphasis on fundamental scientific issues that can be addressed using STM and ballistic-electron-emission microscopy (BEEM). The discussion is organized according to the topics of structure (atomic scale precursors, surface reconstructions, bulk structures, interfaces) kinetics and growth (direct atomic measurement, modelling, stoichiometry, layer and island growth, phase transitions and nanoscale metallization) and BEEM (method, band structure, Schottky barrier, defect scattering, inelastic scattering and surface effects). These topics are described in general terms, then elaborated with specific examples.

The effect of ion-neutral collisions on the structure and ion flux emanating from a steady-state, planar discharge with two negative components is investigated. The positive ion component is modelled as a cold fluid subject to constant-mobility collisions, while the electrons and negative ions obey Boltzmann relations. The model includes the collisionless limit. When the negative ions are sufficiently cold three types of discharge structures are found. For small negative ion concentrations or high collisionality, the discharge is `stratified', with an electronegative core and an electropositive edge. For the opposite conditions, the discharge is `uniform' with the negative ion density remaining significant at the edge of the plasma. Between these cases lies the special case of a double-layer-stratified discharge, where quasi-neutrality is violated at the edge of the electronegative core. Double-layer-stratified solutions are robust in that they persist for moderate collisionality. Numerical solutions for finite non-neutrality verify that the plasma flux varies continuously with collisionality, although the derivative of the flux with respect to collisionality is discontinuous when the discharge structure changes from uniform to stratified. Double-layer solutions are found when the flux predicted for the plasma approximation is double-valued and the flux associated with the smaller plasma edge potential is less than that associated with the larger edge potential. A comparison with numerical non-neutral solutions confirms that the flux is correctly predicted using the plasma approximation when the larger value of the flux is taken in the two-solution regime.

This report describes the epitaxial growth and fabrication of room-temperature InAs_0.89Sb_0.11/InAs_0.48Sb_0.22P_0.30 semiconductor light emitting diodes operating in the mid-infrared wavelength region near 4.5 µm. The InAs_0.89Sb_0.11 ternary material used in the light emitting diode active region has a large lattice mismatch with respect to the InAs substrate layer and in order to accommodate this it was necessary to grow a buffer layer with an intermediate composition (InAs_0.94Sb_0.06). The devices exhibit infrared emission at 4.5 µm and could be effectively used as the basis of an optical sensor for the environmental monitoring of carbon monoxide at 4.6 µm and carbon dioxide at 4.2 µm in various applications.

1.6 wt% Mg-containing Al-1.0 wt% Si films exposed to air during the lifetime test have been investigated for electromigration resistance. The films with Al₂O₃ or MgO dielectrics near the metal surface are more effective for increasing electromigration resistance and, thus, also extend the lifetime and enhance the activation energy up to 0.79 eV.

Epitaxial MgO thin films have been grown on single crystal substrates of M-plane (100), A-plane ( 20), C-plane (0001) and R-plane (102) sapphire by laser ablation of a Mg metal target in a molecular oxygen atmosphere using 1064 nm radiation from a Nd:YAG laser. X-ray measurements indicate that the MgO grows epitaxially on all substrates, with its orientation dependent on the cut of the sapphire substrate. (110)-oriented MgO grows on M-plane sapphire, while (111)-oriented MgO films are found on both the A-plane and the C-plane sapphire. The orientation of MgO found on R-plane sapphire appears to be surface and temperature dependent. The principal growth orientation obtained on the R-plane is (100)-oriented MgO although (100) growth tilted 30° from the [11] Al₂O₃ direction can result. X-ray area mapping has been performed to determine the mosaicity, d value spread and strain present in the films. These data are compared with the in-plane orientation and the mismatch of the MgO and Al₂O₃ lattices in an attempt to relate the preferred orientation to the plane of the sapphire on which it is grown.

The residual stress-tensor components in amorphous ferromagnetic wire produced by an 'in-rotating-water' spinning process are calculated on the basis of the theory of viscoelasticity. The viscoelastic behaviour of the solidifying wire near the point of vitrification is studied and the effect of the relaxation process on the residual quenching stresses is clearly elucidated. The radial dependence of the residual stress-tensor components obtained is consistent with the magnetic behaviour of amorphous wire with small negative saturation magnetostriction. The residual stress amplitude is investigated as the function of wire radius and the parameters of the cooling process.

We report the temperature dependence of magnetic properties and phase transitions in a soft-magnetic Co-based nanostructured alloy obtained from amorphous Vitrovac 6025. Homogeneous nanostructured samples are achieved by suitable thermal treatments below the conventional crystallization temperature and have ultrafine grain structures with average grain size of about 10 nm and volume fraction 0.6. The grains are embedded in a residual ferromagnetic amorphous matrix and this leads to superior soft-magnetic properties which, in our view, can be justified on the basis of the random anisotropy model. The temperature dependence of initial reversible permeability and coercive field emphasizes a different behaviour in the magnetic properties of nanostructured and amorphous samples. The sharpness in the Hopkinson peaks of the initial permeability is the best tool to show the first magnetic transition in both specimens: the results are compared with those detected by thermogravimetric curves and by magnetization intensity measurements. Differential thermal analysis is used to evaluate the transformation enthalpy of the amorphous-to-microcrystalline and the nano-to-microcrystalline processes, which both occur at about 820 K where another ferromagnetic phase appears.

Rubber ferrite composites (RFC) are important since they have useful applications as microwave absorbers and flexible magnets. The mouldability of these composites into complex shapes is another advantage. The evaluation of their dielectric and magnetic properties is important in understanding the physical properties of these composites. Pre-characterized nickel zinc ferrites (Ni_1-xZn_xFe₂O₄ where 0x1 in steps of 0.2) prepared by ceramic techniques were incorporated in to a butyl rubber matrix according to a specific recipe to yield RFCs. The dielectric constant of ceramic Ni_1-xZn_xFe₂O₄ and the butyl rubber composites incorporated with Ni_1-xZn_xFe₂O₄ are studied as a function of frequency, composition, loading and temperature. The observed data indicates that the dependence of the dielectric constant on frequency follows Maxwell-Wagner interfacial polarization. The compositional (zinc content, i.e. x value) dependence shows that the dielectric constant increases initially and reaches a maximum value for the composition corresponding to x = 0.6 and thereafter it decreases. This can be explained on the basis of porosity and alternating current (AC) conductivity. It was also observed that the dielectric constant of the composite material increases with an increase of the volume fraction of the magnetic filler. These observations satisfy some mixture equations, which correlate the dielectric constant of the matrix, filler and the composites. The temperature dependence of the dielectric constant of the ceramic samples as well as the RFCs shows an increase with an increase of temperature at low frequencies. The dielectric constant of the blank butyl rubber was also determined. It was observed that for a blank sample (without filler) the dielectric constant decreases with an increase of temperature. This is due to the decrease in polymer density with increase in temperature. These results suggest that the magnetic and dielectric properties of RFCs can be manipulated by appropriate loading and a judicious choice of the magnetic filler. The modification of these properties will aid in the design of composite materials for microwave absorbers.

Monte Carlo simulations are a very useful tool for simulating many physical processes. The method has often been used for electron microscopy, spectroscopy and microanalysis. In this paper, the backscattering coefficient and coefficient of elastic reflection in the energy range 0.2-5 keV for C, Al, Cu, Ag and Au is simulated and compared with literature data. For the elastic cross sections the program PWADIR was used, whereas for inelastic ones the simple hyperbolic distribution of energy losses was utilized. Due to the hyperbolic shape, one free parameter, the minimal energy loss W_min appears. This quantity hardly influences the dependence of the backscattering coefficient on primary energy, but strongly influences the energy dependence of the coefficient of elastic reflection. By comparison of the calculated and measured values of this coefficient we were able to find the optimal values of W_min, and, due to mutual dependence, the `experimental' values of the inelastic mean free path _in. The calculated and measured energy dependence of the backscattering coefficient and the coefficient of elastic reflection have shown, in the mentioned energy range, reasonable agreement for all the elements studied.

This paper presents a numerical simulation of the temperature field resulting from the impingement of a laser beam on a metal surface. The calculations were based on a hydrodynamic physical model of laser-material interaction, which includes the effect of evaporation recoil pressure on melt flow and the related convective heat transfer. The computations follow an experimentally verified physical model of melt hydrodynamics and the model of laser-induced evaporation developed by Anisimov. The simulations indicate that convective heat transfer which is induced by recoil pressure is significant for absorbed laser intensities from 0.5 MW cm^-2 to 10 MW cm^-2. This range corresponds to laser welding, cutting and drilling due to melt ejection (hydrodynamic drilling). The simulations also show that the motion of the melt from the centre of the beam interaction zone towards the periphery results in a secondary maxima in the temperature distribution, which in turn can lead to instability of the melt flow and temperature field fluctuations. The model also predicts that the cooling rate in and around the fusion zone is strongly influenced by the recoil-pressure-induced melt flow. At the centre of the beam interaction zone, the cooling rate is much higher (~10 times), and at the periphery, much slower than predicted by a pure heat conduction model.

A novel model is presented for shock wave development in axially-biased ferrite-loaded coaxial lines. The one-dimensional transverse electromagnetic plane wave equations for the coaxial line are linked to the three-dimensional Landau-Gilbert gyromagnetic equation describing the coherent ferrite magnetization process. The effects of demagnetization field terms imposed by the ferrite and coaxial line geometry are identified. Numerical time-stepping solution gives the predicted transmission line waveforms directly in the time domain. Simulated and experimental waveforms are compared for a nickel-zinc ferrite-loaded coaxial pulse-sharpening line. The model is shown to give generally good results for the shock front waveform, rise time and propagation characteristics, except that predicted and measured rise times diverge increasingly at very low pulse amplitudes.

A performance analysis of an irreversible regenerative magnetohydrodynamic (MHD) power cycle operating at maximum power for a constant-velocity and a constant-Mach-number type of MHD generator has been carried out. The obtained results are evaluated together with the results of performance analysis at maximum thermal efficiency, and thus the upper and lower bounds of the optimal operating region in terms of power and thermal efficiency are determined. The considered MHD power cycle model includes irreversibilities coming from the electrical losses at the MHD generator, frictions at the compressor and pressure losses in the heater, the cooler and the regenerator. The effects of these major irreversibilities and regeneration application on the general and optimal performance are investigated. The analysis presented in this study may provide a theoretical basis for the optimal design and operating conditions for regenerative MHD power cycles.

Transport coefficients and velocity distribution functions for electron swarms in methane in the presence of uniform static electric and magnetic fields crossed at arbitrary angles to each other are calculated using a multi-term solution of the Boltzmann equation. Particular attention is paid to the effects of the relative orientation of the fields and how this affects the convergence in the spherical harmonic expansion that represents the velocity distribution function. The inadequacies and incorrect assumptions of previous treatments of this problem are also highlighted.

We have calculated the multi-temperature SF₆ plasma composition by two different methods. One method is based on the mass action law whereas the other method uses a kinetic chemical reaction scheme. The first approach is furthermore split up in two different calculations. One is based on the multi-temperature Saha equation and the other is based on the excitation temperature of the involved species.

With respect to the electron density we conclude that the multi-temperature Saha equation results in appreciable higher number densities compared to both the kinetic scheme and the excitation-temperature-based calculations. The difference increases with decreasing gas temperature and increasing electron temperature (i.e. with increasing degree of kinetic non-equilibrium). The kinetic scheme and the excitation-temperature-based calculation yield comparable results for higher gas temperatures. In this region the excitation-temperature-based method is preferable over the kinetic method. However, for the dielectric case (i.e. lower gas temperature) we find different results at higher electron temperatures. The excitation-temperature-based calculation method requires additional modelling at this point.

A model has been developed which follows electrons and ions through assumed time varying potentials in a radio-frequency methane/hydrogen plasma. The system modelled is relevant to a low-pressure (10-90 mTorr) plasma used in the etching of GaAs surfaces. To understand the etching process, a knowledge of the flux and energy distributions of all species at the surface is required and as a first stage it is necessary to derive the spatial ionization distribution function. Inelastic and elastic collisions with neutral gas molecules are included in the model and the results give realistic electron energy distribution functions (EEDFs). These EEDFs are found to be insensitive to the inclusion of rotational modes of excitation, but vary significantly when vibrational modes of excitation are included. The EEDF is seen to change greatly with spatial position between the electrodes. Results also suggest that the form of the spatial ionization distribution function is identical for all ion species, though different in magnitude. Ion fluxes predicted at the anode are in broad agreement with experiment and are most affected by the ionization cross sections used in the simulation. Neutral fluxes to the surface are seen to make a major contribution to the particle flux, especially at the higher pressures. The secondary electron emission coefficient, though having a significant effect on the electron energy distribution function, has little effect on the mass distribution of the flux deposition on the substrate surface.

We present a method for the deconvolution of spectral line profiles - the subtraction of the apparatus function. This inversion problem requires solution of the Fredholm integral equation of the first kind. For this purpose we suggest the use of B-splines. The efficiency of the approach is demonstrated using test examples as well as the deconvolution of the H spectral line profile, measured by means of the Fabry-Perot interferometer. The H line was emitted by a low temperature plasma in RF discharge burning in water vapour at reduced pressure. This deconvolution method is compared with the standard least squares method, where the initial profile is described by the Voigt function.

Infrared (IR) absorption spectroscopy and mass spectrometry have been simultaneously applied to dusty radiofrequency (RF) plasmas in methane, acetylene and ethylene. The combination of IR absorption spectroscopy and mass spectrometry allows the chemical composition and structure of the most relevant plasma-produced neutral species, the ionic plasma composition and the chemical composition of the nanometer-sized particles to be precisely identified. The production of acetylenic compounds (C₂H_x) seems to be a key mechanism for the powder formation in all the investigated hydrocarbon plasmas. Electron attachment to acetylenic compounds and the following ion-neutral reactions might lead to the high-mass carbon anions, which are trapped in the plasma and finally end in powder formation. The hydrogenation of the monomer strongly influences the composition of the ions. Finally the composition of the plasma-produced particles is mainly sp³ bonded carbon and the infrared spectra show similarities to that of polyethylene.

The excitation of the N₂(B ³_g) state in microwave discharges ignited in flowing nitrogen and in their short-lived afterglow is investigated. Simultaneous measurements in the post-discharge on this species and the N₂⁺(B ²_u⁺) ion through emissions, and also on the electronic ground state of N₂ by spontaneous Raman scattering, provide information of their vibrational distribution function. A spatial study has been carried out, showing a quite slow evolution of the vibrational populations along the short-lived afterglow, and in particular the first proof of a steady vibrational distribution of N₂(X ¹_g⁺) from the beginning to the bulk of this ionized region. The N₂(B ³_g vibrational distribution is analysed by a steady-state kinetic model taking into account all the B state excitation processes, i.e. electron collisions and reactions between molecular excited states, and quenching. The reaction between the vibrationally excited N₂(X ¹_g⁺) and N₂(A³_u⁺) is shown to be the pre-eminent excitation mechanism in the short-lived afterglow.

The time afterglow of a pulsed discharge is used to investigate the neutral-particle kinetics in N₂-O₂ low-pressure mixtures. The pressure is in the range 0.5-2 Torr at 300 K and the mixture composition in the range 0-20% of oxygen. Time-resolved emission spectroscopy on N₂(B), N₂(C), NO(A) and NO(B) is employed to monitor energy transfers involving the metastable state N₂(A). The influence of N(⁴S) and O(³P) atoms on the kinetics is accurately treated using absolute concentration measurements by time-resolved absorption spectroscopy in the VUV range. It is shown that by an appropriate choice of the discharge repetition rate, the vibrational excitation of N₂(X) can be neglected. A chemical model, containing few unknown parameters, is developed in order to fit the experimental fluorescences. The NO(X) kinetics are investigated and its absolute concentration is deduced. Furthermore, it is shown that the N2(A) density is probably higher for pulsed discharges than for stationary low-pressure DC discharges.

Results obtained using a pulsed-streamer corona discharge for organic compound removal are reported in this investigation. The removal of phenol and the effects of various parameters on the removal efficiency were studied. The intermediate products and final byproducts were also studied. It was found that the organic contaminants in aqueous solution could be destroyed effectively by the pulsed-streamer corona discharge and that the organic compound removal was greatly influenced by gas injection (bubbling). In addition, a preliminary study of the reaction mechanism was performed. Byproducts were determined for phenol removal. The main intermediate products produced by the pulsed-voltage discharge during the treatment process were hydroquinone, pyrocatechol and 1,4-benzoquinone. These intermediate products disappeared when the residence time was increased. By analysing the total carbon content of the treated phenol solution, it was found that 83% of the total organic carbon was removed without oxygen bubbling and 86% was removed with oxygen bubbling. Carbon dioxide emitted in the gas phase was measured by an absorption method.

The mathematical equivalence between a non-Debye and a frequency distributed process, both having a single activation energy, under non-isothermal and time varying poling, is established using the concept of intrinsic time. The well known time-temperature superposition principle is derived and it is concluded that the vast class of processes obeying this principle might indicate that they are indeed non-Debye processes rather than processes distributed in frequency with a single activation energy.

The dielectric properties of a MgO and Mg(NO₃)₂ doped and undoped talc are measured as a function of the frequency range 260 kHz-40 MHz. The ´, tan and _ac values are given as a function of the firing temperature for a 5.0 MHz constant frequency. In this study the effects of an impurity and the firing temperature on the dielectric properties are also discussed.

HgBr₂ single crystals were grown from the vapour by the moving vapour growth method. By optical spectroscopy measurements in the fundamental energy gap region the energy gap value at room temperature RT was determined, and by electrical measurements the energy levels inside the energy gap were resolved. Investigation of the crystals by a photoluminescence technique using a confocal laser scanning microscope using a Kr-Ar laser (488 nm/514 nm) confirmed the presence of internal inhomogeneities and gave information regarding the position and dimension of the impurity regions in the crystals.

Titanium hydride was successfully synthesized at room temperature by ball milling. X-ray diffraction patterns demonstrated that a transformation of elemental Ti powders into TiH₂ could be induced at room temperature by ball milling under a hydrogen atmosphere. Almost no hydrogen absorption was observed during the initial few minutes of the milling process on the basis of x-ray diffraction patterns and hydrogen pressure in the reservoir system, although the grain size of Ti decreased significantly. A non-stoichiometric titanium hydride with an approximate composition of TiH_1.9 was first formed after about 7 min of milling and existed together with unhydrided Ti flakes. The stoichiometric hydride phase TiH₂ was developed after further milling at room temperature. Due to the trapping of hydrogen by numerous defects and grain boundaries, the dilute solution Ti(H) does not exist in the nanograin milled metal powders.

A physical model of the optical behaviour of aluminium nitride cermet solar coatings has been used to optimize the metal volume fraction and layer thickness of the coatings. A modified photo-thermal conversion efficiency for solar collector tubes is presented and used. The cermet layers are generally deposited by reactive sputtering in a gas mixture of argon and nitrogen. Sputtered aluminium Al^sp is used as a metallic component in the cermet and its refractive index, evaluated in this study, is employed. Due to oxygen contamination, aluminium oxynitride (AlON) is used as a ceramic component in the cermet. Bruggeman approximations are used to calculate the dielectric function for composite materials. An initial ten-layer grade film optimized to one nearly identical to a double cermet film structure when maximizing photo-thermal conversion efficiency at 80 °C under a concentration of 1. The optimized films, F10Lm (for initial ten-layer graded film) and F3Lm (for three-layer film) have an identical solar absorptance of 0.957 and an identical hemispherical emittance of 0.048 at 80 °C. The optimized film consists of one anti-reflection layer and two cermet layers with metal volume fractions of 0.093 and 0.255, and thicknesses of 30 nm and 93 nm, respectively, going from the anti-reflection coating to the infrared reflector layer. The solar performance can be further improved using a lower refractive index anti-reflection layer and a lower emittance infrared reflector. For example, using an Al₂O₃ anti-reflection layer, the solar absorptance increases to 0.974, and using a Cu infrared reflector, hemispherical emittance decreases to 0.033 at 80 °C. For these different anti-reflection and infrared reflector materials, optimized calculations have also predicted that the double cermet layer film structures have the highest photo-thermal conversion efficiency.

It has recently been brought to my attention (M J Sablik, 1999, private communication) that in the above paper, which outlines a pair of differential equations for describing reversible and irreversible effects of stress on magnetization, part of the results shown in figure 9 cannot be obtained as a result of the application of the equations described in the paper.

In particular, while the changes in magnetic induction B with stress under application of either compression or tension are correct, as shown in figure 9, for stress increasing from zero, the loci displayed for removal of stress back to zero are not obtained from the equation described.

Furthermore, it will have been obvious from an algebraic manipulation that equation (21) of the paper has a typographical error in which the second term on the right hand side is missing a multiplication c. This equation should of course read: