Table of contents

In gas discharges at elevated pressure, radiation-less collisional de-excitation (quenching) has a strong influence on the population of excited states. The knowledge of quenching coefficients is therefore important for plasma diagnostics and simulations. A novel time-resolved optical emission spectroscopic (OES) technique allows the measurement of quenching coefficients for emission lines of various species, particularly of noble gases, with molecular hydrogen as collision partner. The technique exploits the short electron impact excitation during the field reversal phase within the sheath region of a hydrogen capacitively coupled RF discharge at 13.56 MHz. Quenching coefficients can be determined subsequent to this excitation from the effective lifetime of the fluorecence decay at various hydrogen pressures. The measured quenching coefficients agree very well with results obtained by means of laser excitation. The time-resolved OES technique based on electron impact excitation is not limited - in contrast to laser techniques - by optical selection rules and the energy gap between the ground state and the observed excited level.

Novel employment of the absorbing a-C:H layer for optical isolation of an a-Si:C:H photosensor in the reflective liquid crystal optically addressed spatial light modulators is discussed. The dependences of electrical resistivity ρ, Tauc gap E_T and an absorption coefficient α at 633 nm on the deposition rate of the a-C:H films in the interval from 1 to 10 Å s^-1 were compared and correlations between these properties were ascertained to match the a-C:H layer with the a-Si:C:H layer electrically. Current-voltage characteristics of ITO/a-C:H and ITO/a-Si:C:H/a-C:H thin-film structures were measured using a mercury droplet contact. Our investigations showed that the absorbing a-C:H layer with α~4×10⁴ cm^-1 at λ = 633 nm^-1, ρ in the range from 10¹⁰ to 10¹¹ Ω cm, and E_T in the range from 1.25 up to 1.5 eV can ensure effective optical isolation and be a well matched couple with the a-Si:C:H photoconductor. The photocurrent and dark current I-V characteristics of such ITO/a-Si:C:H/a-C:H thin-film structures were symmetrical. The ratio between photocurrents and dark currents of such structures was governed by the photosensitivity of the a-Si:C:H layer and depended on the electrical resistance of the a-C:H layer.

We explore the effects of various basic atomic processes on the island shapes of diffusion-limited-aggregate growth on a triangular lattice using kinetic Monte Carlo simulation. Edge diffusion and bond-reducing relaxation of double-coordinated adatoms, as well as single-coordinated adatoms, are proved to be essential to the island shape transition from fractal to irregularly compact and from irregularly compact to hexagonal islands, respectively. Shape transitions of adatom islands on fcc (111) substrates can be understood naturally by this model.

Electrical, optical and structural properties of secondary phase free CuInSe₂ thin films deposited on glass substrates are studied. The activation energy (E_A) has been calculated for the films using the Arrhenius plot and the band gap energy (E_g) has been obtained from the optical transmission spectra. E_A varied between 15 meV and 75 meV and E_g varied between 0.95 eV and 0.98 eV. Structural parameters a and c are calculated for the tetragonal phase. The mean grain size of the polycrystalline films is 0.1 µm.

Numerical solution of the Boltzmann equation with a relaxation collision term is used to study gas-dynamic flows formed under nanosecond pulsed laser ablation. Atoms ejected from the surface of a target are assumed to have a Maxwell velocity distribution corresponding to the surface temperature and the saturated vapour pressure. The surface temperature is obtained from a transient heat transfer equation in the condensed phase. Atomic collisions in the ablation plume orient atoms towards the surface normal and speed up the plume expansion from the target. Atoms backscattered in the gas phase, stick to the target surface and cause back condensation of the vapour at later stages. When the mean free path is much less than the plume dimension, a Knudsen layer, a hydrodynamic flow region, and a low-density tail may be distinguished in the gas phase. The present numerical simulation is in good agreement with the analytical quasi-steady Mott-Smith approach to the Knudsen layer in the case of evaporation and at the early stages of condensation. Comparison with experiment reveals that the model underestimates both the width of the ablated material angular distribution and the amount of high-energy atoms. The difference increases with the laser fluence and may be caused by lateral expansion of the plume, by vapour acceleration due to laser radiation absorption or probably by non-thermal evaporation effects.

A novel method is proposed to determine effective barrier heights in homogeneous nonideal Schottky contact from I-V measurements. This method takes into account the different mechanisms of current flow through the metal-semiconductor interface. The total current has been expressed as the sum of two independent terms which are: (1) the thermionic current where the ideality factor value is equal to one and (2) the contribution of different transport mechanisms. The second term responds to a general expression of the thermionic emission theory where the barrier height and the ideality factor are voltage dependent. The effective barrier height is found by means of subtraction of the transport mechanism terms from the total current. The method was applied to a group of I-V experimental curves which were reported by M Barus and D Donoval 1993 Solid State Electron.36 969.

A wide class of magnetic molecular materials - molecular clusters with high magnetic moment containing 3d transition metals (such as `Fe<sub>8</sub>', `Mn₁₂ac', etc) - have been considered from the point of view of their use as refrigerants in low-temperature regions. The consideration was made in the framework of the model based on the Langevin theory of a superparamagnet. The magnetic entropy change caused by a change in an external magnetic field was calculated for various magnetic clusters. The estimations made show that the magnetic molecular clusters could be promising materials for magnetic refrigeration in low-temperature regions (below about 20 K).

Nanometric Ni wires and I collagen fibres set across holes of Lacey carbon microscope grids have been observed in a lensless point projection field-emission microscope operating between 100 and 300 eV. At sufficiently high magnification specific fringe patterns appear in both of the two cases. With the help of simulations we show that both cases are strongly dependent on the electric charge density of the observed objects. In the case of Ni nanowires we show that the effect of the remanent magnetization on the electron beam is masked by electric charge of residues of polycarbonate track etched membrane. In the case of collagen these patterns are characterized by an odd number of fringes with a central fringe that becomes very bright as the charge increases. The fringe pattern masks the fitting of the collagen molecules and it is necessary to take into account the charge effect in order to interpret this diffraction pattern. With remarkable precision, from these two examples an average diameter and a linear charge density have been obtained by the analysis of the fringes. The limits of the models used for the simulations and those of the data acquisition system are discussed. Finally we emphasize the fact that the interpretation of the point projection images is delicate despite a very simple experimental scheme. The results may be considered as especially important, in particular in so far as the observation of biological objects is concerned, as this may become the major application of point projection microscopy.

The series of epitaxial garnet films of general composition Lu_3-x-yBi_xPr_yFe_5-zAl_zO₁₂ and Lu_3-xBi_xFe_5-y-zSc_yAl_zO₁₂ were grown on (111) oriented GGG (gadolinium gallium garnet) substrates by the liquid phase epitaxy. Their magnetic and magneto-optical properties were studied using both experimental techniques and modelling. All obtained films demonstrated generally a magnetic anisotropy close to the easy-plane type. The Pr-containing films exhibited large negative uniaxial anisotropy and significant cubic anisotropy. The latter causes a distortion of magnetization curves in samples magnetized in a direction normal to the film plane, especially at low temperatures. The large negative uniaxial anisotropy of Pr-substituted iron garnets allows us to increase the saturation field up to 0.5 T at liquid nitrogen temperature. The Sc-doped films displayed small positive uniaxial anisotropy that did not exceed the shape anisotropy. The magnetization curves of these films did not show any distortion due to the cubic anisotropy. The suitability of Pr- and Sc-doped garnets that meet the requirements for indicator layers for magneto-optic visualization at liquid nitrogen temperature is discussed.

Zinc II hexacyanoferrate III, Zn_1.5[Fe(CN)₆]·6H₂O, was prepared by coprecipitation method. The precipitated powder possessed a face-centred cubic structure with a lattice parameter of 10.32 Å. Phase transformation in this cyanide compound was encountered after heat treatment at a temperature at 75 °C or above, from a face-centred cubic to a monoclinic structure. The finding of these different structures could help one to have a better understanding of the magnetic properties of the compound after being subjected to different stages of thermal treatment.

The non-equilibrium boundary layer of a Saha plasma contacting material walls or electrodes is analysed in the parameter regime λ_D<<λ_CX<<λ_i, where λ_D is the Debye length, λ_CX is the mean free path for charge exchange collisions and λ_i is the ionization length. According to this scaling, the boundary layer is split up into the separate model zones of: (i) a space-charge zone (scale λ_D); (ii) a Knudsen layer (scale λ_CX); (iii) a transition layer (scale λ_i). The collision dominated transition layer is analysed in the mobility-diffusion approximation. The Knudsen layer, which is characterized by strong inhomogeneities and by strong deviations from an equilibrium distribution, is treated kinetically. The model zones are related to each other by a consistent matching procedure. Handy formulae for overall quantities are supplied for convenient applications.

Les méthodes existantes de titrage de N et O d'une post-décharge au moyen de l'intensité d'émission de la molécule NO excitée ne permettant pas d'aller au-delà de x = 5% dans un mélange xO₂-(100%-x)N₂, nous présentons une démarche valable pour x⩽20%. Cette technique est fondée sur la mesure de l'intensité d'émission de NO₂(A), en fonction du débit de NO introduit, en relation avec une dérivation analytique des équations des concentrations [N] et [O]. La concentration d'oxygène atomique obtenue par cette méthode est validée de façon indépendante à partir de la mesure du rapport des intensités d'émission de NO(B) et de N₂(B, 11) (celle-ci détectable pour x⩽8%). Enfin, la méthode proposée est mise en oeuvre pour apprécier l'influence de la valeur de la concentration d'oxygène atomique sur le temps de stérilisation dans une post-décharge en flux à partir d'un plasma de N₂-O₂. \engabstract Existing titration methods of N and O in an afterglow based on the emission intensity of the excited NO molecule cannot be used at x values exceeding 5% in the xO₂-(100%-x)N₂ mixture. Our technique extends the x range to 20%. It utilizes the emission intensity measurement of NO₂(A), as a function of the introduced NO flow, in relation with analytically derived equations for the O and N concentrations. The atomic oxygen concentration obtained in this way is validated independently through measurements of the emission intensity ratio of NO(B) and N₂(B, 11) (detectable for x⩽8%). Finally, the proposed method is used to assess the influence of the oxygen atom concentration on the sterilization time in the flowing afterglow of an N₂-O₂ plasma.

Spectroscopic investigations are made on expanding plasma plumes created by a 20 ns, 3 J laser pulse impinging perpendicularly onto targets of boron, boron carbide and boron nitride. The characterization of the plasma plumes is carried out spectroscopically in the wavelength range from 30 to 330 Å, and by taking pinhole pictures at shorter wavelengths. The experimental results are compared with the plasma plume produced from different targets under the same experimental conditions. The calculated plasma temperatures are 50 eV and 30 eV at 1 mm from the target in the cases of boron and boron carbide targets, respectively. Electron temperature measurements were made as a function of laser irradiance, distance from the target surface and time elapsed after the maximum of the laser pulse.

This paper is aimed at calculating the electric field in the point-to-plane electrode system with the plate covered with a dielectric layer. With charge accumulation on the dielectric surface by corona discharge, the field in the dielectric is increased at the expense of a decrease in the gas gap. The charge accumulation on the dielectric surface proceeds to the maximum possible value when the normal component of the surface field vanishes. With the dielectric layer fully-charged, the percentage decrease of the field in the gas gap is maximum at the dielectric surface and declines along the gap axis to vanish at the point tip. The percentage decrease of the field becomes more pronounced with the increase of the diameter of the dielectric layer. The effect of inter-electrode spacing and the dielectric layer thickness on the field distribution is investigated. An accurate method of charge simulation was used for field calculation irrespective of the thickness of the dielectric layer and the gap geometry. With ion flow along the flux lines from the stressed point to the ground plane, the field enhancement factor increases and the volume charge density decreases along the flux lines. The voltages of the ion flow threshold and corona quenching are calculated and compared with previous measurements. The method of calculation is extended to calculate how high the surface potential of the charged dielectric needs to be to trigger a micro-spark in the electrostatic discharges from grounded point electrodes.

Electrical treeing is of interest to the electrical generation, transmission and distribution industries as it is one of the causes of insulation failure in electrical machines, switchgear and transformer bushings. Previous experimental investigations of electrical treeing in epoxy resins have found evidence that the tree structures formed were either electrically conducting or non-conducting, depending on whether the epoxy resin was in a flexible state (above its glass transition temperature) or in the glassy state (below its glass transition temperature). In this paper we extend an existing model, of partial discharges within an arbitrarily defined non-conducting electrical tree structure, to the case of electrical conducting trees. With the inclusion of tree channel conductivity, the partial discharge model could simulate successfully the experimentally observed partial discharge activity occurring in trees grown in both the flexible and glassy epoxy resins. This modelling highlights a fundamental difference in the mechanism of electrical tree growth in flexible and glassy epoxy resins. The much lower resistivities of the tree channels grown in the glassy epoxy resins may be due to conducting decomposition (carbonized) products condensing on the side walls of the existing channels, whereas, in the case of non-conducting tree channels, subsequent discharges within the main branches lead to side-wall erosion and a consequent widening of the tubules. The differing electrical characteristics of the tree tubules also have consequences for the development of diagnostic tools for the early detection of pre-breakdown phenomena.

A model is developed for recombination dominated electronegative plasmas which allows the electron temperature (T_e) to be related to the similarity variable pressure×plasma dimension (pL). It is based on four relationships: (i) T_e>T_ec where T_ec is given by equating ionization and attachment rates; (ii) a relationship between the rate coefficients for ionization, attachment and recombination (k_iz, k_att and k_rec) and the central negative ion/electron density ratio α₀≡n_n0/n_e0 set by the ion fluxes at the centre; (iii) a relationship between the attachment and recombination rates, k_att and k_rec, the electronegativity, α₀, and X₀, the fractional size of the central negative ion-positive ion plasma, obtained by integrating the negative ion flux; and (iv) a plasma balance equation between the generation and loss rates and pL, effectively the integral of the positive ion flux.

Since the underlying equations are nonlinear it is necessary to introduce another parameter. This is chosen to be the ratio of the central electron density (n_e0) to the gas density (n_g).

Detailed results are given for chlorine and comparison is made with simulation and experiment in that, and other, gases.

The paper brings into correspondence two different recent analytical/computational approaches to the subject.

The results are given of detailed experimental studies of the model flue gas mixture irradiation by pulsed electron beams and electron-beam-initiated non-self-sustained discharges. The effects of the electron beam parameters, the external electric field and the composition of irradiated gas on the process of sulphur dioxide SO₂ removal are analysed. We demonstrate the existence of optimum values of the electron beam current density and pulse duration, and of the external electric field strength, at which the energy required to remove one SO₂ molecule is minimum. The energy cost and removal rate are given as functions of the concentration of SO₂, oxygen and water vapour in the mixture. The concentration limit that divides the chain and radical mechanisms of SO₂ removal is determined.

This article presents measurement results of the liquid refraction index achieved with a new kind of instrument called a `capillary-cube'. This instrument is made with a glass cube bored along its whole length and lit by a laser beam. The boring diameter is 1 mm and the beam waist is 0.8 mm. Different kinds of interference patterns are obtained according to the adjustment of the laser beam shift with respect to the capillary axis, and the beam diameter used. These interference patterns have good visibility and stability. A model of working order is proposed with the purpose of explaining the shape of the interference patterns. Measurements of refraction index variation have been realized with distilled water at different temperatures, in order to evaluate the experimental sensitivity of this technique. The obtained values are in accordance with the proposed model and, furthermore, they show that this measurement method can be a precise technique.

The abrupt increase of the damping coefficient of an oscillating microlever in the proximity of a surface is discussed as a direct consequence of a local relaxation. The power law found and the dependence of the damping coefficient on the oscillation amplitude are in good agreement with the experimental data recorded on a graphite surface. We show that a combined study of the resonance frequency shift and of the dissipation should bring new information on the intrinsic mechanical properties of a surface at a local scale.

It is shown that the inhomogeneous polarization of an insulation gap with barrier in an electrical field due to a sharp change in the permittivity and/or conductivity at the interface of `basic dielectric-barrier' is one of possible reasons of the so-called barrier effect in dielectrics. The influence of the preliminary polarization of thin polyethylene terephthalate barriers on the breakdown voltage of an air gap with a needle-plane configuration under ac and dc voltage is determined.