Table of contents

The impulse response method is often used to study the ultrasonic field radiated by planar or gently curved transducers. One of the advantages of this approach is an easy understanding of the diffraction phenomena in the time domain; moreover, it allows the prediction of diffraction effects on the pressure field by a simple temporal convolution between the impulse response for the potential and the acceleration of the surface transmitter. An experimental evaluation of the impulse response for the potential in the case of focused transducers is proposed using a deconvolution of two pressure signals detected by a small hydrophone. One of these two signals corresponds to a measurement at the focus since at this particular point the pressure is directly proportional to the acceleration of the surface source. Because of the finite bandwidth of the measurement system the deconvolution is not an easy operation. In order to restore the missing low-frequency components, a polynomial extrapolation of the transfer function is used to improve the deconvolution results.

An attempt is made to offer a model applicable to adhesive wear prediction. The essence of the approach is the proposal that the rate of wear can be predicted in probabilistic terms. Therefore, the estimation of the probability of wear, which can be regarded as synonymous with the probability of asperity contact, precedes the calculation of the wear rate. Further, recognising the fact that wear takes place within the actual area of contact, it is argued that this area consists of plastic and elastic contacts between asperities which, in turn, have different shear strengths and contribute to the wear process differently. An assumption that only two levels of adhesion occur during asperity interactions combined with the statistical properties of rough surfaces leads to a model of adhesive wear which is a better approximation of the real process than earlier models.

Despite the recognition of several sources of variation of contact angle, both between and within sessile drops on plane substrates, no comment has ever been made on the statistical treatment of observed angles, especially those around single drops. Circumperipheral observations are suggested as essential, and analysis using cos theta in an autocorrelation model is proposed as a general means of handling such data.

For pt.II see ibid., vol.21, p.1605 (1988). Non-linear effects of the field of high-frequency surface waves on the ionisation instability in gas discharge plasma presented previously are applied to gaseous lasers. It is shown that suppression of the ionisation instability by the high-frequency field can be considered as an effective method for improving the output characteristics of He-Ne lasers.

An approximate model for treating multistep ionisation and recombination in inert gas plasmas is presented. The model makes specific use of the typical electronic level structure of inert gases with a large energy gap between the ground state and first excited state, followed by levels which are relatively closely spaced. Formulae for ionisation and recombination rates in argon are derived which can be rapidly evaluated. The method is useful for inert gas plasmas in which radiation effects are secondary in importance or negligible.

The analysis of the Doppler profile of the Balmer H_alpha line in the cathode sheath and negative glow of a hydrogen glow discharge shows an important concentration of anisotropic energetic atoms. This translational energy is regarded as mainly resulting from reactions including ions accelerated by the cathode sheath electric field. The mechanisms involved are gas-phase charge-exchange collisions producing fast atoms moving toward the cathode and simultaneous neutralisation and backscattering of the ions at the cathode surface leading to energetic atoms moving in the other direction. Quantitative analysis shows that most of the emission of the fast atoms results from excitation directly produced by these ion impact reactions.

Arrival-time spectra of the evolution of swarms in the hydrodynamic regime have been studied theoretically starting from the Boltzmann equation and its eigenvalue problem. In order to express the development of the number density of the one-dimensional pulse swarm, a new evolution equation with new transport parameters, which are obtained directly from the arrival-time spectra, is introduced. Relations between the longitudinal transport coefficients and the new parameters are also presented. By means of this theory, it is shown that the distribution functions for the different types of experiments (steady-state Townsend or pulsed Townsend) are distinguished from each other in non-conservative cases, and drift velocities defined by different principles have in general their own values even if the kind of gas and the value of E/N are the same.

The radio frequency glow discharge in Ar at 13.56 MHz has been investigated by using time- and space-resolved emission spectroscopy with a resolution of 0.2 ns. The relative net excitation rate is also obtained by deconvoluting the emission profile considering the effect of the lifetime. From these results, the authors are able to guess the electron transport and the temporal structure of the discharge. ArI(3p₅ to 1s₄), ArI(2p₂ to 1s₅) and ArII(4p⁴D_7/2 to 4s⁴P_5/2) lines are selected for observation. The electrons with low energy, epsilon >14.57 eV, are transported corresponding to the phase of the current. The electrons with higher energy, M>35.05 eV, are observed at the phase corresponding to the applied voltage only when the aluminium electrode is used under the present experimental conditions. It is confirmed that the high-energy electrons are secondary electrons emitted from the electrode. The RF discharge in Ar at 13.56 MHz has two modes with a hysteresis loop as the function of input energy.

The arc movement was examined for tubular copper electrodes using different plasma gases (argon, helium, nitrogen, air, chloride and mixtures of these gases). The normal arc current was 100 A; the arc was moved using an external magnetic field which varied between 10 and 1500 G. The arc velocity followed an aerodynamic type of equation when the cathode surface was contaminated. For clean or heavily contaminated cathode surfaces another opposing force becomes important, the surface drag force. The arc movement was also investigated by using high speed filming and following the arc current fluctuations. The cathode surface was examined using Auger, ESCA and SEM; work function values were obtained using a Kelvin probe. The results were consistent with the model presented for the arc movement.

Glasses in the system XNa₂O+(1-X)SiO₂ with X varying from 0.096 to 0.242 have been prepared by the sol-gel technique. The electrical conductivity of these materials has been investigated in the temperature range 400-600 K over frequencies varying from 10 Hz to 10 MHz. The bulk electrical resistivity has been determined by the complex impedance analysis. The AC relaxation spectrum has been studied by using the complex dielectric modulus M*=1/ epsilon *. The modulus data have been analysed by the Kohlrausch-Williams-Watts (KWW) stretched exponential function. The exponent beta is found to have a value of the range 0.28-0.40 depending on the glass composition. The beta value for a melt-quenched glass of comparable composition is found to be higher than these values and this difference is believed to arise due to the gel route of glass preparation. The activation energy for DC conductivity is found to be appreciably higher than that for AC conduction.

Contact charge exchange measurements have been made on polytetrafluoroethylene (PTFE) films charged by repeated contacts to mercury. The authors have shown previously that charge accumulates with repeated contacts until the surface reaches saturation, and that the saturation potential is strongly affected by impurities in the mercury. In this study they show that molecularly thin adsorbate layers on the mercury surface play a dominant role in the charge exchange. They have demonstrated this effect by chemisorbing 3-methylthiophene onto the metal surface. Thiophenes are known to form stable, strongly adsorbed monolayers on the surface of mercury, and they find that these monolayers diminish the electron transfer from pure mercury to PTFE; in contrast, a 3-methylthiophene monolayer increases electron transfer from a sodium-doped mercury contact, suggesting that the monolayer displaces the tarnish layer from the impure metal surface.

Uniformly thick, conductive, tin oxide thin films ranging in thickness from 3 to 85 nm were deposited by reactive evaporation of tin, in the presence of oxygen, onto heated glass substrates. The conductivity of the films could be increased by doping with molybdenum using a simple, novel technique which is described. Film resistivity was found to increase with the time of exposure to laboratory ambients subsequent to deposition. Mobility and carrier density data for the more conductive films are presented. An approximately constant sheet resistance was found for the undoped films from which it is inferred that conduction is probably dominated by a highly conductive surface layer. The sheet resistance of the doped films increased with decreasing thickness. The films produced consisted entirely of tin oxide phases as indicated by X-ray diffraction results and were highly transparent (T>85%). There was no significant difference between the optical absorption edge (3.6-3.9 eV) for doped and undoped films. A direct optical transition was found to be the dominant absorption process. The direct bandgap was found to lie in the region 4.1+or-0.1 eV.

The linear electro-optic effect in LiNbO₃ has been investigated in the wavelength range from 0.4 to 1.15 mu m at applied field frequencies of 200 Hz and 9.45 GHz. The dispersion of clamped (r^s₂₂) and unclamped (r₂₂^T) electro-optic coefficients is measured. The values of the clamped electro-optic coefficients (r^s₂₂) are determined. Based on the results of measurements and reported data of the second harmonic generation coefficient, an attempt has been made to estimate the value and dispersion of the electronic and ionic contributions to the clamped electro-optic effect.

Thermoluminescence kinetics is discussed within the framework of a band model containing an arbitrary number of types of recombination and trapping centres at an arbitrary correlation of all centre parameters. It is shown that the initial system of kinetic equations is reduced to an equivalent system consisting of two integro-differential equations which permit one to perform an accurate generalisation, in the case of a continuous centre distribution, to their parameters for the description of irradiation and thermoluminescence, taking into account charge carrier redistribution to both types of centre. In addition, if only one electron (hole) channel is taken into account, only one integro-differential equation is obtained. On the basis of this equation a precise algebraic equation is obtained for calculation of the area of an arbitrary part of the thermoluminescence curve (TLC), consisting of one or several peaks, which slightly overlap with other peaks. It is shown that at doses which are less than the saturation dose, when the centres are not completely filled by the charge carriers, the dose dependences of such a part of the TLC may have a non-linear character at a simultaneous linear dependence of the area of the whole TLC. At doses which are greater than the saturation dose, the dose dependences of the area of the whole TLC, as well as of its separate parts, undergo breaks at the saturation doses.

The usefulness of the film thickness determination method based on the principle of white light interference is tested numerically in a wide range of optical constants and thicknesses of a thin film of interest. Numerous cases occurring in practice are included. The factors which could influence the results obtained by the method are analysed. Some limitations of the method are found resulting from both experimental errors and the factors discussed.

A droplet emission mechanism is proposed for the operation of liquid metal ion sources in the so-called 'droplet mode'. The model is based on experimental observations of current oscillograms as a function of beam angle, as well as emission frequency spectra, and is supported by theoretical arguments. It is suggested that the main droplet emission mode is one where the jet-like apex of the cone-shaped liquid anode is driven unstable by oscillation of the cone. Since jet break-up times are much shorter than the cone oscillation period, the cone oscillation frequency determines the frequency of droplet emission.

In a low-pressure plasma-enhanced chemical vapour deposition of boron from B₂H₆+He, the apparent activation energy for the growth ( Delta E) and the film structure were investigated as a function of substrate temperature (T_s) for plasma powers from 0.4 to 1.4 kW. It was found that Delta E makes a transition from a negative (-2.0 to -8.0 kcal mol^-1) to a positive (4.2 to 20 kcal mol^-1) value with the increased T_s. According to a growth model consisting of surface elementary processes, the Delta E transition is attributed to a transition of the rate-limiting surface process from migration to growth reaction; a temperature-dependent morphological variation also supports this hypothesis.

Tellurium dioxide (TeO₂) thin films grown in amorphous phase up to a substrate temperature of 200 degrees C undergo phase transformation in the polycrystalline state, when annealed, from a mixed phase of orthorhombic and tetragonal structure to pure tetragonal structure as a function of annealing temperature. X-ray diffraction and transmission electron microscopy have been used for elucidation of structural details and scanning electron microscopy has been employed to study surface topography of the films.

A theory has been developed to determine solar cell design parameters in the conventional solar cell equation using measured cell I_sc, I_mp, V_mp and V_oc. The theory overcomes the problem of series resistance becoming negative. It is shown that the diode factor a indeed depends upon cell temperature. The theoretical predictions are in good agreement with experimental observations.

If an electron swarm is subjected to a uniform steady electric field thereby drifting through a gas of moderate density, its intensity increases due to an avalanche effect in the direction of the field. The rise in the number of electrons and positive ions was first ascribed to ionising electron-neutral particle collisions. However, since electrons also excite neutral particles to higher energy states of long lifetimes, a significant population of excited particles is now formed so that low-energy electrons can ionise them 'in stages'. The analysis based on single-plus-stage ionisation is compared with earlier interpretations, showing that the inclusion of a non-linear term is associated with an increase of the spatial swarm population for increasing discharge voltage at constant reduced field E/N. Using only analytical solutions may provide a welcome step towards unambiguously apportioning volume and photoelectric (cathode) effects to the swarm current.

The authors derive a simple analytical solution that accurately describes the evolution in time of breath figures, i.e. the fogging that occurs when a vapour condenses onto a 'cold' surface.

Silver island films prepared by oblique evaporation show optical dichroism in the visible and second-harmonic generation at vertical incidence of the 1.06 mu m fundamental from a Nd YAG laser. Investigation by STM reveals that the observed optical properties can be well explained by the submicroscopic structure of the silver particles. Due to the preparation technique the silver particles are strongly tilted towards the direction of evaporation, thereby losing the rotational symmetry with respect to the surface normal.