Table of contents

The investigation of supported size-selected clusters by different surface science techniques requires stringent ultrahigh vacuum (UHV) conditions during cluster deposition especially at low temperatures. The existing high-energy sputter sources used for cluster production in general do not fulfil this demand. We therefore have developed, built and tested a new fully bakeable ultrahigh vacuum-compatible version of the cluster source CORDIS (Cold Reflex Discharge Ion Source). As a first application, we present ultraviolet photoemission spectra (UPS) of size-selected clusters generated by this new cluster source and deposited on highly oriented pyrolytic graphite (HOPG). We show evidence for cluster diffusion and island formation even down to low temperature (T = 70 K). These islands display a polycrystalline structure. After a short annealing, they are transformed into Ag films with (111) surface orientation.

Polycrystalline film has been grown on fused silica by pulsed laser deposition at . The wavelength dependence of the transmittance of the film was determined. We first obtained the band gap . Favourable optical waveguiding properties have been demonstrated by observation of sharp m lines from the TE and TM multimodes. The surface and cross sectional morphology of the film was studied by scanning electron microscopy (SEM).

Direct current magnetron sputtering and evaporation have been used to form thin films with a columnar structure having a chevron-like morphology. This type of morphology is one of the possible types present in sculptured thin films. The tilt angle of the columnar grains was controlled by variation of the angle of incidence of the depositing species, and by carrying out the deposition at a number of different angles of incidence films having a chevron-like microstructure were formed. Films of copper, aluminium and alumina were produced by the technique described here. It is demonstrated that under certain conditions it is possible to grow films that are virtually free of renucleation at the planes where the atom flux changes direction (kink planes). The morphology of the individual grains in the chevron-like films agrees with the zone model developed for more conventional films. The chevron structures were also quite visible in amorphous films, and in this case the `grain boundaries' consist of regions of lower density. A few examples of potential applications for these chevron films are discussed.

The method of Greenwood and Williamson is extended to give a general solution for the coupled nonlinear problem of steady-state electrical and thermal conduction across an interface between two conductors of dissimilar materials, for both of which the electrical resistivity and thermal conductivity are functions of temperature. The method presented is sufficiently general to cover all combinations of conductor geometry, material properties and boundary values provided that (i) the current enters and leaves the conductor through two equipotential isothermal surfaces, (ii) the remaining boundaries of the conductor are thermally and electrically insulated and (iii) the interface(s) between different materials would be equipotential surfaces in the corresponding linear problem. Under these restrictions, the problem can be decomposed into the solution of a pair of nonlinear algebraic equations involving the boundary values and the material properties, followed by a linear mapping of the resulting one-dimensional solution into the actual conductor geometry. Examples are given involving single and multiple contact areas between dissimilar half spaces.

We demonstrate that a carbide/Ni/Ni-silicide sandwich structure can be formed on a silicon surface using focused electron-beam heating. Auger electron spectroscopy depth profile analyses show that the thickness of the carbide layer is about 3 nm and that a Ni silicide is produced between the Ni and the silicon substrate. Since the temperature caused by focused electron-beam heating is not high enough to transform a whole layer of Ni into Ni silicide, part of the Ni remains between the carbide and silicide layers. The carbide layer which is induced in the electron-beam system contributes to protecting the pattern of Ni film during the etching process.

Magnetic flux-leakage (MFL) measurement is the most widely used non-destructive technique for in-service inspection of oil and gas pipelines. The effect of line-pressure-induced hoop stress on MFL signals has been studied for an electrochemically milled pit (50% penetration) in a 9 mm thick steel pipe wall. The existing theoretical models for flux-leakage signals are discussed. Among them the Zatsepin-Shcherbinin and Edwards-Palmer models have been fitted to the axial and radial MFL signals. Both the leakage-flux components under various stresses are scaled to make them stress independent.

Using a Monte Carlo method we have studied the expansion process of an atomic beam ejected from a liquid metal surface in the presence of relaxation between internal and translational energies. We have also made a comparison of simulations and experimental results for the electron beam vaporization of uranium; reasonable agreement was reached.

An extensive Hurst analysis to quantify long-term statistical dependence is performed on time series associated with sliding of blocks on an inclined plane induced by small perturbations. The analysis reveals the existence of a fluctuation phenomenon exhibiting two phases, depending on the angle of inclination, both obeying a Hurst scaling . The first (second) phase has H = 0.70 (0.50) and refers to large (small) inclinations irrespective of the material of the block. These results indicate that sliding of blocks in the first regime exhibits persistence and indicate that there are long-term memory effects on the storage of shear stress at the block-plane interface. In the second regime, the long-term memory effects disappear and the sliding activity is intermittent; that is, it is distributed in bursts separated by variable periods of stasis.

The fact that electrons are accelerated by intense laser pulses in a vacuum is particularly apparent from recently published work, but up to now no high-beam-quality laser vacuum acceleration scheme has been presented. In this paper, numerical simulations demonstrate that free electrons can be accelerated axially (high beam quality) in a vacuum by using two intersecting ultrashort, high-intensity laser pulses. As the maximum intensities are on the laser axes, a hollow channel is formed between the two laser beams. The electrons will be expelled by the transverse ponderomotive force of each beam to the axis of the symmetry, and can be trapped inside the channel until they are accelerated to relativistic energies. This makes possible axial rather than radial acceleration.

A highly realistic Monte Carlo code is used to simulate radiation trapping including effects due to partial frequency redistribution and with a spectral line profile dominated by natural (radiative) broadening resonance collisional broadening and Doppler broadening. Results from 294 simulations are used to determine the fundamental mode trapped decay rate in cylindrical geometry over a very wide range of parameter space including a factor of in gas density, a factor of 1000 in column radius, and a factor of 1000 in Doppler width. The fundamental mode decay rates are fitted to an analytical formula expressed in terms of three dimensionless parameters which can conveniently be applied to a variety of gases, column radii and Doppler widths.

The plasma response to a picosecond multi-terawatt laser pulse is characterized by several distinctly different time scales. In the present report we consider the plasma's evolution on the slow time scale typical for excitations of the ionic component. A relativistically strong laser pulse (-) is shown to drive, under proper conditions, mega-ampere ion currents with a typical ion energy of 50-150 keV, which covers the region of the resonance peak in the cross section of D-T fusion and, hence, provides an opportunity to initiate nuclear reactions directly in the laser focus.

The analysis of measurements of the overvoltage dependence of time delays to electrical breakdown in nitrogen-filled diodes under various illumination conditions confirms the role of long-lived metastable species. After long periods of afterglow the concentration of molecules significantly influences the breakdown probability as well as the magnitude of the glow-discharge current.

Two existing expressions for the leader stepping time for a positive point-plane gap in gas have different pressure dependences, which requires an examination based on a theoretical consideration. A theoretical explanation based on the authors' experimental results of the pressure dependence of the leader stepping time, which can help one to understand the breakdown process in gas better, is presented.

It has been suggested that optical flashes observed in the upper atmosphere above giant thunderstorms (red sprites) are due to streamers. Such streamers are initiated in the lower ionosphere by electron patches caused by electromagnetic radiation from horizontal intracloud lightning and then develop downward in the static electric field due to the thundercloud. The triggering conditions of streamer development are analysed in the paper. Using similarity relations, known characteristics of streamer tips obtained earlier in laboratory conditions are extended to a description of streamers in rare air. Streamer growth in the nonuniform atmosphere is calculated. It is shown that streamers first appear at a height of about 80 km and then grow downward to slightly below 50 km, where they are terminated. This is in agreement with red sprite observations. An altitude distribution of the streamer generated plasma is obtained. The simple models of streamer development presented in this paper could be applied for computations of streamers growing in various other conditions.

By means of the developed technique the local cross sectional size of the cathode plasma jet of a steady-state vacuum arc is measured. The measurements are carried out within a range of distances from the cathode. The found magnitude has been rather less than the measured cross sectional size of a steady-state integral plasma flow. The effect of a contraction of the profile of the plasma density and a simultaneous expansion of the profile of the electron temperature (which resulted from magnification of the value of ) on distances is found out also. It is possible to explain the observed effects by considering the compression of a cathode jet by an intrinsic magnetic field of a current, which in fact controls the form and the parameters of the plasma of a cathode jet at large distances from the cathode.

Investigations of semiconductor heterostructures by x-ray diffraction are characterized by the absence of any direct depth information in the scattering profile. This paper demonstrates a new method of obtaining depth-dependent information about heterostructures by x-ray diffraction. The distances involved here are limited by the absorption of the x-rays in the material. The absorption depth can be varied continuously for asymmetric reflections by rotating the sample around the scattering vector. Two different methods for this rotation as well as experimental set-ups are discussed. We present experimental results that conclusively demonstrate the usefulness of depth-dependent measurements. The data can be used directly for the improvement of semiconductor devices.

The problem of how contact between elastic spheres is modified by the surface forces which act between them is becoming of great interest with the possibility of studying such contacts experimentally in the atomic force microscope and in the surface force apparatus. Experiments are frequently interpreted with the help of the JKR adhesion theory, in which the action of the surface forces is represented by the change in surface energy involved in uniting or separating the two surfaces. This has two drawbacks: it is strictly valid only when the Tabor parameter is large, though in fact it appears that the condition is sufficient, and it gives no help with the attractive forces before contact takes place, which may cause `jumping on'. However, it has one enormous virtue: the results are described by simple algebraic equations, which all can manipulate as required. In contrast, full numerical analyses, such as the pioneering solution by Muller, Derjaguin and Toporov and a more recent solution by Greenwood, give only the results the authors choose to pass on. These establish when the JKR solution is valid and indicate the ways in which it is incomplete; but stop there. For this reason, an analytical solution by Maugis using a simplified force-separation law, that the adhesive stress has a fixed value whenever the separation is less than a critical distance, has proved most valuable. However, it too has a drawback, though a minor one: the shape of the gap involves elliptical integrals, which is inconvenient when the extension to visco-elastic spheres is considered. Here we present an alternative to the Maugis theory, based on the combination of two Hertzian pressure distributions. The results are very close to those of the Maugis model, but the gap shape now involves only elementary functions.

In this paper a theoretical analysis of the product and the detectivity in a GaInAsSb infrared photovoltaic detector is reported, dependent on the four fundamental kinds of noise mechanism and the quantum efficiency. The considerations are carried out for near room temperature and m wavelength. The analytical results show that the noise mechanisms can be reduced, and correspondingly the performance of such detectors can be improved.

We describe a three-dimensional grid electrode system, in which a biological cell can be precisely moved or positioned by positive and negative dielectrophoresis. The electrode system consists of two glass plates, on which parallel strip electrodes are fabricated, placed together with a spacer between them so that their electrodes face each other and cross at right angles to form the grid. The microelectrodes of width and spacing have been fabricated using two different materials and methods. For one method, electrodes of thin gold-on-chrome film on a glass substrate were fabricated using photolithography, whilst the other method employed excimer laser ablation of indium tin oxide (ITO) thin films on glass. The ITO electrodes have the advantage over conventional metal electrodes of higher optical transparency, which allows visual observation of cells' behaviour in three dimensions. It has been demonstrated that a plant protoplast, whose diameter was almost identical to the electrode's size, could be continuously moved between grid intersections by controlling the magnitude and frequency of the ac signals applied to the electrodes.

A simple condition for metallurgical instability, useful in the development of processing maps for analysing high-temperature forming of metals, is suggested following a criterion based on continuum principles as applied to large plastic flow proposed by Ziegler. It can be used for any type of flow stress versus strain rate curve. This criterion has been validated using the flow stress data of a 6061 Al-10 vol% metal matrix composite with microstructural observations. Optimum hot working conditions based on the instability map are suggested for this material.

This paper considers the design of electrorheological (ER) particles. Multi-coated particles suspended in insulating (very weakly conducting) oil are recommended for obtaining high-performance ER suspensions. Only the outer two coatings determine the ER strength. The outermost coating should be a material with high dielectric constant, high electrical breakdown strength and a reasonable level of conductivity. The coating immediately below should be a highly conducting material. The inner coatings, including the core (which can be void), can be of any material having such a density that the composite particle has substantially the same density as the host liquid. Our analysis gives that multi-coated particles can have an ER shear strength as high as 29 kPa when the volume fraction of particles is 0.4 and the applied field is 5 kV . Results in the literature provide support for the concept and analysis.

An endoreversible cycle model of a multi-temperature-level absorption heat transformer is set up and used to analyse the performance of the heat transformer affected by the irreversibility of finite-rate heat transfer. The key performance parameters, such as the coefficient of performance, specific heating load, temperatures of the working fluid in the heat exchangers, heat-transfer areas of the heat exchangers and so on, are optimized. Some new results which are conducive to the optimal design and operation of real heat transformer systems are obtained and several special cases are discussed in detail. Moreover, the endoreversible cycle model is generalized to become an irreversible cycle model. The important results describing the optimal performance of a multi-temperature-level absorption heat transformer affected simultaneously by the internal and external irreversibilities can be obtained simply and conveniently from the corresponding formulae of the endoreversible cycle model because of the introduction both of the equivalent temperatures and of the equivalent overall heat-transfer coefficients.

Li et al published recently a paper presenting a method for the interpretation of x-ray rocking curves to obtain crystal lattice-strain profiles in GaAs superstructures. The method is described as a phase-retrieval technique in x-ray scattering. However, the paper contains a number of questionable theoretical statements which are discussed in this comment.

This is a reply to Nikuli's comment on our paper ( 30 3296 (1997)).