Table of contents

The time-dependent ablation process of a metal surface due to laser irradiation is investigated for the case of high Peclet numbers. An analytical solution of the energy balance equation is found by the method of characteristics. The theory is restricted to the geometrical-optical approximation. There exist self-similar solutions which are piecewise parabolic in space. Their amplitude is damped which finally leads to a smoothed quasi-stationary surface profile. An expression for the decay time of this solution is derived which depends on the amplitude and the length of periodicity.

The ionisation alpha /p₀, electron attachment eta /p₀ and effective ionisation alpha /p₀(= alpha /p₀- eta /p₀) coefficients for monosilane (SiH₄) and disilane (Si₂H₆) were measured by the steady-state Townsend method for 60<E/p₀<270 V cm^-1 Torr^-1, where E is the electric field and p₀ is the gas pressure reduced to 0 degrees C. The results show that the values of alpha /p₀ and alpha /p₀ for SiH₄ and Si₂H₆ are smaller than that of CH₄, and the value of alpha /p₀ for Si₂H₆ is about ¹/₃ of that for SiH₄. The value of eta /p₀ is found smaller than about 5*10^-2 cm^-1 Torr^-1 for the E/p₀ values studied here. It appears that the present work is the first to report the value of these parameters. The limiting E/p₀, at which alpha /p₀= eta /p₀, for SiH₄ and Si₂H₆, is determined to be about 65 and 125 V cm^-1 Torr^-1, respectively. The total secondary ionisation coefficient gamma _T for these gases are also measured for E/p₀>150 V cm^-1 Torr^-1 and found to be less than 10^-5.

Proposes a new magnetoresistor based on composite materials. This device exhibits characteristics comparable to that of conventional magnetoresistors but can be realised with much simpler technology.

The hysteresis effect occurring in reactive sputtering is shown by pumping speed analysis to arise from loss in gettering throughput, consequent on reduction in target yield, without consequent rise in pumping throughput. Change in this balance due to higher pumping speed can result in stable conditions.

A p-CuCNS photocathode coated with methyl violet, lightly platinised and then coated with acridine orange was found to be stable in aqueous KI. Photophysics and photochemistry of the mechanism of stabilisation are discussed.

A calibrated six-channel recording system has been used to detect, locate and characterise acoustic emission events associated with the growth of a fatigue crack in a compact tension specimen of 7010 aluminium alloy. The 3D force dipole representation of each event was deduced from the arrival strengths by inversion of the Green tensor. The principal strain components and the orientation relative to the specimen axes were then calculated. It was found that while all events contained dilatational and shear components, approximately 70% were in good agreement with mode I crack opening, while approximately 30% took the form of shear-like (slip) events. The significance of errors in the measurement and inversion procedures is discussed.

A laser pulse incident on a metal plate produces, by the thermoelastic process, an inward displacement at the epicentre, except for an initial outward-displacing spike. This initial displacement is not predicted by a point source approximation for this source of elastic waves. It is shown that the finite size of the source resulting from thermal diffusion into the bulk of the plate is a major cause of this initial spike in the epicentral waveform. As the rise time of the incident pulse is increased, the duration of the spike increases, buts its amplitude decreases.

A lattice type periodic model formed by finite-spaced point spherical interacting particles, as developed by Pande et al. (1984), is used for the estimation of effective thermal conductivity of loose granular materials, under the effective continuous medium approximation. Calculated values of effective thermal conductivity of these materials have been compared with experimental results. The results are in good agreement. The experimental methods used for the measurements of effective thermal conductivity of these materials are the Transient Hop Strip and Thermal Probe methods.

The thermal creep velocity and the thermophoretic force acting on prolate and oblate spheroids have been obtained. The results, which are of considerable interest in calculating the deposition rate of aerosols on cold surfaces, show that deviation from sphericity can have significant effects on thermophoretic behaviour and should be included in any quantitative study.

Using a finite difference formulation of the Navier-Stokes equation coupled with a mathematical description of electrical charge migration. The authors examined transient electrohydrodynamic (EHD) phenomena in an electrically stressed liquid duct. Perturbations resulting from electrode injection are studied as a function of time by examining the liquid vorticity vector and stream function. The response of local pressure to advancing charge fronts is evaluated. The theoretical simulation is supported by previous laboratory studies of EHD motion. Experimentally recorded flow patterns are compared with the predicted results.

The authors examine the conversion efficiency from electrical energy to fluid kinetic energy. They show that an increase of fluid velocity leads to a higher conversion efficiency. The case of a corona discharge from a moving point in ambient air is investigated experimentally.

The total rate coefficients used in a simplified collisional radiative (SCR) model are calculated for thermal nitrogen plasmas at atmospheric pressure. Most of them cannot be considered as physical quantities depending on only a few parameters such as the electron temperature but are functions of the actual state of the plasma. The variations of these coefficients under the influence of radiative losses, the departure between electron and heavy-particle temperatures and the diffusion processes are studied. The use of these coefficients in a SCR model is discussed.

A numerical analysis of the cathode double sheath-plasma interaction in low-pressure low-current argon discharges with a thermionic cathode was made using a supposition of a presheath which joins the collisionless space-charge sheath with the plasma.

Plasma production is studied with a planar magnetron device using permanent magnets in a DC discharge for thin film preparation. The minimum value of discharge threshold voltages is found to lie in a wide region of pressure*gap length in comparison to that in the normal discharge without the magnets, resulting in a more stable discharge. Strongly inhomogeneous ion density in the radial direction is observed in a region close to the magnetics under the cathode to form a visible ring. Primary electrons and high electron temperatures are observed around the cathode, while the ion temperature is spatially homogeneous. The drift velocity caused by the effect of the electric and magnetic fields is directly measured to keep the value roughly constant in the longitudinal direction. A typical thin film of SnO₂ prepared by the plasma CVD technique in this device is presented.

Both active and passive spectroscopic techniques have been used to investigate the excited state populations in a multiple hydrogen plasma device typical of those under development for negative ion production. Laser fluorescence measurements have been used to obtain the hydrogen n=2 level population directly, and a simple model of the relevant atomic and molecular processes has been used to estimate from this the total atomic density. The density obtained is sufficiently high that lowering of the obtainable H^- density by atomic processes cannot be ruled out. Measurements of emitted visible line radiation have demonstrated the existence of a strong atomic spectrum originating from close to the extraction aperture of the source. This may provide a spectroscopic method of measuring the H^- density.

Spectroscopic measurements have been made to compare the structure of a gas-tungsten arc burning on a molten anode with one burning on a water-cooled anode. Metal vapour emanating from the molten-anode pool can be detected in the arc column near the anode, with concentrations as high as 2500 parts per million. However, it is found that the presence of the metal vapour does not significantly influence the temperature distribution in the arc.

The primary ionisation coefficient alpha of Kr and Xe has been evaluated from measurements of the spatial growth of ionisation currents between plane-parallel gold electrodes for the respective ranges 40<E/n<250 Td and 60<E/n<400 Td. Deviations from Kruithof's values are found, especially at the lower of range E/n. In the same gases the secondary ionisation coefficient gamma has been determined from measurements of the breakdown voltage. It has been clearly established that gamma depends on the electrode separation and a qualitative explanation is proposed.

The steady-state electron swarm parameters in Ar/Ne mixtures are analysed using a Boltzmann equation in which the influence of generation of secondary electrons through Penning ionisation in a discharge is taken properly into consideration. The ratios of the Townsend first ionisation coefficient to the electric field strength, alpha /E, calculated using the present equation for E/N values from 1.7 to 565 Td in neon with various admixtures of argon (ratio of the Ar to total concentrations from 10^-6 to 10^-1) agree well with the experimental values of Kruithof and Penning. It is found that generation of Penning electrons in an electron swarm lowers the mean electron energy appreciably, but influences the drift velocity very little. The values and shape of the effective excitation cross section of the metastable neon, which plays an important role in the determination of the amount of Penning electrons, are discussed.

The dependence on illumination intensity up to 10⁹ W m^-2 of photoconductivity (PC) and photoluminescence (PL) has been studied in ZnIn₂S₄ for two different photon energies (2.07 eV and 2.5 eV; both less than the band gap=2.8 eV). The difference in the PC and PL behaviour for the two photon energies can be attributed to the availability of nonionised acceptors for donor-acceptor (D-A) recombination. The D-A route begins to saturate for excitation with 2.50 eV photons at intensities >or=0.5-20 MW m^-2.

For pt.I see ibid., vol.19, p.1299 (1986). Electrochemical studies (Mott-Schottky plot and J-V characteristics) of an electrodeposited CuInSe₂ film in contact with aqueous S^2-/S₂^2- redox electrolyte have been carried out. The material is n-type and the charge transfer at the interface occurs via the valence band involving the surface states and deep traps also. The values of V_fb, N_D, W, E_c and E_v have been evaluated. Photo-electrochemical solar cells have been fabricated using the above junction and studied for films with different stoichiometry.

The spectral dependences of PbSe optical constants have been obtained by Kramers-Kronig analysis of the reflectance data in the energy range 1 to 4 eV. The thermoreflectance spectra of PbSe films, grown by two different techniques-hot wall epitaxy and laser-assisted deposition, have been investigated in the same energy range at temperatures 40, 80, 100, 200 and 300K. As a result of the quantitative interpretation of these spectra a detailed assignment of the energy levels has been obtained. The simultaneous investigation of the films grown by two different techniques has enabled their optical properties to be compared and some conclusions concerning the laser deposition technique to be drawn.

For pt.I see ibid., vol.19, p.1771 (1986). Reflectance and thermoreflectance spectra of Pb_1-xCd_xSe (x=0.01 and 0.02) have been obtained at different temperatures in the energy range 1.4 to 4 eV. The spectral dependences of the optical constants have been obtained by Kramers-Kronig analysis of the reflectance data. The change of the inter-band transition energies on the cadmium content x have been obtained as a result of the quantitative interpretation of the thermoreflectance spectra. The analysis of the results has led to the conclusion that the cadmium atoms replace the Pb atoms in both octahedral and tetrahedral interstices of the selenium sublattice.

Current density-temperature measurements performed at constant applied voltage have been made on evaporated copper phthalocyanine thin films, with chamber pressure to deposition rate ratio P/R varying in the range 2.6*10⁴-1.3*10⁹ Pa m^-1s. In common with previous measurements these results were interpreted in terms of a space-charge limited mechanism dominated by an exponential distribution of trapping levels. There was a systematic decrease in mobility from 10^-6 to 10^-8 m² V^-1 s^-1 and an increase in trapping concentration from 5*10²² to 6*10²⁴ m^-3 as P/R was increased in the stated range, but the temperature parameter characterising the trapping distribution remained constant at approximately 750K. The variations in mobility and in trapping concentration were ascribed to the scattering and trapping effects of incorporated ambient gas molecules, thus indicating that these parameters are particularly sensitive to deposition conditions.

The thermoluminescence (TL) of LiF (TLD-100) has been studied over the temperature range 90 to 300K. A total of nine glow peaks are observed in this temperature interval. The most intense peak is the one at 137K. Besides this, three other peaks at 149, 254 and 275K are well resolved and their growth with exposure could be followed. No supralinearity could be found in the response of any of these peaks. Similarly, a sensitisation effect is also not observed for any of the glow peaks. Emission spectra were recorded at several temperatures and found to be changing. Up to about 150K, three bands at approximately 250, approximately 305 and approximately 420 nm were observed. The 250 nm band then gradually disappeared and the 420 nm band gained in prominence. At temperatures above about 200K, the emission spectrum contained only the 420 nm band. Phototransferred TL (PTTL) could be obtained in samples irradiated at 90K, warmed up to 300K and then subjected to 254 nm light. The PTTL glow curve showed the repopulation of only two peaks, namely at 149 and at 275K. The emission spectrum of the 149K PTTL peak consists of two bands at approximately 315 and approximately 440 nm and that of the 275K peak only one band at approximately 450 nm. These observations show that the simultaneous recombination of modified V_K mobile holes at two different sites is indicated and their recombination should result in 305/315 nm and 420/440 nm emissions. Optical repopulation studies further show that the two strongest low-temperature peaks have a distinctly different behaviour and may be associated with holes.

Electrical transport and optical properties of magnetron sputtered MoS₂ films have been investigated as a function of process conditions. The results for the as-sputtered films and the influence of thermal annealing have been discussed in relation to the specific film morphologies and their inherent grain boundaries. Electrical resistivity measurements (300-10K) have shown that the substrate temperature (T_s) is the dominant sputtering parameter. At room temperature the resistivity varied between 3.8*10^-2 Omega cm (T_s=-70 degrees C) and 10.1 Omega cm (T_s=150 degrees C). Hall effect measurements indicated very low Hall mobilities (1-5 cm² V^-1 s^-1 at 297K). A check of the thermoelectric power response showed that for both the as-sputtered and annealed films the majority charge carriers are electrons. From optical absorption measurements an energy gap of 1.17 eV has been determined. In contrast to the electrical transport results, the optical properties were weakly influenced by process conditions.