Table of contents

By driving the arc spots over a slotted cathode a new method for determining the spot current density has been tested. From the measured values of spot velocity and current rise rate behind the slit, current densities of the order 10¹² A m^-2 are derived. This confirms estimates from crater studies.

A SiO₂ layer deposited by RF sputtering at room temperature has been employed as an encapsulant for the diffusion of tin from spin-on sources. This encapsulant prevents cracking problems of SiO₂ layers, removes the requirement for thermal ramping before diffusion, and thus increases the process reliability.

A simple theoretical expression is introduced to account for the contribution, to the diffusion process, of double layer interactions between the particles of a colloidal dispersion. This double layer contribution is found to be comparable with that derived from thermal interactions.

Stearic acid, stearylamine, stearyl alcohol and stearic acid methyl ester were deposited on n-type gallium phosphide. The Schottky barrier height was observed to increase by 0.4-0.5 eV depending on the compound. This was interpreted as being due to an interfacial dipole layer at the semiconductor-insulator interface. The dipole layer was due to the intrinsic dipole of the organic headgroups.

Torsion in a ribbon geometry produces longitudinal stresses varying across the ribbon width, giving rise to significant second-order elastic, plastic and magnetoelastic effects. An overall agreement between the experimental curves for torque-torsion angle and inverse Wiedemann effect and theoretical predictions has been found for both as-quenched and for torsionally stress-annealed ribbons. Annealed ribbons exhibit tremendous elastic and magnetic anisotropies which can be easily controlled by varying the dimensions of the cross-section as well as the stress annealing conditions.

A radiative bound exciton transition at 1.36 eV has been observed in the low temperature cathodoluminescence spectrum emitted from InP grown epitaxially by both molecular beam epitaxy (MBE) and by metal-organic chemical vapour deposition (MOCVD) under certain specific growth conditions. In both growth techniques the conditions give rise to a depletion of phosphorus in the epilayers and it is suggested that the defect binding the excitons may be a complex incorporating a phosphorus vacancy.

A study has been made of the relationship between the facetting and surface structure observed upon (111)B and (100)GaAs photocathode surfaces and the mean transverse emission energy of the photoelectrons emitted from these cathodes. It is concluded that the mean transverse emission is largely independent of the degree of facetting and the presence of surface structure; however, a relationship exists between the temperature at which the cathode surface is cleaned and the mean transverse emission energy.

Al_1-xIn_xAs layers have been grown on (100) InP substrates by molecular beam epitaxy (MBE). The substrate temperature region, T>580 degrees C, where In loss is known to occur has been explored. It is demonstrated that the loss of indium can be suppressed by increasing the As₄ overpressure during growth. However, the layers grown in this region have high unintentional impurity levels. These are believed to be due to chalcogen impurities originating in the arsenic source.

Measurements of the relaxation of the magnetisation work have been performed in amorphous Fe₄₀Ni₄₀P₁₄B₆ ribbons, both as-quenched and after 4% thickness reduction by cold rolling. The results are analysed in the framework of an activation energy spectrum for the relaxation. The two energy zones present in the as-quenched sample are broadened by the cold rolling and, as a consequence of this, an increase in the number of low energy processes available for relaxation is produced.

The maximum entropy and Backus-Gilbert approaches to data analysis have been adapted and applied to superparamagnetic measurements to obtain particle size distributions together with realistic estimates of their significance. The particle size distributions of three alumina supported nickel catalysts are presented as an illustration.

Certain improvements can be made to the analytical method presented by Martinelli and Morini (1983) for calculating values and distribution of the magnetic field in superconducting DC magnets. The Fourier method can be modified to include constant, uniform permeability axial iron to a far greater accuracy for the case of finite permeability and an exact solution can be obtained for infinite permeability.

Previous work (1983) which involved finding expressions for the vector potential A for a thick solenoid is extended to include iron. This involves an integral representation (as opposed to a differential formulation) together with the use of a Green's function expansion in cylindrical polar coordinates. The expressions obtained agree with those obtained previously using a Fourier series approach.

The Boltzmann transport equation has been developed for conditions typical of electrical discharges as found in CO₂ lasers excited by a rapidly time-varying field. This has then been solved numerically using cross-section data available in the literature. Curves are presented of electron energy distributions, electron drift velocities, diffusion coefficients mobilities and molecular excitation rates for E/N values ranging from 2.0*10^-16 V cm² to 1.0*10^-15 V cm². It is demonstrated that the electron energy distribution over the frequency range relevant to RE discharge excited CO₂ lasers (i.e. approximately 20 to 400 MHz) is almost identical to that for DC excitation at the same value of E/N. Factors influencing the onset of frequency effects are examined and their influence on the electron energy distribution discussed. It is also shown how the values of frequency at which the electron energy distribution becomes significantly altered can be estimated.

A sensitive technique for the determination of optical gain in a DC discharge is outlined. This technique has permitted an investigation of the radial distribution of optical gain at 633 nm in a 3 mm bore discharge with a spatial resolution of around 120 mu m. Results are presented which illustrate the behaviour of this distribution with gas pressure and current.

The scattering of radiation by aerosol particles can lead to significant anisotropy and this influences the radiation heat transfer through the material. A model scattering function involving backward, isotropic and forward scattering has been developed which enables the equation of radiative transfer to be reduced to an equivalent isotropic form with suitably scaled coefficients. An integral equation is formed for the temperature of the medium and, by means of a variational functional, accurate values are obtained for the heat flux across a parallel plane aerosol of finite thickness as a function of the emissivities of the surfaces and the scattering properties of the medium. General conclusions indicate that in the range of back-scattering -1<or=cos theta <0 differences of up to 50% compared with the isotropic case can arise. In the forward scattering region, typical of large aerosol particles, there can arise order of magnitude corrections which are particularly severe when the ratio omega =K_s/K is of order unity. It is also concluded that the 'transport approximation', in which the unit of length is multiplied by the factor (1- omega cos theta ), leads to an acceptable description of anisotropic scattering over the complete range of cos theta .

The problem considered is that of two rigid surfaces approaching each other with small brittle particles between. The loads required for spherical indenters to penetrate layers of quartz particles and glass spheres (ballotini) have been determined. An electrical resistance technique gave the instant of contact. It is shown that small indenters can penetrate between particles, but larger indenters can only break through the layers by fracturing particles. As analysis is given for an indenter approaching a spherical particle on a flat surface, and the conditions for trapping and sliding of the particle evaluated. The analysis successfully explains the main features of the experimental observations with layers of particles. High speed photographs of a two-dimensional model further confirm the proposed mechanism and conditions for penetration. The relevance of this work to various practical contact and wear situations is discussed.

Presents results of measurements of the electron energy distribution function (EEDF) in the range 0-40 eV and the electron density for the helium plasma of a high-voltage discharge (HVD) in a hollow cathode as used for He-inert gas and He-metal lasers. The EEDF has been measured employing the method of the second derivative of the electric probe current-voltage characteristic. The results show that the shape of the EEDF for the HVD in the hollow cathode is similar to that for the conventional transverse hollow-cathode discharge (HCD) operating at low voltage. However, the increase of the operating voltage in the hollow cathode results in energetic redistribution of electrons and a selective change of the efficiency of atomic processes. It appears then that the relative number of electrons within the energy interval from several eV to 20 eV is lower in the HVD than in the conventional HCD. This is believed to be the result of the lower efficiency of the excitation of metastable states of helium in the HVD. Above the energy of 35 eV the relative number of electrons in the HVD is higher than that in the conventional HCD. Thus ionisation seems to be one of the most favoured atomic processes in the HVD.

The pressure dependence of the intensity of 1.06 mu m radiation required to produce breakdown in Ar and Xe at pressures between 10 and 760 Torr has been studied using single approximately 10 ps Nd³⁺ laser pulses. A photomultiplier was used to detect the onset of breakdown and a weak pressure dependence was found, indicative of multiphoton ionisation. Earlier studies exhibiting a strong pressure dependence are interpreted in terms of expansion of the focal volume with increasing intensity.

In a vacuum spark discharge the electron temperatures of the microplasmas produced from the electrode materials Al, Ti, Fe, Mo and W, have been measured. The values were derived from the X-ray continuum transmitted through different absorber foils. For most discharges two temperature groups were identified, each following a different scaling relation.

The process of image recording by laser-induced oxidation of thin metal films has been mathematically modelled. A numerical simulation of the recording process has been carried out by a computer. In this experiment the optimum conditions for distortionless thermochemical image recording have been established. It has been shown that the nano-second laser operation regime is the most suitable one for recording of complex images at high resolution.

Polycrystalline In-doped CdS thin films have been deposited at 160, 220 and 250 degrees C by simultaneous evaporation of CdS and In from two crucibles. Films with In concentrations, N_In, in the 10¹⁷-10²¹ cm^-3 range have been characterised by Hall measurements. The conductivity, mobility and carrier concentration dependences on In concentration are reported. For the lower In concentrations, the conductivity is essentially determined by the deposition temperature. At N_In=8*10¹⁸ cm^-3 the conductivity presents a minimum, which is caused by a minimum in the mobility. The simultaneous rise of the carrier concentration is not sufficient to compensate the decrease of the mobility, contrary to the behaviour of polycrystalline silicon, where no conductivity minimum appears.

Using an automatic network analyser, broadband dielectric measurements in the frequency range 0.3-16 GHz have been made on various hydrated protein powders: haemoglobin, casein, bovine albumin and bovine tendon collagen. The relaxation spectra revealed show evidence of more than one state of multilayer water when the temperature is raised from 20 degrees C to about 80 degrees C and when the water content is high. Some discussion is presented of the relationship between the dielectric spectra observed and the sorption characteristics of these powders.

CaSO₄:Dy irradiated to doses above 10² Gy, annealed partially, and exposed to UV, exhibits strong photo-transfer thermoluminescence (PTTL) due to transfer of charge carriers from filled deep traps to shallower traps. The PTTL below 280 degrees C is up to two times the reduction in the residual thermoluminescence. The UV irradiation does not appear to produce the free charge carriers from the filled deep traps for the transfer.

Natural impurity layers on Ta, W and Au surfaces have been shown to consist essentially of H, C and O atoms. If they are removed locally by a laser prepulse, recontamination occurs within a time scale of several seconds until saturation is reached. Under the high vacuum conditions used, it is found that recontamination is caused by the surface impurity atoms and that the influence of the residual gas can be neglected.

Semitransparent GaAs films bonded on to glass substrates with a Ga_xAl_1-xAs strain-matching layer have been activated to give negative electron affinity (NEA) characteristics in a commercial UHV apparatus. Quantitative surface analysis of the photocathodes before and after activation show a Cs-O overlayer with a well defined Cs₂O stoichiometry and a thickness of approximately 1.3 nm. Measurements of photoelectron energy distribution curves (EDC) from the cathodes confirm the NEA mode of operation, giving very narrow almost wavelength-independent 'thermalised' electron distributions throughout the photosensitivity range. Measurements of the response times of different GaAs NEA cathodes using a special UHV demountable Synchroscan streak camera gave response times as low as approximately 8 ps, suggesting the feasibility of a medium-high temporal resolution streak camera sensitive in the technologically important 1.3-1.55 mu m spectral region.

In an RF-discharge methane plasma, the dissociation rate of CH₄ in collisions with electrons has been measured using a laser absorption technique. The measured rate increases as the RF power increases and as the gas pressure decreases. The electron energy distribution function f( epsilon ) and the electron density n_e have also been measured by a heated electrical probe. The measured f( epsilon ) is quite different from the Maxwellian distribution and rather close to the Druyvesteynian distribution. The dissociation rate constant deduced from f( epsilon ) and the reported cross-section when multiplied by the measured n_e is consistent with the results of the laser absorption method. Based on these measurements a modelling of the plasma has been performed using a set of rate equations for CH₄ and the various neutral and ionic species produced in the plasma. The calculated degree of dissociation for CH₄ in the plasma agrees well with the measured results. The most abundant neutral radical in the plasma predicted by the model is CH₃, while CH₅⁺ is the most abundant ionic species.

The properties of optical waveguides are utilised in the construction of an optical filter which is then used in a high temperature solar energy conversion device, in which the loss of energy due to re-radiation into the atmosphere is adjusted to maximise the efficiency. Such a device, without any arrangement for concentrating solar radiation into a small area, promises to give an efficiency of about 30%. By using concentrators greater efficiencies can be attained.

Increasing interest in the optics of the sea makes it important to be able to measure efficiently the intrinsic optical properties of sea water. In situ measurements are accurate but slow, costly, and difficult in adverse weather conditions. Passive remote sensing techniques measure only one optical parameter whereas two are needed for adequate characterisation of the water. This paper describes a technique for independent measurement of the two basic intrinsic optical properties of sea water: the absorption and scattering coefficients. The technique uses an airborne system that transmits a laser pulse down to the water and records the signal backscattered upwards from particles in the water. Experimental results demonstrating the feasibility of the system are reported. Comparison with in situ measurements confirm the validity of the technique.