Table of contents

This paper reviews recent developments in highly magnetostrictive terbium-dysprosium-iron alloys. This includes discussion of domain structure, the magnetization process, microstructure, sample preparation, magnetostriction, effects of different chemical compositions on magnetic properties of the alloys, and possible applications of these materials. It has been found over recent years that there has been a steady improvement in material properties, both in terms of increased strain amplitude and reduction in variability from sample to sample. The material is now finding applications in magnetostrictive transducers, actuators and adaptive vibration control systems.

The light scattering behaviour of granular silica aerogels is measured at a fixed wavelength of 633 nm. Two different correlation lengths a_c,bulk and a_c,sur, which characterize the bulk and surface scattering properties, are derived. A value of a_c,bulk,=(45+or-5) nm is typical for the bulk scattering, whereas surface imperfections, such as dust grains and scratches as well as bubbles and streaks, cause a strongly forward-peaked scattering distribution with a_c,sur=(600+or-50) nm. The spectral scattering coefficient in the visible region is also measured. By modelling the data, the contributions of large and small inhomogeneities to the scattering coefficient S can be separated. Typical values are S_bulk=(65+or-10) m^-1 at a wavelength of 633 nm and S_bulk=(115+or-20) m^-1 at 543 nm. For the large inhomogeneities a wavelength-independent scattering coefficient of S_sur=(100+or-30) m^-1 is obtained. We have also measured the directional-hemispherical transmission t_dh of a loose granular fill as a function of granule size. The influence of the aerogel nanostructure on t_dh is determined from radiation transport calculations. Because of the forward-peaked scattering distribution of the large inhomogeneities, t_dh is predominantly affected by the bulk.

The dynamics of cw near ultraviolet (NUV) laser ablation of polymethyl methacrylate (PMMA) films are investigated by recording the time dependence of the reflected light power. A new phenomenon in cw light ablation of PMMA, which is similar to 'incubation' pulses in excimer laser ablation, is recognized as the 'incubation' period of ablation, when polymer fragments are accumulated up to a critical density followed by a volume explosion. The incubation time is found to decrease when the light intensity in the film and/or the film temperature increases. However, for sufficiently high light intensities the incubation time depends only upon the film temperature. A dynamic model of the ablation process during the incubation time is proposed which accounts for the main features of our observations and provides a consistent quantitative description of the mechanism underlying the initial stage of ablation, i.e. before the volume explosion.

Molecular orientation in polystyrene fibres has been studied using polarized Fourier transform infrared spectroscopy. Polymerization of the monomers was carefully controlled to work with the same molecular weight and heterogeneity index. During drawing, the velocity of the fibres was maintained constant and the temperature was varied from 166 to 200 degrees C. Samples were cut from the different fibres. Two polarized spectra, with electric vectors parallel and perpendicular to the drawing direction, were recorded. The molecular orientation function was calculated from the area of the 1028 cm^-1 peaks, which corresponds to the nu _18a in-plane CH bending normal mode of the aromatic ring. Results indicate that the molecular orientation depends strongly upon the drawing conditions. Annealing the fibre at a temperature slightly higher than the glass transition temperature allows one to recover most of the isotropic properties.

The effect of temperature dependent viscosity on the boundary layer flow over a linearly stretching surface is investigated. This solid elastic flat surface remains at a constant temperature higher than ambient, and moves in its own plane at a velocity varying linearly with the distance from the discharge slit. In many liquids, such as water and oils, the variation in viscosity due to temperature change is more dominant than other effects. The authors adopt two approaches to deal with the effect of temperature dependent viscosity by (1) setting a known function describing the mathematical modelling of the viscosity-temperature relation into the governing equations (direct method) and (2) using the perturbation expansion technique for small heat transfer rates (perturbation method). In the direct method the viscosity-temperature relationship follows an exponential function. In the perturbation method a Taylor expansion is used on viscosity, velocity and temperature on a small quantity that is defined as the ratio of temperature differences between the moving surface and the ambient to the ambient temperature. The perturbation expansion method is independent of any choice of-the viscosity-temperature function. Following the similarity transformation, the governing partial differential equations reduce to ordinary equations. The results obtained by the direct method indicate that the velocity and temperature distributions depend upon the Prandtl number as well as the viscosity ratio that implicitly represents the temperature difference between the moving surface and the ambient. The results by the perturbation method can effectively describe the flow and temperature fields with perturbation series truncated after linear terms even though the perturbation quantity is not so small.

Slow motion of a Newtonian fluid past a porous spherical shell has been examined. The flow in the free fluid region (inside the core and outside the shell) is governed by the Navier-Stokes equations whereas the flow in the porous region (shell region) is governed by the Brinkman model. The exact solution has been found under Stokes' approximation. The drag experienced by the shell has been discussed numerically for a range of values of governing parameters. The streamlines for the flow outside of the inner core and around the spherical shell have been depicted graphically and compared with the corresponding streamlines around a non-porous sphere.

The paper presents a set of electroluminescence and other transport parameters calculated using a detailed three-dimensional Monte Carlo method, which simulates the drift of electrons in gaseous xenon (p=760 Torr, T=293 K) under reduced electric fields E/N in the 3 to 16 Td range (E/P approximately 1 to 5 V cm^-1 Torr^-1), which is the region for secondary scintillation production in xenon filled gas proportional scintillation counters. Results are compared with available experimental or theoretical data as well as an earlier one-dimensional Monte Carlo simulation. The calculated parameters are the excitation and scintillation efficiencies and the reduced light yield, together with mean time intervals, mean drift distances and mean number of elastic collisions between xenon excitation collisions. The authors also present electron drift velocities, mobilities and characteristic energies, as well as mean electron energy and electron energy distribution functions.

The author reports on a highly efficient operation of the zirconium cathode in a vortex-stabilized air plasma torch. The measurements show low erosion rates of the zirconium cathode for two methods of holding the zirconium inserts. Reproducible operation of the plasma torch, together with many turn-on-and-off cycles of the arc, is obtained, for 94 A. The author shows that the erosion rate is reduced by more than one order of magnitude as the erosion depth grows. The author presents a possible explanation for this effect. After 50 h of operation of the torch, the erosion depth measured was 3 mm. A formula was obtained that enables one to calculate the specific erosion from the erosion rate. Experimental results of the cathode dissipated power and its connection to cathode erosion are also discussed.

New mathematical algorithms are presented to evaluate the electron velocity distribution function from Langmuir-probe and Thomson scattering measurements. They are based on the Tikhonov regularization method which is applied to the volt-ampere characteristics of the probe and to the spectral intensity of the scattered light. These methods permit the problem to be formulated as regularized integral equations rather than evaluating the electron velocity distribution function by the usual inadequate technique of forming the second or first order derivatives from a sequence of fluctuating experimental points. In addition analytical solutions of such integral equations are found. The undisputable reliability of the Tikhonov regularization method and the simplicity of the described algorithms seem to make it the most appropriate and useful for the evaluation of the electron velocity distribution function from Langmuir-probe and Thomson scattering measurements.

A numerical simulation including kinetics, transport phenomena and temperature gradient is presented to describe axial and radial distribution of ozone in an oxygen-fed ozonizer with cylindrical symmetry. The interdependence of three types of modelling (kinetics, hydrodynamics and electronic injection due to electrical corona discharge) is emphasized and comparisons are made with previous simpler models. The simulation is performed in the case of an industrial cylinder-to-cylinder device for which the temperature gradient is negligible, and in the case of a laboratory wire-to-cylinder ozonizer where an important radial temperature gradient is experimentally observed. Two values of pressure are considered: 760 Torr, where chemical kinetics dominates over diffusion effects, and 10 Torr where diffusion is dominant.

The conservation equations of turbulent gas-blast nozzle arcs based on local thermal equilibrium and on boundary layer assumptions have been solved numerically. A Prandtl mixing length model of turbulence has been adopted because of its simplicity and its success when applied to turbulent rounded jets. The investigation is aimed at an understanding of the arc in a gas-blast circuit-breaker, where the key problem is the arc behaviour during the current-zero period. Emphasis has been placed on quantitative analysis of turbulence-enhanced cooling and its influence on arc behaviour during the current-zero period. The dominant energy transport processes and the critical region for thermal interruption within the nozzle have been identified. The critical rate of rise of recovery voltage as a function of stagnation pressure has been studied and the physical mechanism responsible for this functional relationship discussed. Good agreement between the prediction and experimental measurements has been achieved for the two nozzle geometries investigated.

The inevitable presence of longitudinal end effects occurring in drift-tube mass spectrometers, if not corrected for, can cause significant systematic errors in measurements of ion transport coefficients. In our transverse diffusion measurements of Na+ ions in He and several other ion-neutral pairs, appreciable longitudinal end effects were detected for sufficiently high values of E/N, the ratio of electric field to neutral gas number density. Systematic measurements at various drift lengths and analyses reveal that these end effects are generally more pronounced for ion-neutral pairs with very low or very high mass ratio. They can be either positive or negative and their magnitudes can be quantified by a simple linear relationship of the variance of the transverse ion current density profile with drift length. A systematic study of the Na⁺-He interaction pair, which is known to exhibit such end effects, establishes that it is possible to apply a methodical correction procedure to experimentally obtained values of D_T/K to yield reliable results of an accuracy comparable to those obtained from systems known to be free from such end effects.

A new simulation technique for obtaining the equilibrium electron energy distributions of electron swarms in gases under steady-state Townsend conditions is proposed based on a propagator method. The electron energy distribution function could be obtained from calculations in only one slab in real space, assuming exponential spatial growth of an electron swarm and similarity of the electron energy distribution at any position. The electron energy distributions were analysed in argon at E/N=1414 Td for a case of a positive large value of the effective ionization coefficient alpha and in an argon ((90%)/fluorine (10%) mixture at E/N=56.6, 84.8, 113.1 and 141.1 Td for a case of values of alpha crossing zero. The results agreed excellently with those of a Monte Carlo simulation and a Boltzmann equation analysis. This propagator method was found to deduce stably proper solutions for both positive and negative values of alpha . A relation between the present method for a steady-state Townsend condition and a similar method for a pulsed Townsend condition is discussed.

Calculations have been made of spectral absorptivities of SF₆ plasmas as a function of wavelength from 10 nm to 30 mu m, temperature from 300 to 30000 K and pressure from 1 to 10 atm. Net emission coefficients of radiation have been derived from these absorptivities for cylindrical isothermal plasmas of radii from 0.1 to 2 cm. In addition calculations have been performed as a function of pressure, temperature and radius of the fraction of the emitted radiation that is below the photo-ionization threshold of 130 nm.

The pseudospark discharge in a neon-filled multi-gap chamber was investigated. The rise and decay of internal and external currents were studied. After breakdown, the plasma temperature was determined to be 2.5 eV. Lasing was observed from the Ne(3p(3/2)₂ to 3s(3/2)₂) transition of 614.3 nm. The results were found to be compatible with a model for the pseudospark discharge.

Glow discharge plasma in atomic and molecular gases and their mixtures at low pressure is prone to spontaneous inception of wave instabilities (moving striations). They result from different transport processes between electrons and neutral gases. The predominance of some transport process is reflected in phase and group velocities of a particular wave instability which, in turn, can be used for detecting the internal plasma kinetics. In our case ionization-thermal diffusion waves have been used to monitor the final temporal degradation of a CO₂, N₂ and He mixture in direct current glow discharge.

The random phase approximation method is extended to calculate static electrical conductivity of fully ionized plasmas in the presence of a uniform magnetic field. The components of the corresponding conductivity tenser and other parameters have been obtained for the case of a moderate magnetic field. The results are compared with experimental data available for shock compressed plasmas.

A two-fluid theory of a positive column in a cylindrical geometry having an internal coaxial wall is given. The space charge density throughout the whole plasma and inertia and diffusion controlled conditions are taken into account. An eigenvalue problem with a free boundary is solved. The radial distributions of the number densities and of the drift velocities of the ions and electrons and the profile of the electric potential are calculated. The boundary of the regions of particle collection for the inner and outer walls is exactly determined. The results show that the particle fluxes and the number densities of ions and electrons and, in particular, the floating potential at the inner wall are different from the values at the outer wall. The differences increase with an increasing ratio of the radii of the outer and inner walls. Under these conditions a so called impressed voltage or electromotive force occurs within the plasma which compensates the electric voltage between the inner and the outer wall since no radial electric current flows.

We present the results of crystallization of amorphous thin Ni-Zr alloy films. The alloys were prepared by the sputter-deposition technique over the continuous composition range Ni_92.7Zr_7.3 to Ni_20.7Zr_79.3. The samples were heated in the electron microscope incorporating the microscope heating stage and crystallization was viewed through the diffraction mode. The average grain sizes of the microstructure after cooling to room temperature were measured. It was found that there was no enhancement of the crystallization temperature, T_x, in the region of the eutectics and that the compositional dependence of crystallization temperature bore no relation to features of the equilibrium phase diagram. Comparison of T_x of thin alloys with that of sputtered 5 mu m thick alloys heated in the differential scanning calorimeter yielded inconclusive results from the temperature-thickness relationship. Results of T_x from some melt-quench alloys are also compared with our data.

Copper ion conducting glasses with mixed glass formers and having composition CuI+Cu₂O+MoO₃+B₂O₃ have been studied. The variation of room-temperature conductivity with glass composition has been found to be controlled by the variation in mobility of the mobile species and the number of Cu²⁺ ions present in the glass samples. The temperature variation of conductivity has been found to follow an Arrhenius relation.

The shot noise characteristics of a resonant tunnelling diode are calculated from the model which includes the effects of the effective transmission coefficient of electrons in the conduction band and in the subband of the accumulation layer in the emitter. The transit time is formulated for a double barrier structure. However, the transit time for electrons at the Fermi level and the subband of the emitter are different because of the energy difference. The results show that the shot noise spectrum is independent of the frequency at low and intermediate frequencies and dependent on the frequency at high frequencies.

A polarizing microscope with a Babinet compensator eyepiece has been used to quantify the residual internal mechanical stress in a number of synthetic resin insulators containing an embedded electrode. Markedly different magnitudes and reticulation times of the residual internal mechanical stress are found for epoxy and polyester resins. In combination with a beam bending arrangement the times for electrical tree initiation have been investigated as a function of the mechanical stress at the tip of the embedded electrode. Preliminary results show that initiation is suppressed by a compressive stress along the electrode axis, and enhanced by a tensile stress. It is also shown that a band of zero mechanical stress in the electrical breakdown region of the resin insulator can act as a substantive passivation barrier to electrical tree propagation.

An automatic iterative method by which material constants of piezoelectric ceramics may be determined in complex form is described. Measurements are reported on samples with different shapes: thickness expansion and shear plates, and length expansion bars. With the constants provided by this method, the profiles for the conductance, resistance, susceptance and reactance are calculated, for the same sample, in the frequency range containing resonance and anti-resonance, and very good agreement is obtained with respect to the experimental data.

Optical properties have been studied in amorphous thin films of various Ge-based chalcogenide glasses with a view to investigating the effect of Te isovalence substitution in the Ge-S-Se system. Te either completely replaces S to give Ge_2.5Te_2.5Se₉₅ and Ge₅Te₅Se₉₀ or partially replaces Se to give Ge_2.5S_2.5Te_2.5Se_92.5 and Ge₅S₅Te₅Se₈₅. The changes recorded for the absorption band edge and for the optical gap width are discussed in terms of the different physical properties of Te with respect to S or Se.

The analytical expression for the reflectance of a weakly absorbing wedge shaped thin film supported on a thick transparent substrate has been obtained. The envelopes of reflectance maxima and minima derived from that expression together with known formulae for envelopes of transmission spectrum enable calculation of optical constants and thickness non-uniformity of the film. The calculations have been applied to thin films of amorphous silicon and the results obtained have been compared with those for films considered uniform.

Mossbauer spectroscopy results show that, in 30 nm-grained Fe-Cu-Si-B alloy, boron atoms exhibit two states, 9% of which are as Fe₂B and the remainder boron-enriched species in the interfaces. Also, the solubility of Si atoms in body-centred cubic Fe is calculated to be about 13 at.% and the arrangement of Si atoms in the alpha -Fe(Si) phase is of short-range order. With grain growth the Mossbauer parameters of the Fe₂B phase change slightly. However, the Mossbauer parameters of the alpha -Fe(Si) phase are strongly affected by grain growth, indicating variation of short-range order of silicon atoms in the body-centred cubic Fe phase with grain growth.

In this work surface plasmon resonance studies have been used to characterize the response of a thin film of 18-crown-6 metal-free phthalocyanine upon exposure to NO₂. The phthalocyanine was deposited by spin coating onto a gold film which supported the surface plasmon resonance. By fitting angle dependent reflectivity data to the Fresnel theory for a range of wavelengths it has been possible to obtain the real and imaginary parts of the optical permittivity of the material over the visible part of the spectrum. This procedure has been conducted both before and after exposure to the gas giving an accurate characterization of the changes brought about by the NO₂. Thereby the wavelength at which the system's sensitivity to NO₂ is an optimum has been calculated. As a secondary measure, the optical parameters derived from the surface plasmon resonance experiments have been compared to data obtained by ultraviolet-visible spectroscopy.

In this paper we report on a systematic investigation of the influence of the growth temperature on the current voltage (I-V) characteristics of symmetric AlAs/GaAs double-barrier tunnelling diodes grown by molecular beam epitaxy (MBE) and a correlation between electrical and structural properties. It is shown by high-resolution transmission electron microscopy (HRTEM) that the growth temperature affects the interface roughness and the symmetry of the double-barrier region. Both the microstructure of the interfaces and the Si doping spreading in the double-barrier region have a significant influence on the peak-to-valley ratio (PVR) and the symmetry of the I-V characteristic. The substrate temperature was varied from 480 to 680 degrees C. It is demonstrated that the highest PVRS at room temperature are found for samples grown in the temperature range 580-600 degrees C.

Electronic properties of a-Ge_xC_y:H films plasma-deposited in a new type of audiofrequency reactor with three parallel electrodes have been investigated. It has been found that small changes in coupling capacity in the reactor can cause a drastic step change in the electronic structure of the deposited films. This effect is attributed to a transition from amorphous dielectric to amorphous semiconductor.

The role of oxygen in increasing the carrier concentration of n-type silicon germanium-gallium phosphide alloys was investigated. It was found that silicon germanium-gallium phosphide alloy without additional dopant prepared by mechanical alloying and cold pressing is p-type if heat treated in argon or hydrogen, but is n-type if heat treated in air. The results indicate that carrier enhancement probably relates to the dissociation of gallium phosphide followed by oxidation of the dissociated gallium and phosphorus. A possible mechanism for carrier enhancement in n-type silicon germanium-gallium phosphide alloys is proposed.