Table of contents

A method of acoustic imaging is introduced by which the two-dimensional distributions of the inhomogeneous density and compressibility of a material are found from measurements of the scattered field. The method uses a pulsed source to generate data instead of a continuous wave source which is normally used in diffraction tomography. The use of a pulse source relates the method to conventional acoustic imaging systems. The reconstruction algorithm is based upon a diffraction slice theorem having similar characteristics to the well known projection slice theorem. The algorithm is easier to implement numerically than a reconstruction algorithm based upon the use of CW sources.

For pt.I see ibid., vol.20, p.7 (1987). A method for reconstructing two-dimensional variations in the inhomogeneous density and elastic Lame parameters characterising an elastic material is presented. The directivity pattern of the intrinsic scatter generating parameters is derived enabling the material inhomogeneities to be uniquely determined by three separate experiments. The method is illustrated by performing reconstructions on synthetic data sets, and its applications to non-destructive evaluation of materials is discussed.

To satisfy fundamental thermodynamic requirements and in contrast to the conventional mathematical procedure, a small finite molecular sample is considered as the essential basis for a theoretical structure in gas dynamics. With molecular flux gradients present the conventional particle velocity used in the Euler-Lagrange relationship is then necessarily modified. Further corrections appear in the momentum and energy conservation equations; and it follows that the entropy increment for conventional isentropic conditions is not zero. Although the procedures are developed, with a number of simplifying approximations, for conventional adiabatic non-viscous, one-dimensional flow of a perfect gas at moderate Mach numbers, more complex conditions may be included, and do not fundamentally affect the principles involved. Quantitatively the corrections are significant when gradients are severe. For unsteady flow in a duct it is indicated, contrary to existing theory, that multi-values of the variables do not arise as a pressure wave progresses; and that attenuation of the peak amplitude does occur. It is suggested that the proposed structure provides an improved physical basis for the appraisal of phenomena in gas dynamics.

The Bitter colloid technique has been used in an optical study of field-induced domain changes in polycrystalline Tb_0.27Dy_0.73Fe₂ (Terfenol). Stripe-like domains were observed to form in the bias-field region where the magnetomechanical coupling coefficient of the material is known to maximise and a tentative interpretation of these domain patterns is given. A new colloid technique has been developed for studying domains using scanning electron microscopy and micrographs are presented which show some fine domain details which have not previously been observed.

The rate constants for the collisional decomposition of SiH₄ by photoexcited Hg(6³P₁), k_d, and for the quenching of Hg(6³P₁) by Si₄, k_q, have been measured using spectroscopic techniques. The obtained values for k_d and k_q at a gas temperature of 320 K are (1.7+or-0.5)*10^-10 cm³ s^-1 and (2.1+or-0.5)*10^-10 cm³ s^-1, respectively. It can be inferred from this result that most of the quenching collisions of SiH₄ with Hg(6³P₁) lead to the decomposition of SiH₄. The contribution of the secondary reactions such as the hydrogen abstraction reaction of H with SiH₄ to the decomposition of SiH₄ has also been investigated and it is concluded that the effect is small under the author's experimental conditions.

Discusses the interaction of conditions in the liquid metal surrounding the keyhole which is formed when a laser is used as the source of power for welding, with conditions in the vapor itself. The transfer of power and matter across the interface is considered, and a simple model set up for the energy interchange and vapor flow in the keyhole itself. The principal processes are identified. The model is then used to calculate keyhole shapes, and the variation with depth of the related quantities is found.

The discharge process is experimentally studied from measurements of the temporal evolution of potential formation when a pulsed electron beam is injected into a DC field in a low-pressure argon gas ((3-5)*10^-4 Torr). The V-shaped axial potential profile is found to be formed instantaneously at the moment of beam injection and then collapses at the start of the discharge development. The effective depth of the potential minimum exceeds the ionisation voltage at this moment, resulting in the acceleration of beam electrons with energy enough to ionise the gas. The measured two-dimensional structure of the potential profile reveals that the potential minimum forms a negative potential well in front of the beam source on the central axis.

Examines theoretically two different effects which are sometimes encountered in double probe measurements under continuum large-Debye-length conditions: (i) electron emission due to the de-excitation of metastable atoms and molecules on the probe surface, and (ii) asymmetric double probe characteristics that result from number-density gradients on the plasma. Electron emission is shown to have little effect on the probe characteristics. A method of analysing asymmetric double probe characteristics is discussed.

A theoretical study is made of the development of a gas discharge in which two ionisation processes are present: primary ionisation due to electron impact in the gas and secondary ionisation due to photo-electric emission at the cathode, with negligible delay. The discharge is allowed to develop continuously in time, in contrast to previous work in which the time was considered to progress in discrete steps. Equations are derived which give the probability of finding any number of electrons in the discharge at a given time. These are solved for the particular problem of finding the probability that the discharge should have become extinguished at any time. It is shown that the discrete-time model gives reasonable agreement with these calculations.

A triple-axis X-ray spectrometer has been used to examine the reflectivity and surface scattering from silicon wafers with various thicknesses of oxide surface layers (from 10 to 100 AA). The X-ray techniques provide accurate information on the thickness of the surface oxide layer, the electron density of the roughness of both the surface interface and the substrate to layer interface. The use of a triple-axis spectrometer has two important advantages over the more conventional X-ray reflectivity techniques. It allows accurate reflectivity measurements to be carried out on samples which are, macroscopically, far from flat, such as semiconductor wafers in preparation for device fabrication. It also enables the intensity of the specularly reflected X-ray beam to be measured without contamination from the appreciable small-angle scattering which can occur from a microscopically rough surface. To illustrate this point, reflectivity measurements from a high-quality glass optical flat are also described.

The growth of stress corrosion cracks in aligned glass-reinforced polyester resin was monitored using the acoustic emission technique. The cracks propagated perpendicularly to the fibre direction under very low stress levels. The log-normal distribution function was applied to describe peak amplitude distributions of the acoustic emission signals produced mainly by fibre failure. It was found that the average peak amplitude of the signals is a linear function of the stress intensity K₁ at the stress corrosion crack tip.

A calculation is given of the spatial variation of phi , the volume fraction of particulate material, during one-dimensional steady-state diffusivity transport in an aerosol, taking coagulation of the particles into account. The effect of the latter is to modify the linear variation in phi which would otherwise exist, and detailed analytic results are obtained in the regimes Kn<<1 and Kn>>1. It is shown that if coagulation effects are sufficiently strong the spatial variation of phi can exhibit a maximum, while if they exceed a certain value in the Kn>>1 regime no steady state can be set up. Calculation of numerical values suggests that measurements of phi could provide useful experimental data on coagulation effects.

Quasi-elastic light scattering has been used to study thermally excited capillary waves on free liquid surfaces over a considerably wider range of surface wavenumbers than hitherto. The surfaces of ethanol, water and of aqueous solutions of both ethanol and tertiary-butyl alcohol have been studied. The observed frequencies and damping constants of capillary waves of all wavenumbers are in good accord with theoretical predictions. For all liquids studied the surface tensions and viscosities deduced agreed with literature values to within 0.4 and 1.3% respectively. These results indicate that surface viscous effects postulated for water exist neither for that liquid nor for aqueous solutions of alcohols which are thought to enhance the inherent structure of water.

The effect of oxygen on the photocurrent in poly-1,3,4-oxadiazole (POD) is discussed from the analyses of transient photocurrents, thermally stimulated current and thermally stimulated desorption of oxygen. Oxygen was considered to disturb the electron conjugated system in POD, so that the photocurrent is smaller in oxygen by a factor of 10 to 100 compared with that in vacuum. The colour of POD was different in vacuum, and in oxygen. This is also considered to be due to the same reason.

Thin films of Pt (100-200 nm) were sputtered onto a zirconia substrate and annealed for several days at 700 degrees C. Agglomeration occurred accompanied by loss of continuous electronic conduction for surface coverages below a critical level. AC impedance analysis showed that at elevated temperatures conduction across the surface involved a mixed mechanism of electronic conduction in the metallic islands and ionic conduction in the electrolyte between the islands.

A method of calculating the electric field induced by a photon fluence inside a rectangular conducting box is demonstrated using an analytical solution of Poisson's equation. The calculated field is in the range of 70-140 V mm^-1 for a particular optical path for a deposited dose in nitrobenzene of 9 krad. The calculations show that under some circumstances the field is unexpectedly sensitive to the geometry and material of the experimental apparatus. An experimental technique that would measure the predicted field using the Kerr electro-optic effect is also demonstrated and the field measured is 70_-10⁺⁷ V mm^-1 for a deposited dose of 9 krad in nitrobenzene. However at the 2.2 SD level of significance the measured electric field is <85 V mm^-1 and therefore although it is likely that a radiation-induced electric field has been observed there is a small probability that the chosen optical path had zero or a very small electric field. The observed pulse had an anomalously short decay time constant and radiation-induced conduction needs to be determined in more detail if an optical method of field measurement is to be useful. The radiation-induced current measured in a low-impedance diode agreed with that predicted by McCallum (1968).

The present study deals with the optical properties of copper sulphide films obtained by the spray pyrolysis technique. Films deposited at 250 degrees C substrate temperature have Cu₂S chalcocite as the dominant phase. The films are stable and have good adhesive properties. Optical constants of the films are calculated from the measured reflectance, transmittance and thickness data using the iteration method. The films have an indirect band gap of 1.0 and a direct band gap of 1.9 eV. Films deposited on aluminium substrates work as solar selective absorbers of the semiconductor/metal tandem type with maximum solar absorption ( alpha _s) of 0.89 and minimum thermal emittance ( epsilon _T) of 0.25.

Equations for resolution in the field ion microscope are reviewed and modifications are suggested. Calculations are made of each contributing term, based on a proposed model for the electric field variation. Comparison with experimental results is made. The revised equations strongly indicate that thermal accommodation of He imaging gas prior to ionisation is almost complete. Instead of the gas temperature being 6.5 to 7.3 times the tip temperature as estimated by Chen and Seidman (1971) a factor of only 1.3 to 1.5 now seems plausible.

Aging characteristics of the alloy Al-1.0 wt.% Mn-0.28 wt.% Fe have been investigated in the temperature range 400-550 degrees C by resistivity measurements. Pre-cold working was found to have considerable effects on the isothermal characteristics of the alloy. The activation energy of vacancy migration in the first stage of aging was found to decrease on cold-working, its value ranging between 0.35 and 0.50 eV depending on the thermal and mechanical history of the alloy. A resistivity peak has been observed in the second range of aging and this has been ascribed to a clustering process. The precipitation process in the third stage of aging was found to be enhanced by cold-working, the energy activating this process being reduced by cold-working and ranging between 0.28 and 0.50 eV. Flat needle-like precipitates of Al₆Mn were observed by TEM examination after prolonged aging.

This paper describes some of the yarn deformations and relative movements between yarns which occur when an isolated yarn is pulled through 20 crossover yarns out of an untreated cotton weave which has a relatively open and regular structure. The described deformations are of two kinds: linear extension in the selected yarn and the bending of the yarns which intersect its path. These processes occur at relatively low-tensile forces (<50*10^-2 N) on the chosen yarn and are largely reversible. The imposition of greater forces (>50*10^-2 N) produce irreversible deformations where the pulled yarn begins to progressively slide over the crossover yarns. As the pulled yarn is drawn from the weave the associated force decreases in discrete steps in proportion to the number of residual crossover points. The paper does not deal with the movement of the yarns normal to the direction of the tensile force. A simple linear elastic model is developed to describe the interrelationship between the extensibility of the pull-out yarn and the effective elastic shear resistance of the neighbouring yarns prior to inter-yarn junction slip. The tensile stiffness of the yarn is significantly greater than the stiffness in shear of the locally deformed yarn. The maximum pull-out force is a measure of the static friction or adhesive rupture force of one contact and the decreasing force values after this point provide a value for the sliding friction of one junction; the former is approximately three times bigger than the latter.

A dynamic model of melt ejection by a gas jet in laser cutting is presented. The molten material is removed due to friction forces and the pressure gradient of the gas flow. The solution of the stationary equations yields the thickness of the molten layer and its velocity of flow, dependent on cutting speed, gas jet formation and the viscosities and densities of the melt and the gas. A stability analysis of the stationary flow shows instabilities for a pressure gradient controlled melt removal. It is argued that these instabilities correlate with ripple formation on the cutting surface.

Any desired surface wettability of a plastic surface can be produced by changing the concentration of the plasma gas, which here is a mixture of oxygen and a compound which includes fluorine. In the plasma treatment, the use of a third electrode consisting of a metal mesh for ion trapping can significantly decrease the etching effect. The plastic surface wettability, given by the contact angle of a water drop, does not have any direct relationship with the surface roughness due to etching in this experiment.