Table of contents

Time-reversal of ultrasonic fields allows a very efficient approach to focus pulsed ultrasonic waves through inhomogeneous media. Time-reversal mirrors (TRMS) are made of large transducer arrays, allowing the incident acoustic field to be sampled, time-reversed and re-emitted. The paper introduces the time-reversal approach in a discussion of the techniques used in optics for focusing through inhomogeneous media. The discussion is extended to ultrasonic pulsed fields where adaptive time delay focusing and TRM focusing are the main techniques of interest. Time reversal focusing of a pulsed wave is optimal in comparison with time delay focusing in the sense that it realizes the spatiotemporally matched filter to the inhomogeneous propagation transfer function between the array and the target. Time-reversal processing also permits choice of any temporal window to be time-reversed, allowing operation in an iterative mode. In multi-target media, such a process converges on the most reflective target. In the case of an extended target, automatic resonances can be achieved. Applications of TRM are numerous and include medical applications (imaging, lithotripsy and hyperthermia) as well as non-destructive testing (NDT) and underwater acoustics. Experimental results obtained with 64 and 128 channel TRMS demonstrate the efficiency of this technique.

By using the technique of laser induced fluorescence to measure the velocity distribution function of an atomic uranium beam produced by evaporation from a spherical surface by electron bombardment, the authors have observed the phenomenon of vapour expansion cooling. Agreement between the theoretical analysis and experimental results is satisfactory. Some of the results are compared with those published by Havazelet and Birnboim (1983) and good agreement is achieved.

Using the model of a cylinder-type heat source, the power loss owing to heat conduction in laser cutting and welding of metals is calculated analytically. The case of laser cutting is described by taking into account the influence of the generated cutting kerf using numerical calculations. Both the analytical and the numerical solution for the power loss deposited into the material are well described by approximative formulae. The theoretically predicted power loss into the cut workpiece is confirmed by measurements of the temperature rise within the metal sheet in laser cutting experiments.

Information theory, as developed by Shannon (1948), provides a theoretical framework for describing the transfer of information through a communication channel. A coherent imaging system is a particular form of communication channel which is characterized by a multiplicative noise process termed 'speckle'. Information theory is used to assess the effect of speckle on the information content of coherent images. Particular emphasis is given to relating information theoretic concepts to intuitive descriptions of information based on physical models.

Three calculation methods for the determination of effective constants of composite media are studied in detail. An approximate method for the definite integral makes the transformation field method capable of computing the effective conductivity of composite media with overlapping inclusions. The collocation method is used to calculate the effective thermal conductivity of composite media, which illuminates the validity of the collocation method for composite media with potential discontinuity at phase interfaces.

The predicted rates of rise of water in glass capillaries of diameters 0.2 and 0.4 mm by a theory which includes the effect of inertia on the movement of the liquid are in satisfactory agreement with experimental data. With an immersion depth (dip length) in the liquid of about 3 mm at the commencement of wicking, the predicted effect of inertia on the rate of rise of liquid in the capillaries is negligible. For a dip length of about 1 mm there is a small inertial effect with the larger capillary which is indicated by a slightly more rapid initial rise of the liquid relative to that when the dip length is about 3 mm. Under the test conditions the force of inertia is not an important factor in determining the rate of rise of liquid in the capillaries. The predicted initial volume rate of wicking of the liquid in the larger capillary is several times greater than that of the smaller one. This result will have an important bearing on the design of fabrics with optimum wicking capabilities.

An argon plasma jet is produced from a continuous optical discharge at gas pressures in the range 2<or=P₀<or=8 bar and effective laser power in the range 165<or=P<or=360 W. Plasma production is characterized by measurement of the threshold of the stability region (in terms of laser power-gas pressure). Study of the threshold law given by a simple model allows the core temperature of the plasma to be deduced. Temperatures are also measured by time-of-flight analysis of the neutral beam extracted from the plasma jet. They are compared with averaged temperatures deduced from the nozzle flow rate. From this comparison, a temperature profile over the nozzle radius can be calculated as can the heat flux through the nozzle. The kinetic energy yield of the continuous optical discharge can then be estimated. Values as high as 60% of the input laser power are obtained.

The three stages of anode surface area evolution in an industrial CO₂ laser owing to the formation of oxide films have been recognized. The increase of the current at which the glow-to-arc transition occurs by transition to the final (homogeneous) stage has been established. The disappearance of the anode current spot under presence of the homogeneous low-conducting coating on the anode surface has been shown using 2D simulation. The performed studies enable one to highlight the dominant role of the surface state in anode spot formation in an elevated pressure glow discharge.

Gas breakdown voltages are calculated for a cylindrical spacecraft in the ionosphere, whose axis is parallel to the geomagnetic field. The Townsend breakdown criterion is applied to the neutral gas of 10¹⁷-10²⁰ m^-3 surrounding the cylindrical spacecraft, which is assumed to have a highly negative potential with respect to the ionospheric plasma. Electrons are emitted from the surface and ionize the neutral gas inside the ionic sheath while they rotate around the spacecraft due to E*B drift. The Townsend coefficients of argon gas are calculated by Monte Carlo simulations based on the concept of equivalent reduced electric field. The breakdown occurs in the range 200-10000 V at neutral densities less than 10¹⁹ m^-3.

The total radiative loss in atmospheric argon plasmas is calculated allowing for deviations from local Saha equilibrium LSE. The authors have taken into account non-equilibrium excited state populations using numerical and analytical collisional-radiative models. Simple expressions for the different radiation loss mechanisms are given in terms of the electron density, electron temperature and ionization degree. These quantities together with the heavy particle temperature also define the deviation from equilibrium. In the recombining zones the effect of non-equilibrium will have significant influence on the total radiative loss due to line radiation. The dominancy results from the fact that the electron density in a recombining plasma is much larger than the value predicted by Saha. The results can also be used for non-atmospheric argon plasma provided that n_theta >5*10¹⁹ m^-3 and n(1)d>10²⁰ m^-2 in which d is the plasma dimension.

The diffusion coefficients for ground-state (²S₁2/) copper atoms in He, Ne and Ar and those for metastable (²D₃2/) copper atoms in Ne and Ar were determined experimentally. By producing copper vapour in a low-power hollow cathode discharge, measurements were performed near room temperature. The diffusion coefficients for Cu(²D₃2/) were approximately equal to the corresponding quantities for Cu(²S₁2/).

Results of a mass spectrometric investigation of dissociation of nitrogen in DC glow plasmas in a flowing gas system in the pressure range 1-3 Torr, electron density range (1-8)*10¹⁰ cm^-3 electron temperature range 2.0-2.7 eV and linear flow rate range 1.1-5.8 m s^-1 are presented. The extent of dissociation is of the order of approximately 1%. A simple collisional-radiative model is presented which incorporates several molecular electronic excited states, nitrogen-containing ions, and N atoms. The N atom production is presented as a function of electron density; the predictions of the model are in agreement with the experimental results within an order of magnitude.

Reports electron swarm parameters in SF₆ in uniform E*B fields employing a Monte Carlo simulation technique. The parameters studied are transverse and perpendicular drift velocities, magnetic deflection angle, collision frequency, mean energy and effective ionization coefficients for 30<or=E/p<or=200 V cm^-1 Torr^-1 and 0<or=B/p<or=6*10 T Torr^-1. At a given E/p, application of a magnetic field causes decreases in the transverse drift velocity, the mean energy and the effective ionization coefficient. The validity of the effective reduced electric field concept is also investigated for the evaluated collision frequency, mean energy and effective ionization coefficient.

The Fowler-Milne or normalized spectroscopic method is used to measure the temperature distribution of a tungsten inert gas (TIG) arc with a number of different spectral lines. The results highlight some of the limitations of this method and provide evidence for departures from LTE in the arc. The paper describes the apparatus and data inversion procedures, and compares the results with those from other groups.

A physical model of an SF₆ circuit-breaker arc was developed to study the behaviour of the arc as zero current is reached and to analyse its interaction with the circuit. The role of turbulence, processed using the mixing length model adjusted with experimental results, is an essential factor in the extinction of the arc. The variations of post-arc current were calculated as a function of the rate of rise of recovery voltage (RRRV) and of the time lag before application of this transient voltage. The authors then linked their physical arc model to a circuit model. Calculation of the arc-circuit interaction showed that the disturbances brought about in the RRRV by the presence of the arc should be taken into account, since they modify the post-arc current and thus the interrupting capability of the circuit-breaker.

The intermetallic compounds Ho₂Ni₁₇ and Er₂Ni₁₇ crystallize in the hexagonal structure of Th₂Ni₁₇ with lattice parameters a=8.32 AA, c=8.03 AA and a=8.28 AA, c=8.03 AA respectively. The compounds are ferrimagnetic and the effective magnetic moments per formula unit (fu) determined from the paramagnetic region are mu _eff/fu=16.51 mu _B for Ho₂Ni₁₇ and mu _eff/fu=15.27 mu _B for Er₂Ni₁₇, very close to the theoretical values of 16.44 mu _B and 15.16 mu _B. The critical Curie temperatures for the compounds are T_c=587 K (612 K after heat treatment) for Ho₂Ni₁₇ and T_c=572 K (605 K after heat treatment) for Er₂Ni₁₇.

The decomposition of austenite following the initial rapid formation of bainitic ferrite in a eutectoid steel containing nominally 1 wt.% Ni and 1 wt.% Cr isothermally transformed in the bainitic region of the time-temperature transformation diagram has been studied. The microstructure and kinetics were studied using both optical and electron microscopy. Also, the mean potential difference V_B and the frequency F_B of Barkhausen jumps were measured for various bainitic transformation products. Lower bainite with fine carbides shows the higher values of F_B and V_B. The formation of upper bainite with coarser carbides leads to a considerable decrease of F_B and V_B, while in the region of upper bainite and spiky pearlite F_B and V_B increase slightly. The relative amount of the eddy currents depends on the transformation products.

A detailed study of magnetization processes has been carried out on a new type of hard magnetic material. The material Nd_3.8Fe_73.3V_3.9B_18.0Si_1.0 is unique among hard magnets due to the exchange coupling of two ferromagnetic materials, one of which is soft and the other hard. The combination of the two materials gives a composite material, with a high magnetic saturation and coercivity. The irreversible components of magnetization for the material were examined using the demagnetizing remanence curve (DCD) and the isothermal remanence curve (IRM) for the magnetizing case. The nature of the interactions in the material were also studied using the delta M plot obtained via the DCD and IRM remanence curves. The reversible components were obtained from the M-H loop data, M_rev=M_tot-M_irr. Measurements of magnetic time dependence have also been carried out to provide fluctuation field data, H_f, from which activation volumes of reversal, v, for various fields have been obtained. The mean activation field H_a, obtained by extrapolating the linear portion of the fluctuation field data to zero, is the same as the mean nucleation field H_n obtained from the reversible components, M_rev of the demagnetizing remanence curve. Assuming spherical volumes of reversal, the dimensions of the soft and hard phases, obtained from the mean volume of activation, v(H_n) are close to the critical dimensions of the soft and hard phases.

A new method for modelling stress-strain characteristics of demagnetized polycrystalline ferromagnets is presented, taking into account changes in the magnetic domain structure due to the application of stress. The materials are assumed to be isotropic polycrystalline aggregates with crystallites of cubic symmetry. Introducing a mesoscopic variable, the change of magnetic domain volumes, the analytical form of nonlinear stress-strain behaviour is derived. Together with a quantitative description of the Delta E effect in the uniaxial stress state, a Delta G effect is predicted, which is a decrease in shear modulus G taking place in shear deformations.

The authors used dynamic mechanical and dielectric thermal analyses together with infrared spectroscopy to identify molecular motions in polyaniline films cast from emeraldine base powder dissolved in the solvent N-methyl-2-pyrrolidinone (NMP). These relaxations include a librational ring motion at around -80 degrees C and a glass transition (T_g) centred at 100 degrees C. They have also observed a permanent film hardening process at around 180 degrees C which is ascribed to polymer chain crosslinking, both physical (associated with chain entanglements) and more importantly chemical (chain-chain chemical bonding). Chemical crosslinking can be further identified with chain defects and residual impurities in the polymeric material. The importance of precise control of polyaniline synthesis and processing is therefore strongly emphasised. These results have clear implications for the thermal orientation process in polyaniline.

The long-term fading of peak 5 has been studied using four different annealing regimes in order to isolate the behaviour of peak 5 free from interference effects of peaks 2, 3 and 4. Peak 4 grows over the first nine months of storage and only then begins to decay; peak 5, on the other hand, decays rapidly in the first six months and then stabilizes or even begins to grow. Peaks (4+5) together, however, fade monotonically by approximately 8% per annum. Using thermal cleaning procedures to isolate the behaviour of peak 5, the authors have determined that its mean lifetime at 20 degrees C is 5.8+or-1.2 years (1 SD). This mean lifetime is orders of magnitude smaller than predicted by the exponential extrapolation of the mean lifetimes measured at higher temperatures or via peak shape techniques. The discrepancy suggests that the long-term decay of peak 5 is strongly influenced by other forms of decay not associated with charge carrier detrapping. The short-term fading behaviour of peak 5 alone, on the other hand, does appear to be 'well behaved', at a rate of 3.4+or-0.8% (1 SD) per month (for the first month) irrespective of annealing routine, or the presence of peaks 2 and 3. This suggests the possibility of universal short-term fading corrections in environmental and personnel dosimetry if only peak 5 is used in dosimetric measurements.

Mechanisms of thermoluminescence (TL) in aragonite have been studied using electron spin resonance (ESR) techniques. Natural aragonite shows a prominent TL glow peak at 613 K. Additional gamma irradiation enhanced this glow peak but displaces it slightly to 593 K, and induces two glow peaks at 393 and 493 K. Pre-annealing at 653 K for 10 min and subsequent gamma irradiation also induces and enhances two glow peaks at 393 and 493 K. The authors show that second-order kinetics characterizes all the glow peaks reasonably well. Results of dose calibration in aragonite indicate that the 393 K peak increases linearly with dose, whereas the others exhibit anomalous behaviours. On the other hand, aragonite presents a complex ESR spectrum. The pre-anneal and subsequent irradiation procedure is found to enhance Pb³⁺ and CO₃^- ESR hole centres. An isochronal thermal anneal sequence experiment indicates that these ESR centres can be related to two glow peaks at 393 and 493 K. A model for the recombination mechanism is suggested on the basis of TL and ESR measurements.

When co-doped with La³⁺, Ba₃(PO₄)₂:Eu²⁺ shows an intense thermally stimulated luminescence after X-ray irradiation. The phosphor is also optically stimulable. The stimulated emission is due to the Eu²⁺ ion. In the absence of Eu²⁺ a broad emission band is observed which is tentatively ascribed to a phosphate group close to the La³⁺ ion. EPR reveals the presence of H⁰ after irradiation. It is concluded that the substitution of La³⁺ on a Ba²⁺ site is accompanied by the introduction of H⁺ into the lattice. The latter ion acts as an electron trap; detrapping yields the stimulated luminescence.

The authors report on photoreflectance (PR) measurements of the electric fields at the surface and GaAs/GaAs interface of doped GaAs films prepared by molecular beam epitaxy (MBE). By using He-Ne and He-Cd lasers alternately as the pump light, PR signals from the surface and interface of the films can be effectively separated because of the difference in the penetration depths of these two pump beams. Electric fields at the surface and interface were evaluated from PR spectra. Film interference effects on modulation spectroscopy have been studied. The origin of the electric field at interfaces has also been discussed.

The optical absorption and photoluminescence spectra of multi-alkali photocathodes are presented. The value of the band gap of multi-alkali photocathodes is determined by analysis of these spectra. It has been shown that the band gap E_g is equal to 1.3-1.4 eV. The value of the work function is also determined and the conclusion is drawn that multi-alkali photocathodes are photocathodes with negative or nearly zero electron affinity.

The electron field emission from various kinds of particles ( approximately 20 mu m) placed on plane cathodes has been measured in a SEM modified by the addition of a scanning anode tip. Results are similar for cathodes of Au (oxide free) and Nb (with thin insulating native oxide). In general no emission is found from insulating particles (Al₂O₃, SiO₂) or particles with insulating oxides (Nb,Ti) under fields at least as high as 100 MV m^-1, while oxide free particles (Ag, Au) and those with a conducting oxide (Fe) tend to emit strongly at much lower fields. Anodizing the Nb cathode surface to >200 nm produces no significant change in emission; after initial emission the particle/substrate contact is found to be conducting. With rare exceptions, these results are consistent with a classic metallic projection model.

Highly conducting ( rho =2.5*10^-4 Omega cm) and transparent (92%) tin-doped indium oxide (ITO) films have been prepared by a reactive electron beam evaporation technique; the dependence of these properties on the substrate temperature (150-275 degrees C) and tin doping (0-15% by weight) has been studied. Hall mobility in the films has been found to increase with substrate temperature and decrease with the addition of tin. An attempt has been made to distinguish the different scattering mechanisms responsible for electrical conduction; it is found that the grain boundary scattering is negligibly small in these films. An analysis of the optical data has been made to evaluate the broadening parameter and the Burstein-Moss shift in optical band gaps.

The authors present some results concerning the characterization of an etching triode reactor with a multipolar confinement, excited at 13.56 MHz. Two generators were used, one for creating the discharge and the other for biasing the used sample. This reactor makes the energy of the ions independent of the plasma creation function. They have studied the effect of ion energy and discharge parameters, (flow rate and incident power) on etching kinetics, anisotropy and contaminations of Si(111) samples exposed to SF₆ plasma.

A physical mechanism for the enhancement effect in laser-enhanced arc welding (LEAW) is proposed. The minimum laser power for a CO₂ and a Nd:YAG laser is calculated in the case of Fe-, Al- and Cu-based welding materials. The results are compatible with the pertinent welding experiments.

Suspensions of yeast cells (Saccharomyces cerevisiae) were used as model systems to investigate the electrokinetic behaviour of colloidal particles subjected to travelling electric fields generated using microelectrodes. Measurements were made over the frequency range 1 kHz to 10 MHz and for suspending medium conductivities in the range 6-260 mS m^-1. A theoretical model is developed to provide a good description of the dependence of the observed translational motion, termed travelling-wave dielectrophoresis (TWD), on the dielectric properties of the particle and suspending medium, on the size of the particle, and on the magnitude and frequency of the applied field. Unlike conventional dielectrophoresis, the TWD effect is found to be related to the imaginary, rather than to the real, component of the induced dipole moment. Dielectrophoresis and electrorotation measurements were made to provide a further understanding of the observed effects and to support the theoretical model.

The ability of scanning electron acoustic microscopy (SEAM) to image misfit dislocations in heterostructures is shown in the case of InGaAs/GaAs superlattices. The SEAM images are compared with panchromatic cathodoluminescence (PCL) images of the same samples and the differences are discussed.