Table of contents

This article deals with the second-order magnetic field, which appears significant in inhomogeneous current systems, where the total electric current has a zero value. The origin of this field is affected by the different drift velocities of the charge carriers in the individual parts of the system. The field is of a relativistic nature. This work describes suitable current systems as the origins of this field. Their main component is a semiconductor or superconductor bar, or if need be a beam of accelerated charged particles (electrons). The induction of this field in direct and alternating systems is calculated. The most significant component of this field is the acceleration boundary field, which appears in the thin boundary layers between the conductors with different drift velocities. The electromagnetic induction method of measuring the alternating second-order magnetic field is described. The article also gives an account of a successful experiment, which has confirmed the existence of this field with fast electrons in both a qualitative and quantitative way. The work discusses the possibilities of realisation of the current system with the semiconductor or superconductor and their application in the research of transport phenomena.

Basic principles are established for a method of obtaining range data by measuring the profile of blurs in or near the back focal plane of a lens or lens system resulting from the capture of light from a point or line source situated on an object in the scene. The effectiveness of the method is demonstrated using simulation and practical means. It is suggested that the principle could form the basis of a rangefinder for robotic applications in which objects are illuminated by a coaxial laser projector with the reflected light being captured by an aperture-restricted lens system.

The characteristics of thermal absorption by the workpiece during penetration welding with a laser can be represented by a combination of point and line sources on the axis of the keyhole. This gives an insight into the way in which the beam interacts with the material being welded. The analysis is worked out for a workpiece of finite thickness, and then applied to a series of lap welds in galvanised steel plates. This shows that in many of these welds the absorption is very substantial near the surface of the workpiece, but in some cases it falls rapidly before increasing again with depth. In some examples, there is a pronounced maximum at the junction of the two plates being welded together.

A copper vapour laser (CVL) has been successfully operated with a magnetic pulse compression (MPC) driver. With the MPC circuit the laser output power is increased, and stresses on the thyratron are reduced leading to an increased tube life. A theoretical analysis of the MPC is presented and compared with experimental results.

The dependence of second-order susceptibility on the density rho and the averaged atomic mass M in crystals has been investigated. It is found that the experimental data roughly follow the relation d(2 omega )=A rho ⁶M^-2.75, where A is a constant. The correlation is roughly as good as assuming the constancy of Miller's delta.

Ultrasonic scattering theory is used to relate the ultrasonic velocity and attenuation of emulsions to their physical properties (e.g. droplet size and volume fraction). This theory includes visco-inertial and thermal scattering mechanisms and corrections due to multiple scattering. The physical significance of the scattering mechanisms is discussed so as to highlight the factors which influence ultrasonic measurements in these systems. Theoretical predictions were compared with experimental measurements of velocity and attenuation in sunflower oil in water emulsions with varying droplet radii (0.14-0.74 mu m), mass fraction (0-0.5) and frequency (1.25-10.0 MHz) at 293.1 K. Appreciable excess attenuation and velocity dispersion were observed which could be attributed to thermal scattering. The relationship between the measurable ultrasonic parameters and the physical properties of emulsions means ultrasonic scattering should prove a useful means of investigating these systems.

The assertion by Kendall and Fuller (1987) that biological materials with J-shaped stress-strain curves are not necessarily more difficult to tear than Hookean solids, as suggested by Gordon (1978) is re-examined. While it is true that under certain conditions cracking is easier in materials with J-shaped stress-strain curves, the authors show that these conditions are not at all representative of biological materials. Gordon's hypothesis remains true, therefore, that biological materials are difficult to break because of their lack of shear stiffness.

Electron motion in the cathode sheath region of a glow discharge in argon has been simulated for a linearly decreasing electric field using a one-dimensional computer model. Distribution functions for the electron flux crossing the sheath have been calculated for 12 values of the cathode fall between 180 and 1400 V encompassing the normal and abnormal regimes. Macroscopic variables including the Townsend ionisation coefficient, the electron multiplication factor and the mean electron energy have been determined for cathode fall values between 180 and 1000 V. It is shown that the proportion of electron flux crossing the sheath without collisions increases as the cathode fall is raised. When the cathode fall is in the range 300-900 V, the electron flux at the sheath/negative-glow boundary is found to be highly directional with distinct beam-like characteristics. Beyond 900 V, the largest component in the distribution of electron flux to reach the boundary region is the collision-free group. These electrons enter the negative glow with energies corresponding to the cathode fall potential and are nearly mono-energetic. In this case, the entire cathode sheath/negative-glow region can be referred to as an 'electron-beam' glow discharge.

The electron transport parameters and excitation rate coefficients for individual N₂(X, v-Y, v') transitions (Y denoting an excited electronic state) were calculated for typical operating conditions of a stationary N₂ discharge at low pressures (0.1<or=p<or=2 Torr) and moderate currents (0.5<or=l<or=50 mA) taking into account the coupling between the electron energy distribution function (EDF) and the vibrational distribution function (VDF) of N₂(X) molecules. The coupled EDF and VDF were self-consistently calculated by solving simultaneously the Boltzmann equation and the system of rate balance equations for the vibrational levels including e-V, V-V and V-T energy exchange processes. The calculations are based on a complete set of electron cross sections for the transitions X v-Y, v' generated from a consistent set of cross sections for excitation of the entire states Y from X, v=0 using the Franck-Condon approximation.

The electron energy distribution function, transport parameters and rate coefficients for excitation and ionisation in N₂ under the action of a uniform HF field were calculated by solving the Boltzmann equation together with a set of rate balance equations for the populations in the N_x(X,v) vibrational levels. The treatment is restricted to situations in which the electrons do not lose appreciable energy during a cycle of field oscillation but applies to a broad range of applied frequencies, omega , bridging the gap between the limiting cases of few and many collisions per oscillation. The case of DC applied field previously investigated is obtained from the present formulation for omega =0. The effects of changes in omega and in vibrational excitation on the electron kinetics are investigated in detail, thus opening the way to firmly based theoretical interpretations of the properties of N₂ discharges sustained by HF fields.

The computation of continuous absorption coefficient for SF₆ plasmas has been carried out taking into account free-free radiation, using the scaled Thomas-Fermi potential for the determination of the terms resulting from the radiative recombination. The authors have computed the continuum absorption coefficient K' in SF₆-N₂ mixtures (for pressure values between 1 and 8 bar, in the temperature range 8000 to 24000 K) assuming that the plasma is in local thermodynamic equilibrium, by combining their results obtained in the case of SF₆ with those concerning N₂ from the literature. The evolution of K' with nu is explained in terms of electronic configuration of the species and of the variations of electron and ion densities with pressure, temperature and SF₆ proportion.

Numerical methods have been used to find the electric potential distribution in the stationary ion sheath that initially surrounds wedge-shaped cathodes in a plasma, following the application of a large negative voltage pulse.

The experimental results of the arc lifetime, the arc voltage, the cathode spot velocity and the erosion structure influenced by the cathode surface microstructure for low-current DC vacuum arcs are presented. The correlation between the arc voltage instability peak and the discontinuity in the cathode erosion trace has been shown when applying a transversal magnetic field. With increasing cathode surface roughness, an increase (of about a factor of 100) in the arc lifetime and a 30% decrease in arc voltage at the most, has been found for a drawn arc. The increase of the cathode spot velocity of the retrograde motion by cathode roughness is significant (a hundred times different at most) for triggered arcs. The mechanism of the cathode surface microstructure influence on the DC arc stability is discussed.

Reports a study on the development of primary electron avalanche under a toroidal electric field. The applied toroidal field is an induced electric field of E₀=86.8 V cm^-1 and the gases used are H₂, He and N₂. The avalanches are generated and make steady progress in the torus at 0.8-2.4 Torr in H₂, at 1.6-6.3 Torr in He and at 0.4-1.2 Torr in N₂ under the given field. It is suggested that the considerably high pressure range of He is closely related to the large value of alpha /p at low E/p. The development velocities of avalanche in N₂ and He are nearly equal to the electron drift velocity at the E/p used. The development velocity in H₂ is about twice as large as that of the others. This difference is presumably due to the multiplied effect of the large ionisation coefficient and the small absorption coefficient.

In monosilane-argon mixtures the measured electron swarm parameters, the drift velocity and the longitudinal diffusion coefficient showed distinctive features: humps in the drift velocity and dents in the ratio of the longitudinal diffusion coefficient to the electron mobility. These swarm parameters were analysed using a Boltzmann analysis and these features in the swarm parameters were conveniently used to derive a reasonable energy dependence of the vibrational excitation cross sections of a monosilane molecule. The derived cross sections were also confirmed to be consistent with published electron swarm parameters in pure monosilane gas.

The authors derive a system of equations describing the space-time development of the density, velocity and energy of a stream of electrons flowing through a background medium in the presence of a strong electric field. The equations, which resemble the equations of normal compressible fluid dynamics, provide a convenient description of electron dynamics in situations where the space-time evolution is fast enough to lead to significant non-equilibrium effects.

A spectroscopic diagnostic of the streamer point-to-plane discharge based on the spectrum emitted by the excited molecules and ions of N₂ was used to obtain the spatial electronic temperature distribution in the streamer head. The assumption of a Townsend electrical equilibrium allows the spatial distribution of the local and space-charge fields to be obtained. The electronic temperature presents a maximum of about 9 eV at a distance 2 mm from the point electrode, while the value of the reduced electric field at the same position is about 280 V cm^-1 Torr^-1. The evaluated distribution of the electric field and the geometric field distribution were used to obtain the space-charge field distribution.

Direct measurements were made of the time-dependent electron density around the current-zero period in a scaled-down air blast breaker with an interruption current level of a few kiloamperes, by means of the Thomson scattering of ruby laser light. The effects of interruption parameters such as the electrode material (copper and graphite) and the gas flow rate on the magnitude of the electron density in AC arcs were quantitatively investigated. The results showed that, in an arc current of 1.5 kA, the electron density at current zero in a copper electrode arc was significantly enhanced by a factor of 4-5 compared with that in a graphite electrode arc. Spectroscopy measurements have shown that such an increase in the electron density is brought about by the admixture of copper vapour with a concentration of 1.5% into high-temperature air at current zero. The electron density, however, was suppressed to around half the magnitude by the increase in the forced convection of air from 100 to 250 l min^-1. The inter-correlation between arc interruption performances of the test circuit breaker and the electron density at current zero is discussed. Statistical analysis showed that the success rate of arc interruption of the circuit breaker decreases almost straightforward with increase in the current-zero electron density on the normal probability chart.

The optical gap E_o and photocurrent peak E_p were investigated in amorphous SnS₂ films evaporated onto glass substrates kept at a temperature from 294 to 609 K. For substrate temperatures up to 343 K, the film grew into an amorphous state with E_o=1.05+or-0.1 eV and E_p=2.1+or-0.05 eV. With increasing temperature, E_o increased markedly to 1.93 eV, while E_p decreased to 1.50 eV. By considering the results of radial distribution function obtained from the analysis of X-ray diffraction, the increase of E_o was explained in terms of a transition from an amorphous to a polycrystalline state. The substrate temperature dependence of the density of states was discussed including the result of two-step activation processes of electrical conductivity.

Detailed studies considering the recent developments in the analysis of line shift, line asymmetry and Fourier lineshape have been made on the X-ray diffraction profiles pertaining to the pure copper phase in the composite matrix of Cu and Cu₂Te (hexagonal) phases in Cu-Te alloys with solute tellurium at 3.30, 5.62, 12.02 and 13.08 wt.%. The microstructural parameters such as the coherent domain sizes, the microstrains within the domains, the intrinsic fault density, alpha ', the extrinsic fault density, alpha " and deformation twin fault density, beta , have been evaluated. While the intrinsic fault density, alpha ', and the extrinsic fault density, alpha ", were found to be absent even at higher solute contents, the deformation twin fault density, beta , which is normally absent in Cu and Cu-base alloy systems, is found to be present here in an appreciable quantity. The reason for the presence of this deformation twin fault in the Cu phase may be attributed to the influence of a neighbouring Cu₂Te phase in the matrix.

The effect of sodium to or from silver ion exchange on electrical conductivity has been investigated for some heterophase glasses containing bismuth oxide. The electrical resistivity in ion-exchanged samples decreases by several orders of magnitude when they are subjected to an electrical field of about 10-30 V cm^-1 at around 300 degrees C. Polarisation experiments carried out on samples in such a highly conducting state show that the electrical transport is ionic in nature. The switching to these states is believed to arise as a result of the growth of a silver-rich phase to a percolation configuration. The resistivities of these phases are in the range 10-10³ Omega cm with low activation energy values. The results suggest that these materials may constitute a new class of superionic conductors.

A study is presented of the fixation of collectors on fluorite ore under electromagnetic fields of low and high intensity. Dielectric measurement results are interpreted in terms of physico-chemical interactions between the surface of the solid and the liquid. Electrical modelling of the heterogeneous medium allows the prediction of the distribution of the electromagnetic field. Energy losses are preferentially located on the solid grains or near the surface of the grains. Some experiments of the recovery of fluorite under electromagnetic irradiation are presented. When oleate is the collector, the electromagnetic field increases the fixation of the collector because of the temperature gradient at the interface. This is evaluated on the basis of a simple model. When the collector is dodecylsulphonate, thermal effects have been taken into account to interpret the results.

Thin films of PbTe of different thicknesses have been prepared on glass substrates at room temperature by vacuum deposition. It is found that the electrical resistivity of the air-exposed films is much higher (by about 2 to 3 orders of magnitude) than that of the as-grown (unexposed) thin films. The electrical resistivity temperature behaviours of both the air-exposed and as-grown (unexposed) thin films of PbTe are different but both show hysteresis behaviour during successive heating-cooling cycles. These observations can be explained by considering that the desorption of absorbed gas molecules (mainly oxygen) and creation of defects at higher temperatures during heating influence the electrical conduction. Further, the time factor involved in gas desorption-adsorption can cause the observed hysteresis in temperature-dependent conduction behaviour. The as-grown (unexposed) thin-film conductivity exhibits the expected reciprocal thickness dependence due to the thickness effect, but the air-exposed film conductivity does not. This can be explained to be due to the complete masking of the thickness effect by the gas adsorption effect in air-exposed film conductivity. The reciprocal thickness dependence observed in the case of unexposed film conductivity has been explained by the 'effective mean free path' model. The low value of the 'grain boundary' mean free path obtained by the analysis points to the fact that in polycrystalline films, grain boundary scattering is extensive and controls the film conductivity.

A combined technique of low-energy Na⁺ ion scattering spectroscopy (Na⁺ ISS) and low-energy electron diffraction is applied to study the initial growth process of Ag films on Si(111) surfaces. In comparison with results obtained by Auger electron spectroscopy, the Na⁺ ISS is also found to reflect reasonably well the Stranski-Krastanov growth mode or the Frank van der Merwe growth mode. Moreover, owing to the extreme sensitivity of the outermost surface layer, the Na⁺ ISS gives information on changes in the surface morphology induced by the Ag adsorption and subsequent annealing.

The authors report TSD and TSC measurements on PVK films over the temperature range 85-530 K. The only peak previously reported is found to lie at 250 K at zero drift field, and the corresponding activation energy is deduced to be 0.55+or-0.05 eV. A new peak at 365 K at zero field is attributed to hole traps of effective depth 0.8 eV. Further peaks at 290 and 125 K are ascribed to the known beta - and gamma -dipolar relaxations. The dipolar alpha -relaxation expected at the glass transition temperature of 483 K was not observed but, under prior electron bombardment, a strong peak developed near that temperature. This peak is identified with the release of the implanted electrons from traps >or=1.3 eV deep.

Advantages of a semi-wet route to the synthesis of YBa₂Cu₃O_7-y employing coprecipitation of yttrium and barium as precursor carbonates are presented. It is shown that the precursor carbonate is an intimate mixture of fine barium carbonate and amorphous yttrium carbonate particles. YBa₂Cu₃O_7-y powders prepared under identical conditions by the conventional dry route and the present semi-wet route are analysed for compositional inhomogeneities and coherently scattering domain size (D), using principles of X-ray line profile analysis. It is shown that unlike the powders prepared by the conventional method, which contain compositional inhomogeneities and possess domains of about 2200 AA, powders obtained under identical conditions by the semi-wet route are free from compositional inhomogeneities and possess larger domain size (around 4000 AA). Since the actual particle size in both the cases is several mu m, the loss of coherency in each particle is attributed to the presence of twin interfaces. Better compositional homogeneity and lower density of twin interfaces in specimens prepared by the semi-wet route are believed to be responsible for the improved superconducting behaviour of ceramics prepared by the present method.

The low-field complex susceptibility curves of ferrofluids have in general a Debye-type profile with a loss component which peaks at a frequency from which it is possible to estimate the average relaxation time of the particles in the ferrofluid. The authors report on magnetic measurements on a water-based ferrofluid whose loss curve displayed two distinct peaks. It is concluded that one peak is due to Brownian relaxation and the other is due to a combination of Brownian and Neel relaxation.

The magneto-acoustic emission (MAE) from a 3% Si-Fe single crystal splits into two peaks at the coercive force during half a cycle of the B-H hysteresis loop, one associated with the change of strain due to the nucleation and the other due to the annihilation of domain walls. The emission increases as the magnetising frequency increases; however, the rate decreases above the frequency where the skin-depth effect becomes apparent. The emission from 3% Si-Fe polycrystals increases with tensile stress up to 11.2*10⁶ N m^-2, then it decreases with further stressing. The source mechanism of the MAE is discussed in terms of the change of strains: one is related to the magnetostriction coefficient due to the nucleation and annihilation of domain walls, and the other is related to the magnetostriction due to the 90 degrees domains.

A method has been proposed for exact calculation of the optical parameters n and k of a thin film on a transparent substrate by simultaneously solving the equations for two of the envelopes of the transmission and reflection spectra. The singularity of the solution and the convergency area are proved, and the influence of the inaccuracies in drawing the envelopes upon the errors made during the calculation of n and k is also investigated. The method is most accurate for the envelopes of the transmission minima and the reflection maxima in the region of strong and medium absorption. The accuracy of other methods which use data from the transmission or reflection envelopes has also been investigated. The present method describes correctly the change of n in the region of medium and strong absorption for an amorphous film with composition Ge₂₈As₁₂S₆₀.

The dependence of the electrical and optical properties of reactively evaporated indium tin oxide films on substrate temperature and tin concentration has been studied. Films of resistivity as low as 3*10^-4 Omega cm and high visible transparency of about 91% have been obtained at higher deposition temperatures ( approximately 300 degrees C) for tin concentrations of 10-15 wt.% in the source alloy. Hall mobility in the films has been found to decrease with the addition of tin and increase with deposition temperature. An observed increase in carrier density with substrate temperature has been viewed as being due to the possibility that higher deposition temperature favours the incorporation of tin as Sn⁴⁺ in the films. In the optical band-gap calculation the effect of collision broadening, which is typical for a degenerate semiconductor, has been included and found to be significant.

Results are reported on the substitution of Ca for Ba in type M hexaferrites. Three techniques have been used to determine the degree of substitution: magnetometry; X-ray diffractometry; and the analysis of chemical equilibrium equations. For materials obtained from nominal compositions given by (CaO+6Fe₂O₃+zBaO), magnetic measurements show that the ratio CaO/BaO in the hexagonal phase is constant, independent of the quantity z in the starting mixture. Diffractograms also show that the hexagon alphase is the same up to z=0.56. The analysis of equilibrium equations helps to determine precisely the value of CaO/BaO. For firing temperatures of 1100 and 1150 degrees C hexaferrites have been obtained with CaO/BaO ratios of 1/1 and 5/4 respectively. This has been possible because of the compositions used and the way in which mixtures were prepared. Specific saturation magnetisation for the phase with CaO/BaO=5/4 is 70.9 emu g^-1.

Thermographic NDT is potentially a very useful technique for rapidly locating flaws in large structures without physical contact. The fundamental factors limiting thermography are discussed, and recommendations for the achievement of high-contrast thermograms are formulated based upon a simple one-dimensional analysis of heat diffusion in a defective test piece. The analysis shows that the most effective form of noncontact heating is flux profiled so that the temperature of the front surface of the specimen is maintained at a constant high value. A three-dimensional numerical model has also been developed and some predictions are presented. The model is used to calculate the size and depth of a perfectly insulating defect which can be detected thermographically under ideal experimental conditions.

Self-diffraction and phase conjugation by degenerate four-wave mixing have been observed in a dyed nematic liquid crystal at intensities of a few watts per square centimetre continuous. The change in the extraordinary refractive index due to absorption of laser light, enhanced by the dye, was used. The time response of the non-linearity was measured roughly in the self-diffraction set-up and found to be of the order of a few milliseconds. This is fast compared with effects involving molecular reorientation, but slower than expected from thermal diffusion. Although the diffraction efficiency of the refractive index grating formed was low, typically less than 10^-4, self diffraction was observed with an unfocused 6 mW HeNe laser. The dependence of the diffraction efficiency on liquid crystal temperature and power in the input beams was measured.

Data collected during 5 s operating pulses in a narrow (35 mm wide) plasma centrifuge are presented. Pressure gradients, set up radially in the argon plasma by the combined effects of centrifuging and the ion wind, are unsteady, with the pressure measured at the centre rising and falling several times during a run. From observations of plasma light output, it is found that at times during a discharge when the pressure is rising, there is a high level of nonuniformity in the plasma. The likely explanation is that when strong spoke-type structures are present in the plasma, circulating flows degrade centrifuge performance.

S₂F₁₀ was found as a product resulting from the high-voltage arcing of SF₆. S₂F₁₀ was identified by the observance of the molecular ion S₂F₁₀⁺ and its fragment, S₂F₉⁺, produced at 20 eV electron impact ionising energy as well as the sulphur isotopic AMU values.

The authors investigate the nonradiative lifetime of long-lived metastable states in DC afterglow discharges in He, He+10 PPM Ne and He+1000 PPM Ne at pressures of 13 and 65 mbar. It is shown that the nonradiative lifetime depends upon the pressure of the gas or the mixture in the diode. At a pressure of 65 mbar the nonradiative lifetime is approximately 300 s, while at 13 mbar it is in excess of 1 h. De-excitation of metastable states in He and He-Ne mixtures is studied in a similar fashion.

Ion bombardment at glancing incidence (67 degrees from substrate normal) with 300 eV argon ions during deposition is shown to have a pronounced densification effect on MoS_2-x thin films. Comparison with as-sputtered samples gave a density increase of up to 100%, depending on the ion dose and substrate temperature during deposition. On the other hand, resputtering of the growing film occurs as a result of the off-normal incidence of the ion beam.