Table of contents

An approximate calculation of the heat flow in a plasma with an arbitrary temperature gradient is presented. A Fokker-Planck collision term is employed. The calculated heat flow may well be approximated by the flux-limited classical value.

A comprehensive review was published (Landa et al. 1980), devoted mainly to studies of moving striations in glow discharge, but containing also some remarks on the present author's work (Chapnik 1968) on the theory of stationary striations, which usually appear in molecular gases. Landa et al. (1980) conclude that the results obtained are at variance with experimental data on the variation of electron density and electron temperature in the striated positive column in hydrogen. However, this conclusion, in the present author's opinion, is based on some mis-understandings, perhaps caused by the omission from the original work of detailed discussion of the relation between the theoretical solution and the experimental data. Bearing this in mind and considering also that the existence of stationary striations is still often thought to be poorly understood, the relationship of the proposed theory and the experimental data, are briefly discussed.

The thermoelectric power (TEP) measurements in thin ytterbium and samarium films over the thickness range 500 AA to 1250 AA are reported. The TEP is found to be thickness dependent and increases with film thickness in both cases. The thermopower of ytterbium reaches a constant value of 19 mu V degrees C^-1 for film thicknesses greater than 900 AA, and for samarium its value becomes almost zero for all thicknesses greater than 1100 AA.

The 1.315 mu m atomic iodine laser has been developed for repetitively pulsed operation by the use of a thermally driven fast gas flow and replenishment system. The flow, produced by thermal evaporation and condensation, removes the molecular iodine formed by photolysis which acts as a strong laser quenching agent. An all-glass construction of the laser and flow system gives long life sealed-off operation, and no iodine corrosion problems. Fluorocarbon buffer gases have been evaluated for use in providing flexibility in the operation of temperatures of the flow system in projected larger aperture oscillator systems, and for enhancing energy storage capability by pressure broadening the laser linewidth in Q-switched oscillators and amplifiers.

The pumping mechanism of the N₂⁺ ion laser excited by a rapid discharge in a He-N₂ mixture has been examined experimentally. The dependence of the laser output on temperature and pressure of gas mixture and the time history of the gain of active medium are measured, leading to the conclusion that the electron impact excitation of the Na₂⁺ (B² Sigma _u⁺) state is the dominant excitation process in the case of high speed excitation.

The authors present an investigation on the effect of transverse non-uniformity on the parametric interaction of radially gaussian modes of arbitrary type in the absence of self-focusing. The propagation-distance dependences of the real-valued axial amplitudes, phases, beamwidth parameters and phase gradients have been derived by using the variational technique. The analysis has been numerically illustrated by considering the stimulated Raman scattering in a plasma. It is seen that the pump node propagates like a trapped filament whereas the remaining modes undergo a periodic focusing or defocusing on account of the parametric-interaction-induced energy exchange.

Measurements are presented of voltage generated by turbulent flow of liquids through dielectric tubes. An empirical relation based on these observations is compared with the corresponding classical Helmholtz equation for laminar flow, and it is concluded that the mechanism for charge generation is located at the tube entrance. Practical applications for the measurement of flow rate and fluid resistivity are discussed.

Double layers in a current-carrying magnetised argon plasma are studied experimentally. The layer evolves from an unstable anode sheath when the anode voltage has reached a critical value which depends on the neutral gas pressure. The double layer is an oscillating, three-dimensional structure enclosing both axially and radially a plasma at higher potential which is maintained by a low rate of ionisation due to the accelerated electrons. Axially accelerated electrons and radially accelerated ions have been detected, but not axially accelerated ions. High frequency is detected on the anode side of the layer. Double layers have also been observed in helium and xenon plasmas. Time independent numerical solutions of Maxwell's equation div D= rho in one dimension including ionisation are discussed.

A calculation of the medium- and low-energy part of the electronic distribution is described, together with its indirect comparison to experimental results. The study deals with a plasma created by a proton beam (2 MeV, approximately 2 mu A cm^-2) impinging on a Ne (about 10 Torr) target, with N₂ impurities. Microwave measurements of electronic density and effective collision frequency agree satisfactorily with calculations using a theoretical distribution function.

Electron heat flow has been studied in a double plasma device. Electrons of mean energies 5.1 eV and 2.3 eV were separated by a grid which was then pulsed to create a thermal wave moving in a large temperature gradient. Changes in the electrons' velocity distribution, concentration and energy agreed qualitatively with a simple theory based on transport in neutral gases. Thermoelectric effects reduced the heat flow by a factor of 5. This factor combined with the theory suggests a flux-limiting parameter with an upper limit of 0.15 for laser fusion. This value lies within the range found in experiments on plasmas heated by lasers.

Sparks in iodine vapour have been studied at pressures between 0.08 and 0.80 Torr, and for gaps of the order of 10 cm. Sparks were initiated in non-uniform fields, using voltage pulses between 1 kV and 3 kV, corresponding to overvoltages of up to 300%. Development of the spark proceeds through a glow phase to a filamentary arc. The glow-arc transition is a two-stage process, involving the formation of a cathode spot, with a corresponding drop in cathode fall voltage, followed by constriction of the positive column, during which molecular dissociation occurs. Following construction, the arc expands radially and a temperature of 5000K and electron density of 6*10¹² cm^-3 are reached before decay sets in. During spark decay, a transition occurs from an atomic to a molecular arc phase.

Back-scatter Mossbauer experiments have been conducted to investigate residual and induced anisotropic structures in samples of low-carbon steel rods. Some unusual effects were observed, particularly when the specimens were subjected to mechanical polishing.

For pt.VII see ibid., vol.14, p.1293 (1981). The Swift effect has been considered in relation to the simple torsion mode of deformation. The parameters appearing in a particular increment type constitutive equation for an isotropic incompressible strain hardening elastic-plastic solid with large deformation have been computed assuming a particular form of piece-wise continuous yield condition. These parameters are considered, where possible, in terms of modified Lobes variables. The Swift effect is also considered in relation to the fracture of various metals twisted in simple torsion. A tentative failure criterion for ductile materials is considered in relation to particular values of two of the parameters associated with the Swift effect.

Deep-level transient current spectroscopy (DLTCS) as an alternative of the well-known DLTS capacitive method is suggested to analyse electronic relaxation phenomena in metal-insulator-semiconductor (MIS) structures. The DLTCS spectra of Al-SiO₂-Si diodes are compared with those obtained using the thermal dielectric relaxation current (TDRC) technique. Attempts are presented to estimate the interface states density distribution throughout the band-gap of the semiconductor.

There are significant differences in the field dependence of conductivity in low-density polyethylene as reported by various investigators. These differences are attributable partly to the presence of different kinds of impurities in variable concentrations in polyethylene samples of various origin, but are mainly due to the different measurement procedures adopted. The significance of the time which elapses after the application of voltage to the electrodes, as well as the effect of conditioning, e.g. thermal and voltage stressing of the specimen before and during the measurement, is discussed. Whether it is always necessary to obtain a steady-state current in order to determine the true conductivity is somewhat debatable.

The responses of Cu/Cr-p-type silicon and Au-n-type silicon Schottky barrier solar cells at higher temperatures have been studied. The variations of different factors, such as intrinsic carrier density, energy gap, absorption coefficient, barrier height, effective number of photons etc., have been taken into account. The results show that in the temperature range 300-450K, except for the short-circuit current, all other factors (e.g. open-circuit voltage, fill-factor and conversion efficiency) decrease with increase in temperature.

Interface states in Al-, Au- and Sn-GaAs Schottky barriers were investigated by means of impedance measurements. From the deviation of the Mott-Schottky behaviour under forward bias, an interface-states parallel capacitance C_p' could be derived. For the dependence of C_p' versus forward voltage and C_p' versus frequency, simple laws were found. From C_p' the interface state density multiplied by the capture cross-section could be calculated. Large values were found for the reactive metals Al and Au and lower values for the rather inert metal Sn. In all GaAs Schottky barriers studied, only a thin ( approximately=2 nm) interfacial oxide layer was present, and the interface states were identified as metal-induced states.

An applied electric field has a strong effect on the contact charging of an insulator by a metal. However, the experiments reported show that previous interpretations of the effect of an electric field are probably wrong, and that it is not possible to estimate the depth of penetration of charge from observations of the effect of an electric field on contact electrification.

Sn-doped In₂O₃ films (ITO) have been prepared by coevaporation of In and Sn on to heated glass substrates at 10^-3 Torr O₂ and the electrical properties have been investigated. Tin-doping suppresses the oxidation of In under evaporation, and higher substrate temperatures, above 400 degrees C, are necessary to obtain a visible transparent film (80% at 550 nm) than in the case of undoped In₂O₃ films. By tin-doping, an ITO film with the lowest resistivity rho =6*10^-4 Omega cm has been obtained at tin-doping of Sn/In approximately=0.05 atomic ratio, where the carrier concentration is 7*10²⁰ cm^-3 and the mobility is 15 cm² V^-1 s^-1. All the films show n-type conductivity with complete degeneracy.

Charge transport in steam-grown thin films of silicon dioxide (SiO₂) is studied using a short-duration pulse of energetic electrons for generating hole-electron pairs in the oxide. Within the time resolution of the measuring circuitry ( approximately 1 ns) electron transport follows the excitation pulse exactly. It is shown that electron-beam-induced currents in SiO₂ are limited by geminate or columnar recombination processes and that the incorrect application of the normal photoconduction equation to experimental data leads to invalid thickness-dependent mobility-lifetime products ( mu tau ). The present measurements indicate that for electron mu tau >>1.2*10^-13 m² V^-1 and mu >1.2*10^-4 m² V^-1 s^-1.

Prebreakdown phenomena of KCl (100) in a point-plane electrode system on the application of a high-voltage pulse of 1 ns rise time and 100 ns duration, were investigated by optical methods. Two spike light pulses were detected using a fast photomultiplier just at the instants of application and removal of high voltage for both point polarities. When successive positive pulses were applied to a point electrode the light output decreased monotonically. The light intensity obtained at the second pulse returned to the value of the first one when the time interval between pulses succeeded 3 h. These results can be explained by the effect of space charge associated with the holes, which is formed by positive streamer growth on the application of the preceding pulse.

With ultrasonic fatigue-cycling, a dumb-bell design of specimen is commonly used, some being of a shape for which the well-known stress equations pertaining to a stub dumb-bell would only give a poor approximation to the actual stress endured. In cases where the gauge-fillets are too large to be ignored theoretically, the stress distribution is examined in detail and expressions are found for the maximum stress amplitude S and resonant half-length l. Special requirements for S and l leading to optimum specimen design are also found.

The detection of cracks in fibre reinforced plastics is becoming more important as these materials are being used increasingly in load-bearing applications. The authors describe how the measurement of specific damping capacity can be used to detect very small cracks in unidirectional glass-fibre-reinforced polyester resin. This technique allowed the unambiguous detection of very small 'cracks', invisible to the naked eye, as well as the detection of small visible cracks, which could not be detected by an alternative technique using an infrared camera.