Table of contents

This review surveys the behaviour of graphite under loading conditions in which the stressing is on the whole tensile. The performance of a recently introduced model of graphite failure is reviewed in relation to the models from which it was developed, and against data obtained from a wide range of experiments on the failure and deformation properties of graphite in tension and bend. The new model represents, to some extent, an amalgamation of two earlier models, which in themselves are only capable of describing graphite behaviour to a limited extent. it is demonstrated to be very successful in describing a wide range of experimental failure behaviour of VFT pitch coke and IM1-24 graphites in tensile and three- and four-point bend stress states. The introduction of non-conservative microstructural processes allows strain hysteresis to be predicted. Furthermore, it is demonstrated that such processes may be associated with a component of acoustic emission which, together with a further component attributed to pore extension by cracking, account for the experimentally observed frequency and energy of events from tensile stressed specimens. The possible extension of the new model, for instance to include the effects of fast neutron irradiation and radiolytic oxidation experienced in a nuclear reactor environment or to allow its use in finite element stress analysis, is discussed.

The maximum entropy approach is used to calculate the escape probability, the emergent and transmitted intensities for a finite slab, and the emergent intensity for a semi-infinite medium. Numerical values are given and compared with the exact results which show that the results by the maximum entropy method converge to the exact results.

Induction currents due to a moving flat charge of three types (plane charge, charged plate conductor and space charge layer) between parallel plate electrodes were analytically studied by use of the energy balance principle, where both kinetic energy consumption and potential energy gain were taken into account. A problem concerning application of the energy balance principle asserted by Brignell and Evison (1973) has been resolved.

The CO₂ active medium of an 86 MHz radiofrequency flowing waveguide laser has been investigated. Optical diagnostics in the visible-infrared spectral region, mass spectrometry and the Langmuir probe technique have been employed to study He-N₂-CO₂ discharge at various pressures, compositions and discharge powers. The vibrational distributions of CO(X) and N₂(B and C) electronic states have been measured. Evidence for a moderate vibrational disequilibrium has been found. The N₂(X, v=1) vibrational and gas temperatures have been deduced. CO₂ dissociation yields have also been measured. A very simple technique consisting of trapping CO₂ from the discharge output stream in a liquid nitrogen cooled sample has been applied. Dissociation yield varies from 15% to 90% depending on the flow rate, mixture composition, pressure (35-80 Torr) and power. The electron energy distribution function measured by Langmuir probes at low pressure in a large parallel plate reactor shows that the addition of 6% CO₂ to a He-20% N₂ mixture enhances both the tail and the bulk of the EEDF.

The laminar accelerating flow of a thin film along a vertical wall is examined by using similarity principles and an empirical relation for the boundary layer thickness in terms of the entrance length parameter and the interaction parameter is suggested, which will be useful for comparison of results obtained by any other simple approximate solutions.

Couette flow of a viscous, compressible Newtonian fluid in a channel, bounded by a moving impermeable wall and a stationary naturally permeable bed, is investigated. The effect of the permeability on the velocity and temperature field is discussed, taking viscous heating into consideration.

With the aid of spectroscopic and microwave measurements, the authors developed a simple collisional radiative model (CRM) for helium that they applied in a low-pressure range (<20 mTorr) where atom-atom inelastic collisions are negligible. Exploiting the two main populating channels of the n=3 helium levels, and assuming that they involve two electron groups, they deduced the gas temperature and two electronic temperatures characterizing these two electron groups. The electron energy distribution function tail temperature, higher than the core one, varies between 4.0 and 7.5 eV according to gas composition (Ar-He-F₂), total pressure and coupled RF power values.

A simplified simulation of an arc response is carried out by using energy balance and a rate equation for electrons with ionization and recombination coefficients. An argon arc gap is supposed to be submitted to a re-ignition voltage, which generates a fast increase of the electronic temperature. A comparison of calculated results with arc measurements shows that such an analysis may be useful in order to determine characteristics of re-ignition processes.

Modeling of gas movements in a circuit breaker arc chamber requires an accurate knowledge of thermodynamic functions. The proposed method gives a simplified form of the law of state, in close agreement with the tables. This law has been implemented in the NS2 code developed at Merlin Gerin, France. This code allows detailed investigation of gas flows in circuit breakers.

Two parallel experiments have been undertaken to investigate the mechanisms which govern the recovery of spark gap switches in repetitive applications. Measurement of the neutral gas density is accomplished using a laser Schlieren technique which enables spatial and temporal cooling of the gas to be observed. The rate of rise of voltage recovery is recorded using a double-pulse modulator system which determines the breakdown voltage of the gas at specific time intervals after breakdown by the first pulse. A continuous sweep voltage is applied to the switch in order to establish the influence of ions on the recovery characteristics. Results are presented for SF₆/bin and air which show that voltage recovery is not solely governed by recovery of neutral gas density. There is a significant influence from the residual ion population created by the previous discharge. This influence is dominant for several hundred milliseconds after breakdown. The application of a suitable sweep voltage effectively minimizes the ion population, which results in significantly improved voltage recovery characteristics.

SnO₂:P/SiO₂/n-Si (textured) (SPNT) heterojunctions were prepared by chemical vapour deposition of phosphorus-doped tin oxide films onto heated n-Si single crystals with <100> orientation and a resistivity of 0.1 Omega cm. Before the deposition of SnO₂:P the wafers were textured and a SiO₂ layer was chemically grown onto the n-Si surface. The electrical properties of the SPNTheterojunction were studied by means of current-voltage (I-V) and capacitance-voltage (C-V) measurements. The dark I-V characteristics showed that injected current controls the transport mechanism. The presence of interface states was found to play an important role in the electrical properties of the junction. The C-V measurements at a frequency of 1 kHz showed the abrupt nature of a junction with a built-in voltage V_d=0.82 V. A solar cell of active area 2.0 cm² shows a conversion efficiency of 12.6% under AM1 simulated irradiation.

The initial permeability of soft polycrystalline ferrites exhibits thermal hysteresis when the temperature is cycled from above the Curie temperature T_c to below. The temperature loops are classified into two categories according to the presence or absence of a secondary peak (SP), in addition to the Hopkinson peak, in the thermal spectra. Hysteresis is studied in detail in this work. The role of the transition points T_c and T_SP (SP for secondary peak) is shown. The hysteresis is explained by taking into account domain wall pinning and competition between the intrinsic parameters M_S and K₁ around the transition points. This explanation seems to support the fundamental role of a domain wall mechanism in the magnetization process and the idea of a domain wall topography specific to toroidal soft ferrite samples.

Changes, provoked by poling in the corona discharge and isothermal annealing, in the elemental composition of the surface layer and in the electret properties of PbMg₁3/Nb₂3/O₃ (PMN) single crystals have been studied. Air oxidation has been found to enrich the surface with Mg. This confirms the authors finding that, upon air oxidation, the surface of multicomponent oxide compounds is enriched with the element whose heat of oxide formation is greater relative to one oxygen molecule. Sizeable variations of the PMN surface composition correlate with changes in the electret potential and in its relaxation time. The temperature critical for the changes to occur in elemental composition and electret properties has been found to be 500 degrees C.

Accurate knowledge of the relative transition probabilities is important for deconvolution of characteristic lines in X-ray spectra. A large discrepancy between the anticipated and the true line intensity may even lead to qualitative misinterpretation of a measured spectrum, resulting in a false identification of an element which is not really present in the examined sample. Prediction of the relative line intensities for arbitrary experimental conditions requires the knowledge of the (relative) transition probabilities. Experimentally determined relative transition probabilities are presented here for 57<or=Z<or=68. This transition belongs to the M_zeta X-ray line. The superior spectral resolution of a wavelength dispersive spectrometer is exploited in these experiments. The detection efficiency of the spectrometer is determined from the measured intensity distribution of the Bremsstrahlung radiation. Self-absorption effects are eliminated by extrapolation to the absorption-free limit. The estimated reliability of the presented data is 10%. The authors measured transition probabilities exceed the tabulated values by three orders of magnitude. The present data are in accordance with the few previous observations available.

Au/Ti lateral tunnel diodes with interelectrode gaps less than 50 nm have been fabricated on Si₃N₄ substrates using nanolithographical techniques. The fabricated devices were found to conduct by Fowler-Nordheim tunnelling both in high vacuum and at atmospheric pressure. The electrical characteristics of these devices showed that by using nanometre-size gaps between cathode and anode, vacuum microelectronic devices can be made to operate at ambient pressures much higher than those normally allowed at present.

A modification of an earlier technique, to measure the distribution of space charge in the insulation of a coaxial cable, under both DC and AC electric fields, is described. The mathematical analysis is developed on the basis of acoustic waves propagating in media with cylindrical geometry. In order to measure the charge distribution in AC electric fields at any phase angle, a phase shift technique has been employed. The method of calibration of charge density is given, and the charge distribution in the insulation (2 mm in thickness) of XLPE cable has been measured under DC voltages (+or-40 kV, at room temperature) and AC voltage (17 kV peak, at 24 degrees C and 105 degrees C).

Laser light scattering, associated with molecular beam flux measurements, provide an easy to perform, sensitive, non-destructive technique for the determination of relative mean sizes of molecular clusters in molecular beams formed with different expansion conditions. The results are compared to an absolute method used in the authors' laboratory and show good agreement in the range investigated (n approximately=150-4000).

Distributions of elastic stresses in individual small particles (SPS) compressed normally by closest neighbours in loaded ensembles of SPS are studied in detail. Schemes of two, eight and twelve contacts with closest neighbours calculated by means of the finite element method are considered. On the basis of numerical results, simple algebraic expressions approximating the distributions of the hydrostatic stresses and the intensity of shear stresses in SPS having Hertzian contacts with their neighbours are obtained. An increase in the number of neighbours leads to more uniform stresses inside the SP. The development of a stressed state with an increase in the number of contacts is analysed.

Stable negative ions containing up to sixteen silicon atoms have been measured by mass spectrometry in RF power-modulated silane plasmas for amorphous silicon deposition. These hydrogenated silicon cluster ions reach much higher masses than the positive ions, which have no more then six silicon atoms. This supports the view that negative ions are the precursors to particulate formation in silane plasmas. The time-dependent fluxes of positive and negative ions from the plasma are shown with a 5 mu s time resolution. Possible cluster reaction sequences are discussed and the effect of visible light on the negative ion signal is commented upon.

In the plasma processing of ultrafine particles of material, the heat transfer and force are considerably affected by particle charging. A new model, including thermal electron emission and incorporating the effect of electric field near the particle surface, is developed for metallic spherical particles under the condition of a thin plasma sheath. Based on this model, the particle floating potential, and thus the heat transfer and force, can be determined more accurately and more realistically than previously.

Electrical resistivity and optical properties were studied for reactively RF-sputtered tin oxide films with and without molybdenum doping. In contrast with some earlier results, the authors were unable to find that the molybdenum contributed to a low resistivity.