Table of contents

The dielectric strength of hexafluoropropylene has been investigated under breakdown and prebreakdown conditions in the pressure range from 0.07 to 2 bar. Breakdown voltages measured in a uniform field gap have been found to be in good agreement with intrinsic dielectric strength evaluated from measurements of net ionisation data. The results show that the Paschen rule cannot be applied to this gas-the dielectric strength (E/p)₀ exhibits a marked increase with gas pressure at constant products of pressure times distance (pd).

The Kelvin method has been used to measure the relative surface potentials of multilayers of stearic acid deposited on n-type GaP. The potential varies with the number of layers deposited in a manner consistent with the hypothesis that the region explored consists of a Schottky exhaustion region.

The local field effects on voltage contrast in the scanning electron microscope (SEM) mean that local fields generated by a non-uniform potential distribution on specimen surface cause a variation in the secondary electron (SE) detector current. It causes some errors in the voltage measurement. The authors present a theory to calculate the SE detector current in the presence of the local fields. In the calculation, they assume that the field distribution above the specimen surface (metal electrodes with 8 mu m width and 12 mu m separation) is two-dimensional. Analysed models are a conventional SEM detector system (model A), a retarding-field energy analyser with an extraction field (model B) and a high-resolution energy analyser with an extraction field (model C). The results show that the local field effects could not be neglected even in models B and C with strong extraction fields. The calculated values of local field effects in models A and B agree well with the experimental ones. The dependence of local field effects on the electrode geometry is equivalent to that on the extraction field, though this dependency is not so strong.

Energy-space dependent neutron transport is considered in a homogeneous half-space in forward-backward (Fermi-type) and hard-sphere scattering models. Assuming a mono-energetic unit source at the free surface, analytical expressions are derived as solutions of the transport equation for both the outcoming energy dependent current and the space-energy dependent collision density in the half space.

Considers a system of two superposed fluid layers F₁ and F₂ and studies the linear response of the air-F₁ and F₁-F₂ interfaces to a modulation of the surface tension at the air-F₁ interface due to inhomogeneous heating by infrared radiation. The deflection of the F₁-F₂ interface is shown to be strongly enhanced relative to that of the air-F₁ interface. Making this deflection visible is the principle of a very sensitive IR-visible image converter.

The erosion of a surface by a stream of solid particles is examined by considering the arrival times of the particles on the surface. The treatment first proposes a new concept, an 'impact zone', and then considers the arrival of the particles on one zone as a queuing process. The rate of erosion of ductile metals is predicted using simple arguments from probability theory. A comparison of the mechanisms of erosion has been made using this analysis as a basis for models. Results indicate that not all of the mechanisms are capable of predicting the incubation time effects which have been reported previously. Also discussed is the importance of the duration of the test and the concentration of eroding particles, or flux, striking the target. A recommendation is made for a new standard to be used when quoting flux levels in future publications.

An analogue technique for the solution of the biharmonic equation is used to determine the viscous drag, for Reynolds number between 0 and 30, on cylinders falling between parallel walls. Results obtained show good agreement with numerical solutions and with results by direct experiments. Streamlines are obtained with a maximum error of 3% and vorticities and viscous drag forces of approximately 20%.

Experimental measurements of the drift velocity and the ratio of longitudinal diffusion coefficient to mobility for electrons have been obtained for the range of 2.8<or=E/N<or=566 Td for argon and 14<or=E/N<or=849 Td for krypton (1 Td=10^-17 V cm²). The results obtained from Monte Carlo simulation have been compared with the present and previously obtained data and a good fit has been achieved for the entire range of E/N. The simulation also produced results for the ionisation coefficient and the ratio of radial diffusion coefficient to mobility and these values have also been compared with other theoretical and experimental values, where available. The mean energy has also been computed and compared with the experimental breakdown voltage. This comparison has shown that equilibrium conditions exist in the region in which the present measurements have been made. As a result of this investigation, two sets of cross-sections are given which accurately simulate electron swarm parameters in argon and krypton respectively.

A pulsed drift tube has been employed for measurement of the electron attachment coefficient, alpha _a in pure sulphur dioxide over the E/N range 3.03<or=E/N<or=242.4 Td. A pressure dependence of alpha _a/(SO₂) has been observed for E/N<40.0 Td, indicating the existence of a three-body attachment process, whilst for values of E/N>40 Td the results indicate a dissociative attachment process. Measurements of the mobility of the negative ions formed indicate the presence of at least four ions with reduced mobilities of 0.69, 0.62, 0.55 and 0.39 cm² V^-1 s^-1 respectively. Using a drift tube mass spectrometer system the ion spectra produced in pure SO₂ and in SO₂-O₂ mixtures have been studied. In pure SO₂ at pressures around 1 Torr a number of large cluster ions are observed. At much lower pressures the predominant ion is SO₂^- which rapidly clusters to form SO₂.SO₂^- as the pressure increases. When SO₂ is added to oxygen the following reactions are observed: O^-+SO₂ to SO₃+e; O₂^-+SO₂ to SO₂^-+O₂. The rate for the first reaction has been measured over a range of E/N and has been found to vary between 2.0*10^-9 cm³ s^-1 and 8*10^-10 cm³ s^-1. The second reaction has a rate of 3.9*10^-9 cm³ s^-1 at E/N=117.2 Td.

The electron and ion continuity equations and Poisson's equation are solved for the motion of pulses of (i) electrons and (ii) plasma in an applied electric field of approximately 6 kV cm^-1. For the cases considered in nitrogen at a temperature of 293K and at pressures of 12 kPa, where E/N approximately 2*10^-15 V cm^-2, and drift distances are a few cm, electron drift dominates over diffusion. The relative motion of electrons and ions in a plasma pulse have characteristic features very different from ambipolar diffusion motion characteristic of low fields and pressures. An electron pulse of radius approximately 0.5 cm and density approximately 10¹¹ cm^-3 spreads rapidly due to space-charge fields, regardless of whether a uniform electric field is applied. Space-charge effects enhance the electric field at the leading edge of an electron pulse in a uniform electric field. For an initially neutral plasma of density 5*10⁶ cm^-3 in a uniform electric field, electrons move independently of the ions. When the density is increased, and space charge is significant, the electron distributions have a characteristic steep trailing edge and an almost flat leading edge. At the highest densities considered of approximately 10¹¹ cm^-3 the electric field is reduced almost to zero at the centre of the plasma by space-charge effects.

When the potential across a positive-point-plane gap in air is increased, discharges proceed in order as follows: intermittent streamer corona, glow corona and spark. The streamer is markedly enhanced by adding a small amount of an electronegative gas e.g. Cl₂, NO₂ NO etc. This is because the streamer can be initiated at a higher potential than is corona onset potential due to extinction of the glow corona. This effect becomes more marked with increased concentration of the additive. For a gap system mounted in an enclosed chamber (air), the streamer formation is enhanced by successive discharges and a pronounced fall of the sparking potential is expected to occur. This is due to NO₂ formation resulting from successive discharges.

For pt.I see ibid., vol.9, no.12, p.1809-16 (1976). The total arcing energy for a fuse link of uniform cross-section in an AC circuit of known power factor is evaluated assuming adiabatic heating of the link and a constant voltage fuse arcing characteristic. It is shown that this energy maximises for optimum values of the closing angle and prospective current. The maximum and optimum parameters are expressed in units involving the supply voltage and fuse link properties and presented as a function of fuse arcing voltage and circuit power factor. In particular, for fuse testing applications, the tolerance on closing angle and prospective current is determined in order that the arcing energy should be at least 95% of its maximum value.

There is always a voltage drop between the electrodes of a triggered gas-filled spark gap. It is shown that the magnitude of this voltage, its form and frequency, can have a dramatic effect on the damping of a triggered RLC circuit. Energy can be fed to such an oscillating circuit by externally controlling the frequency of the spark gap voltage drop waveform. The dependence of this energy transfer on the amplitude, frequency and polarity of the gap voltage drop is examined experimentally and theoretically with the aid of numerical calculations. The conclusions of this study are applied to reducing the damping influence of a spark gap and to the construction of a circuit to generate pulses of RF oscillations.

The kinetics of the precipitation and size evolution of Cs colloids in additively coloured CsI, CsBr and CsCl have been studied. In the precipitation stage the dependence of the precipitated volume fraction on the annealing time has been fitted to the expression W=1-exp(-P(t/ tau )^a). Values of a=1 for CsI and less than unity for CsBr and CsCl have been obtained. From the equilibrium between F centres and colloids, values for the enthalpy of evaporation of F centres from colloids of 0.45 eV for CsI, 0.46 eV for CsBr and 0.52 eV for CsCl have been found in the 430-610K temperature range. From optical absorption measurements, and taking into account different size distribution functions, values of the critical radius R_c of the colloids during several sets of isothermal annealings have been obtained. A linear dependence of R_c⁴ on the annealing time has been observed. A mechanism limited by dislocations or grain boundary diffusion has been proposed both for the precipitation and ripening stages.

A dislocation transformation model with three types of dislocations-namely the mobile, the immobile and those with clouds of solute atoms-is considered. Some physically reasonably reactions are postulated, leading to a coupled set of nonlinear differential equations for the rate of change of their densities. The basic idea of Cottrell's mechanism has been incorporated. It is shown that these equations admit a class of periodic solutions called limit cycles which are typical of nonlinear systems, suggesting that nonlinearity plays a fundamental role in the model. The rate equations are solved on a computer to obtain the oscillatory behaviour of the densities and hence leading to steps on the creep curve. The theory predicts that there is a range of temperature over which the phenomenon can occur, in agreement with the experiment of L.N. Zagorukuyko et al. (1977). The theory also reproduces other normal forms of creep curves.

For pt.I see ibid., vol.14, no.4, p.699 (1981). Investigates the mathematical properties of the statistical model for dislocation dynamics introduced in the context of creep. The situation corresponds to a nonstationary process in which all the cumulants depend on the density. Based on expressions derived for the first four cumulants via a series expansion derived in the authors' earlier work, they derive an approximate form for the characteristic function. The solution is shown to be a good approximation. The distribution function is platykurtic in nature. The velocity autocorrelation function is also calculated.

The problem of determining the thermoelectric properties of a randomly inhomogeneous mixture of two thermoelectric materials is reduced to a simpler problem, the conductivity of a randomly inhomogeneous conductor.

An investigation of thermally stimulated conductivity (TSC) for InP:Fe is reported. The magnitude of the observed TSC signal is discovered to be critically dependent upon the nature of the electrical contacts to the sample. Using a variety of analysis techniques, two main traps are found in the material with activation energies of approximately 0.25 eV and 0.50 eV. The ability to fill the deeper level depends on the illumination temperature and it is suggested that this might be explained by a temperature variation in the capture cross-section.

The temperature T, electric field E and pressure p dependence of the switching parameters of trissarcosine calcium chloride (TSCC) such as the switching time t_s and the maximum switching current i_max are measured. The mobility mu of the domain wall motion is expressed by the relation mu varies as exp(-W/kT), where W is the activation energy for the domain wall motion. The value of W is 0.05 eV at p=0 kbar. The effect of pressure on the switching parameters is dominated by the pressure dependence of W and alpha , and delta W/ delta p is 0.03 eV kbar^-1 and delta x/ delta p is 0.14 kV cm^-1 kbar^-1 at -156 degrees C, 0.18 kV cm^-1 kbar^-1 at -161 degrees C and 0.22 kV cm^-1 kbar^-1 at -166 degrees C. The symmetry m of the switching current transient is almost constant ( approximately=0.5) at fields below 2.5 kV cm^-1 and increases approximately with E according to the relation m varies as E^0.3 at fields above 2.5 kV cm^-1. Such a switching current transient is explained by the phenomenological theory of the polarisation reversal.

The dielectrophoretic response of different solids is investigated in liquid media of various permittivities and resistivities. The solids investigated had a wide range of both relative permittivity (3.8 to 350) and resistivity (10^-4 to 10¹⁶ Omega m). The use of dioxane and water solutions provided liquid media of controllable relative permittivity in the range 2.2 to 80. The response, which is found to be frequency dependent, is determined for various combinations of insulating and conducting materials at three frequencies: 10² Hz, 10⁴ Hz and 2.55 MHz. The results show that the resistivities of the materials exercise a control over the response comparable to that of their relative permittivities. The correlation of the observed response with that predicted using a theory developed for non-insulating materials is discussed.

It has been observed that the field emission of electrons from a micropoint carbon fibre cathode is accompanied by the emission of optical photons. Image intensifier techniques have been used both to record the visible spectrum of this radiation and to show that the dependence of its intensity on the applied voltage is consistent with it being electroluminescent in origin. The phenomenon has been qualitatively interpreted in terms of an emission mechanism involving a composite microregime consisting of a metallic-like substrate overlayed by a thin insulating medium.

Experiments are described in which radioactive zinc is diffused into n-type GaAs at temperatures of 700, 900 and 1000 degrees C. Diffusion profiles are plotted. For certain of the experiments the external zinc vapour source is removed after diffusion has proceeded for 15 minutes and the experiment is continued as a simple annealing stage for a further 15 minutes. Remarkably little further movement of the profile is found during this second stage. This effect is considered and an attempt is made to explain the results in terms of the kinetic processes involved in diffusion from a vapour source. In addition, the profiles permit an accurate value to be obtained for the diffusion coefficient of zinc in GaAs at 700 degrees C and a concentration of 5*10¹⁹ cm^-3.

The results of a study of the conversion of pyrite (obtained from coal) and marcasite, when heated to about 485 degrees C in several different atmospheres (vacuum, He, H₂ and CO) are reported. Changes in the magnetic susceptibility chi and in the mass of the sample are noted as the sample is first heated to 485 degrees C and then cooled to room temperature at the rate of about 2 degrees C min^-1. The composition parameter chi of the converted pyrohotites Fe_1-xS is determined from the known location of the various magnetic and structural transitions in Fe_1-xS and from the magnitudes of the observed chi . The mass changes indicated the conversion process FeS₂ to Fe_1-xS. Pyrite, when heated in vacuum and He, forms monoclinic pyrohotites with x approximately 0.11 to 0.13 whereas in H₂ atmosphere it converts to hexagonal pyrohotites with x approximately 0.08 to 0.10. Prolonged heating in H₂ eventually produces FeS (x=0). In CO atmosphere the end product seems to be an antiferromagnetic pyrrhotite with x approximately 0.05. For marcasite, the conversion products in vacuum and H₂ atmospheres are essentially the same as that for pyrite except that the conversion efficiencies are somewhat higher. The magnetic properties of coal pyrites vis-a-vis mineralogical pyrites are also examined.

The small-angle X-ray scattering (SAXS) method has been applied to evaluate various macromolecular parameters such as the specific inner surface, transversal lengths, the length of coherence, the range of inhomogeneity and the percentage of air present in sisal (Agave sisalana) fibre after treating the fibre according to the technically important process of mercerisation. The samples are found to behave as a densely packed two-phase system belonging to a general micelle system. The theories of O. Kratky (1966) and G. Porod (1960) have been utilised to evaluate the macromolecular parameters. The small-angle Kratky camera is used for photographic registration of the scattering intensities. Monochromatised CuK_alpha radiation is obtained by reflecting the copper radiation from a Johansson-Guinier bent-crystal monochromator. Comparison of the scattering curves showed that these treatments dissociate the scattering inhomogeneities without improving the mechanical properties and the appearance of the fibre.