Table of contents

A novel, magnetic way of characterizing the geometric structure of the voids in macroporous material is presented. A system of methane gas confined interstitially in a close-packed assembly of non-absorbent zirconia beads constitutes a two-phase model (pore and matrix) of simple porous media. Nuclear magnetic resonance measurements of the proton spectral linewidth and spin echo decay are reported. They are compared with a theoretical model of the intrinsic, non-uniform, magnetic field and its gradient, which are due to the heterogeneous magnetic susceptibility. Three magnetic parameters are derived, which indirectly characterize the pore geometry and which adequately describe the system over seven-fold ranges of gas pressure and bead size.

The after-effect function, F(t), or magnetization decay of a ferrofluid sample may be determined from frequency-dependent complex relative magnetic susceptibility data of the sample. Here we report on the effect that two distribution functions, namely the magnetic analogues of the Cole-Cole and Frohlich distribution functions, have on the after-effect function of three ferrofluid samples. The samples chosen were those for which good alternating current susceptibility data, chi '( omega ) and xi ( omega ), were available, namely in the lower frequency range in which the frequency of the maximum of xi "( omega ), f_max, is indicative of the presence of aggregates. However, because of a lack of knowledge of the sizes of these aggregates it was not possible to generate a distribution function. By fitting the Cole-Cole and Frohlich distribution functions to the susceptibility data it is shown that the after-effect function is very sensitive to the distribution used, illustrating the difficulty of determining accurate after-effect functions without an adequate knowledge of the distribution function. It is shown that, in all cases, the use of the Frohlich distribution function results in a decay function with the fastest initial rate of decay.

The values of the crystalline electric field (CEF) parameters of rare earth intermetallic compounds Ce(Zn_1-xCu_x)₂ (x=0, 0.25, 1) were calculated from the least-squares fit to the reciprocal paramagnetic susceptibility along principal crystalline axes. The results of the calculation are in good agreement with the experimental values.

Gyroelectric chiral media, via blending the effects of Faraday rotation with those of optical activity, have potential applications in chirality management. In the present paper, a field representation is developed based on the characteristic wave concept and angular spectral expansion for this class of medium. The analysis reveals that the solution of source-free vector wave equation is a sum-integral form of circular cylindrical vector wavefunctions in isotropic media. Applications of the present theory in scattering and waveguides are considered.

An analytical formalism is proposed to describe the energy distributions of an electron beam transmitted at a right angle through solid films of any material and thickness, over the incident energy range 0.5<or=E₀<or=200 keV. Comparisons are established with experiments in order to check the applicability of the proposed formalism. It is found that the best choice of adjustable parameters in our formula is primarily affected by the large scatter of data available in the literature.

Two prototype ballasted electrodes were fabricated and tested with a large-volume, reduced pressure, pulsed CO₂ laser gas discharge, initiated by corona pre-ionization, and pumped by pulse widths up to 19 mu s. The design of segmented, ballasted cathodes is considered and equations are derived to quantify the degree of discharge arcing given partial collapse of the discharge. The number of segments passing the fault current was experimentally investigated as a function of the discharge power and found to increase until each segment passed the fault current. Compared with a planar-planar electrode geometry, under identical pulse excitation conditions, the ballast electrodes gave a factor of four improvement in the maximum discharge power depositions, prior to the development of instabilities. Higher frequencies were obtained with pulse trains below 50 ms, the maximum observed frequency being approximately 5 kHz. Arcing for the ballasted electrode configurations was observed in the middle of the electrodes, and its position corresponded to a measured non-uniform gas velocity.

In previous work the chi ⁽³⁾ nonlinear optical behaviour of blue, thermally evaporated P4BCMU has been investigated at a wavelength of 1.064 mu m using grating coupled waveguide techniques. The measured response was shown to be electronic in origin. In this work similar, though not identical, techniques are used to determine the nonlinear response of a related polymer, PSBCMU. The results show an irradiance-dependent refractive index xi ⁽³⁾ value (Re( chi ⁽³⁾)=-4.4*10^-11 esu (2.0*10^-19 m² V^-2); lm( chi ⁽³⁾)=4.4*10^-11 esu) much higher than some reported values for red spun P4BCMU films but significantly less than for the blue form P4BCMU films. The results rule out the need for higher order terms in the polarization expansion for this P9BCMU film up to an irradiance of at least 6 GW cm^-2. The nonlinearity is shown to be genuinely of electronic origin.

Tunable near infrared (NIR) radiation from 1200 to 1800 nm is generated by non-collinear difference frequency mixing of Nd:YAG and its second harmonic pumped dye laser radiations in a flux grown KTP crystal. In the process, the 'retreation phenomena' in the phase matching curve for difference frequency mixing in KTP is observed for the first time.

A helium gas flowing positive column plasma has been studied theoretically and experimentally. Coupled transport equations for electrons, metastable species, atomic and molecular ions have been solved numerically, and axial and radial species density profiles have been calculated for a steady state cylindrical discharge tube under laminar gas flow. Experimental confirmation has been conducted by measuring the plasma parameter profiles using an electrostatic probe, and results agree well with the present model.

On the basis of a continuum multi-fluid description of a plasma, a theory is developed of an ionization layer between a plasma in ionization (Saha) equilibrium and the space-charge sheath near a surface of a cathode or of an insulating wall. The crucial parameter determining the character of the solution is the ratio of the recombination length to the mean free path ion-neutral species; if this ratio is large, then the developed theory is consistent with a conventional diffusion approach. Numerical and asymptotic solutions of a stated problem are obtained. It is found that the problem is solvable only if the degree of ionization does not exceed a certain value, which varies between approximately 0.6 and 1. Beyond this value, the equation of the pressure balance cannot be satisfied and the plasma as a whole in the considered layer cannot stay in static equilibrium. A simple approximate formula for the ion flux is obtained.

A detailed model of the behaviour of charged species in the afterglow of a nitrogen plasma in a tubular geometry has been developed. The axisymmetric model for the species concentrations (N_k⁺, where k=1, 2, 3 and 4) accounted for radial and axial ambipolar diffusion, homogeneous recombination, homogeneous ionization, ion-neutral species rearrangements and recombination reactions at the sheath boundary at the wall. Comparison of the model results with experimental measurements showed agreement to within the experimental uncertainties. By systematically varying the terms in the model, it was possible to identify the important reactions in the afterglow. In terms of total ion concentrations, the homogeneous ionization reactions were a significant source term, particularly near the entrance of the afterglow, and recombination at the sheath boundary layer was the dominant sink term (homogeneous recombination was negligible). Indeed, the rate of recombination at the sheath boundary was so fast that the overall process is limited by the rate of radial diffusion of ions to the sheath. The ion-neutral species reactions, being charge-conserving, had little effect on the total concentration of ions; however, their importance in determining the relative concentrations was identified. It was also demonstrated that a fully two-dimensional model of the afterglow was necessary to predict the experimental measurements accurately. Truncation of the model to one dimension produced results that disagreed with experiment by several orders of magnitude.

The purpose of this paper is to present an electrical model of the leader current, which is relevant to describe the current resulting from an oscillating impulse voltage. The model allows one to evaluate the leader current, the electrical charge and the potential profile along the leader channel. The physical parameters of the leader are described by making use of an equivalent electrical network. By integrating these parameters in a test circuit, we are able to deduce the current and the electrical charge at any point in the air gap. Our results are then compared with the experimental ones obtained for discharges in large air gaps that are produced experimentally. The current and electrical charge characteristics given by the model are in good agreement with the experimental ones.

The plasma generated by a coaxial plasma puff-gun is studied using a double Langmuir probe and spectrometer. An analytical solution of the plasma sheath motion is also presented. The charge voltage of the energy-storage capacitors of the gun ranges from 1.5 to 4.0 kV, the plasma electron temperature from 10 to 20 eV, the kinetic energy from 45 to 310 eV and the density from 5*10¹³to 7*10¹⁴cm^-3. An approximate solution has been obtained under the assumption of the snowplow model, and the comparison of the predictions of the theory with our experimental results indicates general qualitative agreement.

The number densities of tungsten atoms (W I) and singly charged tungsten ions (W II) in a recovering pseudospark switch were measured by laser-induced fluorescence after current pulses with amplitudes 6-17 kA and 1.6 mu s duration. Atomic and ionic energy levels up to 0.77 eV were taken into account to obtain approximate values for the total number densities. At the end of current flow the density of both neutral species and ions is of the order of 10¹⁸ m^-3. Within 100 mu s the densities of W I and W II drop by three and five orders of magnitude, respectively. Excitation temperatures of W I exceed 4000 K initially, and have dropped to 2000 K by 100 mu s after the end of current flow. The densities of both W I and W II are found to be too low relative to the densities of neutral hydrogen and electrons, respectively, to have a significant influence on the recovery process.

An ion-neutral species collision model with charge exchange is developed for use in particle simulation of a glow discharge based on an extension of the theory of Langevin (1905) and Hasse (1926). The validity of the model is checked by Monte Carlo calculation of drift velocity for He⁺-He, Ne⁺-Ne, Ar⁺-Ar and Kr⁺-Kr collisions. The results show good agreement with the experimental data and the solution by the moment method, especially for heavy gases. By proper choice of the cut-off value of the dimensionless impact parameter in the model, it is applicable to a collision of ions of energy 1 keV.

The properties of the cathode-directed positive streamer discharge, propagating in a uniform electric field, have been investigated. Precautions have been taken to keep the effects of space charge, due to branching, to a minimum, and conditions under which streamers possess minimum initial energy have been obtained. In this way, a threshold field of 440 kV m^-1 for the propagation of a single streamer has been determined. The velocity of streamers in the uniform field has been studied. This rises linearly with the electric field and the results confirm that space-charge effects are negligible. Comparison is made with previously published work.

Measurements are reported of the variation of the electric field required for propagation of positive streamers in air as a function of ambient temperature. Propagation velocities have also been measured as function of field and temperature. The results are discussed in terms of changes of air density consequent upon the temperature changes and comparisons are made with other work in which density has been changed by variation of pressure at constant temperature. There is some evidence that specific temperature effects exist. The variation of streamer velocity with air density indicates that a constant reduced total field E/N is maintained in the avalanches, replicating the streamer tip as the density is changed.

Spatial distributions of plasma temperature and electron density have been measured in a transferred direct current are running at atmospheric pressure in 500 scc s^-1 nitrogen gas flow. The measurements were made at 100-250 A current for two arc lengths of 100 and 200 mm. Optical spectroscopy techniques have been used to determine the plasma parameters using local emissivities of atomic and ion spectral lines, as well as continuum radiation. In the are positive column the parameters agree well, that is the plasma state is close to local thermodynamic equilibrium. Near the cathode some variation has been observed in the measured data, showing a possible deviation from the equilibrium state: emissivities of atomic and ion lines give very different results. The phenomenon can be explained, if one supposes that the plasma ionization composition does not correspond with an equilibrium model of the plasma in this spatial region, with the electron density being higher than would follow from the Saha equation for pure nitrogen plasmas. This is supported by our measurement data of the plasma electron density.

Heat transfer to a thermally isolated graphite anode in a long duration vacuum arc was investigated. The anode bulk temperature was measured as a function of time using two high-temperature thermocouples. The anode surface temperature was optically determined. The visual image of the arc was detected with the same optical system. A surface temperature of 2300 K was obtained in a 340 A arc. A one-dimensional nonlinear heat flow model for the anode was developed. A solution was obtained using a dynamic numerical method and the time-dependent effective anode potential (defined as the ratio between the input power to the anode and the arc current) was determined to decrease from a value of 10, 12 V at t=0 to approximately 6.3 V at steady state observed for t>or=100, 60 s, for arc currents of 175 and 340 A respectively. Input power to the anode decreased on a timescale approximately coinciding with the appearance of an anodic plume which developed during the arcing. The effective potential of the water-cooled copper cathode (defined as the ratio between the net input power to the cathode and the arc current) was determined to rise from 6.6 V near the beginning of the arc to 7.2 V during steady state. The total arc voltage, measured between the electrodes, increased from a value of approximately 21 V to a steady state value of 24, 26 V for arc currents of 175 and 340 A respectively.

Species conservation equations for SF₆⁺, SF₆^-, and electrons have been solved by the method of MacCormack with a Shuman filter and by the flux corrected transport (FCT) method. A detailed comparison of numerical results with analytical solutions to species conservation equations shows that in order to achieve the same accuracy the grid size required by both methods is the same. Compared with the FCT method, the algorithms of MacCormack's method with Shuman filter are the simpler and computational costs are lower.

Donor-doped n-BaTiO₃ polycrystalline ceramics show a strong negative temperature coefficient of resistivity below the orthorhombic-rhombohedral phase transition point, from 10^2-3 Omega cm at 190 K to 10^10-13 Omega cm at <or approximately=50 K, with thermal coefficient of resistance alpha =20-23% K^-1. Stable thermal sensors for low-temperature applications are realized therefrom. The negative temperature coefficient of resistivity region can be modified by substituting isovalent ions in the lattice. Highly nonlinear current-voltage (I-V) curves are observed at low temperatures, with a voltage maximum followed by the negative differential resistance. The I-V curves are sensitive to dissipation so that cryogenic sensors can be fabricated for liquid level control, flow rate monitoring, radiation detection or in-rush voltage limitation.

A model for photoconductivity decay in which initial trapping occurs at surface traps, followed by deep trapping at volume traps, is studied. The Monte Carlo simulations follow the random walk of the electron and hole following excitation. The volume traps are assumed to be randomly dispersed, but their effect on electron motion is simulated by trapping at the boundaries of concentric cubes that span the volume of the microcrystal. The probability of trapping at the boundaries is a function of the radius and concentration of traps. Although this model is a necessary component in any model to study doping effects in AgBr microcrystals, it fails to produce the correct dependence of decay time on microcrystal edge length observed in undoped microcrystals. This failure arises because the lower surface-to-volume ratio in large microcrystals causes electrons to spend less time at surface traps, which makes them more prone to volume trapping. Experimentalists have determined trapping radii of dopants by plotting trapping rates against dopant concentration and extracting the trapping radius from the slope of the linear relationship. A similar linear relationship was seen in the simulation results, but the trapping radius required to reproduce the experimental slopes was a factor of two to three larger than that determined experimentally. This discrepancy is most probably due to neglect of electron-hole recombination in deriving equations used to calculate the experimental trapping radius.

A nuclear magnetic resonance imaging method is demonstrated for measuring diffusion of a paramagnetic tracer in an aqueous gel, and the hydrodynamic dispersion of the same tracer in a porous medium. The fast inversion recovery method for determining the longitudinal relaxation times T₁ is used in conjunction with a standard Fourier imaging scheme to obtain images with null contours, which track particular tracer concentrations. The fast inversion recovery method offers considerable saving of experimental time relative to the standard inversion recovery protocol, but without introducing dependences on transverse relaxation time T₂, which would be hard to quantify. With correct parameter choice, null contours can be localized in the images within two pixels and the corresponding tracer concentrations identified from longitudinal relaxation data only. In a cylindrical gel sample, concentration fronts are tracked and yield diffusivities in agreement with results previously reported for a similar one-dimensional method; in a core of natural sedimentary rock, we image clearly the markedly irregular advance of a concentration front caused by natural heterogeneity in the medium. The method is applicable to a variety of fields involving diffusion or dispersion in microscopically or macroscopically heterogeneous media.

Electrical tree growth (a long-term electrical breakdown process) has been investigated in Araldite CT200 and CT1200 epoxy resins as a function of voltage and material age (defined as the time between manufacture and testing of pin-plane samples). Reproducible and predictable electrical tree growth was obtained for both CT200 and CT1200 epoxy resins provided that (i) the essentially random tree initiation time is removed and (ii) the samples tested were of the same age. The tree growth and time to failure (defined as the time to breakdown from a pre-initiated 10 mu m tree) characteristics as a function of both voltage and sample age show large step changes at a critical voltage and critical age. In particular, the resin physical ageing has a large effect on the tree growth behaviour, with the time to failure varying by three orders of magnitude over a time span of 3 years. Measurements of some of the physical properties (residual internal mechanical stress, surface refractive index, glass transition temperature and dielectric loss) of CT200 epoxy resin all indicate the occurrence of physical ageing of the resin, with structural (network) relaxation as the most important ageing process. However, these measurements are unable to account for the step change (critical age effect) found in the time to failure of tree growth. The fractal nature of tree growth and its relationship with voltage and the long-term changes in the properties of the resin are briefly commented upon.

A model is proposed to explain the superlinear growth of thermoluminescence, based on a consideration of competition both in excitation and during heating between occupation of traps and luminescence centres. Before excitation, it is assumed that a population of electrons r₀ remains in the competing traps while the same number of holes is left in the luminescence centres. On this basis, an implicit expression is obtained for the area under a glow curve, from which several thermoluminescence response curves for different sets of parameters can be obtained. The results show that the area under a glow curve at low doses has a linear dose-dependence for a large r₀ and has a quadratic dose-dependence for a small value of r₀. In the case of early saturation of the competing traps, the dose-dependence is more than quadratic.

Inorganic dusts from different herbs and spices are investigated in this study using both standard thermoluminescence measurements of integrated intensity versus temperature and measurements of the thermoluminescence emission spectra (intensity as a function of both temperature and wavelength). The importance of particular minerals in the composition of glow peaks detected is discussed. Thermoluminescence emission spectra of the samples studied reveal (i) the presence of a broad high-intensity signal in the red to infra-red spectral region in all irradiated polymineral dusts; (ii) the hitherto unsuspected importance of calcite in the polymineral fractions of the dust; (iii) the existence of high-temperature unbleachable peaks from the silicate fraction in the orange to near infra-red portions of the emission spectrum; and (iv) the potential for quantification of absorbed dose. Discrimination between irradiated and un-irradiated spices appears to be feasible.

Chromium nitride layers with CrN and Cr₂N stoichiometry were deposited onto silicon substrates by means of reactive RF-magnetron sputtering at different substrate temperatures (300-830 K). The layer thicknesses were 100 to 1500 nm. Composition, surface roughness, grain size, microstructure and phase formation were analysed using RBS (Rutherford backscattering spectrometry), RNRA (resonant nuclear reaction analysis), PAC (perturbed angular correlation), STM (scanning tunnelling microscopy) and XRD (x-ray diffraction). RBS and RNRA depth profiling of the samples revealed for both nitrides homogeneous Cr and N concentrations over the whole layer depth. For Cr₂N films, indications of a phase transformation due to the increased target temperature from a disordered Cr/N phase to the ordered Cr₂N phase were obtained via XRD and PAC. The lateral grain size measured with STM and the vertical grain size measured with XRD are approximately the same for both nitrides at all deposition temperatures.

This study determined the effect of PMMA addition on the crystalline forms ( alpha and beta phases) of PVDF, crystallized from solution. The changes in the content of the two phases due to addition of different concentrations of PMMA at different crystallization temperatures were detected by IR spectroscopy. DSC analysis revealed that the melting temperature for both phases and the crystallization temperature of the alpha -phase decreased slowly with the increase of PMMA concentration. It was also observed that small concentrations of PMMA (10 and 15 wt%) favoured the beta -phase crystallization.

A qualitative study of the phase shifts in coplanar waveguides (CPWs) with various periodically patterned conductive top covers is assessed experimentally. An increase in the dynamic range of the phase shift has been observed by altering the CPW between needle-like-patterned and uniform conductive top structures. A CPW with a fine-patterned top cover coated with CdS material was examined to demonstrate the potential application as an optically controlled CPW phase shifter.

Solid state batteries using pure and doped sodium yttrium fluoride as the solid electrolytes were fabricated at ambient temperature. The discharge characteristics of the cells for a load of 1 M Omega were studied. The cell with NaYF₄:Dy as the solid electrolyte exhibited a larger plateau region compared with the other cells. Open circuit voltage and short circuit current of the cells range from 2.5 to 2.7 V and 7 mu A to 435 mu A respectively. The performances of the cells under varying loads for a constant voltage of 1.5 V were also investigated. Various cell parameters were evaluated.