Germanium space-charge regions have been studied under conditions of double-depletion achieved by applying a reverse bias to both junctions of a transistor structure. The generation and distribution of carriers between the two junctions are described in terms of models for the thermal generation of carrier pairs and for potential distribution in a cylindrical system. Measurements over a wide range of temperature reveal that generation of carriers occurs through the medium of one or more sets of recombination centers and not by direct transition between the valence and conduction bands even at elevated temperatures. The Shockley-Read recombination-generation theory is applied to obtain the activation energies associated with the recombination centers. The impurity density, base width and junction areas are estimated from measurements of punchthrough voltage and junction capacitance. For some specimens the capacitance measurements made with one junction floating confirm the sharp increase in capacitance at punchthrough noted by Barker. The distribution of current between the junctions when both are equally reverse biased is found to be roughly proportional to their areas. It is also shown that control of the reverse current across one junction may be achieved by variation of the reverse-bias on the other junction. The mechanism of this interaction is considered in terms of the diffusion of carriers between the two space-charge regions. The phenomen of slightly non-saturating reverse current is explained in terms of the expansion with reverse bias of the space-charge region within the base. An expression relating the base current and reverse bias in a cylindrical system shown to give good agreement with experiment for one specimen. For the units in which the increase in current with voltage is appreciable, space-charge expansion cannot account for the increase and two other mechanisms are considered: avalanche multiplication in the bulk and along the surface, and a high generation rate on the surface. The surface conduction channels on the base region are investigated in two ways. Maximum floating junction potentials are calculated from measured values of the common emitter amplification factor (using the Shockley 1949 theory) and these are compared with directly measured potentials. A second method involves direct measurement of a.c. surface conductance between collector and emitter when both junctions are reverse-biased to prevent bulk conduction. Both tests reveal very small degrees of surface conductance on all specimens. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The reflectivity of large single crystals of protonated and deuterated (TMTSF)₂Re0₄ and (TMTSF)₂BF₄ has been measured from ≈ 30 cm-¹ to ≈ 8000 cm-¹ using a Bruker IFS 113V Fourier Transform Interferometer for E / a and E / b' above and below the metal--insulator transitions at 177 K and 39 K respectively. The infrared powder absorption spectra of protonated and deuterated (TMTSF)₂Re0₄ has been measured from 200 cm˗¹ to 2000 cm˗¹. The Kramers-Kronig optical conductivity has been calculated from the reflectivity using Drude extrapolations to high frequency. The results for the conductivity for E / a show a one-dimensional density of states, characteristic of a one-dimensional semiconductor with strong electron-phonon coupling, with the vibrations appearing as resonances below the gap and as antiresonances above. The E / b' conductivity is smaller by almost two orders of magnitude than that for E / a, but displays the same semiconducting behavior. The phonons active in the E / b' polarization appear only as resonances. A normal coordinate analysis has been performed for protonated and deuterated TMTSF⁰ and TMTSF⁺. The results have been used to infer the frequencies of vibration and the deuterium shifts of TMTSF⁺⁰ׄ⁵. The molecular frequencies of vibration have been assigned on the basis of their observed frequencies and optical polarization, as well as their deuterium shifts. Some external phonons have also been assigned. The observation that many of the internal and external vibrations are split is due to the eightfold increase in the size of the unit cell (and subsequent reduction of the Brillouin zone) below the metal-insulator transition. The optical properties of the semiconducting state have been modelled for a one--dimensional molecular conductor with a twofold-commensurate charge-density wave, which accurately reproduces the effects of the lattice dimerization and the potential due to the anion chains. The calculations yield the electron-molecular-vibrational coupling constants for the totally symmetric a[formula omitted] vibrations of the TMTSF molecule. The model also yields a transfer integral of 1400 cm-¹ for both materials and semiconducting energy gaps of 2Δ = 1700 cm˗¹ and 2Δ = 1120 cm˗¹ for (TMTSF)₂ReO₄ and (TMTSF)₂BF₄ respectively. The optical conductivity in the E / b' polarization has been discussed in -terms of a two-dimensional band structure with anisotropic transfer integrals. The band structure calculations show the same general features as the measured spectra. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

An automated ellipsometer was used to study three topics associated with the anodic oxide films of tantalum and niobium. The electro-optic effect was measured on tantalum and niobium oxides and was found to be quadratic. The change in refractive index upon application of a field occurred in two phases: an instantaneous change followed by a slower change. The effects on Nb₂O₅ were greater than on Ta₂O₅. The effect of ultra violet light on the two oxides was found to cause a change in the refractive index before appreciable photo-induced growth occurred. The results indicated that it was unlikely that the u.v.-induced change in refractive index occurred uniformly throughout the film. The effects of annealing and temperature are discussed in relation to the constant field current transient. The change in the refractive index during the transient was monitored with the ellipsometer. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

This thesis is concerned with three aspects of magnetic properties in solids. 1. The mean square value <I(I + 1)> of the proton angular momentum per molecule has been measured to be 3.73 ± 0.18 in solid CH₄ at 4.2 K . The experiment was done by measuring the ratio of the proton magnetic resonance free induction decay signal to that of ¹³C at the same frequency in a sample containing 53% ¹³CH₄ . The measured value of <I(I + 1)> differs appreciably from the high temperature value of <I(I + 1)> =3 and from the value of <I(I + 1)> = 6 when all the molecules are converted into the A spin symmetry species. Experiments at lower temperatures using a sample containing 0.002% O₂ impurity indicated that there may be a slight increase in the value of <I(I + 1)> between 4.2 K and 2.45 K . The presence of 0.05% O₂ impurity shortened the time constant for conversion of CH₄ between different symmetry species. The interpretation of measurements of <I(I + 1)> in the presence of large amounts of O₂ impurities is complicated by changes in the NMR line shapes. These complications are discussed in terms of the theoretical results obtained in the second part of the thesis. 2. The NMR line shape inhomogeneously broadened by paramagnetic impurities in solids was investigated theoretically using the statistical method of Margenau. The general expression for the Fourier transform of the line shape function was derived. The lattice can be divided into an inner spherical cut-off region and its complement. Impurities inside the cut-off region give rise to satellite lines and impurities outside determine the shape of the main line and satellite lines. Detailed numerical calculations were performed for a f.c.c. lattice when the impurities are (a) classical magnetic dipoles, (b) spin 1/2 systems and (c) spin 1 systems. An asymmetry in the line shape is predicted at finite temperatures. The peak intensity is shifted from the centre by an amount linear in the impurity concentration and in the magnitude of the magnetic moment and the overall line shape adjusts itself in such a way to give a vanishing first moment about the centre. The line shape is always Lorentzian at the centre and Gaussian in the wings. It depends on the impurity concentration. At low concentrations, the main line and the satellite lines have the same shape which is predominantly Lorentzian. The shape of a powdered sample is also discussed. 3. General expressions are derived for the first two terms in the expansion of the specific heat of a paramagnetic salt in powers of T⁻¹. It is assumed that the interaction between the paramagnetic ions consists of magnetic dipole and isotropic exchange interactions, and that the g-tensor of each ion is axial and has the same orientation for all ions. The general formulae are evaluated numerically for cerium magnesium nitrate. Comparison with the experimental data of Mess and coworkers indicated that the exchange parameter corresponds to a ferromagnetic interaction. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The present thesis is concerned mainly with space charge and high field effects on the electrical properties of thin amorphous films. A theory of space charge contribution to the polarization current in thin dielectric films is proposed. The transient current on short-circuiting a thin dielectric film is believed to consist of two components, one due to the dielectric polarization and the other due to trapped space charge. The space charge contribution is investigated using a model for a film containing distributed traps. Computed results seem to be consistent with experimental results on Ta/Ta₂O₅/Au diodes, so that space charge effects are more important at low preapplied fields. The applicability of step response techniques to determine low frequency dielectric losses is discussed and the effect of space charge on the dielectric losses is analysed. The theory of thermoluminescence and thermally stimulated currents is extended to the case of traps with distributed binding energies to investigate the possibility of distinguishing between distributed and discrete trap levels. It seems possible to distinguish experimentally between distributed and discrete traps by using different doses of optical radiation to obtain initially different amounts of trapped charges, and by varying the frequency of optical excitation over a suitable frequency range to allow only certain energy levels to be occupied by excited electrons. High field electronic conduction through very thin films sandwiched between two metal electrodes is analysed. In view of the fast tunneling time of electrons through very thin films, MIM structures can be used for microwave detection. It is shown that the maximum responsivity-bandwidth product of such detectors is obtained when they are biased at a voltage equal to the anode work function (in volts), and that the presence of invariant positive space charge increases the magnitude of this maximum. In considering high field switching in thin films of semiconducting glasses, it is suggested that Joule heating, which could account for the delay times observed experimentally, serves only to initiate an electronic switching mechanism. A model for current-controlled negative resistance due to space charge formation is proposed and its dc characteristics are computed. Carrier injection from the electrodes is taken to occur either by Schottky thermionic emission or a Fowler-Nordheim tunneling mechanism. The injected carriers develop space charge regions near the electrodes by impact ionization. The position dependent generation-recombination rate is discussed. The small ac signal equivalent circuit of the model is given. The formation of current filaments is analysed. Memory devices are discussed in terms of filament formation and phase change mechanisms due to excessive heating. Filamentary breakdown has been observed in anodic films grown on Ta, Al, Nb and Ti. A detailed experimental study of film growth and the effects of growth conditions, film thickness, counterelectrodes and temperature on breakdown strength has been carried out. A possible mode of breakdown, in which breakdown can result from thermal effects following a non-destructive electron avalanche, is proposed and its limitations are pointed out. It is concluded that breakdown in thin anodic films would occur due to disruption of the chemical bonds as the applied field approaches the formation field. The product of the molecular dissociation and the presence of energetic electrons could start an accumulative process which might end with the formation of a highly conducting channel. The injected electrons, field distortion and thermal runaway could assist in the channel development. Once the channel is developed, the sample's stored energy starts to dissipate through the channel. The voltage collapse has been found experimentally to occur in a time of less than 200 nanoseconds. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

A novel experimental method to determine the activity of alloy components in very dilute liquid metal solutions has been developed. The method is applied to the measurement of magnesium vapour pressure over nickel alloys to find the thermodynamic properties of magnesium in dilute liquid solutions at 1470°C. The experimental method employs a commercial Atomic Absorption Spectrophotometer to determine directly the vapour pressure of magnesium over the alloys. A radiatively heated Knudsen cell inside a vacuum system contains the metal. Equilibrium constants are given for the reactions, [Formula Omitted] where Al, 0 and Mg are dissolved in liquid nickel. Further, values for the metal-oxygen interaction coefficients [Formula Omitted] and [Formula Omitted] are determined. Finally, a value for the Raoultian activity coefficient at infinite dilution is suggested. A significant change in the activity of magnesium upon the addition of 20% chromium or iron to the dilute liquid nickel alloys was not detected. This is believed due to overpowering magnesium-oxygen interaction at these levels of chromium and iron. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Energy absorption mechanisms have been investigated intensively for the past decades by various authors and institutions, and numerous articles and other literature sources are available in print, as well as on the Internet. Energy absorbers and crashworthiness structures are two main research components in the energy absorption field under investigation today. In this research geometric changes are introduced on Al 6063-T6 circular tubes in the form of horizontal and spiral grooves to asses their influence on energy absorption characteristics. The horizontal and spiral grooves were cut into the tube to a cut depth of half the wall thickness of the tubes. The pitch was varied for both the horizontal and spiral grooves, while the cut width was kept constant. A specially designed static impact sleeve was used to compress the test specimens axially in an Instron 250 kN universal hydraulic testing system. Load vs. displacement graphs were generated from the captured experimental data for the uncut, horizontal and spiral grooved tubes. Energy vs. displacement graphs were created from the experimental data. The final deformed tubes were visually examined to determine the effect the geometric change had on the circular tube form, as well as the deformation pattern of the crushed tube. A Finite Element Method model is presented for each of the experimentally investigated tube impact models. A two dimensional (2D) model for the uncut as well as horizontally grooved tube is generated and analysed using a quasi static loading approach. Non-linear material properties are assigned to the model, and the Riks algorithm is used to model the non-linear post buckling behaviour of the various tubes. The results from the FEM analysis are used to generate load vs. displacement and energy vs. displacement graphs that are compared with the experimental data. Three dimensional (3D) FEM models of the normal, spiral and horizontal cut tubes were also generated in a CAD environment. A dynamic explicit non-linear analysis was done for each of the models to determine the reaction force and energy output values of each of the models. All analyses extend into the plastic material domain. Reaction force vs. displacement and energy vs. displacement graphs are generated from these analyses. A comparison is made between the numerically and experimentally determined gradients of the energy vs. displacement graphs of each of the tubes investigated. This forms the basis for an energy absorber design with application in the transport industry. Unique geometric imperfections were investigated experimentally and numerically for aluminium tubes. A lower buckling load than that for the normal tubes was achieved with the introduction of these geometric imperfections. New deformation patterns on tubes with imperfections not previously observed were described and analysed extensively. The load vs displacement graphs showed a constant increase in the load for the spiral grooved tubes. From the comparison between the numerically and experimentally investigated geometric imperfections a design guide line was esthablished and used in the conceptual design of an energy absorber for the automotive industry. / Prof. L. Pretorius Prof. R.F. Laubscher

Ph.D. (Physics) / Please refer to full text to view abstract.

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This thesis explores the fundamental properties of the contingent.