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A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy School of Physics 2017 / Different experimental techniques have clearly demonstrated that the predominant intrinsic point defects in ionic barium fluoride are anion Frenkel pairs. Positron annihilation technique is utilized in obtaining Doppler broadening and positron lifetime spectra in the temperature range 300 - 900 K. Doppler broadening quantifies the defects whereas positron lifetime components elaborate on the nature of defects. Theoretical approach by density functional theory (DFT) and the generalized gradient approximation (GGA) in the calculation of electron-positron momentum density (or Doppler broadening) spectra at 0 K show that the positron annihilations decay predominantly with barium valence electrons, especially the 5p and 6s electrons and to a lesser extent with core electrons. These annihilations contribute towards the electron-positron momentum density. The annihilations with valence electrons partly contribute toward the short positron lifetime component. The positron-electron annihilations in barium atoms increase steadily with temperature. At 693 K, the annihilation fraction due to the Ba-atom when the anionic Frenkel is formed is found to be 84.44% compared to 15.56% for the fluorine atom. These annihiltions become part of a larger bulk positron-electron annihilations which form a short positron lifetime component. It is also noted that for F-divacancy at 693 K, the annihilation fraction due to 5p and 6s valence electrons in Ba increases by 2.13% to 86.57% indicating the role of defect clusters in the annihilation process. The long positron lifetime decreases in the temperature range from 500 ps at 300 K to 402 ps at 711 K, corresponds to a fractional increase of 22% in the temperature range 300 K to 693 K. The long positron lifetime component is attributed to a delocalized positronium which quickly annihilates through the pick-off (spin conversion) process. Pick-off process seems to be the dominant processes in the long positron lifetime component. The self-diffusion, at all temperature ranges, of cations Ba2+ in barium fluoride is several orders of magnitude smaller than that of F− which has a diffusion constant of 10−9 m2/s at 300 K. Therefore the contribution of cations in superionic conductivity in the temperature range can be ignored. This is also supported by the absence of third lifetime component which is an indication that only anionic vacancies, F−, are generated in the temperature range. The variation of the lattice constant with temperature as determined by X-ray diffraction becomes a major factor in the determination of S-parameters as a function of temperature hence it can reveal the critical temperature at which the formation of anion Frenkel defects commences before entering superionic region. The disordering of fluorine sublattice is found to deviate from linear behaviour at a temperature of 580 K (S-parameter of 0.50622 and lattice constant of 0.623 nm) without observing any appreciable superionic conductivity. X-ray diffraction technique provides a lattice constant of 0.625 nm at 693 K (corresponding to S-parameter of 0.50776) through which an appreciable small activity in conduction is first observed. This is demonstrated through the correlation between the lattice constants and conductivity values at elevated temparatures. This effectively means that lattice constant increases exponentially with temperature. Ilmenite (FeT iO3) which is an ionic conductor in which a permanent dipole moment can be formed by local changes in the environment of Ti4+ ion. It was used to test the validity the positron annihilation spectroscopy in a completely different environment of this corundum structure of space group R-3. The observed long positron lifetime components in comparison with theoretical calculations clearly show that these long positron lifetime components emanate from positron annihilations at metallic vacancies Fe2+. M¨ossbauer pressure effect confirms the increase of Fe3+ at high pressure. At ambient conditions (pressure and temperature), the ratio Fe3+/Fe2+ is small but gradually increase as the pressure increase. The relative intensity clearly shows a dramatic increase of the Fe3+ component with pressure. Further test was carried out using variable positron beam on a 100 keV Ar+ implanted LiF in the fluence range of 1012 − 1016 ions/m2. In the process of ion implantation on alkali halides, ion vacancies in the form of F centers are formed. Using the penetration depth profile, S-parameter at different incident positron beams from 0.03 to 25 keV energies identifies the concentration of defects. This identification was also confirmed by optical absorption which clearly identified the F-band at 242 nm and F2-band at 444 nm. / MT2018

Thesis (Ph. D.)--University of California, 1911. / "Reprinted from the Physical review, vol. XXXIII, no. 3, September, 1911."

Thesis (Ph. D.)--University of Wisconsin--Madison, 1957. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaf 139).

Applying purely thermodynamic arguments it has been shown that temperature fluctuations in a sample can be represented by introducing appropriate 'series temperature generator’ or 'shunt heat current generator'. The temperature fluctuations lead to resistance fluctuations in the sample. These resistance fluctuations due to temperature fluctuations can be detected as voltage-fluctuations (temperature noise) by using a sensing direct current through the sample. Statistical-mechanical arguments are used to obtain theoretical expressions for spectral density of heat current fluctuations in a metallic conductor in terms of the macroscopic properties of the conductor. Since the electrons are carriers of heat and electric currents in a metal, heat and electric current fluctuations are correlated. Spectral density of cross-correlation between electric and heat current is derived. Statistical considerations are extended to the calculation of the steady state spectral density of heat current fluctuations between two black bodies in radiative contact. Temperature noise in a system in which there is only a partial correlation between temperatures at different points along the length at any time (isothermal System) is compared with a system in which there is a complete correlation between temperatures at all points along the length at any time (Single Temperature System). Experimental results indicate that for the metal filament used Nyquist theorem can be applied at the operating temperature. For frequencies close to the characteristic frequency of the system it is observed that there is an increase in noise temperature of the filament due to temperature noise. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

This thesis discusses the minimization of the electric field intensity on electric power transmission line conductors. Two separate cases are considered. One, transmission lines with one conductor per phase, and two, transmission lines with several conductors per phase. Non-circular conductors are considered for the case when only one conductor is used for each phase. Some contours subject to conformal mapping are investigated to determine if conductors of such shapes provide less variation in electric surface charge density than circular conductors. Also a perturbation method is developed which essentially consists in perturbing the boundary of a circular cylinder in such a manner that the electric surface charge density on the cylinder is uniform when it is placed parallel to line charges. Using this method it is found that a solution exists only in one particular case. In considering, several conductors per phase, that is, bundled conductors, the bundle configuration is changed from the symmetrical form normally used until the same maximum electric field intensity exists on all conductors of the same phase. Transmission lines with three and four conductor bundles are treated. The optimum configuration was determined by aid of the Alwac III-E digital computer and the results obtained are indicated. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

This thesis describes work carried out in an attempt to relate the physical properties, in particular, the restivity of certain electrical conductors to the secondary field generated by them in the presence of an alternating primary electromagnetic field. It also discusses, the feasability of determining these properties by the normal methods of electromagnetic prospecting, and relating them to the mineralogical properties of geological conductors. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Thesis made openly available per email from author, 8-2-2015.

Thesis (M.A.)--York University, 2007. Graduate Programme in Music. / Typescript. Includes bibliographical references (leaves 100-103). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&res_dat=xri:pqdiss&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&rft_dat=xri:pqdiss:MR38793

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