Masters Degrees (Physics)

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  • ItemOpen Access
    Spectropolarimetric behaviour of a selection of high-energy blazars
    (University of the Free State, 2022) Barnard, Joleen; Van Soelen, B.
    Blazars form part of the jetted class of Active Galactic Nuclei (AGN), with a relativistic jet closely aligning to the observer’s line of sight. Due, in part, to this viewing angle, these sources are characterised by Doppler-boosted emission across the entire electromagnetic spectrum that is variable on timescales from years down to minutes. These are some of the most numerous extragalactic γ-ray sources, dominated by polarised non-thermal emission produced by relativistic leptons and/or hadrons moving within the jet- component. Blazars are subdivided into two categories: BL Lac-type objects (BLLs), and Flat-Spectrum Radio Quasars (FSRQs). They are classified based on the presence/absence and equivalent widths of optical spectral features, as well as the location of their synchrotron peak frequencies. Blazar SEDs are characterised by two broad, non-thermal components, along with an underlying thermal contribution at optical/ultraviolet wavelengths due to the accretion disc, host galaxy, and broad-line region of the blazar. However, the nature and origin of the non-thermal, high-energy emission remains unclear, as it can be reproduced by two different models, each assuming different particle populations and emission regions that produce the emission. Polarimetry provides a unique diagnostic tool to probe the polarisation signatures of the emission at optical wavelengths. The investigation of the polarisation of blazar emission aides in constraining the particle populations and acceleration mechanisms responsible for the non-thermal emission at lower energies, and constraining the high-energy emission and polarisation. It also places constraints on the accretion disc component and mass of the supermassive black hole, and sheds light on the structure of the jet’s magnetic field. Here, an optical spectropolarimetric study of a selection of high-energy blazars was undertaken. The aims of this project was to: first, investigate the change in the degree of polarisation from high to low states, second, monitor the evolution of polarisation in BLLs and FSRQs over a long period of time, and last, investigate overall trends in the behaviour of the sample of blazars, and compare it to what has been found in other population studies. To achieve these goals, optical spectropolarimetric observations were taken of eighteen blazars (six BLLs and twelve FSRQs) using the Southern African Large Telescope (SALT), complemented by optical photometric data from the Las Cumbres Observatory (LCO), and γ- ray data from the Fermi Large Area Telescope (LAT). For the population of blazars investigated, it was found that FSRQs tend to reach higher degrees of polarisation than BLLs during high states. More regular occurrences of high states and a wider range of polarisation levels were observed for FSRQs. For both BLLs and FSRQs, the average degree of polarisation was higher while the γ-ray fluxes were still on the rise than while it was decreasing to a lower state. In agreement with what was found by the RoboPol monitoring campaign, the degree of polarisation showed no clear correlation with the γ-ray luminosity or redshift of the source, but showed a significant anti-correlation with the location of the synchrotron peak frequency (νsy) of the sources. Two sources with noteworthy results were AP Lib and PKS 1510–089. For AP Lib, long-term variations in the optical and γ-ray fluxes were detected, with the same behaviour mirrored in the degree of polarisation, but lagging by ∼ 54 days. PKS 1510–089 was observed multiple times in 2022, during an unprecedented low-state in which the non-thermal emission from the jet diminished completely. This project serves as a good basis for further investigation of blazar polarisation, both in optical/ultraviolet and X-ray/γ-ray regimes.
  • ItemOpen Access
    The search for high-energy gamma-ray emission from the close binary system AR Scorpii using Fermi-LAT data
    (University of the Free State, 2021-10) Kaplan, Quinton; Meintjes, P. J.; Van Heerden, H. J.
    AR Scorpii (AR Sco) is an enigmatic close binary system which is observable across most of the electromagnetic spectrum. Multi-wavelength emission consists of both thermal and non-thermal components, where pulsed non-thermal emission has been observed from radio to X-ray. The apparent lack of accretion in the system also suggests that the highly magnetic white dwarf is primarily spin-powered, similar to pulsars, where particles can be accelerated to produce non-thermal emission. The multi-wavelength nature from radio to X-ray of AR Sco has been extensively studied and presented since 2016, showing a clear non-thermal synchrotron component from optical to X-ray. However, the parameters and processes towards the high-energy regime still need to be clearly verified and explored. Hence, a search for non-thermal γ-ray emission from AR Sco using archival Fermi data from the past decade has been performed. By using updated Fermi-LAT software and Pass 8 analysis methods, low-level upper-limit emission has been observed above 100 MeV, with a possible low-level pulsed signal at the spin period (Ps = 117 s) of the white dwarf. Given the results, the possibility of non-thermal emission at higher energies can not be excluded. An updated spectral energy distribution using Fermi-LAT data was also produced to serve as a template for future studies using more sensitive Cherenkov telescope detectors like e.g. the Cherenkov Telescope Array (CTA).
  • ItemOpen Access
    The search for pulsed radio and gamma-ray emission from the cataclysmic variable system AE Aquarii using MeerKAT and Fermi-LAT data
    (University of the Free State, 2021-12) Madzime, Spencer Tendai; Meintjes, P. J.; Van Heerden, H. J.
    The nova-like AE Aquarii was extensively studied using 11 years of data and contemporaneous optical data with the aim to establish pulsed gamma-ray emission at the rotational period (33.08 s) of the white dwarf and the first harmonic (16.54 s). In addition the study was also aimed at identifying pulsed radio emissions at or near the fundamental frequency. Therefore the search of pulsed radio and gammaray emission using MeerKAT (radio) data and upgraded Fermi-LAT pass 8 (gammaray) datasets is presented. The L-band 45 mins observation of AE Aquarii resulted in the first detection of this source by the MeerKAT telescope. Also, the pulsed radio emission search resulted in the first detection of pulsed emission modulated at the white dwarf’s rotational period (33.08 s). Further investigation resulted in no indication of any pulsations at the first harmonic (16.54 s). The pulsed signal’s overall strength is confined to lower L-band frequencies at the onset of the flare event. No pulsations were significant at the peak of the flare-like event. The spectral variation investigations revealed an optically thin spectrum. This optically thin spectrum observed in the MeerKAT data appears to be uncorrelated to the overall radio spectrum up to   1000 GHz. Hence, a correlation between the MeerKAT data and observations at higher frequencies could be an exciting topic for in-depth follow-up studies. Investigation for gamma-ray emission from AE Aquarii was conducted using an   11-year baseline of the archived upgraded Fermi-LAT pass 8 dataset. The standard binned and unbinned analyses techniques resulted in no detection of any significant gamma-ray excess in the region of AE Aquarii. However, the search for transient burst-like gamma-ray emission through a light curve revealed sections of the data with an excess significance of 2 s. A search for pulsed gamma-ray emission from these sections resulted in substantial evidence of pulsed signal from some of these sections with excess significance. An additional search for pulsed gamma-ray emission using shorter time bins resulted in detecting more significant pulsations at both the fundamental (33.08 s) and the first harmonic (16.54 s) frequencies. The stacked periodogram displayed clear evidence of particle acceleration from two polar zones of the white dwarf in AE Aquarii, which resulted in a double pulse. Periods contemporaneous to observed optical flares displayed weak pulsations in the noise level. After stacking the periodograms, a significance of over 4 s is seen at the fundamental frequency (33.08 s). A further search for evidence of sporadic burst-like gamma-ray emission from AE Aquarii using refined or more selective filtered data resulted in the detection of AE Aquarii with a significance of   14 s from both the binned and unbinned analyses techniques. The gamma-rays detected from a more constrained refined filtered dataset show clear gamma-ray spectral hardening above the galactic gamma-ray emission. These results have significant implications for follow-up studies of AE Aquarii using ground-based Cherenkov telescopes.
  • ItemOpen Access
    Synthesis and characterization of long afterglow phosphors (Sr Al2iO4:Ce3+ SrAl2iO4:Tb3+CaAlxOy:Tb3+, Y3Al5O12:Eu3+) using solution combustion method
    (University of the Free State, 2011-11) Foka, Kewele Emily; Dejene, B. F.; Swart, H. C.
    This work consists of several aspects of phosphor materials. Strontium, calcium and yttrium aluminate doped with rare earth (Ce, Tb and Eu) have been synthesized by solution combustion method using urea as a fuel for investigations of the luminescent, structure and morphological properties. The phosphors were characterized by several techniques such as Xray diffraction (XRD), energy dispersive electroscopy (EDS), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and Photoluminescence (PL), PL data were collected using a Cary Eclipse Photoluminescence Spectrophotometer equipped with a 150 W xenon lamp. Cerium doped strontium aluminum oxide (SrAhO4:Ce3+) were synthesizes. The effects of different concentration of cerium were investigated. X-ray diffraction results confirmed the formation of the SrAhO4 monoclinic phase (Powder Diffraction Standards (JCPDS) file N0 34-0379). The particle sizes of different peaks were estimated and the average particle size was 47 nm. SEM results showed agglomerated as well as small elongated-egg-like shape on particles when taken to higher magnification. The PL spectra show a broad emission consisting of two bands peaking at 374 and 384 nm, corresponding to the transitions from the lowest 5d excited state to the 2F512 and 2F712 states. The excitation and emission peak position shifted with varying the cerium concentration. This maybe due to uncontrollable electrospinning conditions like air and wetness, which influence the cristal field that surround Ce3+.SrAhO4:Tb3+ XRD peaks confirmed the formation of the SrAhO4 monoclinic phase and some impurities were also observed. The photoluminescence characteristics show the emission peaks at 415, 436 and 459 nm which correspond to the 5D3 to 7F1 (1=5 , 4, and 3) level and 489, 543, 585, and 622 nm corresponding to 5D4 to 7F 1 (J= 6, 5, 4, 3) under excitation at 229 nm and the terbium concentration was varied. The elements of the phosphor SrAhO4:Tb3+ were shown by energy dispersive spectroscopy. The decay curves were also observed and the decay constants show a higher value at a concentration of 0.25 mol% and lower value at a concentration of 2 mol¾. CaAlxOy:Tb3+ green phosphors were obtained at low temperature (500 °C) by a solutioncombustion method. The structural analysis revealed the presence of both monoclinic CaA4O1 and CaAhO4. The main parent structure of CaAhO4 monoclinic was revealed when varying the concentration of terbium. The characteristic luminescence properties were investigated using emission spectra. The emission peaks are from transition of the 5D4 state to the 7F1 (J = 6, 5, 4, 3) state. The optimal intensity was obtained when the concentration of Tb3+ was increased to 2 mol¾. FTIR was used to identify all the chemical bands. Absorption bands of the condensed matter AlO4 located in the range of700 cm·1-900 cm·1 and condensed matter AlO6 at 500 cm·1-680 cm·1 are attributed to AlO4 liberation at 600 cm·1-900 cm·1 The decay curves of the phosphor were investigated and showed a higher intensity and longer afterglow time at higher concentration of terbium 2 mol%.Y3Al5O12 known as Yttrium aluminum garnet (Y AG) phosphor doped with different concentration of Eu was synthesized by the solution combustion method. The crystalline structure, morphology and luminescent properties of the phosphors were studied. The SEM revealed the agglomerated morphology containing small spherical particles around the pores. FTIR spectra reveal all bonds that exist in the phosphor. The emission spectra revealed three major emission peaks at 592, 615, and 628 nm, corresponding to the 5D0-7F1 (592 nm), 5D0-7F2 (615 nm) and 5D0-7F3 (628 nm) transitions respectively. The luminescence intensity increased with an increase in Eu concentration at 0. 7 mol¾ and then decreases with an increasing of concentration further.
  • ItemOpen Access
    Synthesis and characterization of undoped and doped ZnO nanoparticles prepared by sol-gel process
    (University of the Free State, 2009-11) Ali, Abdub Guyo; Dejene, B. F.; Swart, H. C.
    Nanotechnology is an emerging field of engineering that has intrigued a tremendous potential to change today's lives. Nanoparticles represent the transition region between individual atom/molecule and bulk materials have been attracting considerable research input, owing to some unique physical and chemical properties they exhibit. They are already being used in many applications. This is due to their potential useful size and shape dependant. They are provided with a number of quantum size effects that determine specific electro-physical performance. This is one of the reasons why the preparation and study of nanomaterials are important area of research. This work focuses on the synthesis and characterization of semiconducting metal oxides, the most widely used materials in powder and films forms. The aim is to compare the optical performances of zinc-oxides nanostructures obtained by means of sol-gel (SG) method. The microstructure quality (concentration and nature of defects) of materials which is a fundamental parameter to evaluate if the material is suitable for any applications are usually correlated with optical properties (intensity and spectral emission range) of materials. The idea of this work was to carry a basic characterization of the structural (by X-ray diffraction technique and scanning electron microscopy) and optical (photoluminescence measurements) properties of ZnO, ZnO in SiO2 matrix and manganese doped ZnO nanoparticles samples prepared by sol-gel process. Initially, several samples of ZnO nanoparticles were successfully synthesised by varying the pH of the precursors. Secondly, manganese doped ZnO samples were synthesised by varying the concentration of manganese. The analysis of ZnO nanoparticles prepared under varying pH has displayed a dependence of structural quality, morphology as well as optical properties on the pH of the precursors. SEM images of manganese doped ZnO samples reveal nanorods of micrometer-size. XRD patterns confirm polycrystalline wurtzite structure of ZnO and particle sizes of about 7nm. High optical transmittance greater than 80% was achieved in the visible spectral wavelengths with UV-Vis optical absorption and transmission measurements. Photoluminescence spectra of the pure ZnO are composed of two main emission bands peaked at 388nm and 569nm. Both the blue (388nm) and the green (560nm) emissions in ZnO nanostructures were quenched, although to different extent, when doped with Mn2+. Mn-doped ZnO nanorods have several applications as the most suited materials for sensors, spintronics, better insulation materials etc.
  • ItemOpen Access
    Room temperature gas sensing characteristics of titanium dioxide nanostructures: effects of hydrochloric acid on the structure and magnetic properties
    (University of the Free State, 2016-01) Tshabalala, Zamaswazi Portia; Motaung, D. E.; Mhlongo, G. H.; Ntwaeaborwa, O. M.
    Monitoring and detection of toxic and combustible gases such as methane in underground mining, carbon monoxide from burning coal in our homes and odourless gases such as nitrogen dioxide and sulphur dioxide in industries has become the subject of extensive scientific and technological research and this has been motivated by their harmful impact on the environment and human health. Early detection of these gases can help prevent fatal incidences such as fire, suffocation and death. Development of portable gas sensors with higher sensitivity and selectivity, fast response and recovery times, low detection limit and capability of operating at room temperature is one of the challenges facing researchers world-wide. Various materials such as semiconductor metal oxides (MOX), polymers, and carbon nanotubes have been used for gas sensing application. Among all the various materials, MOX such TiO2, ZnO, SnO2, Fe2O3, WO3 are the most preferred materials for gas sensing application due to their noticeable response to any change in electrical resistance when exposed to either reducing or oxidizing gas and also due to their unique properties such as high stability and easy to synthesize. However, among the range of MOX semiconductors mentioned above, TiO2 has emerged as the preferred MOX semiconductor for gas sensing application due to its remarkable features such as nontoxicity, biocompatibility, high photocatalytic activity and affordability. TiO2 occurs in three crystalline forms namely: anatase, rutile and brookite. Anatase and rutile polymorphs are widely studied for technological applications. Therefore, in this study, we investigated the gas sensing properties of TiO2 nanoparticles annealed at 450 C, and that annealed at various temperatures, as well as those doped with various concentrations of Mn. The undoped TiO2 nanoparticles were synthesized from P25 Degussa via a simple hydrothermal method in an aqueous solution of sodium hydroxide (NaOH). The samples were washed with distilled water (H2O) and different concentrations of hydrochloric acid (0.25, 0.5 and 1.0 M) which acted as the morphological controlling agent. TiO2 doped with various concentration of Mn2+ were washed using 1.0 M HCl. To investigate the effect of hydrochloric acid (HCl) as a washing agent on the structure, morphology, optical, magnetic and gas sensing properties, x-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) and Kinosistec gas sensor testing system were used for characterization. Microscopy analysis showed that the sizes of the pure TiO2 nanoparticles were reduced when increasing the HCl concentration indicating that the particle sizes could be easily tailored and tuned by adjusting the HCl concentration. Structural analyses revealed a phase transformation from a mixture of anatase and rutile phases to pure anatase phase at higher HCl concentration. The PL, XPS, EPR and BET analyses disclosed that the 1.0 M sample contained relatively high concentration of oxygen vacancy, Ti4+ and Ti3+ interstitial defects and it also had higher surface area which played an important role in transforming the sensing properties, resulting in higher sensing response, sensitivity and selectivity to NO2 at room temperature. Furthermore, the effect of thermal annealing was investigated on the structural and gas sensing properties of the pure TiO2 nanoparticles washed with H2O and HCl, and annealed at different temperatures (300, 450, 700 °C) in air. Surface morphology analyses revealed that the nanoparticles transformed to nanorods after annealing at 700 C. The results showed that the sensing properties are dependent on annealing temperature. The 1.0 M TiO2 nanostructures annealed at higher temperatures (700 C) revealed improved sensing response to CH4 gas at room temperature due to higher surface area of 180.51 m2g-1 and point defects related to Ti3+ observed from EPR and PL analyses. In addition, the 1.0 M TiO2 sensing material annealed at 700 C also revealed an interstitial defect states which played a vital role in modulating the sensing properties. To improve sensitivity, selectivity and stability of the gas sensing materials, various concentrations (1.0, 1.5, 2.0, 2.5 and 3.0 mol % denoted as S1, S2, S3, S4 and S5, respectively) of Mn-ions were loaded on the TiO2 particles. The nanoparticles were characterised in detail using various analytical techniques. XRD analysis showed that the structure of both pure and Mn-doped TiO2 was tetragonal and no peaks corresponding to Mn or impurities were observed. Raman spectroscopy revealed quenching and peak broadening due to lattice disorder with increasing concentration of Mn. Optical studies revealed that the Mn loaded TiO2 nanoparticles have enhanced UV-Vis emission and a broad shoulder at 540 nm denoting defects induced by substitution of Ti4+ ions by Mn2+. The XPS and the EPR results revealed the presence of Ti4+, Ti3+ and single ionised oxygen vacancies in both pure and Mn loaded nanoparticles. Additionally, a hyperfine split due to Mn2+ ferromagnetic ordering was observed confirming incorporation of Mn ions into the lattice. Gas sensing studies revealed that Mn2+ loaded TiO2 surface improved the NO2 and NH3 sensing performances in terms of response and selectivity. The S1 (1.0 mol. % Mn) demonstrated an improved sensitivity of approximaterly 85.39 ppm-1 at 20 ppm NH3 gas at room temperature. Our findings showed that, the thermal annealing and Mn doping improve the sensitivity and selectivity and stability of the gas sensing materials. The results also validated that our sensing materials are highly sensitive and selective to CH4, NO2 and NH3 at room temperature. The observed room temperature response in this work, suggests that these TiO2 nanostructures are possible candidates for gas sensing application in work places, mining sectors, etc. Moreover, the findings in this work give a possible solution to the issue of energy consumption of metal oxide gas sensors.
  • ItemOpen Access
    'n Ondersoek na die segregasie van fosfor en ander onsuiwerhede in 3Cr12 vlekvrye staal
    (University of the Free State, 2003-11) Vermaak, Christiaan; Roos, W. D.; Terblans, J. J.
    English: One of the main reasons for temper embrittlement in steel is the segregation of impurities like P to the grain boundaries. Segregation can be defined as the diffusion of atoms from the bulk to the surface and grain boundaries in such a way that the total Gibbs free energy is minimized. This means that segregation can take place against the concentration gradient, from a low concentration in the bulk to a high surface concentration. The chemical potential gradient is the driving force behind segregation. The aim of this study is to investigate the segregation behaviour of P and other impurities like S and Sn in 3Cr12 steel. A background theory is founded by using: (i) The semi infinite solutions to the Fick equations (ii) t½ and modified t½ models (iii) the modified Darken model. One of the advantages of the Darken model is that it supported both segregation kinetics and equilibrium behaviour. The multi component model for ternary alloys could be expanded to quaternary alloy systems in this study. Segregation kinetics as well as the equilibrium was described by making use of constant and linear temperature heating. Auger electron spectroscopy was used to investigate the S, P, Cr, N, and Sn segregation behaviour in a Fe matrix. A personal computer was used to control the Auger spectrometer as well as the constant and linear heating runs. Three commercial 3Cr12 samples was investigated during the study. They were numbered according to their P contend as 26P for the sample with 0.026wt% P, 32P for 0.032wt% P and 62P for the sample containing 0.062wt% P. The constant temperature runs indicate that Sn competes with Cr, N and Pin sample 26P. A definite correlation is visible between Cr and N in sample 32P while Sn and S compete with P in sample 62P. The constant and linear heating Darken simulation model was used to give a qualitative description of the experimental segregation behaviour. The behaviour of two segregating species were simulated in a Fe matrix, from which the influence of the segregation parameters could be demonstrated, namely. If the surface concentration of species 1 is higher than that of species 2 during segregation kinetics, it can be said that the diffusion coefficient of species 1 is higher than that of species 2. If the surface concentration of species 1 is less than that of species 2, then the diffusion coefficient of species 1 is less than that of species 2. If the surface concentration of species 1 is less than that of species 2 at equilibrium, then the segregation energy of species 1 is less than that of species 2. If the equilibrium surface concentrations are equal, the segregation energies are equal. When the surface concentration of species 1 is higher than that of species 2, then the segregation energy of species 1 is higher than that of species 2. It is possible to sort the segregation parameters in order of magnitude from the results of the experimental work and the constant and linear heating simulations. The diffusion coefficients of the species could be arranged from high to low (DN > DP > DSn = DS). The segregation energies of samples 26P and 32P could be arranged in the same order, namely ?GS < ?GS n< ?GP
  • ItemOpen Access
    A comparative study between the simulated and measured cathodoluminescence generated in ZnS phosphor powder
    (University of the Free State, 2003-08) Chen, Sheng-Hui; Greeff, A. P.; Swart, H. C.
    In the past few decades cathode ray tubes (CRTs) have dominated the display market because of their excellent image quality, ease and economy of manufacture. However their bulky packaging and high power consumption make them unsuitable for portable electronic devices. Field emission displays (FEDs) show the most potential amongst all other types of flat panel displays (FPDs). These FEDs have several advantages over the FPD market, which is currently dominated by active matrix liquid crystal displays (AMLCDs) and plasma displays (PDPs). FEDs generate their own light by a process referred to as cathodoluminescence (CL) in which phosphor powders inside the screen are excited in a similar manner to those used in CRTs. However, in contrast to CRTs, the accelerating voltage of electrons in FEDs is lowered in order to reduce the bulky packaging and the power consumption. Electrons with the reduced accelerating voltage have a shallower penetration depth and therefore the surface condition of the phosphor powder is critical in order to ensure proper functioning of the display. During the prolonged exposure of the phosphors to an electron beam, the phosphor surface is oxidised to form a non-luminescent layer. This electron stimulated oxide formation is due a chemical reaction between the phosphor and the residual gases in the sealed vacuum, e.g. oxygen and water vapour. Since the CL is dependent upon the energy loss of electrons in the phosphors, the CL decreases with the growth of the oxide layer on the phosphor surface. For high acceleration voltages, this oxide layer has little effect on the brightness of the CL, but as the accelerating voltage decreases as for FEDs, the layer has a much more profound effect. The ZnS:Cu,Al,Au (P22G) is a standard green phosphor commonly found in CRTs. In this study the P22G phosphor powder was bombarded by an electron beam in an oxygen ambient, argon ambient and other mixture of gases. These mixtures consisted of varying concentrations of oxygen, carbon monoxide and argon gas. Auger electron spectroscopy (AES) and cathodoluminescence spectroscopy were used to monitor changes in surface composition and luminescent properties of the P22G phosphor during electron bombardment. When the P22G phosphor powder was exposed to an electron beam in water-rich oxygen gas, a chemically-limited ZnO layer was formed on the surface. The CL intensity generated from carbon free P22G phosphor decreased linearly with the thickness of the ZnO layer. The experimentally measured thickness of the ZnO layer agrees very well with the calculated value of the theoretical simulation. The theoretical simulation of electron trajectories into the ZnO/ZnS powders was based on a Monte Carlo simulation and the CL intensity was quantified from the electron energy loss profile generated during the simulation. According to the results of the simulation, the effect of a ZnO layer on the CL is minimised by the use of a high energy electron beam at a low incident angle. The electron exposure of P22G phosphor powder was also performed in dry oxygen gas. A layer of ZnSO4 was formed on the surface after electron exposure. The sulphate formation decayed exponentially with time and it is postulated that this was due to the diffusion of the charge reactants through the sulfate film to reaction interfaces. The P22G phosphor exposed to the electron beam in argon gas and gas mixtures degraded more slowly than in oxygen gas. Argon gas and carbon monoxide gas may suppress the degradation of the P22G phosphor powder.
  • ItemOpen Access
    An investigation of variability and its associated synchrotron emission in relativistic AGN jets using numerical hydrodynamic simulations
    (University of the Free State, 2017-03) Van der Westhuizen, Izak Petrus; Van Soelen, Brian; Meintjes, Petrus Johannes
    English: Active regions at the centres of certain galaxies known as Active Galactic Nuclei (AGN) are some of the most energetic and violent sources of emission in the universe. Certain types of AGN can produce jet-like emission structures that extend hundreds of kiloparsec in length. The jet-like sources show intricate time dependent structure and are believed to consist of collimated flows of relativistic plasma. Many studies have focused on investigating the structure and emission of these sources. The evolution time scale of the jets are much longer than their recorded history which makes observational studies of their evolution challenging and, due to the relativistic nature of these jets, they have not been accurately reproduced in laboratory experiments. Instead many studies have employed fluid dynamic numerical simulations of these sources to study their properties. To accurately compare a fluid dynamic simulation to that of observational data the emission emitted by such an environment must be modelled. In this study a fluid dynamic simulations of a relativistic jet is constructed and a synchrotron emission model is applied to the simulations to reproduce intensity maps at radio frequencies which is comparable to observational data of AGN jet sources. The numerical fluid dynamic simulation was created and evolved using the PLUTO software and consisted of a three dimensional environment containing ambient medium, into which a jet is injected through a nozzle on the lower z boundary. The injected material consisted of a less dense medium with a super-sonic bulk motion of Lorentz factor T = 10. The simulation reproduced a jet structure containing a relativistic beam of material propagating through the ambient medium. The beam of material was surrounded by a turbulent cocoon region with asymmetric structure. The entire structure was encased in a bow shock. Intensity maps of the three dimensional fluid simulation were created by applying a post-processing code to the simulation data. The emission model estimated the synchrotron emission by assuming that the entire population of electrons in the jet had a power-law energy distribution. The intensity maps were able to reproduce emission structures that resemble those of FR II type radio galaxies with a dominant cocoon region containing time dependent hot spots and laments. To investigate the effects of Doppler boosting, intensity maps were calculated at different polar angles and the results were consistent with the current unified model of AGN and showed a significant increase in the intensity of the relativistic beam at small polar angels. The intensity maps were able to reproduce time dependent emission structures due to fluid dynamic instabilities that formed during the simulation. The time dependent structure led to the production of variability with an amplitude of ≈ 10% in the total intensity. It was therefore shown that some variability observed within these sources occurs due to fluid dynamic instabilities rather than a change in the injection parameters. However, large flares which have been observed from these sources require additional perturbations in the flow. This study serves as a good basis for future in depth investigation of AGN emission.
  • ItemOpen Access
    Study of the structure, particle morphology and optical properties of mixed metal oxides
    (University of the Free State, 2017-01) Mokoena, Pulane; Ntwaeaborwa, O. M.; Kroon, R. E.
    The structure, morphology and optical properties of metal oxides (ZnO, MgO and SrO), their composites (MgO-ZnO, SrO-ZnO) and systems with different x molar concentration values (0.2, 0.4, 0.5, 0.6, 0.7, 0.8) of MgxZn1-xO and SrxZn1-xO, were synthesized via solution combustion method at initial reaction temperature of 600 ˚C for 15 minutes. These properties of the synthesized nanostructures were investigated using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), High resolution transmission electron microscope (HR-TEM) and Photoluminescence (PL) spectroscopy. The ZnO, MgO and SrO phosphors were successfully synthesized via solution combustion method and their crystallization was confirmed by XRD analysis. The ZnO powder crystallized in the hexagonal phase. The diffraction patterns of the ZnO samples became sharper and more intense when synthesis temperature was increased from 600 ˚C to 700 ˚C indicating improvement of crystallinity and an increase in crystallite sizes from 23.3 nm to 30.06 nm of the as-prepared undoped ZnO phosphor powder. The MgO powder had cubic crystal structure with Fm-3m space group and crystallized in rocksalt/sodium chloride (NaCl) type cubic structure and the SrO sample indicated the presence of three well-defined crystalline phases which are SrO, Sr(OH)2 and Sr(CO3)2, with Sr(OH)2 appearing as the most prominent phase. With respect to the following systems: MgxZn1-xO and SrxZn1-,xO and their composites, their XRD patterns revealed the presence of two well-defined crystalline phases, namely MgO or SrO and ZnO, the most prominent phase being ZnO. The SEM images of ZnO showed agglomeration of small particles and flower-like morphology. The HR-TEM images showed that the nanoparticles (NPs) were hexagonally shaped and aggregated into clusters. The SEM images of MgO showed spherical cube-like morphology with the appearance of closely-packed or attached particles in all the SEM micrographs. The HR-TEM images show that the NPs were cubic-spherically shaped and aggregated into clusters. For the SrO sample small and coagulated particles of irregular shapes and different sizes were observed. Pores of different sizes were also observed from the solution combustion synthesis. This is due to the outgassing of the gaseous products, namely N2 and CO2, of this synthesis method. The HR-TEM images showed that the NPs were spherically shaped and aggregated into clusters. The selected area electron diffraction pattern confirmed the observation of a large number of nanoparticles and hence there were many spots within each ring. In the case of the MgxZn1-xO system SEM observations revealed different kinds of particle morphologies such as pyramids clustered together to form flowers with spherical particles grouped together on the sides, triangles grouped together in the shape of a cauliflower, tetragonally shaped particles with some degree of faceting and for the SrxZn1-xO system, flower-like structures, oval-shaped particles and elongated rod-like structures. The photoluminescence results of ZnO exhibits two characteristic peaks: one narrow in the ultraviolet (UV) region at 380 nm which comes from recombination of free excitons, and one broad in the visible region at 639 nm for ZnO synthesized at 600 ˚C and 626 nm for ZnO synthesized at 700 ˚C, which were attributed to electron mediated defect levels in the bandgap. The MgO sample showed three PL emission peaks at approximately 419, 432 and 465 nm and a minute emission peak at 663 nm. The SrO PL spectrum exhibited UV and deep level emission peaks. In addition, there was a narrow peak in the UV region at 397 nm and a broad peak in the visible region at 750 nm. With regards to the MgxZn1-xO system with x ranging from 0.2, 0.4, 0.5, 0.6 and 0.7, a red shift in the emission peaks from 602 to 610 nm was observed for the 0.2 and 0.4 molar concentrations while their luminescence intensity decreased. For a molar concentration 0.5 there was a blue shift in the emission peak from 610 to 551 nm together with luminescence quenching. From molar concentration 0.5 to 0.6 there was a blue shift in the emission peaks from 551 to 539 nm with a luminescence enhancement, but when the molar concentration was 0.7 there was a slight red shift in the emission peak located from 539 to 549 nm together with a luminescence enhancement. With regards to the MgO-ZnO composite sample there was only one broad emission peak at 559 nm in the visible region and luminescence intensity increased significantly. For molar concentrations 0.2 and 0.4 there were emission peaks at 383, 540 and 760 nm. For molar concentration 0.5 there were emission peaks at 383, 514 and 760 nm. For molar concentration 0.6 there were emission peaks at 383 nm, minor humps at 413, 435 and 760 nm and a broad peak at 514 nm. For molar concentration 0.7 there were emission peaks at 383, 514 and 760 nm and for molar concentration 0.8 there were emission peaks at 383, 514 and 760 nm. The emission peak in the UV region (383 nm) was narrow and this was ascribed to recombination of free excitons, while the broad emission peaks at 514 and 540 nm were attributed to electron mediated defect levels in the bandgap.
  • ItemOpen Access
    Oxidation of a segregated MoN layer grown on Fe(100)-3.5wt%Mo-N
    (University of the Free State, 2001-06) Conradie, Rochelle; Roos, W. D.; Swart, H. C.
    English: The oxidation behaviour of the segregated MoN layer on the Fe(100)-3.5wt% Mo-N substrate was investigated in this study. Previous studies suggested the synergetic segregation of the Mo and N from the Fe(100)-3.5wt% Mo-N specimen. It has also been shown that the segregated Mo and N form a MoN surface compound. As an alloy element in stainless steels, the Mo aids in the inhibition of the oxidation and thus prevents corrosion Auger electron spectroscopy (AES) was used to obtain the experimental results. For this study the oxidation of a Fe(100) specimen and a Fe(100)-3.5wt% Mo-N specimen were investigated to establish a point of reference to describe the oxidation behaviour of the segregated MoN layer. Linear temperature ramping was used to segregate the Mo and N from the Fe(100)-3.5wt% Mo-N specimen. The specimens were exposed to an oxygen environment at various temperatures. The partial pressure of the oxygen was monitored with a mass spectrometer and was kept constant at 2 x 10-10 torr. The Auger peak-to-peak heights for the relevant elements in the specimens were measured as a function of the exposure time. Upon oxidation, the low energy Fe AES peak (47 eV) undergoes shape changes. The iron oxide has a dual peak with 42 eV and 52 eV kinetic energy respectively. The Fe(100) specimen surface reacted rapidly with the oxygen environment at room temperature to form an iron oxide, as depicted by the change in the low energy Fe AES peak. The exposures performed at 100°C and 200°C also resulted in oxide formation although the extent of the oxidation decreased with an increase in the temperature. Above 300°C indication of the Mo and N reacting with the oxygen environment. At 100°C and 200°C less oxide formation was detected and above 300°C there was only oxygen adsorption. The segregated MoN layer had a markedly different response to the oxygen exposure. The oxygen exposure performed at room temperature had a strikingly different course of the 0 Auger peak-to-peak height increase compared to that of the Fe(100) and Fe(100)- 3.5wt% Mo-N specimens exposure at the same temperature. The segregated MoN layer retards the surface reaction. A hypothesis formulated describes the MoN layer as a perforated layer that has some Fe exposed. The oxygen reacts rapidly with the exposed Fe. Longer exposures result in the dissociation of the MoN layer and the desorption of the Mo03 and NxOy compounds from the surface. Once the layer has dissociated completely the Fe will continue to react as for the other specimens. Oxidation occurs up to 300°C and at higher temperatures no oxide formation is detected. The changes in the low energy Fe AES peak are used to calculate the fraction oxide and metal contributing to the peak by using the Linear Least Squares method. The low energy Fe AES peak cannot be used for thickness calculations as it is subject to the backscattering term. The experimental data suggests that the backscattering term is a function of the exposure time. A first approximation is to assume a linear change with time. This approximation was applied successfully to the room temperature oxidation of the segregated MoN layer, but the same function could not be applied to the other two specimens, The thickness of the oxide was calculated using the change in the high energy Fe AES peak intensity. The O2 sticking coefficient for the exposure of the Fe(100) and the exposure of the segregated layer was also calculated and the differences in the values were attributed to the effect of the dissociation of the MoN layer on the adsorption of the O2 on the specimen surface. there was no oxide formation detected and therefore there is only oxygen adsorption at these temperatures. The Fe(100)-3.5wt% Mo-N specimen showed similar oxidation behaviour as was seen for the Fe(100) specimen. At room temperature the surface of the specimen reacted rapidly with the oxygen environment to form an iron oxide. There was no
  • ItemOpen Access
    A multi-wavelength study of super soft X-ray sources in the Magellanic Clouds
    (University of the Free State, 2012-02) Odendaal, Alida; Meintjes, P. J.
    English: Supersoft X-ray Sources (SSS) form Cl, highly luminous class of objects that emit more Than ~ 90% of their energy in the supersoft X-ray band, i.e. below 0.5 keV. They are generally believed to consist of a white dwarf with a more massive binary companion, resulting in thermal time-scale mass transfer to the white dwarf and associated accretion. The high accretion rate of material onto the white dwarf is sufficient to drive nuclear burning and accompanying soft X-ray emission on the white dwarf surface, and may imply the presence of an accretion disc and significant mass outflow from some of these sources. However, SSS do not form a homogeneous class and also include objects like planetary nebulae, symbiotic novae and cataclysmic variables exhibiting nova outbursts. To investigate the phenomenon of accretion and the nature of possible mass outflow in SSS. a sample of 3 candidate sources in the Magellanic Clouds were identified for optical spectroscopic and X-ray studies: CAL 83, N67 and SMC 13. The galactic symbiotic nova RR Tel was also included in the study due to the evidence for an accretion disc implied by the double-peaked Raman-scattered 0 VI emission. Signatures of disc accretion and mass ejection in close binary supersoft sources (CBSS) like CAL 83, may provide evidence that such systems can evolve towards another class of binary system, namely the cataclysmic variables. Optical spectroscopic studies of CAL 83, NG7 and RR Tel were performed with the Southern African Large Telescope (SALT) and the SAAO l.9-m Telescope, and archived Chandm and XMM-Newton observations of the sources SMC 13 and CAL 83 were also analysed. The optical spectra of CAL 83 exhibit evidence of line broadening due to radial motion in an accretion disc, and a signature of possible disc outflows is also present. A search for periodicity in the X-ray data of CAL 83 revealed indication of consistent periodic modulations at P ~ 67 s, which could possibly be associated with the rotation period of a spun-up white dwarf. The presence of a fast rotating WD could provide a mechanism to explain the outflow inferred from the optical spectrum. The widths of nebular emission lines of the planetary nebula N67, as well as that of typical nebular lines in RR Tel are consistent with the known expansion velocities of nebulae surrounding the central objects in these systems.
  • ItemOpen Access
    Mode identification in delta δ Scuti stars
    (University of the Free State, 1998-10) Evers, Elizabeth Anne; Balona, L. A.; Meintjes, P. J.
    This thesis discusses mode-identification from multicolour photometry. First, the need for a better mode identification technique for stars that have significant phase differences between the light curves in different colours is addressed. The necessary equation needed for modeidentification from photometry is then derived and briefly discussed. Then, a new, statistically based algorithm for mode-identification is developed by extending and adapting a method that has been applied to pulsating white dwarfs, to include the information provided by the different phases of the light curves in different wavelengths. This new algorithm allows the best estimate of the spherical harmonic degree l to be determined, as well as a confidence level from which the uniqueness of the mode-identification can be ascertained. The algorithm is then applied to a selection of well-observed 8 Seuti stars with the necessary multicolour photometry. It is found that it works well for high amplitude 8 Set stars, but that discrimination between the l = 0 and l = 1 modes is sometimes poor for the low amplitude stars. An algorithm to deduce the effective temperature, luminosity and equatorial velocity from the observed frequencies is also proposed. It is found that some mode-identification is necessary to obtain a unique solution of the stellar parameters. The method is applied to a subset of the 8 Set stars which have a suitable number of frequencies and suitable mode identifications.
  • ItemOpen Access
    The determination of ternary segregation parameters using a linear heating method
    (University of the Free State, 2000-11) Asante, Joseph Kwaku Ofori; Roos, W. D.; Du Plessis, J.
    In this study the segregation behaviour of the ternary system Cu(lll ), Sb, Sn is investigated experimentally, as well as with the modified Darken segregation model. The model, which describes the kinetics as well as the equilibrium of segregation, had been used successfully in various studies of binary systems. A computer program based on this model was developed for ternary systems. A Cu(lll) single crystal was doped with low concentrations of 0,180 at% Sb and 0.133 at% Sn using evaporation and diffusion process.' The experimental results were gathered with the Auger electron spectroscopy technique. This technique was combined with a linear temperature ramp that makes it possible to obtain the segregation parameters in a single run. The traditional method requires various runs at different temperatures. The overlapping of Sb and Sn Auger peaks in the energy regions of interest necessitated the development of a method to successfully extract the true contributions of the elements from the measured spectra. It is clearly shown that the combination of Auger peaks is not linear and that the true contributions of Sb and Sn can be calculated if the peaks overlap in two energy regions and the standard spectra are available. The segregation profiles resulted from the Auger data show clearly the sequential segregation of the two elements (Sn and Sb). From the equilibrium conditions, it is also concluded that an interaction energy between Sb and Sn is present. By simulating the experimental results, using the theoretical Darken model, values for the segregation parameters can be obtained. The initial values for the fits are found mathematically (highenergy regions) and manually (low energy regions). The calculated profiles fit the experimental results very well. The present study confirms that Sn segregate first to the surface with Do = 1.58x10-5 m²s-¹ and E = 170 kJ/mol. Sb with a lower dimsion coefficient (Do = 1.93x10-8 m²s-¹ and E = 150 kJ/mol) segregates at higher temperatures. A further increase in temperature results in the stronger segregate Sb, (with a higher segregation energy ∆G = -74.6 kJ/mol) to displace the Sn (∆G = -59.0 kJ/mol) from the surface. From the simulations, it is clear that the maximum surface coverage for Sn is determined mainly by the attractive interaction (ΩSnCu = -8.25 kJ/mol) between Sn and Cu. The desegregation rate of Sn in this system is determined by the segregation rate of Sb. The segregation profile of Sb is similar to that in a binary system (Cu,Sb) with the desegregation rate of Sb much slower than the segregation rate. The study also shows definite attractive interaction between Sb and Cu (ΩSbCu= -17.05 kJ/mol) This trend was not observed in the studies of binary systems. There is, however, repulsive interaction between the segregates (ΩSnSb = 3.62 kJ/mol). The repeatability of the segregation parameters at different heating rates shows that this experimental method can be used successfully.
  • ItemOpen Access
    Bepaling van die elastisiteitskonstantes van yster enkel-kristalle
    (University of the Free State, 1966-10) Lombaard, J. C.
    Abstract not available
  • ItemOpen Access
    Magnetic, luminescence and gas sensing properties of various zinc oxide nanostructures: the influence of surface modification by gold on the gas sensing properties
    (University of the Free State, 2016-01) Shingange, Katekani; Mhlongo, G. H.; Motaung, D. E.
    Various morphologies of zinc oxide (ZnO) including particles, spheres, flowers and sheets achieved by varying the pH from 7 to 13 were successfully synthesized using the microwave-assisted hydrothermal method. Effect of pH and annealing on morphological, optical and magnetic properties was investigated. Annealing altered the morphology of the ZnO structures obtained at pH levels of 9 and 13 whereby spheres and sheets were transformed into particles and platelets, respectively. The decrease in surface area and porosity of the ZnO structures was also observed with post-annealing. Green emissions assigned to oxygen vacancies (VO) dominated the PL spectra of the as prepared ZnO structures. Whereas for annealed ZnO structures, green emissions only dominated the PL spectra of the ZnO structures produced at lower pH levels (pH 7 and 9) while those of the structures obtained at higher pHs were dominated by blue emissions assigned to zinc interstitials (Zni). The sensing performance of the ZnO nanostructures to CO, CH4, NO2, H2 and NH3 at temperatures ranging from room temperature (RT) to 450°C was investigated. The study conducted on the influence of irradiation time to structural, luminescence, magnetic and sensing properties of ZnO nanorods revealed an increase in the surface area of the rods which correlated with the decrease of the lengths and widths with increasing irradiation time. High sensing response to CO at 350 °C was achieved. Surface defects on the ZnO nanorods were attributed for the high response to CO through the confirmation from PL and EPR analyses. ZnO and Au loaded ZnO nanorods were also synthesized through the microwave-assisted hydrothermal method to study the effect of Au loading on sensing properties. The distribution of the Au nanoparticles on the surface of the ZnO nanorods was controlled by varying the Au concentration as 0.5, 1, 1.5, 2, 2.5 wt%. XRD, SEM, TEM and X-ray photoelectron spectroscopy (XPS) studies confirmed the presence of the Au nanoparticles on the ZnO nanorods surface. It was found that the sensors were selective to NH3 and the 0.5 wt% sensor showed the highest response to NH3 as compared to the other sensors. The mechanisms involved in the improved sensing response of the Au modified ZnO sensors were explained in detail.
  • ItemOpen Access
    Luminescent properties of combustion synthesized BaAl₂O₄:Eu²⁺ and (Ba₁₋xSrx)Al₂O₄:Eu²⁺ phosphors co-doped with different rare earth ions|
    (University of the Free State, 2011-11) Lephoto, M. A.; Ntwaeaborwa, O. M.; Mothudi, B. M.; Swart, H. C.
    Combustion method was used to prepare all the alkaline earth aluminates (rare-earths doped BaAl2O4, BaSrAl2O4 and BaZnAl2O4) phosphor powders in this study. Measurements of these phosphor powders were carried out using various characterization techniques such as X-ray diffraction (XRD), Scanning Electron Microcopy (SEM), X-ray energy dispersive spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FT-IR). The XRD data were collected using a D8 advance powder X-ray diffractometer with CuKα radiation. Morphology and elemental composition were done using JEOL- JSM 7500F Scanning Electron Microscope. The stretching mode frequencies data were collected using Perkin Elmer Spectrum 100 FTIR spectrometer and the elemental composition on the surfaces of the phosphor powders were monitored by the PHI 5400 Versaprobe scanning X-ray photoelectron spectrometer. Photoluminescence (PL) data were collected using 325nm He-Cd laser and decay data were collected using Varian Cary Eclipse Fluorescence Spectrophotometer coupled with a monochromatized Xenon lamp (60-75 W) as excitation source and measurements were carried out in air at room temperature. The thermoluminescence (TL) data were collected using a Thermoluminescence Reader (Integral-Pc Based) Nucleonix TL 1009I. BaAl2O4:Eu2+ phosphor powders co-doped with different trivalent rare-earth (RE= Dy3+, Nd3+, Gd3+, Sm3+, Ce3+, Er3+, Pr3+ and Tb3+) ions were prepared at an initiating temperature of 600oC and annealed at 1000oC for 3 hours. The X-ray diffraction (XRD) data shows hexagonal structure of BaAl2O4 for both as prepared and post annealed samples. All samples exhibited bluish-green emission associated with the 4f65d1→4f7 transitions of Eu2+ at 504 nm. The longest afterglow was observed from the BaAl2O4:Eu2+ co-doped with Nd3+. BaAl2O4:Eu2+, Nd3+, Gd3+ phosphor powders were prepared at different initiating temperatures of 400-1200oC. X-ray diffraction data show the formation of the hexagonal BaAl2O4 structure at the temperatures of 500oC-1200oC. The crystal size calculated from the phosphor powder prepared at 1200oC was found to be 63 nm. Blue-green photoluminescence with persistent/long afterglow, was observed at 502 nm and the highest PL intensity was observed from the sample prepared at 600oC. The phosphorescence decay curves showed that the rate of decay was faster in the case of the sample prepared at 600oC compared to that prepared at 1200oC. The TL glow v peaks of the samples prepared at 600oC and 1200oC were both stable at 72oC suggesting that the traps responsible for the long afterglow were not affected by the temperature. Barium-substituted phosphor powders of (Ba1-xSrx)Al2O4:Eu2+;Nd3+ composition were prepared at an initiating temperature of 500oC. The X-ray diffraction with the composition of x = 0 shows the hexagonal phase of BaAl2O4 and the one for x = 1 shows the monoclinic phase of SrAl2O4. The XRD with the composition of x = 0.4, 0.5 and 0.6 shows the admixture of BaAl2O4 and SrAl2O4 structures. SEM investigations showed some changes on the surface morphology for different compositions. Photoluminescence (PL) studies showed the (Ba1-xSrx)Al2O4:Eu2+;Nd3+ (x = 0) and (Ba1-xSrx)Al2O4:Eu2+;Nd3+ (x = 1) with blue-green to bright-green emissions with peaks at 505 nm and 520 nm respectively. The mixed composition with x = 0.4, 0.5 and 0.6 showed two peaks at 447 nm and 517 nm. Phosphorescence showed higher luminescence for (Ba1-xSrx)Al2O4:Eu2+;Nd3+ at (x = 0) compared to other compositions. (Ba1-xZnx)Al2O4:Eu2+;Nd3+ phosphor powders with the compositions x = 0.2, 0.4, 0.5, 0.6, 0.8 and 1 were prepared at an initiating temperature of 500oC. The X-ray diffraction showed the cubic structure for the compositions of x = 0 and x = 1. The SEM images of the phosphor samples showed different kinds of morphologies for the compositions x = 0, 0.5 and 1. The PL emission of the phosphor powder clearly showed a shift from green to blue regions. The highest PL emission and the long afterglow ascribed to trapping and detrapping of charge carriers were observed from (Ba1-xZnx)Al2O4:Eu2+;Nd3+ with x = 0.2.
  • ItemOpen Access
    Low-cost thermoluminescence measurement using photodiode sensing
    (University of the Free State, 2015-05) Mbongo, M.; Ocaya, R. O.
    Many branches of scientific and industrial research require precise instrument(s) for control and measurement. Such instruments tend to be prohibitively expensive. In the current global economic climate the funding to procure research equipment is fast dwindling. One current interest that our institution has is the synthesis and thermoluminescence (TL) characterization of phosphors, polymers and nano-materials. TL measurement requires precise control and measurement of sample temperature as a function of output intensity. In the present research, we describe the design and construction of a low-cost TL instrument that allows automatic control of various steps of the experiment while logging instantaneous intensity output. This work started with two fundamental considerations. Firstly, whether a low-cost thermoluminescence equipment is feasible. Secondly and more importantly, whether a photodiode can form the intensity sensing apparatus. We answer these questions affirmatively by first putting together a course of research and assimilating the necessary tools needed. Using the the resulting demonstrable TL instrument, we demonstrate the versatility for temperature sequencing, range and heating control of the sample over the temperature range of 23 to 600 ± 0.5 ◦C. A comparable instrument in the institution operates at a maximum ceiling of 300 ◦C. Additional refinements to the prototype instrument enable the sample temperature to be held constant at any temperature within this range with the aid of software tuned Proportional-Integral-Derivative (PID) control. The intensity measurements are made using a temperature-compensated, large area photo-diode operated in photovoltaic mode and covering a wavelength range 400 to 1100 nm. The interfaces of the instrument that made the instrument easy to use were developed con-currently. For instance the Universal Serial Bus (USB) handler, the Visual BASIC.NET control program that also logs the temperature and intensity data, and the PIC18F2520 micro-controller firmware code that was written in the C-language. Several other tools, listed in the body of the dissertation were also used. Finally, we present various results of temperature control and measurement and a demonstration measurement on a ceramic sample.
  • ItemOpen Access
    Narrowband Ultraviolet B emission from gadolinium and praseodymium co-activated calcium phosphate phosphors for phototherapy lamps
    (University of the Free State, 2014-01) Mokoena, Puseletso Pricilla; Ntwaeaborwa, O. M.; Swart, H. C.
    Different phases of calcium phosphates co-doped with gadolinium and praseodymium were prepared by co-precipitation, urea combustion, citrate-gel combustion and microwave-assisted methods. Ca5(PO4)3OH:Gd3+,Pr3+ phosphors were prepared by the co-precipitation and citrate-gel methods, and were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), High resolution transmission electron microscopy (HRTEM), Energy dispersive x-ray spectrometer (EDS) and photoluminescence (PL) spectroscopy. The XRD pattern was consistent with the hexagonal phase of Ca5(PO4)3OH referenced in JCPDS Card Number 73-0293. The XPS data indicated that Ca2+ occupied two different lattice sites referred to as Ca1 and Ca2. The P5+ is surrounded by O2- ions in the tetrahedral arrangements. Each tetrahedron contains oxygen atoms designated as O1, O2, and O3. The particle morphology was analyzed using SEM and HRTEM. SEM shows that the powder was composed of an agglomeration of irregular particles. HRTEM revealed faceted edges forming a hexagonal shape. PL data exhibited a narrowband emission located at 313 nm, which is associated with the 6P7/2→8S7/2 transition of the Gd3+ ion. This emission is classified as ultraviolet B (UVB) and it is suitable for use in phototherapy lamps to treat various skin diseases. The PL intensity of the 313 nm emission was enhanced considerably by Pr3+ co-doping. The crystallographic structure of Ca5(PO4)3OH:Gd3+,Pr3+ and possible mechanism of energy transfer from Pr3+ to Gd3+ are discussed. Ca5(PO4)3OH:Gd3+,Pr3+ phosphor exhibited a single thermoluminescence peak between 339-363 K. The peak shifted towards high temperature with an increase in dose. The shift shows that the trap system is more complicated than a single trap obeying first order kinetics. The calculated activation energy (EA) was found to be 0.91 eV when the using initial rise method. The activation energy values were further calculated using the peak shape method. The calculated activation energies for , and , were 0.75, 1.03, and 0.42 eV respectively. There was a peak shifting to higher temperatures with an increase in heating rate which is attributed to recombination that is slowing down due to electron-phonon interactions. The peak intensity increased with an increase in heating rate from 0.6 to 2.0 °C.s-1 and started to decrease from 3.0 to 5.0 °C.s-1, the decrease maybe due to thermal quenching as the peak shift to higher temperatures. The calculated activation energy by heating rate method was found to be 0.60 eV. This value is comparable to other calculated values of activation energies by various methods mentioned above. Ca3(PO4)2:Gd3+,Pr3+ phosphors with different concentrations of Gd3+ and Pr3+ were successfully prepared by urea combustion process using metal nitrates as precursors and urea as fuel and also by the microwave assisted method. XRD exhibited a rhombohedral phase of Ca3(PO4)2 referenced in JCPDS Card No. 70-2065. The PL excitation spectra of Ca3(PO4)2:Gd3+ and Ca3(PO4)2:Pr3+exhibited peaks at 220-280 nm and 300-490 nm associated with the f-f transitions of Gd3+and Pr3+ respectively. The UVB emission resulting from the 6P7/2→8S7/2 transition of Gd3+ was observed at 313 nm when the Ca3(PO4)2:Gd3+ phosphor was excited at a wavelength of 274 nm using a monochromatized xenon lamp. Upon Pr3+ co-doping, the excitation peaks due to Gd3+ and Pr3+ f-f transitions were suppressed and an intense broad excitation peak ascribed to the 4f4f5d transitions of Pr3+ was observed at 227 nm. The peak intensity of the UVB emission at 313 nm was shown to improve considerably when the Gd3+ and Pr3+ co-doped systems were excited at the wavelength of 227 nm suggesting that the Pr3+ is a good sensitizer of the 313 nm narrow line UVB emission from Gd3+.
  • ItemOpen Access
    An investigation into the nature of the relativistic compact object in the micro-quasar system LS 5039 : a multi-wavelength study
    (University of the Free State, 2007-11-30) Van Soelen, Brian; Meintjes, P. J.
    English: LS 5039 is a high mass binary system that shows multi-wavelength broad non-thermal emission. It is also a very high energy gamma-ray emitter, with TeV energy gamma-rays detected by H.E.S.S. (High Energy Stereoscopic System). The nature of the compact object is unknown, but a mass > 1:44M is implied by the lack of an X-ray eclipse. The presence of radio jet-like structures and a proposed mass of 3:7M , under the assumption that the system is pseudosynchronized, has led to the system's classiffication as a microquasar. Another model, in terms of a pulsar wind has also been proposed for the system. This study undertakes a model independent investigation of LS 5039 (neither microquasar nor pulsar), to attempt to determine what conclusions can be drawn from the system from first principles. A brief review of certain aspects of high mass binary theory is first presented, including accretion, binary motion, non-thermal radiation and mass out ow processes. The analysis looks at thermal evaporation from a disc structure in a black hole system, showing that this is unlikely, given the required temperature and the lack of thermal emission observed. The required conversion efficiency > 20% of accretion power in the black hole scenario also suggests that an additional reservoir of power is needed. The presence of a rotating magnetized neutron star, provides not only the magnetic field required to produce the non-thermal emission, it also supplies an additional power source, i.e the rotational kinetic energy of the neutron star. The magnetic field strengths and electron energies (for single particles) required to produce the very high energy gamma-rays is considered. An analysis of a fast rotating magnetosphere suggests that the centrifugal force exerted on the wind material could prevent accretion in the system. The power for the system is then extracted by a turbulent MHD process near the Alfv en radius.