Masters Degrees (Physics)

Permanent URI for this collection

http://hdl.handle.net/11660/455

Browse

Now showing 1 - 20 of 51

Open Access
Bepaling van die elastisiteitskonstantes van yster enkel-kristalle
(University of the Free State, 1966-10) Lombaard, J. C.
Abstract not available
Open Access
Cathodoluminescence degradation and surface characterization of SrGa₂S₄:Ce³⁺ power and thin films
(University of the Free State, 2011-05) Moleme, Pulane Adelaide; Ntwaeaborwa, O. M.; Swart, H. C.
The structure, morphology and luminescent properties of commercial SrGa2S4:Ce3+ phosphor powder and thin films were investigated. The phosphor shows bright blue under ultraviolet (UV) excitation. Measurements were carried out using various characterization techniques such as Xray diffraction (XRD), scanning electron microcopy (SEM) and X-ray energy dispersive spectroscopy (EDS). The XRD data were collected using a D8 advance powder X-ray diffractometer with CuKα radiation. Morphology and elemental composition were done using Shimadzu Super Scan SSX-550 coupled with EDS. Photoluminescence (PL) data were collected using Varian Cary Eclipse Fluorescence Spectrophotometer with a monochromatized Xenon lamp (60-75 W) as excitation source and measurements were carried out in air at room temperature, and cathodoluminescence (CL) data were collected with S2000 Ocean Optics Spectrometer. The absorption spectra were recorded using Perkin Elmer Lambda 950 UV-VIS spectrometer. The same characterization tools were used to characterize the thin films. XRD data confirmed the orthorhombic structure of SrGa2S4 that was consistent with the standard JCPDS file no. (77-1189). The SEM images of the SrGa2S4:Ce3+ powder showed particles with irregular shapes and EDS detected presence of the major elements. Both PL and CL showed the broad emission peaks around 444 nm and 485 nm which are due to Ce3+ radiative transitions (5d (T2g) → 4f (2F5/2) and 5d (T2g) → 4f (2F7/2)). Cathodoluminescent ageing characteristics of the SrGa2S4:Ce3+ powder and thin films under prolonged electron beam bombardment were studied and presented. The cathodoluminescent intensity with increasing Coulomb loading was observed to degrade under different primary electron beam voltages for the powder. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used to monitor the surface chemical changes both during electron beam bombardment and after the degradation process. Auger peak to peak heights monitored during the ageing process suggest a decrease in S and C Auger peak intensity and an initial increase in oxygen concentration on the surface. XPS results indicate the formation of an SrO overlayer due to electron stimulated surface chemical reactions (ESSCRs). For preparation of films, silicon (Si) (100) substrates were used. A pellet was prepared from the standard SrGa2S4:Ce3+ powder. The Lambda Physik EMG 203 MSC 309 nm XeCl excimer laser was used to grow the films. The films growth was carried out in a chamber which was first evacuated to a base pressure of 8 x 10-5 mbar before backfilling to pressures of 1.0 x 10-2 mbar Ar and 1.0 x 10-2 mbar O2, where Ar and O2 were used as cross pulse gases. The films were deposited at different substrate temperatures ranging from 400°C to 600°C with 28 800 and 57 600 pulses respectively. The laser beam was operated at 8 Hz repetitive rate. The substrate temperature, number of pulses and the working pressure are the parameters that were varied during the preparation of the thin films. A highly crystalline SrGa2S4 layer was obtained at the growth temperature of 400°C. XRD patterns also showed that the properties of the films were sensitive to substrate temperature. PL and CL spectra were characterized by a broad band that can be fitted by two Gaussian peaks according to the two Ce3+ radiative transitions. At high substrate temperature a shift to Ce3+ emission in SrS occurred as well as in Ar atmosphere for both UV and high energy electrons excitation. The atomic force microscopy (AFM) images before annealing exhibited smooth surface at low substrate temperature, which became rough at high substrate temperature and after annealing in vacuum at 700°C temperature. Non-uniformity in particles (big and small) of the films and smooth films were observed from the SEM images.
Open Access
Characterization of Y3(Al,Ga)5O12:Ce3+ phosphor thin films prepared by pulsed laser deposition
(University of the Free State, 2013-11) Dlamini, Sipho Thapo Solomon; Swart, H. C.; Ntwaeaborwa, O. M.
The morphological and luminescent properties of Y3(Al,Ga)5O12:Ce3+ powder phosphor were investigated. Scanning Electron Microscopy (SEM) revealed the phosphor’s agglomerated particles with a size ranging from 0.4μm to 1.4μm. The X-ray diffraction (XRD) indicated a cubic polycrystalline phosphor with an average crystal size of 80 nm. Excitation peaks for the powder were obtained at 439, 349, 225 and 189 nm and emission peaks at 512 and 565 nm. Emission wavelength at 512 nm was also used to approximate the Al/Ga ratio within the crystal. Photoluminescence (PL) data also revealed that the addition of the Ga into the YAG:Ce3+ matrix caused a blue-shift in the emission spectra. The UV-VUV excitation and emission spectra of the Y3(Al,Ga)5O12:Ce3+ were also recorded and an energy diagram was constructed from the values. The phosphor powder was used as target material for Pulsed Laser Deposition (PLD). SiO2/Si(100) was used as substrates and thin films were deposited in the presence of different background gases. XRD indicated that better crystallization took place for films deposited in a 20 mTorr O2 atmosphere. Atomic force microscopy (AFM) revealed an RMS value of 0.7 nm, 2.5 nm and 4.8 nm for the films deposited in vacuum, O2 and Ar atmospheres, respectively. The highest PL intensity was observed for films deposited in the O2 atmosphere. The thickness of the films varied from 120 nm to 270 nm with films deposited in vacuum having the thin layer and those in Ar having the thick layer. The stoichiometry of the powder was maintained in the film during the deposition as confirmed by Rutherford backscattering spectroscopy (RBS). Luminescent properties of Y3(Al,Ga)5O12:Ce3+ thin films prepared by PLD at different substrate temperatures in an O2 background atmosphere were also investigated. XRD indicated that the films have the same cubic polycrystalline phase structure as the powder. AFM revealed poorly defined grain growth for films ablated at a substrate temperature of 22°C and 500°C but well defined grain growth was observed for films ablated at a 300°C substrate temperature. Auger electron spectroscopy (AES) depth profile of the film ablated at 500°C indicated that Si has diffused into the thin film. The highest PL intensity was observed for films deposited at the substrate temperature of 300°C. A slight shift in the wavelength of the PL spectra was obtained for the thin films with respect to the powder due to a change in the crystal field. The maximum PL intensity was obtained from the film deposited at the substrate temperature of 300⁰C in an O2 atmosphere. In addition, the films with well-defined grains (rougher surfaces) showed higher PL intensity compared to films with poorly-defined grains (smooth surfaces) as confirmed from AFM data
Open Access
CL degradation of Y2SiO5:Ce thin films coated with SnO2
(University of the Free State, 2006-05) Coetsee, Elizabeth; Swart, H. C.; Terblans, J. J.
The degradation of the cathodoluminescence (CL) intensity of cerium-doped yttrium silicate (Y 2SiO 5:Ce) phosphor thin films and commercially available Y2SiO5:Ce phosphor powders from Phosphor technology, England, were investigated for possible application in low voltage field emission displays (FEDs). Thin films of Y2SiO5:Ce were pulsed laser ablated on Si (100) substrates by using a XeCl (308 nm) excimer laser, in an oxygen (O) ambient gas pressure of 7.5 x 10-4 Torr, with laser energy of 81.81 mJ, repetition rate of 10 Hz, substrate temperature of 400°C, target to substrate distance of 3.7 cm and by using 6600 pulses. Some of the phosphor thin films were coated with tin oxide (SnO2), with the same deposition parameters as for the Y2SiO5:Ce phosphor layer except for the amount of pulses that was reduced to 1200 pulses. A SnO2 layer was ablated onto some of the thin films in order to investigate the effect of the coated layer on the surface and on the degradatio n of the CL intensity. Rutherford backscattering (RBS) was used to measure the film thicknesses. The results showed a non uniform Y2SiO5:Ce layer covered with a 58 nm thick SnO2 layer. Scanning electron microscopy (SEM), atomic force microscopy (AFM) and energy dispersive spectroscopy (EDS) were use to study the surface morphology of the thin films. The results indicated that the Y2SiO5:Ce phosphor was ablated onto the Si (100) substrate surface as micron-sized spherical particles and that the SnO2 layer was ablated as a uniform coated layer covering the surface of the substrate and the randomly distributed spherical Y2SiO5:Ce particles. SEM was also use to study the surface morphology of the Y2SiO 5:Ce phosphor powders and the results showed that the particles were agglomerated. X-ray diffraction (XRD), that was used to measure the crystal planes of both the thin films and the powders, revealed the monoclinic crystal structure of Y2SiO5:Ce. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and CL spectroscopy were used to monitor changes in the surface chemical composition and luminous efficiency of the Y2SiO5:Ce phosphor powders and thin films (coated and uncoated) . AES and CL spectroscopy measurementswere done with 2 keV energy electrons and with beam current densities between 26.3 mA.cm-2 and 52.63 mA.cm-2 , in high vacuum and in oxygen pressures of 1 x 10-8, 1 x 10-7 and 1 x 10-6 Torr. AES indicated adventitious carbon (C) on the surface before CL measurements were made. C was depleted from the surface during electron bombardment. Residual gas mass analysis (RGA) showed that C was removed from the surface as volatile gas species. RGA with the electron beam on resulted in a higher intensity of CO2, CO and H2O gas specie s, compared to when the electron beam was off. This is consistent with the electron stimulated surface chemical reaction (ESSCR) model, whereby the electron beam dissociates the oxygen gas species into reactive atomic species, which then reacts with the carbon on the surface to form the volatile CO2 and CO gas species. Auger peak to peak heights (APPH) for oxygen and silicon on both the uncoated thin film and the powder surface stayed almost constant. The CL intensity (measured at 440 nm) increased within the first 300 C.cm-2, which is the result of the depletion of the carbon from the surface, and then it stayed constant for prolonged electron bombardment. The carbon results in an extra layer on the thin film surface that increases the energy loss of the incoming electrons. This results in the creation of fewer electron – hole pairs for photon emission during radiative recombination. The CL emission spectrum resulted in the characteristic double shoulder peak of Y2SiO5:Ce with the two main peak positions at 440 and 500 nm (blue light) before and after 24 hr s of electron bombardment for the uncoated thin film, coated thin film and for the powders. Light emission in the rare earth, Ce3+, is due to the 5d ? 4f transition due to the splitting effects of the 4f energy level. The 4f energy level splits due to the effect of the crystal field in Y2SiO5 as the host material, into the 2F7/2 and the 2F5/2 energy levels. The broad band emission of Y2SiO5:Ce is the result of the different splitting effects due to the crystal field. The relatively high CL intensity of the thin films is attributed to the spherically shaped phosphor particles grown on the surface of the Si (100) substrate. The SnO2 was also successfully ablated as a coating layer. The SnO 2 coating layer increases the energy loss of the incoming electrons which results in a lower CL intensity. The CL intensity for the uncoated thin film was therefore higher than for the coated thin film. The CL intensity stayed almost constant for the 24 hr s of electron bombardment of both the coated and uncoated thin films. The CL intensity for the phosphor powders, however, behaved differently. The intensity showed an increase after about 300 C.cm-2. The CL emission spectrum showed an increase in a second broad band at a wavelength of 650 nm after 24 hr electron bombardment. It was proved with XPS that this second broad band is due to the formation of a luminescent silicon dioxide (SiO2) layer on the surface of the Y2SiO5:Ce phosphor powders, as a result of the electron surface stimulated reactions (ESSCR). The increase in the CL intensity is thus due to the luminescent SiO2 layer that was formed as a result of electron beam irradiation that causes the Si-O bonds to break and to form intrinsic defects at 1.9 eV (650 nm) and 2.7 eV (459 nm). XPS also indicated that the Ce concentration on the surface layer increased during the degradation process and the formation of CeO2 and CeH3 also resulted from the degradation process. The phosphor powders degraded from a blue light emitting phosphor before electron bombardment to a whit ish light emitting phosphor after 24 hr, as a result of the luminescent SiO2 layer formed during degradation.
Open 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.
Open 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.
Open Access
The development of a UFS-Boyden Photometric pipeline to facilitate the observational study of accretion driven systems
(University of the Free State, 2005-05-30) Calitz, Johannes Jacobus; Meintjes, P. J.
After the retirement of professor A. Jarrett in 1986, the 1.5-m telescope at Boyden Observatory stood idle for a decade. With the appointment of Dr P. Meintjes, steps were taken to refurbish the telescope with an updated drive control and camera system, which would eventually enable the telescope to be operated as an astrophysical research instrument. After funding became available, upgrading of the drive mechanisms were undertaken by DFM during August and September 2001 and the new SpectraVision 1k 1k CCD camera, that was on loan from Lawrence Livermore National Laboratory (LLNL), was installed during February 2002. After 16 years, the telescope was ready to be used for gathering data for research projects. The camera was installed with only demonstration software. Software was needed to control the camera and also for data reduction and a photometry pipeline. During this project, the problems encountered with the baes, electronics and collimation in the telescope were analized and xed where needed and possible. Manuals were written for the general use of the telescope, as well as the reduction and photometry pipeline. Extinction coecients for Boyden Observatory were determined. Software were developed to control the PixelVision CCD camera. A CCD reduction routine that is easy and automatic as far as possible was written and implemented. A photometry pipeline that can be used with vast amounts of data, while producing a high level of accuracy were developed. The research elds that are making use of the software include gravitational microlens observations, accreting compact objects and Gamma Ray Burst afterglows. A brief overview of these elds are given.
Open Access
The development of an IRAF-based scientific photometric package for the UFS-Boyden 1.5-m telescope
(University of the Free State, 2009-03-10) Van Heerden, Hendrik Jacobus; Meintjes, P. J.
English: In this dissertation there will be looked at the development of an (Image Reduction and Analysis Facility) IRAF-based scientific photometric package for the UFS / Boyden 1.5-m telescope. The dissertation consist of a discussion on the history of Boyden Observatory and its instruments, with specific emphasis on the Rockefeller 1.5-m telescope. The discussion will include information on the upgrades and improvements the telescope underwent to compete and do research on an international level. In the proceeding chapters charged coupled devices (CCDs) will be discussed, as well as how to characterize CCD photometric observation systems, like the 1.5-m telescope. The chapters will include experimental procedures and results obtained during characterization experiments. Chapters on photometry techniques will follow thereafter as well as the development of the Boyden-IRAF photometric data-analysis system. It will include an overview of IRAF, as well as a more in depth discussion of the Boyden-IRAF package. The discussion will specify as to why and how it was developed and how it works. A final chapter will be presented on the testing of the Boyden-IRAF package through the determining of the Boyden atmospheric extinction coefficients using the newly developed package. With this project, i.e. the development of an IRAF-based photometric program, an attempt is made to fill a void that exists related to the in-house photometric capabilities. A reliable and user-friendly photometric program will definitely also result in Boyden Observatory playing an important role in student training and research programs. Finally a conclusion will be drawn as to the success of the new developments, the IRAF-based photometric package, and what this means for the development of Boyden Observatory and the UFS Astrophysics group i.t.o. research and development.
Open Access
The effect of nitrogen on the cosegregation of molybdenum in a Fe-3.5wt%Mo-N (100) single crystal
(University of the Free State, 2006) Jordaan, Werner Albert; Terblans, J. J.; Swart, H. C.
In this study the cosegregation of molybdenum and nitrogen to the (100) plane of an iron single crystal was investigated. Ternary systems are considerably more complex than binary systems in that there are seven segregation parameters to determine, as opposed to three. However, a novel approach was undertaken to minimize the amount of variables, by first analysing a similar binary system that was exposed to a nitrogen ambient. Two single crystal were selected for this purpose, i.e. a Fe- 3.5wt%Mo(100) binary system and a Fe-3.5wt%Mo-N ternary system. By exposing the binary crystal to a nitrogen ambient at high temperatures it was observed that molybdenum segregated to the surface. The segregation profiles of the two systems were acquired at constant temperatures from 797 K - 888 K and Auger Electron Spectroscopy was used to monitor the surface concentrations of the relevant species. Since accurate surface temperature measurements are essential to segregation studies, a calibrated infrared thermometer was used. The segregation profiles were generated by measuring time and the Auger signal simultaneously. From the segregation profiles, initial estimates for the diffusion coefficients of Mo were first determined for the binary system by applying Fick’s equation to the segregation profiles. From these values the pre-exponential factor, D0, was determined to be 1.2x10−4±2 m2/s and the activation energy, E, as 258±33 kJ/mol. The diffusion coefficients determined thus, were used as estimates for obtaining the Darken seggregation profiles. In this case the D0 value was found to be 2.4x100±1 m2/s and the E value, 323±16 kJ/mol. The segregation energy, G, of Mo was calculated as -38 kJ/mol. In both cases it was observed that the diffusion coefficient of Mo deviated from the expected value at high temperatures due to the desorption of nitrogen from the surface. Using thermodynamic theory, an expression for the segregation energy of Mo in terms of the nitrogen surface concentration was derived. The Darken fits were repeated and it was found that the high temperature diffusion coefficient values fell on the the Arrhenius linear regression lines. For this special case, the D0 value was calculated as 5.5x101±1 m2/s, the E value as 345±18 kJ/mol. The segregation parameters determined for the binary system were then used as initial values for fitting the experimental data of the ternary system. Using Fick’s equation, the diffusion coefficients of Mo and N in Fe were determined. From the Arrhenius linear regression, the pre-exponential factor for Mo was calculated as 3.6x10−2±1 m2/s and that of N as 4.1x10−1±2 m2/s. The activation energies were 308±20 kJ/mol and 210±40 kJ/mol for Mo and N, respectively. The segregation parameters of the ternary system were then determined via the Darken method. In this case the pre-exponential factors were 1.9x10−4±1 m2/s for Mo and 2.8x100±3 m2/s for N. The activation energies were 271±11 kJ/mol and 323±43 kJ/mol. The segregation energy of Mo was calculated as -32 kJ/mol and for N, -19 kJ/mol. The interaction coefficient between Mo and N was calculated as -19 kJ/mol.
Open Access
Experimental and computational study of S segregation in Fe
(University of the Free State, 2012-06) Barnard, Pieter Egbert; Terblans, J. J.; Swart, H. C.; Hoffman, M. J. H.
A systematic study was conducted to investigate the diffusion and segregation of S in bcc Fe using (i) DFT modelling and (ii) the experimental techniques Auger Electron Spectroscopy (AES) and XRay diffraction (XRD). The aim of this study was to obtain the activation energies for the segregation of sulfur (S) in bcc iron (Fe), both computationally and experimentally in order to explain the diffusion mechanism of S in bcc Fe as well as the influence the surface orientation has on surface segregation. The Quantum ESPRESSO code which performs plane wave pseudopotential Density Functional Theory (DFT) calculations was used to conduct a theoretical study on the segregation of S in bcc Fe. To determine the equilibrium lattice sites of S in bcc Fe, the tetrahedral-interstitial, octahedralinterstitial and substitutional lattice sites were considered. Their respective binding energies were calculated as -1.464 eV, -1.660 eV and -3.605 eV, indicating that the most stable lattice site for S in bcc Fe is the substitutional lattice site. The following mechanisms were considered for the diffusion of S in bcc Fe: tetrahedral-interstitial, octahedral-interstitial, nearest neighbour (nn) substitutional and next nearest neighbour (nnn) substitutional with migration energies, Em, of respectively 4.438 kJ/mol (0.046 eV), 22.48 kJ/mol (0.233 eV), 9.938±6.754 kJ/mol (0.103±0.007 eV) and 96.49±0.579 kJ/mol (1.000±0.006 eV). According to the binding and migration energy calculations, S will diffuse via a substitutional mechanism with a migration energy of 9.938±6.754 kJ/mol (0.103±0.007 eV). The three low-index planes of bcc Fe were investigated to determine the stability, the vacancy formation energy and the activation energy for each surface. Structural relaxation calculations showed that the surfaces in order of decreasing stability are: Fe(110)>Fe(100)>Fe(111) which is in agreement with surface energy calculations obtained from literature. The formation of a vacancy in bcc Fe was modelled as the formation of a Schottky defect in the lattice. Using this mechanism, the vacancy formation energies, Evac, for the Fe(110), Fe(100) and Fe(111) surfaces were respectively calculated as 267.4 kJ/mol (2.772 eV), 256.8 kJ/mol (2.662 eV) and 178.2 kJ/mol (1.847 eV). The activation energy, Q, of S diffusing via the substitutional mechanism for the Fe(100), Fe(110) and Fe(111) surfaces were respectively calculated as 277.4 kJ/mol (2.875 eV), 266.8 kJ/mol (2.765 eV) and 188.1 kJ/mol (1.950 eV). Thus it was found that the vacancy formation energy is dependent on the surface orientation and thus the structural stability of the Fe crystal. Experimental values for the activation energy of S in bcc Fe (232 kJ/mol (2.40 eV) and 205 kJ/mol (2.13 eV)) were obtained from literature confirming the nearest neighbour substitutional diffusion mechanism of S in bcc Fe. No indication is given regarding the orientation of the crystal in which the value of 232 kJ/mol (2.40 eV) was obtained while the value of 205 kJ/mol (2.13 eV) is for a Fe(111) crystal orientation. For the experimental investigation of the Fe/S system polycrystalline bcc Fe samples were studied. These samples were prepared by a new doping method by which elemental S is diffused into Fe. In order to prepare the samples by this method a new system was designed and build. Auger depth profile analysis confirms the successful doping of Fe with S using the newly proposed doping method. It was found that the S concentration was increased by 89.38 % when the doping time was doubled from 25 s to 50 s. An Fe sample doped for 50 s was annealed at 1073 K for 40 days after which the effects induced by S and the annealing of the sample were investigated by Secondary Electron Detector (SED) imaging. Results showed a 36±11 % decrease in the grain sizes of the polycrystalline Fe sample due to the presence of S. It was found that the re-crystallization rate of Fe is increased due to the presence of S. Using XRD, the Fe (100), Fe(211), Fe(110), Fe(310) and Fe(111) orientations were detected for both the un-doped and the annealed S doped Fe samples. The annealed sample showed the following percentage changes in the concentrations of the respective orientations compared to the un-doped sample: -5.180, +2.030, +16.41, +0.400, -13.66. Taking the calculated trend in surface stability for the three low-index orientations of Fe into consideration, it was found that the more stable Fe(110) orientation had increased in concentration during annealing, while the less stable Fe(100) and unstable Fe(111) orientations had decreased in concentration during annealing. AES measurements on the two samples were performed using the linear programmed heating method. The segregation parameters of S for the un-doped Fe sample are: D0=4.90×10-2 m2/s, Q=190.8 kJ/mol (1.978 eV), ΔG=-134 kJ/mol (-1.39 eV) and ΩFe/S=20 kJ/mol (0.21 eV). The segregation parameters of P obtained for the un-doped Fe sample are: D0=0.129 m2/s, Q=226.5 kJ/mol (2.348 eV). For the S doped Fe sample the segregation parameters of S were determined as: D0=1.79×10-2 m2/s and Q=228.7 kJ/mol (2.370 eV), ΔG=-145 kJ/mol (-1.50 eV) and ΩFe/S=8 kJ/mol (0.08 eV). These results showed that for the doped sample, with an increased concentration in the stable Fe(110) and a decreased concentration in the less stable Fe(100) and unstable Fe(111) orientations, a higher activation energy was obtained. Comparing the measured activation energies to the calculated values indicates that the diffusion of S occurs via a vacancy mechanism, where the S atom occupies a substitutional lattice site. Despite the fact that polycrystalline samples were analysed, the activation energies are still in the same order as the calculated activation energies of the single crystals. This confirms the theoretical prediction of a substitutional diffusion mechanism of S in bcc Fe. During this study the diffusion mechanism of S was determined as the substitutional diffusion mechanism whereby a S atom would diffuse from a substitutional lattice site to a nearest neighbour vacancy. The different Fe orientations considered in the calculations can be arranged from highest to lowest activation energy as Fe(110)>Fe(100)>Fe(111). These calculations are in agreement with the AES results which showed an increased activation energy for the doped sample having a higher Fe(110) concentration and lower Fe(111) and Fe(100) concentrations.
Open Access
Growth and characterization of ZnO nanoparticles by sol-gel process
(University of the Free State (Qwaqwa Campus), 2015-01) Ungula, Jatani; Dejene, F. B.
Solid state lighting technology is of particular interest in application of semiconductors. To this end, ZnO nanostructures have gained great attention in the research community, in part because of its requisite large direct band gap. The stability of the exciton (binding energy 60 meV) in this material, can lead to lasing action based on exciton recombination and possibly exciton interaction, even above room temperature. Therefore, it is very important to realize an optimized growth of ZnO nanostructures and investigate their properties. The main motivation for this thesis is not only to successfully realize the controllable growth of ZnO nanoparticles by sol-gel method, but also to investigate the structure, optical and electrical properties in detail by means of scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy), UV-Vis spectroscopy, X-ray diffraction (XRD) and other techniques. The influence of various growth parameters on the morphology, optical and electrical properties of the nanoparticles were also systematically studied. These include the growth temperature, volume ratios of water to ethanol solvent and different dopants effects. By controlling these parameters different shapes of nanoparticles, like spherical particles, nanorods and nanoflowers are demonstrated. XRD indicated that all the as-grown and annealed nanoparticles produced at temperatures between room temperature and 75 °C crystallize in the wurtzite structure and post growth annealing enhanced the crystalline quality of the materials while the band gap energy reduces. The crystallite size, obtained from XRD analysis, of as prepared ZnO nanostructures was found to decrease from 24 to 12 nm with the increase in volume ratio of ethanol in the solvent as peak intensities and sharpness increase with volume ratio of water. Thus in order to have smaller particles more volume ratios of ethanol solvent is favourable at growth temperature of 35 °C. The dopants were also observed to have slight effect on the grain sizes .No traces of zinc hydroxide were observed even in materials grown at lower temperature as reported by some authors. The optical quality of the nanostructures was investigated using PL. Both UV and defect related emissions have been observed for all as-grown and annealed samples of nanostructures. Photoluminescence spectra showed a strong ultra-violet emission, for annealed ZnO nanoparticles, which was centred on 385 nm and weak green emission at 550 nm confirming that the samples possess good optical properties with less structural defects and impurities. The effect of post-growth annealing on the optical quality of the nanostructures was carefully examined. Annealing at a temperature of 600 °C enhances the UV emission and suppresses defect related deep level emission for all samples. The PL spectra showed strong, broad and intense emission in visible region for Ce-doped ZnO samples while other dopants suppressed this green emission. The reflectance spectra of the annealed products show that the percentage absorption in visible range increases with annealing temperature. UV measurements depict a shift in absorption edge confirming the changes in particle sizes with varying ratios of solvents (water and ethanol). The band gap decreased from 3.31 to 3.17 eV with an increase in the ethanol composition in the solvent, implying that the optical properties of these materials are clearly affected by the precursor compositions. The SEM micrograph of ZnO revealed that the surface aspect depends on both the dopant used and annealing temperature. The characterization of the nanoparticles with Scanning Electron Microscopy (SEM) showed that at low temperatures (35 °C and 45ºC) clearly defined spherical particles are formed while at higher temperatures agglomerated irregular and diminished nanoparticles were observed.
Open Access
The identification of detectable gravitational wave signatures within the Einstein formalism for various classes of galactic sources
(University of the Free State, 2014-01) Maritz, Jacques; Meintjes, P. J.
English: A central result of this thesis is the prediction of short period (transient) GW signatures, using General Relativity. This thesis focused on various LIGO and SKA Gravitational Wave (GW) sources such as collapsing supernovae, rapidly spinning magnetars, the coalescence of compact binary objects and the stochastic Gravitational Wave backgrounds produced by Super Massive Black holes. Upper limits for the GW amplitudes and frequencies were predicted by means of numerical and analytic methods. Finally, the prospects of detecting Gravitational Waves from the galactic center will be discussed.
Open Access
Influence of the shape and size of a quantum struture on its energy levels
(University of the Free State, 2006-05) Harris, Richard Anthony; Terblans, J. J.; Swart, H. C.
In this study the importance of the luminescent properties of low-dimensional quantum structures are investigated focusing on the change in the exciton binding energy with a change in the size of the low dimensional QuantumWell or Wire. With a reduction in dimensionality, moving from bulk semiconductor materials through Quantum Wells, Wires and ultimately Quantum Dots, the band structure as well as the density of states for these low-dimensional structures change appreciably going from quasi-continuous in bulk semiconductors to discrete in Quantum Dots. This leads to an increase in the energy gap (compared to the bulk material), with a decrease in size for a low-dimensional structure. An interacting electron-hole pair in a Quantum Well-Wire is studied within the framework of the Effective-Mass Approximation. A mathematical technique is presented which investigates the quasi-two-dimensional, quasi-one-dimensional behavior of a confined exciton inside a semiconductor as the bulk material is reduced in dimensions to form a Quantum Well and Wire. The technique is applied to an infinite Well-Wire confining potential. The Envelope Function Approximation is employed in the approach, involving a three parameter variational calculation in which the symmetry of the component of the wave function representing the relative motion is allowed to vary from the one- to the two- and three-dimensional limits. A quasi–two-dimensional behavior occurs on reducing the well width as the average electron-hole distance decrease leading to an increase in the binding energy. However, when the well width is smaller than a critical value, the leakage of the wave function into the barriers becomes more important and the binding energy is reduced until it reaches the value appropriate to the bulk barrier material for which L = 0. As the electronic industry progress from micro-technologies to nanotechnologies whereby devices are designed in the nanometer range, it becomes increasingly necessary to address the concern of the exciton losing its enhanced effects in the ultra- small quantum structures, due to the increased penetration of the exciton wave function into the barrier regions in the direction of diminishing spatial confinement. A trial wave function is employed; written as a product of three wave functions. The first two are corresponding to the single particle wave function of an electron and a hole in the Quantum Well-Wire and the third represents a free exciton whose radius is adjusted as a variational parameter. This method can be suitably adapted for any particular choice of variational wave function. The choice of this wave function is only limited by the users’ qualitative knowledge of the system under consideration and how this knowledge is imbedded into this trial wave function. Results to this numerical calculation are presented. Quantitative comparisons with previous calculations for quantum wells was made (in the wire limit where Lz → ∞) and it was found that there exists a good agreement between this infinite- and other finite- as well as infinite - potential models up to a point of 100 Å. A plot of the binding energy vs. the variational parameter λ revealed that the electron in the exciton has a very similar behavior than the electron in the Hydrogen atom (or for that matter any particle trapped inside a radial decreasing (i.e. V~1/r) potential field). However on reducing the size and dimensions of the quantum structure, it seems that the screening of the other electrons surrounding the hole start to play a very important role and the shape of a plot of binding energy versus λ is very similar to that of an alpha particle trapped in an atomic nucleus. It is concluded from this that for accurately predicting the behavior of systems like these it is important to include in such a model not only the different dielectric constants for the barrier and the well-wire materials, but also to include the change in dielectric constant due to a change in size, i.e. ε = ε (L), i.e. to take into account the decrease in the amount of electrons in the valence band due to a decrease in size of the Quantum Well-Wire.
Open 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.
Open 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.
Open 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.
Open 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) Annah, Lephoto Mantwa; Ntwaeaborwa, O. M.; Mothudi, B. M.; Swart, H. C.
A 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 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.
Open 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.
Open 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.
Open 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.