Masters Degrees (Physics)

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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
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.
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
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
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
'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
Open Access
A Monte Carlo program for simulating segregation and diffusion utilizing chemical potential calculations
(University of the Free State, 2004) Joubert, Heinrich Daniel; Terblans, J. J.; Swart, H. C.
Bulk-to-surface segregation plays a major role in the engineering of alloy surfaces. An increase in surface sensitive analysis techniques in recent years have led to big advances in the engineering of surface properties. The focus of this study is the development of a Chemical Potential Monte Carlo (CPMC) model which is based on the modified Darken model. This model is capable of simulating diffusion and segregation in crystals with a uniform concentration as well as crystals consisting of thin layers. The chemical potential equations used for the calculations by the modified Darken model are rewritten to include the segregation energy associated with the surface layer. The change in chemical potential directs atomic motion and simulations involving the change in chemical potential are performed on a 2-dimensional matrix containing two elements: the solute and the solvent elements. A random selection of an atom inside the matrix initiates the model. The change in chemical potential due to an atomic jump of a randomly selected atom to an adjacent layer is calculated. The largest change in chemical potential directs the atomic motion, complying with the conditions associated with the lowering of the Gibbs free energy; the driving force of atomic motion is therefore the lowering of the total crystal energy. Inclusion of the segregation energy (for jumps involving the surface layer) limits the number of atomic jumps from the surface layer to the bulk. Simulated segregation profiles generated by the CPMC model were compared with profiles calculated with both the modified Darken and Fick model. The comparisons show that the CPMC successfully describes both the kinetic and equilibrium conditions associated with surfa ce segregation. A reduction in calculation time was also achieved by implementing the CPMC model in parallel.
Open Access
Simulating ion sputtered depth profiles in Auger electron spectroscopy
(University of the Free State, 2004-05) Yohannes Tesfamicael, Biniam; Roos, W. D.; Terblans, J. J.; Wang, J. Y.
Recent developments in advanced materials technology are mainly based on the progress in surface and interface science. These surface and interface properties of materials greatly affect and control the overall properties of the materials. The reliable performance of multilayered thin- film structures in many technological applications like microelectronics, for instance depends upon the mechanical and chemical stability of the interfaces. Hence, appropriate study and analysis of the interfaces is an important aspect that has to be carried out with great precision. Depth profiling is one of the most powerful mechanisms in the analysis of surface and interfaces of thin multilayered structures. This depth profiling is accomplished by surface analytical techniques like AES and XPS accompanied by ion sputtering. The principal aim of this depth profiling is to investigate the distribution of elemental concentration with depth. The ion etching of the sample during the depth profiling, however, imposes some effects on the shape of the profile. The major causes for the profile distortion comes from Atomic mixing, Interface roughness, Information depth of the secondary emission and preferential sputtering in multicomponent systems. A model (MRI) that is often used in literature to simulate depth profiles in Auger electron spectroscopy takes into account the effect of atomic mixing, interface roughness and information depth. One of the radiation-induced factors limiting depth resolution is preferential sputtering. In this study the model was modified to incorporate the effect of preferential sputtering on the distortion of the depth profile. Although preferential sputtering is an exponential function it was treated as independent of the other contributing functions and in such a way as to add to the total depth resolution in quadrature, according to an error propagation law. One application of the model is in the determination of interdiffusion parameters in annealed multilayered thin film structures. In the experimental part of this study a Cu/Ni multilayer structure was evaporated onto a silicon substrate. The samples were annealed for different times in the temperature range 250 to 350ºC. This was followed by Auger depth profiling using Ar + sputtering with 3 keV primary ions at an angle 60º to the surface normal. Deconvolution of the overlapping Cu and Ni Auger spectra were performed followed by the calibration of the depth and concentration scales. In the process of simulating the measured depth profile the modified model yielded the contributions of atomic mixing, information depth, interface roughness and the ratio of the sputtering yields of Cu and Ni. The value of the interface roughness, expected to be a function of annealing temperature and time, was used to calculate the interdiffusion coefficient. The diffusion parameters Do = 4x10 -14 m 2 /s and the activation energy Q=69kJ/mol agrees excellently with values available in literature where grain boundary diffusion is the dominant diffusion process. These results confirm the successful modification of the MRI model.
Open Access
Secondary star surface magnetic activity and mass transfer in cataclysmic variables
(University of the Free State, 2005) Jurua, Edward; Meintjes, P. J.
In this study it is shown that secondary star magnetic fields influence the mass transfer process in close interacting binaries, especially cataclysmic variables (CVs) and thus play a fundamental role in the whole mass transfer process, and evolution of these systems. The Mestel and Spruit (1987) stellar wind theory is used to model the surface magnetic field of the secondary star in CVs, particularly the intermediate polars, constraining the angular momentum that is required to drive the observed mass transfer rate through Roche lobe overflow. This in turn allows solving for the mass transfer rates, via magnetic braking, and the surface polar magnetic field of these stars. These field strengths are used to study and constrain magnetic advection from the secondary star to the primary star, and its effect on the mass flow in the funnel in magnetic CVs. This has important consequences for the so-called magnetic viscosity in the accretion discs of disc accreting magnetic cataclysmic variables, which are fed by these magnetic secondary stars. It is shown that the mass transfer rates in these systems vary with orbital period, with lower mass transfer rates in more compact systems than in the wider systems. It is also shown that advection of magnetic flux into the funnel results in severe magnetic viscosity at the L1 region. The advected magnetic field into the funnel flow results in a magnetized flow and enhanced magnetic pressure in the L1 region. Since the magnetic pressure in the L1 region exceeds the flow ram pressure, continuous flow of material through the L1 region is prevented. It is shown that matter can easily cross the funnel if pressure builds up behind the barrier. This therefore implies that the mass transfer in these systems is not continuous but fragmented in the form of blobs.
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 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
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
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
Oxygen-induced segregation during batch annealing of industrial steel coils
(University of the Free State, 2006-05) Wurth, Etienne; Swart, H. C.; Terblans, J. J.
The development of diffusion welds between spirals of steel coils, during batch annealing, is of particular interest because it preve nts the coils from being unwound for further use. The physical metallurgy of iron and steel is exceedingly complicated and many of the complications arise from the behaviour of solutes, which segregate to surfaces and interfaces, which alter the mechanical behaviour. Segregation studies were done by measuring the APPH’s (Auger Peak to Peak Heights) of the segregating species (P, S, C and Ti) against annealing time during the annealing of an ultra low carbon (ULC) Ti stabilized steel between 550 and 800oC. The modified Darken model was used to describe the complex segregation behaviour of the species involved during annealing of the industrial steel. This was done by comparing the initial changes in fractional surface concentration of the segregating species against annealing time to the trends in the surface concentration changes as describe by the Darken model for a ternary alloy. Calculations were done, using Langmuir-McClean equations, to determine the change in effective segregation energy as a function of oxygen surface coverage. Oxidation was allowed after sputtered cleaning and segregation, these oxidation results were compared with each other. No C segregation occurred without oxygen in the system. Oxygen induced-segregation of Ti and C occurred at 700oC and 800oC. Oxidation occurred at 700oC and 800oC. It was found that the adsorption of oxygen on the surface profoundly influence the segregation rate of the species involved. The modified Darken model was successfully used to describe the oxygen induced-segregation process. The induced segregation may act as a possible source of the diffusion welds during batch annealing.
Unknown
TEM investigation of rapidly deformed Cu and Mo shaped charge liner material
(University of the Free State, 2007-05) Cronje, Shaun; Kroon, R. E.; Roos, W. D.
The strength and ductility of metals is a vast and important research area in which certain trends are well known, but where it is difficult to predict results with a high level of certainty, especially under extreme conditions e.g. high strain rates and very small grain sizes. Results may also be strongly influenced by impurities. All of the above factors play a vital role in the performance of shaped charge liners. Of particular interest is the material used in the manufacturing of liners. The microstructure and extended defects of copper and molybdenum shaped charge liners were investigated. Samples were extracted from the liners by electric discharge machining, to minimize any microstructural damage. Chemical testing revealed a higher than expected impurity concentration. Samples were annealed under two different annealing conditions, in order to obtain a variety of starting microstructures. Copper samples were annealed at 300°C for 30 minutes and 500°C for 30 minutes. Molybdenum samples were annealed at 1200°C for 30 minutes and 1200°C for 3 hours. These samples were then deformed at high strain rates using a split Hopkinson pressure bar. Two strain rates were used, the higher strain rate being approximately twice that of the lower strain rate. For both the copper and molybdenum the lower strain rate was on average 700 s-1, while the higher strain rate was on average 1550 s-1 and 1650 s-1 in the case of copper and molybdenum respectively. In the case of the molybdenum, the results showed a strong strain rate dependency of the yield strength which is typical of body centred cubic materials, whereas no such strain rate dependency could be detected in the copper results. Both materials show significant softening due to annealing, but relatively small changes between less and more intense annealing procedures. The unannealed samples showed significant variation in the stress-strain results, which is attributed to them originating from different parts of the liner. The uniformity of results after annealing indicates that the stress-strain properties of both materials after annealing are not strongly dependent on their prior straining history. The microstructure of these samples was examined using an optical microscope as well as a scanning electron microscope. The grain size was determined using the Heyn method. The as-received copper material had an elongated and heavily deformed microstructure. The lower annealing temperature produced a recrystallised grain structure, having an average grain size of 5 μm. The higher annealing temperature allowed grain growth with grains averaging 9 μm. The annealed copper samples contained annealing twins. In the case of molybdenum, the as-received material consisted of large (200 μm) grains. Annealing under both annealing conditions produced the same recrystallised, non-uniform grain structure with grains ranging from 47 μm to 92 μm. Transmission electron microscopy investigations of the samples revealed that deformation twinning occurred in the annealed and strained copper samples. This twinning occurred at a lower strain rate than expected. Dislocations in an annealed but unstrained copper sample occurred in entangled networks separated with areas containing no dislocations. These mixed dislocations were found to have Burgers vectors of the type b = a/2<110>. Pure edge dislocations with a [100] projected direction in the (110) plane with a Burgers vector of the type b = a/2[110 ] were also found. These dislocation arrays appear as ripple like structures. No evidence of twinning was found in the molybdenum samples. Some dislocations with Burger vectors of the type b = a/2<111> were found in the molybdenum samples. There are however exceptions, which is difficult to explain. This is an important observation, and further research would have to be performed.
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
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
Sol-gel synthesis of and luminescent properties of Pr³⁺ in different host matrices
(University of the Free State, 2009-11) Mbule, Pontsho Sylvia; Ntwaneaborwa, O. M.; Swart, H. C.
Luminescent ZrO2:Pr3+ , SiO2:Pr3+, ZnO:Pr3+ and ZnS:Pr3+ nanophosphors were synthesized by a sol-gel method, dried, ground and annealed in air at 600oC (SiO2:Pr3+, ZrO2:Pr3+, ZnO:Pr3+ and ZnS:Pr3+) or 280oC (ZrO2:Pr3+). The chemical composition of the powder phosphors was analyzed by energy dispersive x-ray spectrometer (EDS). The structure and particle sizes were determined with x-ray diffraction (XRD) and particle morphology was analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SiO2:Pr3+ was amorphous even after annealing at 600oC. ZrO2:Pr3+ annealed at 280oC showed an amorphous structure but the material crystallized when the annealing temperature was increased to 600oC. The particle sizes estimated from the XRD peaks were ∼2±0.2 nm (dried ZnS and ZnO) and ∼8±0.1 nm (ZrO2:Pr3+ annealed at 600oC). Particle sizes increased to ∼17-20±0.2 nm in diameter for annealed ZnS:Pr3+ and ZnO:Pr3+. The UV-Vis spectrophotometer was used to determine the absorption properties of the nanophosphors and their band absorption showed a blue shift compared to their bulk counterparts. Powder phosphors were also irradiated with 325 nm (He-Cd) laser to study photoluminescence (PL) properties. PL spectra were obtained for both undoped and Pr3+ -doped nanophosphors. A broad emission band was observed at 498 nm with a shoulder at 416 nm from SiO2:Pr3+ annealed at 600oC. ZrO2:Pr3+ annealed at 280oC showed two emission bands in the visible range at 459 nm and 554 nm. A broad green emission band at 567 nm and a shoulder at 607 nm were observed for dried ZnO:Pr3+ nanophosphor and the shoulder at 607 nm was enhanced significantly when the Pr3+ concentration was increased. Annealed ZnO:Pr3+ (280oC) nanophosphor showed a green emission band centered around 533 nm and a shoulder at 624 nm. Dried ZnS:Pr3+ nanophosphor showed a blue emission centered at 445 nm and the PL intensity increased with an increase of Pr3+ ions concentration. All these emissions were coming from the host matrices and not from the Pr3+ ion when the powders were excited by 325 nm (3 eV) photons. SiO2 and SiO2:Pr3+ powder phosphors were subjected to prolonged 2 keV electron beam irradiation in an ultra high vacuum (UHV) chamber at a base pressure of 1x10-9 torr. The surface reactions and degradation of cathodoluminescence intensity were monitored using Auger electron spectroscopy (AES) and cathodoluminescence (CL) spectroscopy respectively. CL emission of SiO2 showed a maximum emission peak at 451 nm and a shoulder at 478 nm and SiO2:Pr3+ showed a multiple peak emissions located at 510 nm, 614 nm, 730 nm, 780 nm and 970 nm which are attributed to the transitions in the Pr3+ ions. The SiO2:Pr3+ CL intensity decreased with time as a result of continuous exposure to 2 keV electrons. The Auger peak-to-peak height as a function of energy spectrum showed that there were changes on the surface chemistry of the powders as a result of prolonged irradiation by 2 keV electrons. It is most likely that non-luminescent layers were formed on the surface and they contributed to the CL intensity degradation. A high concentration of volatile gas species, which might have contributed to the CL degradation, was detected with a residual gas analyzer (RGA). Cathodoluminescence was not measured for ZnO:Pr3+,ZnS:Pr3+ and ZrO2:Pr3+ due to charging of the powder phosphors and ZrO2:Pr3+ did not emit light under high energy electron exposure (2 keV).
Open Access
Synthesis and characterization of Ce³⁺ doped silica (SiO₂) ) nanophosphors co-doped with Al³⁺ or Mg²⁺ ions
(University of the Free State (Qwaqwa Campus), 2009-11) Koao, Lehlohonolo Fortune; Dejene, B. F.; Swart, H. C.
In recent studies, amorphous silica (SiO2) has been used as a host matrix for rare-earth ions to prepare luminescent materials that can be used in various light emitting devices. Sol-gel glasses have the potential to hold up to ≥10% dopants without losing their amorphous structure. However, before rare earth (RE) - doped sol-gel glasses can be used as luminescent material, several fluorescence quenching mechanisms must be overcome. There are several quenching mechanisms which are present in all materials that are more serious in sol-gel glasses. The first is cross relaxation which involves energy transfer between RE elements; the others are energy transfer through lattice vibrations and to hydroxyl (OH) groups which are present due to the use of water as the solvent during the preparation process. A few studies have demonstrated that the luminescence intensity of rare-earth doped silica can be improved through incorporation of co-dopants such as Al, TiO2, B and by annealing at high temperatures (e.g. > 500ºC). Following their footsteps and in order to make comparisons, we used aluminum as the codopant in some samples to investigate the effects on luminescence yield for various RE concentrations. We also investigated the effects of magnesium co-dopant and high temperature annealing on the luminescence intensity of rare-earth doped silica. In this work, the highest emission intensity was observed for the sample with a composition of 0.5 mol% Ce3+. Cerium doped silica glasses had broad blue emission corresponding to the D3/2- FJ transition at 445 nm but exhibited apparent concentration quenching after higher concentrations of 0.5 mol% Ce3+. Silica containing Mg2+ or Al3+ ions displayed an increase in luminescence intensity as the Mg2+ or Al3+ to Ce3+ ratio increases for the range investigated but significant luminescence enhancement was observed for Mg2+:Ce ratio greater than 20, while that of Al3+ co-doping had the highest luminescent intensity when the ratio of Al:Ce is 10:1. This enhanced photoluminescence was assigned to an energy transfer from the Mg nanoparticles, to result in enhanced emission from Ce3+. The Al3+ or Mg2+ ions disperses the Ce3+ clusters, enhancing 2F5/2 and 2F7/2 emissions due to increased ion-ion distances and decreased cross-relation.
Open 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.