Physics and Engineering

Permanent URI for this collection

https://hdl.handle.net/10530/407

Browse

Now showing 1 - 20 of 55

The O+ 1 -> O+ g monopole transition in 112Cd
(1985) Rohwer, Erich Gottfried
Internal conversion is a nuclear electromagnetic deexcitation process by which an electron is emitted from an atomic elec¬tron shell. This process competes with the emission of gam¬ma-rays from the nucleus- Recently a mini-orange electron spectrometer, for the recording of internal electron spectra, was designed and built at the University of Zululand. The spectrometer has proven to be a very effective apparatus for the detection of internal conversion electrons. The work presented in this dissertation essentially consists of three main parts. Firstly a theoretical study of electro-magnetic deexcitation processes, with special emphasis on in-ternal conversion, is presented. In this dissertation the main field of interest is internal conversion monopole transi¬tions, for which gamma-radiation is strictly prohibited. The monopole matrix element is defined and Its relevance to nu¬clear structure is emphasized. Secondly, a brief description of the mini-orange spectrometer used in this work is presen¬ted, together with a discussion on the transmission charac¬teristics of the apparatus. The method used to determine the transmission curve for a specific configuration, is described in detail, and some experimental results of transmission cur¬ves are given. Thirdly, the details of the experimental arrangement and conditions for the recording of the *AiiCd internal conversion spectrum are given, as well as the ob¬tained results. The 0+1 0+g monopole transition in u*Cd was detected. The recorded spectrum, was analyzed and the results were used to calculate the nuclear structure parame¬ters p(E0) and X(E0/E2). The obtained results compare well with similar results presented in the literature. The results were also compared to some theoretical predictions of these parameters, and it has become evident that none of these nuclear model predictions are in satisfactory agreement with the experimental values. This stresses the significance of experiments of this kind in gaining more information on the structure of the nucleus.
Phase formation sequence at metal-germanium interfaces in thin film systems.
(1996) Ndwandwe, O. Muzi; Pretorius, R.; Spoelstra, B.
Germanium thin films are used in integrated circuit electronic devices. Interest in germanium is due to the high mobility of its electrons and holes. It can therefore be used among other things as a high speed complementary transistor as well as in other devices. It is also used for making Ohmic contacts in GaAs devices. In strained layer (Ge,Si)/Si heterostructures, the band structure can be modified, which leads to interesting electronic properties. This thesis concerns itself mainly with phase formation in metal-germanium systems. Characterisation of samples after heating in a vacuum furnace was done by Rutherford Backscattering Spectrometry (RBS) and X-Ray Diffraction (XRD). Ti-Ge, Pd-Ge, Zr-Ge, Fe-Ge and Cr-Ge thin film systems were investigated. First phases found in these systems were Ti6Ge5l Pd2Ge, ZrGe, FeGe. and CruGe8 respectively. Subsequent phase formation was also investigated. Results obtained were compared to the predictions of the Effective Heat of Formation (EHF) model. Nucleation and phase skipping was also investigated. In germanides non-congruent phases with more negative effective heats of formation tend to form first in some systems (e.g. CoGe in the Co-Ge system). The two systems Ti-Ge and Ni-Ge were chosen for detailed investigation because they have non-congruent phases (viz. TiGe2 and NiGe) with more negative effective heats of formation. An attempt is made to nucleatethese phases as first phases. A statistical view to phase formation is also introduced. In this model phase formation is described using elementary probability theory. Probabilities for atoms to meet in correct ratios to form phases are derived and used to predict first phases. Fractional heats which are closely related to effective heats of formation are also introduced. The model also makes use of diffusion a! and rotational activation energies to describe stability of phases against breaking into constituent parts. These activation energies are not readily available, which is a disadvantage of this model. Many models of phase prediction have been proposed in the past. Those of particular note are the Walser-Bene model, kinetic model of Gosele and Tu, the Zhang and Ivey model as well as the EHF model of Pretorius. These models as well as the statistical model developed as part of this work are described and compared to each other. The EHF model is found to be the most successful of the lot. It can predict phase formation in silicides, germanides and metal-metal systems. It can also predict phase formation sequence, phase decomposition, and the effect of impurities on phase formation. The success of this model is due to it's direct use of thermodynamic data, such as heats of formation (AH°), and kinetics through the effective concentration of interacting species at the growth interface. It can also explain why different experiments produce different first phases in the same binary system at times.
Solid state interaction between thin metal films and SiO2 substrate.
(2000) Hlatshwayo, Qinisile Y; Pretorius, R.; Ndwandwe, O.M.
Thin films of Cu, Ir, Mo, Pt, Re, Y, Yb and Zr were vacuum deposited onto Si02 substrates of thermally oxidized Si< 100 > wafers and were annealed in a vacuum tube furnace. Samples were then analysed using Rutherford Backseattering Spectrometry (RBS) and X-Ray Diffraction (XRD) to determine whether solid state reactions took place and to identify the compound phases formed. It was found that Y, Yb and Zr reacted with the S1O2, while none of the other metals reacted even after heating for hours at high temperatures of up to 900° C. These results as well as all the other metal-S1O2 interaction studies that could be found in the literature were summarized and extensively analysed. In all the cases where the metal reacted with the S1O2 (Hf, Nb, Ta, Ti, V and Zr) with the exception of the rare-earth metals Y, Tb and Yb, it was found that a thin layer of metal silicide formed sandwiched between the S1O2 substrate and a top layer of metal oxide. From this configuration the following model for the diffusion process could be formulated. Metal atoms diffuse to the SiC^/silicide interface where interaction with and dissociation of SiC>2 takes place. The released oxygen atoms then diffuse through the silicide layer to form a metal oxide on top of the silicide. It could therefore be concluded that oxygen diffuses more readily through the silicide than silicon through the metal oxide. If the opposite was true the sample configuration after reaction would have been SiC^/MO^/MSiy. In the case of the rare-earth metals, solid-state reaction always took place, but with all the elements M, Si and O being present throughout the reaction region. In the case of Yttrium ternary phases were identified, whereas XRD showed that a mixture of silicides and oxides formed in the case of Ytterbium. In this study an extensive table of heats of formation for the silicides and oxides was compiled, from which heats of reaction could be calculated. The theoretical thermodynamic predictions were found to be consistent with experimental observations. This study also showed that the results could be correlated with the mean electronegativity of the metal which offers a convenient empirical method of predicting whether a metal will react with Si02 or not. It is found that metals with an average electronegativity (average of Allred-Rochow, relative compactness and Pauling electronegativities) of less than 1.45 on the Pauling scale react with Si02- An even better correlation with electronegativity could be obtained when using the Miedema electronegativity parameter (
The effect of capping layers, diffusion barriers on TiSi2 phase formation
(2002) Nkosi, M.M.; Ndwandwe, O.M.; Theron, C.C.
Metal silicides continue to be an important part of the design philosophy in semiconductor technology due to their application as contacts or interconnect. Among these silicides, titanium disilicide is currently the most widely used as it possesses the lowest resistivity (\4\in~cm) of all silicides, a low contact resistance and a good thermal stability. Titanium-silicide formation with and without capping layers, through diffusion barrier layers and on stressed Si (100) substrate was studied. The oxidation of titanium makes accurate kinetics measurements very difficult. Capping layers, namely Al203 and MgO, and sandwich technique were used in attempting to limit the unwanted oxidation. It was found that both capping layers reduced the oxygen concentration in the unreacted Ti film by 30% compared to the uncapped samples. It was also found that the reaction was faster in the MgO capped samples and slower in the Al203 capped samples. In the sandwich technique, the oxygen concentration was found to be less by 35% and the reaction was fast at high temperatures compared to the silicide formation in the unsandwiched samples. In attempting to control the metal concentration at the growth interface diffusion barrier layers were used. Cr and Zr were used as diffusion barrier layers. Both barrier layers were found to react with silicon to form CrSi2 and ZrSi2 respectively. It was found that both barrier layers moved deeper into the samples as more silicide was formed on top, which indicates that Si is the dominant diffusing species during Ti-silicide formation. Both barrier layers delayed the formation of TiSi2. Tensile and compressive stresses were induced by depositing different thicknesses of Si3N4 and Si02 on the backside of Si{100) wafers. The reflection of laser light was used to measure the radius of curvature which was related to stress using Stoney's equation. It was found that the silicide phase formation was faster in samples under tensile stress and slower in samples under compressive stress. The cube root of time gave the best fit to the data for all sample groups. Therefore the TiSi2 formation kinetics was neither diffusion nor reaction limited.
Development of fission suppression devices for afrodite
(2005) Hlatshwayo, Thulani Thokozani; Bark, R.A.; Ndwandwe, O.M.
The discovery of the third minimum in various heavy mass nuclei, for example, in the U-Th region, has developed interest in the nuclear physics community to study the behaviour of the nucleus in this minimum. The nuclear shape that is predicted to exist in this minimum is known as a hyperdeformed shape, which corresponds to 3:1 major to minor axis ratio. A hyperdeformed nucleus could be created when a heavy target is bombarded with a light beam, for example, when an a-particle beam bombards 232Th. However in such a reaction, the strongest channel is fission, which occurs almost 99% of the time. Due to the high fission background, which reduces the experimental sensitivity for picking out y decay of hyperdeformed states from the background, many experiments have failed to observe hyperdeformed bands in this minimum. Therefore, this thesis focuses on the development of two fission suppression devices, namely a recoil detector and a solar cell array at iThemba LABS (South Africa).
Measurement of the fusion barrier distribution for the 86Kr + 208Pb reaction
(2005) Ntshangase, Sifiso Senzo; Bark, R.A.
The main object of this work is to investigate a method of determining the total capture cross-section without passing through separate measurements of quasi-fission, fusion-fission and evaporation residue, using the fact that the sum of the flux in the various final channels is equal to the total incident flux. Thus capture is complementary to the flux reflected from the barrier. Here we present an experiment performed at the cyclotron facility at iThemba LABS to determine the fusion barrier distribution for the ^Kr + 208po reaction using the method of measuring quasi-elastic scattering proposed by Dr. N. Rowley.
Determination of the inclusive (polarized proton, alpha) analyzing powers in the investigation of the Ca-40 (polarized proton, proton-alpha) reaction with 100 MeV polarized protons
(2005) Lakaje, Tsepiso Emily; Neveling, R.; Ndwandwe, O.M.
Alpha particle formation in the 40Ca nucleus was investigated by means of the (~p*,pa) reaction with a 100 MeV polarized proton beam. Coincident and singles analyzing power data were acquired for three different quasi-free angle pairs. The knocked out a particles were measured using a silicon detector telescope and the protons were measured using a K600 magnetic spectrometer. Only the singles analyzing power data for the silicon detec¬tor telescope were analyzed in this thesis. This data served as an important consistency check for the coincident results between different datasets, and it was also used in the investigation of the clustering phenomenon. The experimental analyzing power results were compared with the theoretical calcu-lations which were done using the THREEDEE computer code. This computer code is based on the Distorted Wave Impulse Approximation (DWIA). Experimental analyzing power results are not in good agreement with the theoretical analyzing power results.
Production of C, N and O fragments in the interaction of 12C with 12C at an incident energy of 200 MeV.
(2006) Dlamini, Mxolisi Justice; Fortsch, S.V.; Ndwandwe, O.M.
The double differential cross sections of different isotopes of carbon (C), nitrogen (N), and oxygen (0) emitted in the interaction of C with C at an incident energy of 200 MeV were measured in the angular range of 8- 45'. The high-energy part of the spectra was measured with a Si AE-E detector while the corresponding low energy part was measured with a Bragg Curve Detector (BCD). The standard AE-E technique was used for particle identification which allowed good separation of different isotopes of the emitted fragments with Z > 3. The BCD could only resolve charge of different fragments. Energy spectra of the isotopes of C, N, and O indicate that at forward angles (8' -20*), the high-energy part of the spectra is produced by transfer reactions of few neutrons or protons from the projectile to the target nucleus or vice versa. At lower emission energies pre-equilibrium emission is mainly governed by nucleon coalescence and further by evaporation. As the emission angle as well as the mass of the fragments increases the evaporation process dominates the spectra. The fragments heavier than 17O are almost entirely produced as evaporation residues.
Radiation shielding calculations by means of MCNPX
(2006) Lukhele, Mbongeni France; Ndwandwe, O.M.; Buthelezi, E.Z.
Monte Carlo radiation transport calculations were performed to re-evaluate the configuration of the horizontal target bombardment station of the ra-dionuclide production at iThemba LABS. Calculated neutron and photon dose attenuation factors for this shield were compared with those previously obtained by means of the multigroup discrete ordinate code. Isotropic and anisotropic source terms were assumed. Furthermore, two geometries were considered, namely, spherical and cylindrical The neutron and photon total dose rates and dose attenuation factors were calculated at 1.5 m from the center of the target for a shield config¬uration composed of a combination of homogeneous materials such as iron, paraffin wax containing boron carbide for the attenuation of neutrons, and lastly, lead as an outer photon shield layer. The thickness of lead necessary to attenuate photons transmitted from the neutron capture reactions and those from the decay of the residual nuclei was about 4 cm. The amount of boron carbide needed to mix with paraffin wax was determined by adding. by weight percent, small increments of boron carbide into paraffin wax until about 10% of the boron carbide was eventually added into the wax. The thickness of iron and wax needed to attenuate fast neutrons to thermal neu¬trons was determined by fixing the total thickness of the shield 60 cm and the position of iron, with respect to iron/wax interface was determined by mov¬ing iron across the shield configuration. Where available, the results obtained by means of MCXPX were compared with those previously published.
The study of high-spin states in 196HG
(2006) Msezane, Bhekithemba Michael; Lawrie, J.J.; Ndwandwe, O.M.
High-spin states in the oblate deformed even nucleus of mHg have bees studied by y-ray spectroscopy following the t98Pt (a, 6n) 196Hg reaction at an incident energy of 65 MeV. The experiment was performed at iThemba LABS using the AFRODITE array. The y-y coincidence data in two different experiments using different geometries of the y-detectors are obtained. The analysis of results, concentrating on the measurement of Directional Correlations in Oriented (DCO) state ratios and anisotropy polarization lead to extension of the level scheme up to energy E=8.3 MeV and spin I=29h. The dipole bands consist of Ml and E2 cross-over transitions were observed at higher excitation energies and spins.
Micro PIXE analyses of Furnace and Converter Mattes for the determination of trace element distribution and concentrations
(2006) Govender, Shaun; Nell, J.; Przybylowicz, W.; Ndwandwe, O.M.
Analyses of the constituent matte phases by Particle Induced X-ray emission (PIXE) were performed to investigate the distribution and concentration of trace elements. These levels of sensitivity are beyond that of the electron microprobe. The objective of these analyses was to examine low level concentrations (ppm or ppb) in phases such as alloys, oxides and sulphides. The analyses were done at the nuclear microprobe chamber of Material Research Group in iThemba LABS. Samples consisted of slow cooled furnace and converter mattes which were prepared at Mintek. The samples were first analysed using SEM/EDX techniques to characterise the phases present, XRD was also performed to characterise the sample but due to the complexity of the samples it was unsuccessful. The SEM images were used to position the samples during experiments in the nuclear microprobe. Each phase was analysed to determine the distribution and concentration of the major and trace elements. The proton beam can be scanned in the same way as the electron microprobe, generating a map; this map indicates concentration as a function of position, the map includes a distant bar and concentration bar which is in weight percent. Also included are tables which contain the results of all the phases which were analysed. To confirm the presence of low. level trace elements, inductively coupled plasma mass spectrometry (ICP-MS) was performed on the samples at the University of Stellenbosch, this technique was also unsuccessful. Interesting information about PGEs and Au was obtained, as well as information about other elements such as Mo, W. Se, As. Bi. Pb. Sn, Sb and many more. Information such as there distribution and concentration in the phases, and the association they have with each other.
The design, development and fabrication of a Microwave Proton Ion Source at iThemba LABS
(2006) Ndlangamandla, Ceboliyozakha Leonard; Celliers, P.J.; Ndwandwe, O.M.
A low-power microwave ion source for the production of protons in the iThemba LABS Van de Graaff accelerator at Faure has been developed. The main advantages of the source are its simple construction, its low power consumption, the small number of parameters that have to be adjusted and the fact that the discharge starts easily. The construction and the operational conditions of the source are discussed. The beam intensities are compared with those of a duoplasmatron ion source in use at iThemba LABS.
Nanostructured diamond-like carbon by dual pulsed laser ablation-pulsed gas feeding
(2007) Sibiya, Patrick Sipho; Maaza, M.; Ndwandwe, O.M.
Diamond-like carbon films is a metastable form of carbon containing mixture of sp3 and sp2 hybridization. In the previous decades Diamond-like carbon has been studying widely due to its unique properties resembling those of diamond. These properties exhibit the high hardness, high wear resistance, low friction coefficient, chemical inertness, high electrical resistance, and optical transparency in the IR region. These properties make DLC films a good candidate in various applications such as the mechanical, optical, coating magnetic hard dicks, and biocompatibility in the replacement of hip joints, heart valves, stents, as well as zinc sulphide for IR windows. In the present work nano-structure diamond-like carbon was deposited at room temperature by Pulsed Laser Ablation in a methane atmosphere on corning glass and silicon substrate. The structures of Diamond-like carbon film such the surface morphology and the composition has been studied by the scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The structural properties of DLC films have been studying by were investigated by Raman spectroscopy. The vibrational mode of C-H molecules and the composition of carbon, oxygen and hydrogen have been investigated by Fourier transformations Infrared Absorption, Rutherford backscattering, Elastic Recoil Detector. The optical and the surface topography of the films have been studied by Ultraviolet Visible spectrophotometer, Zygo interferometer, and Stylus Profiler. SEM shows that DLC films deposited in a high vacuum peel out of the silicon substrate whereas the films deposited on glass shows the dark yellow color depending on the thickness of the films. Raman results indicate the depended of DLC films on deposition time, the sp3 fraction increase from 21% to 97.1% and the peak position changes with respect to time. XPS result shows excellent films produced by pulsed laser ablation with Cls in the range 81.5%-88.8 % with the surface roughness less 30nm. These smooth film shows promise applications on hard and medical biocompatibility. DLC films deposited on have refractive (n) in range of 1.7 to 2.2 suitable for optical applications.
Nuclear reaction analysis cross-sections measurements for Boron and Carbon.
(2007) Miya, Senzo Simo; Segonyane, S.P.; Franklyn, C.B.; Ndwandwe, O.M.
The aim of the project was to measure nuclear reaction cross-sections for selected light elements such as Boron and Carbon for the database at Radiation Utilisation Group of Necsa. Deuteron-fnduced and "He-induced reactions were performed at the 4MV Van de Graaff accelerator at detection angles of 0= 150cand 135° for incident beam energies between 1.8 and 2.8 MeV. Immediate application of this study will help in determining: 1. The Boron contaminants or impurities in the raw material used for the operation of the Pebble Bed Modular Reactor 2. The isotopic composition of boron carbide, which is used as a neutron shielding at the SAFARI-1 research reactor.
In-situ gamma-ray mapping of environmental radioactivity at Ithemba Labs and associated risk assessment
(2007) Hlatshwayo, Israel Nkululeko; Ndwandwe, O.M.; Newman, R.T.
In February and July 2005, following up on June 2004, the Environmental Radioactivity Laboratory (ERL) of /Themba LABS performed in-situ and ex-situ measurements of environmental radioactivity on the /Themba LABS (/TL) site. The ERL's Multi-Element-Detector-for-Underwater-Sediment-Activity (MEDUSA) and high-purity germanium (HPGe) detector systems were used to make in-situ and ex-situ measurements, respectively. MEDUSA consists of a CsI(Na) crystal (length 15 cm, diameter 7 cm) for y-ray detection. MEDUSA was mounted - 0.5 m above the ground on the front of a 4x4 vehicle to traverse [at ~ 2 m.s"'] the accessible portions of the /TL site. Spatial data (via a GPS receiver mounted above the crystal) were acquired every 1 s, and y-ray spectra (0-3 MeV) every 2 s. Maps of count rate were produced from the MEDUSA data to show the spatial distribution of radioactivity on the site. The lead-shielded HPGe detector is a Canberra p-type model with built-in pre-amplifier, crystal diameter 62.5 mm, and length 59.9 mm. The HPGe was used to measure the radioactivity in soil (and also in some grass) samples collected at particular spots on the /TL site. The sampled spots include six identified high activity spots ("hotspots") and two ''calibration spots". The activity concentration results that were obtained for the naturally occurring radioactive materials (NORM) [from both HPGe and MEDUSA data from off-road /TL site sections] have the ranges 7.8 - 39.0, 5.9 - 21.4? and 29.8 - 63.3 Bq.kg"1 for 238U, 2j2Th, and "^K, respectively. The absorbed and effective doses (from external gamma irradiation) that were calculated for the NORM radionuclides have the ranges 10.4 -27.2 nGy.h"1 and 12.8 - 33.4 /iSv.y"!r respectively. The average NORM activity concentration results (from February and July 2005) for the jTL road are 30.3, 29.6, and 345.0 Bq.kg1 for 238U, 232Th, and 40K, respectively- When averaging over the off-road sections of the iTL site, and excluding data from the hotspots, the average results obtained are 10.6, 8.5, and 42.4 Bq.kg"1. When averaging over the off-road sections plus hotspots, the average results are 23.1, 8.0, and 50.4 Bq.kg"1. The activity concentration results that were obtained for the anthropogenic radionuclides have the ranges 1.2 - 6533.5, 0 - 100.8, 0 - 20.6, 0 - 0.7, and 0,2 -192.5 Bq.kg'1 for 68Ga, 652n, ^Na, I37Cs, and 54Mn, respectively. The absorbed and effective doses (from external gamma irradiation) due to anthropogenic sources were calculated for two plane source depth locations, namely, 1 and 10 cm. The absorbed and effective dose results that were obtained at 1 cm plane source depth have the ranges 0.2 - 152.7 nGy.h"1 and 0 - 187.3 ^Sv.y"1. The absorbed and effective dose results that were obtained at 10 cm plane source depth have the ranges 0-38.1 nGy.h" 1 and 0 - 46.7//Sv.y1. The count rate maps showed that the six identified hotspots consistently appear at the same locations for June 2004, and February and July 2005 maps. The maps also showed that the radionuclide deposition at the hotspots is not a typical result of radionuclide spread via irrigation. The activity concentration results showed that the radioactivity' level due to NORM sources on the /Themba LABS grounds is below the world average of 35; 30, and 400 BqJcg"1 for 238U, ^Th, and ^K, respectively. The effective dose results showed that the maximum effective dose to humans on the iThemba LABS grounds as a result of external exposure to natural and anthropogenic radionuclides is well below the regulatory 1 mSv per year per member of public.
Hydrothermally grown Pb2+ doped ZnOnanorods for hydrogen and acetylene gas sensing
(2008) Ngqondo, Sipho Tumi; Ndwandwe, O.M.
Randomly orientated ZnO nanorods were deposited hydrothermally on Si <100> wafers. The precursor solutions used for ZnO nanorods deposition were zinc chloride + aqueous ammonia and Zinc nitrate + HMT. ZnO nanorods were deposited on Si<100> wafers to establish the optimal PH and concentration which yield the best substrate coverage for each precursor solution. ZnO thin films were deposited on glass substrates using pulsed laser deposition. These thin films were used for both gas sensing and as seed for orientated ZnO nanorods deposition. Rutherford Backscattering was used to measure the thickness and stochiometry of the deposited ZnO films. Orientated ZnO nanorods were then deposited on glass slides with ZnO thin films using the optimal PH and concentration for reagents established when Si<100> wafers were used as substrates. Pb doped ZnO nanorods were prepared by adding Lead (II) Nitrate as the Lead source into Zinc nitrate + HMT precursor solution. These Pb doped ZnO nanorods were then deposited on glass substrates with a ZnO thin film. The crystal structure and preferred plane orientation of all samples was studied using X-ray diffraction. Morphological and elemental analysis of samples was investigated using Scanning Electron Microscopy equipped with EDX. All samples showed good ZnO stochiometry with no foreign atoms except the samples with Pd doped ZnO nanorods. Ag contacts were deposited on the samples using EDWARDS vacuum coater. The gas sensing characteristics were investigated on all samples using 20% hydrogen and acetylene as test gases while 80% nitrogen was used as a carrier gas. ZnO thin films were found be at optimum for hydrogen gas sensing in the range 300-350oC, the minimum temperature for acetylene sensing was 400oC. Orientated ZnO nanorods also operated at their optimum in the 300- 350oC range like thin-films, however they exhibited better sensitivity in this range than thin films. ZnO nanorods started sensing acetylene at 350oC, slightly lower than ZnO thin films. Pb doped ZnO nanorods were able to sense hydrogen with 67% sensitivity at 250oC. These Pb doped ZnO nanorods were also found to be cable of sensing acetylene in the whole temperature range.
Vanadium dioxide thermochromic thin films correlation between microstructure, electrical and optical properties
(2008) Msomi, Velaphi; Nemraoui, O.; Ndwandwe, O.M.
Thermochromic thin films of vanadium dioxide (VO2) were successfully prepared using two physical vapour deposition methods namely Pulsed Laser Deposition (PLD) and Inverted Cylindrical Magnetron Sputtering (ICMS). We used these two techniques in order to comparatively study the effect of deposition temperature on VO2 thin films microstructure, electrical and optical properties. Prior to the deposition, we first optimized the deposition conditions for both techniques. Optimized conditions were then used to systematically study the effect of substrate temperature, ranging from 350oC to 550oC, on the microstructure and the switching properties of VO2 thermochromic thin films. All films prepared by PLD were observed to be crystallized, giving a first peak at 27.87o corresponding to (011) plane indicating the existence of VO2 [5,9,13,18,21]. We also observed (by Debye Shrerer’s formula) that the film grain size increases with the increase in substrate temperature. AFM was employed so as to study the surface roughness. We observed the electrical and optical properties for VO2 behaviour using four point probe and UV-VIS spectrophotometer. The same procedure used for characterizing VO2 thin film synthesized by PLD was also repeated for ICMS results. The films deposited by ICMS were also found to be crystallized, showing strong texturation corresponding to (011) plane indicating VO2 existence [5,12,13,18,21]. We observed that optical and electrical properties exhibited by VO2 thin films are almost the same with substrate temperature increase. ICMS technique appears to be a better technique to as compared to PLD to synthesize or deposit high quality films which would show clear substrate temperature dependence as it appears to be important effect in the study.
Optical determination of the temperature of a laser heated industrial diamond
(2008) Masina, Bathusiwe Nelisiwe; Forbes, A.; Ndwandwe, O.M.; Hearne, G.R.
Diamond has the highest thermal conductivity among known materials. In order to heat diamond to high temperature and measure the temperature, one needs to have a laser heating system with sufficiently good power stability, beam position and suitable wavelength for absorption. In this dissertation we heated, and measured the temperature of a diamond sample by using a CO2 laser. Temperatures are difficult to measure with the accuracy and stability required for many potential applications. Temperature sensors such as thermocouples that depend on attainment of thermal equilibrium with a surface via conductive or convective heat transfer are problematic because: (i) they require intimate contact with the surface which affects the local surface energy balance, especially when minute regions of the sample are to be probed, and (ii) they are limited to temperatures of <1500 °C. Similar restrictions apply to pyrometric, non-contact, temperature determination of hot-spots where the colour of a hot object is referenced against that of a heated tungsten filament. However these limitations do not apply in the case of non-contact infrared thermometry (spectro-radiometry), involving the grey-body (Planck) emission envelope emanating from a hot spot, which may even be well below sub-millimeter dimensions. A model of the surface temperature was successfully developed and qualitatively compared to the experimental results. CO2 and Nd: YAG laser heating was employed for heating two different industrial diamonds at extreme high power levels. The industrial diamonds were characterised using Raman spectroscopy, Scanning electron microscopy and X-ray diffraction. We show that there was some changing (physical and chemical) in the industrial diamond after heating.
Investigation of physical properties of mesoporous NANO-Ti02 for dye-solar
(2008) Mhlongo, Hlengiwe Gugu; Maaza, M.; Ndwandwe, O.M.
The nanocrystalline dye sensitized solar cells, so called «Gratzel cells* are based on a wide band gap «3.0-3.2eV» semiconductor TiO2 in its nano-scaled form. These nano-sized TiO2 are sensitized by a dye monolayer so to increase its spectral response and are immersed in a redox electrolyte for charge transfer. At the present moment, the best overall sunlight to electrical energy conversion efficiency under direct and diffuse sunlight laboratory devices is about 7-11%. It is well acknowledged in the literature that the low efficiency of standard large dimension Gratzel solar cell which varies in the range of 2-5% is caused by the excitonic fast recombination. In order to improve this conversion efficiency and reach values comparable with standard PV technologies, it is necessary to minimize this charge recombination at TiO2-dye interface in addition to an optimized optical path management. The conversion efficiency is related to the Incident Photon-Current Efficiency «IPCE» via the charge carrier free mean path «L» and the absorption coefficient or of the dye-TiO2 system. IPCE = 7—f Where L is related to charge carrier lifetime «£» and its diffusion constant «D» and mobility «u»: Hence nano-TiO2 with substantial shape anisotropy such as TiO2 nanorods/ nanotubes or 1-D porous TiO2 structures would exhibit less interfacial scattering as a result of a larger lifetime and efficient light trapping. This trapping effect would offer a better electron transport in the nano-TiO2 electrodes as a result of faster photoresponse and higher electron collection efficiency. This research project focuses mainly on engineering meso-porous 1-D type TiO2 n a no-structures with an obvious shape anisotropy for possible application in DSCs application which is expected to minimize the fast electron/hole pair recombination process. In this work we report the high-density arrays of mesoporous anatase TiO2 nano-structures derived using the combination of Evaporation Induced Self-Assembly (EISA) and sol-gel methods using Pluronic P123 «EO23 -PO70-EO23» as a surfactant and Titanium isopropoxide «Ti-O'Pr4 » as a starting material . The mesoporous TiO2 films were prepared by dip coating method using ITO coated float glass as a substrate. The phase and crystallinity information was determined from small angle grazing incident-XRD. The average crystallite size of the TiO2 nanopartides was estimated using Scherer's equation. X-ray reflectivity (XRR) and grazing incidence small angle x-ray scattering were used to follow the order and disorder in the formation of the mesoporous nano-structured TiO2 and also to track the evolution of the films structure before and after calcinations. Optical analyses were done using UV-VIS-NIR spectroscope. The Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) were used to observe the surface morphology and surface roughness of the mesoporous TiO2 nanostructures, respectively. Elemental analyses were done using X-Ray Photoelectron Spectroscopy (XPS). Photoluminescence (PL) measurements were done using an Argon laser source at an excitation wavelength of 476 nm in order to study the defects present in the samples.
Synthesis and characterization of graphene: a raman study of the effect of electromagnetic and proton irradiations on graphene
(2009) Mbuyisa, Puleng Nontobeko; Maaza, M.; Ndwandwe, O.M.
Graphene, a single atomic layer of hexagonally arranged sp2–hybridized carbon atoms, with a thickness of only 0.34 nm, exhibits unique properties. The current interest in graphene can be attributed to three main reasons. Firstly, various forms of graphite, nanotubes, buckyballs and others can all be viewed as derivatives of graphene. Secondly, the scalability of graphene devices to nano-dimensions makes it a promising candidate for applications in nano-devices. Thirdly, its electron transport properties are described by the Dirac equation which allows access to quantum electrodynamics in a simple condensed matter experiment. The methods for obtaining individual graphene sheets have progressed, from ripping it with adhesive tape, or gently pushing small graphite crystals along a hard surface to produce high quality graphene by cleaving of graphite, to oxidation of graphite using the modified Hummers method developed by M. Hirata et al. (Carbon 42(2004)). In this research the graphene was synthesized using the modified Hummers method which resulted in a suspension of graphene oxide flakes in distilled water. The graphene oxide (GO) was then chemically reduced to produce graphene or reduced graphene oxide (rGO). The physical properties of the resulting graphene films were characterized and the effects of irradiation by an excimer pulsed laser (UV radiations), visible light (green) and proton irradiation were investigated. During the irradiations the dose of the radiation was varied in order to track the changes in the properties of the materials as a function of the flux. The different spots were then characterized using Raman spectroscopy to measure the created disorder. The Raman spectra of the samples irradiated by light displayed the D and G mode. The Raman spectra characteristics for the UV irradiated sample were similar to that of the proton irradiated sample. In both cases there was a splitting of the G mode. It was then concluded that the splitting of the G band similar to that found in semiconducting Single Walled Nanotubes is an indication that the samples are semiconducting. However the G line shape is highly sensitive to whether the SWNT is metallic or semiconducting and in the UV irradiated sample there was a transition from semiconducting to more metallic as the irradiation was increased.

Browse

Browsing Physics and Engineering by Issue Date

Results Per Page

Sort Options