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Computational modelling and simulation in materials science uses mathematical abstractions of particle-particle forces to postulate, develop and understand materials that are organized as particle systems. Real particle systems occupy macroscopic scales and can be costly to simulate in terms of hardware and software tools and simulation time. Even the most basic simulation can generate large amounts of intermediate data that requires innovative further processing to decipher its underlying physics or to answer fundamental questions about its material properties. These questions are increasingly being asked due the present furious academic and industrial interest in nanosized crystalline lattices. Of particular pertinence are questions of whether or not the properties of the nanostructures are identical to those of their macrostructures. In the light of this the primary focus of this contribution is the development of a tool to simulate face-centered cubic (fcc) particle systems on a “standalone” hardware platform, and to apply it to a specified particle system. The studied particle systems are to range from nanostructures to macrostructures. This thesis is thematically divided into two main parts. In the first part, comprising the first five chapters, we conduct a detailed survey of the current in computation, followed by a definition of the kinds of systems to which the study is applicable, and then we provide a detailed but not overwhelming description of the tool development, with numerous actual codes and examples. This part culminates in a working tool, abbreviated VSV. In the second part, comprising the subsequent chapters, we apply the VSV and associated tools to solve actual physics problems in nanostructures thereby offering new approaches and results to answer the current questions. In developing VSV, we discuss the pairwise-particle potential and its integration into an embedded atom model (EAM) approach that is a cost-effective way to simulate fcc metall

In this study, the surface segregation in a Ni-Cu alloywasinvestigated by modeling the segregation process theoreticallyand also measuring it experimentally. This was performedfor abulkcrystaland thin films. In addition to the segregation measurements, the Ni/Cumultilayer thin films werealso used to study the interdiffusion of Cu and Ni with AES depth profiling.TheMRI model was used toquantitatively evaluate the sputtering-induced surface roughness and depth resolution for AES and SIMS depthprofiling. Depth profiles ofthe Ni/Cu polycrystalline multilayer thin films were performedby AES and SIMSin combination with ion sputtering. The measured depth profiles data were quantitative analyzed with the MRI model. The sputtering-induced surface roughness and depth resolutionwere evaluatefor sputtering with (i) a stationary sample, (ii) a rotating sample and (iii)a stationary sample with two ion beams simultaneously. The results showthat the depth resolution is smaller when profiling with dual-ion beam vs. a single-ion beam. It was also found that profiling with a lower ion energy result in a better (smaller) depth resolution. Rotation of the sample during ion sputtering had the better (smaller) depth resolution. Depth profiling with Cs+ ion sputtering had the best depth resolution compared to Xe+ and O2+ ion sputtering. The MRI model was also used for extracting the interdiffusion coefficients for the AES depth profiles of a Ni/Cumultilayer. The interdiffusion parameter for Cu/Ni multilayer thin films was characterized for the first interface Do =6.2×10-13 m2/s and Q =101.4 kJ/mol, and the last Cu/Ni interface Do=6.3×10-14 m2/s and Q =79.0 kJ/mol. It was clearly showing that the depth-dependent interdiffusion coefficients are depth-dependent. The segregation of Cu and S from a ternary Ni-Cu(S) bulk alloy was measured with AES using linear temperature programmed heating and constant temperature heating. The segregation data were fitted with the modified Darken model and the, Cluster program of the University of the Free State, South African Research Chairs Initiative of the Department of Science and Technology

English: English: Nanotechnology research has expanded notably, with a wide range of applications from catalysis in fuels, to optics. A key factor in manufacturing these particles is understanding diffusion and segregation of dopants and impurities in the nanocrystals, as segregation of these impurities influences which atom is exposed to the surface of the nano-particle, and able to react. Understanding these processes in terms of the shape and size of the particle, as well as the effects of temperature, are all important factors for nano-material manufacture. Molecular Dynamics software is uniquely able to study the dynamics inside particles of up to several thousand atoms. The Sutton-Chen potential, in particular, is able to simulate the reactions of face-centred cubic (FCC) metals and model bulk modulus, elastic constants, lattice parameters, surface energies, phonon dispersion, cohesion energy and vacancy formation energy. It is ideally suited for studying the diffusion and segregation dynamics of the large clusters of atoms that make up nanocrystals. In this study, a Molecular Dynamics model using the Sutton-Chen potential was built. This model implements the Verlet Velocity scheme to simulate the kinetics of the atoms, and uses the Berendsen thermostat to keep the system at a constant temperature. The model was tested on six FCC metals, namely Al, Ni, Cu, Pd, Ag and Pt, and, making use of periodic boundaries in order to simulate bulk crystals, calculated the cohesion energy to confirm the effectiveness of the model. The model further confirmed surface orientation dependence for low index surfaces. The relationship for vacancy formation energy Ev(111) > Ev(100) > Ev(110) of applied to all the FCC metals studied. The effects of temperature on other diffusion-related energies in the crystals were also studied. It was further found that the diffusion activation energy of FCC metals has the same relationship of Q(110) < Q(100) < Q(111) Equipped with this informati, Afrikaans: Nanotegnologie navorsing het geweldig uitgebrei, met 'n wye verskeidenheid van toepassings van katalise in brandstof, tot optika. 'n Belangrike faktor in die vervaardiging van hierdie deeltjies is die begrip van diffusie en segregasie van doteermiddels en onsuiwerhede in nano-kristalle, aangesien segregasie van hierdie onsuiwerhede beïnvloed watter atoom word blootgestel op die oppervlak van die nano-deeltjie, en in staat is om te reageer. Begrip van hierdie prosesse in terme van die vorm en grootte van die deeltjies, sowel as die effekte van temperatuur is almal belangrike faktore vir die vervaardiging van nanomateriale. Molekulêre Dinamika sagteware is uniek in staat daarin om die dinamika binne deeltjies van tot 'n paar duisend atome te bestudeer. Die Sutton-Chen potensiaal, in besonder, is in staat om die reaksies, volumemodulus, elastisiteitskonstantes, roosterkonstantes, oppervlakenergie, fononverstrooiing, kohesie-energie en leemte-vormingsenergie van vlakgesentreerde kubiese metale te modelleer. Dit is ideaal vir die bestudering van diffusie en segregasie dinamika van groot groepe atome waaruit die nano-kristalle bestaan. In hierdie studie is 'n Molekulêre Dinamika model, wat van die Sutton-Chen potensiaal gebruik maak, ontwikkel. Hierdie model maak gebruik van die Verlet Snelheid skema om die kinetika van die atome te simuleer, en maak gebruik van die Berendsen termostaat om die sisteem teen 'n konstante temperatuur te hou. Die model is getoets op ses FCC metale, naamlik Al, Ni, Cu, Pd, Ag en Pt, en deur van periodiese-randvoorwaardes gebruik te maak om grootmaatkristalle te simuleer, die kohesie-energie is bereken om die doeltreffendheid van die model bevestig. Die model bevestig verder die oppervlak afhanklikheid vir lae-indeks oppervlaktes. Die verhouding vir leemte-vormingsenergie van Ev(111) >Ev(100)>Ev(110) is van toepassing op al die FCC metale wat bestudeer was. Die effek van temperatuur op ander diffusie-verwante energieë in die kristall, Nano Cluster of the University of the Free State, National Research Foundation

Platinum (Pt) is an important catalyst for applications such as catalytic converters. In this thesis the formation of platinum nanoparticles was investigated by means of simulations. For the first part of the thesis a molecular dynamics simulation using the Sutton-Chen potential was implemented. This program was used for the simulations. Low energy structures were found. It was found that the number of nearest neighbours are maximised in the low energy structures. The energy barriers that have to be overcome as atoms move around the structures were also calculated. A model is proposed for the prediction of energy barriers. The model is useful for understanding the factors that influence the energy barriers and thus the mobility of atoms. The model will also be useful for Monte Carlo simulations. Simulations were done modelling physical vapour deposition onto the Pt(111) surface and a graphite surface represented by the Steele potential. It was found that higher temperatures and lower evaporation rates lead to lower energy structures. The smaller interaction between the graphite surface and the Pt leads to structures that have more layers. The parameters of the Steele potential that determine nearest neighbour distance and interaction strength between Pt and the substrate were adjusted to simulate other materials. It was found that a mismatch between the nearest neighbour distance of the substrate and Pt causes an increase in the mobility of the Pt atoms on the surface. The results of the simulations will enable the choice of suitable substrate and experimental parameters for the growth of Pt nanoparticles of desired shapes., National Research Foundation (NRF), University of the Free State, Cluster Programme, Inkaba yeAfrica programme

A systematic study was conducted to investigate the diffusion and segregation of S in bcc Fe using (i) DFT modelling and (ii) the experimental techniques Auger Electron Spectroscopy (AES) and XRay diffraction (XRD). The aim of this study was to obtain the activation energies for the segregation of sulfur (S) in bcc iron (Fe), both computationally and experimentally in order to explain the diffusion mechanism of S in bcc Fe as well as the influence the surface orientation has on surface segregation. The Quantum ESPRESSO code which performs plane wave pseudopotential Density Functional Theory (DFT) calculations was used to conduct a theoretical study on the segregation of S in bcc Fe. To determine the equilibrium lattice sites of S in bcc Fe, the tetrahedral-interstitial, octahedralinterstitial and substitutional lattice sites were considered. Their respective binding energies were calculated as -1.464 eV, -1.660 eV and -3.605 eV, indicating that the most stable lattice site for S in bcc Fe is the substitutional lattice site. The following mechanisms were considered for the diffusion of S in bcc Fe: tetrahedral-interstitial, octahedral-interstitial, nearest neighbour (nn) substitutional and next nearest neighbour (nnn) substitutional with migration energies, Em, of respectively 4.438 kJ/mol (0.046 eV), 22.48 kJ/mol (0.233 eV), 9.938±6.754 kJ/mol (0.103±0.007 eV) and 96.49±0.579 kJ/mol (1.000±0.006 eV). According to the binding and migration energy calculations, S will diffuse via a substitutional mechanism with a migration energy of 9.938±6.754 kJ/mol (0.103±0.007 eV). The three low-index planes of bcc Fe were investigated to determine the stability, the vacancy formation energy and the activation energy for each surface. Structural relaxation calculations showed that the surfaces in order of decreasing stability are: Fe(110)>Fe(100)>Fe(111) which is in agreement with surface energy calculations obtained from literature. The formation of a vacancy in bcc Fe was modell

In this study the surface segregation of In from a dilute Cu(In) alloy to the surface was investigated. The Cu(In) alloy was prepared by diffusion doping. A thin layer of In was deposited onto the backside of a polycrystalline Cu crystal. The In layer was covered with a thin Cu layer to prevent the In from melting and evaporating from the Cu crystal. The Cu crystal was annealed in an Ar gas atmosphere. During annealing the temperature was increased stepwise from 150 to 900 to prevent the In layer from melting. Finally the Cu crystal was annealed at 900 for 456 hours. As a result, the polycrystalline Cu crystal was successfully doped with an In concentration of 0.059 at%. Using Auger electron spectroscopy (AES) coupled with an Ar+ ion gun (for sur- face sputter cleaning) and a programmed crystal heater inside the ultra high vacuum (UHV) chamber, the segregation profiles where recorded by monitoring the surface condition of In and S on the Cu crystal during linear heating and constant temperatures heating. The constant temperature segregation measurements were performed in the tem- perature range 460 to 580 . The segregation profiles showed that both In and S segregates from the Cu crystal. In segregates to the surface at a much higher rate than S. In reached a maximum surface coverage of 25 % for the temperatures 460 ,490 and 520 and 16 % for temperatures 550 and 580. Both the In and S segregation profiles were fitted with semi-infinite solution of Fick’s equation and the segregation parameters (Q and D0) were obtained for In as Q = 191.9 kJ.mol−1, D0 = 1.1 × 10−5 m2.s−1 and for S as Q = 201.1 kJ.mol−1, D0 = 4.4 × 10−2 m2.s−1. The linear temperature measurements were carried out from the temperature range 100 to 800 at heating rates of 0.025; 0.05; 0.1 and 0.2.s−1. All seg- regation profiles showed In segregation to the surface that reached a surface maxi- mum coverage of 23−25 %. The segregation of In was accompanied by a slow seg- regating S that replaced the In co

English: Red glowing phosphors were prepared by adding Pr3+ ions as activators to several oxide host matrixes; CaTiO3, LaTaO4, YTaO4, and GdTaO4. The perovskite CaTiO3:Pr3+ compound is a phosphor that glows with a single red emission around 613 nm at room temperature upon irradiation with UV light of 230 – 360 nm wavelengths or an electron beam. The source of the single red emission is the intervalence charge transfer between Pr3+ and Ti4+ ions, which opens up a channel to completely depopulate the 3P0 state, by populating the 1D2 state. This leads to a dominant emission coming from the 1D2 → 3H4 transition. The dynamics of Pr3+ in YTaO4, LaTaO4, and GdTaO4 have not been explored excessively, and the resulting emission of these compounds doped with Pr3+ comes from both 3P0 and 1D2 states of Pr3+. The compounds were prepared by solid state reaction at 1200 oC and CaTiO3 was prepared by directly firing TiO2 (Anatase phase) and CaCO3 for 4 hours. The compound was doped with several mol% concentrations of Pr3+ from PrCl3 compound to optimize the output emission intensity. The rare-earth tantalate phosphors were prepared by directly firing Ta2O5 with Y2O3, La2O3, or Gd2O3 for 4h to obtain LaTaO4, YTaO4, and GdTaO4 respectively. The tantalates were doped with 0.5 mol% concentration of Pr3+ from PrCl3 and the synthesis was carried through in the presence of 30 wt% Li2SO4 flux agent. The role of the flux agent in this instant was to increase the reaction rate by acting as an intermediate that converts the reagents to reactive species, lower the reaction temperature required for the final compound to form and to facilitate crystallinity and to control particle sizes. The phase of the phosphor compounds was identified by using X-ray diffraction (XRD, Bruker AXS D8 Advance). The XRD patterns of CaTiO3 with different Pr3+ concentrations match that of the standard orthorhombic CaTiO3 (JCPDS card no. 22-0153). The XRD patterns of LaTaO4, YTaO4, and GdTaO4 with 0.5 mol % of Pr3+ s, National Research Foundation (NRF)

Under Ultra violet (UV), cathode-ray and X-ray excitation, terbium activated rare earth oxysulphide (Gd2O2S:Tb3+) phosphors shows bright green luminescence. Due to its superior luminescent performance, Gd2O2S:Tb3+ phosphor is used in the manufacturing of TV screens. The degradation of commercially available Gd2O2S:Tb3+ phosphor powder and pulsed laser deposited (PLD) thin films were studied with Auger Electron Spectroscopy (AES) and Cathodoluminescence (CL). The surface reactions were monitored with AES while the light output was measured with a PC2000-UV spectrometer. The CL of the Gd2O2S:Tb3+ was excited with a 2 keV energy electron beam with a beam current density of 26 mA/cm2. The CL and AES were measured simultaneously while the sample was bombarded with the electrons in an oxygen atmosphere. A comparison between the low energy peaks of the AES spectra before and after degradation showed significant differences in the shape of the peaks. A linear least squares (LLS) method was applied to resolve the peaks. Elemental standards from Goodfellow were used in conjunction with the measured data to subtract the S and Gd peaks. A direct correlation between the surface reactions and the CL output was found for both the thin films and the powder. The adventitious C was removed from the surface as volatile gas species, which is consistent with the electron stimulated surface chemical reactions (ESSCR) model. The CL decreased while the S was removed from the surface during electron bombardment. A new non-luminescent surface layer that formed during electron bombardment was responsible for the degradation in light intensity. X-ray photoelectron (XPS) indicated that Gd2O3 and Gd2S3 thin films are formed on the surfaces of the Gd2O2S:Tb3+ powder and thin films during prolonged electron bombardment. Luminescent Gd2O2S:Tb3+ thin film phosphors were successfully grown by the PLD technique. The effects of oxygen pressure and substrate temperature on the morphology and the PL emis, South African National Research Foundation

English: From Icarus’ mythical flight to escape Crete to manned space flight to the moon, mankind’s dream to fly has impacted this world immensely. Technological advancements made in metallurgy and alloy development has played a huge role in realizing this dream. Developing materials and superalloys with higher melting temperatures and greater strength has allowed for the design of the modern turbine jet engines. Economical and (today more than ever) environmental concerns continue to provide ample motivation for operating the engines at ever increasing temperatures, thereby improving the thermodynamic efficiency and reducing pollutant emissions. One of the most aggressive man made environments is that of the high pressure turbine section of a modern gas turbine engine. During operation, after combustion, highly oxidizing gas enters the turbine. This happens at temperatures exceeding 200 °C above the melting point of the superalloy turbine blade. Newer generations of civil aircraft will have turbine entry temperatures (TET) that will exceed 1800 K at take-off. Increased power and improved fuel consumption remains a continuing demand in modern aero-gas turbine engines as this result in an increase in TET. One strategy to achieve this goal is by coating the turbine blades with a thin film composed of alloy material. These films can be engineered to have specific heatresistant, oxidation-resistant properties. Two coating techniques that show promise in achieving these goals are pulsed laser ablation (PLD) and electron beam physical vapour deposition (EB-PVD). These techniques are investigated in this study in particular of platinum-aluminium alloys. The appearances of droplets on the thin film surface that arise due to the pulsed laser ablation technique itself are investigated. A suitable technique to minimize the appearance of these droplets by using ambient gas and ambient gas pressure is discussed. The stoichiometric transfer of material from the target to a substr, Afrikaans: Sedert Icarus se mitiese vlug om van Kretense af te ontsnap tot die meer onlangse bemande ruimte ruise na die maan het die mensdom se droom om te kan vlieg die wêreld geweldig beïnvloed en verander. Tegnologiese ontwikkeling in mettalurgie en allooi ontwikkeling het ‘n groot rol gespeel om hierdie droom te verwesenlik. Die ontwikkeling van materiale en super allooie met hoër smeltpunte en beter drywingsvermoëns het ‘n groot rol gespeel in die ontwerp van die moderne turbine vliegtuig enjins. Ekonomiese en omgewings faktore is vandag die grootste motiveerder om hierdie enjins teen selfs hoër temperature te laat werk. Hierdeur word die termodinamiese effektiwiteit verhoog en die emissie van afvalstowwe verlaag. Een van die aggresiefste mensgemaakte omgewings is in die höe druk turbine seksie van ‘n moderne gasturbine enjin. Hier word hoogs oksiderende gasse oor die turbine laat vloei teen temperature wat 200 °C hoër is as die smeltpunt van die super allooi turbine lemme. Van die nuwer generasie van siviele vliegtuie sal turbine ingangstemperature (TIT) hê wat hoër is as 1800 K wanneer die vliegtuig opstyg. Daarom is die verhoging in drywing en verbetering in brandstof verbruiking van vliegtuie ‘n vereiste in moderne gas turbine enjins. Een van die strategiëe wat gebruik word om hierdie doelwit te bereik is om die turbine lemme te bedek met ‘n dunfilm wat bestaan uit ‘n allooi material. Hierdie dun films kan so ontwerp word dat dit spesifieke hittebestande en oksidasie-bestande eienskappe het. Twee van die beddekkingstegnieke wat belowend lyk om hierdie doelwitte te bereik is gepulseerde laser deponering (PLD) en elektron bundel fisiese damp deponering (EB-PVD). In hierdie studie word hierdie tegnieke ondersoek in spesifiek op platinum-alluminium allooie. Films wat met gepulseerde laser deponering vervaardig word het klein druppel-vormige deeltjies op die oppervlakte. Hierdie druppels sowel as ‘n gepaste tegniek om die voorkoms van druppels op die dun film o

A literature study showed that the rate of segregation to a Cu(110) surface is higher than to a Cu(111) surface. The difference is mainly due to a change in the vacancy formation energy which determines the diffusion coefficient. The diffusion coefficient is a very important factor during kinetic segregation and determines the flux of atoms to the surface. The experimental verification of the calculations is very difficult due to the high number of parameters involved during measurements. In this study, the segregation parameters for Sb in a Cu bi-crystal, with (111) and (110) surface orientations, were determined. A unique experimental setup and measuring procedure was used to determine the concentration of the segregant as a function of temperature. This setup ensures exactly the same experimental conditions for both orientations allowing the researcher to directly compare the segregation parameters. The Auger Electron Spectroscopy (AES) spectrometer, used to measure the Sb enrichment on the Cu bi-crystal surfaces, was specially modified for these measurements. The deflection plates in the primary e- gun were physically aligned, horizontally and vertically, relative to the laboratory frame of reference. A computer program was developed to control the deflection of the e- beam during the measurements. Because it was decided on diffusional doping of the crystal an annealing system was designed and built. The system is consistently successful in annealing specimens at high temperatures for long periods of time in non corrosive atmospheres. Because of concerns that the grain boundary can influence the segregation, a secondary study was done on the migration of grain boundaries in polycrystalline Cu specimens. These studies indicate the inhibition of grain boundary mobility with small additions of Sb. The concentration build up on both surface orientations was monitored while the crystal was heated linearly with time, at different rates. The experimental data were fitt, National Research Foundation (NRF)

Thin film diffusion studies often involve a surface sensitive analysis technique combined with ion erosion to produce a depth profile of a sample. Such studies compare the depth profile of a reference sample to the depth profiles of samples that were annealed at different temperatures and times. The extent to which atoms of one layer diffuse into an adjacent layer, for a particular temperature and time, yields information on the diffusion process involved and allows quantification of the diffusion coefficient. The drawback to using an erosion type system is the effect of the incident ions on the surface being probed. The Mixing-Roughness-Information model attempts to compensate for this effect and is often employed as a means of quantification of measured depth profiles by means of profile reconstruction. Used in conjunction with Auger electron spectroscopy, the Mixing-Roughness-Information (MRI) model is a useful tool to reconstruct the ion erosion depth profiles as well as extracting inter-diffusion parameters from these depth profiles. The first part of the study focuses on the extraction of the diffusion coefficient of classically annealed samples of Ni in Cu from Ni/Cu depth profiles obtained from ion erosion Auger electron spectroscopy. The resultant depth profiles were reconstructed with the MRI model. The diffusion coefficient for Ni diffusing in Cu was obtained from the MRI fit and it compared well to values available in literature. From an Arrhenius graph a value of 9 2 -1 D0 6.49 10-9 m2 .s-1 for the pre-exponential factor and Q =130.5 kJ.mol-1 for the activation energy was calculated. The second part of the study involves linear ramping as an annealing technique. In previous studies, linear temperature ramping was used to determine diffusion coefficients from bulk-to-surface segregation experiments of a low concentration solute. Thin film diffusion studies usually employ a classical heating regime, where a sample’s annealing time is taken as the time bet, National Research Foundation (NRF)

Luminescent lead sulphide (PbS) nanoparticles embedded in an amorphous silica (SiO2) matrix were synthesized at room temperature by a sol-gel process. The prepared nanocomposite materials were crushed into powders and annealed in air at 200oC. The chemical composition of the powders was analyzed with an energy dispersive x-ray spectrometer. Particle sizes, crystalline structure and morphology of the PbS nanoparticles were determined with transmission electron microscopy (TEM) and x-ray diffraction (XRD). The crystal particle sizes estimated from the XRD peaks and the TEM images were in the range of 10 to 50 nm in diameter. The SiO2:PbS powders were then irradiated with 325 nm (He-Cd) and 458 nm (Ar+) lasers for photoluminescence (PL) measurement. PL spectra were obtained for pure SiO2 as well as the encapsulated PbS nanoparticles at room temperature. Two strong broad bands, blue (450 nm) and yellow-orange (560 nm) from bulk SiO2 and PbS nanoparticles, respectively, were observed. The PL data show a blue shift from the normal emission wavelength at 3200 nm in bulk PbS to 560 700 nm in nanoparticulate PbS powders. The blue-shift of the emission wavelengths is attributed to quantum confinement of charge carriers in the restricted volume of nanoparticles. Energy transfer from ZnO nanoparticles to PbS nanoparticles was also observed. The possible mechanism for the energy transfer is reported. The powders were also subjected to prolonged 2 keV electron beam irradiation in a vacuum chamber at and different O2 pressures (5 × 10-8 2 × 10-7 Torr O2). The cathodoluminescence (CL) was measured with Ocean Optics S2000 spectrometer, and showed the emission peak to be at a wavelength of 680 nm. Changes in the CL brightness and the corresponding change in the surface chemical composition were investigated with Ocean Optics S2000, Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS). The oxygen Auger peak-to-peak height decreased simultaneously with the CL in

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.

MSSA (Mittal Steel South Africa), which produces electrogalvanised steel for the local automobile industry, experiences a problem with white spot formation when their steel is phosphated. The addition of nickel inhibits white spot formation but produces an unacceptable discolouration of the surface layer. Furthermore the locally produced steel exhibits blister formation when heated to 300ºC. The substrates, electrogalvanised coatings, phosphated samples and annealed samples are studied with Glow Discharge Optical Emission Spectroscopy, Scanning Electron Microscopy, Energy Dispersive Spectroscopy, as well as X-ray diffraction. Combining the results allows for an interpretation of the morphology, topography, composition, crystalline structure, quantitative depth profile as well as the spatial distribution of the elements The white spot formation on the electrogalvanised surfaces is closely related to the presence of contaminants on the electrogalvanised surface and at the interface between the substrate and the electrogalvanised coatings. The accelerated phosphate reaction results in complete dissolution of the electrogalvanised surface, thereby exposing the iron substrate. The white spot consists of an anomalous protruding perimeter with elongated crystals that grow towards the centre of the spot, present inside the spot. Partial dissolution is required in order for the phosphate process to occur. Complex phosphates deposit on the surface comprised of various cations, such as zinc, manganese and nickel. The zinc dissolution is the preferred reaction and therefore there is a slight enrichment of nickel in the sublayers of the phosphate. The manganese deposited on the surface must not be confused with the manganese present in the substrate. The addition of other cations to the electrogalvanised layer results in a change in the structure of the phosphated layer. The presence of cobalt and copper in the electrolyte results in an increase in the deposition of manganese ph

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

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 concentra, National Research Foundation, National Metrology Laboratory

English: A phase miscibility gap has as jet, not been observed experimentally for ternary alloys. To prepare a ternary alloy in such a way that a phase miscibility gap can be observed, the segregation parameters of the two segregating species should be known. These parameters are the activation energies, the diffusion constants, the segregation energies and the interactions. In this investigation a theoretical model was developed which can calculate the segregation energies of the segregating species in a binary system. The model makes use of the sublimation energies for the calculations and takes the orientation of the surface into account. The model clearly shows that the true driving force for segregation is not the difference in surface tension (between the surface tension of the pure element and the surface tension of the element in the alloy) but the difference in surface energies. The segregation energies as calculated by the model can be used as initial values for simulations. The validity of the calculations was confirmed experimentally for the three low index orientations of copper. The segregation parameters for a ternary system can be obtained by changing the temperature of the system linearly with time. A ternary alloy was prepared by evaporating silver on to an existing binary copper antimony alloy, and annealing the system at 920 °C for 30 days. Segregation profiles were measured by the surface sensitive technique Auger electron spectroscopy. From the simulations of the profiles the following segregation parameters were obtained successfully: For antimony in copper the pre exponential diffusion coefficient was (1.4 ±1.0)x10 -5 m2/s, the activation energy: was 197±7 kJ/mol, the segregation energy was 80±3 kJ/mol and the interaction between antimony and copper was 12±2 kJ/mol. For silver in copper the pre exponential diffusion coefficient was 8 (1.3±0.4) x10 m2/s, the activation energy was 215±22 kJ/mol, the segregation energy was 30±3 kJ/mol, the inter, Afrikaans: Die verskynsel van ‘n fasemengbaarheidsgaping is nog nie eksperimenteel in ternêre legerings waargeneem nie. Om ‘n ternêre legering voor te berei sodanig dat ‘n fasemengbaarheidsgaping eksperimenteel waargeneem kan word, moet die segregasieparameters van die twee segregerende spesies bekend wees. Hierdie parameters is die aktiveringsenergieë, diffusiekonstantes, segregasie-energieë asook die interaksies. In hierdie ondersoek is ‘n teoretiese model ontwikkel wat die segregasie-energie van die segregerende spesie in ‘n binêre sisteem kan bereken. Die model maak van die sublimasie-energie gebruik vir die berekening en neem die orientasie van die oppervlak in ag. Die model toon duidelik dat die werklike dryfkrag vir segregasie nie ‘n verskil in oppervlakspannings (tussen die oppervlakspanning van die suiwer element en die oppervlakspanning van die element in die legering) is nie, maar die verskil in oppervlakenergieë. Die segregasie-energieë soos bereken deur die model kan as aanvangswaardes vir simulasies gebruik word. Die geldigheid van die berekeninge is vir die drie lae indeks oriëntasies van koper eksperimenteel bevestig. Die segregasieparameters van ‘n ternêre sisteem kan eksperimenteel verkry word deur die temperatuur van die sisteem lineêr met tyd te verander. ‘n Ternêre legering is voorberei deur silwer op ‘n bestaande binêre koper-antimoon legering te damp en die sisteem by ‘n temperatuur van 920 °C vir 30 dae uit te gloei. Segregasieprofiele is met behulp van die oppervlaksensitiewe tegniek Augerelektronspektroskopie gemeet.Uit simulasies van hierdie profiele is die volgende segregasieparameters suksesvol verkry: Vir antimoon in koper is die pre-eksponensiële diffusiekoëffisiënt (1.4±1.0) x10 -5 m2/s; die aktiveringsenergie 197±7 kJ/mol; die segregasie-energie 80±3 kJ/mol; en die interaksie tussen antimoon en koper 12±2 kJ/mol. Vir silwer in koper is die pre-eksponensiële diffusiekoëffisiënt 8 (1.3±0.4) x10 m2/s; die aktiveringsenergie 215±22 kJ/mo

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 spectroscop

Nanotechnology has, without a doubt, ushered in a new era of technological convergence and holds the promise of making a profound impact on the way research in physics, chemistry, materials science, biotechnology etc. are conducted. The novel properties of materials at the nanoscale (or nanostructures) make them useful in a variety of applications, from catalysis to the medical field and electronics industry. However, to exploit these properties at the nanoscale, precise control over the morphology and size of nanostructures is required. One strategy that may be explored to tailor nanostructure morphology and size is vapour deposition. A lot of further insight can be gained from computer simulations of the processes governing the growth of nanostructures during vapour deposition. A method that shows promise in simulating thin film growth through vapour deposition is kinetic Monte Carlo (KMC). Therefore, in this study, a KMC model was developed to describe growth through vapour deposition. A gold on graphite system was simulated to test the model. In this KMC model, substantial effort was devoted to developing the model in different stages, each stage being more robust than the previous one. The assumptions made at each stage and possible artefacts (unphysical consequences) arising from them are discussed in order to distinguish real physical effects from artificial ones. In the model, data structures, search algorithms and a random number generator were developed and employed in an object-orientated code to simulate the growth. Several simulations were performed at different growth conditions for each of the stages. The results are interpreted based on the kinetic constraints imposed during the growth.

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 multilay

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., National Research Foundation (NRF)