Doctoral Degrees (Physics)

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Browsing Doctoral Degrees (Physics) by Subject "Antimony"

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    ItemOpen Access
    Experimental studies and modelling of surface loss during segregation
    (University of the Free State, 2015-01) Cronje, Shaun; Roos, W. D.; Kroon, R. E.
    Afrikaans: Metallurgiese produkte speel ‘n onontbeerlike rol in die alledaagse lewe. Die soeke na metale met beter eienskappe soos sterkte, slytasie- en korrosieweerstand asook metodes om vervaardigingskostes en – tye te verminder duur steeds voort. Die invloed van onsuiwerhede in metale is van besonderse belang. Tydens die vervaardigingsproses van metallurgiese produkte is dit algemeen om een of ander hittebehandelingsproses te volg. By hoë temperature is die atome van onsuiwerhede, sonder uitsondering, mobiel en kan dus na korrelgrense en ander oppervlakke diffundeer, wat weer ‘n groot invloed op die eienskappe van die materiale het. Hierdie herverdeling van atome tussen die oppervlak en die bulk van die materiaal staan bekend as segregasie. Die segregasie van Sb uit ‘n Cu(100)-kristal, gedoteer met 0.05 at% Sb, is eksperimenteel gemeet. Tydens ‘n liniêre temperatuurveeg, is die oppervlak-konsentrasie van Sb met behulp van Augerelektronspektroskopie gemeet. Die segregasieprofiel is gepas met behulp van die Gemodifiseerde Darkenmodel en die volgende segregasieparameters is onttrek: 𝐷0 = 1.5x10-5 m2.s-1, 𝐸 = 177.0 kJ.mol-1, Δ𝐺 = -89 kJ.mol-1 en Ω = -3 kJ.mol-1. Hierdie data is ‘n waardevolle toevoeging tot vorige gemete segregasieparameters van Sb na Cu-oppervlakke met verskillende oriëntasies. In ‘n aantal binêre-legerings is die oppervlakverdamping tydens segregasie-eksperimente gerapporteer. Die invloed van verdamping tydens segregasiestudies het egter baie min aandag gekry. In besonder het onlangse segregasiemodelle die invloed van segregant-verdamping geïgnoreer. In hierdie studie is ‘n gemodifiseerde weergawe van die Hertz-Knudsenvergelyking gebruik om die Gemodifiseerde Darkenmodel te verbeter. Hierdie rekenaarprogrammetuur stel navorsers in staat om beide die kinetika- en ewewigsegregasie, met inagneming van die oppervlakverdamping van die segregant, te voorspel. Die invloed van die verdampingsparameter wat in die Hertz-Knudsenvergelyking ingevoer is, word bespreek en toon die sensitiwiteit van die segregasieprofiel vir selfs baie klein verdampingstempos. Dit word duidelik getoon dat indien verdamping geïgnoreer word onakkurate segregasieparameters gegenereer kan word. Riglyne word gegee hoe om segregasieparameters, wat met behulp van die Gemodifiseerde Darkenmodel, sonder verdamping onttrek is, te korrigeer. Die interpretasie van die segregasieparameters, in terme van verdamping, word ook bespreek. Veranderinge is aangebring aan ‘n Augerelektronspektroskopie-instrument om verdamping met behulp van ‘n Inficon XTC/3s deponeringsbeheerder te meet. Hierdie veranderinge en voorgestelde eksperimentele prosedures, maak voorsiening vir die verstuiwing en analise van die oppervlak wat vir die verdampingsstudies gebruik word. Deur van hierdie sisteem gebruik te maak is die verdampingstempo van suiwer Sb gemeet. Alhoewel daar kwalitatiewe ooreenkomste rakende die verdampingsgedrag verkry is, is gevind dat die oorspronklike Hertz-Knudsenvergelyking die verdampingstempo ver oorskat. Geen verdamping van Sb kon vanaf die gedoteerde Cu(100)-oppervlak gemeet word nie, aangesien die verdampingsvloed laer as die deteksielimiet van die instrument was. Dit word toegeskryf aan die baie lae verdampingstempo van Sb, soos in hierdie studie vir suiwer Sb waargeneem is, indien vergelyk word met voorspellings deur die Hertz-Knudsenvergelyking. Nogtans toon vergelykings tussen die eksperimentele segregasieprofiel en segregasieprofiele wat met behulp van die opgedateerde rekenaarprogram gegenereer is, dat Sb verdamping wel vanaf die Cu(100)-oppervlak tydens segregasie plaasvind. Hierdie studie toon duidelik hoe belangrik dit is om verdamping tydens segregasiestudies in ag te neem. Die studie lê ‘n belangrike fondament vir toekomstige verdampingstudies vanaf die oppervlakke van metale tydens segregasie, deur die nodige apparatuur in ‘n Augersisteem te monteer, ‘n eksperimentele protokol daar te stel en die opgradering van ‘n rekenaarprogram van die bestaande Gemodifiseerde Darkenmodel om segregasieprofiele te simuleer.
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    ItemOpen Access
    Growth of antimony on copper : a scanning tunneling microscopy study
    (University of the Free State, 2012-01) Ndlovu, Gebhu Freedom; Hillie, K. T.; Roos, W. D.
    English: The thesis deals with adsorption, self–assembly and surface reactions of Sb atoms on solid Cu(111) substrates. It is of genuine interest in materials science and technology to develop strategies and methods for reproducible growth of extended atomic and molecular assemblies with specific and desired chemical, physical and functional properties. When the mechanisms controlling the self-organized phenomena are fully disclosed, the self-organized growth processes can be steered to create a wide range of surface nanostructures from metallic, semiconducting and molecular materials. The experimental technique used to study ordered phases and phase transitions of Sb on Cu(111) substrates was the Scanning Tunneling Microscopy (STM) – an outstanding method to gain real space information of the atomic scale realm of adsorbates on crystalline surfaces. It is a general trend to conduct studies on well known structures before one begins working on complicated systems. Therefore, in this study, Si(111) Cu(111) and HOPG surfaces were studied in atomic detail to confirm the calibration and the resolution capabilities of the instrument. The acquired data were comparable to the reported theoretical and experimental data in literature. The investigated Cu(111) – Sb system is characterized by a complex interplay between adsorbate interactions and adsorbate substrate interactions which in this study manifests through self–assembly processes. Both low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) were utilized to determine the substrate cleanliness prior to the growth of a submonolayer Sb coverage (0.43 ± 0.02 ML Sb as calculated from the acquired STM data). The freely diffusing Sb adatoms on the copper surface were thermally excited from a random distribution of Sb atoms after growth to a finally rearrangement to more energetically stable configuration. The experimental results illustrated the presence of a surface alloy after annealing at ~360°C. The Cu – Cu spacing increased from 0.257 ± 0.01 nm (atomically clean Cu(111)) to 0.587 ± 0.02 nm after annealing at 360°C. At that temperature, the STM images showed the surface protrusions of different sizes and contrast, attributed to Cu and Sb atoms. In addition to the conventional ( 3 × 3)R30°–Sb structural phase acquired at ~400°C, new metastable structural phases: (2 3 × 2 3) R30°–Sb and (2 3 × 3)R30°– Sb were obtained for the first time after annealing at 600°C and 700°C, respectively. STM data after annealing at 600°C and 700°C was best described by a structural model involving an ordered p(2×2) and p(2×1) overlayer structures superimposed onto the ( 3 × 3)R30°–Sb surface, respectively. At elevated temperatures LEED showed ring shaped diffraction patterns composed of twelve equidistant spots which are consistent with the growth of a hexagonal film forming three equivalent rotational domains. All the superstructures were found to favour a structural model based on Sb atoms occupying substitutional rather than overlayer sites within the top Cu(111) layer. Other than the dissolution of Sb onto Cu(111), the study report also on the segregation of Sb on Cu together with STS measurements. The surface chemical reactivity on an atom–by–atom basis of the Cu sample surface was studied by current imaging tunneling spectroscopy (CITS). The local density of states (LDOS) were derived from dI/dV maps at low tunneling voltages by a simultaneous measurement of high resolution topographic micrographs. The use of surface sensitive techniques (LEED, AES, STM, STS) in studying the surface alloy in question has enabled more precise statements to be made about the surface structure of the system at various temperatures. Based on the experimental results, a comprehensive study of the adsorption and segregation behaviour of Sb on Cu(111), including the mechanisms for phase formation at the atomic scale is presented in this study.