This research, is initiated to systematically study the microstructure of AISI 4340 steel prior to impact, after impact and after post-impact annealing to determine the effect of the pre-deformation microstructure on the nucleation and initiation of ASBs, and the mechanism of evolution of ASBs during impact. This study used state-of-the-art microstructural characterization techniques such as the FIB and STEM/HRTEM to reveal that initial microstructural inhomogeneity produces nucleation sites for the initiation of ASBs during impact. It was observed that double misfit interfaces and boundary layers, formed around precipitated carbides (interface between reinforcements and matrix), increased the volume fraction of dislocation sources within the pre-impact specimens. It is demonstrated that the intersection of an activated dislocation source with the direction of maximum shear (regions of stress concentrations) within the specimens during impact, is a necessary condition for the points of intersection to act as possible sites for the nucleation and initiation of ASB depending on the rate of dislocation generation, local strain and strain rate. In addition, the structure that evolves after strain localization starts out with elongation of the grains in the shear direction with the initiation of random and transverse dislocation boundaries along the elongated grains. The elongated grains break along the initiated dislocation boundaries as strain/strain rate increases resulting in the creation of smaller elongated-broken grains and nanograins. Boundary refinement of the broken grains occurring through grain rotation and adiabatic heating results in the evolution of refined grains, subgrains and nanograins. The presence of elongated grains, broken grains, refined grains, subgrains and nanograins within the evolved shear band structures demonstrate that the local deformation is dependent on the imposed local strain and strain rate and that these mechanisms occur concurrently during impact. The results obtained, which are specific to the behavior of BCC ferritic Pearlitic hardenable steels, lead to the conclusion that the evolution of ASBs is a simultaneous layering of microstructures initially driven by dislocations which produce the final structures observed in the shear bands at the end of passage of the stress wave. / February 2015

Die vorliegende Arbeit beschreibt die Implementierung ionenstrahlanalytischer Methoden zur Charakterisierung der Probenzusammensetzung in einem Helium-Ionen-Mikroskop mit einem auf unter einen Nanometer fokussierten Ionenstrahl. Zur Bildgebung wird dieser im Mikroskop über Probenoberflächen gerastert und die lokale Ausbeute an Sekundärelektronen gemessen. Obwohl sich damit ein hoher topografischer Kontrast erzeugen lässt, lassen sich weder aus der Ausbeute noch aus der Energieverteilung der Sekundärelektronen verlässliche Aussagen zur chemischen Zusammensetzung der Probe treffen. Daher wurden in dieser Arbeit verschiedene ionenstrahlinduzierte Sekundärteilchen hinsichtlich ihrer Eignung für die Elementanalytik im Helium-Ionen-Mikroskop verglichen. Zur Evaluation standen der Informationsgehalt der Teilchen, deren Analysierbarkeit sowie deren verwertbare Ausbeute. Die Spektrometrie rückgestreuter Teilchen sowie die Sekundärionen-Massenspektrometrie wurden dabei als die geeignetsten Methoden identifiziert und im Detail untersucht. Gegenstand der Untersuchung waren physikalische Limitierungen und Nachweisgrenzen der Methoden sowie deren Eignung zum Einbau in ein Helium-Ionen-Mikroskop. Dazu wurden verschiedene Konzepte von Spektrometern evaluiert, erprobt und hinsichtlich ihrer Effizienz, Energieauflösung und Umsetzbarkeit im Mikroskop bewertet. Die Flugzeitspektrometrie durch Pulsen des primären Ionenstrahls konnte als die geeignetste Technik identifiziert werden und wurde erfolgreich in einem Helium-Ionen-Mikroskop implementiert. Der Messaufbau, die Signal- und Datenverarbeitung sowie vergleichende Simulationen werden detailliert beschrieben. Das Spektrometer wurde weiterhin ausführlich hinsichtlich Zeit-, Energie- und Massenauflösung charakterisiert. Es werden ortsaufgelöste Rückstreuspektren vorgestellt und damit erstmalig die Möglichkeit zur Ionenstrahlanalytik im Helium-Ionen-Mikroskop auf einer Größenskala von ≤ 60 nm aufgezeigt. Das Pulsen des primären Ionenstrahls erlaubt es zudem, die Technik der Sekundärionen-Massenspektrometrie anzuwenden. Diese Methode bietet Informationen zur molekularen Probenzusammensetzung und erreicht für einige Elemente niedrigere Nachweisgrenzen als die Rückstreuspektrometrie. Damit konnten erstmalig im Helium-Ionen-Mikroskop gemessene Sekundärionen-Massenspektren sowie die ortsaufgelöste Elementanalyse durch spektrometrierte Sekundärionen demonstriert werden. Die Ergebnisse dieser Arbeit sind in der Fachzeitschrift Ultramicroscopy Band 162 (2016) S. 91–97 veröffentlicht. Ab Oktober 2016 werden diese auch in Form eines Buchkapitels in dem Buch „Helium Ion Microscopy“, Springer Verlag Heidelberg zur Verfügung stehen. / The present work describes the implementation of ion beam analysis methods in a helium-ion-microscope for the determination of sample compositions with a focused ion beam of < 1 nm size. Imaging in the microscope is realized by scanning the focused ion beam over the sample surface while measuring the local secondary electron yield. Although this procedure leads to a high topographical contrast, neither the yield nor the energy distribution of the secondary electrons deliver reliable information on the chemical composition of the sample. For this purpose, in this work different ion beam induced secondary particles were compared with respect to their suitability for the analysis of the chemical composition in the helium-ion-microscope. In particular the information content of the particles, their analysability and their yield were evaluated. As a result, the spectrometry of backscattered particles and the mass spectrometry of sputtered secondary ions were identified as the most promising methods and regarded in detail. The investigation focused on physical limitations and detection limits of the methods as well as their implementability into a helium-ion-microscope. Therefor various concepts of spectrometers were evaluated, tested and validated in terms of their efficiency, energy resolution and practicability in the microscope. Time-of-flight spectrometry by pulsing the primary ion beam could be identified as the most suitable technique and has been successfully implemented in a helium-ion-microscope. The measurement setup, signal processing and data handling as well as comparative simulations are described in detail. Further the spectrometer was characterized explicitly in terms of time, energy and mass resolution. Spatially resolved backscattering spectra will be shown demonstrating the feasibility of performing ion beam analysis in a helium-ion-microscope for the first time on a size scale of ≤ 60 nm. By pulsing the primary ion beam the technique of secondary ion mass spectrometry becomes automatically accessible. This method provides information on the molecular composition of samples and can reach higher detection limits than those from backscattering spectrometry. For the first time, in a helium-ion-microscope measured secondary ion mass spectra and spatially resolved elemental analysis by spectrometry of secondary ions, could be demonstrated. The results of this work are published 2016 in the scientific journal Ultramicroscopy, volume 162 on pages 91 to 971. In October 2016 there will be another publication as a book chapter in „Helium Ion Microscopy“ (publisher: Springer Verlag Heidelberg).

Thesis (MTech (Environmental Management))--Cape Peninsula University of Technology, 2018. / The iThemba Laboratory for Accelerator Based Sciences (iThemba LABS or iTL), a facility governed by the National Research Foundation (NRF) based in Cape Town has embarked on a project called the South African Isotope Facility (SAIF). Part of the SAIF will be a project called the Low Energy Radioactive Ion Beam (LERIB) for the production of radioactive beams with the Separated Sector Cyclotron (SSC). The LERIB project is anticipated to generate significant prompt sources of ionising radiation such as neutrons displaced by protons during bombardment of targets. At present, the facility has 3 - 5 m thick concrete walls designed to shield against 200 MeV neutrons. The floor thickness varies from each vault according to the behaviour and characteristics of the produced radiation. The LERIB facility concrete floor is envisioned to be 1 m thick. Insufficient floor shielding might expose the sub-surface strata below the floor with ionising radiation. Some matter in the exposed strata might undergo activation and consequently produce undesirable radioactive isotopes. Exposure to radioactive isotopes can pose potential risks to the general public and the receiving environment. In the sub-surface, the activated isotopes can spread through the soil and groundwater bodies. Groundwater being mobile in nature might act as conduit for activated isotopes to migrate in the subsurface. The half-lives of the formed radionuclides were used to estimate the distance the radionuclides can migrate in the sub-surface before decay. The X-Ray Fluorescence (XRF) and Inductive Coupled Plasma - Mass Spectrometry (ICP-MS) were used to quantify the mass fraction of elements from soil and groundwater samples. The Inductive Coupled Plasma - Optical Emission Spectrometry (ICP-OES) was used to measure dissolved metals of major and minor oxides in groundwater samples. These scientific techniques were applied in order to measure mass fractions of elements present in the sub-surface anticipated to be exposed with neutron radiation. Results revealed that the sub-surface in the study site constitutes Si, Al, Na and Fe in abundance. This suggests that, these elements might have a high probability to undergo activation compared to those occurring at lower concentrations, such as minor and trace elements. However the neutron capture cross-section for these elements varies and hence some elements such as Si and Al were not activated. To investigate elements measured from the ICP-MS, ICP-OES and XRF which might undergo activation, soil and water samples were irradiated for an hour with neutrons from a Beryllium target from a range of 0 – 66 MeV energy. The results revealed that activating the sub-surface with neutron radiation will result to the formation of short and long-lived unstable isotopes. However, isotopes with long half-lives such as 22Na, 52Mn, 54Mn and 46Sc only occurred in soil samples and none measured from the water samples except a short-lived isotope 24Na. These results agrees with the reviewed literature that, water being a product of two hydrogens and one oxygen, with 1H from H2O considered to be a good neutron moderator should have absorbed the neutrons so as to reduce neutron capture cross-sections.

O grafeno é um material que apresenta propriedades físicas e químicas superiores aos materiais tradicionalmente utilizados na fabricação de diferentes dispositivos. Porém, para substituir tais materiais, é imprescindível o conhecimento e controle de processos de adsorção e incorporação de diferentes compostos na superfície do grafeno, crescido ou transferido, sobre diferentes substratos. A investigação da síntese e caracterização de filmes de grafeno com a finalidade de controlar as propriedades físico-químicas e elétricas é um esforço da comunidade científica atualmente. Nesse trabalho, tivemos o objetivo de investigar o processamento térmico do grafeno em vapor de água e em óxido nítrico e sua síntese pela técnica de MBE, caracterizando as estruturas resultantes através de técnicas de análise por feixes de íons, espectroscopia de fotoelétrons e espectroscopia Raman. No caso do processamento em vapor de água, as amostras foram submetidas a tratamentos térmicos em ampla faixa de temperatura (200-1000°C), sendo possível distinguir três diferentes regimes de interação do grafeno com a água. A baixas temperaturas de processamento (200-400°C), nenhuma modificação considerável é observada. Na faixa entre 400-500°C, a estrutura plana do grafeno é corrugada para acomodar os novos grupamentos funcionais formados. A partir de 600°C, os domínios cristalinos diminuem e observa-se alto nível de dopagem oxidativa. Já no processamento em NO, evidenciou- se a introdução de N na rede cristalina e etching do grafeno em altas temperaturas. Quanto ao crescimento de grafeno sobre substratos de Si3N4 e AlN(0001) por MBE, este trabalho mostrou a viabilidade de se obter filmes de nanografeno com uma taxa de crescimento de 1 monocamada por minuto, onde a qualidade cristalina do filme formado e a espessura dependem do tempo de crescimento. / Graphene is a material which presents physical and chemical properties superior than the materials traditionally used in different devices. In order to use graphene rather than those materials, it is necessary to understand and control the adsorption process and incorporation of different compounds on graphene, grown or transferred, on different substrates. The scientific community has made efforts to control the physicochemical and electrical properties of graphene films. The objective of thesis is to investigate the physicochemical modifications of graphene induced by annealing in water vapor and nitric oxide and to grow graphene by MBE. Resulting structures were characterized by ion beam analysis, x-ray spectroscopy and Raman spectroscopy. Concerning samples treated in water vapor, annealings were performed at temperatures ranging from 200 to 1000ºC. Results evidence three different regimes with respect to annealing temperature. In the low temperature range (200–400°C), no prominent modification of graphene itself is observed. At higher temperatures (400–500°C), to accommodate newly formed oxygen functionalities, the flat and continuous sp2 bonding network of graphene is disrupted, giving rise to a puckered layer. For 600°C and above, shrinking of graphene domains and a higher doping level take place. Regarding graphene processing in NO, results show N incorporation in graphene crystal network and etching of graphene at higher temperatures. Concerning graphene synthesis on Si3N4 and AlN(0001) by MBE, this work demonstrates the feasibility of growing nanographene with a growth rate of one monolayer per minute. In addition, the crystal quality of the films and their thickness depends on growing time.

Neste trabalho foi desenvolvido um estudo sistemático dos mecanismos de difusão responsáveis pelo switching de resistência em memórias resistivas. Essas memórias possuem estrutura semelhante a de um capacitor, a qual sofre uma transição de resistência induzida pela aplicação de um campo elétrico. A transição é provocada pela formação de filamentos condutivos no interior da matriz semicondutora. Os filamentos podem ser constituídos por metal originado de um dos eletrodos (ECM) ou por regiões do óxido deficientes em oxigênio (VCM), geradas pela difusão de vacâncias de oxigênio. Dispositivos de TiO2 foram construídos e sua resposta elétrica foi adquirida através de medidas elétricas do tipo I-V para diferentes metais de eletrodo. Técnicas de análise por feixe de íons, como retroespalhamento Rutherford por micro-feixe e perfilometria com reação nuclear ressonante, foram usadas para detalhamento dos processos de difusão. Constatou-se uma dependência do comportamento elétrico em função do método de deposição da camada semicondutora, sua espessura e os parâmetros da medida de tensão. No caso do filamento ser composto por átomos de metal, espectros de micro-RBS foram adquiridos para identificar a sua estrutura no interior do óxido. Ainda, observaram-se bolhas na superfície do eletrodo superior dos dispositivos com difusão de vacâncias de oxigênio após o tratamento elétrico. Nesse mesmo contexto, foi medida a difusividade e energia de ativação da difusão de oxigênio em filmes finos de TiOy. / In this work we developed a systematic study of diffusion mechanisms which are responsible for resistance switching in resistive memories. The structure of these memories is similar to a capacitor which suffers resistance transition induced by electrical field. The transition is caused by the formation of conductive filaments inside the semiconductor matrix. The filaments may be constituted by metal from one of the electrodes (ECM) or by oxygen deficient areas (VCM), generated from oxygen vacancies diffusion. Devices of TiO2 have been built and its electrical response was acquired through electrical measurements (I-V) for different electrode metals. Ion beam techniques such as micro-probe Rutherford Backscattering and Nuclear Reaction Profiling were used to detail the diffusion processes. It was observed a dependence in the electrical behaviour with the semiconductor layer deposition method, its thickness and bias measurement parameters. In the case which filaments are composed by metal atoms, measurements of micro-RBS were performed to identify its structure inside the oxide. Also, bubbles have been observed over the surface of top electrode in devices with oxygen vacancies diffusion after the electrical treatment. In this context, it was measured diffusivity and activation energy for oxygen diffusion in thin TiOy films.

No description available.

Lithium ion batteries (LIBs) have gained increasing popularity due to their high potential, low self-discharge, zero priming and minimal memory effect. However, the emergence of electrical vehicles and hybrid electrical vehicles in the automobile industry, where LIBs are predominantly in use, instilled a need to improve LIB batteries by experimenting with new materials. Graphite, the commonly used anode material for LIBs suffers from low theoretical capacity (372 mA h g-1) and torpid rate performance. Germanium (Ge) seems to be a promising substitute of carbon due to its high theoretical capacity, high Li+ diffusivity and electrical conductivity. However, Ge undergoes large volumetric change (±370%). This causes deboning of the thin film Ge electrode from the substrate current collector, causing a rapid decrease in the electrolytic performance. The process of ion beam mixing claims to have overcome this problem. In our current study, the adhesion strength of Ge thin film over Nickel (Ni) substrate (with and without ion beam mixing) is being measured using nanoindentation and the superlayer indentation test. Nanoindentation is one of the popular techniques to measure the mechanical properties and adhesion of thin film coatings. In this technique, a very small indenter of a desired geometry indents the film/substrate pair and the work of adhesion is calculated by knowing the plastic depth of indentation and the radius of indentation. Superlayer indentation is analogous to normal indentation but with a highly stressed superlayer on top to restrict the out-of-plane displacements, it reduces the plastic pile up around the indenter tip. The results from our study strongly suggest the possibility of dramatically increasing the adhesion strength by ion bombardment, which can be achieved by atomic level intermixing of the film/substrate pair. These, in turn, suggest that Ge could be an effective successor to graphite in the near future.

The deformation mechanisms operating in diamond-like carbon (DLC) coatings on (100) and (111) Si, has been investigated. The effect of coating thickness, indenter geometry, substrate orientation and deposition technique on the deformation of DLC coatings and the underlying substrate was studied by undertaking nanoindentation followed by subsurface microstructural characterization. Uncoated (111) Si was also investigated for comparison. The observed microstructural features were correlated to the indentation response of the coatings and compared with simulation studies, as well as observations on uncoated Si. In uncoated (111) Si, phase transformation was found to be responsible for the discontinuities in the load-displacement curves, similar to (100) Si. However, slip was activated on {311} planes instead of on {111} planes. Moreover, the density of defects was also significantly lower and their distribution asymmetric. The coatings were adherent, uniformly thick and completely amorphous. The load-displacement curves displayed several pop-ins and a pop-out, the indentation loads for the first pop-in and the pop-out depending primarily on the thickness of the coating. The coatings exhibited localized compressive deformation in the direction of loading without any through-thickness cracks. The extent of this localized deformation increased with indentation load. Hardness and thickness of the coatings and the geometry of the indenter influenced the magnitude of compressive strains. Harder and thinner coatings and a blunt indenter exhibited the minimum degree of deformation. Densification by rearrangement of molecules has been suggested as the mechanism responsible for plastic compression. At indentation loads corresponding to the first pop-in, (100) and (111) silicon substrates initially deformed by <111> and <311> slip respectively. Higher indentation loads caused phase transformation. Therefore, unlike in uncoated Si, dislocation nucleation in the Si substrate has been proposed as the mode responsible for the first pop-in. Subsequent pop-ins were attributed to further deformation by slip and twinning, phase transformation and extensive cracking (median and secondary cracks) of the substrate. The pop-out, however, was ascribed to phase transformation. Extensive deformation in the substrate, parallel to the interface, is attributed to the wider distribution of the stress brought about by the DLC coating. Good correlation was obtained between the nanoindentation response, microstructural features and simulation studies.

<p>The work describes the development of a reliable device for profiling anion beam in the intensity cross section. A sensor head consisting of a Faradaycup in combination with a Channel Electron Multiplier was designedand built together with electronics including power supply and front endelectronics. The design was chosen considering financial and long term lifeaspects. Testing, first calibration and error analysis were done using the ionbeam facilities where the unit is supposed to be installed permanently. Theprofiling system performed as designed and the profile of the ion beam couldbe measured reliably with an accuracy down to the femto ampere range.</p>

Piezoelectric actuators (PEA) hold the most promise for precision positioning applications due to their capability of producing extremely small displacements down to 10 pm (1 pm = 10-12 m) as well as their high stiffness and force output. The piezoelectric-driven stick-slip (PDSS) actuator, working on the friction-inertia concept, has the capacity of accomplishing an unlimited range of motion. It also holds the promises of simple configuration and low cost. On the other hand, the PDSS actuator has a relatively low efficiency and low loading capability, which greatly limits its applications. The purpose of this research is to improve the performance of the PDSS actuators by employing specially-designed working surfaces.<p> The working surfaces, referred as anisotropic friction (AF) surfaces in this study, can provide different friction forces depending on the direction of relative motion of the two surfaces, and are used in this research to accomplish the aforementioned purpose. To fabricate such surfaces, two nanostructure technologies are employed: hot filament chemical vapour deposition (HFCVD) and ion beam etching (IBE). The HFCVD is used to deposit diamond on silicon substrates; and the IBE is used to etch the diamond crystalloid with a certain angle with respect to the coating surface to obtain an unsymmetrical-triangle microstructure. <p> A PDSS actuator prototype containing the AF surfaces was developed in this study to verify the function of the AF surfaces and characterize the performance of PDSS actuators. The designed surfaces were mounted on the prototype; and the improvement in performance was characterized by conducting a set of experiments with both the normal isotropic friction (IF) surfaces and the AF surfaces, respectively. The results illustrate that the PDSS actuator with the AF surface has a higher efficiency and improved loading capability compared to the one with the IF surfaces.<p> A model was also developed to represent the displacement of the novel PDSS actuator. The dynamics of the PEA and the platform were approximated by using a second order dynamic system. The pre-sliding friction behaviour involved was investigated by modifying the LuGre friction model, in which six parameters (Note that three parameters are used in the LuGre model) were employed to represent the anisotropic friction. By combining the PEA mechanism model, the modified friction model, and the dynamics of end-effector, a model for the PDSS actuator with the AF surface was developed. The model with the identified parameters was simulated in MATLAB Simulink and the simulation results obtained were compared to the experimental results to verify the model. The comparison suggests that the model developed in this study is promising to represent the displacement of the novel PDSS actuators with AF surfaces.