Clostridioides difficile is a Gram positive, spore-producing, anaerobic bacterium. It is considered a nosocomial pathogen due to high incidence rates of C. difficile infections (CDI) in hospitals. However, research reveals an increase in community-associated cases. CDI is most common in the elderly, immunocompromised, or those taking a course of antibiotics. These individuals are more vulnerable to experiencing gut dysbiosis, allowing C. difficile to colonize the colon. CDI are an urgent threat due to their ability to sporulate. Spores are hardy and not eradicated by common disinfectants. They persist on surfaces the patient may have contact with during CDI. Current decontamination methods include the utilization of bleach-based disinfectants followed by ultraviolet (UV) light and adherence to a strict quality control protocol. However, spores may remain in the environment even after this process, thus allowing the pathogen to spread through surface contact. Another problem is the cost incurred by the hospital over time in terms of inpatient and equipment turnover related costs. These reasons make it imperative a better means of disinfection is developed. Silver is a known antimicrobial agent and utilized in clinical settings for burn treatment. Cerium oxide nanoparticles (CNP) possess antioxidant properties and can be used for drug delivery. Preliminary studies show silver-modified CNP (AgCNP) possess antiviral activity against COVID-19 and rhinovirus. To study the antimicrobial effects of the AgCNP against vegetative cells, a three-day time-kill assay was performed. Two strains of C. difficile, R20291 and NAP1, were cultured in BHIS (brain heart infusion, supplemented) and grown overnight. Glass coupons coated with AgCNP1 or AgCNP3, CNP1 or CNP3, or left uncoated were used in this study. They were incubated with cell culture at a variety of time points. The results indicate AgCNP3 may possess bactericidal activity. Further research should be conducted to determine the extent of this activity.

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En esta Tesis se estudia la interacción de las moléculas de metanol yetanol con diferentes sitios de la superficie de óxido de magnesio, con el propósito de determinar propiedades catalíticas relativas a óxidos de metales alcalinos. La metodo-logía teórica empleada está basada en el formalismo DFT, para una base localizada de tipo gaussiano, aplicada a un modelo de agregado de átomos, con un embedding de cargas puntuales y potenciales efectivos. Los efectos electrostáticos no balanceados se minimizan haciendo nulos la carga y el mo-mento dipolar del agregado. Inicialmente se estudió la adsor-ción de una molécula de metanol sobre la cara (100) de MgO, la que ocurre sin pérdida de integridad y con una deformación muy pequeña, como resultado del ajuste de la configuración del metanol libre a la estructura del campo eléctrico superfi-cial. El estudio de la adsorción de dos moléculas del metanol en un borde de MgO según la dirección [001] mostró que la primera molécula adsorbida se comporta como un nuevo defec-to de superficie y torna más reactivo al sitio de MgO; y se observa que los metanoles interactúan entre sí mediante estructuras complejas de enlace puente de hidrógeno (ePH). Por otro lado, la interacción de grupos de moléculas sobre la cara (100) de MgO muestra que tetrámeros de metanol de-sarrollan enlaces puente de hidrógeno entre ellos y con la superficie, y enlaces iónicos parcialmente covalentes con los Mg de superficie. Las configuraciones de equilibrio tienen la misma simetría de la red superficial, lo que sugiere la forma-ción de una capa molecular con propiedades del metanol sóli-do. Esta conclusión es consistente con la formación de una rígida capa de metanol sobre una superficie de MgO clivada, tal como se observa experimentalmente. Luego se estudió la adsorción de etanol sobre la cara (100) de MgO y sobre los defectos de borde y esquina. Es de notar que el átomo de C del metanol y el grupo CC del etanol se equilibran de manera distinta en el campo eléctrico superficial del MgO, y que el etanol establece enlaces ePHc(CmolHOsus) adicionales. En este trabajo, se asume la hipótesis central de que un conjun-to de modelos de superficie con las propiedades de carga y momento dipolar nulos, proporcionan configuraciones de equili-brio de la molécula adsorbida que se corresponden con aque-llas para una superficie extendida. Esta afirmación posibilita re-alizar comparaciones entre las diferentes configuraciones de equilibrio, explorar la adsorción de grupos de moléculas, obser-var las interacciones intermoleculares y cotejar estos resulta-dos con la información experimental; además implica sostener el carácter altamente localizado del proceso de adsorción. Los estudios realizados, junto con los trabajos previos y datos experimentales son consistentes con esta hipótesis. Esto implica a su vez que es posible considerar un esquema de análisis que da cuenta de los diversos resultados de manera unificada. En este sentido se introdujo la definición de Trián-gulo de Enlace y de un enlace Mg−ePH. La reacción MgePH ocurre en el proceso de adsorción, cuando el Omol interactúa con la base iónica de MgO mediante un doble enlace: un en-lace iónico-covalente (OmolMg) y un ePH (OmolHOsus). / In this doctoral thesis the interaction of methanol and ethanol molecules with different sites of a magnesium oxide surface is studied, in order to establish the catalytic properties proper of alkaline earth oxides. The theoretical methodology used here has its grounds in DFT formalism, with a local basis set of Gaussian type, applied to a cluster model, combined with an embedding of punctual charges and effective potentials. The non compensated electrostatic effects are minimized ma-king zero the charge and dipolar moment of the cluster. First, the adsorption of only one methanol molecule on the (100) face of MgO was studied; in this case, no methanol dissocia-tion is observed, with only a very small deformation. This is a consequence of the adjustment of the free methanol molecule geometry to the surface electric field. The consideration of two methanol molecules adsorbed on a MgO step oriented along the [001] direction showed that the first adsorbed molecule behaves like a new defect on the surface, making more reactive this site of MgO surface; moreover it is noticed that the methanol molecules interact between them through complex structures produced by hydrogen bonding (ePH). On the other hand, the interaction of several molecules on the (100) face of MgO shows that the methanol tetramers have hydrogen bonds between them and with the surface, and ionic- partly covalent bonds with thwe Mg atoms of this surfa-ce. The equilibrium configurations have the same symmetry of the surface lattice, suggesting the formation of a molecular layer with bulk methanol properties. This conclusion is in agreement with the formation of a rigid methanol layer on cleaved MgO, as it is experimentally observed. Afterwards,the adsorption of etanol on the (100) faceof MgO and on the step and corner defects was considered. It is to be noticed that that the C atom of methanol and the C-C group of ethanol equilibrate differently o the surface electric field of MgO, and that the ethanol molecule establishes additional ePHc(CmolHOsus) bonds. In this work, the assumed central hypothesis is that an ensemble surface models with zero charge and zero dipolar moment give equilibrium configurations of the adsorbed molecule that correspond with those for a extended surface. This affirmation allows us to make comparisons between different equilibrium configurations, to explore the adsorption of several molecules, to observe the intermolecular interac-tions and to test these results with the experimental informa-tion; moreover, it supports the highly localized character of adsorption process. The performed calculations, in conjunc-tion with the previous studies and experimental data are con-sistent with this hypothesis. This implies that a scheme can be developed to analyse these observations in order to ratio-nalize the different results in unified way. In this direction the definitions of Bond Triangle and o Mg-ePh bond were introdu-ced. The MgePH is produced in the adsorption process, when the Omol interacts with the ionic basis of MgO through a double bond: a ionic-covalent bond (OmolMg) and an ePH (OmolHOsus).

Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: Terahertz (THz) radiation is often used in many promising applications such as information and communication technology and airport security. Optimized and adapted terahertz fields hold huge promise for leading this technology further. This study is focused on terahertz time domain spectroscopy (THz-TDS). In THz-TDS the electric field is measured, therefore both amplitude and phase information of the THz pulse can be obtained. We used the pump-probe technique in order to measure a THz pulse from the photoconductive antenna. A pulsed fiber laser with FWHM of ' 100 fs was used for this. The frequency spectrum of the measured THz pulse was obtained via a fast Fourier transform. We studied the principles of the THz pulse generation as well as detection, with a photoconductive antenna as emitter and detector. In this study terahertz spectroscopy has been used to investigate the refractive index and absorption coefficient of different types of materials in the terahertz region. The last part of this study deals with a simple process for material parameter extraction of a polymer sample using commercial software called Teramat1.0. It uses the sample thickness, the reference THz pulse and the transmitted THz pulse to retrieve the complex refractive index of the sample. / AFRIKAANSE OPSOMMING: Terahertz (THz) straling word gereeld gebruik vir belowende toepassings soos inligting en kommunikasie tegnologie en lughawe sekuriteit. Geoptimeerde en aangepaste terahertz velde dra by tot die bevordering van die tegnologie. Hierdie studie fokus op terahertz tyd domein spektroskopie (THz-TDS). In THz-TDS word die elektriese veld gemeet en dus word beide amplitude en fase inligting van die THz puls verkry. Ons gebruik ’n pomp en toets tegniek om die THz puls deur ’n fotogeleidende antenna te bepaal. ’n Gepulseerde vesel laser met FWHM van 100 fs word hiervoor benut. Die frekwensie spektrum van die gemete THz puls word bereken deur ’n vinnige Fourier transvorm te bereken. Die beginsels van die generering en deteksie van THz pulse is bestudeer met ’n fotogeleidende antenna as sender en ontvanger. In die studie is terahertz spektroskopie gebruik om die brekingsindekse en die absorpsie koeffisiënte van verskillende materiale in die terahertz gebied te bepaal. Die laaste gedeelte van die studie handel oor ’n eenvoudige proses om die materiaal parameters van ’n polimeer te bepaal deur gebruik te maak van kommersiële sagteware Teramat 1.0. Die monster dikte, die THz verwysingspuls en die deurgelate puls word gebruik om die komplekse brekingsindeks van die materiaal te bereken.

Thin films are structures with thicknesses ranging from the atomic scale to the mesoscale that are used to alter the properties of a surface and/or serve as functional layers in devices. Thin metal films deposited from the vapor phase on weakly-interacting substrates, including oxides (TiO2, ZnO, SiO2 etc.) and two-dimensional (2D) materials (graphene, MoS2, etc), are relevant for a wide array of technological applications, such as optical devices, nanoelectronic components, sensors, and catalytic devices. The weak interaction between deposit and surface in these film/substrate combinations leads to three-dimensional (3D) metal-layer morphological evolution in an uncontrolled manner; which often constitutes an important challenge toward integrating metal layers in key enabling devices. Thus there is a need for efficient growth manipulation strategies, such that metal films with controlled 3D and 2D microstructures and morphologies can be synthesized. Surfactants, i.e., minority metal, non-metal, and gaseous species which are deployed to the growing surface together with film-forming species, have been shown to enable growth manipulation in a multitude of homo- and heteroepitaxial metal/metal and semiconductor/semiconductor systems. This work explores the viability of N2 and O2 surfactants to manipulate growth in model weakly-interacting Ag/SiO2 and Au/SiO2 systems. Au and Ag are deposited by direct current (DC) magnetron sputtering on Si substrates covered with a 500 nm thick thermally grown SiO2 layer. Gaseous N2 and O2 surfactants are introduced to the sputtering atmosphere either continuously during deposition or at well-defined points during growth, such that specific film-formation stages as targeted. Using a combination of in situ/real-time diagnostic tools and ex situ characterization techniques, it is shown that O2 and N2 cause Ag and Au, respectively, to grow flatter, i.e., 2D growth morphology is promoted. Moreover, by deploying surfactants selectively during early or late film growth stages and studying their effect on film morphological evolution, it is concluded that N2 and O2 effectively suppress the rate of island coalescence promoting formation of flatter films. The overall results of this study are the first step toward establishing an atomic-scale understanding of the effect of surfactants on morphological evolution of metal films on weakly-interacting substrates. The knowledge generated herein is relevant for designing growth manipulation strategies in a wide range of technologically important film/substrate systems.

Les progrès récents concernant la fabrication des substrats de Ge mono- et poly-cristallins, ainsi que des substrats « Ge-sur-isolant », combinés au transfert des technologies des isolants « high-k » et des contacts ohmiques de la technologie Si vers la technologie Ge permettent d’envisager le développement d’une microélectronique à haute performance basée sur une technologie utilisant le Ge en remplacement du Si. Toutefois, afin de respecter les restrictions liées à la fabrication de la prochaine génération de dispositifs microélectroniques miniaturisés (MOSFETs à canal court), il est nécessaire d'améliorer nos connaissances sur le dopage et sur la formation des contacts ohmiques sur Ge, en particulier pour le Ge de type n. Le principal objectif de cette thèse était d'étudier la redistribution atomique ayant lieu pendant certains procédés impliqués dans la fabrication de la structure [métal premier niveau / contact ohmique / Ge-dopé] localisée sur chacune des zones actives (grille, source et drain) des transistors. Notre travail s’est concentré sur le sélénium et le tellure en tant que dopant de type n, ainsi que sur le gallium et l'aluminium comme dopants de type p. Le Palladium a été choisi pour la fabrication des contacts ohmiques. Notre travail comprend l’étude des interactions entre dopants et défauts étendus, de la formation de précipités, et de la diffusion des dopants dans le Ge(001) pendant un recuit post-implantation. La formation et la stabilité des films minces de germaniure de Pd sont également étudiées dans le but d’évaluer et d’optimiser l’utilisation du composé PdGe comme contact ohmique sur Ge. / The recent progress concerning the fabrication of large Ge mono- and poly-crystalline substrates, as well as the fabrication of Ge-On-Insulator (GOI) substrates, combined with the successful transfer from the Si technology to the Ge technology of the high-k dielectric and of the ohmic contact fabrication technologies support the development of a future high-performance Ge-based microelectronic technology. However, in order to meet the restrictions for the fabrication of the next generation of miniaturized microelectronic devices (short-channel MOSFETs), it is necessary to improve our knowledge concerning Ge doping and contact fabrication, in particular for n-type Ge. The main goal of this PhD was to investigate the atomic redistribution occurring during some of the fabrication processes involved in the fabrication of the structure [first-level metal / ohmic contact / doped-Ge] found on each active zone (gate, source, and drain) of transistors. Our work focused on selenium and tellurium for n-type doping, as well as on gallium and aluminum for p-type doping. Palladium was the metal chosen for the fabrication of ohmic contacts.This work includes the study of extended defect interactions with dopants, dopant clustering, and dopant diffusion in Ge(001) during post-implantation annealing. The formation and stability of Pd germanide thin films are also investigated, in order to evaluate and optimize the use of the PdGe compound as ohmic contact on Ge. Finally, dopant redistribution in PdGe thin films and in the Ge substrate during ohmic contact fabrication is also investigated.

The ability to extract accurately the stress intensity factor and the T-Stress for fractured engineering materials is very significant in the decision-making process for in-service engineering components, mainly for their functionality and operating limit. The subject of computational fracture mechanics in engineering make this possible without resulting to expensive experimental processes. In this thesis, the Finite Block Method (FBM) has been developed for the meshless study of interface stationary crack under both static and dynamic loading in bi-materials. The finite block method based on the Lagrangian interpolation is introduced and the various mathematical constructs are examined. This includes the use of the mapping technique. In a one-dimensional and a two-dimensional case, numerical studies were performed in order to determine the interpolation error. The finite block method in both the Cartesian coordinate and the polar coordinate systems is developed to evaluate the stress intensity factors and the T-stress for interface cracks between bi-materials. Using the William's series for bi-material, an expression for approximating the stress and displacement at the interface crack tip is established. In order to capture accurately the stress intensity factors and the T-stress at the crack tip, the asymptotic expansions of the stress and displacement around the crack tip are introduced with a singular core technique. The accuracy and capability of the finite block method in evaluating interface cracks is demonstrated by several numerical assessments. In all cases, comparisons have been made with numerical solutions by using the boundary collocation method, the finite element method and the boundary element method, etc.

The shrinkage during solidification of aluminium and iron based alloys has been studied experimentally and theoretically. The determined shrinkage behaviour has been used in theoretical evaluation of shrinkage related phenomena during solidification. Air gap formation was experimentally studied in cylindrical moulds. Aluminium based alloys were cast in a cast iron mould while iron based alloys were cast in a water-cooled copper mould. Displacements and temperatures were measured throughout the solidification process. The modelling work shows that the effect of vacancy incorporation during the solidification has to be taken into account in order to accurately describe the shrinkage. Crack formation was studied during continuous casting of steel. A model for prediction of crack locations has been developed and extended to consider non-equilibrium solidification. The model demonstrates that the shrinkage due to vacancy condensation is an important parameter to regard when predicting crack formation. The centreline segregation was studied, where the contributions from thermal and solidification shrinkage were analysed theoretically and compared with experimental findings. In order to compare macrosegregation in continuous casting and ingot casting, ingots cast with the same steel grade was analysed. However, the macrosegregation due to A-segregation is driven by the density difference due to segregation. This is also analysed experimentally as well as theoretically.

<p>The shrinkage during solidification of aluminium and iron based alloys has been studied experimentally and theoretically. The determined shrinkage behaviour has been used in theoretical evaluation of shrinkage related phenomena during solidification. </p><p>Air gap formation was experimentally studied in cylindrical moulds. Aluminium based alloys were cast in a cast iron mould while iron based alloys were cast in a water-cooled copper mould. Displacements and temperatures were measured throughout the solidification process. The modelling work shows that the effect of vacancy incorporation during the solidification has to be taken into account in order to accurately describe the shrinkage. </p><p>Crack formation was studied during continuous casting of steel. A model for prediction of crack locations has been developed and extended to consider non-equilibrium solidification. The model demonstrates that the shrinkage due to vacancy condensation is an important parameter to regard when predicting crack formation. </p><p>The centreline segregation was studied, where the contributions from thermal and solidification shrinkage were analysed theoretically and compared with experimental findings. In order to compare macrosegregation in continuous casting and ingot casting, ingots cast with the same steel grade was analysed. However, the macrosegregation due to A-segregation is driven by the density difference due to segregation. This is also analysed experimentally as well as theoretically.</p>

In order to make much stiffer, light weight and high performance material products, polymer nanocomposites play an emerging role in the material innovation. Unlike other thermoplastics, polymer nanocomposites are fabricated by introducing a small amount of solid nano-scale fillers (normally less than 5 wt%) such as nanoclay, carbon nanotubes or nanofibres into a plastic resin to dramatically enhance its stiffness, strength and thermal properties. The difference between nanocomposites and conventional fibre composites is that the added fillers are extremely small, only one-millionth of a millimetre thick, and provide a much larger interface area per unit volume for greatly improving the interfacial bonding effect between nanofillers and the polymer matrix. More importantly, polypropylene (PP)/clay nanocomposites have quite a high potential to form such innovative materials and replace the conventional plastics in many automotive and packaging applications. Nevertheless, the growth of PP/clay nanocomposites faces an obstacle of hydrophobic polymer’s low interactions with hydrophilic clay. Maleic anhydride (MA) grafted PP (MAPP), commonly used as a compatibiliser, has been proven to facilitate a good clay dispersion within the PP matrix through its functionalised MA groups. But despite the great attention from the manufacturers and researchers in recent years,commercial PP/clay nanocomposites with reliable material properties are still limited in availability. The major problem stems from the complex influences of the material selection and processing methods. The present work developed a comprehensive approach from the material formulation and processing, experimental characterisation to the numerical modelling of PP/clay nanocomposites based on the finite element analysis (FEA) of micro/nanostructures. Initially, effects of the material selection including the clay type and content, MAPP content and PP matrix viscosity were investigated for the mechanical property enhancement of PP/clay nanocomposites. These nanocomposites were prepared using twin screw extrusion and injection moulding processes with a well-known Taguchi design of experiments (DoE) method in order to statistically detect the significant factors for influencing their mechanical properties. The preferred material formulations were then determined by Pareto analysis of variance (ANOVA) with the technical and economic considerations. The fundamental material characterisation was also conducted on those formulated nanocomposites using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). Overall mechanical properties of neat PP and corresponding nanocomposites were determined by the general tensile, flexural and impact tests. Finally, computational models were established by implementing both the representative volume element (RVE) technique and innovative object-oriented finite element (OOF) analysis to predict the tensile moduli of PP/clay nanocomposites in comparison to the experimental data and available composites theoretical models. / This research work was sponsored by Foundation for Research, Science and Technology (FRST), New Zealand under the grant #UOAX 0406 and financially supported by Tertiary Education Commission (TEC), New Zealand through Bright Future Top Achiever Doctoral Scholarship to Yu Dong.