In this thesis, a novel in-situ method for incorporating nanoscale ceramic particles into metal has been developed. The ceramic phase is introduced as an organic-polymer precursor that pyrolyzes in-situ to produce a ceramic phase within the metal melt. The environment used to shield the melt from burning also protects the organic precursor from oxidation. The evolution of volatiles (predominantly hydrogen) as well as the mechanical stirring causes the polymer particles to fragment into nanoscale dispersions of a ceramic phase. These “Polymer-based In-situ Process-Metal Matrix Composites” (PIP-MMCs) are likely to have great generality, because many different kinds of organic precursors are commercially available, for producing oxides, carbides, nitrides, and borides. Also, the process would permit the addition of large volume fractions of a ceramic phase, enabling nanostructural design, and production of MMCs with a wide range of mechanical properties, meant especially for high temperature applications. An important and noteworthy feature of the present process, which distinguishes it from other methods, is that all the constituents of the ceramic phase are built into the organic molecules of the precursor (e.g., polysilazanes contain silicon, carbon, and nitrogen); therefore, a reaction between the polymer and the host metal is not required to produce the dispersion of the refractory phase. The polymer precursor powder, with a mean particle size of 31.5 µm, was added equivalent to 5 and 10 weight % of the melt (pure magnesium) by a liquid metal stir-casting technique. SEM and OM microstructural observations show that in the cast structure the pyrolysis products are present in the dendrite boundary region in the form of rod/platelets having a thickness of 100 to 200 nm. After extrusion the particles are broken down into fine particles, having a size that is comparable to the thickness of the platelets, in the 100 to 200 nm range, and are distributed more uniformly. In addition, limited TEM studies revealed the formation of even finer particles of 10-50 nm. X-ray diffraction analysis shows the presence of a small quantity of an intermetallic phase (Mg2Si) in the matrix, which is unintended in this process. There was a significant improvement in mechanical properties of the PIP-MMCs compared to the pure Mg. These composites showed higher macro-and micro-hardness. The composite exhibited better compressive strength at both room temperature and at elevated temperatures. The increase in the density of PIP-composites is less than 1% of Mg. Five weight percent of the precursor produced a two-fold increase in the room-temperature yield strength and reduced the steady state creep rate at 723 K by one to two orders of magnitude. PIP-MMCs showed higher damping capacity and modulus compared to pure Mg, with the damping capacity increasing by about 1.6 times and the dynamic modulus by 11%-16%. PIP-composites showed an increase in the sliding wear resistance by more than 25% compared to pure Mg.

[pt] Os compósitos Cu-Al2O3 possuem excelente resistência a recozimentos em altas temperaturas bem como altas condutividades térmica e elétrica. Uma dispersão nanométrica uniforme de partículas cerâmicas na matriz metálica confere características únicas ao material, possibilitando a sua utilização como, por exemplo, resfriadores ativos. Por outro lado, estas propriedades são essencialmente dependentes da microestrutura do material, que por sua vez, varia de acordo com o método de preparação adotado. Os principais objetivos do presente trabalho são a introdução de um novo método de síntese e a caracterização microestrutural dos nano-compósitos Cu- Al2O3 e Ni- Al2O3. Este método é dividido em dois processos, ambos combinando as características de uma rota química para a preparação de uma mistura em pó de CuO ou NiO e Al2O3, com as vantagens do processamento in-situ de materiais, através da redução preferencial com H2 do CuO ou NiO. No processo 1, o Al2O3 é formado in-situ através da adição de uma solução de Al(NO3) 3 ao pó de CuO ou NiO. No processo 2, tanto o CuO ou NiO como o Al2O3 são formados in-situ a partir de uma solução contendo os nitratos de Cu ou Ni e Al. Os estudos termodinâmicos e cinéticos apresentados mostraram que as reduções do CuO para Cu e do NiO para Ni são viáveis, mesmo em baixas temperaturas (200-400oC). Amostras de Cu- Al2O3 (0,5, 1 e 5% em peso) foram analisadas por difração de Raios-X, microscopia eletrônica de varredura (MEV), e microscopia eletrônica de transmissão (MET) convencional, de alta resolução e de varredura. Os cristais de Cu da matriz variam de 50 a 250/300 nm para o Cu- Al2O3 (5% em peso)-processo 1 e possuem um tamanho médio de 500/600 nm para os compósitos contendo 0,5 e 1% em peso de Al2O3, também preparados pelo processo 1. O diâmetro das partículas de Al2O3 varia de 10 a 60/70 nm. Os nano- compósitos Cu- Al2O3 (0,5, 1 e 5 % em peso)-processo 2 possuem uma microestrutura formada por uma distribuição homogênea de Cu, Al e O. Os nano-compósitos preparados por ambos os processos apresentaram a formação de uma terceira fase, que pode ser CuAlO2 ou CuAl2O4. Nano-compósitos Ni- Al2O3 (0,5% em peso)-processo 2 também foram obtidos com sucesso, apresentando uma microestrutura similar a do Cu- Al2O3. Ligas Cu-Ni também foram obtidas em baixas temperaturas (400oC) através da redução por H2 de uma mistura de CuO-NiO preparada através do processo 2. / [en] Cu-Al2O3 composites are reported to have excellent resistance to high temperature annealing as well as high thermal and electrical conductivities. The uniform dispersion of nanometric ceramic particles in the metallic matrix provides unique characteristics to the material, enabling their application in high temperature and corrosive atmospheres. The special physico-chemical and mechanical properties are essentially dependent on the material`s microstructure, which in turn, will vary according to the composite preparation method. The main objectives of the present work are the introduction of a novel method for the preparation of Cu-Al2O3, Ni-Al2O3 nano- scale composites and their characterization. The preparation method is divided into two processes. In process 1, Al2O3 is formed in-situ by the addition of Al (NO3)3 solution to CuO powder, while in process 2, CuO or NiO and Al2O3 are formed in-situ from a water solution containing the dissolved nitrates of Cu or Ni and Al. Both the processes combine the advantages of chemical routes with that of in-situ processing, through the preferential H2 reduction of the CuO or NiO, contained in the mixture. The thermodynamics and kinetics studies presented have shown that the reductions of CuO to Cu and NiO to Ni are viable at a very low temperature (200-450oC). The Cu-Al2O3 (0.5, 1 and 5 wt%) specimens thus prepared have been examined by X-ray diffraction, scanning electron microscopy (SEM) and conventional, high resolution and scanning transmission electron microscopy (CTEM, HRTEM and STEM). The Cu crystals range from 50 to 300 nm for the Cu-Al2O3 (5 wt%)-process 1 and have an average grain size of 500/600 nm for the Cu-Al2O3 (0,5 and 1 wt%)-process 1, while the Al2O3 particles range from 10 to 60/70 nm in all cases. The Cu- Al2O3 (0.5, 1 and 5 %Peso)-process 2 composites are composed of a homogeneous dispersion of Cu, Al and O. Composites prepared by both the processes, have exhibited the formation of a third phase, which is suggested to be CuAlO2 and/or CuAl2O4. The Ni-Al2O3 (0.5 wt%) nano-scale composites have also been successfully prepared through process 2 and their characterization revealed a microstructure similar to that of the Cu-Al2O3 samples. By applying process 2, it has also been possible to co-form CuO and NiO. This co-formed oxide mixture has been reduced in H2 atmosphere at a low temperature of 400oC to produce a homogeneous nano-powder of a Cu-Ni (50 at%) alloy.

Magister Scientiae - MSc / The main hypothesis in this study is the development of a nanocomposite mediated amperometric biosensor for detection of hydrogen peroxide. The aim is to combine the electrochemical properties of both polyaniline and ferrocenium hexafluorophosphate into highly conductive nano composites capable of exhibiting electrochemistry in non acidic media; shuttling electrons between HRP and GCE for biosensor applications. / South Africa

abstract: Traditionally, for applications that require heat transfer (e.g. heat exchangers),metals have been the go-to material for manufacturers because of their high thermal as well as structural properties. However, metals have some notable drawbacks. They are not corrosion-resistant, offer no freedom of design, have a high cost of production, and sourcing the material itself. Even though polymers on their own don’t show great prospects in the field of thermal applications, their composites perform better than their counterparts. Nanofillers, when added to a polymer matrix not only increase their structural strength but also their thermal performance. This work aims to tackle two of those problems by using the additive manufacturing method, stereolithography to solve the problem of design freedom, and the use of polymer nanocomposite material for corrosion-resistance and increase their overall thermal performance. In this work, three different concentrations of polymer composite materials were studied: 0.25 wt%, 0.5 wt%, and 1wt% for their thermal conductivity. The samples were prepared by magnetically stirring them for a period of 10 to 24 hours depending on their concentrations and then sonicating in an ice bath further for a period of 2 to 3 hours. These samples were then tested for their thermal conductivities using a Hot Disk TPS 2500S. Scanning Electron Microscope (SEM) to study the dispersion of the nanoparticles in the matrix. Different theoretical models were studied and used to compare experimental data to the predicted values of effective thermal conductivity. An increase of 7.9 % in thermal conductivity of the composite material was recorded for just 1 wt% addition of multiwalled carbon nanotubes (MWCNTs). / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2020

No description available.

The many advances of nano technology extensively revolutionize mechanics. A tremendous need is growing to further bridge the gap between the classical mechanics and the nano scale for many applications at different engineering fields. For instance, the themes of interdisciplinary and multidisciplinary topics are getting more and more attention especially when the coherency is needed in diagnosing and treating terminal diseases or overcoming environmental threats. The fact that how mechanical, biomedical and electrical engineering can contribute to diagnosing and treating a tumor per se is both interesting and unveiling the necessity of further investments in these fields. This dissertation presents three different investigations in the area of nano mechanics and nano materials spanning from computational bioengineering to making mechanically more versatile composites. The first part of this dissertation presents a numerical approach to study the effects of the carbon nano tubes (CNTs) on the human body in general and their absorbability into the lipid cell membranes in particular. Single wall carbon nano tubes (SWCNTs) are the elaborate examples of nano materials that departed from mere mechanical applications to the biomedical applications such as drug delivery vehicles. Recently, experimental biology provided detailed insights of the SWCNTs interaction with live organs. However, due to the instrumental and technical limitations, there are still numerous concerns yet to be addressed. In such situation, utilizing numerical simulation is a viable alternative to the experimental practices. From this perspective, this dissertation reports a molecular dynamics (MD) study to provide better insights on the effect of the carbon nano tubes chiralities and aspect ratios on their interaction with a lipid bilayer membrane as well as their reciprocal effects with surface functionalizing. Single walled carbon nano tubes can be utilized to diffuse selectively on the targeted cell via surface functionalizing. Many experimental attempts have smeared polyethylene glycol (PEG) as a biocompatible surfactant to carbon nano tubes. The simulation results indicated that SWCNTs have different time-evolving mechanisms to internalize within the lipid membrane. These mechanisms comprise both penetration and endocytosis. Also, this study revealed effects of length and chirality and surface functionalizing on the penetrability of different nano tubes. The second part of the dissertation introduces a novel in situ method for qualitative and quantitative measurements of the negative stiffness of a single crystal utilizing nano mechanical characterization; nano indentation. The concept of negative stiffness was first introduced by metastable structures and later by materials with negative stiffness when embedded in a stiffer (positive stiffness) matrix. However, this is the first time a direct quantitative method is developed to measure the exact value of the negative stiffness for triglycine sulfate (TGS) crystals. With the advancements in the precise measuring devices and sensors, instrumented nano indentation became a reliable tool for measuring submicron properties of variety of materials ranging from single phase humongous materials to nano composites with heterogeneous microstructures. The developed approach in this chapter of the dissertation outlines how some modifications of the standard nano indentation tests can be utilized to measure the negative stiffness of a ferroelectric material at its Curie temperature. Finally, the last two chapters outline the possible improvements in the mechanical properties of conventional carbon fiber composites by introducing 1D nano fillers to them. Particularly, their viscoelastic and viscoplastic behavior are studied extensively and different modeling techniques are utilized. Conventional structural materials are being replaced with the fiber-reinforced plastics (FRPs) in many different applications such as civil structures or aerospace and car industries. This is mainly due to their high strength to weight ratio and relatively easy fabrication methods. However, these composites did not reach their full potential due to durability limitations. The majorities of these limitations stem from the polymeric matrix or the interface between the matrix and fibers where poor adhesion fails to carry the desired mechanical loadings. Among such failures are the time-induced deformations or delayed failures that can cause fatal disasters if not taken care of properly. Many methodologies are offered so far to improve the FRPs' resistance to this category of time-induced deformations and delayed failures. Several researchers tried to modify the chemical formulation of polymers coming up with stiffer and less viscous matrices. Others tried to modify the adhesion of the fibers to the matrix by adding different chemically functional groups onto the fibers' surface. A third approach tried to modify the fiber to matrix adhesion and at the same time improve the viscous properties of the matrix itself. This can be achieved by growing 1D nano fillers on the fibers so that one side is bonded to the fiber and the other side embedded in the matrix enhancing the matrix with less viscous deformability. It is shown that resistance to creep deformation and stress relaxation of laminated composites improved considerably in the presence of the nano fillers such as multiwall carbon nano tubes (MWCNTs) and zinc oxide nano wires (ZnO- NWs). The constitutive behaviors of these hybrid composites were investigated further through the use of the time temperatures superposition (TTS) principle for the linear viscoelastic behavior and utilizing phenomenological models for the viscoplastic behavior. / Ph. D.

<p>The main hypothesis in this study is the development of a nanocomposite mediated amperometric biosensor for detection of hydrogen peroxide. The aim is to combine the electrochemical properties of both polyaniline and ferrocenium hexafluorophosphate into highly conductive nano composites capable of exhibiting electrochemistry in non acidic media / shuttling electrons between HRP and GCE for biosensor applications.</p>

Die vorliegende Dissertation beschäftigt sich mit Wachstum und Aufbau von Fluorapatit-Gelatine-Nanokompositaggregaten. Diese Aggregate werden im sogenannten Doppeldiffusionsversuch biomimetisch erzeugt und ihre äußere Form bzw. Formentwicklung lässt sich anhand eines fraktalen Modells bis ins Detail nachvollziehen. Sie zeigen einen komplexen inneren Aufbau, in dem die Makromoleküle der organischen Komponente einerseits im Zentrum jeder Nanoeinheit und andererseits zu Strängen, den sogenannten Fibrillen, zusammengelagert am Aufbau der Kompositaggregate beteiligt sind. Im Fall des Kompositkeims ist die innere Architektur in hoher Detailstufe verstanden, auch wenn -- insbesondere bezüglich der späteren Wachstumsphasen -- eine Reihe ungeklärter Fragestellungen verbleibt. Ein zentrales Ergebnis der vorliegenden Arbeit bildet die Entdeckung eines weiteren Wachstumstypen, der im Vergleich zu den bekannten, fraktalen Kompositaggregaten grundsätzliche Unterschiede bezüglich des inneren und äußeren Aufbaus zeigt. Der Grund für die andersartige Formentwicklung liegt in der Versteifung der organischen Komponente durch eine vorangegangene Einlagerung von Calciumionen, wie sowohl experimentell als auch mit atomistischen Computersimulationen gezeigt werden konnte. Aufgrund der hohen Komplexität des Systems ist es bislang allerdings nicht möglich, lokale Ionen-Konzentrationen und pH-Werte vor bzw. während Nukleation und Wachstum der Kompositaggregate im Doppeldiffusionsversuch zu bestimmen. Deshalb wurde ein Ersatzversuch -- der sehr ähnlich strukturierte Aggregate erzeugt, sich aber mit rechnerischen Methoden analysieren lässt -- entworfen und untersucht. Anhand dieser Ergebnisse konnte erstmals die &amp;quot;Geschichte&amp;quot; von Fluorapatit-Gelatine-Nanokompositaggregaten detailliert nachvollzogen werden. Da über die Rolle der Gelatine beim Wachstum der Kompositaggregate nur wenig bekannt ist, wurde eine Reihe von Versuchen durchgeführt, in denen Gelatinen mit verschiedenen Molekülmassenverteilungen eingesetzt wurden. Es stellte sich heraus, dass für selbstorganisiertes und insbesondere fraktales Wachstum der Kompositaggregate lange, möglichst wenig gestörte Makromoleküle von zentraler Wichtigkeit sind. Um die Funktion der organischen Komponente für das Kompositwachstum näher zu untersuchen, wurden Oberflächen von Kompositkeimen mit rasterkraftmikroskopischen Methoden studiert. Durch Säuberung der Oberflächen konnten Austrittsstellen der organischen Komponente durch die Oberfläche der Kompositkeime identifiziert werden. Damit konnte gezeigt werden, dass die organische Komponente aus dem Inneren des Festkörpers teilweise durch die Oberfläche dringt und somit während des Wachstums weit in das Gel hineinreichen sollte. Für die mesoskopische Strukturbildung der Kompositaggregate spielen intrinsische elektrische Felder eine essenzielle Rolle. Deshalb wurde bislang eine Wirkung externer elektrischer Felder auf das Wachstum der Kompositaggregate vermutet. Im Rahmen der vorliegenden Arbeit wurde herausgearbeitet, dass es zwar zu keiner direkten Beeinflussung kommen kann, jedoch in den elektrodennahen Bereichen des Gels eine Ordnung der organischen Moleküle durch externe elektrische Felder zu erwarten ist. Dies könnte eine Wirkung auf wachsende Kompositaggregate zeigen. Da diese Effekte auch aufgrund der elektrischen Felder um die dipolaren Kompositaggregate zu erwarten sind, könnte eine ähnliche Strukturierung der Gelatine in der Nähe der wachsenden Kompositaggregate stattfinden. Insgesamt wurden in dieser Arbeit eine Reihe grundlegender Beiträge zur Erforschung der biomimetisch erzeugten Fluorapatit-Gelatine-Nanokompositaggregate geleistet. Es konnten neue Erkenntnisse zur inneren und äußeren Architektur der Kompositaggregate, zu Mechanismen der Morphogenese und deren wichtigsten Einflussgrößen sowie zum Verständnis der chemisch-physikalischen Vorgänge auf atomarer Größenskala gewonnen werden. Als besonders fruchtbar erwies sich die Verbindung von Experimenten mit theoretischen Untersuchungen, so dass dieser Weg auch in Zukunft grundlegende Erkenntnisse bei der Erforschung der Biomineralisation verspricht und weiterhin verfolgt werden sollte.

<p>The main hypothesis in this study is the development of a nanocomposite mediated amperometric biosensor for detection of hydrogen peroxide. The aim is to combine the electrochemical properties of both polyaniline and ferrocenium hexafluorophosphate into highly conductive nano composites capable of exhibiting electrochemistry in non acidic media / shuttling electrons between HRP and GCE for biosensor applications.</p>

Integrally moulded hinges and tension bands are important features in packaging components for plastic closures and their function is critically dependent on the flow induced micromorphology in the hinge section. Polymer characteristics and processing of the hinge also have an influence on the hinge properties obtained. This study is aimed at obtaining interrelationships between polymer characteristics, in-cavity flow, microstructure development and hinge properties, to produce hinges with enhanced functional properties. Three different virgin polypropylene (PP) grades were investigated (homopolymer PP-H, random copolymer PP-RC and impact copolymer PP-IC) and injection moulding simulation was carried out using Moldflow software. In-cavity data acquisition has been carried out for different sets of injection moulding conditions, using high performance transducers and a data acquisition system. A comparison between Moldflow simulation and practical injection moulding data suggests that, for thin wall injection moulded components the real time pressure data are in close agreement during the injection stage. During the packing stage there is some disagreement between these data, since the thickness of hinge and tension band sections are 0.4 mm and 0.5 mm respectively, suggesting that these dimensions are extending the capability of the software. An extensive study using a design of experiments (DoE) approach was carried out on both practical and predictive data. Injection velocity and melt temperature were the most influential factors on the component mechanical properties. From the optical micrographs it is observed that PP-RC has a finer micro-structure compared to PP-H and PP-IC and some micrographs confirm Moldflow simulation results in which hesitation effects are evident, as the flow converges into the thin hinge and tension band sections. PP-clay nanocomposites (PP-CN) were prepared using a twin screw compounder. Transmission electron microscopy (TEM) has shown some evidence of dispersion and exfoliation of the clay particles in the PP matrix. However, X-ray diffraction (XRD) results show a reduction in inter-layer spacing of PPCN s possibly due to clay compaction. The addition of nano-clay however has not resulted in any significant improvements in the mechanical properties of hinges and tension bands. The high degree of molecular orientation induced in the hinge and tension-band sections appears to mask any improvements attributed to the addition of nano-clay. From the reprocessed and post consumer recyclate (PCR) study conducted on hinges and tension bands, it is seen that with an increase in both the re-processing and PCR content there is a decrease in the component strength of around 14%, giving scope to potentially use PCR in future packaging applications. Investigations conducted on colour pigments (violet and green) reveal that the onset of crystallisation for green pigmented mouldings is considerably higher (16°C) than for natural and violet mouldings. Optical micrographs also reveal a finer microstructural texture for green components, indicating a high nucleating capability of the green pigment. Irrespective of the colour, both for hinges and tension bands, the yield stress values were around twice as high as the values quoted in the manufacturer s data sheet for isotropic PP, due to the high levels of molecular orientation in the hinge and tension band sections. In order to industrially validate the findings from the DoE study, commercial closures were produced in industry on a production tool then characterised. In the case of tension bands, there was a good agreement between the results obtained from lab scale and industrial study due to the relatively simple geometry. For hinges this agreement is not so clear. Finally a comparison of mechanical properties of the 3 PP grades shows that PP-H has a higher yield stress compared to PP-IC and PP-RC and yield stress is significantly higher (yield strain values are lower) than values quoted by the manufacturer. The PhD study has confirmed the process conditions that are able to optimise all the interactive effects to improve functional properties in high volume parts in the packaging industry.