[pt] O aumento do interesse em materiais nanoestruturados, nos anos recentes, tem incentivado o desenvolvimento de materiais compósitos de matriz metálica reforçados com nanotubos de carbono. No presente estudo foi produzido um material nano compósito de matriz de cobre contendo nanotubos de carbono (CNT 2% peso), a partir de síntese por métodos químicos. O procedimento começa pela dissociação do nitrato de cobre na presença de CNT e um tensoactivo aniônico a 250°C e sua posterior redução in-situ com atmosfera de Hidrogênio sobre pressão de 1 atm. a 350°C. A análise por difração de Raios X confirmou a formação de CuO puro no momento da dissociação, assim como de cobre metálico após a redução. A presença dos CNT foi detectada nas duas etapas por essa técnica. Análises por Microscopia Eletrônica de Transmissão (MET)mostram que o tamanho médio de partícula do óxido e de 30nm em quanto que para o material reduzido está na faixa de 150-300nm, apresentando-se boa dispersão dos nanotubos. O material reduzido foi compactado, em forma de pastilhas, por pressão uniaxial a frio sob 25MPa e, posteriormente, por pressão isostática a 150MPa. O material compactado foi sinterizado em atmosfera de Argônio a 650°C por 15 min. Análise por Microscopia Eletrônica de Varredura (MEV) assim como TEM do material sinterizado, mostrou uma distribuição heterogênea de tamanho de grão na faixa de 100nm a 4 μm. Medidas de resistividade elétrica mostram que o compósito apresenta uma resistividade sensivelmente menor a baixa temperatura (2x10(-6) ? .cm) a 83°K que o cobre sem nanotubos (5.9x10(-6) ? .cm). / [en] The increasing interest in nanostructure materials in recent years has provided incentive to develop nanostructure composite materials with metal matrix, reinforced with carbon nanotubes. In the present work, copper matrix nano composite with carbon nanotubos (2% wt) was produced by chemical synthesis method. The procedure begins by the copper nitrate dissociation containing SWCNT and anionic tensoactive agent at 250°C, followed by in-situ reduction at 350°C, under hydrogen atmosphere at pressure of 1atm. CuO and Cu formation was confirmed by X ray diffraction at the moment of dissociation and reduction respectively. CNTs presence was detected at both steps by this characterization method. Transmission Electron Microscopy analysis, estimate particles grain size of 30nm for CuO powder while Cu powder particles were observed to be in the 100-300nm range, showing good dispersion of CNT. Bulk nano-composite pellets of the reduced material were obtained by pre-compactation under uniaxial pressure of 17 MPa followed by issostatic pressure of 150MPa. Sinterizing of the compacted material was carry out at 650°C under Argon atmosphere by 15 min. Scanning Electron Microscopy and Transmission Electron Microscopy analysis of the sinterized material showed an heterogeneous grain size distribution in the 100nm to 4 ìm range. Electric resistivity measures show that the nanocomposite material has lower resistivity at low temperature (2x10(-6) ? .cm) at 83°K than the copper without carbon nanotubes (5.9x10(-6) ? .cm).

Ce travail de thèse a pour objectif d’élaborer de nouveaux revêtements composites en utilisant les nanocristaux de cellulose (NCC) comme additifs biosourcés. Pour pallier au problème d’incompatibilité entre les charges hydrophiles et les matrices hydrophobes, une méthode simple à partir des esters de vinyle a été développée pour fonctionnaliser la surface des NCC. Une étude préliminaire réalisée à partir de l’acétate de vinyle utilisé comme réactif modèle, a d’abord permis d’optimiser les conditions de réaction. Ce protocole expérimental a ensuite été étendu à d’autres esters de vinyle fonctionnels, confirmant le caractère polyvalent de la méthode. Les NCC non modifiés et acétylés ont été dispersés dans des matrices acryliques (latex) ou polyuréthane (réticulable), afin d’étudier leur impact sur les performances mécaniques et barrières des composites. Une amélioration des propriétés mécaniques et barrières à l’oxygène a pu être observée dans certains cas, mais l’acétylation de surface des NCC n’a pas conduit à de meilleures performances. Une solution bicouches constituée d’un film 100 % NCC acétyles recouvert de polymère a alors été envisagé et a d’augmenter fortement les propriétés barrières à l’oxygène des différents matériaux. Certains NCC fonctionnalisés ont également été dispersés dans une matrice polydiméthylsiloxane, potentiellement utilisable comme revêtement protecteur pour l’aérospatial. Une amélioration notable de la stabilité thermique et optique sous irradiations UV dans des conditions géostationnaires a alors été observée. / The objective of this research work consist in the elaboration of novel compositecoatings using cellulose nanocrystals (CNC) as biobased additives. To palliate the problem ofincompatibility between the hydrophilic filler and the hydrophobic matrices, a simple methodbased on vinyl esters was developed to functionalize the CNC surface. A preliminary studyperformed with vinyl acetate selected as model reactant first allowed optimizing the reactionconditions. This experimental protocol was subsequently extended to other functional vinylesters to confirm the versatility of the method. The unmodified and acetylated CNC weredispersed in acrylic polymers (latex) or polyurethane (cross-linked resin) matrices, to studytheir impact on the mechanical and barrier performances of the composites. An improvementof the mechanical and barrier properties could be observed in some cases, but the CNCacetylation did not improve further the performances. A bi-layer approach consisting in afilm of 100 % of acetylated CNC coated with the polymer was then envisaged, and allowedincreasing significantly the oxygen barrier properties of the different resins. Some of thefunctionalized CNC were incorporated into a polydimethylsiloxane matrix, for a potential useas protective aerospace coating. A significant improvement in thermal stability and in opticalstability under UV irradiation in geostationary conditions was then observed.

Carbon fiber reinforced plastic (CFRP) composites have extensive value in the aerospace, defense, sporting goods, and high performance automobile industries. These composites have huge benefits including high strength to weight ratios and the ability to tailor their properties. A significant issue with carbon fiber composites is the potential for catastrophic fatigue failure. To better understand this fatigue, there is first a huge push to measure strain accurately and in-situ to monitor carbon fiber composites. In this paper, piezoresistive nickel nanostrand (NiNs) nanocomposites were embedded in between layers of carbon fiber composite for real time, in situ strain monitoring. Several different embedding methods have been investigated. These include the direct embedding of a patch of dry NiNs and the embedding of NiNs-polymer matrix nanocomposite patches which are insulated from the surrounding carbon fiber. Also, two different polymer matrix materials were used in the nanocomposite to compare the piezoresistive signal. These nanocomposites are shown to display repeatable piezoresistivity, thus becoming a strain sensor capable of accurately measuring strain real time and in-situ. This patch has compatible mechanical properties to existing advanced composites and shows good resolution to small strain. This method of strain sensing in carbon fiber composites is more easily implemented and used than other strain measurement methods including fiber Bragg grating and acoustic emissions. To show that these embedded strain gages can be used in a variety of carbon fiber components, two different applications were also pursued.

The mechanical and thermal properties of natural short latania fiber (SLF)-reinforced poly(propylene)/ethylene-propylene-diene-monomer (SLF/PP/EPDM) bio-composites reinforced with nano-clays (NCs), pistachio shell powders (PSPs), and/or date seed particles (DSPs) were studied using experiments and machine learning (ML) predictions. This dissertation embraces three related investigations: (1) an assessment of maleated polypropylene (MAPP) coupling agent on mechanical and thermal behavior of SLF/PP/EPDM composites, (2) heat deflection temperature (HDT) of bio-nano-composites using experiments and ML predictions, and (3) fracture toughness ML predictions of short fiber, nano- and micro-particle reinforced composites. The first project (Chapter 2) investigates the influence of MAPP on tensile, bending, Charpy impact and HDT of SLF/PP/EPDM composites containing various SLF contents. The second project (Chapter 3) introduces two new bio-powderditives (DSP and PSP) and characterizes the HDT of PP/EPDM composites using experiments and K-Nearest Neighbor Regressor (KNNR) ML predictions. The composites contain various contents of SLF (0, 5, 10, 20, and 30wt%), NCs (0, 1, 3, 5wt%), micro-sized PSPs (0, 1, 3, 5wt%) and micro-sized DSPs (0, 1, 3, 5wt%). The third project (Chapter 4) characterizes the fracture toughness of the same composite series used in the second project, by applying Charpy impact tests, finite element analysis, and a ML approach using the Decision Tree Regressor (DTR) and Adaptive Boosting Regressor (ABR). 2wt% MAPP addition enhanced the composite tensile/flexural moduli and strength up to 9% compared with the composites with zero MAPP. In addition, energy impact absorption was profoundly increased (up to78%) and HDT (up to 4 Co) was improved upon MAPP addition to the composites. SLF, NC, DSP and PSP could separately and conjointly increase HDT and fracture toughness values. The KNNR ML approach could accurately predict the composite’s HDT values and, Decision Tree Regressor (DTR) and Adaptive Boosting Regressor ML algorithms worked well with fracture toughness predictions. Pictures taken through a transmission electron microscope, scanning electron microscope and X-Ray proved the NC dispersion and exfoliation as one of the factors in HDT and fracture toughness improvements.

No description available.

No description available.

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