In fabrication of nanoparticle-reinforced polymers, two critical factors need to be taken into account to control properties of the final product; nanoparticle dispersion/distribution in the matrix; and interfacial interactions between nanoparticles and their surrounding matrix. The focus of this thesis was to examine the role of these two factors through experimental methodologies and molecular-level simulations. Carbon nanotubes (CNTs) and vinyl ester (VE) resin were used as nanoparticles and matrix, respectively. In a parametric study, a series of CNT/VE nanocomposites with different CNT dispersion conditions were fabricated using the ultrasonication mixing method. Thermomechanical properties of nanocomposites and quality of CNT dispersion were evaluated. By correlation between nanocomposite behavior and CNT dispersion, a thermomechanical model was suggested; at a certain threshold level of sonication energy, CNT dispersion would be optimal and result in maximum enhancement in properties. This threshold energy level is also related to particle concentration. Sonication above this threshold level, leads to destruction of nanotubes and renders a negative effect on the properties of nanocomposites. In an attempt to examine the interface condition, a novel process was developed to modify CNT surface with polyhedral oligomeric silsesquioxane (POSS). In this process, a chemical reaction was allowed to occur between CNTs and POSS in the presence of an effective catalyst. The functionalized CNTs were characterized using TEM, SEM-EDS, AFM, TGA, FTIR and Raman spectroscopy techniques. Formation of amide bonds between POSS and nanotubes was established and verified. Surface modification of CNTs with POSS resulted in significant improvement in nanotube dispersion. In-depth SEM analysis revealed formation of a 3D network of well-dispersed CNTs with POSS connections to the polymer. In parallel, molecular dynamics simulation of CNT-POSS/VE system showed an effective load transfer from polymer chains to the CNT due to POSS linkages at the interface. The rigid and flexible network of CNTs is found to be responsible for enhancement in elastic modulus, strength, fracture toughness and glass transition temperature (Tg) of the final nanocomposites. / Includes bibliography. / Dissertation (Ph.D.)--Florida Atlantic University, 2016. / FAU Electronic Theses and Dissertations Collection

This thesis presents the results of a study focussed on the elucidation of the mechanisms responsible for determining the structural integrity of carbon coatings on ductile substrates. Through elucidation of these mechanisms, two different coating systems are designed; a multilayered coating and a functionally graded coating. While concentrating upon carbon coatings, the findings of this study are applicable to a broad range of hard coatings on ductile substrates. The thesis concludes with a chapter outlining a brief study of the effects of gold coatings on silicon under contact load at moderate temperatures to complement the major part of the thesis. Carbon coatings with differing mechanical properties were deposited using plasma enhanced chemical vapour deposition (PECVD), filtered assisted deposition (FAD) and magnetron sputtering deposition methods. Combinations of these techniques plus variation of deposition parameters enabled composite multilayered and functionally-graded coatings to also be deposited. Substrates were ductile metals; stainless steel and aluminium. Characterisation of the coating mechanical properties was undertaken using nanoindentation and nano-scratch tests. The same techniques were used to induce fracture within the coatings to allow subsequent analysis of the fracture mechanism. These were ascertained with the assistance of cross-sectional imaging of indents prepared using a focussed ion beam (FIB) mill and transmission electron microscopy (TEM) using specimen preparation techniques also utilising the focussed ion beam mill. A two dimensional axisymmetric finite element model (FEM) was built of the coating systems using the commercial software package, ANSYS. Substrate elastic-plastic properties were ascertained by calibrating load-displacement curves on substrate materials with the finite element model results. Utilising the simulation of spherical indentation, the distribution of stresses and the locations for fracture initiation were ascertained using finite element models. This enabled determination of the influence such factors as substrate mechanical properties, residual stresses in the coatings and importantly the variation of elastic properties of the different coating materials. Based upon the studies of monolithic coatings, simulations were undertaken on multilayer and functionally-graded coatings to optimise design of these coating types. Based on the results of the modelling, multilayered and functionally graded coatings were then deposited and mechanical testing undertaken to confirm the models. Three major crack types were observed to occur as the result of the spherical nanoindentation on the coatings; ring, radial and lateral cracks. Ring cracks were found to initiate from the top surface of the film, usually at some distance from the edge of the spherical contact. Radial cracks usually initiated from the interface between the coating and the substrate directly under the symmetry axis of indentation and propagated outwards in a non symmetrical star-like fashion. Lateral cracks formed either between layers in the multilayer coatings or at the interface with substrate. Ring and radial cracks were found to form upon loading whereas lateral cracks formed upon both loading and unloading depending upon the crack driving mechanism. Pop-in events in the load displacement indentation curve were found to be indicative of the formation of ring cracks, while the formation of the other types of cracks was not signified by pop-ins but rather by variations in the slope of the curve. The substrate yield strength was found to influence the initiation of all crack systems while compressive stresses in the coating were seen to only influence the formation of ring and radial cracks. However, it was also noted that the initiation of one form of crack tended to then hinder the subsequent formation of others. In multilayer coatings, the lateral cracks were suppressed, as opposed to the monolayer coating system, but a ring crack was observed. This drawback in the multilayer system was successfully addressed by the design of a graded coating having the highest Young??s modulus at the middle of the film thickness. In this coating, due to the graded nature of the elastic modulus, the stresses at the deleterious locations (top surface and interface) were guided toward the middle of coating and hence increased the load bearing capabilities. The effect of substrate roughness upon the subsequent surface roughness of the coating and also upon the fracture process of the coating during indentation was also investigated. For the coatings deposited on rough substrates, the radial cracks were observed to form initially and this eventfully delayed the initiation of ring cracks. Also the number of radial cracks observed at the interface was found to be proportional to the distribution of the interfacial asperities. In summary, the study elucidated the fracture mechanisms of monolayer, multilayer and graded carbon coatings on ductile substrates under uniaxial and sliding contact loading. The effects of the yield strength, surface roughness of the substrate, along with the residual stress and elastic modulus of the coatings on the fracture of coatings were investigated. The study utilised finite element modelling to explain the experiments observations and to design coating systems.

The discovery of unique properties of graphene has led to the development of graphene for a variety of applications like integrated circuits, organic electronic devices, supercapacitors, sensors, and composite materials. Fluorination of graphene enables control of its physical, chemical, and electronic properties. Our initial studies demonstrated the viability of sulfur hexafluoride plasmas to fluorinate epitaxial graphene as a safer alternative to the commonly reported techniques of fluorination that include exposures to fluorine and xenon difluoride gas. Formation of carbon-fluorine bonds after SF6 plasma-treatment was confirmed by x-ray photoelectron spectroscopy. Raman spectroscopy and low-energy electron diffraction studies suggest that the framework of sp2-hybridized carbon atoms remains intact after the plasma-treatment. Increase in work function after the fluorination was determined by ultra-violet photoelectron spectroscopy. The findings of our subsequent investigation to controllably modify the work function of epitaxial graphene via plasma-fluorination indicate that the work function of fluorinated epitaxial graphene is controlled by the polarity of carbon-fluorine bonds. Further studies to investigate the effect of the surface topography of epitaxial graphene on the work function of plasma-fluorinated epitaxial graphene were performed using scanning Kelvin probe microscopy (SKPM). The results of SKPM characterization of plasma-fluorinated epitaxial graphene demonstrated that the increase in the work function of epitaxial graphene after plasma-treatment is independent of its surface topography, but non-uniform fluorination may result from non-uniformities in plasma density.

Thesis (Ph.D)--Polymer, Textile and Fiber Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Kumar, Satish; Committee Member: Garmestani, Hamid; Committee Member: Graham, Samuel; Committee Member: Griffin, Anselm; Committee Member: Shofner, Meisha. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Carbon Nanotubes are thought to have tremendous potential as reinforcements for the next generation of composite materials. In the past decade, the enhancing effects of the nanotubes on the mechanical, electrical as well as multi-functional properties of polymer composites have been reported. However, the same nanotubes/polymer composites investigated by different research groups, in many cases, do not show a good agreement with one another. The root cause of this variability is believed to lie in the processing methodology employed to prepare the composites. Before one can propose an ideal and systematic processing condition, it is imperative to have a fundamental understanding of the effect of these nanotubes on the processing of the nanotube-based composites. In this study, the effect of 0.2wt.% functionalized single walled carbon nanotubes on the various thermo mechanical and thermo chemical properties of aerospace grade epoxy was investigated. Namely, the thermal degradation, rheological behavior, cure kinetics as well as the thermal expansion behavior of the epoxy were addressed. In addition, the effect of the application small quantity of nanotube/epoxy composite film on the interlaminar shear strength (ILSS) of a conventional laminated carbon fibre/epoxy prepregs was also investigated. The characterization results show that the presence of the nanotubes has a very significant effect on some of the inherent physical and chemical properties of the epoxy. The presence of these nanotubes leads to a delay in the degradation temperature of the epoxy. The viscosity sees a seven-fold increase at room temperature and the resin also gels at a lower temperature in the presence of the nanotubes. At the same time, the total heat of reaction is also lowered on addition of the nanotubes. The mechanical test, however, shows that the addition of the nanotube/epoxy film does not affect the ILSS of the laminated carbon fibre/epoxy composite. This ILSS value is also found to be dependant on the controlled alignment of the nanotubes and the method of application of the film at the interfaces of the laminates. Finally, it was observed that the nanotubes, when used in such low contents, also had no effect on the thermo mechanical properties of the epoxy. / Les nanotubes de carbone sont considérés comme ayant un potentielénorme pour assurer le rôle de renforts dans la prochaine génération de matériauxcomposites. Dans les décennies précédentes, les effets des nanotubes surl'amélioration des propriétés mécaniques, électriques et multi-fonctionnelles despolymères ont été révélés. Par contre, dans la plupart des cas, les études réaliséespar différents groupes de recherche et portant sur les mêmes composites faits depolymère renforcé de nanotubes ne présentent pas toutes des résultatscomparables. La cause majeure de cette variabilité est la méthodologie utiliséelors du procédé de fabrication de ces composites. Avant que quiconque ne suggèreune méthodologie idéale et systématique, il est impératif de comprendre les basesfondamentales de l'effet des nanotubes sur le procédé de fabrication. Dans cetterecherche, les effets des nanotubes de carbone à paroi simple sur les propriétésthermo mécaniques et thermo chimiques d'une résine époxy de gradeaéronautique ont été investigués. Les caractéristiques étudiées comprennent ladégradation thermique, le comportement rhéologique, la cinétique depolymérisation, ainsi que l'expansion thermique. L'effet de l'application d'unfilm de nanotube/époxy sur la résistance interlaminaire au cisaillement aégalement été étudié avec un préimprégné conventionel de fibre de carbone etd'époxy. Les résultats de caractérisation montrent que les nanotubes ont un effetsignificatif sur certaines propriétés physiques et chimiques inhérentes à l'époxy.La présence des nanotubes crée un délai dans la température de dégradation del'époxy. La viscosité de la résine est 7 fois plus élevée à la température de la pièceet sa température de gélification est inférieure. De plus, la chaleur totale deréaction est diminuée. Par contre, les tests mécaniques montrent que l'applicationd'un film de nanotube/époxy ne cause pas de changement dans la résistanceinterlaminaire au cisaillement d'un préimprégné de fibre de carbone et d'époxy.Par ailleurs, cette valeur de résistance est dépendante de l'alignement desnanotubes et de la méthode d'application du film sur les interfaces du laminé.Finalement, il a été observé que les nanotubes n'ont aucun effet sur les propriétésthermo mécaniques.

The objective of this thesis is to study the effect of process conditions on structure and electrochemical properties of polyacrylonitrile (PAN)/carbon nanotube (CNT) composite film based electrodes developed for electrochemical capacitors. The process parameters like activation temperature, CNT loading in the composite films are varied to determine optimum process conditions for physical (CO2) and chemical (KOH) activation methods. The PAN/CNT precursors are stabilized in air, carbonized in inert atmosphere (argon), and activated by physical (CO2) and chemical (KOH) methods. The physical activation process is carried out by heat treating the carbon precursors in CO2 atmosphere at activation temperatures. In the chemical activation process, stabilized carbon precursors are immersed in aqueous solutions of activating media (KOH), dried, and subsequently heat treated in an inert atmosphere at the activation temperature. The structure and morphology are probed using scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. The specific capacitance, power and energy density of the activated electrodes are evaluated with aqueous electrolytes (KOH) as well as organic electrolyte (ionic liquid in acetonitrile) in Cell Test. The surface area and pore size distribution of the activated composite electrodes are evaluated using nitrogen absorption. Specific capacitance dependence on factors such as surface area and pore size distribution are studied. A maximum specific capacitance of 300 F/g in KOH electrolyte and maximum energy density of 22 wh/kg in ionic liquid has been achieved. BET surface areas in excess of 2500 m2/g with controlled pore sizes in 1 - 5 nm range has been attained in this work.

This thesis presents the results of a study focussed on the elucidation of the mechanisms responsible for determining the structural integrity of carbon coatings on ductile substrates. Through elucidation of these mechanisms, two different coating systems are designed; a multilayered coating and a functionally graded coating. While concentrating upon carbon coatings, the findings of this study are applicable to a broad range of hard coatings on ductile substrates. The thesis concludes with a chapter outlining a brief study of the effects of gold coatings on silicon under contact load at moderate temperatures to complement the major part of the thesis. Carbon coatings with differing mechanical properties were deposited using plasma enhanced chemical vapour deposition (PECVD), filtered assisted deposition (FAD) and magnetron sputtering deposition methods. Combinations of these techniques plus variation of deposition parameters enabled composite multilayered and functionally-graded coatings to also be deposited. Substrates were ductile metals; stainless steel and aluminium. Characterisation of the coating mechanical properties was undertaken using nanoindentation and nano-scratch tests. The same techniques were used to induce fracture within the coatings to allow subsequent analysis of the fracture mechanism. These were ascertained with the assistance of cross-sectional imaging of indents prepared using a focussed ion beam (FIB) mill and transmission electron microscopy (TEM) using specimen preparation techniques also utilising the focussed ion beam mill. A two dimensional axisymmetric finite element model (FEM) was built of the coating systems using the commercial software package, ANSYS. Substrate elastic-plastic properties were ascertained by calibrating load-displacement curves on substrate materials with the finite element model results. Utilising the simulation of spherical indentation, the distribution of stresses and the locations for fracture initiation were ascertained using finite element models. This enabled determination of the influence such factors as substrate mechanical properties, residual stresses in the coatings and importantly the variation of elastic properties of the different coating materials. Based upon the studies of monolithic coatings, simulations were undertaken on multilayer and functionally-graded coatings to optimise design of these coating types. Based on the results of the modelling, multilayered and functionally graded coatings were then deposited and mechanical testing undertaken to confirm the models. Three major crack types were observed to occur as the result of the spherical nanoindentation on the coatings; ring, radial and lateral cracks. Ring cracks were found to initiate from the top surface of the film, usually at some distance from the edge of the spherical contact. Radial cracks usually initiated from the interface between the coating and the substrate directly under the symmetry axis of indentation and propagated outwards in a non symmetrical star-like fashion. Lateral cracks formed either between layers in the multilayer coatings or at the interface with substrate. Ring and radial cracks were found to form upon loading whereas lateral cracks formed upon both loading and unloading depending upon the crack driving mechanism. Pop-in events in the load displacement indentation curve were found to be indicative of the formation of ring cracks, while the formation of the other types of cracks was not signified by pop-ins but rather by variations in the slope of the curve. The substrate yield strength was found to influence the initiation of all crack systems while compressive stresses in the coating were seen to only influence the formation of ring and radial cracks. However, it was also noted that the initiation of one form of crack tended to then hinder the subsequent formation of others. In multilayer coatings, the lateral cracks were suppressed, as opposed to the monolayer coating system, but a ring crack was observed. This drawback in the multilayer system was successfully addressed by the design of a graded coating having the highest Young??s modulus at the middle of the film thickness. In this coating, due to the graded nature of the elastic modulus, the stresses at the deleterious locations (top surface and interface) were guided toward the middle of coating and hence increased the load bearing capabilities. The effect of substrate roughness upon the subsequent surface roughness of the coating and also upon the fracture process of the coating during indentation was also investigated. For the coatings deposited on rough substrates, the radial cracks were observed to form initially and this eventfully delayed the initiation of ring cracks. Also the number of radial cracks observed at the interface was found to be proportional to the distribution of the interfacial asperities. In summary, the study elucidated the fracture mechanisms of monolayer, multilayer and graded carbon coatings on ductile substrates under uniaxial and sliding contact loading. The effects of the yield strength, surface roughness of the substrate, along with the residual stress and elastic modulus of the coatings on the fracture of coatings were investigated. The study utilised finite element modelling to explain the experiments observations and to design coating systems.

Thesis (Ph. D.)--University of Alabama at Birmingham, 2006. / Additional advisors: Andrei V. Stanishevsky, Derrick R. Dean, Gregg M. Janowski, Yogesh K. Vohra. Description based on contents viewed Feb. 11, 2009; title from PDF t.p. Includes bibliographical references (p. 132-151).

Thesis (M.S.) -- Mississippi State University. Department of Mechanical Engineering. / Title from title screen. Includes bibliographical references.

Thesis (M.S.)--Brigham Young University. Dept. of Civil and Environmental Engineering, 2006. / Includes bibliographical references (p. 121-122).