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Numerous alternative energy sources are being researched for sustainable energy applications, but their overall benefit is still too costly for them to be considered viable. Commonly produced temperature gradients created by the environment, or are man-made, can be converted into useful energy by using thermoelectric materials. Inorganic semiconductors are the most commonly used thermoelectric materials, but have raised concerns due to toxicity issues, rarity of heavy elements used, and high fabrication temperatures. These concerns have led research efforts into electrically conductive polymer composites prepared in ambient conditions from aqueous solutions. By combining polymer latex with carbon nanotubes (CNT), electrical conductivity can resemble metals while thermal conductivity remains similar to polymers. Using different CNT stabilizers for these fully organic composites can tailor the thermoelectric properties and harvest thermal gradients from previously inconceivable places (e.g., body heat converted into a voltage). A semiconducting CNT stabilizer, meso-tetra(4-carboxyphenyl) porphine (TCPP), was used to investigate the influence stabilizers have on composite thermoelectric properties. As TCPP was compared to a similar system containing an insulating stabilizer, sodium deoxycholate (DOC), the multi-walled carbon nanotube (MWNT)-filled composites showed a 5x increase in the Seebeck coefficient (S). TCPP did not have a distinct effect on the electrical conductivity (σ), demonstrating the tailorability of S with this molecule. An intrinsically conductive polymer, poly(3,4-ethylenedioxythiophene) :poly(styrene sulfonate) (PEDOT:PSS), was used to stabilize highly conductive double-walled carbon nanotubes (DWNT) and demonstrate the promise of fully organic composites as thermoelectric materials. This combination of CNT and stabilizer produced metallic electrical conductivity (200,000 S m-1) and power factors (S2σ) within an order of magnitude of commonly used semiconductors (~400 μW m-1 K-2). Electrical conductivity was doubled by stabilizing single-walled carbon nanotubes (SWNT) with PEDOT:PSS in a thin film without the insulating polymer latex. To further demonstrate the tailorability of polymer composites, a dual stabilizer approach using semiconducting and intrinsically conductive stabilizers was used. This approach effectively provided the high electrical conductivity from PEDOT:PSS and the enhanced Seebeck coefficients of TCPP. By using multiple stabilizers for CNTs within the same composite, power factors among the highest reported for fully organic composites are achieved (~500 μW m-1 K-2). These water-based, flexible composites are becoming real competition as their conversion efficiencies, when normalized by density, are similar to commonly used semiconductors.

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An investigation of the effect of metal counterface composition on the tribological performance and behaviour of a PTFE filled polymer composite using surface analysis techniques. / The high specific strength, chemical resistance and processability of polymer composites have made them an attractive alternative to traditional metals and ceramics in many industries. For tribological applications, polymer composites also have the ability to eliminate the need of lubricants and lower maintenance costs. The use of carbon fiber, carbon black and polytetrefluoroethylene (PTFE) are well established in the literature as effective reinforcement agents and solid lubricants respectively but not many studies have explored the tribochemical interactions that occurs during sliding. This study investigates the tribochemical interactions between a polyphenylene ether (PPE) and high impact polystyrene (HIPS) blend based composite and different metal surfaces. Four common metals used in industry were chosen for this study: carbon steel C1018, naval brass 485, Inconel 625 and stainless steel 316. In order to isolate the effect of tribochemical interactions between the polymer composite and counterface metals, consistent pressure and velocity (PV) settings were used for all tests. Frictional forces and temperature data were recorded during testing and the wear rates were determined by weighing samples before and after testing. The polymer sand metal washer surfaces were then examined under scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) for their PTFE surface morphology and transfer film composition, respectively. The surface roughness of both polymer and metal samples were also measured. It was observed that tribological performance of the polymer composite was affected by the composition of the metal counterface, and each metal had a different tendency to operating in a stable and unstable state. The surface morphology of the PTFE phase and the transfer film composition on the metal washers also differed between each polymer-metal system. SEM micrographs reveal agglomeration of PTFE domains on the polymer surface and each system had a different domain size distribution and PTFE surface coverage. The polymer-brass system was found to be the most consistent and give the most stable operations with the highest PTFE coverage on the polymer sample’s surface due to brass’ relatively high reactivity. This was explained by tribochemical reaction that occurs at the interface and the reactivity of each metal alloy. Adhesion must be high in order to enable a thicker and more uniform transfer film to adhere, which provides a smooth asperity-free surface for the polymer to slide against, resulting in a stable and low wear operation. A reactive interface allows the introduction of carboxyl groups on both the surfaces and increase electrostatic adhesion between the polymer transfer film and metal surface. Overall, the reactivity of each metal alloy correlated well with the number of stable tests that each polymer-metal system demonstrated as well as the resulting surface coverage of PTFE. This was taken as evidence of the tribochemical interactions. / Thesis / Master of Applied Science (MASc) / The field of friction, wear and lubrication, also known as tribology, traditionally focuses on metal and ceramic components that have high maintenance and lubrication costs. In recent years, there has been growing interest in plastic composites as a replacement material for tribological applications. The high strength, light weight, chemical resistance and self-lubricating properties of these plastic composites makes them an attractive substitute for metals and ceramics. Understanding the chemical interaction between plastic and metal during sliding is necessary to exploit their properties and performance for specific applications. In this study, a plastic composite based on a blend of polyphenylene ether (PPE) and high impact polystyrene (HIPS) filled with polytetrafluoroethlyene (PTFE), carbon black and carbon fiber was used to investigate the effects of different metals on tribological performance and behaviour. Four common metals used in industry were chosen for this study: carbon steel C1018, naval brass 485, Inconel 625 and stainless steel 316. The tribology tests conclusively showed that friction and wear results differ between polymer-metals systems, with each system displaying a stable and unstable behaviour. Surface analysis revealed that the surface PTFE morphology of the polymer composite and the transfer film composition on the metal washer also differ between metals. Tribochemical reactions and the reactivity of each metal explained the results and behaviour of each polymer-metal system.

The primary goal of this research is to investigate the mechanical reinforcing efficiencies of carbon nanofibers in a thermoset polymer material (vinyl ester), considering the presence of the three-dimensional interphase region between nanofiber and matrix, as well as the waviness of the nanofibers. The elasticity-driven response (buckling) and energy absorption efficiency (crush performance) of the structures made of those composites are investigated. The structural/material optimization problem is solved for both buckling and energy absorption. Due to the nondeterministic nature of the influential parameters (fiber, matrix, and interphase geometric and material properties) on the overall properties of the composite, this study considers the probabilistically distributed random variables associated with the material constituents. The uncertainties associated with the material constituents are propagated to the overall properties of the composite material as well as the performance of composite structures made of such nanocomposites. Finally, the design optimization of a composite structure under uncertainty of material constituents is performed for both buckling and energy absorption as structural performance.

<P> Responsive polymer composites were developed by incorporating a functional component into a nonpolar amorphous polymer. The response of the polymer composite is the change in color observed upon exposing the composite to different acids. One application could be a device to monitor the diffusion of different acids in different polymers. </p> <p> The research contained within this thesis deals with an investigation of basic properties of polymer composites. This was accomplished, first through the preparation of a composite of phenolphthalein disodium and Linear Low Density Polyethylene (LLDPE), second, by monitoring the decolorizing process and the aspects that affect it. The investigations included the extrusion parameters, types of acid, acid concentration and indicator concentration, and lastly by quantifying the process through the comparison of empricial diffusion coefficients and corresponding diffusion rates. </p> <p> This study has shown that decolorization occurs at a fast pace in the presence of acetic acid and slow in the presence of hydrochloric acid. Further as the indicator concentration increases, the decolorization process becomes slower. Techniques used to monitor the properties were Scanning Electron Microscopy (SEM) micrographs of freeze fractured composites, Differential Scanning Calorimetry (DSC) scans for the starting materials as well as the composites, and photography of the cross-sections of sample composite cylinders. </p> / Thesis / Master of Science (MSc)

Carbon fiber composite cryogenic fuel tanks are very attractive to the aerospace industry. More information is needed on micro-cracking and how different composite formulations perform at cryogenic temperatures. In this study a cryogenic bulge test fixture was developed to rapidly screen small scale composite samples that are easily formulated in the laboratory. The design goal was to develop a simple fixture that induced thermal and mechanical strains in the same fixture. The pressure decay rate of helium gas through the composite sample after cryogenic operation gives a measure of the amount of micro-cracking induced. Uncertainty analysis techniques were employed to determine the resolution of the pressure decay determined from the bulge test.

Carbon/epoxy laminates are receiving greater attention by the infrastructure, marine, and offshore oil industries due to the need for superior performance capabilities. Such applications generally involve subjecting materials to harsh temperature and moisture conditions. The objective of this study was to provide a greater understanding of how temperature and moisture affect the strength and fatigue behavior of carbon/epoxy composites and the issues involved in modeling these effects. Results from thermal analysis and quasi-static testing on neat resin and unidirectional laminates as a function of temperature and moisture are presented which provide insight into how material properties vary with temperature and moisture and form the inputs necessary to evaluate composite strength and damage models. Fatigue life and damage accumulation testing results provide further insight into the effects of temperature and moisture and also provide a means for model validation. Generally, composite strength was found to be compromised by temperature but enhanced with moisture, while fatigue life was reduced by both temperature and moisture. Crack density with fatigue cycles was found to decrease with temperature but increase for immersed fatigue. Testing also revealed discrepancies between the edge replication and radiography methods for determining crack density. The analytical phase of the work considered a composite strength model and a damage evolution model to predict crack density. The composite strength model was found to provide an accurate dry, room temperature prediction which could be extended to an accurate prediction of wet specimen strength, but the results at elevated temperature fell conservative. The validation of the damage model proved inconclusive as it was found that the results are very sensitive to quantities such as thermal residual stresses and first ply failure. Currently, no reliable methods are available in the literature to determine these values accurately. However, the model was able to predict the decrease in crack density at elevated temperatures. The increase in crack density for immersed fatigue was not predicted. Overall, the study revealed that a more basic understanding of "in-situ" ply properties are needed before one can consider the use of predictive models in practical applications, especially in varying environments. / Ph. D.

Vacuum-Assisted Resin Transfer Molding (VARTM) processes are becoming promising technologies in the manufacturing of primary composite structures in the aircraft industry as well as infrastructure. A great deal of work still needs to be done on efforts to reduce the costly trial-and-error methods of VARTM processing that are currently in practice today. A computer simulation model of the VARTM process would provide a cost-effective tool in the manufacturing of composites utilizing this technique. Therefore, the objective of this research was to modify an existing three-dimensional, Resin Film Infusion (RFI)/Resin Transfer Molding (RTM) model to include VARTM simulation capabilities and to verify this model with the fabrication of aircraft structural composites. An additional objective was to use the VARTM model as a process analysis tool, where this tool would enable the user to configure the best process for manufacturing quality composites. Experimental verification of the model was performed by processing several flat composite panels. The parameters verified included flow front patterns and infiltration times. The flow front patterns were determined to be qualitatively accurate, while the simulated infiltration times over predicted experimental times by 8 to 10%. Capillary and gravitational forces were incorporated into the existing RFI/RTM model in order to simulate VARTM processing physics more accurately. The theoretical capillary pressure showed the capability to reduce the simulated infiltration times by as great as 6%. The gravity, on the other hand, was found to be negligible for all cases. Finally, the VARTM model was used as a process analysis tool. This enabled the user to determine such important process constraints as the location and type of injection ports and the permeability and location of the high-permeable media. A process for a three-stiffener composite panel was proposed. This configuration evolved from the variation of the process constraints in the modeling of several different composite panels. The configuration was proposed by considering such factors as: infiltration time, the number of vacuum ports, and possible areas of void entrapment. / Ph. D.

O desenvolvimento da sociedade humana somente atingiu o estágio atual devido à utilização de materiais compósitos de diferentes misturas, que vem desempenhando um papel fundamental no desenvolvimento da sociedade moderna. Com a finalidade de explorar o crescente interesse nas pesquisas de novos materiais, este trabalho apresenta um estudo sobre as propriedades do compósito elastomérico a base de poli(éter-siloxano) e sólido particulado cerâmico de titanato de bário (3 BaTiO). Amostras com quantidades adequadas dos componentes, matriz polimérica não curada e particulada de titanato de bário, foram curadas em diferentes concentrações (puro, 10, 20, 30, 40 e 50 %) por meio de mistura em massa em uma câmara glove box em ambiente de nitrogênio, até obtenção de mistura homogênea. O copolímero puro e uma série de compósitos com diferentes concentrações foram submetidos a ensaios de inchamento, análises térmicas gravimétricas, calorimetria diferencial exploratória, caracterização morfológica por microscopia eletrônica de varredura, ensaios mecânicos de tração e medidas elétricas. Por meio dessa série de técnicas de caracterização foram observadas propriedades bem distintas entre o polímero puro e os compósitos, caracterizado pelo aumento das ligações cruzadas. Neste trabalho foi também construído um capacitor de placas paralelas para teste de capacitância elétrica dos materiais preparados, visando o estudo do comportamento elétrico dos materiais em estudo. / The present development of the human society should be credited to the use of the composite and blend materials. In this dissertation composites of an elastomer, poly(ether-siloxane), along with barium titanate are prepared and their properties are studied. Nanoparticles of barium titanate were incorporated in the elastomer in the range 0-50 wt% through bulk mixing under nitrogen atmosphere in a glovebox, using a porcelain mortar and pestle. The mixtures thus prepared were placed in molds and allowed to cure in order to prepare specimens for properties analyses. The composites were submitted to swelling tests, thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), mechanical strength tension test, electrical measurements and electron scanning microscopy (SEM). It was possible to make a correlation between the observed properties and the morphology and composition of the composites. Additionally, a parallel plates capacitor for injection and test of small samples was built and used to evaluate the electrical capacitance of the composites prepared in this work.