Élaboration et caractérisation de composites hybrides thermoélectriques / Synthesis and characterisation of thermoelectric composites

A cheaper access to space is needed in current times and new technologies need to be developed to reduce the cost of space access to increase productivity. This thesis presents a study on carbon fiber reinforced polymer (CFRP) composites which is an enabling technology for cost reduction in space vehicles. A literature review of the behavior of CFRP composite has been conducted and it was found that the currently used IM7/977 carbon fiber reinforced epoxy composites do not microcrack at a lower number of thermal cycles. Nano-composites and Thermoplastic matrix composites have been found as two promising alternatives for cryogenic applications. With the use of nano sized inclusions in currently used epoxy resins, coefficient of thermal expansion can be reduced while increase in strength and fracture toughness can be achieved. Some thermoplastics were found to have non-linear stress-strain relationships with signs of ductility even at 4.2K. Both of these resin systems show promise in reducing microcracking at cryogenic temperatures.

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The relationships between fabrication variables, microstructure and selected properties of carbon bonded magnesia-graphite refractory composite materials have been investigated. A novel optical microscope method of characterizing the morphology of flake graphites was developed and used to determine distributions of length and thickness and average aspect ratios for the four graphite samples used in the study. The compaction behaviour of magnesia alone and in combination with the flake graphites has been studied in some detail and the microstructures of the products elucidated. It is shown that the amount of magnesia of small particle size plays a significant role in determining the graphite-graphite contact area in the structure. An irreversible volume expansion is observed on firing composites, the magnitude of which can be related to the microstructure and the graphite content. A phenolic resin binder restricts this expansion. It is shown that the carbon binder does not bond to the graphite phase and only weakly, if at all, to the magnesia. Consequently the strengths and moduli are low and show only a small variation with graphite type. The effect of adding graphite to carbon-bonded magnesia is to lower the strength slightly, but increasing the graphite content from 20-30% causes a small increase in strength. Increasing the amount of carbon bond from pitch has little effect on strength at levels of 5-15% whereas over the range 5-13% the resin binder has a more pronounced effect. The most significant factor affecting the strength and modulus of fired composites is the amount of silicon or aluminium, added as oxidation inhibitors, which react to form carbide and nitride phases. Finally, a brief study of slag penetration shows that this can be reduced by decreasing the amount of oxide fines in the composite because of the changes in microstructure that, result.

The ceramic composite system containing Al₂O₃ and SiCw has seen multiple uses depending on its processing method. Both the thermomechanical properties and the electrical properties have piqued the interest for potential commercial applications. These include cutting tool inserts and microwave heating elements. Composite samples made by extrusion were pressureless sintered and characterized as a function of frequency, bias, sample length, and whisker orientation. Dry pressed percolated specimens containing different amounts of sintering aids were hot pressed and spark plasma sintered. The microstructure and electrical properties were compared.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Fiber reinforced composites (FRCs) are increasingly used in dentistry with their tooth-matching mechanical, physical and aesthetic properties. The most commonly used FRC products consist of E- or S-glass fibers and methacrylate-based resin matrices. The commonly used methacrylate monomer bis-phenol A glycidyl methacrylate (bis-GMA) has a very high viscosity and possibly leads to a low degree of conversion (DC). In this PhD study, the much less viscous urethane dimethacrylate (UEDMA) was chosen as a replacement for bis-GMA, diluted by hydroxypropyl methacrylate (HPMA) to build up the resin system. This thesis comprises four studies. In study 1, three study groups of 70.6wt%, 49.0wt% and 27.4wt% UEDMA mixed with HPMA in the resin, all reinforced with unidirectional E-glass fibers (42.0vol%), were light-cured for 40 s. The control group had a resin blend of 70.6wt% bis-GMA and 27.4wt% methyl methacrylate (MMA). The prepared specimens were stored dry for 24 h or in deionized water at 37 ℃ for 1, 3, 6 and 12 months. Water sorption up to 40 d and Vickers hardness together with flexural properties after each storage period were investigated. The results suggested that water storage at 37 ℃ induced certain degradation to the mechanical properties of FRC. The group with the highest UEDMA content (70.6wt%) exhibited the lowest 40 d water sorption, the highest hardness and the highest flexural strength and modulus after most storage periods among the UEDMA-containing experimental groups, while the control group had an even better performance in most aspects tested. From study 2 to study 4, the UEDMA content was raised to 70.6wt%, 80.6wt% and 90.6wt%. The three resins, without fiber reinforcement, were tested for the DC after 40 s light-curing and monomer leaching in a 75% ethanol:water solution. Specimens prepared of these three resins reinforced with unidirectional E-glass fibers were tested for 40 d water sorption and mechanical properties (Vickers hardness, flexural properties) after 24 h dry storage, 1 and 3 month deionized-water storage at 37 ℃ and thermal cycling between 5 ℃ and 55 ℃ for 6,000 times. The control group was the same as in study 1. The experimental resins showed higher DC compared to the control resin, while higher UEDMA content was accompanied with lower DC. The resin with highest UEDMA content (90.6wt% UEDMA + 7.4wt% HPMA) exhibited the lowest monomer leaching. The UEDMA-based experimental groups exhibited higher 40 d water sorption than the bis-GMA-based control group. While the group with lowest UEDMA content (70.6wt% UEDMA + 27.4wt% HPMA) showed lowest water sorption among the experimental groups. The bis-GMA-MMA-based FRC specimens showed a superior performance in most tested mechanical properties than the UEDMA-HPMA-based FRCs, and with a higher UEDMA content the mechanical properties were more promising for dental use. Water storage at 37 ℃ might induce some decrease in the mechanical properties, while thermal cycling influenced the mechanical properties in a more complicated way. There is a high potential that a new FRC with adequate chemical and mechanical properties could be developed. Biocompatibility should, however, be carefully arranged in the near future. / published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy

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