Ces travaux de thèse ont porté sur l’élaboration d’électrodes déformables pour la récupération d’énergie houlomotrice. En effet, la conversion de l’énergie mécanique des vagues en électricité est possible via un système entièrement souple et basé sur la technologie des polymères électroactifs (ou EAP). Ces matériaux ont la capacité de se déformer sous stimuli électrique, d’où la nécessité de développer des matériaux conducteurs déformables. Le matériau EAP choisi pour l’étude est un élastomère silicone. La formulation de composites à matrice élastomère silicone chargée en particules conductrices carbonées (graphite, nanofeuillets de graphite et nanotubes de carbone) est ainsi la piste suivie pour composer des électrodes déformables. Deux méthodes de mélange, en voie fondu, ont été explorées. La première utilise un mélangeur planétaire, et la seconde utilise en plus un mélangeur tri-cylindre. L’influence sur les propriétés électriques des composites, de la méthode de mélange, de la nature de la charge conductrice ainsi du taux de charges, a été analysée. Aussi, l’étude de la percolation électrique ainsi que l’étude des mécanismes de conduction mis en jeux dans les différents composites ont été réalisées, et complétées par l’observation de la morphologie en microscopie optique et en microscopie électronique. Le comportement mécanique des composites en traction a également été analysé. Enfin, les propriétés couplées électro-mécaniques des composites les plus prometteurs ont été testées. Les mesures permettent de proposer une formulation à base de nanotubes de carbone comme électrode déformable. / This PhD work presents the development of stretchable electrodes for wave energy harvesting. Indeed, it is possible to convert the mechanical energy of the waves into electricity thanks to a flexible system based on electroactive polymer (EAP) technology. As EAPs have the ability to deform under electrical stimuli, deformable conductive materials are needed. In this study, the chosen EAP is a silicone elastomer. Composites formulated with silicone elastomer matrix filled with carbonaceous conductive particles (graphite, graphite nanoplatelets and carbon nanotubes) were thus developed. Two mixing methods, by melt compounding, have been explored. The first uses a planetary mixer, and the second uses a three roll-mill. The influence of the mixing method, the nature of the fillers and the filler rate on the electrical properties of the composites has been analyzed. The morphology, as well as the percolation and the conduction mechanisms have been studied. The tensile properties of the composites were also analyzed. Finally, the electromechanical coupled properties of the most promising composites were tested, allowing us to propose a formulation as a stretchable electrode.

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Conventional microelectronic and power electronic packages based on Si devices usually work below 150°C. The emergence of wide-bandgap devices, which potentially operate above a junction temperature of 250°C, results in growing research interest in high-density and high-temperature packaging. There are high-temperature materials such as encapsulants on the market that are claimed for capability of continuous operation at or above 250°C. With an objective of identifying encapsulants suitable for packaging wide-bandgap devices, some of commercial high-temperature encapsulants were obtained and evaluated at the beginning of this study. The evaluation revealed that silicone elastomers are processable for various types of package structure and exhibit excellent dielectric performance in a wide temperature range (25 - 250°C) but are insufficiently stable against long-term aging (used by some manufacturers, e.g., P²SI, to evaluate polymer stability) at 250°C. These materials cracked during aging, causing their dielectric strength to decrease quickly (as soon as 3 days) and significantly (60 - 70%) to approximately 5 kV/mm, which is below the value required by semiconductor packaging. The results of this evaluation clearly suggested that silicone needs higher thermal stability to reliably encapsulate wide-bandgap devices. Literature survey then investigated possible methods to improve silicone stability. Adding fillers is reported to be effective possibly due to the interaction between filler surface and polymer chains. However, the interaction mechanism is not clearly documented. In this study, the effect of Al₂O₃ filler on thermal stability was first investigated by comparing the performance of unfilled and Al₂O₃-filled silicones in weight-loss measurements and dielectric characterization. All test results on composites filed with Al₂O₃ micro-rods indicated that thermal stability increased with increasing filler loading. Thermogravimetric analysis (TGA) test demonstrated that the temperature of degradation onset increased from 330 to 379°C with a 30 wt% loading of Al₂O₃ rods. In isothermal soak test, unfilled and 30-wt%-filled silicones lost 10% of polymer weight in 700 and 1800 hours, respectively. The dielectric characterization found that both Weibull parameters, characteristic dielectric strength (E₀, representing the electric field at which 62.3% of samples are electrically broken down) and shape parameter (β, representing the spread of data. The larger the β, the narrower the distribution) can reflect the thermal stability of polymers. Both of them were influenced by microstructure evolution, to which β was found to be more sensitive than E₀. The characteristic dielectric strength of unfilled silicone decreased significantly after 240 hours of aging at 250°C, whereas that of Al₂O₃/silicone composites exhibited no significant change within 560 hours. The shape parameter of Al₂O₃-filled silicone decreased slower than that of unfilled silicone, also indicating the positive effect of Al₂O₃ micro-rods on thermal stability. Improved thermal stability can be explained by restrained chain mobility caused by interfacial hydrogen bonds, which are formed between hydroxyl groups on Al₂O₃ surface and silicone backbone. In this study, the effect of hydrogen bonds was investigated by dehydrating Al₂O₃ micro-rods at high temperature in N₂ to partially destroy the bonds. Removal of hydrogen bonds impaired thermal stability by increasing the initial weight-loss rate from 0.025 to 0.036 wt%/hour. The results explained the importance of interfacial hydrogen bond, which effectively reduced the average chain mobility, hindered the formation of degradation products, and led to higher thermal stability. The main discoveries of this study are listed below: 1. Al₂O₃ micro-rods were found to efficiently improve the thermal stability of silicone elastomer used for high-temperature encapsulation. 2. Characteristic dielectric strength and shape parameter obtained from Weibull distribution reflected the change of material microstructure caused by thermal aging. The shape parameter was found to be more sensitive to microscale defects, which were responsible for dielectric breakdown at low electric field. 3. Hydrogen bonds existing at filler/matrix interface were proven to be responsible for the improvement of thermal stability because they effectively restrained the average chain mobility of the silicone matrix. / Ph. D.

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