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Elaboration and optimization of carbon nanotube-based polymer composites for electrical energy storage

Abstract : Ever-increasing energy requirement and exhaustion of fossil fuels demands improving efficiency of energy usage as well as seeking sustainable and renewable resources. Energy storage capacitors are devices that could take this responsibility, and have been the focus of increasing attention due to their advantages such as environment friendliness and very fast energy uptake and delivery. As the requirements grow for a low-cost and high-efficiency capacitive storage system, there is great need for the development of materials with high dielectric permittivity. Polymer composite dielectrics are arousing increasing attention due to their large tunability in dielectric performances. Polymer composites filled with ceramic particles have been used in some energy storage capacitors. Still, their applicability for practical devices is severely hindered by the low dielectric permittivity and deteriorated mechanical and processing properties due to the high content of rigid ceramic particles in the flexible polymer matrix. By replacing ceramic particles with conductive particles in the polymer composites, the percolative polymer composites can be made with the dielectric permittivity dramatically increased in the vicinity of the percolation threshold. Among the conductive fillers, carbon nanotubes (CNTs) have been most intensively studied, as their large aspect ratio coupled with high conductivity can lead to percolation levels in composites at much low loading. One of the greatest challenges for CNT usage in composites is to debundle pristine CNTs and realize uniform dispersion into polymers. This thesis focused on increasing the dielectric permittivity of CNT-based polymer composites by both carefully optimizing the dispersion of nanotubes as well as controlling the microstructure of the composites. The dielectric permittivity increment in the previous composite systems originated from the formation of microcapacitors. However, CNTs were always frizzy in the CNT/polymer composites, which was not beneficial in forming parallel pair electrodes of microcapacitors. To overcome this problem, we proposed a microarchitecture of hybrid SiC-CNT as conductive filler. Such micro/nano hybrids were produced by floating catalytic chemical vapor deposition. The organization mode of CNTs on SiC particles could be effectively tuned by adjusting synthesis conditions. The results showed that asymmetric surface properties of 6H-SiC were prone to led to “single-direction” growth of CNTs on SiC particles, while the competition between the substrate nature and the experimental conditions can resulted in a “multi-directions” hybrid structure. Resultant SiC-CNT hybrids were further incorporated into PVDF to prepare percolative composites. It was found that the SiC-CNT hybrid can significantly improve the dielectric permittivity of SiC-CNT/PVDF composite with an extremely low CNT loading. CNTs on each SiC microplate are oriented along an axis and separated by a thin polymer matrix, giving rise to a network of microcapacitors. As a result, a large dielectric permittivity of more than 8700 and 2100 at 100 Hz could be obtained at a low CNT loading of 2.30 vol% and 1.48 vol% in the “multi-directions” and “single-direction” composites respectively.
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Jinkai Yuan. Elaboration and optimization of carbon nanotube-based polymer composites for electrical energy storage. Other. Ecole Centrale Paris, 2012. English. ⟨NNT : 2012ECAP0034⟩. ⟨tel-00784198⟩

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