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Matériaux super-isolants thermiques à propriétés thermoélectriques intégrées

Abstract : In the search of new sustainable energies, the issue of energy harvesting is essential. Heat loss is involved in most of the industrial processes, thus thermoelectricity has its full role to play in this search through the Seebeck effect which consists in converting a temperature gradient into an electrical current. A good thermoelectric material requires a high electrical conductivity σ and Seebeck coefficient α and a low thermal conductivity λ. However, despite recent advances in the field, the use of conventional thermoelectric materials on a large scale becomes difficult due to their toxicity, low abundance and high cost. The development of new materials that respect environmental considerations has thus become necessary. Hence, with the emergence of a new family of materials, namely organic thermoelectric materials, based on conductive polymers and gels (aerogels/xerogels), new perspectives are now possible. In the frame of these new advances, the aim of this work is to functionalize thermal super-insulating materials with a very low thermal conductivity by adding thermoelectric properties. This was first done by numerical simulations based on density functional theory (DFT) and classical molecular dynamics (MD), via different modules included in the Materials Studio software. This allowed us to numerically represent and validate the structure of our thermal insulating material, the Resorcinol/Formaldehyde (RF) xerogel. A doping process with 5 % in iron particles was then performed using MD calculations in order to evaluate the dispersion of the charges within the RF network and to represent numerically the doped material for a future study of its thermoelectric properties via a Boltzmann formalism.In a second step, the objective was to identify the optimal synthesis protocol as a function of the different synthesis parameters and the different conductive dopants. The study of the influence of a thermal treatment by pyrolysis then allowed the improvement of the electrical conductivity of the pure material having a very low figure of merit ZT=2.7×〖10〗^(-16), (ZT=α^2 σT/λ is a measure of the efficiency of the thermoelectric conversion). A study of doping was then carried out during the gelling process according to different loading rates in order to reach a percolation threshold. A figure of merit ZT=2.4×〖10〗^(-3) was then obtained with a doping level of 60 % in graphene oxide (GO). However, this type of dopant generates a very high synthesis cost, which explain why we investigated other types of charges, namely electrically conductive fibers. In that case, we obtained a ZT= 8.0×〖10〗^(-4) with a doping level of 10 % in oxidized polyacrylonitrile fibers (PANOX). The assembly of the module and the realization of a test bench have made it possible to characterize the thermoelectric performance of our different materials. A power density of the order of 2 mW.m-2 was then obtained with the PANOX fiber-reinforced RF xerogel with a thickness of 1 cm and an surface area of 50 cm² for a temperature difference of 30°C. Thanks to this materials, we have identified an application as part of the thermal insulation of a hybrid vehicle battery in order to detect a failure associated with a vacuum loss. Finally, a study based on theoretical models has shown the interest of continuing research activities in order to improve the thermoelectric properties. We then considered the assembly of modules composed of 1000 junctions (pp) then (np) with target materials in order to reach higher power density levels of several W.m-2 and output voltages of several V to produce enough energy for the supply of auxiliaries such as sensors for example.
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Jérémy Guazzagaloppa. Matériaux super-isolants thermiques à propriétés thermoélectriques intégrées. Autre. Université Montpellier, 2019. Français. ⟨NNT : 2019MONTS086⟩. ⟨tel-02498427⟩

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