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Élaboration et caractérisation fine de pérovskites thermochromes

Abstract : An efficient solar absorber should possess both high solar absorption (> 90%) in the wavelength range of visible (0.4 < λ <0.8 μm) and near infrared (0.8 < λ <2.5 μm) and low infrared emissivity (<10%) in the wavelength range between 6 and 10 μm. At the same time, it is possible to regulate the temperature inside the collector using thermochromic materials, able to reversibly change their optical properties as a function of temperature owing to metal-insulator transition that occurs at a specific temperature (TMI). Previous works have highlighted the suppression of overheating when using a selective absorber coating based on VO2. However, the transition temperature of this material (68 °C) is too low resulting in poor performance of the collector during normalization tests. This brings us to study different systems with optical behavior similar to that of VO2, but the thermochromic effect in which would be initiated at a TMI close to 100 °C. For this work we considered four rare-earth perovskite systems LaCoO3, LaNiO3-δ, NdNiO3-δ and SmNiO3-δ. Thin films of these materials were synthesized by magnetron sputtering followed by annealing in air during several minutes. Their physical properties were then analysed using different techniques such as SEM, XRD, TEM, FTIR, and IR camera. First, the study of the cobaltite LaCoO3 demonstrated a strong dependence of the optical properties of the material on the post-annealed thickness of the film; significant emissivity variations (Δε maximum of 64%) were achieved. We also demonstrated the heterogeneity of La and Co stoichiometric distribution for the manufacturing process on a semi-industrial scale, which requires homogenization by diffusion of the chemical elements during the crystallization annealing. The second part of this study was devoted to rare-earth nickelates. Indeed, LaCoO3 demonstrates a higher emissivity variation than VO2 we have not succeeded in lowering its TMI. Therefore, a nickelate is a good candidate for the considered application since TMI for materials of this family can be adjusted to 100 °C. However, this type of materials require stabilization of nickel in its + III oxidation degree resulting in oxygen deficient structures of the ReNiO3-δ type. After optimization of the experimental conditions, XPS and RBS studies allowed to evaluate the chemical composition and the degree of oxidation for thin films of NdNiO3-δ et SmNiO3-δ.
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Daria Kharkhan. Élaboration et caractérisation fine de pérovskites thermochromes. Science des matériaux [cond-mat.mtrl-sci]. Université de Lorraine, 2019. Français. ⟨NNT : 2019LORR0280⟩. ⟨tel-02746663⟩

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