Résumé
INTRODUCTIONConcentrating solar power (CSP) is a promising and sustainable technology to improve the efficiency of the solar-to-electricity conversion. In this system, the solar absorber plays a key role in the global CSP system performance. As the efficiency of solar thermal power plants increases with increasing working temperature, the absorber must be able to maintain good mechanical, physical and chemical properties under severe operating conditions (~1000°C).Silicon carbide (SiC) is the material commonly used for this application because of its high resistance to oxidation and its good sunlight absorbance, and despite its high emissivity, leading to a poor spectral selectivity. Transition metal carbides of column IV and notably TiC possess low emissivity compared to SiC1. However, their major limitation is their low resistance to oxidation2. To improve the spectral selectivity of SiC and the oxidation resistance of TiC, we propose to associate them in a nanocomposite structure.EXPERIMENTAL/THEORETICAL STUDYThe nanocomposites were prepared using a semi-molecular route3 involving hydrolysis and polycondensation of alkoxides (TEOS and TTIP) as metal oxide precursors and sucrose as carbon source. The resulting powders were subjected to two heat treatments to decompose the sucrose into carbon (800°C) and then for the carbothermal reduction of oxides into carbide (1550°C). The reactant proportions were adapted to obtain nanocomposites having the following molar fraction of TiC: 0, 10, 20, 30, 50 and 100. The synthesized products were thoroughly characterized by several techniques (Thermogravimetric Analyses, X-Ray Diffraction, Carbon Analyses and SEM-EDX). The final powders were densified by Spark Plasma Sintering (SPS) and the resulting pellets were polished to optical grade. Then, the optical properties were evaluated by measuring the hemispherical reflectance in the 0.24-15 µm wavelengths range. The absorbance () and emissivity () were calculated in the 0.24-2.5 µm and 2.5-15 µm wavelengths ranges respectively. The spectral selectivity of the material was evaluated by the ratio (α/ε). RESULTS AND DISCUSSIONTiC-SiC nanocomposites powders were successfully synthesized. XRD analyses showed the diffraction peaks characteristic of these two phases. Carbon contents were close to the ones expected for stoichiometric compounds (± 3%). SEM observations showed TiC and SiC particles homogeneously mixed with an average size in the order of a hundred nanometers (Fig. 1-a). Both types of particles had the same global morphology. SPS sintering lead to pellets with of a density ranging from 74% to 100%. The hemispherical reflectance increased with the proportion of TiC in the whole analyzed wavelengths range (Fig. 1-b) leading to a decrease of emissivity. The emissivity decrease represents a real advantage since it induces a reduction of the optical losses. Despite a small decrease of the absorbance, the spectral selectivity (α/ε) increases with the increase of the TiC proportion. CONCLUSIONIn this study we have successfully synthesized TiC-SiC nanocomposites with spectral selectivity. The increase of the TiC content globally increased the efficiency of the material despite a small decrease of its absorbance. The remaining task is now to determine whether if these materials are resistant enough to oxidation and how their absorbance can be improved. REFERENCES1. M. Coulibaly et al, Sol. Energ. Mat. Sol. C. 143, (2015) 2. A. Onuma et al, Solid State Ion. 149, (2004)3. M. Coulibaly PhD University of Montpellier (2015)ACKNOWLEDGMENTSThis work was supported by the RBPCH project from CEA and by the Carapass ANR from the CNRS. We would like to thank Cyrielle Rey, Bruno Corso, Johann Ravaux and Renaud Podor for assistance with the TGA, XRD and SEM analyses respectively