In view of the possible application of SiC‐based ceramic laminates as critical components of hydrogen generation plants based on sulfur‐iodine or hybrid‐sulfur thermochemical cycles, the effect of corrosion on this kind of ceramics has been investigated. For this purpose, the material was exposed for 1000 h at 850°C to the corrosive action of a gaseous atmosphere simulating the environment of a sulfur‐based thermochemical hydrogen generation plant (a gaseous mixture containing several aggressive species: O2, H2O, SO2, SO3, and H2SO4, with relative concentrations representative of the inlet of an SO3 decomposer reactor). Specimens of multilayer SiC were prepared according to the following processing path: tape casting of a SiC‐based slurry, building of the green multilayer by stacking 11 sheets, final de‐binding and sintering treatments. The material microstructure and its flexural strength and modulus were studied before and after corrosion to investigate degradation phenomena. The microstructure was investigated by XRD, SEM‐EDS, XPS, and density measurements. Flexural strength and modulus were measured by four‐point bending test. The corrosive atmosphere displayed an oxidizing effect that resulted in the formation of silica, oxycarbides, and gaseous products of carbon oxidation. Nevertheless, the corrosion was hindered by the formation on the sample surface of a passivating silica layer. Bending strength and modulus were found unchanged or even improved after corrosion.
The oxidation resistances of different kinds of SiC‐based laminates are compared. The materials under investigation are produced by tape casting of green ceramic sheets, followed by stacking of the sheets in a multilayer structure and laminate consolidation by de‐binding and sintering. Three kinds of specimens are tested: multilayer SiC with fully dense layers, multilayer SiC integrating porous layers and multilayer composites made by stacking SiC/Cf composite layers. Two kinds of chopped carbon fibres (polyamide coated and uncoated) are used for the manufacture of the composite sheets. The oxidation behaviour is investigated by simultaneous TGA–DTA–MS analysis. Specimens are also submitted to a long‐term oxidation treatment (30 h at 1 600 °C in flowing air) and their microstructure and mechanical behaviour compared before and after oxidation. This assessment shows that the integration of porous or composite layers in the multilayer architecture does not worsen the oxidation resistance. In every case the formation of a surface passivating layer prevents major degradation phenomena, so that only small changes in the mechanical features are found after oxidation.
Silicon carbide multilayer composites containing short carbon fibers (Csf/SiC) were prepared by tape casting and pressureless sintering. The C fibers were dispersed with dispersants into a solvent mixture firstly and then mixed with SiC slurry to make green Csf/SiC tapes. Triton X100 was found to be the best dispersant for Toho Tenax HTC124 fibers. Fibers resulted homogeneously distributed in the tape and tended to align fairly well along the tape casting direction. The addition of short C fibers hindered the shrinkage in the plane containing the fibers during sintering. The resulting microstructure of the composite materials was investigated. This kind of composite layers could be integrated in a thermal protection system (TPS) structure, since the outer dense SiC layers can provide excellent oxidation resistance and good heat conductivity in the plane, while Csf/SiC layers in the middle of the multilayer architecture could grant low thermal conductivity through the TPS thickness.
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