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RESEARCH PRODUCT

Study of low-pressure suspension plasma spray nanostructured coating : structural characteristics and application in solid oxide fuel cell

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Suspension plasma spraying (SPS) has attracted more and more attention in terms of the preparation of nanostructured / sub-microstructured ceramic coatings. However, conventional SPS techniques are conducted under atmospheric pressure, which inevitably causes some disadvantages. Recently, a novel suspension spraying technology – low-pressure suspension plasma spraying (LPSPS) – was proposed, in which the sus-pension spraying process is conducted under low environmental pressure. Benefit from the significant impact of low pressure on the coating deposition, LPSPS is expected to improve the disadvantage of SPS as well as to obtain distinct coating structures not achievable in conventional SPS coatings. Currently, the reported LPSPS coatings commonly have a much denser structure compared to the SPS coatings. However, their adhesion and mechanical strength are quite low. More importantly, the structural characteristics of the LPSPS coating are still not fully understood. Its practical applications, especially the application in the electrolyte of SOFC, have not been fully studied and verified as well.In this thesis, the authors further developed and studied LPSPS on the basis of limited references. We first investigated the effect of environmental pressure on the microstructure of the yttria-stabilised zirconia (YSZ) coating prepared by suspension plasma spraying. The results revealed that as the pressure decreased, the coating was transformed from a column-like structure to a vertical crack structure; a dramatic de-crease in surface roughness was also observed. The size of the particle in the coating was significantly decreased as the pressure decreased. More nanoparticles were formed in the coating prepared under lower pressure. The required pressure for LPSPS cannot be too low; otherwise, the powders would be separated from the plasma jet, dramatically decreasing the quality of the coating. The optimal pressure for LPSPS in this thesis was 200 mbar.Next, the microstructures of the LPSPS YSZ coating were comprehensively tailored by varying the spraying conditions. These spraying conditions included environmental atmosphere (argon versus air), oxygen content in the environment, spray distance, suspension solid content and solvent type. Out of them, the environment atmosphere played the most critical role in the microstructure of the coating. The argon atmosphere was not suitable for LPSPS. The evolution modes of suspension in different atmospheres (air versus argon) were proposed. Finally, a high-density YSZ coating and a vertical crack structured YSZ coating were successfully tailored by adjusting the preparation parameters.In the last part of the thesis, we attempted to deposit YSZ electrolyte and lanthanum silicate electrolyte by using the developed LPSPS processes in order to examine the application of this technology in the solid oxides fuel cell (SOFC). The first results showed that the density of the lanthanum silicate electrolyte was higher than that of the YSZ electrolyte and that the gas permeability of the former was lower than that of the latter. Both electrolytes exhibited a low gas permeability in the order of magnitude of 10-17 m2, even in a rather low thickness (30~50 μm). Moreover, they all exhibited a high crystallinity, and no amorphous phase was formed in the lanthanum silicate electrolyte. These results indicate that LPSPS has a huge potential in the preparation of the electrolyte of SOFC.