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Densification of Mn-Doped YSZ for the Solid Oxide Membrane Process and its Surface Corrosion under Fluoride Salt
J. Kanghee, K. Buyoung, R. Jiseung, K. Hyeong-Tae, L. Heesoo
Abstract
The densification-induced changes in the crystal structure and microstructure of x mol% Mn-doped 8YSZ (xMn-YSZ, x = 0, 0.5, 1.0, or 2.0) were investigated, and the mechanical and electrical properties of the samples were determined to evaluate their chemical degradation resulting from chemical stress loading. The xMn-YSZ samples synthesized with a solid-state reaction method possessed cubic crystal structures, similar to 8YSZ. The 0.5Mn-YSZ (111) peak shifted to a lower angle compared to that of YSZ. As the content of Mn increased, the xMn-YSZ peak gradually shifted to higher angles. The peak shift was attributed to lattice shrinkage (after expansion) with increasing Mn content. The grain size and porosity of YSZ were 9.07 μm and 1.21 %, respectively, and those of 0.5Mn-YSZ were 21.90 μm and 0.63 % respectively, revealing that Mn doping led to densification. The hardness increased to 2065.92 Hv (2.0Mn-YSZ) with increasing Mn content, similar to density. Because of the complex effects of densification and lattice distortion, the polarization resistance decreased to 48 % of YSZ for 0.5Mn-YSZ; however, the resistance gradually increased with increasing Mn content. After NaF-CaF2 was loaded as a chemical stress, the cubic and tetragonal phases destabilized and the porosity increased to 2.80 % and 5.44 % for 1.0Mn-YSZ and 2.0Mn-YSZ, respectively. On the other hand, 0.5Mn-YSZ without the tetragonal phase showed a porosity of 2.25 %, indicating that a Mn doping content of > 2.0 mol% worsened YSZ corrosion.
Keywords
Mn-doped YSZ, sinterability, polarization resistance, fluoride salt, surface corrosion, destabilization
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