| Summary: | 碩士 === 國立交通大學 === 影像與生醫光電研究所 === 100 === Two types of sliver-based interlayers, Ag-Cu-Pd and Ag-Cu-Ti, were selected to join yttria-stabilized zirconia (YSZ) and stainless steel for solid oxide fuel cells (SOFCs). The microstructural characterization on the interfaces of joints was performed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), each equipped with an energy dispersive spectroscope (EDS). The bonding strength and hermeticity of steel/metallic interlayers/YSZ joints were also evaluated. Microstructures and mechanical properties of joints were correlated to evaluate which metallic interlayers are suitable for SOFC.
As for the silver-based interlayers, two 58Ag–32Cu–10Pd and 65Ag–20Cu–15Pd interlayers were chosen to join steel and YSZ. A Cu-rich phase was formed in the vicinity of the YSZ with a significant amount of Zr and minor amounts of Fe and Cr in solid solution. The Cu3Pd ordered phase with L12 crystal structure was formed within the interlayer matrix near the steel. In the steel/58Ag–32Cu–10Pd interlayer/YSZ joint, Ag particles were precipitated in the Cu3Pd phase. However, a α-Fe(Cr) needle-like phase existed within the Cu3Pd phase in the steel/65Ag–20Cu–15Pd interlayer/YSZ joint. At the interlayer/YSZ interfaces of both joints, SiO2 (impurity in YSZ) and Ti3O5 (from the reaction of YSZ with Ti impurities in the steel) were observed.
In the steel/68.8Ag–26.7Cu–4.5Ti interlayer/YSZ joint, the Fe from the steel reacted with Ti of the interlayer to form an Fe2Ti phase. In the 68.8Ag–26.7Cu–4.5Ti interlayer/ YSZ interface, two reaction layers of Ti2O3 and Fe2Ti4O were observed in sequence from the YSZ to the interlayer. While Titanium was very active and reacted with oxygen from YSZ to form titanium oxide (Ti2O3), Fe diffused toward the YSZ to react with titanium oxide to form the Fe2Ti4O layer.
The hermeticity of the joints was measured using He leakage tests. The leak rates of steel/65Ag–20Cu–15Pd interlayer/YSZ joint and steel/68.8Ag–26.7Cu–4.5Ti interlayer/YSZ joint wewe 2.3±3.2 ?e10-7 and 1.5±1.1?e10-9 mbar•l/s, respectively, indicating that the Ti additive in the silver-based interlayer could improve hermeticity.
The interfacial bonding strength of the steel/metallic interlayers/YSZ joint was evaluated according to the “ISO 13124 Test method”. The tensile and shear strength of the steel/65Ag–20Cu–15Pd interlayer/YSZ joint were measured as 12.1±1.9 and 36.2±10.4 MPa, respectively. For the steel/68.8Ag–26.7Cu–4.5Ti interlayer/ YSZ joint, the tensile and shear strengths were 16.7±4.8 and 40.2±8.2 MPa, respectively. It was believed that the formation of Ti2O3 and Fe2Ti4O reaction layers in the 68.8Ag–26.7Cu–4.5Ti interlayer/YSZ interface could increase the bonding strength.
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