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Research Papers

Ag–Cu–Ti Braze Materials for Sealing SOFCs

[+] Author and Article Information
Worawarit Kobsiriphat, Scott Barnett

 Northwestern University, 2220 Campus Drive, Evanston, IL 60208

J. Fuel Cell Sci. Technol 5(1), 011002 (Jan 16, 2008) (7 pages) doi:10.1115/1.2784279 History: Received November 09, 2005; Revised July 11, 2006; Published January 16, 2008

Two Ag–Cu–Ti brazing alloy compositions were studied as gas seals for solid oxide fuel cells (SOFCs). Yttria-stabilized zirconia pellets were bonded using Ag–35.25Cu–1.75Ti (Cusil-ABA) or Ag–20.1Cu–1.0Ti (average composition; Ag-Cusil). Both as-brazed filler metals contained metallic Ag and Cu phases, along with continuous titanium oxide layers at the filler metal-zirconia interfaces. The brazed joints were then aged at 700°C for up to 100h in air or while separating H2 and air atmospheres. After annealing in air for 100h at 700°C, degradation in microstructure and hermeticity was more pronounced in Cusil-ABA. However, in H2-air atmosphere, Cusil-ABA performed better than Ag-Cusil. Ag-Cusil aged in H2 air delaminated from zirconia and its microstructure showed large interconnected pores in Ag-rich regions. The results were confirmed by single-cell tests using each filler metal as a gas seal. Observations of the post-test Cusil-ABA microstructure revealed little degradation. These results indicate that Cusil-ABA is better suited than Ag-Cusil as a SOFC gas seal.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 4

Graph of log pressure (Pa) obtained by vacuum pumping a PSZ pellet sealed to a tube using either Cusil-ABA or Ag-Cusil. Aged samples were subjected to 50h at 700°C in air or H2-air atmosphere.

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Figure 1

Schematics showing test geometry for (a) H2-air atmosphere aging experiments and (b) open-circuit voltage tests on PSZ-supported SOFCs (figures not to scale)

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Figure 2

Structure and x-ray maps of PSZ-PSZ joint brazed with (a) Cusil-ABA and (b) Ag-Cusil at 880°C for 30min

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Figure 3

X-ray diffraction patterns obtained from as-brazed samples of (a) Cusil-ABA and (b) Ag-Cusil

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Figure 5

SEM images and corresponding x-ray maps of PSZ-PSZ joint aged in air at 700°C for 100h: (a) Cusil-ABA and (b) Ag-Cusil

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Figure 6

SEM images and x-ray maps of joints aged in H2-air atmosphere for 100h at 700°C: (a) Cusil-ABA and (b) Ag-Cusil

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Figure 7

XRD data obtained from the surface, middle, and bottom parts of Cusil-ABA on PSZ after aging for 100h at 700°C in H2-air atmosphere

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Figure 8

Interconnected pores formed in a region of Ag-Cusil braze layer after aging in H2-air atmosphere for 100h at 700°C

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Figure 9

Open-circuit voltage (Voc) versus time for fuel cell sealed with Cusil-ABA and Ag-Cusil tested at 700°C for a total of 100h

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Figure 10

SEM micrographs of post-cell test samples: (a) Cusil-ABA and (b) Ag-Cusil

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