Design and Fabrication of a Functionally Gradient SOFC Electrode with Controllable Porous Microstructure

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A controllable and gradient porous ceramic structure is desirable for many applications including solid oxide fuel cells (SOFC), ceramic filters or gaseous component separation. There are numerous ways to fabricate porous ceramic structures, including the use of pore formers, particle size control or freeze casting. The process we used was string-coating. Strings with various materials’ coating and coatings on various strings could be a controllable element for assembling a planar or tubular ceramic microstructure. In this research, ceramic microtubes were made using removable templates covered by layers of ceramic materials using sol-gel technology. A low-cost device was designed and built for coating multiple layers of slurry onto templates efficiently. Tested templates included silk (single and triple strands), cotton thread, and angel hair pasta coated with 1, 5, 10, 15, 20, or 25 layers of slurry and then fired with a 5-stage heating cycle up to 1,100°C/1,450°C over a period of 20 hours. Samples were analyzed using a scanning electron microscope (SEM). Average diameters were calculated by measuring 30 diameters from different locations of each tube. The 20× and 25× coatings on the single silk strand, sintered at 1,450°C, resulted in the strongest and most density-uniform microtubes by decreasing the porosity. The single silk strand with 20× coating under 1,450°C firing formed a 25µm diameter hole and an average outer diameter (OD) of 39.90 µm with a standard deviation of 0.97 µm, while the 20× coating under 1,100°C resulted in an average 67.46 µm OD with a standard deviation of 1.14 µm. The 25× coating resulted in a 49.89 µm OD with standard deviation of 0.99 µm when fired up to 1,450°C. The success of this process set a foundation for assembling the individually controllable single microtubes to form ceramic planar or tubular shapes that their microstructure is controllable in terms of composition, pores size, porosity and connectivity.