Mechanical Properties of Micro-Tubular SOFCs

A fundamental issue with micro-tubular Solid Oxide Fuel Cells (SOFCs) is improvement of the mechanical strength of the cell. Fabricated using extrusion and co-firing techniques, the approximately 1.7 mm diameter tubes are composed of NiO-Gd-doped ceria (GDC) as an anode (support tube), GDC as an electrolyte and La_0.8Sr_0.2Co_0.6Fe_0.4O₃ (LSCF)-GDC as a cathode. The mechanical properties of SOFCs are analyzed through burst testing, c-ring testing, and micro- and nano-indentation testing; the burst test is an especially important parameter because of improved power efficiency at increased fuel pressures. Results from micro- and nano-indentation tests performed on electrolytecoated Ni-GDC anode pellets indicate that the hardness of GDC is comparable or greater than that of YSZ. To develop a trend for the mechanical behavior of micro-tubes in relation to variations in fabrication techniques, several parameters were varied. The standard tubes, used as a baseline for variations had four key design parameters as follows: they were not reduced, contained 40% pore former, were sintered at 1400 °C and had a wall thickness of approximately 0.64 mm. A variation on each of the four parameters was performed. The four variations were 1) to reduce the standard tube, 2) to increase the percent pore former to 60%, 3) to decrease sintering temperature to 1350 °C, and 4) to decrease the wall thickness to approximately 0.56 mm. An average burst strength of 9.6 ± 0.6 MPa was observed for the standard tubes, 14.6 ± 7.0 MPa for the reduced tubes, 5.4 ± 3.2 MPa for the increased pore former, 12.9 ± 4.1 MPa for the decreased sintering temperature and 11.7 ± 2.4 MPa for the thinner tubes.