Creating Dense, Constrained Ce 0.9 Gd 0.1 O 1.95 Films at Low Temperature for SOFC Applications
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1023-JJ05-09
Creating Dense, Constrained Ce0.9Gd0.1O1.95 Films at Low Temperature for SOFC Applications Jason D. Nicholas1,2, and Lutgard C. De Jonghe1,2 1 Materials Science and Engineering Department, University of California at Berkeley, 210 Hearst Memorial Mining Building, Berkeley, CA, 94720 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS 62-203, Berkeley, CA, 94720 ABSTRACT We have investigated the effect of various dopants on the sintering characteristics of Ce0.9Gd0.1O1.95 (CGO) and found that 99% dense electrolyte pellets can be produced at the record low temperature of 800°C (as opposed to the 1400°C typically needed) by sintering Ce0.9Gd0.1O1.95 with as little 3mol% lithium. Our studies indicate that doping the CGO surface with lithium nitrate, as opposed to using alternative lithium salts, produces the largest decrease in sintering temperature. Unlike other dopants that lower the sintering temperature by altering the near grain boundary vacancy concentration, lithium lowers the sintering temperature through the formation of an intergranular liquid phase. This liquid phase allows fully dense, completely constrained CGO films to be produced on inert substrates at temperatures as low as 950°C. INTRODUCTION Due to itís high oxygen ion vacancy conductivity in the relatively low (compared to traditional SOFCs) 500-700∞C temperature range, Ce0.9Gd0.1O1.95 (CGO) is a promising electrolyte material for metal supported solid oxide fuel cells. Unfortunately, the development of metal supported CGO SOFCs has been complicated by the high temperature required to densify pure CGO, >1400°C, and the fact that an invariant metal support during firing produces constraining stresses which result in cracked and/or low density CGO films[1]. Recently, many groups[2-18] have focused on using sintering aids to lower the sintering temperature of CGO. In a previous paper,[19] we suggested that a dopantís ability to lower the CGO sintering temperature could be predicted based on its radius, its charge, and the high temperature vaporization behavior of the its oxide. This analysis identified lithium as one of the dopants most likely to induce low temperature sintering in CGO. The present study examines the best way to chemically introduce lithium into a commercial CGO powder, mentions some of the evidence suggesting lithium nitrate doped CGO densifies via a liquid phase sintering mechanism, and shows that dense constrained lithium doped cerium gadolinium oxide (Li-CGO) films can be produced at low temperature. EXPERIMENTAL Appropriate amounts of soluble lithium salts were weighed, assuming a final composition of Li0.03Ce0.873Gd0.097O1.9065, and dissolved in approximately ~200mL of distilled water using a magnetic stirbar. Appropriate amounts of commercially available, nano-sized, ultra high surface area (7-10m2/g) Ce0.9Gd0.1O1.95 powder (Rhodia) from a single batch of powder were added to the solutions and the water was driven off by stirring under a heat lamp for several hours. The
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