17

cra on Tt must be investigate further. The intrinsic stresses of both kinds of films are compressive, especially, the magnitude of cm in La^Sr^MnQ, films is bigger than a^ in it. These compressive intrinsic stresses prevent the delamination of films from the low TEC substrates such as A1203 after depositing For adopting these films for SOFC, It will be necessary to investigate the behavior of films in operating condition further.

Conclusion

La, jSr.CrOj and LaMSrxMn03films were fabricated by RFsputtering method. From the measurement of residual stresses in the films, the elastic moduli, intrinsic stresses and TECs of the both films were estimated. It was clarified that compressive stresses are induced in the films and this stresses prevent the delamination of films from the low TEC substrates.

Reference

1. T.Kato, A.Momma, S.Nagata, Y.Kasuga, "Residual stress analysis of SOFC," Proc. 2nd Int. Fuel Cell Conf., pp461-p464, Feb. 1996.

2. P.Jin and S.Maruno, "Evaluation of Internal Stress in Reactively Sputter-Deposited ZrN Thin Films," Japan. J. Applied Phys., Vol.30, No.7, ppl463-1468,1991.

3. T.Hanabusa, K.Kusaka, K.Tominaga and H.Fujiwara, "Dependence of Substrate and Temperature on Residual Stress in A1N Films Deposited by sputtering," J. Soc. Mat Sei., Japan, Vol.42, No477, pp627-p633,1993.

4. N.Sugiura, S.Otoshi, A.Kajimura, M.Suzuki, H.Ohnishi,H.Sasaki and M.Ippommatsu, "Thermal Expansion Properties of A-Site Deficient La(Sr)Cr03 " J. Ceram. Soc. Japan, VollOl, No7, pp769-772, 1993. .

5. S.Srilomsak, D.P.Schilling and H.U.Anderson, "Thermal Expansion Studies on Cathode and Interconnect Oxides," Prop. 1st Int. Symp. SOFC, ppl29-140,1989.

6. J.Mizusaki, H.Nambu, C.Nakao, H.Tagawa,H.Minamiue and T.Hashimoto, "Relationship of Crystal Structure, Lattice Parameters and Thermal Expansion to Nonstoichiometry in La^MnOj^ and La, jSr^MnO^ Determined by High-temperature X-Ray Diffraction under Controlled Oxygen Partial Pressures," Proc. 4th Int. Symp. SOFC, pp444-453, June, 1995.

CHARACTERIZATION OF CERIA-BASED SOFCs

Rajiv Doshi1, Jules Routbort2, & Michael Krumpelt'

'Electrochemical Technology Program Chemical Technology Division 'Energy Technology Division

Argonne National Laboratory 9700 S. Cass Avenue, Argonne, IL-60439

Solid Oxide Fuel Cells (SOFCs) operating at low temperatures (500-700°C) offer many advantages over the conventional zirconia-based fuel cells operating at higher temperatures. Reduced operating temperatures result in:

-Application of metallic interconnects with reduced oxidation problems -Reduced time for start-up and lower energy consumption to reach operating temperatures -Increased thermal cycle ability for the cell structure due to lower thermal stresses of expansion mismatches

While this type of fuel cell may be applied to stationary applications, mobile applications require the ability for rapid start-up and frequent thermal cycling.

Ceria-based fuel cells are currently being developed in the U.K. at Imperial CoI!ege[l], Netherlands at ECN[2], and U.S.A. at Ceramatec[3]. The cells in each case are made from a doped ceria electrolyte and a La|.xSrxCO|.yFey03 cathode.

Electrolyte Properties

A reduced operating temperature requires using either of a very thin form (about 1pm) of the conventional zirconia electrolyte or a different electrolyte with higher conductivity is. Using a 1-pm-thick electrolyte poses problems in fabrication and in integrity during cycling and operation. A cerium oxide electrolyte doped with a rare earth like gadolinium exhibits higher ionic conductivity in air than the zirconia (fig. 1).

Unlike zirconia however, ceria exhibits significant electronic conductivity above 500°C in the fuel atmosphere because of the reduction of cerium oxide. The fraction of the Ce4+ ions reduced to Ce3+ as a function of temperature and oxygen partial pressure (p02) is shown in fig. 2. The partial reduction of cerium oxide generates mobile electrons and causes electronic conductivity in the

Fig. 1 Conductivity of electrolyte in air and H2. Required thickness for 0.2 i2-cm' shown on right

Fig. 1 Conductivity of electrolyte in air and H2. Required thickness for 0.2 i2-cm' shown on right

0 0

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