Magnesium Diffusion in Silica Enhanced by Na and Retarded by Al
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employs a yttrium-stabilized zirconia (YSZ) electrolyte, kept thin (5–10 µm) to reduce ohmic losses at lower temperatures and supported by a planar Ni-YSZ anode (0.5 mm thick). However, cell performance of such SOFCs may suffer at intermediate temperatures because of the reduced electrochemical activity of the cathode. Researchers S.P. Simner, J.F. Bonnett, N.L. Canfield, K.D. Meinhardt, V.L. Sprenkle, and J.W. Stevenson investigated the use of La-ferrite and La-nickelite cathode compositions, rather than the much-researched La cobaltite, to facilitate matching of the coefficients of thermal expansion (CTEs) between the cathode and other cell components (thus enhancing thermal cycling characteristics) and to reduce chemical interactions. Cobaltites are well known for their high CTE and reactivity with YSZ to form electrically insulating zirconate phases. As described in the July issue of Electrochemical and Solid-State Letters, the researchers prepared La0.8Sr0.2FeO3–δ(LSF-20), La0.7Sr0.3Fe0.8Ni0.2O3–δ(LSFN-3020), and LaNi0.6Fe0.4O3–δ(LNF-6040) in addition to a mature interlayer material, Ce0.8Sm0.2O1.9 (SDC-20) to be incorporated between the cathode and YSZ electrolyte. Sintering temperatures were optimized for each cathode: LSF-20 (1150°C for 2 h), LNF-6040 (1200°C for 2 h), and LSFN-3020 (1150°C for 2 h). Power densities were based on a screen-printed cathode with a diameter of 22 mm. Current-collection was achieved by the use of screen-printed platinum grids with embedded platinum gauze for the cathode and screen-printed NiO grids with embedded Ni gauze for the anode. Despite its lower electrical conductivity, as compared with LNF-6040 and LSFN-3020, LSF-20 exhibited the highest cell performance at each temperature (850–950 mW/cm2 at 750°C and 0.7 V). The researchers suggest that possible reasons for this result might be that LSF-20 has a greater activity for the reduction of oxygen or that its finer grain size can catalyze the reduction of oxygen through greater surface area. An interesting observation, unresolved at this time, is a powerdensity performance increase during the first 40 h of operation that levels off by 200 h and remains constant after that until the end of the test at 300 h. CTEs for the new cathode materials showed that LSF20 had the best match with the anode while the others had slightly higher values. Thus, LSF-20 shows comparable performance to higher-temperature cathodes while resisting cell delamination by matching the CTE of the anode.
Magnesium Diffusion in Silica Enhanced by Na and Retarded by Al Silica-forming ceramics can be beset by certain cations present in the ceramics either as impurities or as sintering additives. Both alkali and alkaline-earth cations are known to break the silica network structure, allowing nonbridging oxygen anions to form, thereby weakening the silica network. For example, researchers have learned that in NBD 200 silicon nitride, a commercial ball-bearing ceramic pressed with ~1 wt% magnesia, sodium impurities are responsible for increased oxidation, comp
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