Low-temperature co-sintering for fabrication of zirconia/ceria bi-layer electrolyte via tape casting using a Fe2O3 sintering aid

Bilayer electrolytes have potential in solid oxide cells to improve ionic conduction whilst blocking electronic conduction. GDC/YSZ bilayer electrolyte processinghas provenproblematic due to thermochemical instability at high sintering temperatures. We first match the shrinkage profile of the two bulk materials using a Fe2O3 sintering additive. Additions of 5 mol% of Fe2O3 in the GDC layer and 2 mol% of Fe2O3 in the YSZ layer prevents delamination during co-sintering. The addition of Fe2O3 promotes densification, enabling achievement of a dense bilayer at a reduced sintering temperature of 1300 ◦C; ∼150 ◦C below conventional sintering temperatures. Elemental analysis showed the compositional distribution curves across the bilayer interface to be asymmetric when Fe2O3 is employed. The Fe2O3 increases the total conductivity of the bilayer electrolyte by an order of magnitude; this is explained by the effect of Fe2O3 on reducing the resistive solid solution interlayer at YSZ/GDC interface from ∼15 to ∼5 m

Tailoring SOFC electrode microstructures for improved performance

The authors thank EPSRC for support through the research grant EP/M014304/1.The key technical challenges that fuel cell developers need to address are performance, durability, and cost. All three need to be achieved in parallel; however, there are often competitive tensions, e.g., performance is achieved at the expense of durability. Stability and resistance to degradation under prolonged operation are key parameters. There is considerable interest in developing new cathodes that are better able to function at lower temperature to facilitate low cost manufacture. For anodes, the ability of the solid oxide fuel cell (SOFC) to better utilize commonly available fuels at high efficiency, avoid coking and sulfur poisoning or resistance to oxidation at high utilization are all key. Optimizing a new electrode material requires considerable process development. The use of solution techniques to impregnate an already optimized electrode skeleton, offers a fast and efficient way to evaluate new electrode materials. It can also offer low cost routes to manufacture novel structures and to fine tune already known structures. Here impregnation methodologies are discussed, spectral and surface characterization are considered, and the recent efforts to optimize both cathode and anode functionalities are reviewed. Finally recent exemplifications are reviewed and future challenges and opportunities for the impregnation approach in SOFCs are explored.PostprintPeer reviewe

Effect of cosolutes on polysaccharides gelation.

In the first stage of the investigation, the effect of high levels of sugars (mixture of 50% sucrose with 351/'o glucose syrup) on agarose (0.7 w-L °'o) was characterized by low amplitude oscillatory measurements of storage modulus (G'), loss modulus (G") and loss tangent (tan 6" as well as large deformation techniques. Samples were prepared at 90°:, and measured immediately, or after Storage at 5°C. The combined Williams-T andel-Ferry W F; /free volume theory was used to derive the glass transition temperature, the fractional free volume, and the thermal expansion coefficierit of the glass. Solution of high concentrations of sucrose Crystallizes, but addition of the polymer encourages Bitei- olecular interactions, which transfoý the mixture into a high v iscosi Ly glass. The mechanical properties of glucose syrup follow ýýý. iý behavior in the glass transition region and revert to an Arrhenius type prediction in the CrIa-SSy State. feasurements on sugar Samples and aýarose-Sugar ii 1x üres were resolved into a bas iiriCuuu Gi temperature alone and a basic function of equency (time) alone. T ie former traces the energetic cost of vitrification, which increases sharply with decreasing tempera ure. The layer, at long time scales, is goes erned by the mtinite molecular weight of the äcräiGSZ ne ork. In the region of S1 O1 tImes, tie effect of Ze vole ie is active rega dless of In a continuation of, investigating the significance of polymer-cosolute interactions, the effect Gi sucrose, glucose, fructose, sorbitol, hylitol, glycerol and ethan-1,2-dial on gelation of high methoxy pectin was studied under different experimental conditions. The main changes in procedure in comparison with the work on agarose were: (i) the polymer concentration was increased from 0.7 to 1.0 wt °'o, (ii) the mixtures prepared at pH AT and subsequently acidified to pH 3, rather than beinte prepared at neutral pH, (i11) the cosolute concentration was varying from 50 to 65 w IL °'o and (vi) the mixtures were studied through rheology, calorimetry and optical rotation. The samples were prepared at 95°C and changes in storage modulus (G') and loss modulus (G") during cooling to 5°C, heating to 900 and re-cooling to 5°C, at 1°C/min, were measured at 1 rad s"1 and 0.5% strain. In all cases, the onset temperature for gelation during cooling and the moduli recorded at 5'C increased With increasing concentration of cosolute. However, both values were substantially lower for the liquid cosolutes than for mixtures with solid cosolutes at the same concentrations. The difference is attributed to inhibition of pectin-pectin interactions by pectin-cosolute interactions, which in turn are inhibited by cosolute-cosolute interactions. On heating there was an initial reduction in modulus, with the same temperature-course as the increase on cooling; for the solids, this was followed by an increase attributable to hydrophobic association of methyl ester substituents. No SUCH increase was seen with the liquid cosolutes, but DSC studies showed two reversible I. thermal ti ansitions in all cases, one over the temperature-range of the initial gelation process ý nfý r 'A f+ .ýýn on cooling and t he oLlleICoincidenf t with the increase in modulus ulon hea ing in the presence o1- ý solid COSO1UteS. The absence of any detectable increase in modulus on heating with the liquid COSOtUteS is a trlbuted to accumulation of cosolute around tiie polymer chains promoting hydrophobic association between methyl ester groups on the same Chain; Or within Cl sters o chains, v with, therefore no contribution to network strucLarz. At high concentrations of the solid cosolutes, the increase in modulus on heating was followed by a decrease at higher ýempei atur e; this -was attributed to excessive aggregation, and was reflected in lower moduli on subsequent re-coolIný to 5'C, in coast to the enhancement in gel strength after heating aril cooling observed at lower concentration of the same COSül teS. in the presence of fructose as cosolute, calorimetric studies showed an intense endotherm follovýied immediately by an intense exotherm on hea ng. These transitions occurred over approximately the same teMperat'ure-rande as initial gelation on cooling and increased in mag liLude with increasing concentration of the sugar. The displacement of both transitions to progressively hid her temperature as the rate of heating was increased was much Greater than anticipated from a simple thermal lag, indicating that the undelying structural chancres are slow. The proposed interpretation is that fructose is capable of site-binding to pectin in both the ordered and disordered stat

Total electrical conductivity and defect structure of ZrO2-CeO2-Y2O3-Gd2O3 solid solutions

The use of a bilayer composite YSZ-CGO (yttria-stabilized zirconia-gadolinia-doped ceria) electrolyte for intermediate-temperature solid-oxide fuel cells (SOFCs) is still problematic, due to solid-state reaction and interdiffusion phenomena occurring between YSZ and CGO. To overcome this problem, a gradation in the microstructure between the single materials is proposed. To optimize the composition of the interlayer with respect to conductivity, different compositions of the system CGO(x)YSZ(1-x) were prepared and characterized in order to study their transport properties. Conductivity was found to reach a minimum for solid solutions where Zr and Ce are present in equimolar quantities. The latter are, however, the most appropriate candidates to be applied as interlayers between the two single materials. Keeping the Ce/Zr atomic ratio approximately equal to one, different solid solutions were prepared and characterized by varying the concentration of the trivalent cations present, in order to clarify their role in the transport properties of the material. For one of the better conducting systems the cell performance of the composite YSZ-interlayer-CGO film electrolyte is simulated and compared to that of the YSZ-CGO film electrolyte. (C) 2000 Elsevier Science B.V. All rights reserved

On the variation of the direct solar radiation during the annular solar eclipse of April 29, 1976

Measurements of the direct solar radiation during the annular solar eclipse of April 29, 1976 are given in three wavelength bands (centered on 800, 680, and 450 nm). A decrease of the solar radiation after the last contact was observed, especially in the red and infrared. This is considered in connection with the results of other direct light and zenith light measurements. © 1979 D. Reidel Publishing Co