CONSECUENCIAS DEL DIVORCIO

temperatures

Figure 3.13 Back-scattered electron (BSE) images of anode from LSCTA- powder (a)

calcined at 1100oC (20wt.%) and 1250oC (80wt.%) with 40wt.% graphite, (b) calcined at 1100oC (15wt.%) and 1250oC (85wt.%) with 40wt.% graphite, and (c) calcined at 1100oC (10wt.%) and 1250oC (90wt.%) with 30wt.% graphite. The weight percentage of graphite is relative to the mass of the LSCTA- ceramic powder used in

the slurry. The laminated tapes shown above were sintered at 1350oC in air for 2h.

The picture of half cell made from LSCTA- powder calcined at 1100oC (20wt.%) and

1250oC (80wt.%) plus 40wt.% graphite in the inset of Figure 3.13(a) reveals the formation of the wrinkles on the edge of YSZ surface. The BSE image of the cross section on the edge of the sample is shown in Figure 3.13 (a), demonstrating a good connectivity between YSZ and LSCTA- but coupled with the crack and arch formed

on the YSZ electrolyte. The optimized half cell using LSCTA- powder calcined at

1100oC (15wt.%) and 1250oC (85wt.%) with an addition of 40wt.% graphite is

2cm

(a)

(b)

(c)

YSZ electrolyte

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presented in Figure 3.13(b). Compared with the former sample, the latter sample has a longer wrinkle on the edge of YSZ surface, but the arch of YSZ layer is less evident. When 30wt.% graphite was added into the LSCTA- powder calcined at 1100oC

(10wt.%) and 1250oC (90wt.%) and an alumina plate was applied on the top of the green tapes during sintering, a high-quality cell was obtained. The BSE image in Figure 3.13(c) shows a good adherence of the different layers.

Therefore, the anode material we used in the slurry is the mixture of 10wt.% LSCTA-

calcined at 1100oC and 90wt.% LSCTA- calcined at 1250oC in respects of the matched

dilatometric curves with YSZ electrolyte upon sintering. The dense YSZ green tapes were fabricated using the same chemicals without pore former. To maximize the bonding of LSCTA- anode and YSZ electrolyte, co-casting of LSCTA- and YSZ green

tapes was employed, where the mixed suspension of LSCTA- was cast on the top of

the thin YSZ green slip. The recipes of YSZ and LSCTA- tapes used in our

experiments are summarized in Table 3.6 and Table 3.7.

Table 3.6 Recipes of dense/porous YSZ tapes using aqueous tape casting technique

Stage Ingredients Dense YSZ tape (g) Porous YSZ tape (g)

Stage 1 milling YSZ 20 20 Graphite - 12.3 Deionised water 12 18 Hypermer KD6 0.28 0.42 Stage 2 milling PEG 1.2 2.0 Glycerol 2.4 4.0 PVAa 12 20 Defoamer 0.28 0.42 a

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Table 3.7 Recipes of co-cast/supported LSCTA- tapes using aqueous tape casting

Stage Ingredients Co-cast LSCTA- layer (g) LSCTA- support (g)

Stage 1 milling LSCTA- (1100oC) 1.5 2.5 LSCTA- (1250oC) 13.5 22.5 Graphite 4.5 6.0 Deionised water 14 23 Hypermer KD6 0.32 0.48 Stage 2 milling PEG 1.5 2.3 Glycerol 3.0 4.6 PVAa 14 21 Defoamer 0.32 0.48 a

: 15wt.% PVA dissolved in deionised water

Figure 3.14 TGA curve for the porous YSZ tape at a heating ramp of 3oC/min

0 200 400 600 800 1000 30 40 50 60 70 80 90 100 Temperature (oC) T G (%) combustion of organics combustion of graphite

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Figure 3.15 Temperature program for co-sintering of the green tapes

After drying overnight at room temperature, the green tapes were cut to an appropriate size and laminated, followed by co-sintering at 1350oC for 2h in air. The sintering program is crucial for the formation of a defect-free ceramic cell, and can be determined by the TGA of the porous YSZ tape with temperature at a heating ramp of 3oC/min, as shown in Figure 3.14. The mass of the tape starts to decrease at 120oC, when the organic chemicals added to the tape as plasticizer and binder begins to burn out. This process is required to be gentle to avoid a rapid increase of gas pressure, otherwise, it can cause damage to the cell, such as deformation, crack and stresses. The mass reduction above 600oC can be attributed to the combustion of graphite.

In terms of temperature dependence of TGA, the sintering program is settled and shown in Figure 3.15. In order to avoid the formation of the defects resulting from the combustion of organics and graphite, a slow ramping rate of 1oC/min up to 750oC was used. Then a dwelling stage of 2h at 750oC was followed to burn off graphite and organic residues completely. The temperature was raised to 1350oC at a faster ramp rate of 3oC/min and kept for 2h to allow the final shrinkage and produce a fully dense electrolyte. At the cooling stage, the temperature was cooled down to room temperature at a ramp rate of 3.5oC/min. The sintered samples of 2cm in diameter were subjected to a reduction process at 1000oC for 12h in a 5% H2-Ar atmosphere.

Upon reduction, the colour of LSCTA- changes from light yellow to black.

0 3 6 9 12 15 18 21 24 27 0 300 600 900 1200 1500 2h Cooling 3.5oC/min Heating 3oC/min T emerature/ o C Time/h Heating 1oC/min 2h

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The cathode side of pre-reduced samples was impregnated with precursor solutions containing La0.8Sr0.2FeO3 (LSF) and La0.8Sr0.2CoO3 (LSC) successively and fired at

450oC in air to decompose nitrate while avoid the re-oxidation of LSCTA-. The

loadings of impregnates into the YSZ cathode scaffold were 15wt.% LSF and 5wt.% LSC. Anode catalyst materials including CeO2 and Ni were impregnated into the

porous LSCTA- anode scaffold using solutions from Ce(NO3)3.6H2O and

Ni(NO3)2.6H2O. Multiple cycles of impregnation and calcination at 450oC were used

until the desired loading of impregnates of oxides was reached. Figure 3.16 shows the BSE image of the cross-section of LSCTA- anode/YSZ electrolyte interface after the

impregnation of ceria and nickel in the anode side and the firing process. It can be seen that the impregnated catalysts have successfully diffused into the interfacial area across the thick anode support.

Figure 3.16 (a) Back-scattered electron (BSE) image of the cross section of the LSCTA- anode/YSZ electrolyte after impregnation of CeO2+Ni and firing

LSCTA-

CeO2+Ni

YSZ electrolyte

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