4. Composite SOFC anodes with YSZ and A-site deficient titanates
4.5. The influence of the calcination temperature on the composite anodes
4.5.2. Influence on the microstructure
The microstructure of the composite anodes composed of YSZ and LST44 or LST44Mn6 and calcinated at different temperatures was investigated with the SEI technique of a SEM microscope like described in chapter 2.2.2.
As described in chapter 4.1.1 and illustrated in figure 4.2 the YSZ skeleton is covered in a way forming regular shaped grains at a level of 10 wt.% A-site deficient perovskite impregnated, and this structure is partly covered by a flat blanket layer without visible grain structure if the amount of impregnated perovskite is increased to 40 wt.%.
Figure 4.29: SEI images of a SEM microscope of composite anodes of YSZ and 40 wt.% LST44 fired after impregnation at temperatures of 1059 °C (a) and 1209 °C (b) under oxidising conditions and 30000x magnification.
Figure 4.29 shows that the size of the grains in the composite anode is a function of the firing temperature, in an YSZ/LST44 anode fired at 1059 °C the grains have a diameter of 100-150 nm, while they have a diameter of over 500 nm in an anode of the same composition fired at 1209 °C. Figure 4.30 shows a schematic of the development of composite anodes with increasing firing temperature on the one side and of increasing amount of impregnated perovskite on the other side.
Figure 4.30: schematic of composite anodes with a YSZ skeleton and different amounts of impregnated perovskite phase subsequently fired at different temperatures.
In composite anodes with YSZ and LST44Mn6 the development of the size of the grains from small grains of around 100 nm to big grains of around 500 nm with increasing firing temperature happens in the same way as in anodes of YSZ and LST44, but generally at temperatures around 50 °C lower. In a composite anode of YSZ/LST44 fired at 1100 °C the size of the grains is around 100 nm or smaller, fired at 1159 °C they show an intermediate size of around 200 nm and fired at 1209 °C the average size is around 500 nm or larger. The intermediate grain size of around 200 nm is reached at a firing temperature of 1100 °C in a composite anode of YSZ/LST44Mn6, and a grain size of 500 nm is reached at a firing temperature of 1159 °C, as shown in figure 4.31. Theoretically substitution of 6% of the titanium at the B-site of the perovskite by manganese reduces the lattice energy because oxide ions are removed from the lattice to maintain charge neutrality, and because the bonding energy between manganese and oxygen is lower than the bonding energy between titanium and oxygen [18]. The resulting decrease of sintering temperature is consistent with the more advanced development of the microstructure for LST44Mn6 compared with LST44 at the same firing temperature, as found in the experiments in
Figure 4.31: SEI images at 15000x magnification of composite anodes of YSZ/LST44 (left column) and YSZ/LST44Mn6 (right column) calcinated at different temperatures showing the different stages of grain development from small grains at the top over grains of an intermediate size in the middle to large grains in the bottom images.
The microstructure of the composite anodes looks very similar after reduction at 900 °C for 6 hours in an atmosphere of 5 % hydrogen and 95 % argon. As shown in figure 4.32 the size of the grains after reduction is a little bit bigger and the terrace structure of the grains is more distinctive. This might be related to the lower lattice energy in the reduced state, caused by the larger number of oxygen vacancies, causing stronger sintering effects at the same temperature.
Figure 4.32: SEI images of composite anodes of YSZ/LST44Mn6 calcinated at 1159 °C before and after reduction at 900 °C at 15000x magnification.
4.5.3. Conclusions
Button cells with composite anodes of YSZ/LST44 and YSZ/LST44Mn6 were fired after impregnation at temperatures of 1009 °C, 1059 °C, 1100 °C, 1159 °C or 1209 °C. Their composition was investigated by XRD and their microstructure by SEI.
The secondary phase peaks in the XRDs assigned to the tetragonal distortion of YSZ upon impregnation of LST44 do not occur below a firing temperature of 1100 °C and are larger if the firing temperature is higher. Since the tetragonal distortion of the YSZ unit cell has been related to the migration of ions from the impregnated perovskite phase into the YSZ phase of the composite anode and their integration into the YSZ lattice [3], a higher temperature might help this migration and integration to happen faster and into deeper layers of the YSZ grains.
Basic refinement of the cell parameters of YSZ and the perovskites seem to indicate that the cell parameter of YSZ is neither influenced by the firing temperature, nor by the kind of titanate perovskite impregnated into the skeleton or by reduction at 900 °C. The cell parameters of LST44Mn6 are around 0.002 to 0.005 A larger than the cell parameters of undoped LST44 depending on the firing temperature. The reduced state of both perovskites show cell parameters around 0.005 A larger than their oxidised states. That was to be expected because the manganese ions in valence 2 and 3 have larger ionic radii than the titanium (IV) ion, and the ions of lower positive charge have bigger ionic radii.
The microstructure of the composite anodes generally shows grains covered by a flat blanket layer. The size of the grains is small at low firing temperatures and increases with increasing firing temperature. A certain size of the grains can be observed at temperatures around 50 °C lower if the impregnated perovskite is LST44Mn6 instead of undoped LST44, which is related to a lower lattice energy in LST44Mn6. The grain size in reduced composite anodes is larger than the same composites fired at the same temperature under oxidising conditions.