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edfas.org ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 23 NO . 2 30 REFERENCE CELL During sintering, an amount of excessively doped Mn is expected to diffuse out of the LSM and dissolve into the adjacent YSZ because of higher Mn solubility in YSZ (5.5 cat.%) with lower Mn solubility in LSM (1.5% atomic fraction) at the sintering condition. The occurrence of Mn diffusion fromLSM into YSZ is verified by the EDS analysis, which detects ~5.0 cat.%of Mn dissolved in the YSZ grains. LSM-YSZ INTERACTION ON EXPOSURE TO AIR AT 750°C For the air-tested condition, Mn solubility in both LSM and YSZ suffers a great decrease in comparison to the sintering state (from ~1.5% atomic fraction down to ~-0.05% atomic fraction in LSM and from 5.5 cat.% down to 2.1 cat.% in YSZ). The negative excess Mn solubility in LSM suggests that all excess Mn ions would diffuse out of the LSM. This is supported by the EDS result in Fig. 4, which reveals a similar concentration between theMn and La near the surface of the LSM particle. At the same time, some Mn ions, which have dissolved in the YSZ during the sintering, wouldmigrate out of the YSZ. As a result, all the excess Mn ions diffusing out fromboth YSZ and LSM could only migrate along the LSM/YSZ interfaces and finally precipitate out as Mn-rich grains as observed in Fig. 3. The loss of excess Mn in LSM implies the loss of origi- nal A-site deficient stoichiometry of LSM and therefore, destabilizes the LSM phase. Consequently, the activity of La 2 O 3 in the vicinityof the LSM/YSZ interface increases. This results in the Zr ions fromYSZ interacting with chemically active La 2 O 3 (or LaMnO 3 ) in LSMand forms themore stable La-O-Zr bonding structure because of its large heat of for- mation. [6,8,22,23] Because the Zr +4 ion is larger than the Mn +3 ion, the formationof a La-O-Zr bond couldpossibly expand the lattice structure on the surface of the LSMgrain, which further leads to later Zr ion diffusion into the LSMgrains. Accordingly, the stronger driving force of La 2 O 3 -Zr interaction coupled with an enlarged La-O-Zr lattice structure could attract more Zr ions to diffuse from the YSZ into the LSM, resulting in the formation of Zr-rich particles inside the LSM grain. As the observed Zr-rich phases are in their early formation stage, which involves relatively small size and low quantity par- ticles, precise chemical analysis by EDS and phase identification by XRD are extremely difficult. However, by employing the STEM- EELS techniques, valuable electronic infor- mationof thenano-sizeZr-richparticles hasbeenobtained with indication that they are possibly a precursor or a transition phase of LZO. The most interesting evidence is that theOelement in this Zr-rich phase exhibits zirconate- like electronic structure, which is essentially the backbone structure for the zirconate phase. It is likely that the elec- trical properties of the precursor of LZO are similar to the LZO and therefore, the formation of the Zr-rich phase is responsible for the severe cathode degradation in the air-tested cell. LSM-YSZ INTERA CTION ON EXPOSURE TO OXYGEN AT 750° C In the oxygen-tested cell, someMn ions are expected to diffuse out of the LSM to the LSM/YSZ interface as a result of reduction of excess Mn solubility in LSM in comparison to the sintered state (from 1.5% atomic fraction to 1.0% atomic fraction). Meanwhile, someMn ions that dissolved in the YSZ during sintering will migrate out of the YSZ due to also a large decrease of Mn solubility in the YSZ compared to the sintering condition (from ~5.5 cat% to ~1.7 cat%). As the predicted excess Mn solubility in LSM is still positive in this case (Fig. 11), it suggests that some Mn-rich clusters could precipitate out adjacent to the LSM/YSZ interface. These Mn-rich clusters are likely to interact with theMn ions fromYSZ in formingMn 3 O 4 grains at the LSM/YSZ interface. This is strongly supported with the fact that 1/3 of the Mn ions in the Mn 3 O 4 phase are Mn +2 , whose primary source should be from the YSZ as majority of Mn ions dissolved in YSZ are Mn +2 . [20,21] In addi- tion, Mn 3 O 4 formation under the oxygen-tested condition is in good agreement with the stability diagram of LSM and various manganese oxides determined by Backhaus- Ricoult et al. [24] As described above, the excess Mn solubility in LSM is calculated to be positive under the oxygen-tested Fig. 11 (a) MnO x solubility in YSZ and (b) excess Mn solubility in LSM for the sintering temperature and testing temperature as a function of P O 2 . The P O 2 values for sintering, oxygen-testing, and air-testing conditions are marked with arrows. Note that the P O 2 for the testing condition is the estimated value in the vicinity of cathode/electrolyte interface. (a) (b)