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edfas.org 23 ELECTRONIC DEV ICE FA I LURE ANALYSIS | VOLUME 23 NO . 2 TEM work was performed using a JEOL 3000F, a Schottky style field-emission analytical 300 kV TEM equipped with a Gatan Imaging Filter (GIF) and a scanning TEM (STEM) unit. Chemical analysis was performed using both point and line scan utilizing 1 nm probe size. All concentration profiles are plotted using cation percentage by excluding theOcontent. Elemental distributionmapswere acquired by energy-filtered imaging (EFI) using the three-window technique, [11] where the Zr- M 4,5 edge (180 eV), Mn- M 4,5 edge (49 eV), and La- N 4,5 (99 eV) edge were used. EFI phase maps were also constructed by superimposing individual Zr (red), La (green), and Mn (blue) maps together. Both Y and Sr EFI maps are disregarded in this study due to their low concentrations. Electronic structure study was per- formed on O- K and Mn- L 2,3 edges by employing electron energy-loss spectroscopy (EELS). The same background- subtracted windows were applied to all spectra acquired from both point EELS spectra and STEM-EELS spectra. Theoretical calculations ofMn solubility in8YSZandMn excess in LSMwere performedwith respect to awide range of oxygen partial pressures at two temperatures, 750°C corresponding to the testing temperature, and 1050°C to the cathode sintering temperature. The calculations were carried out using the commercially available software Thermo-Calc [12] by employing the thermodynamic data- bases of La-Sr-Mn-O and of Mn-Y-Zr-O systems developed byGrundy et al. [13] andChen et al. [14] The databases contain Gibbs energy functions of the phases in La-Sr-Mn-O and Mn-Y-Zr-O and were developed utilizing the CALPHAD (CALculation of PHAse Diagram) methodology. [15] RESULTS REFERENCE CELL Examination of the reference cell provides information regarding interfacial microstructure and diffusion profiles in the as-fabricated cells. The TEM results are presented in Fig. 1, including thebright-field (BF) image overlooking the cathode/electrolyte interface, the zero-loss image, and the corresponding EFI covering several LSM and YSZ grains. The cathode’s porous structure of LSM and YSZ compos- ite and the dense YSZ electrolyte are clearly observed in Fig. 1a. The distribution of the LSM and YSZ grains in the cathode are illustrated by the EFI map shown in Fig. 1c. All the phases in the EFI map are also confirmed withcomplementaryEDSanalysis. The results inFig. 1 indi- cateno thirdphase formationduring the sinteringprocess. EDS analysis detects ~5.5 cat.% Mn content in the YSZ grains. The value is consistent with the one reported by Mitterdorfer and Gauckler. [6] The result that YSZ started without any Mn but received it from LSM during test suggests Mn diffusion from LSM to YSZ has occurred during sintering. AIR-TESTED CELL Figure 2 shows the TEM bright-field and dark-field images with the cathode/electro- lyte interface in the field of view. Here, the LSM grains (identified by EDS) are in con- tact with the YSZ electrolyte. The cor- responding diffraction patterns are also shown. Fig 2c is acquired fromthe LSMphase only, while Fig. 2d is obtained on the region with the embedded particles inside the LSM. The dark-field image in Fig. 2b was acquired by positioning an objective aper- ture on the diffraction spot marked with dashed circle in Fig 2d. It is shown that sev- eral crystalline particles with a diameter in a range of 25 to 35 nm are embedded inside the LSM grains along the LSM/ YSZ interface. The dark-field image with higher magnification in Fig 2b illustrates Fig. 2 (a) The bright-field TEM image across the LSM/YSZ electrolyte interface in the fuel cell tested in dry air. (b) The dark-field TEM image showing twoparticles formedalong LSM/YSZ interface. (c) Thediffractionpattern acquired from the LSM phase. (d) The diffraction pattern obtained on the region with embedded particles inside the LSM, (air flow cell). Fig.1 (a) The TEMbright fieldoverview image of cathode/electrolyte interface in the reference fuel cell. (b) The zero-loss image of several LSMand YSZ grains. (c) The corresponding energy-filtered imaging phase map by superimposing the Zr, La, andMnmap; Zr (red), La+Mn (green) (as-fired cell).