Structure analysis of thermoelectric materials by TEM-ALCHEMI and STEM

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It is well known that Ca_3Co_4O_9 is a good thermoelectric material and addition of dopants is an effective way to improve its properties. To gain insight on the role of Sr-doping, we used scanning transmission electron microscopy(STEM), energy-dispersive X-ray spectroscopy(EDX), and electron energy loss spectroscopy(EELS). STEM images and EDX analyses indicated that Sr substitutes in the rocksalt layer. It is estimated from the EELS results that the Co valence values are(3. 48±0. 10)+ in the rocksalt layer and(3. 02±0. 08)+ in the CoO_2 layer, respectively, which are similar to the non-doped Ca_3Co_4O_9 case. It is inferred from these results that Sr-doping effectively enhances the figure of merit by decreasing the thermal conductivity in the rocksalt layer, whereas it does not significantly alter the electrical properties.