Synthesis and structural characterization of Ca₃Co₄O₉ based thermoelectric oxide

Full text: Among all oxide the misfit layered oxide Ca₃Co₄O₉ exhibits one of the highest figures of merit been one of the most promising thermoelectric material. In one of its layer the cobalt atoms are surrounded by other six oxygen atoms in an octahedral site, forming the compost CoO₂. In another layer, Ca, Co and O form the compost Ca₂CoO_3+δ in a face centered cubic structure [1]. Crystals with this kind of structures are considered natural super lattices. Interesting electronic properties of these materials result from charge transfer between the two subsystems, leading to a mixed valent cobalt state within the CoO₂ layer. At high temperatures, the electronic properties of many oxide materials are influenced by reduction: thermal creation of effectively positively charged oxygen vacancies and the accompanying change in the concentration of electronic charge carriers. In this work, nanostructured Ca₃Co₄O₉ samples with different pHs (pH = 0, 0.5, 1, 1.5 and 2) were prepared by the method Pechini. The effects of the pH on the Ca₃Co₄O₉ structure were analyzed. The samples were characterized by X-ray diffraction, thermal analysis, scanning electron microscope (SEM), RAMAN spectroscopy and Hall effect measurements. The parameters of the synthesis were determined by the thermal analysis. The X-ray diffraction and the RAMAN results show that the Ca₃Co₄O₉ has been successfully obtained. As the pH increase the SEM data reveals that the granularity of the samples reduces drastically, resulting in an opened structure. Thermal treatments in oxygen atmosphere over the prepared samples revels that the opened structure were more suitable to changes in the oxygen stoichiometry. These data were confirmed by appearance of oxygen vacancy related mode at the RAMAN spectra and changes in the carrier density changes determined by Hall measurements. Reference: [1] Miyazaki, Y., et al. Journal of the Physical Society of Japan, 71(2), 491-497 (2002). (author)