Microstructure of Ca 3 Co 4 O 9 Single Crystals and Thin Films
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Microstructure of Ca3Co4O9 Single Crystals and Thin Films Y. F. Hu1, E. Sutter2, W. D. Si1, Qiang Li1 1 Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, U.S.A. 2 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY 11973, U.S.A. ABSTRACT We present a comparative study of the microstructure of Ca3Co4O9 single crystals and c-axis oriented Ca3Co4O9 thin films grown on glass substrates. Though both crystals and films have similar values of Seekbeck coefficient and electric resistivity at room temperature, their microstructures are rather different. Extensive high resolution transmission electron microscopy (TEM) studies reveal that the films grown on glass substrates have abundant stacking faults, which is in contrast to the perfect crystalline structure found in the single crystal sample. The caxis lattice constants derived from the x-ray diffraction (XRD) and TEM measurements for the single crystal sample and the thin film are virtually the same, suggesting that the thin film on the glass substrate was not strained.
INTRODUCTION Thermoelectric (TE) materials can convert heat flow into electric energy directly without producing carbon dioxide gas, radioactive substances, or other emissions, and are thus considered to play a key role in clean energy generation. Thermoelectric oxide materials have been widely studied in the past few years due to their high chemical stability at high temperatures, and are promising to have profound impact in the thermoelectric applications such as power generation and Peltier cooler. Among them, layered cobaltates have been extensively investigated since the discovery of simultaneously high electric conductivity and thermoelectric power in NaCo2O4.1 After this discovery, many other layered cobaltates which have misfit structure have been found and most of them show reasonable TE properties.2-6 The misfit layered cobaltates Ca3Co4O9 are especially promising in the thermoelectric applications due to their high thermoelectric power and stability in air,7-11 and thus have been extensively studied in single crystalline and bulk polycrystalline forms.1, 8-11 On the other hand, the studies of TE properties of cobaltate films are somewhat limited.12-15 In many TE applications, such as thermochemistry-on-a-chip, bio-thermoelectric chip, and active cooling for microelectronic processor, film devices are required that allow localized cooling/heating at points of interest. High quality TE thin films, having the crystal structure of single crystals, can have low resistivity and high Seebeck coefficient intrinsic to their electronic band structures. At the same time, thin films are expected to have thermal conductivity lower than that of the single crystals due to strong phonon scatterings at both surfaces and film/substrate interfaces.16, 17 Glass is an amorphous material and has very low thermal conductivity, therefore, cobaltate films on glass substrates will have additional advantages in potential T
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