Enhanced high temperature thermoelectric properties of Bi-doped c-axis oriented Ca_3Co_4O_9 thin films by pulsed laser deposition

摘要

Ca_(3-x)Bi_xCo_4O_9 (x = 0-0.4) thin films were deposited on single-crystal sapphire (0001) substrates by pulsed laser deposition. Structural characterizations indicated that these thin films exhibited perfect c-axis orientation and were well crystallized. Surface chemical states analysis confirmed Bi-substitution for Ca in the thin films with x <0.4. For the thin film with x=0.4, excessive Bi were found isolated within the film. Due to their perfect orientation, in-plane electrical properties of these thin films measured from 300 to 740 K were found to be comparable to those of the single crystals. Furthermore, Bi-substitution was noted for the reduced electrical resistivity and enhanced Seebeck coefficient. The above superior properties resulted in a high power factor of 0.81 mW m~(-1) K~(-2) at 740 K for thin film Ca_(2.7)Bi_(0.3)CO_4O_9, which was about 29% improvement as compared to that of pure Ca_3CO_4O_9 thin film. The results suggested that Bi-doped Ca_3CO_4O_9 thin films could be a promising candidate for thermoelectric applications at elevated temperatures.