Effect of heating cycle on cobalt-antimonide-based thin films for high-temperature thermoelectric energy conversion applications.

Abstract In this study, a comparative analysis of the structural and thermoelectric properties of all possible Co-Sb-based phases in thin-film form for high-temperature applications is presented and discussed. A series of thin-film samples were prepared via radiofrequency co-sputtering on an oxidized silicon substrate. For improved crystallinity, the thin-film samples were subjected to heat treatment in the form of a high-temperature heating cycle where the temperature was slowly ramped up to about 760 K. The films were characterized using X-ray diffraction for phase identification and crystalline quality. Energy-dispersive spectroscopy was performed to determine the stoichiometric composition and homogeneity, whereas the surface morphology and microstructure were examined using scanning electron microscopy and atomic force microscopy. To investigate film performance at high temperatures, the electrical resistivity and Seebeck coefficient were measured at regular temperature intervals, and results were correlated with identified phases, composition, and crystalline quality. The CoSb phase was observed to have metallic features, the CoSb 2 phase had doped semiconductor characteristics, whereas the CoSb 3 -phase thin films showed typical semiconductor behavior. The CoSb 2 and CoSb 3 samples shows a transition of the Seebeck coefficient from positive to negative at a temperature range of 500–550 K and 550–600 K respectively. The thermoelectric power factor was evaluated from the measured parameters and maximum values of 3.2, 7.8, and 3.8 mWm−1·K−2 for the CoSb, CoSb 2 , and CoSb 3 thin films, respectively, are reported. We observed a wide range of structural and thermoelectric properties in the series of samples, demonstrating a route to tailoring and improving properties for maximal performance. Highlights • Thermopower depends upon both the carrier concentration and electron effective mass. • The presence of impurity phases shapes the magnitude of the Seebeck coefficient. • Seebeck coefficient retained the p to n type transition in the second heating cycle. • A Seebeck coefficient as high as −227 μV/K was achievable for CoSb 3 thin films. • CoSb 3 skutterudite thin films achieved a sound maximum power factor of 4 mW/m.K2. [ABSTRACT FROM AUTHOR]