Theoretical and experimental evaluation of thermoelectric performance of alkaline earth filled skutterudite compounds

Skutterudite compounds are one of the most promising thermoelectric materials that can be used for intermediate power generation applications because of their excellent thermoelectric and mechanical properties. In spite of extensive research efforts during the past two decades, there is still a lack of full understanding of the electronic and thermal transport in these compounds. In the present study, we carry out a combined experimental and theoretical investigation of the electronic and thermal transport properties of alkaline earth-filled (Ca,Sr,Ba)(0.2)-Co4Sb12 skutterudites in the temperature range from 300 K to 800 K. The experimental work includes structural properties probed by PXRD and EMPA for phase purity and composition analysis. Theoretical values of the temperature dependent electronic transport parameters were determined using the nearly-free electron approximation, and various phonon thermal conductivity contributions were studied using the Debye isotropic continuum model and the theory of Price for the bipolar contribution. The theoretical and experimental transport parameters were found in good agreement with each other. They confirm that both Ba and Sr filled skutterudites are highly degenerate semiconductors, while the heat transport in the Ca filled compound is dominated by the bipolar contribution at temperatures above 300 K. The Sr filled skutterudite shows the highest Seebeck coefficient, likely due to its lowest donor ionisation energy. From our detailed theoretical investigations, it is seen that the key mechanism to control the phonon thermal conduction is originated from the interaction between mass defects and phonons. The maximum ZT value obtained 0.14 for Sr0.2Co4Sb12 compound at 800 K likely indicate that Sr is more effective filler among the alkaline earth metals.