Microstructures and Thermoelectric Properties of Heusler Fe2VAl Alloys Containing Oxide Nanoparticles.

The Heusler-type Fe₂VAl alloy is a promising candidate for use in fabricating a thermoelectric power generation device because of its large Seebeck coefficient and high electrical conductivity. However, the high thermal conductivity of this alloy, as a thermoelectric material, degrades its power generation capacity. In this study, to reduce its thermal conductivity, the microstructure of a sintered Fe₂V1.08Al0.92 alloy prepared via a powder metallurgical process was modified by adding oxide nanoparticles. Via the dispersion of Al₂O₃ nanoparticles, a sintered Fe₂V1.08Al0.92 alloy with fine grains of approximately 200nm in size was obtained due to the pinning effect on grain growth during sintering. The thermal conductivity was reduced from 16 to 11W/mK. Upon La₂O₃ addition, the grain size of the Fe₂V_1.08Al_0.92 alloy was reduced to approximately 100nm and the thermal conductivity was further reduced to 10W/mK. The difference in grain refinement could be caused by the lower stability of La₂O₃, which facilitated dispersion during ball milling, compared to that of Al₂O₃. As these microstructure refinements negatively affected the electronic properties, the thermoelectric performance of the Fe₂V_1.08Al_0.92 alloy could not be enhanced. However, partial microstructure refinement with sparsely distributed La₂O₃ could slightly enhance the thermoelectric performance due to an appreciable reduction in the thermal conductivity without a considerable degradation in the electronic properties. By using these thermoelectric properties, a simple estimation of thermoelectric power generation, assuming a thermal resistance between the heat sources and thermoelectric module, was conducted. Remarkably, the results suggested that the reduction in thermal conductivity could enhance the output power density and conversion efficiency and reduce the optimal leg length. Thus, practically, controlling the balance between the electronic and thermal properties via microstructural modification is favorable in improving the practicability of the Fe₂VAl alloy by enhancing the power generation capacity and reducing the sizes and masses of thermoelectric devices. [ABSTRACT FROM AUTHOR]