Improved power factor in low thermal conductive Fe2VAl-based full-Heusler thin films by composition-control with off-axis sputtering method

We demonstrate the improved power factor in full-Heusler Fe2VAl1−xSix thin films using precise composition-control with the off-axis sputtering method. The valence electron concentration per atom was varied from 5.9 to 6.1 by manipulating the target substrate off-axis distance in addition to changing the sputtering target composition, resulting in an improved power factor up to 3.0 mW/K2 m in the off-stoichiometric composition of Fe1.93V1.05Al0.77Si0.24. The films had a polycrystalline structure with an average grain size of 40–50 nm. The cumulative lattice thermal conductivity calculation as a function of phonon mean free path revealed that the small grain size in the thin film contributed to a lowered lattice thermal conductivity of 3.8 W/Km. As a result, the figure of merit ZT of 0.15 at 50 °C was obtained, and it is the highest value in the Fe2VAl1−xSix system.