Improving the Seebeck Coefficient in Fe₂VAl Material System Guided by Cluster Expansion Computational Modeling

Fe₂VAl is a low cost, non-toxic, and mechanically robust promising thermoelectric (TE) material for converting waste heat to electricity. Currently, the Seebeck coefficient and figure of merit (zT) values are comparatively low, which restricts its energy conversion efficiency. From our previous experimental studies, two strategies can be applied to increase the Seebeck. (1) Decreasing the order-disorder phase transition temperature by doping/alloying with other elements, the material will have a higher Seebeck coefficient due to favorable changes in band structures when introducing atomic site disorder. (2) Increasing the bandgap in a finite range can also increase the Seebeck in TE material.<br/>However, experimentally testing the order-disorder phase transition temperatures and band structure with different dopants, stoichiometry and processing conditions is time consuming and low throughput. Theoretical modeling will help greatly in screening for promising candidates. Cluster expansion (CE) is a powerful tool to study the phase transition temperature in an alloy as a function of stoichiometry. Atomic site occupation data shows that the occupation is not 100% even at 1000 K below the L21 to B2 phase transition temperature, which suggest that non-equilibrium processing such as quenching can better preserve the favorable atomic site disorder at the TE working temperature of 300-500 K. By Monte Carlo simulation, the phase transition temperature reaches the maximum when the Al/V ratio is between 1.2 and 1. If we increase the Al composition or V composition beyond this range, the phase transition temperature will decrease, and the Seebeck coefficient will increase. For example, at Al/V=1.3 the order-disorder transition is drastically decreased by 500 K. We also calculated the band structure for the Al-rich and V-rich samples. They both show a finite pseudo bandgap. Therefore, we propose that the uneven distribution of Al and V atoms not only decreases the phase transition temperature, but also increases the Seebeck in Fe₂VAl.<br/>Based on these theoretical predictions, we fabricated the base alloy, Al-rich and V-rich samples and tested the heat capacity and Seebeck coefficient. The trend in temperature-dependent heat capacity matched our calculation quite well in order-disorder transition temperatures, and the Seebeck test confirmed that enhancements in Al-rich and V-rich samples compared to the base alloy. Therefore, our calculation was verified by the experiments. Furthermore, thermal conductivity was decreased due to atomic disorder, which notably improved the zT values.<br/>Our research provides a useful tool to increase the Seebeck coefficient in the Fe₂VAl Heusler alloy. Similar methods to decrease the phase transition temperature, such as external doping or ball milling, can be used to further increase the Seebeck coefficient in the future.