Atomic Disorder Engineering of Thermoelectric P-Type Fe₂VAl Doubling Previously Reported ZT

The L2₁ intermetallic compound Fe₂VAl is an ecofriendly, low cost, and easy to process potential replacement for conventional low temperature (250 - 500 K) thermoelectric materials used for waste-heat energy harvesting, such as Bi₂Te₃, which is brittle and uses toxic and expensive elements. The main issue with Fe₂VAl is its high thermal conductivity (~25 W/mK at 300 K) which leads to a low figure of merit, ZT. The aim of this project is to investigate off-stoichiometric compositions and engineer atomic disorder to enhance both the thermal and electrical properties of bulk p-type Fe₂VAl.<br/>While the highly ordered L2₁ phase is the ground state, the higher temperature B2 (Al-V site swapping) and A2 (fully disordered) phases, whose transition temperatures are identified using DSC, can be retained through quenching. Notably, by replacing V with Al the B2 phase transformation temperature is lowered causing more retained disorder after quenching. The effect is a thermal conductivity reduction &gt;60% to 9 W/mK. Al-rich compositions also demonstrate a flattening of the conduction band and widening of the bandgap resulting in an increased effective carrier mass and lower mobility. The flattened conduction band edge, however, converges with a second band increasing the carrier density and enhancing conductivity while maintaining the Seebeck coefficient. The net result is a far off-stoichiometry composition, namely Fe₂V_0.7Al_1.3, that exhibits a ZT &gt;0.3 at temperatures above 400K which is nearly a 2x improvement compared to existing p-type Fe₂VAl.