Résumé
Thermoelectric properties were determined in self-substituted Fe2VAl Heusler alloys (Fe2V1+xAl1-x, - 0.1 < x < 0.1), pursuing the goal of their optimization. A parabolic band model fitted to experimental plots of Seebeck coefficient versus charge carrier concentration at 220 K yielded values of the density of states (DOS) effective mass, m*ν = 3.2me and m*c = 13.7me for the holes and electrons, respectively (me is the bare electron mass). The measured Sommerfeld coefficient of the electronic specific heat is consistently smaller in p-type Fe2V0.92Al1.08 (γp = 7.8 mJ mol-1 K-2) than in n-type Fe2V1.07Al0.93 (γn = 11.5 mJ mol-1 K-2). First principles calculations of the DOS lead to the theoretical values m*ν = 2.4me, m∗c = 13.0me and γn/γp = 1.9, in good agreement with the experimental values. These direct comparison of calculations with experiments unambiguously show that the heavy electrons arise from flat Fe eg conduction bands. Calculations of the optimum thermoelectric power factor (PF) show that it is nearly reached experimentally in n-type Fe2V1.03Al0.97 (PF = 6.6 mW m−1 K−2 for n = 1.4 × 1021 cm-3) whereas ptype Fe2V0.985Al1.015 (PF = 2.7 mW m−1 K−2 at p = 6.7 × 1020 cm-3) is not yet optimum. The easier optimization of the thermoelectric properties in n-type self-substituted Fe2VAl can be traced back to the larger effective DOS mass of its electrons.