The full-Heusler VFe2Al has emerged as an important thermoelectric material in its thin film and bulk phases. VFe2Al is attractive for use as a thermoelectric materials because of it contains only low-cost, non-toxic and earth abundant elements. While VFe2Al has often been described as a semimetal, here we show the electronic and thermal properties of VFe2Al can be explained by considering VFe2Al as a valence precise semiconductor like many other thermoelectric materials but with a very small band gap (Eg = 0.03 ± 0.01 eV). Using a two-band model for electrical transport and point-defect scattering model for thermal transport we analyze the thermoelectric properties of bulk full-Heusler VFe2Al. We demonstrate that a semiconductor transport model can explain the compilation of data from a variety of n and p-type VFe2Al compositions assuming a small band-gap between 0.02 eV and 0.04 eV. In this small Eg semiconductor understanding, the model suggests that nominally undoped VFe2Al samples appear metallic because of intrinsic defects of the order of ∼1020 defects per cm-3. We rationalize the observed trends in weighted mobilities (μw) with dopant atoms from a molecular orbital understanding of the electronic structure. We use a phonon-point-defect scattering model to understand the dopant-concentration (and, therefore, the carrier-concentration) dependence of thermal conductivity. The electrical and thermal models developed allow us to predict the zT versus carrier concentration curve for this material, which maps well to reported experimental investigations.
ASJC Scopus subject areas
- Materials Chemistry