Boosting the thermoelectric performance of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Fe</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>VAl</mml:mi><mml:mtext>−</mml:mtext><mml:mi>type</mml:mi></mml:mrow></mml:math> Heusler compounds by band engineering

Abstract

Linking the fundamental physics of band structure and scattering theory with macroscopic features such as measured temperature dependencies of electronic and thermal transport is indispensable to a thorough understanding of thermoelectric phenomena and ensures more targeted and efficient experimental research. Regarding ${\mathrm{Fe}}_{2}\mathrm{VAl}$-based compounds, experimental work has seen mostly qualitative and often speculative interpretations, preventing this class of materials from tapping their full potential when it comes to applications. In this paper, the temperature-dependent Seebeck coefficient and electrical resistivity of a set of $p$-type and $n$-type samples with the composition ${\mathrm{Fe}}_{2}{\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Ta}}_{x}{\mathrm{Al}}_{1\ensuremath{-}y}{\mathrm{Si}}_{y}$ are presented from 4 K up to 800 K as well as the Hall mobility and carrier concentration from 4 K to 520 K. We attempt a quantitative analysis of our data using a parabolic two- and three-band model and compare the model results with those from density functional theory calculations. Our findings show an increase of the band gap ${E}_{\text{g}}$ from almost zero in undoped ${\mathrm{Fe}}_{2}\mathrm{VAl}$ toward ${E}_{\text{g}}\ensuremath{\approx}0.1\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ with increasing Ta substitution, consistent with results from first-principles calculations. Due to the resulting enhancement of the Seebeck coefficient, the maximum power factor is boosted up to 10.3 mW/${\mathrm{mK}}^{2}$, which is, to the best of our knowledge, the highest value among $n$-type bulk semiconductor systems reported near room temperature up until now. We further show that for the $p$-type ${\mathrm{Fe}}_{2}{\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Ta}}_{x}\mathrm{Al}$ compounds, the dominant scattering mechanism of electrons is intrinsically different compared to the $n$-type samples, for which acoustic phonon scattering can well describe the temperature-dependent Hall mobility in a broad range of temperatures.

Topics & Concepts