ABSTRACT
In recent years, bismuth-rich Mg3(Sb1-xBix)2 (x = 0.5-0.8) compositions have generated significant interest due to their excellent thermoelectric (TE) performance near room temperature, making them potential applicants for recovery of low-grade waste heat. The superior performance in these materials is due to its complex electronic band structure (EBS) with presence of multiple near degenerate bands close to the conduction band edge. The position and curvature of these bands strongly depend on the alloy composition, doping amount as well as temperature. Thus, identifying optimal material compositions to get the best TE performance depends on an understanding of the temperature dynamics of EBS and forms the objective of this work. Mg3Sb0.6Bi1.4 (x = 0.7) is chosen for this study due to its reported high near room temperature performance, and compositions with varying doping concentrations (Te used as dopant) have been synthesized. EBS parameters like effective mass and deformation potential of bands, interband separation and band gap values have been estimated using a recently developed refinement approach. Refinement results indicate that the interband separation between conduction bands to be a function of both temperature and doping concentration. Further, thermal conductivity (κ) was estimated for all of the compositions. Utilizing the EBS and κ information, predictive 3D maps indicating the variation in zT (TE figure of merit) with doping concentration and temperature have been generated. The 3D maps reveal an interesting surface topography with a broad peak zT region. This observation explains why these materials have high TE performance and are less sensitive to doping inhomogeneities. Our results provide detailed EBS information and fundamental insights on the TE properties of Mg3Sb0.6Bi1.4. Further, the proposed technique can be utilized to probe other Mg3(Sb1-xBix)2 compositions and TE materials.
ABSTRACT
Mg3Sb2-xBix solid-solutions represent an important class of thermoelectric (TE) materials due to their high efficiency and variable operating temperature range. Of particular significance for midtemperature applications is the Mg3Sb1.5Bi0.5 composition whose superior thermoelectric (TE) performance is attributed to the complex conduction band edge in conjunction with alloy dominated phonon scattering. In this work, we show that microstructure also plays a significant role in lowering the lattice thermal conductivity which in turn affects the overall TE performance (change in peak zT values between 1.1 and 1.4 have been observed). Temperature dependent TE properties of Mg3+xSb1.5Bi0.5 compositions with varying nominal Mg content (x = 0.2, 0.3, 0.4) have been studied. A marked reduction of the lattice thermal conductivity (κL) is observed in compositions with low nominal Mg content (x = 0.2), which is due to the presence of lamellar structures within the grains. These lamellar regions are isostructural to the matrix with a low misfit angle and represent compositional fluctuations in the Bi to Sb ratio. Both the size (200 nm-500 nm) and the interfacial strain contribute to the enhanced phonon scattering. A quantitative estimate of κL reduction due to these structures have been carried out using a mean free path (MFP) spectrum analysis which reveal a good match with experiments at room temperature. Further, the electrical properties are not influenced by these lamellar structures as observed from the similar power-factor (S2σ) and weighted mobilities in all of the compositions. This is due to their similar orientation to the adjacent matrix region. Thus, the zT parameter in the various compositions with similar carrier concentration can be significantly altered (â¼25%) by adjusting the nominal Mg content. The results demonstrate that preferential phonon scattering by microstructure modification can be a new route for property improvement in Mg3+xSb2-yBiy solid-solutions.
