Several approaches can be used to optimize the efficient of thermoelectric materials. Particularly the lattice thermal conductivity kl can be reduced effectively by create a high phonon scattering at point and mass defect or interfaces. This can be achieved by forming a solid solution, nanstructuring or phase separation. We focus her to reduce the thermal conductivity of half-Heusler compounds by targeted phase separation for n-types as well as p-types.
The microstructures of phase separated half-Heusler Ti1−xHfxCoSb0.85Sn0.15 are displayed in the backscattered electron (BSE) images in Figures 1(a)-(f). The amount and shape of the second Heusler phase depends on the ratio of Ti to Hf. Samples with a single element Ti (x = 0) or Hf (x = 1) exhibit homogenous distribution indicating existence of single phase. Samples with 0.75≤ x ≤0.25 show a fine network of the second phase within the matrix.
An element-specific EDX mapping of the composition Ti0.25Hf0.75CoSb0.85Sn0.15 (as shown in Figure 2) confirm the phase separation of this composition into a Ti-Sn-rich phase and a Hf-Sb-rich phase.
Figures 3a-d shows the temperature dependence of the thermoelectric properties of the series Ti1−xHfxCoSb0.85Sn0.15. All compounds exhibit a positive S, with a maximal value of +278 µV/°C at 700°C for TiCoSb0.85Sn0.15. All samples exhibit a metallic behavior of s, which increase from x =1 (Ti) to x = 0 (Hf). The thermal conductivity k values are highest (4.10 WK-1m-1) for single-phase samples with x = 0 and x = 1 and effectively (> 40%) suppressed by substituted composition reaching a minimal value of 2.30 WK-1m-1 at 980°C for x = 0.25. The presence of the intrinsic microstructure phase separation leads to additional boundary scattering at the interfaces and hence to a reduction of kl. Fine-tuning of the Ti–Hf ratio has a significant impact on zT. The selection of the optimal electronic properties in combination with the lowest thermal conductivity leads to the maximum figure of merit of zT = 1.2 at 710°C for Ti0.25Hf0.75CoSb0.85Sn0.15 as shown in Fig. 3d.