Boron strengthened GeTe-based alloys for robust thermoelectric devices with high output power density

High-performance thermoelectric (TE) devices require not only a high figure of merit (ZT) but also mechanical strength and thermal stability. Here, a simultaneous enhancement of ZT as well as mechanical properties is obtained in GeTe-based alloys by adding boron. This material is then assembled with n-type CoSb${}_3$ skutterudite into TE modules. The improved ZT values result from the increase in charge carrier mobility due to the reduced interfacial barrier height. A peak ZT of 2.2 at 773 K can be achieved in Ge${}_{0.84}$Pb${}_{0.1}$Sb${}_{0.06}$TeB${}_{0.07}$, which shows a negligible change in the coefficient of thermal expansion upon the phase transition from the rhombohedral to the cubic phase, ensuring good thermal stability of the device. Resulting from the boron-induced grain refinement and dispersion strengthening, the average compressive strength and Vickers hardness of Ge${}_{0.9}$Sb${}_{0.1}$TeB${}_{0.07}$ can be enhanced to ≈227 MPa and ≈202 H${}_v$, respectively. The improved mechanical properties facilitate the assembly of devices and lower the interfacial contact resistance. As a synergy of increased ZT and mechanical strength, a high output power density of ≈1.76 W cm${}^{-2}$ at ΔT = 425 K and an energy conversion efficiency of 7.4% at ΔT = 477 K can be achieved in the TE modules. (© 2021 Wiley-VCH GmbH)