Power generation from the Cu₂₆Nb₂Ge₆S₃₂-based single thermoelectric element with Au diffusion barrier

In this study, we have developed a diffusion barrier for thermoelectric colusite Cu₂₆Nb₂Ge₆S₃₂ and evaluated the conversion efficiency of the Cu₂₆Nb₂Ge₆S₃₂-based single thermoelectric element. This Cu₂₆Nb₂Ge₆S₃₂-based single element with metal diffusion barriers (Ti, Pt, Ni, and Au) was prepared through hot pressing. Microstructural investigations revealed microcrack formation at the interface between the Ti/Pt diffusion barriers and Cu₂₆Nb₂Ge₆S₃₂ due to the mismatch of the coefficient of thermal expansion between them. Although no cracks were observed at the interface between the Cu₂₆Nb₂Ge₆S₃₂ and Ni diffusion barriers, secondary phases of Ni-S, Ni-Ge, and Cu-S were formed around the interface. A good match in the coefficient of thermal expansion between Cu₂₆Nb₂Ge₆S₃₂ and Au resulted in a crack-free interface. Moreover, no secondary phases were found around the Cu₂₆Nb₂Ge₆S₃₂ and Au interface. Therefore, the Au diffusion barrier allows a reduced specific contact resistance of 4-5 × 10^-10 Ω m². In the conversion efficiency evaluation, the radiative heat transfer was compensated by using silica glass as a reference. The maximum thermoelectric conversion efficiency (η_max) of ∼3.3% was estimated at the hot-side temperature (T_h) of 570 K and the cold-side temperature (T_c) of 297 K for the Cu₂₆Nb₂Ge₆S₃₂-based single element with a Au diffusion barrier. Three-dimensional finite-element simulations for the Cu₂₆Nb₂Ge₆S₃₂-based single element predicted the η_max of ∼4.5% at T_h and T_c of 570 K and 297 K, respectively. Therefore, there is further scope for improvement in the performance of the Cu₂₆Nb₂Ge₆S₃₂-based element.