Conducting glasses as new potential thermoelectric materials: the Cu–Ge–Te case

Abstract

Recent approaches to improve performance of bulk thermoelectric (TE) materials show that they should have complex structures, include inclusions and impurities, possess mass fluctuations, disorder and be based on heavy elements. Glasses can possess these properties. In order to identify glasses with interesting TE potential, attention should be focused on small gap semiconducting or semimetallic glasses. Chalcogenide glasses with Cu_x+yGe_20−xTe_80−y (0 ≤ x ≤ 20; 0 ≤ y ≤ 10) compositions were prepared by melt spinning. Their powder X-ray diffraction analyses point to a short-range order analogous to Ge₂₀Te₈₀, with Cu atoms most likely replacing Ge atoms in the GeTe₄ structural unit. It also indicates, together with the differential scanning calorimetry results, a reduction in the glass stability with the increase in Cu concentration. The enhancement of Cu content dramatically reduces (five orders of magnitude) the electrical resistivity, while keeping the Seebeck coefficients at large values (∼400 μV K⁻¹). As a consequence, a huge increase in the power factor is observed, up to a maximum value of 60 μW K⁻² m⁻¹ for the Cu_27.5Ge_2.5Te₇₀ glass at T = 300 K. Ge₂₀Te₈₀ has extremely low lattice thermal conductivity values (∼0.1 W K⁻¹ m⁻¹ at 300 K), which points to relatively high values for the figure of merit ZT for this family of glasses, and indicates Cu_x+yGe_20−xTe_80−y based glasses as good candidates for obtaining high performance thermoelectric materials.