Secondary phase induced electrical conductivity and improvement in thermoelectric power factor of zinc antimonide films

Abstract

The urgent global need to develop sustainable energy conversion methods and to minimize green-house gas emissions has intensified interest in finding more efficient and non-polluting means for power generation. Thermoelectric materials directly convert heat into electricity which can be used for power generation and waste heat recovery. The waste heat is mostly in the intermediate temperature range and current materials are economically unviable for the recovery of heat loss. Zinc antimonide, β-Zn4Sb3, has one of the highest figures of merit (ZT) owing to its phonon-glass-electron-crystal (PGEC) structure with potential application in the intermediate temperatures (500 K–900 K). It shows poor thermal stability and often includes secondary phases zinc antimony (ZnSb) and zinc (Zn) which deteriorate its thermoelectric properties. Zn4Sb3 films were prepared by ion beam sputtering and their electrical and thermoelectric properties were studied following annealing at temperatures up to 400 °C. Rutherford backscattering spectroscopy (RBS) and electron diffraction spectroscopy results showed formation of Zn-rich Zn4Sb3 films. X-ray diffraction (XRD) and Raman results corroborated the formation of the β-Zn4Sb3 phase along with the presence of a ZnSb secondary phase. Annealing the zinc antimonide films increased electrical conductivity (σ) by three orders of magnitude with σ ∼ 3.7 × 104 Sm−1 for films annealed at 400 °C. Increased conductivity is associated with increased charge carrier density and mobility due to decomposition of the ZnSb secondary phase and improved crystalline quality of the films. The highest Seebeck coefficient of α ∼171 μVK−1 was found for films annealed at 100 °C. The power factor (α2σ) increased with annealing temperature, and the maximum value of α2σ ∼7.5 μWm−1K−2 was obtained for films annealed at 400 °C due to the correspondingly high electrical conductivity. Our findings indicate that the thermoelectric properties of Zn4Sb3 films can be tuned via annealing-induced suppression of the high-resistance ZnSb secondary phase, leading to multifold changes in the electrical conductivity and Seebeck coefficient.