Thermoelectric properties of aliovalent Zn doped Cu1.8S polycrystalline materials

Cu1.8S-based materials are regarded as promising thermoelectric materials due to their super ionic nature. The Cu1.8-xZnxS (x = 0, 0.01, 0.03, and 0.09) samples were synthesized by the hydrothermal method followed by cold pressing and sintering. The powder X-ray diffraction pattern has confirmed the phase purity and rhombohedral crystal structure of Cu1.8S. Additionally, FE-SEM micrographs along with the EDX elemental mapping detected the existence of CuS as the secondary phase in the Cu1.8S matrix and uniform distribution of the constituent elements. Upon doping the Zn atom, the Seebeck coefficient subsequently increases owing to the decrement in the carrier concentration, whereas the electrical conductivity gradually reduces. Essentially, the presence of CuS secondary phase and the defects induced by the Zn atom effectively scattered low-mid wavelength phonons, where a very low thermal conductivity of 3.65 W/mK was achieved for Cu1.71Zn0.09S at room temperature. Ultimately, an overall maximum zT of 0.07 at 573 K was observed for Cu1.77Zn0.03S, which is about 3.5 times greater than that of the pristine sample.