Investigating phase transition and morphology of Bi-Te thermoelectric system

Abstract

The optimization of secondary phase in thermoelectric (TE) materials can help to improve the efficiency of the material. Being a potential candidate for lower temperature TE application, bismuth telluride (Bi2Te3) nanoparticles were synthesized via different routes and profiles to optimize their pure single phase. Systematic characterizations were performed with the help of X-ray diffraction (XRD), Rietveld refinement, and field effect scanning electron microscopy (FE-SEM) for structural and morphological behaviors, while TE properties such as Seebeck coefficient, electrical conductivity, and power factor were measured for the purest sample chosen. Rietveld refinement in the XRD pattern of the samples revealed that only a small amount (∼ 1.6%) of Bi2Te3 was formed in the coprecipitation method, while the hydrothermal technique increases this phase with increment in synthesis duration. This work focused on the phase evolution of Bi2Te3 with increasing synthesis duration time at constant temperature and vice versa. XRD and Rietveld refinement revealed that the hydrothermal technique (150°C for 48 h) can synthesize purest samples (84% Bi2Te3 phase in this case). FE-SEM and energy-dispersive X-ray analysis also indicated that the impure phase in the system is decreased along with an atomic percentage of oxygen from 37% to 11% and decreases in particle size with increase in temperature in case of hydrothermally synthesized samples. Decrease in the size of the particle, with an increase in synthesis temperature, shows a decrease in percentage abundance of Bi2Te3 phase due to surface Te desorption. The observed electrical conductivity is ∼20 times greater, while Seebeck coefficient is ∼3 times lower than that of the pure Bi2Te3 phase. The detailed analysis has generalized the growth mechanism in Bi2Te3 phase evolution by the diffusion of Bi into Te nanorods to fabricate hexagonal Bi2Te3, and Te-desorption from the surfaces of these particles.