Investigating Phase Transition and Morphology of Bi-Te Thermoelectric System
Abstract
The optimization of secondary phase in thermoelectric(TE) materials can helps in improvisation of material's efficiency. Being a potential contender 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 behavior, 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 co-precipitation 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 for 48 hours) can synthesize purest samples (84\% Bi2Te3 phase in this case). FE-SEM and Energy Dispersive X-ray analysis unveiled that the impure phases in the system were quantitatively reduced, and it supported by decline in atomic percentage of oxygen from 37\% to 11\%, in addition to this, it was also found that particle size was also decreased with increase in temperature. The observed electrical conductivity of the chosen sample is 20 times greater, while Seebeck coefficient is 3 times lower than that of 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.