Investigating Phase Transition and Morphology of Bi-Te Thermoelectric System

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(Bi$_2$Te$_3$) 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 ($\sim$ 1.6\%) of Bi$_2$Te$_3$ 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 Bi$_2$Te$_3$ with increasing synthesis duration time at constant temperature and vice-versa. XRD and Rietveld refinement revealed that the hydrothermal technique (150 $^\circ$C for 48 hours) can synthesize purest samples (84\% Bi$_2$Te$_3$ 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 $\sim$20 times greater, while Seebeck coefficient is $\sim$3 times lower than that of pure Bi$_2$Te$_3$ phase. The detailed analysis has generalized the growth mechanism in Bi$_2$Te$_3$ phase evolution by the diffusion of Bi into Te nanorods to fabricate hexagonal Bi$_2$Te$_3$.