Investigation of thermoelectric properties of Sn-Te based alloys with microstructural studies

Abstract

The basic concept of thermoelectricity is the conversion of dissipated heat into electricity with applications in the areas such as automobile engineering, refrigerating coolants, satellites, etc. The thermoelectric figure of merit,zT=((S^2 σ)/κ) T, is the defining factor for thermoelectric materials, which attributes to the power factor (P=S2σ, σ-electrical conductivity S-Seebeck coefficient) enhancement and thermal conductivity reduction. Power factor can be improved by bandgap tuning and carrier concentration, whereas thermal conductivity can be reduced through grain size reduction, point defects, and dislocations. These can be achieved experimentally through microstructural engineering and processing. Microstructure can be tuned by varying volume fraction, morphology variation, inclusion of nano, 2D structures etc. Fine-tuning of microstructure is one of the key factors in improving thermoelectric properties. Creating interfaces through incorporation of multiple phases, self-organised eutectic composites are efficient ways to reduce thermal conductivity. In this thesis, we have chosen to investigate the thermoelectric properties of Sn-Te based alloys with microstructural studies which can be divided into three parts as follows. The synthesis and properties of thermoelectric off eutectic and eutectic composites of the Sn-Te binary system are presented in the preliminary part of the investigation. Hypo eutectic alloys are SnTe rich, and hyper eutectic alloys are towards the phase diagram’s Te rich side with Te and SnTe phase mixture. Interfaces between SnTe and Te phases and the presence of primary and eutectic phase mixture do affect the mobility of charge carriers giving rise to optimized electrical transport properties. The second part consists of the influence of morphological variation of a eutectic composite of SnTe and Te on thermoelectric properties. Microstructural variations obtained by solidifying eutectic composite via furnace, air, oil, water, and (water + glycol) were characterized to understand the microstructural evolution. Novel result of transformation from rod-lamellar was revealed in the eutectic microstructures. Thermoelectric potential of these alloys were explored. Various factors contributing to increased power factor and decreased thermal conductivity was analysed. The effect of cooling rate on overall Figure of merit (FOM) was studied. The third part consists of the third element addition to the Sn-Te binary system. Upon the addition of Sb to the Sn-Te system, ternary eutectics have displayed improved electron transport properties. Contribution of each phase present in the alloy towards the improvement of electron transport properties was explored. Further due to microstructural complexity, thermal conductivity is expected to be reduced. This work consists of robust microstructural modification along with improving thermoelectric properties.