Optimization Of Thermoelectric Transport Properties Of Some Chalcogenides

The modern society is demanding clean, efficient and environmental friendly sources of energy to overcome the depletion of fossil fuels. Green energy is now getting more attention because of environmental concerns and global warming issues associated with consumption of fossil fuels. One of the major issues associated with low efficiency of different energy systems is the loss of energy in the form wasted heat. However, thermoelectric materials have ability to convert waste heat into electrical energy without involving complex mechanical systems which makes these materials noiseless and environment friendly. Here, we report synthesis and thermoelectric properties of pure and composites of BiCuSeO and Cu2Se-based chalcogenides. We have systematically investigated the effect of Cd-doping at Bi site and Ag-doping at Cu site of BiCuSeO in order to optimize low conversion efficiency of BiCuSeO. By controlling phase purity and doping site, our experimental results reveal the excellent linkage between the improvement in transport properties and the structure of BiCuSeO. We obtained some novel results for the Ag substituted at Cu-site, which was considered as unsuccessful approach by employing other dopant such as Cd and Zn.We also report the structural and thermoelectric properties of composite structures of binary chalcogenides like Cu2-xSe, CU2S and ZnTe, in which, two different series of Cu2Se-based composites were fabricated by dispersing nano-phases of Cu2S and ZnTe in the matrix of Cu2Se. The observed improved performance is a direct result of simultaneous improvement of Seebeck coefficient (S) because of the external strains induced by Cu2S nano-inclusions in the matrix of Cu2-xSe which declined the total thermal conductivity by suppressing both electronic and lattice thermal contributions. As a result, a higher ZT value of 0.90 at 773 K was achieved with 10 wt.% of Cu2S. On the other hand, the ZnTe nano-inclusion embedded in the matrix of Cu2-xSe resulted in a substantial improvement of 32% in the electrical conductivity of Cu2-xSe-ZnTe composite. The increase in electrical conductivity is at the expense of Seebeck coefficient which slightly decreased the power factor of composite samples than that of pure Cu2-xSe. Further, introduction of secondary phase facilitates in declining the total thermal conductivity of Cu2-xSe-ZnTe composites up to 34% by suppressing the lattice thermal contributions. Thus, the moderate power factor and lower thermal conductivity values resulted in an improved ZT value of-0.40 in mid-range temperature (750 K) for Cu2-xSe-ZnTe composite containing 10 wt.% of ZnTe, which is about 40% higher than that of its pure counterpart. Hence, it is believed that the incorporation of nano-inclusions in the matrix of Cu2-xSe may be realized as an important route to improve the thermoelectric properties of Cu2-xSe based compounds.