ENHANCEMENT OF THERMOELECTRIC PROPERTIES OF LOW-TOXIC AND EARTH-ABUNDANT CHALCOGENIDES

Abstract

Thermoelectricity is a promising technology that directly converts heat energy into electricity and finds its use in enormous applications. This technology can be used for waste heat recovery from automobile exhausts and industrial sectors, and convert the heat from solar energy to electricity, especially in hot and humid areas such as Qatar. This work aims to develop high-performance Chalcogenide-based thermoelectric (TE) materials using sustainable sources that would further the ongoing activities in developing a cost-effective TE system. This research revolves around two main parts as follows: The objective of the first part is to investigate the optimum sintering method and temperature that can improve the efficiency of bismuth telluride cold compact pellets for thermoelectric applications. Different p-type and n-type bismuth telluride cold compact pellets were treated using three different sintering techniques and conditions, namely pressure less (conventional), microwave, and tube (using argon environment) at temperatures 250 °C, 300 °C, 350 °C, and 400 °C. The structural, microscopic, electron transport, thermal, dielectric, and mechanical properties of the non-sintered or pristine and sintered samples were examined. Even though each type of sintering has its own merits and demerits, results showed that the optimum conditions for enhanced electric and thermal properties could be obtained using the microwave sintering method, followed by the tube and conventional ones. Low thermal conductivity of 0.4 W/m/K was observed in the samples sintered at 250 °C while the increase in sintering temperature in all the three sintering methods from 250 °C to 400 °C improved the crystallinity of the material. These samples exhibited also excellent thermal and mechanical stability. In the second part, we enhanced the thermoelectric properties of p-type copper selenide (Cu2Se) which is a low toxic earth-abundant intermediate material for thermoelectric applications, by a low-cost simple, and fast microwave-assisted metallurgy route. Hot pressed copper selenide pellets were treated at various temperatures in the range of 250 oC to 425 oC in a microwave furnace. We analyze the variation of several characteristics of the samples, such as electrical, thermal, structural, microscopic, dielectric, and mechanical properties when the samples were exposed to different annealing conditions. Broadband dielectric spectroscopy (BDS) analysis was used to understand the variation with temperature and frequency of the AC electrical characteristics of the samples such as AC conductivity, dielectric permittivity storage, dielectric loss, and AC capacitance. The results indicate exceptional electrical, thermal, and mechanical properties for samples annealed at 375oC with room temperature electrical conductivity as high as 538.3 S/cm and ultra-low thermal conductivity as low as 0.59 W/mK. The crystallinity of samples with the increase in annealing temperature from 250 oC to 725 oC along with the formation of oxides above 400 oC annealing temperature. The method used in this research to develop high-performance chalcogenide-based thermoelectrics is a low-cost, simple, and quick process that has produced TE materials with excellent electrical, thermal, and mechanical characteristics.