Abstract
Thermoelectric materials, which convert thermal energy into electrical energy and vice versa, have long been considered as a prospective solution for energy harvesting. Bismuth telluride based compounds are widely used in commercial thermoelectric devices due to their superior thermoelectric figure-of-merit at room temperature regime. In this research, a systematic study of the nanostructured n-type Bi-Se-Te compound prepared by powder metallurgy and subsequent electric-current assisted sintering process has been conducted. The effects of electric current stressing on the microstructure and thermoelectric transport properties of the sintered Bi-Se-Te materials are the main investigated subject. The experimental results shows significant improvement of thermoelectric properties after electric current sintering: the electrical conductivity and the Seebeck coefficient of Bi-Se-Te compounds are greatly improved which could double the overall zT value. The electrical stressed Bi-Se-Te material appears to have lower carrier concentration but much higher mobility than the untreated ones. These enhancements due to electric current stressing are so significant that cannot be simply attained by heating the sample and sintering the sample at the similar temperature. Thus, the electric current should play an important role in modulating crystal defects in the material. Our study found that during electric current stressing, the highly local temperature gradient between the interfaces and grains and the electric current induced decrease in the mobility activation energy of defects enhance the motion of atoms and defects. Therefore the crystal defects are effectively eliminated by easy motion toward defect sinks as grain boundaries, leading to lower carrier concentration and much higher electron mobility than simply thermal annealed one. Based on the mechanism involving electric-current induced atomic migration, the material can be further optimized by the electric-current assisted sintering process, leading to higher thermoelectric efficiency.