Regulation Of Thermoelectric Performance Of Bismuth-Telluride-Based Compounds

Thermoelectric materials are based on the three thermoelectric effects,and the related research is important in the development of new energy and the effective utilization of waste heat.The Bi₂Te₃-based thermoelectric materials have excellent room-temperature thermoelectric properties and have successfully achieved commercial applications.The n-type Bi₂Te₃-based alloys generally have low thermoelectric properties compared to p-type Bi₂Te₃-based alloys,which limits the performance improvement of the thermoelectric device.To promote the commercial application of the Bi₂Te₃-based compounds,it is necessary to effectively improve the thermoelectric properties of the n-type Bi₂Te₃-based alloys.Extensive studies show that Bi₂Te_2.7Se_0.3 has the best thermoelectric properties in the n-type Bi₂Te₃-based alloys.In this paper,n-type Bi₂Te_2.7Se_0.3 is systematically studied from the perspective of improving the thermoelectric performance of Bi₂Te₃-based compounds,and the main results are as follows:（1）Since Bi₂Te₃-based thermoelectric materials are applied near room temperature,improving the room temperature thermoelectric performance of n-type Bi₂Te₃ is crucial to its practical application.This study demonstrates that Cd is a very effective dopant that can optimize the room-temperature thermoelectric properties of the samples by reducing the carrier concentration.First,the doped Cd results in the reduced carrier concentration,increased Seebeck coefficient and reduced electrical conductivity.Secondly,the significant decrease in the electrical conductivity causes a large decrease in the electronic thermal conductivity.Finally,the lattice disorder caused by the replacement of Cd for Bi enhances the phonon scattering and results in a decrease in the lattice thermal conductivity.The mutual reduction in the electronic and lattice thermal conductivity results in a significant reduction in the total thermal conductivity of the samples.Meanwhile,the decrease in the carrier concentration results in the optimal thermoelectric properties of the samples shift toward room temperature.As a result,in the vertical hot pressing direction,the Cd_0.004Bi_1.996Te_2.7Se_0.3 sample reaches a peak z T of 0.97 at 448 K.This result suggests that the room temperature thermoelectric properties of the n-type Bi₂Te_2.7Se_0.3 samples can be optimized by reducing the carrier concentration.（2）Bi₂Te₃-based alloys have a large number of intrinsic defects,and the thermoelectric properties of the materials can be improved by regulating intrinsic defects.In this paper,the regulation of intrinsic defects is achieved by introducing excess Bi in Bi₂Te_2.7Se_0.3samples.First,excessive Bi results in the increased antisite defects(Bi_Te^′,Bi_Se^′),which in turn causes a decrease in the carrier concentration.The decrease in the carrier concentration resulted in the increased Seebeck coefficient and reduced electrical conductivity of the samples.Secondly,the significant decrease in the electrical conductivity causes a large decrease in the electronic thermal conductivity;the increase in antisite defects leads to an enhancement of phonon scattering and a decrease in the lattice thermal conductivity.The simultaneous reduction in the electronic and lattice thermal conductivity results in a significant reduction in the total thermal conductivity of the samples.As a result,the optimal thermoelectric properties of the samples shift toward room temperature,and the Bi_2.008Te_2.7Se_0.3 sample reaches a peak z T of 0.88 at 348 K in the vertical hot pressing direction.This result suggests that the thermoelectric properties of the n-type Bi₂Te_2.7Se_0.3 samples can be optimized by regulating the intrinsic defects.