Hydrometallurgy

[1401-10-10] A research for high performance and low cost Bi₂Te₃ thermoelectric materials

○Tatsuichiro Nakamoto¹, Shun Yokoyama¹, Kenichi Motomiya¹, Koichi Harata¹, Hideyuki Takahashi¹, Kazuyuki Tohji¹ (1. Tohoku University)

Chairman: Kazuya Koyama (Chiba Institute of Technology)

Keywords:thermoelectric materials, Bi2Te3, nanoparticles, nanostructure

Thermoelectric materials, which can convert from thermal energy to electric energy, are needed for improving sustainability of our society based on electricity. Although it is known that Bi₂Te₃ is a good thermoelectric material for low temperature, its efficiency is insufficient for practical use. The performance of a thermoelectric material is evaluated by a dimensionless figure of merit; ZT which is defined as ZT = S²σT/κ, where S, σ, κ and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. In order to decrease κ while maintaining high σ, a prospective approach is to introduce nano-sized crystal boundaries in bulk Bi₂Te₃ by consolidating Bi₂Te₃ nanoparticles. This approach makes use of phonon scattering in the boundaries. Although the optimal crystal size is predicted to exist, the relationship between the crystal size and ZT value has not been investigated well because of difficulties to control the crystal size precisely during synthesis process and prevent crystal growth completely during sintering process. In this study, we controlled crystal size by controlling synthesis and consolidation conditions of the Bi₂Te₃ nanoparticles to investigate the relationship.
Bi₂Te₃ nanoparticles were prepared by our previously reported low cost synthesis method based on chemical reduction reaction. Ascorbic acid was used instead of toxic reducing agent. The driving force of the reduction reaction was controlled by changing the concentration of ascorbic acid, resulting in size controlled Bi₂Te₃ nanoparticles. The Bi₂Te₃ nanoparticles were consolidated into bulk by using spark plasma sintering: SPS. The optimal sintering temperature was found from observation of Bi₂Te₃ nanoparticles behavior during sintering, leading to nanostructured bulk Bi₂Te₃ with high density and low crystal growth. Size effect of nanostructure in bulk Bi₂Te₃ on thermoelectric properties will be shown in our session.