Preparation of n-Type Bi₂Te_2.85Se_0.15 Thermoelectric Semiconductor without Harmful Dopants

Abstract

Undoped n-type Bi₂Te_2.85Se_0.15 compounds without harmful dopants were prepared by mechanical alloying (MA) and sintered by hot pressing (HP). Samples of the Bi₂Te₃-based materials had a nominal composition of Bi₂Te_2.85Se_0.15. Dopants were not added to control the carrier concentration. The constituent elements, i.e., Bi (5 N), Te (6 N), and Se (5 N), were weighed according to the target composition and milled with silicon-nitride balls. MA was carried out for 30 h. The resulting MA powder was sintered by HP in the temperature range 573–673 K under a mechanical pressure of 147 MPa in an argon atmosphere. The Seebeck coefficient α and electrical conductivity σ were measured in the temperature range 300–473 K. The thermoelectric performance was evaluated from the power factor P, where P = α²σ. The obtained samples exhibited n-type conduction and the single-phase of Bi₂(Te, Se)₃. The power factor for an undoped sample sintered at 623 K was 4.4 × 10⁻³ W m⁻¹K⁻² at 313 K. This power factor was 88% of the value of 5.0 × 10⁻³ W m⁻¹K⁻² reported for single crystals of n-type doped Bi₂Te_2.85Se_0.15. These results indicated that it is not necessary to dope Bi₂Te_2.85Se_0.15 prepared by an MA–HP process with harmful halide dopants to achieve carrier control.