Co/Bi2Te3 interfacial reactions and Bi-Co-Te phase equilibria

Abstract

Background

Bi₂Te₃ is the most frequently used thermoelectric compound, while Co exhibits promise as a barrier layer candidate. To provide fundamental information, the Bi-Co-Te phase equilibria isothermal sections at 500 and 400 ℃, as well as the Co/Bi₂Te₃ interfacial reactions at 500, 400, and 300 ℃, were determined.

Methods

The Bi-Co-Te alloys and Bi₂Te₃ substrates were synthesized using high-purity constituent elements. To form the Co/Bi₂Te₃ couples, Co was electroplated onto the Bi₂Te₃ substrates, after which they were subjected to reactions at specific temperatures.

Significant findings

In the Bi-Co-Te ternary system, no ternary compounds were found, and all the binary compounds have limited solubility for ternary elements. In the Co/Bi₂Te₃ couples, significant interfacial reactions were observed. The results suggest that Co alone is not a suitable barrier layer. The reaction paths are Co/CoTe/(Bi₂)_m(Bi₂Te₃)_n/Bi₂Te₃ when reacted at 400 and 300 ℃. Although 500 ℃ is below the melting points of both Co and Bi₂Te₃, surprising liquation occurred, and the reaction path is Co/CoTe/Co+ (Bi₂)_m(Bi₂Te₃)_n +liquid (I)/Bi₂Te₃ at 500 ℃. Based on the morphologies and results of the reaction path, it is concluded the dominating diffusion species is Te.